102 Feedback City

[TheHandsomeOrk]

experienced a similar thing with an old P/P 6v6 amp from the 60s. Owner wanted it recapped as it was all original and in need of a service. The amp had amazing sustain and had the harmonic feedback thing happening on some notes even at very low volume. Turned out one of the (v1)cathode caps was leaky and a microphonic coupling cap. Did the recap at owners request, amp was louder more power generally, cleaner but without the mojo it previously had. Owner was happy as this is what he wanted, not interested in a great blues amp. Not saying this is whats going on with the 102 amp, just saying that's all ,but I wouldn't be surprised if there was something with the preamp caps.

[CHIP]

What causes a coupling cap to be microphonic as opposed to a normally operating cap? Is this trait detectable before the cap is installed in the amp?

[erwin_ve]

The amp had amazing sustain and had the harmonic feedback thing happening on some notes even at very low volume. Turned out one of the (v1)cathode caps was leaky and a microphonic coupling cap.

thanks for sharing, for me this is the most interesting hypothesis in the feedback topic.
How would you implement such a thing as a wanted and controlled item at v1 stage #102 copy?
Edit: maybe lower the cathode resistors?

[martin manning]

The amp had amazing sustain and had the harmonic feedback thing happening on some notes even at very low volume. Turned out one of the (v1)cathode caps was leaky and a microphonic coupling cap.

thanks for sharing, for me this is the most interesting hypothesis in the feedback topic.
How would you implement such a thing as a wanted and controlled item at v1 stage #102 copy?
Edit: maybe lower the cathode resistors?

A microphonic cap would by definition need some acoustic coupling to the output. I thought that was ruled out?

[erwin_ve]

The amp had amazing sustain and had the harmonic feedback thing happening on some notes even at very low volume. Turned out one of the (v1)cathode caps was leaky and a microphonic coupling cap.

thanks for sharing, for me this is the most interesting hypothesis in the feedback topic.
How would you implement such a thing as a wanted and controlled item at v1 stage #102 copy?
Edit: maybe lower the cathode resistors?

A microphonic cap would by definition need some acoustic coupling to the output. I thought that was ruled out?

Hi Martin, I was responding to the leaky cathode cap, you responded to the microphoonic coupling cap

[TheHandsomeOrk]

What causes a coupling cap to be microphonic as opposed to a normally operating cap? Is this trait detectable before the cap is installed in the amp?

Don't know what causes it and don't think you'd be able to tell prior to installing it.I only notice it after I changed out the filter caps
while testing that the amp still ran ok I decided to poke around with a chop stick and found that this one cap ( mustard) was more sensitive to being poked at so I changed it. The electrolytic cathode caps were changed out as a matter of course along with the filter caps it was only because I had my old cap tester handy that I tested a few of the removed caps for leakage that I discovered the leaky cathode cap.
It was a few years ago and at the time I thought " well this how this amp should sound, like it's fresh from the factory" but I much preferred it before the recap.
Which cap was responsible for the before sound, I'm not sure but I know it didn't have the same sustain or feedback after. I recall changing a couple of resistors as well can't remember why perhaps they had drifted. All I know is that leaky, microphonic caps are a bad thing and should be replaced.
I'd forgotten about this experience until this thread and thought I'd mention it, maybe others have had similar or might be able to shed some light.

[TheHandsomeOrk]

Noticed in Tony's video 11 at 43 seconds the runaway pickup truck and the driver chasing it, LoL.

[ayan]

I think it's a dude jogging, but very funny nevertheless.

Gil

Noticed in Tony's video 11 at 43 seconds the runaway pickup truck and the driver chasing it, LoL.

[Structo]

In my limited experience with the ODS amps, the tone stack caps all seem to be a bit microphonic to an extent.

If you tap on them with a chopstick you will hear it.

I think it is due to their location in the gain structure that causes this.

And, as others have stated, they are condensers and just like a condenser microphone they will make noise when tapped on.

[talbany]

Just Finished covering the new cab for it..

Tony

[ayan]

Does it feedback better in that cabinet?

Gil

Just Finished covering the new cab for it..

Tony

[Structo]

Nice cab Tony!

Yeah, so does it still feedback in the cab?

[talbany]

Seems like it's a bit more sensitive in the cab,although I did re position the amp..Still there.. Clean no boost and no lator!!

http://www.youtube.com/watch?v=ZzMOBIrEPNI

Tony

[tjauernig]

This amp feeds back in a similar nature to the ODS that Kimbo Davis owned (may he RIP). That amp always amazed me how at living room volumes it would feedback. It was nearly impossible to play and did take getting used to. Kimbos amp had 6L6's with 704VDC measured at the plates. I always wondered if that had something to do with it. Lot's of "goop" was inside Kimbos amp so it was hard to measure a lot. Kimbo was always nervous about poking around in it as well for fear that something would change. Kimbo always told me the amp had the "rock city" mod done to it in the late 80's, and that's when it started doing the feedback thing.

Amazing amp you have there, and IMO you're getting closer to "the" sound

[talbany]

T. Jauernig

Thanks for the cool story on Kimbo's amp..Can you tell us a bit more about Kimbo's "Rock City mod" amp..For instance was this originally a classic converted to a skyliner and did Dumble change out the preamp board for the mod and if so was the entire board covered in goop?
Also did Dumble tell Kimbo that the feedback he got (after the mod) was a part of the so called Rock City mod?..

Thanks!!

Tony