Class A Cathode & Class AB Bias Mod

[letloverule]

hey guys, i'm currently thinking of doing this mod on my DRRI and wanted to know if you guys thought it was worth it. Basically the tech installs a switch to switch between the two biases. I know very little about biasing and am wondering if I should just do a regular bias with one of the Weber Bias-rite kits or is this a worthy improvement?

[vibroverbus]

Well if your tech thinks Cathode Bias = Class A, you might want to start shopping for a new tech. Bias is one thing, Class of operation is another. He's probably talking about a bias switch, which you might very well like quite a bit - Cathode bias tends to have some nice dynamic differences when driven hard (compresses/damps a bit differently when near clipping).

Doubtful that he's talking about really doing a Class A conversion, and putting in the circuits to make that work. Most (not all) 'Class A' advertised amps are marketing Bull$h!t (although any single tube amp - for example but not limited to - is inherently Class A).

Here's some good techno info for you so you can be down with the Truth, hit his other pages on biasing and single-ended vs. push-pull because all these things are related but NOT the same...

http://www.aikenamps.com/ClassA.htm

[letloverule]

Ahh man and here I am all happy that i'll be making my amp sound sweeter! SO i guess there's no advantage to this then just getting a bias kit from weber or so and doing it myself. I guess that's that. I guess the only way to have a sweeter sounding deluxe is just to save up some money and get the real thing.
thanks you I think you might have saved me some money and aggravation. Anyone else have experience in getting their DRRI modded with a cathode bias?

[AdmiralB]

Cathode bias is a poor idea for a Deluxe since the plate voltage is so high.


Huh? Cathode bias would be more forgiving, since the cathode voltage 'subtracts' from the plate potential.

You certainly aren't going to get a DR into class A, but cathode bias is doable. Probably won't sound much if any different, but it's doable.

[letloverule]

Actually the tech I am talking to said it would be a rebuilding of the circuit and adding the components required for running the DRRI in Class A. Actually If I understand correctly he would be adding a switch where i can operate the amp in either Class A or Class AB. Anybody ?

[Blue Strat]

Definitely possible, still a PC board unless he replaces it with a tag board (figure at least $500 for parts and labor), and depending on the cathode resistor value used it may or may not sound different.

It probably won't be radical, but he could get it so the amp would compress and break up a bit sooner.

[57paf]

I've added a mid-pot (Tonequest), a variable negative feedback pot, and a DPDT switch to go from fixed to cathode bias on my SFDR. When playing a low volume levels, I love boosting the mids and cutting the NF to make it sound like it does at higher levels. When I switch bias, however, I don't really hear an appreciable difference. You can hear it in an A/B comparison, but normally you wouldn't really be aware of it unless you do a direct comparison.
I used to own a Groove Tubes Soul-O-30 Class A amp. About 2 years later, GT came out with a Soul-O-50 which is the same amp, but could switch to fixed bias mode. Takes a big power supply to do Class A w/6L6s. As one would expect, this amp was very saggy. Personally, I prefer a real Vox for creamy, squishy tones and a Marshall Plexi type amp for the dynamic punch. Which is what I have now.

[AdmiralB]

Actually If I understand correctly he would be adding a switch where i can operate the amp in either Class A or Class AB. Anybody ?


Then your tech doesn't understand class A. He/she would not be first -what you're probably talking about is simply toggling between cathode- and fixed-bias. That could be done pretty easily, even on a reissue.

But cathode-bias isn't necessarily class A. The plate voltage in a DR is at least 100V too high to get anywhere close to class A.

[letloverule]

Either way, from the comments here it sounds like it wouldn't result in a big tonal improvement in the amp. I wanted something to make the clean fuller, more alive. I'm guessing now that this isn't really worth it and i'd be better in investing in transformers / better tubes. I know this isn't really the topic of the thread but any of you tried the mercury magnetics transformers? Would NOS tubes be better than the JJ's I have. Thanks again.

