Sub miniature Blues Express - blatty distortion

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flood
Posts: 212
Joined: Wed Jan 30, 2008 3:24 pm
Location: Bombay, India

Sub miniature Blues Express - blatty distortion

Post by flood »

So I built a Blues Express based amp (with a voxy tone stack though). Schematic is attached. I followed Merlin's instructions with respect to cathodyne PI design, using a 1M grid stopper and 100k grid leak with the diode... and it sounds nasty for some reason.

Initially, the cathodyne PI was AC coupled to gain stage 3. It sounded fantastic for a while, very wrecklike going from clean to full bore gain and bloom with the guitar volume, but the sound would cut off intermittently. Then, without warning, the sound cut off altogether. I swapped the preamp tubes, did away with the coupling cap from V3a and upped the grid stopper from 470k to 1M, and now, the breakup is just nasty, blatty, gated. any opinions? thanks!

EDIT: if it makes a difference, all tubes are powered with 7806-regulated 6V DC. The regulator is heatsinked to the chassis. I'm not sure this affects operation though.
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In the interest of full disclosure, I am Animal Factory Amplification.
flood
Posts: 212
Joined: Wed Jan 30, 2008 3:24 pm
Location: Bombay, India

Re: Sub miniature Blues Express - blatty distortion

Post by flood »

ok, swapping the tubes again didn't help. i'm wondering if i should go back to AC coupling. also thinking of increasing the grid leak resistor, R9, at V2A from 68k to 100k-220k, i have a feeling that 68k resistor is a bit too low. something has gone awry here, not sure why the sound just cut out like it did the first time around.

would a low filament voltage cause these issues? although 5.9V is not "low" per se...

UPDATE: made a couple of mods just a minute ago, switched the cap between V1B and V2A from 22n to 2n2 and the 100k grid-leak resistor is 168k now. put a 10k resistor in series from the "rise" pot to ground. i'm also large gridstoppers as used by El_Martin in his build - using 100k. let's see if this helps...
In the interest of full disclosure, I am Animal Factory Amplification.
Ian444
Posts: 210
Joined: Mon Nov 22, 2010 10:19 pm
Location: Australia

Re: Sub miniature Blues Express - blatty distortion

Post by Ian444 »

flood wrote:Initially, the cathodyne PI was AC coupled to gain stage 3. It sounded fantastic for a while, very wrecklike going from clean to full bore gain and bloom with the guitar volume, but the sound would cut off intermittently. Then, without warning, the sound cut off altogether. I swapped the preamp tubes, did away with the coupling cap from V3a and upped the grid stopper from 470k to 1M, and now, the breakup is just nasty, blatty, gated. any opinions? thanks!
It sounds like the design was working well and then you had something go wrong. It's not the design, it's a fault in the build or a faulty component I would think, otherwise it would not have worked well in the first place. Best to measure voltages at each tube pin in this case.
flood
Posts: 212
Joined: Wed Jan 30, 2008 3:24 pm
Location: Bombay, India

Re: Sub miniature Blues Express - blatty distortion

Post by flood »

thank you ian, think i found the problem - the 7806 regulator must have crapped out at some point, taking heat damage. filament voltage was down to 4V and not 6V as initially measured.

going for AC heaters with a 100R/100R virtual CT referenced to the junction of a 220k+100k bleeder series pair (try saying that really fast, it's a mouthful). that should take care of the noise.
In the interest of full disclosure, I am Animal Factory Amplification.
El_Martin
Posts: 183
Joined: Sun Jun 11, 2006 8:49 am

Re: Sub miniature Blues Express - blatty distortion

Post by El_Martin »

Hi flood! (No, its not High Flood) :shock:

Sorry for responding not so fast...haven't been here for a while.

Does your amp behave now? Is it sounding like a TW?

Ciao&good luck!
Martin
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