Dropping Voltage at PT

[captntasty]

Hello. I am working on a Traynor amp from the 70's. The amp is in great condition but as it was designed for use with 115v at 122v coming out of my wall all voltages are high. Normally I wouldn't mind but on this one right after the BR I'm at 436vdc where the schematic shows 400vdc and that voltage is going into a 450v cap. It is being recapped and the new ones are 500v so I'm not really worried about that... it's just that all the way down the line voltages are high - IMO too high.

I have searched and found threads about zener diodes, limiting resistors, etc. but I'm not sure if these fit the bill.
How can I drop voltage right from the PT - where the schematic shows 310vac I'm getting 336vac. Is it possible to drop this ~20v to get all the voltages down the line lower?

[mbratch]

I've used the zener diode method with success. One or more high-wattage zener(s) in line with the HV CT to ground.

Sounds, though, like your voltages are within 10% of nominal, which may not be a problem anyway. Unless you're having trouble getting a good bias setting or you have other sensitive components close to the limit you may not need to be concerned.

[captntasty]

So the zener would be placed at the ground connection/CT of the rectifier diodes, right? The PT does not have a center tap. What wattage would be sufficient?

[Chuck H]

For starters, odds are good that the original amp was a little above spec. @ 115 wall volts. Hammond trannies often are.

Your viltage difference at the wall should result in 427 volts instead of 400 IF the PT was always in spec AND you have the amp biased correctly. If the bias is too cool your plate volts will be too high. NOW... Based on your spec. voltage I'll guess this is an EL84/6bq5 power amp. In that case I do think 427 (or 436) volts is too high.

Not sure about the best method for voltage reduction using a bridge rectifier though. If you don't mind a little sag (compression, slower attack envelope) you could use a big fat series resistor right after the rectifier before the first filter cap. Probably 47-68 ohms if it's a four el84 amp and about 100-150 if it's a two el84 amp.

Keep in mind that any mod to drop the B+ won't have any effect on your filament circuit. If your filament circuit is too high that is also bad. It should be within 10% of 6.3 volts. IME most 115V primary Hammond PT's are within the 10% filament spec. up to 125 wall volts.

If this isn't an EL84 amp I wouldn't worry about it at all. I'd just bias it up proper and play it.

Chuck

[captntasty]

Yep, it's 2 el84... the filament is at about 6.85v, so you can see it's right at the edge there of that 10%. If I could drop the AC wall voltage I would to correct it all!!! I'm just trying to get this to the point where my guy won't have to worry about this stuff. I'll have to see how this shakes out after I recap. I haven't even played with the bias... my gut was that it was just too high all over the place. Thnx

[ThermionicScott]

Sounds like a job for a Vintage Voltage Adaptor.

[captntasty]

Sounds like a job for a Vintage Voltage Adaptor.

Absolutely. I've been wanting to make one of those and now I may have a reason! I'm not sure the customer would be interested though. He just wants to not have to worry about this kind of stuff...

[Steve Conner]

So the zener would be placed at the ground connection/CT of the rectifier diodes, right? The PT does not have a center tap. What wattage would be sufficient?

For a bridge rectifier, you can insert the zener in the same place Chuck says to put the sag resistor. Just get it the right way round, or it'll only drop 0.7V.

The Vintage Voltage adaptor only needs a small transformer that can be hardwired in and hidden inside the chassis, this has worked for me in the past.

[captntasty]

What wattage would be sufficient?

I've seen some say 5w and other say 50w... This is a bit new to me and my gut says that 5w wouldn't handle it.

???

[MWJB]

"If your filament circuit is too high that is also bad. It should be within 10% of 6.3 volts - With respect Chuck, I have to disagree. Low filament voltages (e.g. 6.3VAC less 10%) will sound awful. Many vintage Fenders are around the 7VAC mark and sound great. Even the typically conservative tube data sheets state that 6.3-6.9VAC is within the accepted normal range. Amps are designed to run >6.3VAC with a full current draw, so I'd be more inclined to look at 6.5-6.6VAC as the middle ground.

I would typically be suspicious of any amp with a filament voltage less than 6.3VAC (may point to excess current draw elsewhere on the secondaries), and I wouldn't fret if you had 7VAC.

I'd have a chat with some tube vendors, there is the 13.5W Russian EL84M/E84L that is supposed to take huge voltages for that kind of tube?

[captntasty]

Bias

So after measuring bias current this puppy definitely needs some help. This is all preliminary as this amp will be recapped and various components modified and will get a new set of el84's but it's a little scary. Right now it's in excess of 15w plate dissipation - I believe el84's are 12w max...

I subbed in a 130R 25w resistor from rectifier to first filter cap and it helps but could be dropped even more. I suppose the Sovtek el84m's are an option... 14w max.

I'm tempted also to see if the client wants to convert to adjustable bias...

Just thinking out loud...

Any of your thoughts and opinions on strategies for this one are appreciated.
P

[ThermionicScott]

If hiding a VVA transformer somewhere on the chassis isn't an option , I would personally add enough series resistance to the heater circuit to bring the voltage under 6.9V or so. Who's to say that wall voltages won't continue to climb?

I would favor a Zener to get the B+ voltage down -- adding a bunch of series resistance to the front of the B+ chain could affect the "feel" of the amp in a way the owner doesn't like. Which Traynor is this, by the way?

- Scott

[captntasty]

YBA-2B... the VVA is an option... I'm going to have a talk with my customer, give him some options and see what he can live with. It would seem this amp would be running up around max plate dissipation to begin with before higher AC wall voltages... depending on a few other variables.

[MWJB]

"I'm tempted also to see if the client wants to convert to adjustable bias..." - If you rebias colder remember that the plate voltage will climb further.

[captntasty]

Of course... I guess what really bothers me is that even if this were running at the published voltages on the schematic, wouldn't it already be exceeding the max plate voltage and every other max of an el84? Am I not taking something else into account?