power supply in stereo 6V6 amp

[pjd3]

Hi, hope you all had a pretty good years ending, I'm from Reading Ma and "Limpy our town turkey" got hit by a car and died today so, my town is very sad! But,

While the bank is restocking to finish the Marshall dual channel, I'm in the process of designing up a two-channel 6V6 stereo amp for a fretless guitar I made (2 independent preamp and power sections).

In terms of the power supply, I see that a Twin reverb power transformer meets the electrical specs for HV current and heater current and would still have plenty of breath left over. Out of the gate, what elements of the power supply do you think need to be met in order to properly feed the voltage/current to 2 channels of 6V6's including separate preamps? Can one solid state rectifier and one set of primary filter caps be sufficient for 2 preamps and 2 power stages? Or, would you see a need or benefit to double up on either rectification, filter caps or both? For some reason, I picture the preamps benefiting from independent plate filter caps but, thats just a head theory.

Each of the two preamps will have either 2 or 3 gain stages before a long tailed pair phase inverter. I'm looking at a very clean signal path on this stereo amp, keeping distortion as low as possible. I may even be using mostly 5751's, 12ay7's and 12au7's for this reason. Another thing is that each of the two preamp/amps will have 2 inputs - one for the direct guitar signal and the other for a line level effect. I found some nice Jensen transformers and circuits that should work well for the "effects configuration" that I have planned for this.

So, I guess what I'm trying to do is supply typically robust power to everything without being redundant where it doesn't need to be. I'm not too concerned with saving money per say, only if it doesn't compromise performance.

Of course, your thoughts about this will be very valuable and interesting to me.
Thanks so much and happy new Year!
Phil Donovan

[Stevem]

Well the first thing to do is nail down if the output stages will both be class A biased or class AB biased , or a combo of both because the current needs will be different.

Each preamp tube will need .003 amps each and in class AB each 6V6 will need 37 ma each for a output of about 22 watts with about 350 volts on the plate, with a 8000 Z output transformer.
If running fixed bias you will need another 12 ma of current for that and then you need a reserve of 20 to 25% more peak current to not have the amp play spongy and have poor dynamic responce.

[pjd3]

Thanks Stevem,

The amp will be A/B fixed bias. I'm looking to achieve a pretty tight power application to keep the dynamics tight and not spongy.

Its looking fair that a single power transformer can manage the 2 pairs of 6V6's with associated preamps and PI. The Twin Reverb PT is approximately 320-0-320 @ 450mA with heater max being 5.5A. My math shows that should cover the electrical requirements pretty well. With diode rectification this could put around 430V on the plates of the 6V6's so, I many need to consider that, and if the tubes I choose will handle that voltage. I believe there are certain 6V6's that will.

Whats keeping me awake at night is whether to have a single diode rectification that will feed the 2 amps, and then if a single primary filter cap section will do, just following the rectification. Of course this will all be much more closely considered before circuits are set up and switch on so, this is just the thought process phase for me right now. There is probably a couple of ways to implement a power section for this project, and I'm willing to optimize things for the greatest benefits. I"m having a mental road block with the main filter cap section however - can't think through how that would best work. But, that's why I"m here! Somebody always seems to fill in the blanks for me over here, and that's why my first build was successful

Thanks Stevem!
Phil D

[Stevem]

6V6s are run at 425 volts all the time in Fender deluxe black face amps , but then you can idle them at no more then .018 amp in class A/B mode.

JJ electrons 6V6s are a very hardy tube and I have run them at over 500 volts with a issue in a High watt amp.

[pjd3]

Thanks Stevum.

I should be good to go now with providing reasonable voltage and bias to the 6V6's.

My main consideration is clarifying what needs to be considered with preamp tube voltage and filter application. For example, If one was to attempt to apply preamp tube plate voltages out to two preamps from a single resistor/divider node, wouldn't the amount of filtering need to be changed to accomodate the added impedance or resistance of the second preamp tube plate that it would be serving? Its tempting to just fan out the plate node voltages to another added preamp channel but, that addition is going to add a load to the power supply and node section. My shoot from the hip instinct tell me that since it would be a paralleling type of configuration, that the voltages would remain the same but, twice the capacitance would be needed in order to acclimate mathematically to the "new resistance".

Am I even in the right ballpark here ya think?

Thanks for your patience!
Phil donovan

[pjd3]

It also occurrs to me that the current consumption of a 12ax7 type tube is so low that it would be fine to do with no worries. But, not 100% sure about that.

Phil Donovan

[pjd3]

OK, I also just found a schematic with sorta 2 preamp sections that take the plate voltage from a single node but each gain stage maintains its capacitance value. That is sort of in line with the idea that preamp node filter capacitance would need to increase proportionatly for every 12ax7 plate that it was going to feed.

Seem reasonable even a little bit?

thank you

Phil Donovan

[TUBEDUDE]

I would parallel the supplies after the main filter, which i would double. Large filter caps can suck treble from the tube plates. Paralleling them will have the plates filtered as normal yet isolated from the larger capacitance by the resistor string. Better one stage with 22uF than two stages with 50uF as an example.