B+ to high on homebrew

[billybillybilly]

i have a homebrew pushpull 2x6l6gc amp ive built . its basically a fender style design that i tinkered with the preamp , 2x 12ax7 . my present problem is my voltages are too high (the PT is from an old peavey hybrid running 4x6l6gc to 110w , secondary is 350-0-350) . the output tube plates are getting around 515-520+vdc and screens are at around 500+vdc . i realize that my circuit doesnt draw as much current as the one the PT was designed for . i only have 6l6gc output tubes and no money ... but i do have a crate of semiconductors !
any suggestions on a way to drop the voltage approx 30v ? a diode from CT to ground ? i know ive seen threads on this but ill be darned if i cant find em now . any help is definitely appreciated !

[Albert Kreuzer]

Hi Billy,

big fat zener from CT to ground.
Alternatively, a power transistor or a MOSFET with a small zener (see below). Use chassis as heatsink.

Cheers,
Albert

[olddawg]

Another way is to simply put the zenes in series with the B+ line. Three 10v 5 watt zeners should do it. Just mount them where they can breath.

[black_labb]

you could always leave the plate supply at the higher voltage and drop the screens and the following preamp. wouldnt be a fender style design, but good if you want some extra power.

[MWJB]

You don't say what plate current the 6L6s are drawing, can you safely increase the current (say up to 30mA), this may bring down the B+ a few volts. If not, no problem, any decent 6L6 will take the voltages you quote.

[billybillybilly]

thanks for the all responses !!

ok , so im thinking 100ohm resistor from the CT in series w/ the zener to ground . would that work with a bridge rectifiers CT as oppossed to the one from the transformer ? and Albert , when you say 'big fat' ... the biggest i have are 5w .
also , a question on zeners in series with the B+ : that works because the diode doesnt pass electricity until its threshold is reached there by removing that amount from the voltage supply ? kinda like the reverse of clipping an audio signal ?
a side note - the tubes are the cheapy chinese 6l6gc AES said they couldnt blow up . i have it biased pretty cold i think , -55 . it doesnt sound bad at all and its super loud ! any ideas as to how long those tubes will last under those conditions ?
maybe i could put a switch to choose from a lower power w/ the zener or high power w/ the CT straight to ground ?

[MWJB]

"and Albert , when you say 'big fat' ... the biggest i have are 5w ." Er, they won't handle it, 25-50W would be more like it. Zener in series with OT CT is not usually used with a bridge rectifier...hang on, if you have a 350-0-350VAC PT why are you using a bridge rectifier? Something here doesn't scan, please clarify.

"i have it biased pretty cold i think , -55 . it doesnt sound bad at all and its super loud !" -55v at 515-520vdc doesn't sound cold to me, you need to tell us how much plate current they are drawing, negative voltage is not enough (varies with brand).

"maybe i could put a switch to choose from a lower power w/ the zener or high power w/ the CT straight to ground ?" How would you account for the deviation in plate current?

[billybillybilly]

"and Albert , when you say 'big fat' ... the biggest i have are 5w ." Er, they won't handle it, 25-50W would be more like it. Zener in series with OT CT is not usually used with a bridge rectifier...hang on, if you have a 350-0-350VAC PT why are you using a bridge rectifier? Something here doesn't scan, please clarify.

yea i didnt think 5w were enough . im using a fullwave , was just curious if it would work on a bridge .

"i have it biased pretty cold i think , -55 . it doesnt sound bad at all and its super loud !" -55v at 515-520vdc doesn't sound cold to me, you need to tell us how much plate current they are drawing, negative voltage is not enough (varies with brand).

i dont have it right in front of me . if i remember correctly it was around 20ma . tubes arent red plating .

"maybe i could put a switch to choose from a lower power w/ the zener or high power w/ the CT straight to ground ?" How would you account for the deviation in plate current?

a dpdt (on-off-on) ; one pole for the CT switch and the other for standby , so it would be in standby when the CT was switched ?

[MWJB]

See what plate voltae you get at 30mA per tube?

Regarding the 'zener switch', you can in some cases bias to a reasonable compromise so an amp will work well with 5U4/5AR4, or 5AR4/SS...maybe sometimes even 5U4/SS. But then we are talking usually about a 15-20v max difference in B+ for a 2x6L6 fixed bias amp, so what I'm saying is, that if you bias for a decent plate current at the higher voltage (+30vdc), when you switch to the lower voltage the tubes might be on the cold side.

Mind you, that's just raised another suggestion = a cathode/fixed bias switch. If you used say a 600/680ohm 10W cathode resistor, you would get a voltage drop (50vdc-ish) and the tubes would still be biased OK, back to fixed bias & full voltage. OK, it's not "like for like" but it's an option.

[billybillybilly]

great suggestion !

i guess i shoudve mentioned its a SS fullwave rectifier (6x1n4007).

i know the voltages are a bit above the recommended maxs . would an HT fuse prevent any major component damage if a tube were to fail ?

but , i really like the idea of the cathode/fixed bias switch . the cathode resistor is shared right ? and the switch would be a dpdt (on-on) with one pole to connect the cathodes to ground or to the resistor , and the other pole would connect/disconnet the B- respectively ? where would be a good spot to pull the B- from the circuit , and would i need to remove the 220k divider resistors as well ?

[MWJB]

Plenty of Brown & BF Fenders run similar voltages to your amp. It's not at significantly any more risk of failure than any other amp. The correct fuse in the primary is the first stop, you could use 5W screen grid resistors to prevent them blowing if you had a screen failure. If you have a virtual CT on the heaters then the resistors here will normally blow before anything else. If a tube fails the primary fuse will blow, if not the amp will make unusual noises...either is a clue to stop using it & address the problem. PTs & OTs don't fail very often (unless you make your own fuses out of tin foil). You can add an HT fuse if you want...most amps do OK without them.

Check out Kevin O'Connor's londonpower.com website, search under Tube Amps FAQ, there's a schem for a fixed/cathode switch there.

[billybillybilly]

Plenty of Brown & BF Fenders run similar voltages to your amp. It's not at significantly any more risk of failure than any other amp. The correct fuse in the primary is the first stop, you could use 5W screen grid resistors to prevent them blowing if you had a screen failure. If you have a virtual CT on the heaters then the resistors here will normally blow before anything else. If a tube fails the primary fuse will blow, if not the amp will make unusual noises...either is a clue to stop using it & address the problem. PTs & OTs don't fail very often (unless you make your own fuses out of tin foil). You can add an HT fuse if you want...most amps do OK without them.

Check out Kevin O'Connor's londonpower.com website, search under Tube Amps FAQ, there's a schem for a fixed/cathode switch there.

good to know about the voltages , also , awesome info on that londonpower page , thank you !
i have the 5w screen resistors , no elevated heaters ... and ive upgraded from the tinfoil fuses to the 16ga copper buss wire type !

excellent forum ! glad youre here !! well , back to my tinkering and thanks again !