Pre-amp fixed biased. Anyone else do it this way ?

[mooreamps]

Just curoius.

-g

[Blue Strat]

Not sure I understand your question. Can't think of any preamp tube circuits I've seen that weren't cathode biased.

[mooreamps]

Not sure I understand your question. Can't think of any preamp tube circuits I've seen that weren't cathode biased.

Mine are not cathode biased. I started doing this way as far back as Jan of 07. I know this because that's one reason I keep those youtube videos up there. I also sent a full set of prints to KOC, just to see what he thought of it.

-g

[Blue Strat]

Who's KOC (I know who KFC is, but I prefer Popeye's) and what other information do you have to offer (or is this an "I know something you don't know" thread) :D

Wow, as far back as Jan '07? Isn't that like, the dawn of tube amp history? :D

BTW, I like your work on "Still Got the Blues For You". ;)

[embot]

I've read an article about Mike Zaite and Dr. Z amps. The article mentioned that some of Dr. Z Amps have fixed-biased phase inverters.

[hasserl]

Fixed bias for cathodyne phase inverters is pretty common. On the net it's known as the Paul C mod. Another fairly common method of biasing preamp tubes is called grid leak bias. Fixed bias for preamp tubes would require a low negative voltage source (around -1 to -2 volts), seems like a lot of hassle for what benefit?

[phsyconoodler]

Most preamps are cathode biased except some octals which are grid-leak biased.Can't really see why anyone would want to make them fixed bias.
Tell us what you are getting at?

KOC is Kevin O'Connor,Canada's reining amp guru.I think he eats a lot of KFC.

[xk49w]

Another fairly common method of biasing preamp tubes is called grid leak bias.Grid leak bias - I thought that was an RF power amp thing. You have to draw grid current and charge em up. What audio amp does that?

[mooreamps]

A friend of mine wrote me an e-mail, saying they were discussing this on another forum, and possibly talking about it over on ax84. Obviously it must not be a common practice. I do it on all my amplifiers, in every gain stage in the pre-amp, and on my boot-strapped cathode-followers. One thing I have noticed, it does dump most if not all the hiss from the loud speaker. I'll post a drawing if there is any interest in discussing this further. Thanks...


-g

[VacuumVoodoo]

Nothing new. That's how the first triode ever made was biased. Some high end phono preamps have small batteries to bias the triodes. The very top end guys use supercapacitors for same purpose.

[darkbluemurder]

I believe Jim Kelley used fixed bias on the split load phase inverters in his amps. If I am not mistaken Mesa Boogie used fixed bias long tail phase inverters in the Studio 22+ and Caliber models. One possible reason is to make the amps louder and cleaner but as I have never tried it I am not sure.

I would be very much interested in keeping this discussion going.

BTW: "KOC" is Kevin O'Connor, author of the "The Ultimate Tone" book series.

Cheers Stephan

[mooreamps]

Nothing new. That's how the first triode ever made was biased. Some high end phono preamps have small batteries to bias the triodes. The very top end guys use supercapacitors for same purpose.

Yes, I know.
An A battery for filament voltage.
A B battery for plate voltage.
and a C battery for grid bias ; or fixed bias.

-g

[RedMan]

It seems to me that it would eliminate the possibility of biasing stages differently throughout the preamp and the ability to tone shape gain stages by partially bypassing or unbypassing cathode resistors. I can see all the disadvantages but the only advantage I see is a simpler circuit. I don't see why it would cut down on hiss unless you're biasing the preamp stages really cold. Maybe I am missing some other advantages. Why do you suppose the hiss is being reduced?

[UR12]

:munch:horse

[mooreamps]

It seems to me that it would eliminate the possibility of biasing stages differently throughout the preamp and the ability to tone shape gain stages by partially bypassing or unbypassing cathode resistors. I can see all the disadvantages but the only advantage I see is a simpler circuit. I don't see why it would cut down on hiss unless you're biasing the preamp stages really cold. Maybe I am missing some other advantages. Why do you suppose the hiss is being reduced?

When you remove the cathode resistor, you remove the contribution of the thermal noise it generates, even the low-noise metal film types.

