Ampeg V3 voltages high

[Randall]

Just got a dead V3 working, replaced OT, re-cap, new 7027A's, few other things. I notice my PS V's are high. Get 551V on the plates, a volt or two less on the screens, and -64V bias. Both 7027A's are pulling around 27mA, so at 551v that puts them at less than 15 watts dissapation. That is quite cold is it not? Max Diss rated at is 35 Watts.

My wondering is do I want to try to pull the bias V back closer to the -54V on the schem, or is running cold a desirable thing given the plate and Screen V's are so high? The data sheet I am looking at puts Max plate V at 600v, so I'm good there, but Max screen grid at 500V, which is about 50v lower than where this amp is at. All the PS resistors measure within tolerance out of circuit.

I am thinking about putting a bias pot in it, but am unsure where the best place to put it would be.

What to do, what to do.....

Here is the Joe Piazza version. The Ampeg drawing that is inside it shows plate node at 500v, screen node at 490v, and bias at -54v.

http://schems.com/manu/ampeg/v3poweramp.pdf

[g1]

What kind of 5AR4 are you using? What is your line voltage? Is the PT stock?
Those amps are biased quite cold and that's how they survive the high plate voltage. I would not recommend increasing the bias unless there is very objectionable crossover distortion.
The specs you saw for 7027 are for old production. New production are 6L6 with added pin, so 500V plate, 450V screen (JJ spec sheet).

[Randall]

All the paint is gone on the rectifier, but it looks to be an RCA and knowing the history of this amp, probably original. I swapped it out with a used 5AR4 to check it, they are a few volts apart. The PT I'm going to say is stock. My line voltage is 121VAC.

At the newer ratings, it would seem the screens are getting hammered. Is this just the way it is, or should I be looking for a problem?

The amp sounds good, BTW.

[g1]

IMO that's the way it is, especially since you say it sounds good. Sounds like the PT secondary is kind of hot, which is why the bias voltage is also higher. There's plenty of Ampeg stuff out there with around 540V at plates and screens running 6L6 with no problems (till someone messes with the stock bias ) so I wouldn't worry too much about that. Are any of the HV caps getting voltage more than they are rated for?
To double check if it's the PT responsible, what are the 6.3 and 5V heaters reading?

If it turns out they are also hot, and you really want to do something about it, check out RG Keen's vintage voltage adapter (bucking transformer).

[Leo_Gnardo]

I run 7027A's a little hotter say 35-40 mA. You could put a 5U4B in there and get s pinch less B+ the easy way.

[g1]

Leo, are you talking NOS 7027A's, or modern production (6L6 as I refuse to call them 7027 ) ?

[Leo_Gnardo]

Leo, are you talking NOS 7027A's, or modern production (6L6 as I refuse to call them 7027 ) ?

Originals - deafinitely. But not many of them left so...

Modern "fakers" like JJ 7027A which I'm told are relabeled up-priced 6L6. Yes, them too. I see a pretty good "kink" in the test sine waveform at zero-crossing point with 25 to 30 mA. Looks mo betta at 35 to 40.

[Randall]

Are any of the HV caps getting voltage more than they are rated for?
To double check if it's the PT responsible, what are the 6.3 and 5V heaters reading?

Nope. The caps are all within spec. The heaters are running at 5.3vac and 6.3vac respectively. So sounds like I should stop worrying about it and button her up?

[g1]

To me it sounds good to go. Especially since you said it sounds good.

[Randall]

It sounds great. Now if I could just find the top chassis panel so I can box it up and get paid!