Quadcaster build report

[runmikeyrun]

hey guys,

Sorry about the schem... I made it in paint and i'm not sure what's up with it.  If you PM me your email i'll email you one. 

The stripboard layout I used is the same one for the Twincaster, I just doubled it up and changed the values where specified. 

As for the last cap, yes, 1uF.  I'm not the best with schems!  lol

Mrmoo, I'm not sure what you're getting at... there's no switch in mine between stages...?  Not too sure, I didn't read all 137 pages either!!   

[mrmoo1337]

The schemo shows stage two's anode (pin 6) connected to stage three's anode (pin1) when it should go to the grid of stage three (pin 2).  You'll also need a grid leak resistor from pin 2 to ground.

[rutabaga bob]

it's also that way between stage 6 to stage 7...

[runmikeyrun]

Gah, I suck at this!!  Jesus.  I'll fix and modify the original post.  Sorry guys.

here's the fixed schematic... sorry it's still small, photobucket is resizing it. 

Gallery image (larger!)  http://www.aronnelson.com/gallery/main.php/v/diyuser/quadcaster+schematic.jpg.html?g2_imageViewsIndex=1

Smaller original schem:

[asatbluesboy]

F'in awesome.

[jerrymcdougal]

Sounds Ef'n great! Ever try more than one 12AT7? Ive got two AU7's and two AT7's that need a doomy home. 

[Paul Marossy]

Ha ha, that RAWKS! 

[jerrymcdougal]

Well I just built this beast and its sounds freaking great, but I think I have some issues to iron out. If anyone can offer some advice to a newb, it would be greatly appreciated!

1.) First and foremost is noise. It just screams high pitched noise when not playing. Even with the trimpots at like 40% or less. It's even very noisy with the trimpots all the way down.

2.) If the trimpots are over 50% it squeals and shrieks like crazy. Something has to be screwy.

3.) When the pedal is off, the volume knob produces a low amplitude high pitched squeal when turned nearly all the way up.

4.) If the gain knob is past 85-90% the pedal goes quiet.

I used a 1.8A laptop power supply that puts out 12.6v. I thought it might be the issue, so I tried a known good 9v supply with the same outcome.

I swapped tubes around a little with no difference. I'm using a Tung Sol 12at7, two Tung Sol 12au7's, and one EH 12au7.

As far as I can tell, everything is grounded well. Poking around produces no changes in sound which would suggest bad connections.

Any idea where I should look first? I will upload any pics or sound clips if requested. THANKS!

[amptramp]

You have a very large amount of gain in this circuit.  If there is any feedback from the plate of one stage to the grid of the previous stage, you have an astable multivibrator and the relatively small amount of capacitance required would make it scream at a high (but audible) frequency.  Shielding between all stages may be necessary.

The electrolytic across the B+ to ground may need additional help in the form of a film capacitor across it.  At high frequencies, it may act inductive and make matters worse.

[jerrymcdougal]

You have a very large amount of gain in this circuit.  If there is any feedback from the plate of one stage to the grid of the previous stage, you have an astable multivibrator and the relatively small amount of capacitance required would make it scream at a high (but audible) frequency.  Shielding between all stages may be necessary.

The electrolytic across the B+ to ground may need additional help in the form of a film capacitor across it.  At high frequencies, it may act inductive and make matters worse.

Oi. That sounds unpleasant. I suppose it wouldn't help that I built this all on the backs of the sockets rather than on a pcb. Of course, maybe not, since all the leads are technically shorter than running wires to a pcb. Would pics of the guts be of any use? Would sound clips help diagnosis at all?

I'll try a film cap across the 'lytic as you mentioned. Thanks a lot for helping!

[jerrymcdougal]

Alright, I went through and reflowed each solder joint and popped in a .047uF film cap across the main lytic. It may have helped a bit, but it's still pretty noisy.
Here's some gut shots. Let me know if anything seems amiss or if you have any ideas on how I could improve the high noise issues. I'd appreciate it.

The orange drops are 600v pieces, hence the huge size. Just what was available. I assume the high voltage rating isn't causing any issues right?




Here's a sound clip I recorded showing the noise floor. It's just a single string plucked and then muted to show how the noise floor fades in after muting. Thought it may be a clue. You can see the fade in on the waveform.

http://soundcloud.com/jerry-mcdougal/quadcaster-noise

[jerrymcdougal]

Just audio probed the gain stages. No single stage seems to be the issue. The idle noise starts right after the first stage and gets progressively louder through the stages. I'm assuming this means my issue is most likely that I need to use shielded wire between stages/tubes?