-g

[mooreamps]

:munch:horse

You Got Something You Want To Say ???????
or Are You Trying To Get Me Kicked Off This Forum Too !!!!!!!!!!!

[UR12]

You Got Something You Want To Say ???????
or Are You Trying To Get Me Kicked Off This Forum Too !!!!!!!!!!!

Nope.......... I'm good. Just going to sit back and soak all of this up.

Do you have any clips of one of your amps that has the diodes installed that shows how quiet the amp hiss is just by replacing those cathode resistors with diodes? That might be an excellent demo to show the rest of the amp building world how quiet your amps are with your superior technology.

[Mach Schnell]

I'm confused. What the hell is this thread about?...........:confused:

[mooreamps]

These are the best clips I have showing my prototype 10 watt amp using fixed bias in the pre-amp.



http://www.youtube.com/watch?v=FV59lHvFQwo&feature=user

http://www.youtube.com/watch?v=fL8adKftJ2Y&feature=user



-g

[mooreamps]

curious. so you are grounding the cathode and putting a negative voltage (or whatever voltage) to the grid resistor?

I replace the cathode resistor with an ultra-bright led. It also serves as a bite lamp for that gain stage. I also have a refinement to my power brake not shown on this video.

-g

[conundrum]

Isn't that still cathode bias'd though? Same idea that Mesa had on the Mark I with a zener diode on the first stage.

[kt77]

These are the best clips I have showing my prototype 10 watt amp using fixed bias in the pre-amp.



http://www.youtube.com/watch?v=FV59lHvFQwo&feature=user

http://www.youtube.com/watch?v=fL8adKftJ2Y&feature=user



-g

OMFG!!! That is SMOKIN! That dude can play, man. What is that amp? It has a 6BQ5 AND a 6v6? Wow. How do I get one? That has to be THE BEST amp tone I've ever heard.....the sustain is magnificent. Nice clean tone. Reminds me of Billy Gibbons early tones. And Dicky Betts.
Is there a website to order?
Man, I am blown away. That blows away my '67 plexi AND my tweed bassman hands down. In that itty bitty amp, no less. It's easily worth $5k...no $10k.....
Gary, you're a genius...an outright freakin genius....a true god ampbuilder......
Stand back Jim Marshall, Dave Reeves, Mark Sampson, Leo Fender, et al.......Gary 'AMPGOD' Moore has arrived. And he's packin' heat in the form of an Epi VJ......might as well turn over your adolescent daughters now....he wants em'...and he'll get em.....w/ this amp.

[conundrum]

OMFG!!! That is SMOKIN! That dude can play, man. What is that amp? It has a 6BQ5 AND a 6v6? Wow. How do I get one? That has to be THE BEST amp tone I've ever heard.....the sustain is magnificent. Nice clean tone. Reminds me of Billy Gibbons early tones. And Dicky Betts.
Is there a website to order?
Man, I am blown away. That blows away my '67 plexi AND my tweed bassman hands down. In that itty bitty amp, no less. It's easily worth $5k...no $10k.....
Gary, you're a genius...an outright freakin genius....a true god ampbuilder......
Stand back Jim Marshall, Dave Reeves, Mark Sampson, Leo Fender, et al.......Gary 'AMPGOD' Moore has arrived. And he's packin' heat in the form of an Epi VJ......might as well turn over your adolescent daughters now....he wants em'...and he'll get em.....w/ this amp.
:munch
This thread is about to get awesome.

[judyisjudy]

he he he !!!!! You just keep on working on your projects gary. I'm sure things will turn around for you.

[mooreamps]

OMFG!!! That is SMOKIN! That dude can play, man. What is that amp? It has a 6BQ5 AND a 6v6? Wow. How do I get one? That has to be THE BEST amp tone I've ever heard.....the sustain is magnificent. Nice clean tone. Reminds me of Billy Gibbons early tones. And Dicky Betts.
Is there a website to order?
Man, I am blown away. That blows away my '67 plexi AND my tweed bassman hands down. In that itty bitty amp, no less. It's easily worth $5k...no $10k.....
Gary, you're a genius...an outright freakin genius....a true god ampbuilder......
Stand back Jim Marshall, Dave Reeves, Mark Sampson, Leo Fender, et al.......Gary 'AMPGOD' Moore has arrived. And he's packin' heat in the form of an Epi VJ......might as well turn over your adolescent daughters now....he wants em'...and he'll get em.....w/ this amp.


