Gibson EH-150 with a loud hum -- Need some help

[Long Distance Call]

I have a 1942 Gibson EH-150, the last version with the EH-185 circuit. I haven't used it for a while, so a few days ago I decided to get it going again. I did a soft start using a variac and a light bulb limiter, and while the amp was working fine the last time I used it, it now has quite a loud 120 Hz hum.

It's quiet with just the 6N7 PI (input grid grounded) and output tubes, but there's a loud hum when V2, V3, or both V2 and V3 are installed (V2 & V3 are DC coupled). V1 being in or out of the amp makes no difference.

Both channels work, meaning both amplify and pass signal. By design, the Instrument channel is a bit polite, and the Mic channel has much higher gain. It sounds an open circuit somewhere being amplified -- or what an open guitar cable in the input jack sounds like.

The Hum is constant with both volumes off. The Instrument channel hum level stays constant as it's volume is turned up. When the Mic channel is turned up, the hum first increases, then nulls out around 4 or 5. Past 5, the hum increases with the Volume control (being amplified) and rapidly gets seriously loud. The tone of the hum is affected by the tone control.

So far I've subbed in new filter caps, tried different 6SQ7's, subbed in new coupling caps, and, just to rule it out, replaced the field coil speaker with an external speaker and a 750 ohm 20 watt resistor. None of these changes made any difference.

I'm running out of things to try, so I thought I'd post it here to see if someone here might have solved a similar problem or maybe some ideas / suggestions to help solve the mystery.

Thanks in advance,

LDC

[Phil_S]

How does the mic channel behave when you pull only V2? Then, with V1 and V2 pulled, how does the mic channel behave? I am not getting clarity on this from what you posted.

[Long Distance Call]

Hi Phil, thanks for the question. I pulled V2 and the mic channel still behaves as I described it: first the hum increases, then nulls out around volume 4 or 5. Past 5, then it increases with the volume control (being amplified) and gets louder. With V1 and V2 both pulled, the mic volume control still affects the hum. I grounded the input grid of V3 to see what happens and the hum is still there but the volume control doesn't change it.

Thanks again for the help,

LDC

[Phil_S]

You're not getting other bites on this so I'll opine on it. Please be aware of my amateur status. I am not nearly as knowledgeable as many of the guy you find here, meaning, take what I say with a grain of salt, maybe a few grains.

I'm guessing here. I would look between V3 and the PI. That the "100M" (what we label today as 100K), all the way to the grid of the 6N7, including the grid leak. Make sure all the parts are in-spec, connected to the right place, and properly grounded. Look for unintended connections where you were working.

I think I'd start by lifting the .1uF cap from the plate of V2. Then reconnect it and lift the other cap, looks like .00075uF. You might have a bad pot.

These are just some thoughts. I don't know if any of this will help.

Good luck.

[David Root]

Have you tried replacing the 5U4G rectifier with a known good one.

[Stevem]

By trying different coupling caps do you mean jumping them in , or doing it the right way by lifting one end and then jumping a cap in?

If you can measure anything over .200 Vdc on tube grid then you will have a hum issue !

[Stevem]

Also if you have more than 200 mv of ac power supply ripple on any of the first gain stage plate load resistors input side than you will have some level of hum taking place.

[Long Distance Call]

Thank you all for the suggestions. I've tried them all and so far, nothing has changed. In answer to Stevem's questions, all of the original caps were completely lifted out of the circuit and new caps were jumpered in. Also, there is no measurable ripple on any of the first gain stage plate load resistors input side. Any other suggestions will definitely be appreciated. V2 is still removed so the Instrument channel is out of it. The problem seems to be somewhere in that Mic channel...

Thanks again for all the help,

LDC

[Stevem]

With V2 removed there is nothing coming in from V1 or V3 do there out of the picture and not part of the issue!


Please pull the output tubes out and measure the resistance from the OT center tap to each plate and report back.

[Phil_S]

Steve: I'm not sure I understand what you said about pulling V2. From what I see, that only disconnects the instrument channel. The mic channel still has a clear path to the pot that's in front of the PI from the plate of V3 because the plates of V2 and V3 are a parallel connection.

Long Distance Call: I was asking if you would disconnect the two caps (.1uF and .00075uF) right before the 6N7. Specifically, if you lift one end (the V2 side) of each cap, one at a time, not at the same time (so one is still in-circuit), what happens to the hum?

I'm looking at this part of the schematic and I am thinking the problem lies in this area of the amp. This includes the 2 caps, the pot, and the 100K grid leak (100M is old-style designation). I'm doubtful the 1K cathode resistor is the problem, but it's in the picture ;-}

[Long Distance Call]

As Phil said, with V2 out of the circuit, only the instrument channel is affected and the mic channel is still intact. Also, following your suggestion Phil, I went back and disconnected the .1uF and the .00075uF, first one and then the other (on the V2 side) and the hum did not change. I'm going to try swapping out the 100K grid leak resistor and see what happens.

Thanks again for all the suggestions,

LDC

[Theashe]

Have you check that all your grounds are solid? I once left the speaker output jack ungrounded on my amp and it still worked, but had really weird hum behaviour when you played with the amp volume, especially the master volume.

[JazzGuitarGimp]

Is your heater winding properly centered and grounded?

[Stevem]

Sorry for the mistake, too early !
You need to resistance check all of the terminal strip grounds as even if they are riveted down with age they can read 20 ohms or more and be the sourse of the hum!