More fun with Deluxe Reverb project (PT replacement, plus blackface)

Muttlyboy

Gold Supporting Member
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3,094
Hi everybody,
A few months back I got a Deluxe Reverb which needed some work.

The amp when I got it, had a few circuit mods done to it including the 3rd stage cathode bypass cap on the push-pull volume pot (re-purposed pull boost).

A couple weeks ago, I swapped out the Power Transformer for a Weber replacement which dropped the B+ voltage about 20 volts.

With a 5R4 rectifier and both B+ dropping resistors changed out to 10k (BFDR values) the amp is getting voltages very close to what is shown on the schematic (415V on the plates, tubes biased to about 22mA, and about 175 on v1 and v2 plates.)

Since them I replaced the previously installed 22 uf filter caps with 16uf to match the schematic, and put the 3rd stage cathode back to stock.

The only real mod right now is that I re-purposed the pull boost to a vibrato disconnect switch.

This thing breaks up real early now. Even the normal channel breaks up at about 4 on the dial and the Vibrato channel starts to breakup at about 2 and 1/2.

My question is where does the distortion originate from?

...
I'm nowhere near loud enough at 2.5 to be distorting at the power tubes (or am I?)

Is it the lower voltage at the preamp tubes?

Is the phase inverter easier to distort now with the lower voltages?

It's all very interesting and a fun experiment.
 

HotBluePlates

Member
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14,057
... This thing breaks up real early now. Even the normal channel breaks up at about 4 on the dial and the Vibrato channel starts to breakup at about 2 and 1/2.

My question is where does the distortion originate from? ...

Check d.c. volts at all tube pins and compare to the schematic. Pay close attention to bias voltages throughout.

You need a test signal and either a meter or scope. Apply a guitar-sized test signal (maybe 100mV peak). Turn up the amp's Volume knob until you hear a small amount of distortion. Maybe turn the Volume back down until the point you feel the distortion is just starting.

Now check the signal level at each tube grid, starting at the output tubes. Compare to what you knew to be bias voltage from d.c. voltage checks with no signal applied. If the applied signal peaks (measured with meter or scope) approach/exceed that stage's bias, the stage in question is probably distorting heavily. If the grid signal at the output tubes is much less than bias (say, 20v peak signal but -35v bias), you can guess something is distorting prior to the output tubes.

If you had a Listening Amplifier (scroll to the bottom of the linked page), you could hear where in the amp circuit the signal begins distorting and how much.
 

neteraser

Member
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1,993
Hard to guess without seeing the amp\listening to the problem. But you could check the OT and the NFB, but if it was the NFB losing connection, then the amp should have been a little louder.
In other words, with no tools: listen closely and start checking things is the only way. These amps are pretty serviceable, so there couldn't be an amp-life threatening issues.
 

neteraser

Member
Messages
1,993
Another thing is to make sure you're not disconnecting the intensity pot, because then the voltage divider between the preamp and the poweramp will be broken, but this also should result in a louder signal.

This could be fixed.
 

zenas

Member
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8,871
Just a thought from out in left field for the guys that know more than I do.
Is it possible the Weber PT is sagging voltage as the amp is being pushed harder?
Just thinking that because the country those are made in isn't exactly known for consistency. (although I've used probably that same PT in several amps and they're still working, just not a DR)
 

Muttlyboy

Gold Supporting Member
Messages
3,094
Check d.c. volts at all tube pins and compare to the schematic. Pay close attention to bias voltages throughout.

You need a test signal and either a meter or scope. Apply a guitar-sized test signal (maybe 100mV peak). Turn up the amp's Volume knob until you hear a small amount of distortion. Maybe turn the Volume back down until the point you feel the distortion is just starting.

Now check the signal level at each tube grid, starting at the output tubes. Compare to what you knew to be bias voltage from d.c. voltage checks with no signal applied. If the applied signal peaks (measured with meter or scope) approach/exceed that stage's bias, the stage in question is probably distorting heavily. If the grid signal at the output tubes is much less than bias (say, 20v peak signal but -35v bias), you can guess something is distorting prior to the output tubes.