I don't know. I was thinking I was going to finish building these last two or three amps, and then go back to doing rocket science. I was really hoping just to make a connection with other players and other builders, but it would seem the only thing I've managed to accomplish was to alienate myself, from everyone. I'm also not real happy how much my wife interfered with this whole thing, and made some matters from bad to worse. Anyway, thank you for complient. Biulding equipment is hard work, maybe it's not been a total loss....

-g

[phsyconoodler]

The playing was real choppy at best,however,the amp did sound good.That guitar sucked bad!
Gary,are there any better players where you are?
To get your product some better reviews you need better guitar players.
Seriously,I am not a good player compared to a few friends I have,but I guarantee I can play better than that guy with no effort at all.
There are some good tones in that amp,but it doesn't come through with that poor attempt.
It's not your amp,it's the player.You actually have some cool stuff going on in that amp that I know nothing about.
That speaker cabinet is suspect too.It looks like an MDF cabinet and MDF is the worst for guitar amps.Great for the home stereo,but not for a guitar amp.
The video was much better and the sound reproduction was better too.
But holy moley Gary,that player was lousy at best.
Does anyone live near Gary that can actually
play a guitar and wants to give him a few minutes of their time?
I want to hear the amp for real.

[phsyconoodler]

By the way KT77,that kind of remark was really not needed.

[judyisjudy]

gary, dear.... You don't really mean that do you ? Don't you see you are already starting to make some connections ? If this is something you want to talk about, you can talk to me about this. We would really like to see you continue with your work, as we would encourage anyone with your talents to continue working in this field.

[douglasamps]

I don't know. I was thinking I was going to finish building these last two or three amps, and then go back to doing rocket science. I was really hoping just to make a connection with other players and other builders, but it would seem the only thing I've managed to accomplish was to alienate myself, from everyone. I'm also not real happy how much my wife interfered with this whole thing, and made some matters from bad to worse. Anyway, thank you for complient. Biulding equipment is hard work, maybe it's not been a total loss....

-g
Just "KEEP ON Truckin" and if someone isn't happy with your stuff.........
:hidehttp://i303.photobucket.com/albums/nn150/douglasamps/complaint.jpg

[mooreamps]

Just "KEEP ON Truckin" and if someone isn't happy with your stuff.........
:hide



Hmmmm... an appealing notion, but perhaps a little too strong for me. I was talking to some of my friends. One of them reminded me every one who came a somebody in the music business, started out as a nobody, like me. What they didn't do is throw in the towel, and that one came from someone pretty high up there. So, if they are seeing this, I'm still in....

-g

[kt77]

By the way KT77,that kind of remark was really not needed.

I know...you're right :o
I shouldn't be pickin on the kids on the short bus.....
I'm sorry.....
:stir

Hey Gary...you know my real father was Jack Parsons....
He was a rocket scientist, too
http://en.wikipedia.org/wiki/Jack_Parsons
Handsome..isn't he...you know he is, don't you....yes you do.....who's your daddy? Huh? What did he do in the war? Yeeesss, that's my widdle gawy......your daddy was handsome too.....yes he was.....here comes the tickle monster....he he he he he...the tickle monster is goign to get gawy....yes he is....]

http://i236.photobucket.com/albums/ff320/HerrUuns/eeevill.gif

He was GOD

[booj]

Can you tone shape the stage by varying the bias? I know changing the RC values in the cathode bias circuit have a definate influence in the tone, as well as the operating voltage. But if you take that influence out of the circuit, then you've got just coupling caps and operating voltage. (as well as input and output impedance considerations), which means your options are...
Just show me an example.

