First schem, feedback requested.

[mumford]

I've just finished my first schematic and was hoping some folks would be willing to take a look at let me know if they see potential problems with it--either mistakes or even just "that's a bad idea" type things. I appreciate any feedback anyone can offer.

[selloutrr]

ok I'm looking at the power section.
what is the PT voltage?
any reason the pilot light is not powered by the heater filaments? The legs you are using for the pilot do not have a center tap, typo?

if the PT has a center tap you don't need 200ohm resistors to ground


retifier section. usually when adding caps over the Diodes you also add a resistor.. frankly both are over kill use the larger 1000v diodes and you are fine.

voltages on your filter caps? 450+Volts

your voltage drops between the first filter cap, second filter cap, and third are not consistent though the filter values are the same. I would double check the math.

Bias are you sure you want fixed and not fixed/adjustable.

tone stack / volume are typical to 12ax7 preamp tubes you may want to experiment with cap values to voice the amps controls.

you may also want to experiment with the 33K input resistor.

[David Root]

I would put 470R/2W screen resistors on the 6V6s, especially if they are expensive NOS.

[mumford]

I really appreciate you guys taking a look, thank you.

The PT is 355-0-355, 100mA.

Pilot Light. I was using the 5v tap from the PT for the light out of simplicity. I wasn’t using the 5v tap, so I figured it was easier. I wouldn’t think I’d need an artificial CT just to run the light, am I misunderstanding what you’re saying?

200 ohm resistor. I was using the 200 ohm resistor on the CT to induce sag with the SS rectifier. My understanding is that it will also drop the secondaries by about 20v. It was just one of the ways listed in the merlin power book to get sag.

Voltage Drops. The drops listed on the schem are odd because I listed the quiescent bias voltage by the plate resistors. I’ll have to plead ignorance if that’s a weird thing to do–it’s my first schematic.
Basically, my intent is to have HT after the 2nd dropping resistor to be 200v, although that was calculated based on using a tube rectifier, which I have decided not to do (I know I’ll have to either bump everything up or pull down the power somewhat). I would not be shocked if my math was wrong, however–I’ve been developing an excel file to run my calculations because I took my last math class in about 1988. (By the way, if anyone wants a copy, PM me.)

Tone Stack/33k input resistor. I’ll definitely consider both. I played around with the duncan tone stack calculator, but came to the conclusion that I don’t know enough about what I want to hear to be able to make anything more than semi-arbitrary changes. I figure if I can get the rest working, then I can tweak that stuff.

Fixed Bias. For now, I’m going with a stock 5E3 power amp section. I’m new to all of this stuff, so I figured I would be satisfied with designing the preamp and power supply this time, and leave the power amp section for later. I know my OT and PT will work with 2 6V6's, so the deluxe seemed an easy choice.

[selloutrr]

for the ease of bias consider adapting the fixed bias to a variable fixed bias. this will save you from having to solder in resistors everytime you bias. or at least make the resistor easy to access and change.

mock it up and see what happens.

[mumford]

Thanks!

[Alexo]

What's withthe 6FS5's?

The DC-coupled cathode follower is cool, but with the volume control placed where it is, you very likely might have a hard time getting a clean sound out of the amp, especially with humbuckers, you might just overdrive the CF stage.

I've messed around with this kind of thing before and wound up DC-biasing the cathode follower and putting the volume pot in front of it.

...I'd also use a 50 volt rated cap on the 6V6 cathodes.

[tubeswell]

If you find the level-shifted CF stage doesn't work properly, you can always adjust the voltage divider once its up and running, You can make the bottom leg of the voltage divider adjustable with a pot wired as a variable resistor - but you may want to put a cap between the pot's input and the knee of the divider to stop DC crackle getting into the signal path,

- or go to an AC-coupled CF stage (with either cathode-bias or fixed-bias - but you probably want a bootstrapped grid load resistor for really high input impedance).

Not having looked as a datasheet for a 6SF5, I don't know what the h-k insulation breakdown voltage rating is, so I don't know whether it would handle re-working into a direct-coupled DC CF stage. Is that the reason you went to a level-shifted stage in the 1st place?

[mumford]

Max h-k for a 6sf5 is 100v.

I'm using 6SF5s because this started as a conversion, and i didn't see any reason not to use them after the concept changed. They seem to be basically half of a 12ax7.

[tubeswell]

Well then I'd build it like you have it on the schematic, and then make any adjustments to the cathode resistor or inter-stage voltage divider that you need in order to get the stage biased properly once its up and running

[tictac]

Is there any reason you're not using a 1MA pot as a volume/gain control between the 6SF5 stages?

You'd be able to dial in the amount of grit or clean you want and you could get rid of the 560k/1M network.

The other volume control would become your master volume.


TT

[mumford]

Is there any reason you're not using a 1MA pot as a volume/gain control between the 6SF5 stages?

It's DC coupled, wouldn't I get crackle?

[Structo]

You shouldn't get much if any crackle because of the coupler earlier on.

[tubeswell]

That's not a coupling cap earlier on, take another look - its a bypass cap bypassing the upper leg of the interstage voltage divider. So there will still be DC present, which shouldn't be a problem if the grid in the next stage is at the right voltage, but it requires biasing that stage accordingly. AC coupling of the CF stage would probably be simpler

[jjman]

Nothing wrong with cathode biasing the output. Only thing a little different for me is the biasing method for the PI. I'm used to seeing it's grid biased via a tap to a string of 2 cathode resistors. Like on a 5E3 Fender Deluxe for example. Using the power supply and a 2 meg resistor seems more risky to me.