If you had a Listening Amplifier (scroll to the bottom of the linked page), you could hear where in the amp circuit the signal begins distorting and how much.
I didn't check any cathode or grid voltages, but all of the plate voltages are within a couple volts of the schematic.

When you say to check bias throughout the amp, does that mean that I should measure voltage across the cathode resistors on all of the the triodes?
I don’t have a scope or signal generator, but I can borrow a keyboard and read the AC at the grids...this sounds like an interesting experiment.

I'm just wondering at what point on the volume knob (and also, which part of the circuit i.e. power tubes, phase-inverter, 2ND stage, 1St stage...) does the break-up/grind start happening in a properly functioning AB763 Deluxe Reverb set up to run at the voltages listed on the schematic (415 at the plates and screens, 170v to 180v at all the pres, and about 190various at the phase inverter)
 
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Muttlyboy

Gold Supporting Member
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3,094
Just a thought from out in left field for the guys that know more than I do.
Is it possible the Weber PT is sagging voltage as the amp is being pushed harder?
Just thinking that because the country those are made in isn't exactly known for consistency. (although I've used probably that same PT in several amps and they're still working, just not a DR)
I didn't even think of the PT sagging, but I kind of think that's not the issue at this point because I didn't have the amp up very loud.

I'm not even sure if there is a problem...I'm wondering if maybe what I'm hearing is the difference between the amp running with 440 volts(with 5R4 rectifier) B+ to running with 415v B+
 
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Muttlyboy

Gold Supporting Member
Messages
3,094
Another thing is to make sure you're not disconnecting the intensity pot, because then the voltage divider between the preamp and the poweramp will be broken, but this also should result in a louder signal.

This could be fixed.
Yeah, I fixed it to be able to disconnect the intensity pot on a switch.

With the intensity pot disconnected, the amp has quite a bit more gain.

But, at this point I don't know if I have a problem.

The amp is a 1977 Deluxe reverb that originally was set up to run with a 5U4 rectifier, had less resistance in the B+ dropping resistors, and different resistor values in the phase-inverter.

Also, here in the States, the supply voltages from utility companies has risen to 125vac in many areas.

It appears on the schematic for the AB763 Deluxe Reverb, that the amp was designed for use with 117vac.

With the typical "wall voltage" of 125vac, my amp had a B+ of about 460 volts DC. (40v to 45v above what is on the schematic)

With the new Power transformer and use of a 5R4 rectifier, and the change of the B+ dropping resistors to 10k (as on the schematic), all of the plate voltages are within a couple volts of the schematic.

Maybe what I'm hearing is the difference between the "correct" as per schematic voltages, and the elevated voltages the amp had when I first got it.
 

pdf64

Member
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8,914
With a 5R4 rectifier
5R4 have a lot of sag under load.
I suggest to try a 5AR4.
When reporting Vdc inside the amp, it's helpful to also provide the context of the mains and heater Vac at that time, and the power tube plate or cathode current at idle.
I'm nowhere near loud enough at 2.5 to be distorting at the power tubes (or am I?)
Is the trem disconnect switch engaged?
What type of pick ups does the instrument used have, eg humbuckers or low output single coils?
All being well, with NMV amps and control settings suitable for use on stage, as the signal level rises from 0, the first point of signal clipping is at the power tube control grids.
Have you checked that the amp is correct to its schematic, and that its feedback loop is in place and is indeed negative, ie reduces gain.
Is the OT spec suitable? If the secondary has more than 2 terminals, are the correct ones being used, both for the negative feedback and the speaker?
Check the taper of the volume controls, eg disconnect the hot / input wire, set it halfway (~5.5 on a 1-10 skirted knob) and measure 3 resistances, ie full track and wiper to each track end.