[mooreamps]

I know...you're right :o
I shouldn't be pickin on the kids on the short bus.....
I'm sorry.....
:stir

Hey Gary...you know my real father was Jack Parsons....
He was a rocket scientist, too
http://en.wikipedia.org/wiki/Jack_Parsons
Handsome..isn't he...you know he is, don't you....yes you do.....who's your daddy? Huh? What did he do in the war? Yeeesss, that's my widdle gawy......your daddy was handsome too.....yes he was.....here comes the tickle monster....he he he he he...the tickle monster is goign to get gawy....yes he is....]



He was GOD

My father was a lumberjack. My mother was a housewife. We have no interest in the Occult. We just a plain simple old Irish family. My interest is only on electrical theory, in all it's forms.

But, this is off topic. So, I'll take this answer as ; no you do not biuld amps this way.

-g

[mooreamps]

This is more like it. Very good questions.

Can you tone shape the stage by varying the bias?

My opinion is, I don't think so. Changing the bias is going to affect the first approximation of the gain of the stage, and affect at what point that stage can be driven into saturation.



Pre-amp voicing can still be accomplished by the combination of plate coupling cap with respect to plate load resistor.

-g

[donnyjaguar]

I remember years ago there were some hi-fi tube amps that put a 1.5V battery between the ground and cathode on the driver stage. I never tried this myself and really thought of it as an exercise in pointlessness. FWIW, batteries produce hiss when being discharged anyway. You still see MC preamps using batteries to reduce hum, but having designed and built many a regulated DC power supply I can assure you hum can be totally removed anyway.

[RedMan]

I remember years ago there were some hi-fi tube amps that put a 1.5V battery between the ground and cathode on the driver stage. I never tried this myself and really thought of it as an exercise in pointlessness. FWIW, batteries produce hiss when being discharged anyway. You still see MC preamps using batteries to reduce hum, but having designed and built many a regulated DC power supply I can assure you hum can be totally removed anyway.

Have you ever used switchers for the heaters?

[donnyjaguar]

No. But you can bastardize a computer power supply to produce much more voltage and use it to power a tube amp. The 5V and 12V supplies can be tweaked to provide 6.3Vdc easily enough. Cheap is the name of the game, but it helps a LOT to have the schematic before attempting.

[RedMan]

No. But you can bastardize a computer power supply to produce much more voltage and use it to power a tube amp. The 5V and 12V supplies can be tweaked to provide 6.3Vdc easily enough. Cheap is the name of the game, but it helps a LOT to have the schematic before attempting.

You can get the proper supplies cheaply enough, I just wondered if you had done it so we could compare notes. I already do my heaters that way. I am thinking about designing my own supply but I'm not sure it's worth the trouble.

[mooreamps]

Cheap is the name of the game, but it helps a LOT to have the schematic before attempting.

Cheap is part of the name of the game. Thus, implimenting fixed bias in the pre-amp gain stages without using additional power supply circuitry, and only adding a couple extra dollars to the parts list.

-g

[xk49w]

IMO fixed bias isn't worth the trouble. The usual preamp cathode bias resistor is bypassed by 25uF or so. That should take care of any Johnson noise from the cathode resistor.

[WesKuhnley]

Perhaps a more interesting endeavor is the use of mosfet current sources for plate load or in place of the cathode resistor to boost the amount of gain a single stage can produce.

Fixed bias pre-amp stages are too much fukkin around for me...I'd rather attempt actual innovation, at least as far as the guitar amp world is concerned...

[RedMan]

IMO fixed bias isn't worth the trouble. The usual preamp cathode bias resistor is bypassed by 25uF or so. That should take care of any Johnson noise from the cathode resistor.

KY works great for Johnson noise as well. :AOK

[mooreamps]

KY works great for Johnson noise as well. :AOK

Perhaps this is one of those times you guys need to sit down, and listen to the rocket scientist. You think I do this for your "entertainment" !!

-g

[Deric]

Perhaps this is one of those times you guys need to sit down, and listen to the rocket scientist.

OK, Mr. Rocket Scientist, first question...

I also sent a full set of prints to KOC, just to see what he thought of it.


