20w pp build big problem(s)

[joshdfrazier]

finished this a few hours ago: http://www.ax84.com/static/corepoweramp ... ematic.pdf

used 6bq5/el84 in place of 6v6, and the PT and OT (and same power tubes, actually) from a hammond organ. most of the component values are identical to those in the schematic. i used .02 caps where .01 were called, as thats what i had on hand.

did the smoke test, and got no sound. some pops when i probe parts inside the amp, but nothing besides that.

getting some pretty insane voltages, too. doesn't make sense considering the size of the PT (kind of small) and that it was designed to power an el84 amp.

my b+ voltages are:

1.429
2.428
3.421
4.417

el84: (2)

pin2.-50vdc~
pin7.433 (plate)
pin9.432

PI 12ax7:

pin1.416
pin2.346
pin3.395
pin6.416
pin7.345
pin8.395


so these voltages are way too high, yeah. what i really don't understand is the nearly 400v on the cathodes of the 12ax7. i'm sure i've missed something really obvious here.

[rock_mumbles]

Where are the grounds for the cathodes and power supply?

[joshdfrazier]

power supply is on top of the chassis, everything is grounded at the input jack via star grounding. that little red cap above the two bias capacitors takes the place of the presence knob, and is grounded.

[martin manning]

power supply is on top of the chassis, everything is grounded at the input jack via star grounding. that little red cap above the two bias capacitors takes the place of the presence knob, and is grounded.

When you say "takes the place of the presence knob" do you mean that it's as if the presence is all the way up, except there is no presence pot there? If so then you need a 5k resistor across the cap for a DC ground. That seems to fit with your high PI voltages.

[Tillydog]

Fix what Martin said first, then:

You're biased heavily into cut-off for the EL84s - likely you will want to get down to about -10 to -12 V, but go gently, that's a lot of volts!

I know that you said the PT is the same one as used in the organ - did the organ have tube or SS rectifier? Fixed or cathode biased EL84s? (Tube rectifier and cathode bias could make a difference of 30V or more on the voltage those EL84s see.)

[joshdfrazier]

Thanks Martin, obviously a serious "duh" moment for me. I am getting amplification, and PI voltages are good! Tillydog, thanks for the bias suggestion. increased v- to about -18. something else overlooked.
However, all that's getting amplified is a very loud sine wave which I can adjust the frequency of via the tone (treble) pot. To my ears, it's adjustable between 800hz-2khz+.

I used 10uf caps instead of 25 on the preamp grounds, not sure if that's worth mentioning.

[joshdfrazier]

Tilly, forgot to reply to your whole message. The organ was tube rectified. 6ca4 if I remember correctly. Can't remember the details of the amp, as it was dismantled almost immediately. Knew the voltages would be a bit higher, but I do love punishing my tubes.. Lol

[martin manning]

...all that's getting amplified is a very loud sine wave which I can adjust the frequency of via the tone (treble) pot.

I'm tempted to say swap the OT primary leads, but what treble pot? There is none on the schematic you linked... its only a power amp.

[joshdfrazier]

Sorry, Martin. I've built a modified ax84 "simple preamp" from the building blocks section of the site.

[Tillydog]

...all that's getting amplified is a very loud sine wave which I can adjust the frequency of via the tone (treble) pot. To my ears, it's adjustable between 800hz-2khz+

I'm guessing you've built in a couple of gain stages around an additional 12AX7 to the right of the PI (wiring looks like that, anyway).

Further, I guess that your treble pot is the one on the left in the photos: In which case, try moving all the connections to it well away from the PI (it looks like the coupling cap to the treble pot is almost touching one of the plate resistors of the PI).

Might help to twist the OT primaries together, too to reduce coupling back to the rest of the circuit - it looks a little snug in there!

[rock_mumbles]

Are the 12AX7 cathodes still at almost the same voltage as the plates?

If so measure the resistance from pin 3 to ground NOT to the other side of the resistor ... etc

[joshdfrazier]

PI cathodes are now at 73vdc. resistance between pin 3/8 and ground is 27k ohms.

it is inevitable that lead dress and interference will be an issue with a tiny chassis like this. wouldn't haven chosen one this small unless i already had it laying around!

however, absolutely no sound from the preamp? i suppose this will just take a bit more troubleshooting.

[ampgeek]

Uber high cathode voltages on the pre-amp tube(s) imply no tube conduction due to no/poor connection to ground. 27K ohms to ground from pin(s) 3&8 pretty much confirm that.

Triple check grounding path from downstream of the preamp tube cathode resistors. Make up some alligator clips and jumper from downstream of the cathode resistors to a known good grounding point if needed to temporarily achieve preamp tube conduction if need be.

Good luck!
Dave O.

[joshdfrazier]

r7 was changed to 2k, allowing me to get anywhere between -1 and -40vdc.

i'm not even ashamed to admit that my real problem was that i was using a broken guitar cable.... alright. maybe a little.

some modification need to be made to the tone stack, but it sounds great.

[Structo]

Been there, done that.

Sometimes we are too smart to check the simple things first.