I didn't even think of the PT sagging, but I kind of think that's not the issue at this point because I didn't have the amp up very loud.
The PT HT winding Vac will sag when the amp is putting out full power, caused by the power tubes drawing more current than they do at idle (assuming that the bias is suitable); are you saying that the amp seems low on power when it breaks up?
Or that the amp seems to have more gain than expected (ie causing the power amp to overdrive at low settings of the volume control)?
Or both?
 
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eolon

Gold Supporting Member
Messages
507
Remember that Fender designed all the Deluxe series amps to operate at the hairy edge of ratings. This was when wall power was 110 or 117 VAC. The filter capacitors (especially), the transformers, and the 6V6s routinely ran with much higher voltage and current than their design limit.

So, A typical Deluxe owner plugs into 128VAC and gets great, pleasing distortion above volume 4, but them wonders why his tubes don't last very long and his filter caps blow up.

If you have compensated for the voltage with the power transformer, then you are probably close to the expected performance of the amplifier as it was delivered in 1977. These amps were designed to be loud, but they were never turned up much beyond 4 or 5.

If you do not like the tone you are getting, then I would look at your bias first; assuming you have an adjustable bias, ignore the voltage/current readings and try adjusting the pot while playing through the amp. When you find a sweet spot that you really like, then that is your bias setting. It may take a while. Then go back and measure what the tubes are dissipating, if you are curious.

The Deluxe Reverb is a very robust design, it puts up with a lot of abuse from electrons. Many customers run nearly 100% dissipation on the tubes. Yikes! But, no red-plating, and they love the tone. So...

As to the question: What's distorting? - Hot Blue Plates answered that very well, as usual.

Best Regards,

Don
 

HotBluePlates

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14,057
I didn't check any cathode or grid voltages, but all of the plate voltages are within a couple volts of the schematic. ...

The first thing to do when troubleshooting an amp is to measure the d.c. volts at every tube pin (except heater pins). Okay, the real first thing to do is a visual inspection & make sure everything is connected properly and nothing is obviously-burned. But your 2nd first thing to do is check d.c. volts.

Voltages tell you if anything is way-wrong with the operation of any stage. Plate voltage might look okay, but you could have a leaky coupling cap putting some d.c. on a grid and creating early distortion. For the long-tail pair, you're best not measuring the tube grids but the voltage at the junction of the 4 resistors in the cathode circuit: a pair of 1MΩ, the 470Ω and the 22kΩ resistors.

When you say to check bias throughout the amp, does that mean that I should measure voltage across the cathode resistors on all of the the triodes? ...

Yes. "Bias" is grid-to-cathode voltage of a tube. Doesn't matter if it's a fixed-bias output tube with the cathode tied to ground and -45v (45v of bias) or a cathode-bias preamp tube with the cathode sitting at +1.8v and the grid tied to ground through a 1MΩ resistor (1.8v of bias).

... I'm just wondering at what point on the volume knob ... does the break-up/grind start happening in a properly functioning AB763 Deluxe Reverb ...

They're all different. You have 10% and 20% tolerance resistors, the tubes are at least 20% tolerance (or more), exact voltages vary, guitar pickup output varies, and how hard the player whacks the strings varies. I'd think somewhere around 4 with your guitar volume maxed with a Strat or Tele sounds about right. But you might have very gainy tubes, or tubes with idle way down in the curved part of their characteristics and distort earlier than typical.

... I'm just wondering at ... (and also, which part of the circuit i.e. power tubes, phase-inverter, 2ND stage, 1St stage...) does the break-up/grind start happening in a properly functioning AB763 Deluxe Reverb ...

Output tubes distort first. If Fender did anything else, you wouldn't get all the clean watts from those 6V6's, and they worked hard (different OT primary impedance, long-tail inverter, bigger power transformer) to get the DR to make a few more watts than the Princeton Reverb.

... As to the question: What's distorting? - Hot Blue Plates answered that very well, as usual. ...

Thank you Sir!
 

Muttlyboy

Gold Supporting Member
Messages
3,094
Hi guys, thanks for the replies.

I'm actually not complaining about the sound...it actually sounds pretty damn good.

