Anyone want to help me with my schematic?

[YetzerHarah]

In response to Bruce

At least I seem to have gotten the most basic of the design bugs out (IE: it shouldn't light up the night sky like a 1972 Ford Pinto) As far as the sounding like a stompbox, that is more or less the idea of the build, I want something that sounds very dirty (hum factor omitted). I was thinking of leaching off some of the gain with negative feedback (any feedback on my the idea of feedback?)

Tell me if I'm missing any of your suggestions: I see the 20k resistor in place of the 68k on the input, the added 6.8k resistor in the bias circuit (I assume the best way to deal with the bias issue you were talking about is to build it & calculated the needed resistor based on the actual running specs w/ & w/o signal), & the .1mfd resistor before Vol2 & the .022mfd moved to after the pot.

[Bruce / Mission Amps]

Actually I don't think you'll get any negative bias voltage at all, since with a bridge rectifier, neither end of the transformer will ever make it more than 0.7V below ground.

Those little 6DX8s (what I know as ECL84s) would be suited to cathode bias just fine anyway. The pentode part is a pretty high-gain, sharp-cutoff tube that doesn't need much bias voltage, just like an EL84.

DOH!! I forgot all about the FWB, although you even mentioned it before!!

I've never used a 6DX8.... so how much bias is needed in two of these in PP @ about 250vdc, -5v to -10v?
I think you are right though, a cathode biasing resistor of 150 to maybe as low as 47 ohms is where I'd start and forego the fixed bias schem... especially since he is considering or looking for preamp buzz and massive power tube overdrive.
I wonder if he could hold the FWB up with a 5 to 10 v 5 watt reverse biased zener diode, (bypassed with a positive grounded 22uF50v filter cap).. you know, under the anodes of the FWB and steal some negative voltage for a bias supply? Again never done it but I have done the reverse biased zener to get a few negative volts of DC to run an LED for a switch.

[Enzo]

Bruce, do you want the PI loads that imbalanced? 100k vs 56k? As opposed to 56k and 56k or something?

[Macguyver]

On top of all the major mistakes, I think even after you get this thing fired up, the problem will be tone. Using the corrected schematic Bruce drew out, it looks as if the amp will be very dark and undynamic to say the least, it will probably suffer from blocking distortion due to the size of your output caps, cathode resistor/cap values and lack of attenuation to compensate for the bass heavy response you set your gainstages at. You should shave off alot of that low end especially right after the first gainstage, or maybe across the entire preamp? Its really up to you how to do it, but my advice would be to do some research, check out how other amps generate distortion through out 3-4 gainstages, theres typically alot of low end roll off, especially earlier in the signal path, the low end is added in the powerstage. I wish you luck in your persuit, and I'm sure your amp will eventually work and along the way, you'll hopefully learn from your mistakes.

[Bruce / Mission Amps]

Bruce, do you want the PI loads that imbalanced? 100k vs 56k? As opposed to 56k and 56k or something?

Well, I'm not doing to well with my quick answers am I?
I didn't draw any of the schematic and I only inserted items in his artwork that, with a couple casual glances, were obviously problematic to me.
But yes, 56K for plate and cathode would be better in the cathodyne PP driver.
I think "as is" the amp will be a bit tone challenged and the preamp gain structure is going turn the power pentodes inside out anyhow but we've been warning him so I think he knows that.

[Steve Conner]

I agree that high gain tends to need low-cut. It may well sound like the proverbial "Foghorn with your head wrapped in wax paper" without it. But once the dude gets it working, he can always mess with the tone shaping components.

Putting a zener in the power supply like Bruce suggested is not really any different to putting the same zener between the tube cathodes and ground. You lose the same amount of B+ and dissipate the same heat.

[YetzerHarah]

I'm supprised that no ones noticed...

...the huge error that I made while redrawing the schematic. Ignore the fact that the hot is now connected to the hot instead of the ground out of the bridge.

[Bruce / Mission Amps]

...the huge error that I made while redrawing the schematic. Ignore the fact that the hot is now connected to the hot instead of the ground out of the bridge.

Where was it drawn wrong? I didn't see anything wrong.
The cathodes of the FWB go to your B+ rail and the anodes were at the ground buss line.


oops I see it now... glad you caught that.

[log1982]

Bruce, It looks to me like your phase splitter is unbalanced, with the gain of the inverting side being almost twice the gain of the non-inveritng side. Did you you do this on purpose? (Doh! I see someone already covered this. Sorry for piling on.)

I think Macguyver is right about the tone, but it's still a good first project, and can be modified later...

If you want a negative bias for the output grids, just rectify one of the legs of the heater secondary (CT grounded), slap a pot, or better yet, a zener/pot combination. Viola, negative bias, easily cabable of several volts. According to the data sheet, bias of about - 2V to -4V oughta do the trick.

[YetzerHarah]

Bias off the heater supply?

Log, what's the effect on the heater supply voltage/amperage if I take the bias off one side of the heaters?

[YetzerHarah]

Chassis pic

Here's another pic of the chassis now that it's comming along, it's going to be a real pain stuffing the power trans in there.
http://img508.imageshack.us/img508/6651/pic00003up7.jpg

[log1982]

Log, what's the effect on the heater supply voltage/amperage if I take the bias off one side of the heaters?

Not enough for anyone to care.