[Blue Strat]

Either way, from the comments here it sounds like it wouldn't result in a big tonal improvement in the amp. I wanted something to make the clean fuller, more alive. I'm guessing now that this isn't really worth it and i'd be better in investing in transformers / better tubes. I know this isn't really the topic of the thread but any of you tried the mercury magnetics transformers? Would NOS tubes be better than the JJ's I have. Thanks again.

NOS Brimar 6V6GTs are big sellers for these amps where the user desires a fuller sound. Replacing V2 and V4 preamp tubes can help too.

Speaker upgrades are also less common for players trying to remove the ice pick tendencies in the stock Jensen speakers.

[cochese]

I have an old Siverface Deluxe Reverb that was modified to Blackface specs and later had a Cathode Bias switch installed. I pretty much always use the Cathode bias setting. I just prefer the tone and response. It really makes the amp jump from the word go.

[AdmiralB]

Admiral just out of curiousity, what resistor value do you use and where exactly does the plate dissipation end up?

I've never converted a DR to cathode-bias. I've built a couple 5E3s with higher-voltage iron, though. I'd probably start around 360 ohms for a shared cathode, and see what that gets. Assuming 6V6GTA tubes, I'd probably shoot for 10W dissipation, per.

I've done some 5E3-types with switchable fixed/cathode bias, though, and the differences are REALLY subtle.

[letloverule]

NOS Brimar 6V6GTs are big sellers for these amps where the user desires a fuller sound. Replacing V2 and V4 preamp tubes can help too.

Speaker upgrades are also less common for players trying to remove the ice pick tendencies in the stock Jensen speakers.

yeah for the speaker i'm leaning towards the Eminence speakers right now. I'll definitely be looking at the new tubes as well. I think that's pretty much all i can do with this amp. I'm still a little confused about the whole Class A/AB stuff. Most people seem to think that its just a switch for the cathode vs fixed bias. i'll ask. The tech tells me i'll get a more compressed sound, sweeter at low volumes in that mode. He says that the Class A mod is a complete rebuild of the bias circuit and adds power resistors, caps and other components in order to operate the amp in Class A. I still don't think from your feedback that it'll be worth the money. I hear from you guys that the difference will only be VERY VERY subtle.

[phsyconoodler]

There is a lot of real estate in the DRRI for placing a nice sag resistor.But the best tones you can get out of that amp come from a speaker change.
Try an Eminence Red/White and Blues or a Govnor.Both sound killer in a DRRI.
The JJ's are just fine the way it is,just make sure they are biased right.I like the DRRI a little on the hot side.
If your tech is thinking of repalcing the entire circuit board,that is not a bad thing.Then he can put some decent parts in it.The factory DRRI has cheesy parts inside.

[cochese]

Why not just get a set of THD Yellow Jackets. Although I have the Cathode bias mod in my Deluxe and really like it I feel the Yellow Jackets will probably give you more of a drastic tonal change (with the EL84's) and will be a much cheaper alternative.

[Blue Strat]

BTW, to achieve a similar effect to cathode biasing (more sag/compression) you can change rectifier types to a 5R4 or even a 5Y3. For best sag producing results, it's best to rebias but it's not absolutely necessary. It's a lot less expensive than modding the amp.

[AdmiralB]

I have, to my own DRRI back in 1999. The volts in those amps are higher than on the 1996 schematic. To get a reasonable dissipation you need something closer to 500 Ohms. Its not remotely like class A, and it doesn't sound good.

That doesn't surprise me.

The RIs I've been in have all been right around 420V, somewhat less than the originals.

[letloverule]

Why not just get a set of THD Yellow Jackets. Although I have the Cathode bias mod in my Deluxe and really like it I feel the Yellow Jackets will probably give you more of a drastic tonal change (with the EL84's) and will be a much cheaper alternative.