With the power tubes biased to 22mA (voltage drop across OT pimaries divided by the DC resistance of the OT pimaries) all of the voltages are about right where the schematic says it should be.

I originally put in the 5R4 with the stock power transformer to bring the voltage down from about 460 (with 5U4) to about 440v with the 5R4.

This is all with re-adjusting the bias for the tubes to run at about 9 watts each plate disipation at idle.

The new power transformer has two primary taps (120v, and 125v) and I used the 125v tap. With the 5U4 and the new transformer the B+ voltage was alost 440 vdc.
My wall voltage is about 125vac, and if I remember correctly the filament voltage is 6.3 to ground
 

Muttlyboy

Gold Supporting Member
Messages
3,094
To pdf64,
Does the amp seem to have low power when it breaks up?...I don't know...it does seem to breakup quicker than before, but I don't know if it breaks up in an unusual way.

Would the new lower voltages (about 20 volts lower across the board) do that? (Again, I'm not complaining ) maybe that is to be expected?

The amp does seem to have a bit more gain (even on the normal channel) now than it did before
 
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Muttlyboy

Gold Supporting Member
Messages
3,094
The first thing to do when troubleshooting an amp is to measure the d.c. volts at every tube pin (except heater pins). Okay, the real first thing to do is a visual inspection & make sure everything is connected properly and nothing is obviously-burned. But your 2nd first thing to do is check d.c. volts.

Voltages tell you if anything is way-wrong with the operation of any stage. Plate voltage might look okay, but you could have a leaky coupling cap putting some d.c. on a grid and creating early distortion. For the long-tail pair, you're best not measuring the tube grids but the voltage at the junction of the 4 resistors in the cathode circuit: a pair of 1MΩ, the 470Ω and the 22kΩ resistors.



Yes. "Bias" is grid-to-cathode voltage of a tube. Doesn't matter if it's a fixed-bias output tube with the cathode tied to ground and -45v (45v of bias) or a cathode-bias preamp tube with the cathode sitting at +1.8v and the grid tied to ground through a 1MΩ resistor (1.8v of bias).



They're all different. You have 10% and 20% tolerance resistors, the tubes are at least 20% tolerance (or more), exact voltages vary, guitar pickup output varies, and how hard the player whacks the strings varies. I'd think somewhere around 4 with your guitar volume maxed with a Strat or Tele sounds about right. But you might have very gainy tubes, or tubes with idle way down in the curved part of their characteristics and distort earlier than typical.



Output tubes distort first. If Fender did anything else, you wouldn't get all the clean watts from those 6V6's, and they worked hard (different OT primary impedance, long-tail inverter, bigger power transformer) to get the DR to make a few more watts than the Princeton Reverb.



Thank you Sir!
Thanks for all of this information

Maybe I can get back to this tomorrow after work, but I'll check all of the voltages and maybe this weekend borrow a keyboard and try to measure the signal level at the grids (I've never done that before, should be interesting)
 

HotBluePlates

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14,057
... borrow a keyboard and try to measure the signal level at the grids (I've never done that before, should be interesting)

Hopefully your meter has a peak-reading function. The peak is what you care about.

If you were using a known-pure sine wave and a regular RMS meter, you could use known conversion factors to know how much RMS voltage will equal your ceiling peak voltage (For a sine, Vpeak * 0.7071 = V RMS, or V RMS * 1.414 = Vpeak).
 

Blue Strat

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30,686
Maybe I missed it, but have you measured idle CURRENT (bias current) of your power tubes? If this is way out (should be around 20mA) you can get early breakup. There are several other things. Check ALL voltages against the schematic. Check the plate voltages of the PI tube. If one side of the PI isn't working correctly you'll get very early distortion. You'll usually see a noticeable difference in plate voltages on the tube halves if there's a problem.

