Monkeymatic Shasta Princeton Reverb pine combo

[xtian]

something else you can experiment with, if you're inclined: http://www.el34world.com/Forum/index.php?topic=15087.0

I saw that on Merlin's page, replacing the cathode K and R with a red LED. Very cool.


Next issue, getting some hum from the reverb circuit. I have experimented with moving the reverb tank around, and I'm happy it's not the tank picking up EM. Next, I moved the ground from the Reverb pot over to the grounds for all the other reverb stuff at the RCA jacks--no help. I'd be happy rolling off lower frequencies for the 'verb. Is there a way?

[xtian]

Hey, Milkman, how do you ground your reverb? My RCA jacks are isolated from the chassis, so I have the ability to move the ground anywhere. Right now, all four RCA jacks are grounded together, to the chassis right near the RCA jacks, along with the black wire from the reverb tranny, and the 220K ground ref from the reverb IN jack. And the reverb pot is grounded to the buss bar behind the pots. How do you do yours?

[Milkmansound]

I use isolated RCA jacks as well (sometimes 1/4" for my bigger amps that go to touring guys) the grounds go to a bus on the rear which starts at the isolated speaker outputs and lands near the reverb transformer

For a 20W or lower amp reverb return cathode mounts on the board and grounds to the front bus along with the pot and preamp grounds. With a larger amp and chassis I take that cathode and run it to the rear to keep noise down. You can dime my reverb with just a little tube hiss.

Getting reverb to be silent is very hard to do - even fender amps often had noisy reverb circuits. My method seems to work reliably every time

[xtian]

Thank you! What about your reverb pot? Where is it grounded?

[Milkmansound]

it gets its own ground along with the tremolo pots - to the chassis with a short piece of bus wire

you can run it down to the input jack area but 50% of the time it will be too far. I chalk that up to the grain of the galv. steel chassis - sometimes ok but to be safe it should have a nice short ground path.

[xtian]

Yeah, that's all the way I currently have it grounded, except for the output jack--mine is not isolated.

[Milkmansound]

that's fine. I only isolate mine so that it does not affect functionality when it loosens - I have never had a hum from grounding speaker outputs to the chassis. Since I sell the amps and never want to see them again, I go for reliability. I think my personal amp has grounds to the pots and to the speaker jacks

[xtian]

Finally got a chance to turn it up. And the distortion sounds good, but has a nasty component, blatty, like maybe cutoff or grid conduction. Plugged it into my 2x12 cab (4ohms) to rule out the speaker. Then I blew it up.

I see I have a burnt 1K 2W dropping string resistor. Hmm. Original schem shows only 1W there. Why did it burn?

I was plugged into a 4ohm cab at the time, making my primary 4K instead of 8k. Is that enough to run into trouble? I'm using a Hammond 1750E OT.

Bias tremolo was not on at the time, BTW.

Also, 2a fuse did not blow.

Guess the question is why was it drawing so much current?

[xtian]

I think I fried my reverb transformer. I heard an ugly noise from that area, is what drew my attention. After replacing the 1K dropping resistor, there was still an extreme current draw, even with ALL tubes removed. So I lifted the black wire of the reverb transformer from ground so I could measure the coils. The secondary (black-green) reads good, at less than one ohm. But the primary (red-blue) reads 1Kohm. Which sounds bad. Toast. WTF?

[xtian]

Well, crap. I discovered an unsoldered connection to the second filter cap. That was probably the ugly distortion I was hearing. Soldering didn't fix my immediate problem of current drain, though.

Removing the reverb transformer didn't help. Even with my lightbulb limiter in line, the 1K dropping resistor gets too hot to touch in under 10 seconds. With NO TUBES plugged in. Where is my short?


Feel free to chime in any time.

[JazzGuitarGimp]

Did you inspect the tube sockets for carbon tracks?

[xtian]

Did you inspect the tube sockets for carbon tracks?

Thanks Lou, but no tubes are plugged in. Just inspected all pins for debris/wires/shorts. All passes.

There is no path to ground from the power string. That is, I see infinite resistance between grounds and power string.

[JazzGuitarGimp]

Do you have a choke in the build? Could it possibly have developed a short to its frame?

Oh, I see you have infinite resistance from power string to GND. That's a strange one...

[xtian]

Theory question: How can there be a current drain if there is infinite resistance between + and -? That is, if there is no load?

There can't! There must be a short, right? But then why can't I see it with a multimeter?

[JazzGuitarGimp]

Theory question: How can there be a current drain if there is infinite resistance between + and -? That is, if there is no load?

There can't! There must be a short, right? But then why can't I see it with a multimeter?

Two possibilities come to mind, assuming you're using a digital meter:

- if there's a residual charge on one of the caps. Easy to determine, just set your meter to DCV and see if there is any voltage. DCV will throw the ohmmeter for a loop.

- if the build has a choke, and the choke is in the path you're measuring for resistance, that can throw a digital ohmmeter for a loop too.