What did he think of it?

I'll post a drawing if there is any interest in discussing this further.

Since we're all still here, I think there IS some interest. Post away...

I replace the cathode resistor with an ultra-bright led.

As someone else allready asked, "isn't that still cathode biased"?

[booj]

I've worked with superbright LED's on overdrive box circuits. I know they have an extremely sharp "knee", which means they drop almost exactly the same voltage whether just turned on or full on. That means the tube essentially is operating in fixed bias mode, since the DC cathode to grid voltage remains unchanged regardless of the idle current allowed by the plate resistor.
Gota try it.

[UR12]

I've worked with superbright LED's on overdrive box circuits. I know they have an extremely sharp "knee", which means they drop almost exactly the same voltage whether just turned on or full on. That means the tube essentially is operating in fixed bias mode, since the DC cathode to grid voltage remains unchanged regardless of the idle current allowed by the plate resistor.
Gota try it.

The cap across a resistor does the same thing to a point. It tries to maintain a constant voltage and that is still cathode bias. The big problem with the led is that if you have a tube short out, now you have also taken out the LED and the amp will require surgery instead of just a tube replacement.

Perhaps this is one of those times you guys need to sit down, and listen to the rocket scientist. You think I do this for your "entertainment" !!

-g

Now you guys have done it!!!! Where is JudyisJudy when you need her.

:munch:mob:horse

[mooreamps]

When was the last time you saw a pre-amp tube "short-out" ? I'm assuming you mean plate to cathode.

[mooreamps]

OK, Mr. Rocket Scientist, first question...



What did he think of it?





As someone else allready asked, "isn't that still cathode biased"?






I spoke with him on the phone. Basically, he was unmoved by it, which is fine. But, it still abates the thermal noise induced into the cathode - grid circuit by removing the bias resistor.


If using a lamp in the cathode circuit, then it's considered cathode biased. If using a -1.5 volt tap from the PA tube negative bias power supply, then it's considered fixed bias.

The primary design goal of doing it this way was to lower the noise floor of the pre-amp.

[UR12]

When was the last time you saw a pre-amp tube "short-out" ? I'm assuming you mean plate to cathode.


While it is not an everyday occurence it does happen. I have seen it mostly happen when the glass envelope cracks and the tube looses vaccum. This is usually caused by amps being transported to cold temperatures without giving them time to cool down or someone bouncing the amp around unprotected in the back of a band truck or trailor. I have replaced a lot of cracked preamp tubes over the last 35 years. But to answer your questuion I had an old Sears Silvertone just last month that had a cracked and shorted preamp tube . Evidently the guy had hit the tube shield with his hand while putting the power cord in the back of the amp and bent a pin breaking the vacuum seal between the pin and the glass. I also saw a guy put a bottle of water on his SVT once. The water vibrated off and splashed cold water into the back of the amp (while it was on) and cracked a few tubes including 3 of the 6550 outputs and a 12by7. He was back up and running in 15 minutes with a couple of replacements. I have to give Ampeg credit though. The amp went into protect mode,turned on a little red LED and didn't even blow a fuse. Now that is a BITE light.

Do you not have any experience with preamp tubes going bad and shorting out?

[WesKuhnley]

The cap across a resistor does the same thing to a point. It tries to maintain a constant voltage and that is still cathode bias. The big problem with the led is that if you have a tube short out, now you have also taken out the LED and the amp will require surgery instead of just a tube replacement.



Now you guys have done it!!!! Where is JudyisJudy when you need her.

:munch:mob:horse

Where is she?

Technically (I think this has been mentioned already...) the LED is considered cathode biased due to the positive voltage developed on the cathode. Someone mentioned that the bias voltage doesn't shift with current draw due to the LED's sharp knee, this is a moot point because of the simple fact that a class A amplifier (as 99% of pre-amps are) draws the same amount of current through the full cycle of the wave. The common method of cathode biasing with a resistor provides the same stable bias voltage that a LED would.

