1967 Super Reverb B+ Help

[Tom Phillips]

64ES335,
Welcome to the Forum!

I wouldn't hurt to try another GZ-34 rectifier.

I do my amp setups with a 120V line setting and I have the following set of data taken on a 1968 Super Reverb that has a stock part number 125P50 Fender PT. (However the EIA code was 606443 so the transformer was make in the 43rd week of 1964. It's interesting, but not all that unusual, that and older transformers got installed in the 1968 amp.
The measurements were per the attached table. Note that these are records that I keep for my own reference so it may seem confusing at first to others. However, the quality of the data comes with a money back guarantee.
Let me know if you have questions. Overall I don't think there is anything "wrong" with your SR transformer.
Cheers,
Tom

[Helmholtz]

The correct PT should be a 125P5D (used up to AB568).

If measurements were taken with a line voltage of 110V, all secondary voltages should increase by roughly 9% when connected to 120V.

[64ES335]

Hi Again Guys,

Yessir...the amp is loaded with a 125P5D.

Swapped the rectifier again with no significant change. Brought the line up to 120V and it almost seems like the amp wants to live there.

B+ in standby is 495.9VDC. Drops just under 450VDC when switched out of standby with the tubes biased to 70% dissipation (40mA).

Think I'm going to follow Helmholtz and Tom's advice and accept as-is, unless there are any other clever suggestions to check.

Grateful to everyone that has helped!

[The Dude]

FWIW: Fender wouldn't have biased the amp that hot, but that's your choice.

[Chuck H]

Really awesome that whomever replaced that PT got it right. That said... With a 350-0-350 (for the same part number as what is supposed to be a 360-0-360 and later a 375-0-375) you could expect lower voltages. Also, AC mains voltages were changing at that time from 115Vac to 117Vvac (and thence to 120Vac). Haed to say what primary spec was done for that particular PT. As I mentioned, this PT was seemingly maliable over time and incidence.
I've actually seen a couple of cases where these amps (Concert's, SR's, Bandmasters, Bassmans, etc.) have low-ish voltages with period correct PT's. Though it usually goes the other way with high voltages this isn't ALAWAYS the case. And as Helmholtz noted the loaded voltage drop you're seeing is not at all abnormal (in fact it's lower than typical as far as I can tell). I think The Dude had it right in post 2 and you're actually fine.

[Tom Phillips]

... Drops just under 450VDC when switched out of standby with the tubes biased to 70% dissipation (40mA)...

I agree with The Dude that 70% is actually pretty hot bias and I think it would be interesting to you to try bias in the range of 50% to 60% and determine how you like the sound. The 70% "guideline" is really is more internet lore than a design fact.

[pdf64]

I don't think that the 'idle at 70%' guideline is applicable to valve types that have a design maximum rating unless, as is intended with the design maximum system, the mains supply voltage is tweaked up to its upper tolerance limit.

With the mains voltage at nominal, using the voltages noted on the AB763 Super Reverb schematic https://el34world.com/charts/Schemat..._schematic.pdf
plugging those anode, screen grid and control grid voltages into a calculator / valve info corresponds to about 32mA anode current at 460V anode voltage = just under 15W idle anode dissipation.

[g1]

We should start calling 70% as 100% for class AB push pulls.

[64ES335]

Thanks everyone. Yes understood. The 70% dissipation was really just to see how far the B+ got loaded down when biased hotter. I’ll be dialing it back and adjusting by ear while monitoring the measurements. Thanks again!

[Tom Phillips]

Thanks everyone. Yes understood. The 70% dissipation was really just to see how far the B+ got loaded down when biased hotter. I’ll be dialing it back and adjusting by ear while monitoring the measurements. Thanks again!

I'm interested to know your evaluation results when they are available.

[64ES335]

Hey Tom, Sure thing. I’ll probably get back to it this weekend and will post the voltages etc. I end up with.

[64ES335]

Hi Guys, Sorry for the delay in follow up. Ended up running the amp at 120V as mentioned. Voltages are listed below. Ended up running the bias current at about 39mA (17.5W).

Thanks again for everyones help!

[Chuck H]

We should start calling 70% as 100% for class AB push pulls.

Good idea In fact because I work a lot with el84's most of the known guitar amp lit applies to cathode bias. Anyone here that knows me is aware that I bias them hot but I push them hard and the cathode bias (self bias as it were) sort of auto corrects for high current. That said it can still end up with crossover distortion at high drive levels so I sometimes (not always) incorporate a clamping diode across the bias resistor. It's really not a hard dance to learn and I've read a few reports where people on other forums have read and used my ideas. Just happy to help. Of course fixed bias is another matter but that was sort of my point. The clamping diode does "fix" the bias for the "working voltage". Usually at something around the 70% range? I've never bench tested for that but I'm not blowing up tubes either so I'm probably ok

[Chuck H]

As a side note... It wasn't uncommon once upon a time for techs to bias 6V6 Fenders a bit on the hot side just to reduce plate voltage. I'm not a repair tech but it's clear that what was discovered was that the higher voltages on these amps relative to modern AC outlets was worse than biasing for additional current relative to tube failure rates. So biasing hot to reduce standing plate voltage seemed to reduce tube failure. At the very least in some circumstances and observations. I only remember reading about it but it seems to apply.?.