1967 Super Reverb B+ Help

[64ES335]

Hi everyone,

Looking for some help on a 1967 Super Reverb. So far, I’ve replaced all the electrolytic caps with the correct values, including bypass caps. Checked all resistor and cap values (including dropping resistors ) and measured for leakage. Replaced as needed. Overall, the amp seems to be running OK.

My question is about the B+. I’m running a 116 VAC line supply, in order to achieve 6.3 VAC on the heaters. Measuring the secondary of the PT, I’m getting 350 VAC on pins 4 and 6 of the GZ34. Close to what the original schematic indicates.

When the amp is in standby, I’m measuring a B+ of 475VDC at the rectifier. When I switch out of standby, the B+ drops 35 to 40 V.

I understand that a drop is to be expected once the amp is pulling current. However, this seems to be a bit more than I am used to seeing. This even happens when the tubes are biased colder at 40-50% disapation (I’ve also tried changing entire sets of tubes, including the rectifier, with no effect).

I understand that the real world voltages will not align perfectly with the original schematic. But this seems off to me. By the time I get the output tubes up to 60% dissipation, the voltage drop when hswitching out of standby increases to 40-50V, leaving me around 420v on the plates.

Assuming this voltage drop is more than I should be seeing, any suggestions I want to check next? The filter caps are new, but maybe one is bad? Maybe a dropping resistor is changing value when voltage is applied? Power transformer, choke, or output, transformer issue?

I’ve included some of my voltages below in case it helps. Really appreciate anyone’s input. Thanks very much!

[The Dude]

FWIW: I don't think that your voltage drop engaging B+ is horribly unreasonable. The one thing I MIGHT be concerned about is possible oscillation. If the amp is oscillating, there may be excessive current draw even at lower bias points. Look at the output on a scope and make sure there's no oscillation. You might also try pulling the PI tube and see what happens to B+. Removing the PI will usually stop oscillation, if there is any. If you don't detect any oscillation, I would say 420V B+ at amp idle is fine.

[xtian]

This does sound odd. The heater elements in your tubes draw the most current from the PT. The B+ winding far less. When I power up a tube amp with NO tubes installed, I'm used to seeing high, "unloaded" voltages. But when all tubes are installed and heaters are warmed up, flipping a stand by switch on doesn't make much of a difference in B+.

The Dude posted while I was typing. Oscillation is an excellent guess.

[g1]

Is the B+ low when in play mode (from schematic value)? If not, I don't think B+ in standby is relevant.
Priority should be value in 'play' mode, they don't give values for standby.
I expect a bigger drop between modes when tube rectified (compared to solid state diodes).

[mozz]

This does sound odd. The heater elements in your tubes draw the most current from the PT. The B+ winding far less. When I power up a tube amp with NO tubes installed, I'm used to seeing high, "unloaded" voltages. But when all tubes are installed and heaters are warmed up, flipping a stand by switch on doesn't make much of a difference in B+.

The Dude posted while I was typing. Oscillation is an excellent guess.

Not all standby switches are set up the same.

[pdf64]

How exactly are you measuring the 6L6 dissipation?

Well done with the voltage survey; how much anode or cathode current was each 6L6 drawing at the time of the survey.

What V AC do you measure at the rectifier pins 4&6 in standby? I expect it's a bit above the 350V you measure at idle?

[Chuck H]

The schematic for that amp shows a 360-0-360 HV secondary. Are you sure the PT is stock? If it's not then a replacement with more secondary DCR will drop more voltage than stock. Even if it IS stock the PT for that model varied quite a bit even though it always kept the same model number.

[Helmholtz]

Some data:

The AA1069 SR schematic specifies a B+ of 510V in standby and 440V running, with a line voltage of 120V. So a delta of 70V.
This amp uses a 5U4GB rectifier. With a GZ34 the difference might be 15V less meaning a delta around 55V.

My own AA 1270 Vibrolux reverb (using a GZ34) measures 468V in standby and 422V on. So a delta of 46V.
HT winding is 2 x 340VAC. Tubes biased to 52%. Heater winding is 6.43VAC.
I tried a few different NOS GZ34s and with some the delta was a few volts more.

If you still think there's something wrong with the power supply, you should measure clean output into a 2R resistor.
Expect around 40W.

[64ES335]

Wow, what a great community. Thank you for everyone’s quick replies and support!