Dana, your supposition that the cathode bypass cap "does the same thing" isn't quite correct either. To oversimplify, the cap holds the cathode at AC ground, eliminating feedback inside the tube, thus increasing the gain. The topic at hand is DC bias, and the cap has virtually nothing to do with the DC bias point of the tube. Don't mean to "call you out", just want the "viewers" to understand the difference.
:dude

[UR12]

Where is she?


Dana, your supposition that the cathode bypass cap "does the same thing" isn't quite correct either. To oversimplify, the cap holds the cathode at AC ground, eliminating feedback inside the tube, thus increasing the gain. The topic at hand is DC bias, and the cap has virtually nothing to do with the DC bias point of the tube. Don't mean to "call you out", just want the "viewers" to understand the difference.
:dude
I think we are both kind of saying the same thing. There is the AC component for sure, but the cap will also ( depending on it's size) smoothe out the Audio AC component developed across the cathode resistor and try and hold a more constant dc bias voltage on the cathode. We are in agreement that by supplying a positive voltage on the cathode you are making the grid more negative in respect to the cathode and this is cathode bias, unlike grounding the cathode and making the grid more negative in respect to the cathode which in my mind would be fixed bias.

[mooreamps]

Do you not have any experience with preamp tubes going bad and shorting out?

If you mean having a triode like tube lose it's glass seal and therefore loseing it's vacuum ? Yes. Plenty of them inside those old FRC-39 and FRC-40 HF transmitters I used to work on at the Navy base on Adak. When they lose their vacuum, the tube would just simply fail to conduct plate current. I've never seen a vacuum tube fail in such a way as to take out the cathode resistor and/or the cathode bypass cap. I'd think you would have to have a plate to cathode short to do that.

But also, something more relevent. A few months back, Mick Osborn brought in his Fender Twin for repair, on the day of a show. One of the power tubes had the glass separate from the base, thus also losing it's vacuum. In this case, the power tube just sat there, in circuit, not conducting any plate current. It didn't seem to hurt the HV power supply, because the amp was still working. So I gave him a fresh set of power tubes, and installed for him a 12 inch EV Force Loud speaker, and sent him on his way.

[phsyconoodler]

So I guess using the LED would be a good way to reduce noise if you run the amp flat out all the time,or dime all the tone controls.I just don't see it's value in a well-layed out amp that has almost no noise as is.
Maybe in a single ended amp it would be a good thing.
But having the LED mounted on the front panel would be kind of cool looking to be sure.

[mooreamps]

So I guess using the LED would be a good way to reduce noise if you run the amp flat out all the time,or dime all the tone controls.I just don't see it's value in a well-layed out amp that has almost no noise as is.
Maybe in a single ended amp it would be a good thing.
But having the LED mounted on the front panel would be kind of cool looking to be sure.


It's most useful in the very first gain stage of the pre-amp, which like the first gain stage in a radio receiver, tends to set the noise floor for the whole unit. I found, in my pre-amps, the hiss content in the loud speaker, dropped to almost nothing, even at full volume.

Secondly, I am believing by pulling the cathode bias resistors from the pre-amp does make the tone a little less dull sounding, and adds a little more timber and chime to the tone.


LED,s mounted on the front panel. Ya, well. I've had mixed reviews on that one. No grey area here. Some really liked them, and others really did not.


-g

[phsyconoodler]

So what LED are you using?I want to try this out.I can always use less hiss.Sounds like a great idea.
I have also never seen a preamp tube short by losing it's vacuum.Most of the time a preamp tube just quits conducting current.

[donnyjaguar]

I'm of the mind there is a practical limit as to how high-tech you want to get with a vacuum tube musical instrument amplifier. Dare I say it, if you set out to build the perfect beast and engineer out all the shortcomings of a tube amplifier you'd end up with a fully regulated solid state design.
:)

[phsyconoodler]

You're probably right.We all build these amps to get away from high-tech and solid state anyway.We can live with a little hiss.

