Reducing Voltage On Laney Supergroup 60 Watt

[Gahoo]

Working on a Laney Supergroup 60 Watt for a customer. It's got the original Partridge transformers which I'd like to keep using. The problem is the power transformer is currently supplying about 700VDC for the B+ (unloaded with output tubes).

What's the most elegant solution? Try dropping the B+ with a high-power mosfet or zeners? Seems like a lot of power to try and dissipate...
Leave the plate voltage super high and throttle the screen grid voltage way down?
Or should I just start leaning towards a replacement PT? :/

Any advice would be appreciated, thanks!

Here's a schematic, the preamp I've got is slightly different but the power amp and supply looks the same.
Laney-60.pdf

[GainFreak]

It looks like either there's something wrong with your DMM or the center tap doesn't make contact with ground.

[Enzo]

Unloaded. OK, but what is the B+ when all the tubes are in and it is fully loaded? That is the voltage that matters.

Besides, what is wrong with 700v? EL34s are good for 800v anyway. Of more concern to me would be the screen voltages. And the main thing, dissipation.

I am less in touch these days, but I am not aware of these Laneys being tube eaters. WHy do we feel the need to change it?

[GainFreak]

Working on a Laney Supergroup 60 Watt for a customer.

Since it's a 60W amp I assumed "normal' voltages of up to 500V max. Also it's a "regular" power amp schematic. 700V even unloaded sounds like a lot.

[Gahoo]

Unloaded. OK, but what is the B+ when all the tubes are in and it is fully loaded? That is the voltage that matters.

It came in with a shorted EL34 so I only just now got to fire it up with two tubes installed. Actual B+ with tubes installed is 666 VDC. Screen voltage is 648 which seems like it's definitely exceeding the max rating for EL34s. Grid voltage is -46.

Also to add, a previous tech installed 1K resistors for the screen grids so the 470 Ohm ones indicated in the schematic are not accurate.
Is increasing their value further to drop the voltage the next step? or is there a more elegant way to reduce it?

I appreciate the help, as always

[J M Fahey]

Increase screen resistors even further to 2k2 5W (or 10W if available).

Those screens are geting >600 volts at idle, but as soon as they are being driven and start pulling current voltage will drop thanks to the increased resistors.

FWIW some go even up to 4k7 there, go figure, although sounds somewhat extreme to me.

One problem is that although that *old* transformer has various options to cater to 200V something mains, it only has a 110V tap , while USA mains voltage today is around 120V.

You might add a bucking transformer (120V>12V@4A) to drop current mains voltage to "vintage levels" which will correct not only +V but also, in a way more important, 6.3V filament voltage which certainly must be too high today.

[Gahoo]

Alright, replaced the old screen grid resistors with two 5 Watt 1K's in series. I'm running it off a variac "tuned" to the filament voltage of 6.3; it's putting 107 volts on the primary.

Now my plate voltage is 600V, screens are at 582V idle, and the voltage across the 10R cathode resistor is .4, so I'm assuming an idle current of @20mA per tube.

As I increase the signal, the screen voltage drops as expected. Once the signal starts to clip the screen voltage is around 400V but until that point it's more like 450V. Over the 425V max rating in any case. Think I should further increase the screen grid resistance?

Thanks!

[J M Fahey]

I suggest you "downgrade" the amp, in the name of a good cause of course, by replacing the choke with a resistor, which will lower all voltages downstream, including screens, to a more reasonable level for modern tubes.
Do not remove the choke, just unsolder and insulate wires.
You may try 2k2 10 or 20W.
Also suggest you glue a small note inside: "Choke replaced by resistor to tame *excessive* screen voltage" so a future Tech does not try to reverse it.

A 12V 4A bucking transformer in (reversed) series with mains should reduce it to very near the 107VAC it seems to like.

[Gregg]

Or you can just use couple of 5W zeners in series with the screen resistors in order to lower that voltage to safe levels.

[Gahoo]

Cool, I ended up setting the screen grid resistor to 3K which kept the dissipation of the screen in spec. I also added some flyback diodes to the plates, seems there had been a bit of arcing from the plate to the heaters.

Thanks for the good advice all!