The 5R4 is lowering all your voltages, increasing sag, and will make the amp break up sooner than normal. It's not unusual for a stock DR to have 450V on the plates with the correct rectifier tube. Also, if your volume pot taper is off (not unusual) or the knob isn't oriented correctly (so that a 2.5 reading is really 3) that can throw things off. DRs can break up at 3 with hot pickups.
 

JJman

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994
My '71 distorted above 3 until I changed out the original speaker. Have you tried a different speaker/cab?
 

theprofessor

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Messages
305
The 5R4 is lowering all your voltages, increasing sag, and will make the amp break up sooner than normal. It's not unusual for a stock DR to have 450V on the plates with the correct rectifier tube. Also, if your volume pot taper is off (not unusual) or the knob isn't oriented correctly (so that a 2.5 reading is really 3) that can throw things off. DRs can break up at 3 with hot pickups.
What Mike said. I'm hardly a tech-y expert, but I do have a SFDR and have done a bit of fooling around with lowering the voltages myself, including inserting a 5R4 briefly. Here's what I found during my experiment:

I set a baseline with the stock 5U4GB in there. Wall voltage was 122 v. AC. I was putting 452 volts on the plates of some old Tung-Sol 6V6GT's with them biased at 68% mpd, or 18 mA. I put in the 5R4GYB, which immediately dropped my bias down to 15.6 mA. So I adjusted it to 68% mpd as I had done with the 5U4GB in there. I biased it to 18.2 mA, and the plate voltage went down to 444. So with the plate current on the tubes at the same spot, I moved about 7-8 volts off the plates. I noticed that the sound was definitely "browner" and thicker. The treble was not as high (one could also say less sparkly; more "plain"), and there was less definition between the notes. The bass notes started to get fat and break up earlier on the dial. If all I was playing was blues, and I had no pedals, this might be a good sound for me. But I preferred the tighter, more sparkly sound with the 5U4GB.

As far as SFDR voltages, I really wouldn't worry about 450V on the plates of the 6V6s, as long as you have some good vintage ones or as long as you use the new JJ6V6Ss (which are sure stout, but they have their own sound). Sure, that's above spec, but it's typical for a SFDR.
 

Muttlyboy

Gold Supporting Member
Messages
3,094
What Mike said. I'm hardly a tech-y expert, but I do have a SFDR and have done a bit of fooling around with lowering the voltages myself, including inserting a 5R4 briefly. Here's what I found during my experiment:

I set a baseline with the stock 5U4GB in there. Wall voltage was 122 v. AC. I was putting 452 volts on the plates of some old Tung-Sol 6V6GT's with them biased at 68% mpd, or 18 mA. I put in the 5R4GYB, which immediately dropped my bias down to 15.6 mA. So I adjusted it to 68% mpd as I had done with the 5U4GB in there. I biased it to 18.2 mA, and the plate voltage went down to 444. So with the plate current on the tubes at the same spot, I moved about 7-8 volts off the plates. I noticed that the sound was definitely "browner" and thicker. The treble was not as high (one could also say less sparkly; more "plain"), and there was less definition between the notes. The bass notes started to get fat and break up earlier on the dial. If all I was playing was blues, and I had no pedals, this might be a good sound for me. But I preferred the tighter, more sparkly sound with the 5U4GB.

As far as SFDR voltages, I really wouldn't worry about 450V on the plates of the 6V6s, as long as you have some good vintage ones or as long as you use the new JJ6V6Ss (which are sure stout, but they have their own sound). Sure, that's above spec, but it's typical for a SFDR.
I guess my story is kind of similar...

With the stock power transformer and 5R4, the amp was at a bit under 440v on the plates and I liked it pretty well.

The amp also had all 22uf filter caps which may also effect things.

With the Weber PT I have the amp at 415V on the plates with the 5R4 and now all 16uf filters.

I biased the output tubes to be at about 65%.

I haven't had the chance to crank up the amp (apartment living), but when I get to take the amp out, I'llbe able to find out if the current setup will work for me.

If not, I can always stick a 5U4 back in and pick up 20 volts or so, and maybe swap the 22uf filters back in
 




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