Princeton Reverb B+ problem

[CB Amp]

Hello,

I just built a PR clone, and I can't get a normal B+ of 410 VDC. The maximum I get is 360 VDC, so the amp sounds but without headroom.
I build amps for years, and of course I checked the circuitry, the connections, the grounding, the tubes, tried different ones (new, NOS), and nothing changes. I really don't understand what happens and it drives me nuts. So I would appreciate your help.

Here are the voltages I measure : (I added a standby switch using the typical Fender circuitry, just after the first power cap).
PT, unloaded : 2x385 VAC - 5.21 VAC - 6.84 VAC
Loaded with tubes, standby off : 2x362 VAC - 5.02 VAC - 6.44 VAC => B+ = 505 VDC
Loaded with tubes, standby on : 2x322 VAC - 4.85 VAC - 6.22 VAC => B+ = 360 VDC - screen grids = 350 VDC

I precise that I've got the same problem with or without the preamp tubes, so they shouldn't be involved. I isolated the power amp, and even changed the PS caps and resistors, same problem.
I'm using brand new Mercury Magnetics transformers. Could it be a PT issue which do not handle the load ? The heater of the rectifier tube is a bit low (4.85 VAC) with standby on, but it could be something else that soaks power up.
Thanks for your help !

[Leo_Gnardo]

I'm using brand new Mercury Magnetics transformers. Could it be a PT issue which do not handle the load ? The heater of the rectifier tube is a bit low (4.85 VAC) with standby on, but it could be something else that soaks power up.

"Something else" could be excessive bias current thru the output tubes. Princeton 6V6 should pass 18 to 30 milliamps each, depending on the tube. Definitely no more than 30 mA, no matter what.

Also, what's your AC line voltage? If the transformer is spec'd at say 240VAC, and your line is 220, all the secondaries will be running a little low. But that doesn't explain the severe loss in high voltage.

Other than that, there's your Mercury transformer. Mercury puts the K in Kwality. And a hole in your pocket. The only thing that impresses me about Mercury is their ability to sell ordinary or even substandard crap, and get 2 or 3 or 4 times the price of any other competent transformer. They have an excellent advertising department: ex-carnival barkers.

[CB Amp]

I just read your answer, thanks a lot.
I precisely just measured the bias current : 60mA...
But I can't understand, everything looks good, tubes, connections, etc... I think I'm crazy.

About Mercury, you're certainly right But I used to buy a lot of their transformers, and they have interesting sales policies, so I don't pay so much.

[Leo_Gnardo]

I just read your answer, thanks a lot.
I precisely just measured the bias current : 60mA...
But I can't understand, everything looks good, tubes, connections, etc... I think I'm crazy.

About Mercury, you're certainly right But I used to buy a lot of their transformers, and they have interesting sales policies, so I don't pay so much.

A little discount on an overpriced transformer. If that floats your boat, who am I to argue?

If the bias current is 60 mA totaled for both 6V6, shouldn't be any problem. If it's 60 mA each, OW! You'll have to adjust the bias voltage so the current isn't excessive.

Let's for the moment take 60 mA for both, that's 30 each, should be no strain on the transformer and you should see expected voltages. Then I would have to say your hi voltage winding is lacking some turns. (I almost went to French there - Il manque.... after that, le lepin est perdu.) You'll have to accept a low power Princeton, not such a bad thing. Or get a replacement transformer that's wound correctly.

[CB Amp]

It was 60mA each tube !
I just rebuilt the bias circuit, but I had to swap the original 100K resistor (from the high voltage winding) for a 56K.
I get 404 VDC on the plates and 27mA each tube, it's a bit hot for 6V6's but it works. It sounds good now.
56K vs 100K, it's far from the original value. It's the first time I have to adjust so much. You're perhaps right about Mercury trans.

A little discount on an overpriced transformer. If that floats your boat, who am I to argue?

I don't want to defend them, there are some better manufacturers, but I just looked at my bills. I paid $75 for the PT and $50 for the OT, a bit more expensive than current trannies but you know, guitar players like famous names

Anyway, thanks for your help (and your french, I don't really understand "le lepin est perdu", you mean "the rabbit is lost" ? I don't know this expression...).
Cyril.

[Leo_Gnardo]

It was 60mA each tube !

Sacre' bleu! Yes that's way too much.

I just rebuilt the bias circuit, but I had to swap the original 100K resistor (from the high voltage winding) for a 56K.
I get 404 VDC on the plates and 27mA each tube, it's a bit hot for 6V6's but it works. It sounds good now.

No worries, I often see Princetons in the 400 to 440V range. 404V plate and 27 mA is a good balance.

56K vs 100K, it's far from the original value. It's the first time I have to adjust so much.

Again no worries. I vary the bias load resistor, usually put a trim pot - our UK friends call a "preset" - plus a fixed resistor to adjust bias. But it's just as good the way you did it.

You're perhaps right about Mercury trans.

A little discount on an overpriced transformer. If that floats your boat, who am I to argue?

I don't want to defend them, there are some better manufacturers, but I just looked at my bills. I paid $75 for the PT and $50 for the OT, a bit more expensive than current trannies but you know, guitar players like famous names

D'accord - when I looked into it, Mercury would offer a discount of 25% after you ordered so many transformers, I forget but it was 25 or so. Then you're a "special favorite customer." You must be on the SUPER special list to get 'em at those prices. You're getting a way better deal than I could so that's good for you.

Anyway, thanks for your help (and your french, I don't really understand "le lepin est perdu", you mean "the rabbit is lost" ? I don't know this expression...).
Cyril.

Happy to be of assistance Cyril, and "le lepin... " was what I learned in French class @ 1966, meaning "I lost track of what I was talking about" kind of like, the hunter is on the trail, and the rabbit goes to ground and disappears. Maybe it's a regional expression, or it's something only the old folks might say. I've run across that sort of thing before in several languages.