Elk Amp reverb issues

[5thumbs]

Hey Gang,
I'm trying to restore a friend's Elk Amp from the 70's (?). It is an EB-101 and the closest schematic I can find is for an ES-30. It had a master volume mod and other "mods" that I have removed. I was getting no reverb and found the coupling cap was leaking causing the tube to over bias and shut off. Now when the reverb is turned all the way down, the amp works fine with no reverb, as I turn up the reverb knob, the amp shuts down to noise and the power tubes start to red plate. It seems maybe the reverb signal and amp signal are out of phase and canceling each other out? Anybody had this before? Did previous mod-er change the phase somewhere when he added the master volume?

Thanks for any help!

Also, if anyone has the correct schematic it would really help!

[52 Bill]

If the amp is similar in design to the posted schematic, then I'd guess that the reverb return circuit is going into oscillation as you turn it up.

[Chuck H]

^^^^^^^^^^^^^^^^What he said^^^^^^^^^^^^^^^^

[5thumbs]

Thanks 52 Bill,
What components of the reverb return circuit should I focus on? This thing is laid out on a dual eyelet board and the signal jumps back and forth over itself 2-3 times ;-)

Tim

If the amp is similar in design to the posted schematic, then I'd guess that the reverb return circuit is going into oscillation as you turn it up.

[Enzo]

I agree with them. Not only is your reverb not out of phase and canceling something, it is IN phase and causing oscillation at RF. My working impression is that you have a DEcoupling cap not working, maybe one of your filter caps is not grounded to the right place.

Got a scope?

[5thumbs]

Got a scope?

If your talking about mouthwash, then yes. One for a rifle? no.

I have a multimeter, will that help?

So a Decoupling capacitor is another name for a filter cap? When I got the amp working there was no reverb. I found the .02 coupling cap at the point where the reverb is pulled off the signal to be leaking DC voltage. When I replaced this component, I started getting the issue of the amp going quiet with static when I increased the reverb knob. Then I noticed the el84's getting bright and shut it down until I knew what was causing it. The voltages at the + side of the filter caps seem appropriate for this type amp I am guessing, so how would I test a filter cap?

[Enzo]

By its function. We need to find out if any sort of signal is getting onto the B+ rails. You might monitor B+ and see if it does anything as it slips into this. Or even just tack a parallel filter cap at each node going down the row, to see if it changes things.


Other than the stage oscillating, and sending large RF signals through the amp, I don;t see any way for the reverb circuit to affect the power amp.

[5thumbs]

Thanks Enzo for your help. I tacked another filter cap to each cap can section on the B+ rails and nothing. I moved the ground of the single 22uf final (preamp) filter cap on the eyelet board to chassis ground after I tried tacking on a parallel cap and still no help. No DC voltage on the output side of coupling caps or next stage preamp screens anywhere. I was getting a very small DC voltage (~.05VDC) on the secondary of the reverb transformer (should there be?) so I disconnected it and tacked in a Fender Reverb Transformer and still have the amp going quiet when the reverb pot is turned up. I don't know what else to try...

Tim

[nickb]

Thanks Enzo for your help. I tacked another filter cap to each cap can section on the B+ rails and nothing. I moved the ground of the single 22uf final (preamp) filter cap on the eyelet board to chassis ground after I tried tacking on a parallel cap and still no help. No DC voltage on the output side of coupling caps or next stage preamp screens anywhere. I was getting a very small DC voltage (~.05VDC) on the secondary of the reverb transformer (should there be?) so I disconnected it and tacked in a Fender Reverb Transformer and still have the amp going quiet when the reverb pot is turned up. I don't know what else to try...

Tim

Does the reverb work at all? I'm wondering if the output coil of the tank is open. You can check the DC resistance with your meter.

EDIT 1: There is some Elk info here about the EB102 if that is any help Echo Elk Musical Instruments: Echo Sonic (EB-102)
Edit II:might not be reliable - see later.

Also, I wonder if the foot switch cable is picking up any signal - does it work? What happens if you remove it?

[5thumbs]

Hi nickb,
I checked the reverb tank and it seems fine. I also substituted a known good accutronics tank and still got the same result. The amp is missing the footswitch, but I moved the ground on the RCA jack (ground loop?) and tried shorting the jack to stop the feedback to no luck. All the Elk schematics I have found seem very similar in design, unfortunately they don't show proper voltages to compare with what I have in front of me.

BTW,
Why would they design a phase inverter with one side coming from a 12ax7 and the other being fed by a 6aq8? Those tubes seem very dissimilar in their specs. Are they trying to make the long tailed pair act more like a cathodyne PI?

[52 Bill]

Nick asks some good questions. Was everything working before you removed the earlier mods?

EDIT: There is some Elk info here about the EB102 if that is nay help Echo Elk Musical Instruments: Echo Sonic (EB-102)

I wouldn't trust the schematics drawn at this site, the one I looked at has errors.

Without having the actual schematic for your specific amp, it will be difficult to directly guide you to an solution. Plus the fact that the amp has been modified and un-modified, I'm sure doesn't help matters.

Lead dress is always something that you need to look at when you are chasing a problem like this. Do you happen to have an old AM radio? Sometimes they can "hear" the oscillations that our ears can't. If a radio picks up the noise as you turn up the reverb control, then we'll know that in fact the amp is oscillating.

Maybe if you post a few photos of the chassis someone will see something that is amiss.

[nickb]

Hi nickb,
<snip> I checked the reverb tank and it seems fine. I also substituted a known good accutronics tank and still got the same result. The amp is missing the footswitch, but I moved the ground on the RCA jack (ground loop?) and tried shorting the jack to stop the feedback to no luck. <snip>

I smell a rat. Turning the reverb pot to zero shorts the grid to ground. Having the reverb up and shorting the reverb jack should be pretty much the same. Is the jack properly grounded? Are there long, poorly dressed wires between the reverb pot and the grid and/or jack?

[5thumbs]

I mis-spoke to nickb, it would stop the problem when I shorted the RCA jack. I ultimately gave the amp back with the reverb problem. I just ran out of free time and the person wanted to have it fixed asap, so I sent him on his way with the amp. The Elk was a very odd amp to say the least. Thanks for everyone's help!

Tim