[WesKuhnley]

I'm of the mind there is a practical limit as to how high-tech you want to get with a vacuum tube musical instrument amplifier. Dare I say it, if you set out to build the perfect beast and engineer out all the shortcomings of a tube amplifier you'd end up with a fully regulated solid state design.
:)



It's all about knowing which "hi-tech" things to add to vintage style circuits to enhance their function without regulating the life out of them. Guitar amps, and to a certain extent pro-audio gear, sound the way they do because of several *key* shortcomings, not because of *all* their shortcomings.

Using a diode or LED to bias a triode is certainly not a new idea, and may be a good way to lower the noise floor of a first stage without having to develop a 2 volt supply for fixed bias operation, but it doesn't allow you to bias the amplifier differently without changing the plate voltage fairly drastically.

[mooreamps]

So what LED are you using?I want to try this out.I can always use less hiss.Sounds like a great idea.
I have also never seen a preamp tube short by losing it's vacuum.Most of the time a preamp tube just quits conducting current.

OK. Here's something you can try, real quick. On every gain stage of the pre-amp :

* lift the cathode resistor and cap
* insert a red or yellow led. LED cathode to ground ; LED anode to tube cathode. Forward voltage drop should be about 1.5 volts.


that's it.

Now, if you want gain stage bite status, use a red or yellow ultra-bright. Don't use a blue one unless you want a cooler bias on the stage ; of -2.8 volts.

-g

[mooreamps]

It's all about knowing which "hi-tech" things to add to vintage style circuits to enhance their function without regulating the life out of them.

I'm not using vintage style circuits. I'm in the process of developing my own circuits. For example, every tremolo oscillator circuit I've seen is designed on a plate follower configuration. Well, that's OK I suppose. Seems to me a more stable oscillator configuration would be based on a cathode follower configuration. Make the input impedance much higher, like say between 4 to 5 meg ohms, and of course buffer the output off the low impedance port.

-g

[WesKuhnley]

I'm not using vintage style circuits. I'm in the process of developing my own circuits. For example, every tremolo oscillator circuit I've seen is designed on a plate follower configuration. Well, that's OK I suppose. Seems to me a more stable oscillator configuration would be based on a cathode follower configuration. Make the input impedance much higher, like say between 4 to 5 meg ohms, and of course buffer the output off the low impedance port.

-g

In my book, anything with a vacuum tube ought to be considered "vintage style". A cathode follower is just as old as an anode follower circuit. Figure out how to bias an output stage with a microprocessor and I'll consider it innovation.

[VacuumVoodoo]

I'm not using vintage style circuits. I'm in the process of developing my own circuits. For example, every tremolo oscillator circuit I've seen is designed on a plate follower configuration. Well, that's OK I suppose. Seems to me a more stable oscillator configuration would be based on a cathode follower configuration. Make the input impedance much higher, like say between 4 to 5 meg ohms, and of course buffer the output off the low impedance port.

-g

Every tremolo circuit I've seen used a generic text book phase shift oscillator. Three RC links in feedback loop around a triode. Could you please refer me to an amp schematic with anode follower oscillator? Myself, I used oscillator based on a Wien bridge. Pretty sine waves.

[RedMan]

The only real advantage I can see (aside from the claimed lower noise) is that you could replace all the cathode resistors and cathode bypass caps in the preamp with just one LED or diode. That would be a cost savings. I just don't like biasing all my stages the same and I really like what a partial bypass does on some stages. So you would be neutering the sound somewhat but you'd save a few bucks. Although if you're building a vintage Fender style amp most of the gain stages are pretty generic so maybe it would work for some designs. As far as worrying about the plate shorting to the cathode (very rare) the plate resistor should limit the current enough to keep the diode from harm.

[xk49w]

Regarding the shorting of tube elements, AC coupling the guitar input, which is often connected directly to the grid in old designs, would seem to prudential.

[mooreamps]

Every tremolo circuit I've seen used a generic text book phase shift oscillator. Three RC links in feedback loop around a triode. Could you please refer me to an amp schematic with anode follower oscillator? Myself, I used oscillator based on a Wien bridge. Pretty sine waves.

A Wien bridge ? Really !! A very nice old circuit, developed in the early 1890's is I recall. Operates very close to class A with very low distortion.


I have the schematic captured on my hard drive. I will need to wire it up and test it, before I should post it.