That's a very interesting suggestion. I don't want my speaker to break up too much but this is definitely worth a shot. Always was curious about those yellow jackets. Which model do you suggest (if i'm not mistaken there are 2 models)? Thanks.

[cochese]

You should probably just contact THD or Ed DeGenaro is on TGP and he works for them. The YJ's are pretty cool and it's a totally passive mod. As I've said I do have a cathode bias switch on my converted Silverface/Blackface Deluxe and it was done a few years ago. I can't predict how it would sound in a reissue. I tend to run it in cathode bias mode most of the time. I would say if you are going to run the amp at lower volumes it's pretty cool. As for tona differences I listened last night and would say it's more of a response difference with the 6V6's. With the the YJ's it will probably give you a different tonal response as well.

As for Cathode Bias versus Class A there are many opionions on this but in terms of guitar amps I think it's a bit silly. Class A in the Audiophile purest view is that it is an uncompromised signal and a very pure form of amplification. Guitar amps as we use them today are not typically used in this fasion. We "want" distortion and at this point the argument becomes somewhat moot. In the end it really comes down to what sounds good to you.

[Free]

BTW, to achieve a similar effect to cathode biasing (more sag/compression) you can change rectifier types to a 5R4 or even a 5Y3. For best sag producing results, it's best to rebias but it's not absolutely necessary. It's a lot less expensive than modding the amp.

+1
That's the way to go. You'll notice more sag from the voltage drop of a 5Y3 than cathode-biasing (cathode biasing imparts more of a tonal richness than sag necessarily), and it is indeed WAY simpler.

[billyguitar]

I have converted two 1957 Bassmans and a BF Vibrolux Reverb, a BF Super Reverb and a BF Princeton all to cathode bias. With a 5AR4 they sound almost stock but by using other rectifiers you can control the sag and bloom when you play them loud enough, say at 2/3rds volume. With the Bassmans it made them much easier to gig and also easier on the old speakers. Same for the blackface Fenders. It wasn't hard to do, I followed the instructions in a Gerald Weber book. It's an easily reversible mod also. Cathode bias works for me and on most amps, I prefer it over fixed bias, for both clean and overdrive situations.

[burner]

Interesting read here.

I have a few questions though regarding some of what I've read here.....

-Is a 5E3 circuit a class A or simply an A-B that is cathode biased?
-It was mentioned that some "Class A" amps are just marketing hype....
Are the Peavy Classics, particularly the Classic 30, a true Class A amplifier?

Thanks

[AdmiralB]

-Is a 5E3 circuit a class A or simply an A-B that is cathode biased?

Class AB, cathode-bias


-It was mentioned that some "Class A" amps are just marketing hype....
Are the Peavy Classics, particularly the Classic 30, a true Class A amplifier?


The only class A amps I know of are most single-ended amps (there are exceptions - Vibrochamps and some Champs don't clip symmetrically, so technically they're not really class A), and some boutique amps.

You have to have relatively low plate voltages to run class A. In general, if the plates are over 300V, it's not really class A.

[billyguitar]

I had a guy working on an old Princeton Reverb for me. While he was in there I wanted him to change it to cathode bias. He is a very knowledgeable scientist type guy who works on amps and builds his own amps in his spare time. He was insistent about calling cathode bias Class A. I tried to correct him but he wouldn't hear of it. In the end he said my old PR had a long ago replaced power transformer that couldn't handle the cathode bias load! I just had him fix the obvious problem it had and got it back from him as soon as possible. This guy will not get any more work from me.
I wish there was a good amp man in Kansas City!

[phsyconoodler]

He may be a good tech but his statements are typical 'defense' mechanism remarks.
He may not have theory under his belt and still be able to fix an amp properly.Just don't get him to mod anything.