The Dude - Thanks for the input. I’ll definitely throw it back on a scope and look for any abnormal oscillations. I I did actually pull all the preamp tubes, including the phase inverter to see what the effect was on the B+. No significant change. Maybe rose a few volts.


xtian - I’m also used to seeing somewhat of a drop in B+ when switching out of standby. But this seemed a bit more drastic. I agree oscillation is some thing that I definitely need to check. Thanks.


g1 - I hear you. I normally disregard B+ value when in standby. But since the plate voltage was quite a bit lower than the schematic value (with the same rectifier voltages), and most other techs report values close to the schematic (460V) it raised an eyebrow for me. If I was running the output tubes at 70% dissipation with plates at 440V to 450V I wouldn’t bat an eye. But it seems odd when all the other voltages are generally in line with the schematic, and I’m running the output tubes relatively cool.


mozz - Fair point. Maybe I’m used to a different configuration.


@pd64 - I’m using a shunt method. Measured the resistance of each OT secondary to the center tap. Then measured the voltage from the center tap to each plate, divided to get the current, then multiply the current by the plate voltage to get the dissipation. I also double checked my calculation with a comp-u-bias, which was in the same ballpark.

Re: voltage chart, thanks! I don’t recall off the top of my head, the plate current. It wasn’t excessively high. I can check later when I get home. It may have been 25 to 35mA.

I also don’t recall on the rectifier plate voltages between standby and play mode. Can check that later as well.
Thank

Chuck H - fair point on the PT. It is actually not original, however, it’s a Schumacher (606) 022798 (125P5D) from 1966, which is the correct model for this AB763. Though a few Volts lower (350-0-350) it’s still within the rough tolerance as you point out.


@Helmholz - Great info. Interesting you’re also seeing 40-50V delta on your Vibrolux. Maybe it is nature of the beast. Just doesn’t sit right with me though. Will have to order a 2 ohm power resistor and take that measurement.



I guess what I’m having trouble wrapping my head around is that even though my PT secondary voltage seems to be in line with the Fender values, and the rest of the preamp voltage seem to be in line, the B+ on the plates of my output tubes are comparatively low, even though biased cooler. Plus I would normally only expect a 15 to 20 V drop with a GZ 34.


Thanks to everyone for their help!

[Helmholtz]

@Helmholz - Great info. Interesting you’re also seeing 40-50V delta on your Vibrolux. Maybe it is nature of the beast. Just doesn’t sit right with me though.

But why?
I think your voltage drop is completely normal and my examples confirm it.
Don't forget that Leo allowed for a 20% tolerance with schematic voltages.

....Plus I would normally only expect a 15 to 20 V drop with a GZ 34.

B+ sag is not only due to the rectifier tube.
Rather there are 3 additive effects:

1) Effective PT resistance.
2) Filtering efficiency lowering with increased current.
3) Voltage drop across rectifier.

In other words the tube rectifier sag just adds to the sag you would have with a SS rectifier.

[64ES335]

But why?
I think your voltage drop is completely normal and my examples confirm it.
Don't forget that Leo allowed for a 20% tolerance with schematic voltages.

Probably because I need to be in therapy. LOL. I hear you and don’t disagree.

B+ sag is not only due to the rectifier tube.
Rather there are 3 additive effects:

1) Effective PT resistance.
2) Filtering efficiency lowering with increased current.
3) Voltage drop across rectifier.

In other words the tube rectifier sag just adds to the sag you would have with a SS rectifier.

What you say above makes perfect sense. I was considering that a variation in the PT winding resistance may be the root cause. I may play around with the amp a little more and then let it go. Thanks very much for your help.

[g1]

One thing you didn't mention was which schematic version you were using as a reference.
So setting up line voltage to give 6.3VAC heaters may not give the same AC going to the rectifier tube. Or was 350-0-350 the spec. on the schematic you were using?
I find heater voltages can vary quite a bit off the 6.3V spec., so tend not to use that as a hard target when setting line voltage with variac.

[64ES335]

One thing you didn't mention was which schematic version you were using as a reference.
So setting up line voltage to give 6.3VAC heaters may not give the same AC going to the rectifier tube. Or was 350-0-350 the spec. on the schematic you were using?
I find heater voltages can vary quite a bit off the 6.3V spec., so tend not to use that as a hard target when setting line voltage with variac.

Was referencing the AB763 dwg. which indicates 360-0-360. I figured the 350-0-350 was well within tol. Maybe I’ll carefully increase the line until I get the 360-0-360 and see where I end up on the B+. Afterall, the heater voltage is subject to the same 20% tol., as you point out. Thank you.

[mozz]

Heater voltage is usually 10%, per manufacturer data sheets.

[64ES335]

Heater voltage is usually 10%, per manufacturer data sheets.

Noted. Thanks, Mozz. I’ll play around with it a bit more, then update the thread for “posterity”.

Any other comments or suggestions are still welcome!