-g

[mooreamps]

So you would be neutering the sound somewhat but you'd save a few bucks.


As far as worrying about the plate shorting to the cathode (very rare) the plate resistor should limit the current enough to keep the diode from harm.

No, actually the tone is quite clear and chimmy. But, others need to try it and see what their listening impressions are.


Yes, if the plate shorted to cathode, and if the plate resistor is 100K tied to 300 volts, then mathematically the shorted current should be clamped to only 3 mills. The led is rated for a max of 20 mills. So, perhaps no major surgery required in this case.

-g

[xk49w]

It started Hewlett-Packard. HP 200A. Patent No. 2,268,872. 1942.

[RedMan]

No, actually the tone is quite clear and chimmy. But, others need to try it and see what their listening impressions are.


Yes, if the plate shorted to cathode, and if the plate resistor is 100K tied to 300 volts, then mathematically the shorted current should be clamped to only 3 mills. The led is rated for a max of 20 mills. So, perhaps no major surgery required in this case.

-g

No doubt it sounds a bit Fendery like I mentioned earlier. Have you tried using one LED to replace ALL the cathode resistors and bypass caps in your preamp?

[mooreamps]

It started Hewlett-Packard. HP 200A. Patent No. 2,268,872. 1942.

Yes, HP made it work in their 200A. I was referring to the original circuit which was developed by Max Wien in 1891.


-g

[BJF]

Hi,

Yes well connecting a diode across the cathode resistor or or replacing the cathode return with an LED in a common cathode circuit- no I do not see an anodefollower, as simply the anode does not follow the grid but amplifies outof phase- but anyway this has been done by Marshall and in an effort to give a constant prebias as would be evident from the amplification formula of one triode. It would be evident that such a connection would be equal to a capacitor large enough not to allow any change across the prebias and yes you can measure the transients across the cathode and see how those can be limited by the maximum allowable gridcurrent as when the grid is forwarded it acts as a diode and you'd get a halfway rectifier as viewed at the cathode,now the LED would be similar to a zenerdiode at appropriate voltage and yes there once was a time when all red LED's would have a forward bias of 1V7.

I also asked my local submarine engineer a few years back, to study this and draw graphs of typical LED's which showed deviation especially at lower currents.

Back in the 80's you'd find many transistor poweramps that had as voltage reference an LED. In tube amps,yes it has been done and it could easily be done again at great effect.
Right if the tube would go into shortcircuit the current through the diode would be limited by the anodeload.....and the voltage across the whole contraption

Indeed the most tremolo oscillators are using the phase lead configuration while arguably the lag version would give lower distortion. The Wienbridge has wider range but requires more closely matched components also as seen from the formula for such an oscillator and then finding a bulb these days isn't as easy as it used to be..hehehe

Please excuse that I would automatically assume that an anodefollower would be a common cathode circuit, just like I would assume a collector- or drain follower would be a common emitter or source circuit from reasoning that the output must be in the same phase and close in amplitude to be called a follower.

For the record my father was a blacksmith, not that that would count as anything at all.

I do remember though one time at kintergarten when the boys would gather at a stone and discuss who's father put the stone there...........

Oh well indeed have fun gent's:hide
BJ

BJF Electronics
Sweden
www.bjfe.org
www.mpamp.com

[mooreamps]

No doubt it sounds a bit Fendery like I mentioned earlier. Have you tried using one LED to replace ALL the cathode resistors and bypass caps in your preamp?

I have tried that with two gain stages, and it does not work. Therefore, each gain stage must be fully de-coupled from each other. There is only one other builder, that I know of, who fully de-couples each gain stage, and that would be Kevin O'Conner of London Power.

-g

[judyisjudy]

So gary, is that the reason why my sister just goes nuts over some of the tones we hear on youtube ?

[mooreamps]

Well, I don't know what it is she is hearing. If it's the clean tones she likes, then it's the layout of my pre-amp with the fixed biasing of the gain stages. If it's the over-drive tones, then it's going to be the distortion from over driving the power tubes using my electronic power brake.

-g