[cochese]

I guess the debate could go on for ever but I did find this explanation on the net that was very close to the explanation one amp designer gave me a while back.
Copyright 2006 © David B. Lamkins back
The "Class-A" illusion

While I don't dispute the technical definitions of the operational classes, I believe that they have negligible predictive or descriptive value when it comes to guitar amps. When amp classes were defined (a long time ago, in the `30s or `40s) everyone concerned assumed that the goal was linear amplification, free of distortion insofar as the technology allowed. I don't believe that operational classes are useful in describing guitar amps because the goal is for their power stages to be operating nonlinearly during normal operation.

Suppose you have a guitar amp that's built to operate in Class A (single-ended or push-pull - the argument applies to both) up to the limit of its unclipped power output. If you push the amp harder, distortion will rise dramatically as the output stage clips and the power output will continue to increase. At this point the amp is not operating in Class A because an output tube is either saturated (unable to conduct any more current) or cut off (not conducting at all) for some portion of the cycle. The important distinction is that the cutoff and saturation points are symmetrical, otherwise the amp would be - by definition - Class AB. Our clipping Class A amp is still technically Class A because it will deliver full unclipped power with the output tube(s) conducting for a full 360 degrees.

Now consider a Class AB push-pull amp. Full unclipped power will be more than the true Class A amp of comparable topology; letting one tube cut off for a portion of the cycle allows the output stage to swing more voltage into the load. The key difference between Class AB and Class A is that conduction occurs for less than 360 degrees even when the amp is still operating linearly.

This is where the A vs. AB distinction becomes a matter of degree. Traditionally, Class AB amps were designed to maximize available power while still providing acceptably low distortion while operating in their linear range. They had to be biased hot enough to reduce crossover distortion (remembering always that low distortion was an implict goal), but not so hot as to approach the lower power output of Class A operation.

What if you're running a push-pull amp designed for true Class A operation, but the output tubes have aged symmetrically and are unable to pull their full rated output current? That case is simple: the full output before clipping will be reduced. The amp will still be running Class A.

OK, next case: Consider the same amp (designed for true Class A operation), but now the output tubes are mismatched such that the amp delivers full unclipped power with one tube not conducting for a full 360 degrees? Clearly you'll get an uptick in second-order distortion as the power increases above the point where both tubes are still conducting. Our mismatched tube is cutting off earlier than it should, yet can still push full rated output before clipping. The other tube is conducting for a full 360 degrees. This corresponds better to the definition of Class AB than it does to Class A. The operating class definitions say nothing about symmetry, although it's implied by the desire for minimal distortion. But symmetry is not required by definition, so our Class A amp with a weak tube is really - by definition - a Class AB amp.

Now consider an ideal hot-biased Class AB amp running matched tubes. The hotter the bias, the closer it will approach Class A operation while still producing unclipped output. At some point, any differences between 360-degree conduction and some lesser coverage of the full cycle will have an inconsequential effect on the perceived tone and behavior of the amp. Remember that we're still looking at unclipped output. The difference between this case and the case in the preceeding paragraph is that third-order distortion will increase as the output power exceeds the point where both tubes are not conducting for a full 360 degrees, because the output transformer tends to cancel second-order distortion due to the symmetry of the push-pull circuit. We're still not clipping the output.

So where do you draw the line? Do we say that a Class A amplifier is only running Class A so long as it's running perfectly matched tubes? If not, then why not admit that a hot-biased Class-AB amp is "close enough" to Class A at some point? What if the difference was down to twenty degrees of non-conduction? How about ten degrees? Five? One? The only detectable difference between a perfectly balanced hot-biased Class AB amp and an unbalanced Class A amp is in the amount of second-order vs third-order distortion products as the amplifier approaches full unclipped power output.

The problem with the class definitions is a matter of intent. No one ever considered a hot-biased Class AB amp at the time the class definitions were conceived. Such an amp would have been rejected as a poor design in those days. The entire reason for the existence of a Class AB amp was to increase the output power available from a pair of tubes without unduly compromising low distortion. If your intent was to minimize distortion you'd build a Class A amp. To maximize power you'd build a Class AB amp and set the bias just high enough to give the negative feedback a fighting change to eliminate crossover distortion.

