Stupid preamp question

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va3dxv
Posts: 26
Joined: Sat Jan 06, 2018 6:37 am
Location: Ottawa, Canada

Stupid preamp question

Post by va3dxv »

I'm building an amp from scratch. I've based it on the 5E3, except instead of a 12AX7 in the PI I'm using a 6SN7/6SL7.

Anyways, all that works fine, I can feed a signal into the 6SN7 grid and get sound output. Low volume but it's there. Then I add in a pre-amp tube. I can feed the first grid of a 12AX7, couple the plate to the grid of the 6SN7 via a pot and I get good clean high volume sound and adjustable volume. No problem.

The problem is when I try to expand the pre-amp a bit.. I feed the first plate of the 12AX7 into it's 2nd grid (with a coupling cap and volume pot in between), I get nothing. Total dead air. It's like the 2nd half isn't turning on. I've tried several tubes... Both plates on the 12AX7 are at about 100V. I'm sure I'm missing something obvious. Any ideas?
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didit
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Joined: Thu Sep 25, 2008 3:37 pm
Location: Canada

Re: Stupid preamp question

Post by didit »

Schematic & some photos?

Best .. Ian
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martin manning
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Re: Stupid preamp question

Post by martin manning »

If your PI has component values similar to the original 12AX7, a 6SL7 will function much better than a 6SN7. Re your other issue, check cathode voltage on both 12AX7 triodes to be sure they are biased correctly, but it sounds like the signal is getting lost or grounded out between the first and second triodes.
va3dxv
Posts: 26
Joined: Sat Jan 06, 2018 6:37 am
Location: Ottawa, Canada

Re: Stupid preamp question

Post by va3dxv »

The problem was I forgot to give the first grid of the 6SN7 a DC path to ground. Thanks to all who replied. I guess I'll add myself to the random acts of stupidty thread :mrgreen:
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