Anyone want to help me with my schematic?

[YetzerHarah]

This is a project I've been working on for a while I just want to get it finished, I was wondering if anyone would be willing to check it over & see if they can find any mistakes or suggest any improvements (for example a variable bias circiut, I haven't found one that's fitting) The circuit is located at:

http://img412.imageshack.us/img412/586/schem2so5.gif

[tboy]

Well, for starters I don't see any grid resistors on the first preamp stage and the phase inverter input.

[YetzerHarah]

updated

http://img19.imageshack.us/img19/7720/schem3vz6.gif

& some pics of the chassis:
http://img527.imageshack.us/img527/9312/redcompoj9.jpg

[Enzo]

6DX8-1 triode has no grid resistor. That grid will float up to some voltage you won't like.

6DX8-2 triode will have a cathode voltage of like 50-90 volts or something, but the grid will be sitting at ground potentail. That represents 50-90 volts of bias to overcome with a signal. Look at other phase splitter circuits for examples of how to get the grid up to something near the cathode.

Start there.

[log1982]

The other gentlemen are correct. Let me add my .02.

You still don't have the grid of 6DX8-1 at ground potential in your updated schematic.

With no input connected the grid of 12AX7-A is not ground referenced. Not advisable.

Your tone control (treble roll-off) as wired will be interactive with your volume. You may prefer to wire it to be more independent. (Connect wiper to one side of the tone pot). No biggie, see which one you like better.

Bias the grid of the phase splitter at about 1/4 V+.

Rather than self bias the output stage, consider adding negative bias to Vg1, and short the Cathodes to ground. (more gain, less wasted power, and more power delivered to the load.) Personally, I wouldn't want to tie g1 to a floating cathode, anyway.

How 'bout a HT fuse?

You don't really need 320 uF on the cathodes of the triodes, 25 or 50uF is good enough, unless this is a bass amp.

I would probably connect the CT of the OT primary to the other side of the 3K resistor.

Make sure you wire the 12AX7 heaters in parallel.

Do you really need an AC Volt Select?

Lastly, and most importantly, DO NOT connect the CT of the PT 190 Volt secondary to ground when used with a bridge rect!

[YetzerHarah]

Another Update

http://img371.imageshack.us/img371/9225/schem4cf6.gif

[Enzo]

You still have not added grid resistors in the preamp stages and phase splitter.

ANd you will blow fuses the instant you turn it on if you don't correct the high voltage rectifier circuit as log stated.

EITHER you ground the high voltage center tap and eliminate the two diodes on the left side of your bridge, OR you do not connect the center tap, and then you ground the left point of the bridge. The first way will produce half the voltage as the second way. Your choice, about 270V or 540v.

[YetzerHarah]

Yet another attempt!

The "improved," (slightly, at least I hope) schematic:
http://img526.imageshack.us/img526/5775/schem4er5.gif

& the silvertone schematic that I took the driver that everyone seems to tell me won't work from:
http://img519.imageshack.us/img519/682/silv1263dy5.jpg

[Bruce / Mission Amps]

I guess you aren't paying enough attention to what everyone is telling you.
You stillhave NO grid load on the first triode... only a 68K grid stopper which will do nothing without the grounded grid load.
And, your cathodyne driver is not drawn correctly... but it is drawn right in the Silvertone schematic.

You have both of your grids tied together... wrong.
If you are trying to DC couple the driver triode to the PI, you would connect it's plate to the grid of the next triode, which then would be the PI.
Your 470K/1M level control would be connected to the input side (side that is not the cathodyne driver)... you sort of have that voltage divider right but it needs to be the grid load for that triode and you have to undo your two grids.
Your .022uF coupling cap could be the AC connection to the next triode grid if that grid also has a grid load resistor.... oh, this is too hard....I can tell this is too hard for you to get in text too...
I think you just need someone to re-draw it for you.

[Enzo]

I am guessing you don't intend the two grids of the 6DX8 triodes to be connected in the center of the two on the drawing, thought that is what it looks like.

Look carefully at the Silvertone, and then at yours to see what Bruce points out. You took a signal from the left triode at its plate, then ran it out through a cap to the volume control and THEN back to the grid of the phase splitter. The SIlvertone took the plate of the first triode DIRECTLY to the grid of the second. That way that second grid will be at the elevated vvoltage it needs to be.

If you want to use the layout as drawn, you need to bias the grid of the phase splitter triode. Look at Fender drawings for Bandmaster 5E7 or Harvard 5F10 as examples of how to do it that way. Note the extra couple resistors to set the bias between grid and cathode.

[YetzerHarah]

Yet another attempt!

Let's see if I managed to understand you guys this time!
http://img524.imageshack.us/img524/3420/schem5bx1.gif

[Enzo]

No.
Furst, if you want to add bias supply for fixed bias operation, then the 250 ohm cathode resistor for the outputs has to go. Ground the cathodes instead.

That bias tap off the transformer only works if the center tap is grounded, and if that is the case, then the two diodes on the left side of the bridge MUST go. You MUST decide whuch type of rectification you are going to use. You have wired it for both ways at once, and that will blow fuses.

6DX8-1 pin 1. There is still no resistor to ground. There MUST be a resistor to ground for DC to gave a path to drain away.

6DX8-2 pin 1. You still need to address this. Either change to a direct couple from the other 6DX8 plate (and clear up a couple details) OR add a couple resistor to bring the grid up to the same sort fo voltage the cathode has. See those Fender examples I mentioned.

[YetzerHarah]

Hmmm...

http://img515.imageshack.us/img515/6241/schem6vx4.gif

So I redrew it a little to try & make it the same layout as the Fender schematics to try & make it slightly easier to reference. Hopefully that helped. If the cathodyne or the bridge rectifier circuit still isn't correct, then I hope that someone's willing to draw it for me to finally clear up all this confusion. As for the resistor the ground on 6DX81a I wasn't exactly sure of the nessisary value, so tell me if 1M is ok, or if I need something more like 470k.

That's all for now,
-Dan

[Bruce / Mission Amps]

I think you'll need another B+ node with a filter cap to separate and keep those triodes nodes off the OT center tap lead.
Also, I bet you have way too much gain with this and will need to reduce it quite a bit or it will sound like a stomp box, on full gain-buzz most of the time.
And you'll end up with too much negative bias voltage with the way it is drawn so you'll be messing with the resistors in the bias supply.
Look over what I re-drew with your art work....

[Steve Conner]

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.