If you've followed my arguments so far, you'll see that I'm advocating that there's no important difference between Class A and hot-biased Class AB guitar amps. The distinctions, considering that guitar amps provide little if any facility to balance the output stage, are a matter of degrees (pun intended). Sonically, there's too little difference between the cases to be noticeable, especially considering that a guitar amp is intended not as a linear amplification device, but rather as a non-linear tone-modification device.

Within the closed universe of guitar amplification, "Class A" has become a marketing term implying the presence of a cathode-biased output stage. As such, it's a useful distinction. A cathode-biased guitar amp exhibits different behavior than a fixed-bias guitar amp.

The technical definition of Class A operation should not, IMO, be applied to guitar amps at all because

the intent of guitar amplifiers is not linear signal reproduction, upon which the definitions of operational classes are predicated, and
any sonic differences between Class A and Class AB (applying the technical, not marketing definition) power stages are swamped by other distortions (preamp, speakers) in the overall system and therefore have no predictive or descriptive value.

[Rosewood]

Don't let that post above get out, could make some people sick. AB at 70% is hot enough for me, maybe too hot, I'll spend the rest of my money on tubes and speakers.

[AdmiralB]

LOL...that's like saying that it should be OK to contaminated water "pure" because, well, people have been drinking it forever and they're not sick...

Standard terminology is standard terminology.

[cochese]

Not really. The argument is a good one. Guitar amps as they are designed today don't really fit into the design criteria that was established when tube amplification first came into being. How many guitarists on this forum that own a true class A amp can honestly say they are operating it in the fasion it was meant to be run at, an unclipped signal. Part of the reason early Fender Silverface amps were so disliked is that the old school designers were trying to get rid of that nasty distortion that the Blackface amps had. This was the sound that many players fell in love with. The sound of a distorted guitar amp was really more of an accident that came to be accepted and later became a design goal. You really have to think outside the box on this stuff. This is why I will take a new amplifier design over an old one because many modern amps were designed by amp designers that were also guitarists. They weren't trying to shoehorn a tube design that was meant to cleanly reproduce music without distortion.

I use a Guytron amp which has a cathode biased 20 Watt El84 power section with a long tailed PI functioning as a preamp feeding a 100 watt EL34 class AB power section. One well known amp designer told me he thought it was an interesting amp but is not designed properly because a typical guitar amp doesn't have an output section as a preamp. Some people can't think outside the box. What would this amp be called in standard terminology. What would a Fuchs or a Boogie with a high gain cascaded preamp section be called with this standard terminolgy?

[AdmiralB]

It's a terrible argument.

If you want to make up a new term for guitar amps, that's fine. You don't change standard engineering/specification terms because a small subset chooses to apply them incorrectly. The argument that class in general doesn't matter to guitar amps is another discussion, and I'd probably agree with the author as far as that goes. But...

They're called 'standards' for a reason.

Your examples have nothing to do with your argument. Doesn't matter whether you dime your 'true class A' amp or not - if the power section clips symmetrically, it's class A, and if it doesn't, it's not. It's not thinking inside or outside of the box, it's using terminology correctly. There's a name for what the article discusses, and it's "cathode bias". Since that term exists, is accurate, and has meaning...why not use it? Why use another term that is inaccurate...just because?

The step from "operating class doesn't matter", to "since it doesn't matter, let's call cathode-bias 'class A'" is stupid, IMO.

[cochese]

The point is that the terminology is somewhat irrelevant for the purpose of guitar amplification. The distortion or clipping this standard terminolgy relates to is by no means even close to the type of "intentional" distortion guitar amps produce.

If you go to the Aiken site he on one hand would probably agree with you but he also comes to the same type of conclusion that the author of the "class a illusion" author comes to. Also you seem to be incorrect (at least according to Aiken) regarding Class A amps only clipping symetrically.

