Microphonic amp

[Bob-I]

I thought I was doing so good. I have the power amp and channel 1, Fender type clean, and the reverb completed. While testing I started getting a microphonic feedback. Tapping the chassis makes it worse until eventually it won't stop. I eliminated the tubes by swapping all of them with known good tubes. I moved all of the speaker ouput wires including the impedance switch and jacks. Since the preamp controls make no difference I'm thinking reverb or later.

I just can't get this to stop. Any suggestions?

Here's what it sounds like, be patient it takes awhile to start.

http://www.patmedia.net/bob-ingram/Dumb ... phonic.mp3

And here's what it looks like.

[Normster]

Not a clue, Bob. Ordinarily, I'd say it's a lead dress issue, but from the pics I don't see any parallel runs. Have you tried adjusting it out with the PI trimmer? You could also just bite the bullet and throw a cap between the PI plate resistors like Marshall does.

[Bob-I]

Not a clue, Bob. Ordinarily, I'd say it's a lead dress issue, but from the pics I don't see any parallel runs. Have you tried adjusting it out with the PI trimmer? You could also just bite the bullet and throw a cap between the PI plate resistors like Marshall does.

Yes, I tried the trimmer. This tone is so low frequency that a small cap in the PI will not help. Maybe I'll try that anyway. I've jiggled wires around but it doesn't seem to be a lead dress issue. I followed the lead dress that I used on the Super Reverb build so it should be sound.

I'm beginning to think that I don't need that extra gain stage after the reverb.

Oops I forgot to post the schematic.

http://www.patmedia.net/bob-ingram/Dumb ... ematic.jpg

[mlp-mx6]

Did your tube swapping include the power tubes?

[Bob-I]

Did your tube swapping include the power tubes?

Yes I did. I swapped out 3 sets.

[Normster]

Kind of a long shot, Bob, but how about a loose speaker jack?

[Bob-I]

Kind of a long shot, Bob, but how about a loose speaker jack?

You know, it's most likely something really simple so I'll check that. I'm thinking that maybe it's something electronic though, because if you let it cool off, it's gone until it heats up again. Maybe it's a cap.

[mlp-mx6]

Time for a "cap tap" routine?

[Icetech]

Bob do you happen to have any cool spray laying around?

[mlp-mx6]

Based on a read at Randall Aiken's site, check your feedback loop (from the output xfmr). Here's the link - http://www.aikenamps.com/GlobalNegativeFeedback.htm

He says that if you have too much global negative feedback the amp can oscillate. A long shot, perhaps, but, hey... check EVERYTHING, right?

[Bob-I]

Mlp,

I tried the cap tapping routine. The amp is so sensitive that any tapping at all sets it off. I then swapped out the filters and no change. I also disconnected the NFB, same thing.

Icetech, no I don't have any cooling spray and all the local shops are closed now.

[mlp-mx6]

Does it happen with the first tube out? How about the first tube in the dirty side? Does it happen with the reverb disconnected?

[Bob-I]

Does it happen with the first tube out? How about the first tube in the dirty side? Does it happen with the reverb disconnected?

First of all, I only have channel 1, the Fender channel operational but yes, it happenes with V1 out and with the reverb disconnected.

Here's what I did.

I orignally had written the schematic to share Rk and Ck on the reverb return and the next gain stage, just like a BF Fender. When I built this amp, I seperated them and instead of 820k/22uF I used 1.5K/22uF on each side. Apparently this is too much gain. I reduced Rk on the gain stage to 820 and removed the bypass cap. The microphonic sound is gone and the tone is still ok.

I still have lots of noise in this amp, 60Hz hum mostly so I'll have to double check my grounding and maybe shield a few leads in the PI and output section.

Here's a few pics....

[Normster]

Your amp is looking great, Bob!

As for the hum, I remember reading quite a few posts on ampage about the Weber relay board doing that as well. (Same basic design?) In fact, now that I think about it, I had the same problem on the amp I built for Loren. On his amp, we just insulated the footswitch jack from the chassis and all was right with the world.

If you've already done that, the only other thing I can think of is to seperate the rectifier circuit from the relay board and move it to the tranny side of the chassis. (Dumble style.) As it is, you've got a couple of AC wires in very close proximity to your tone stack caps.

BTW, forgot to mention...I loved the clean tone in the clip you posted. Glad you got the feedback issue cured.

[Bob-I]

Your amp is looking great, Bob!

As for the hum, I remember reading quite a few posts on ampage about the Weber relay board doing that as well. (Same basic design?) In fact, now that I think about it, I had the same problem on the amp I built for Loren. On his amp, we just insulated the footswitch jack from the chassis and all was right with the world.

If you've already done that, the only other thing I can think of is to seperate the rectifier circuit from the relay board and move it to the tranny side of the chassis. (Dumble style.) As it is, you've got a couple of AC wires in very close proximity to your tone stack caps.

Actually, I don't even have the footswitch jack installed yet. That microphonic sound was before I installed the relay and relay supply. The relay supply is on the relay board, 20" from the rectifier.

BTW, forgot to mention...I loved the clean tone in the clip you posted. Glad you got the feedback issue cured.

Thx, but I wasn't even trying. I know it can be even better.

My goal is 3 distinct tones. The Fender bright clean, the Dumble fat clean and Dumble ODS. I need to troubleshoot the hum, which I believe is from the last gain stage or the PI. I think some shielded leads on the grids will fix it.