Q: Is it true that the only true class A amplifier is a single-ended amplifier?
A: No. You can design a true class A single-ended or push-pull amplifier. The presence of a phase inverter tube does not automatically mean the amplifier is class AB, just as the presence of a cathode resistor doesn't automatically mean the amplifier is class A. It is all a function of where the output tubes are biased, and under what voltage/impedance conditions they are operating. In fact, unless you know the plate voltage, plate bias current, and output transformer reflected impedance, you can't tell the class of an amplifier just by looking at the schematic. A push-pull class A amplifier differs from a single-ended class A amplifier tonally in that it cancels even-order harmonics generated in the output stage (but passes through even harmonics generated in the preamp stages, of course). It also has inherent power supply rejection for lower hum and noise levels than a single-ended class A amplifier. Typically, a push-pull class A amplifier will clip rather symmetrically, while a single-ended class A amplifier usually clips asymmetrically, most often rounded on one side while hard-clipped on the other. While both amps are indeed true class A amplifiers, their tones are dramatically different. This further illustrates the fact that there is really no such thing as a "class A" tone.

[AdmiralB]

Also you seem to be incorrect (at least according to Aiken) regarding Class A amps only clipping symetrically.

By 'symmetrically' I meant that both sides of the waveform clip simultaneously...this is the simplest definition of a class A circuit, PP or SE.

What the waveform actually looks like when that happens...varies, as Randall illustrates.

[cochese]

Perhaps this should be another thread. This debate has gone on for years. I guess a better solution rather than engaging in semantics would be to give some examples of "true" (by your definition) class A guitar amp recordings. Considering the amount of recordings at our disposal perhaps you or someone could site some well known examples of Class A guitar tones and the differences between them and cathode bias type tones and class AB tones.

[AdmiralB]

Where did I make any claim about "class A tone"?

I agree with pretty much every point the author of that article makes...with the notable exception of it being OK to use standard terminology incorrectly "because everybody does it".

[bluesaxe]

Back on the original topic -

In my experience the biggest tone changer in a BF / SF power amp circuit is to make the negative feedback adjustable. There is a lot of NF on the old Fender circuits (trying to maximize clean headroom) and removing a little of the feedback can make a huge difference in the way the amp sounds and responds.

Changing over to cathode bias will make some difference, but IMHO this difference is mostly from the reduction of the voltage drop across the output tube plates by putting a postivive voltage on the output tube cathode. As always, YMMV, so try it, you might like it!

[Free]

"Typically, a push-pull class A amplifier will clip rather symmetrically, while a single-ended class A amplifier usually clips asymmetrically, most often rounded on one side while hard-clipped on the other. While both amps are indeed true class A amplifiers, their tones are dramatically different. This further illustrates the fact that there is really no such thing as a "class A" tone."

I know we're off topic here, but the theme has switched in this direction, and the original question has been well-adressed:

Couldn't a Class A PP amp with unmatched output tubes theoretically come close to a single-ended amps distortion character?

Also, as Cochese also mentioned, could someone point out some tonal examples (songs or clips) of these differing distortion characters?

Thank you!

-Mike

[letloverule]

Wow, I never knew my initial question would create such a debate . Nevertheless, i'm impressed by the wealth of knowledge from the users of the site. Clips would be by far the most useful in trying to decipher the tonal changes that each class (or cathode bias, etc.) creates.

s

[booj]

with 450 - 500 volts on the plates use a 400 ohm 10 watt in parallel with a 200uf/100v cap. Bam! Cathode biased. I meant that figuratively.

[jetlag]

I have a switchable cathode bias scheme in my BF Pro Reverb. It's very effective - knocks a lot of thump off the amp and adds a lot compression. Kinda sounds/feels like you are playing a VR with a 15. The key, to me, is in the cathode bias scheme, OMIT the bypass cap. That really makes the difference between the CB and FB modes stand out, making the whole mod worthwhile. My amp guy puts a bypass cap defeat switch in as well if you want three options (not necessary IMO).