102 Feedback City

[talbany]

Here is a quick vid of the 102 showing how this thing wants to take off on virtually any note..The D-lator is hooked up and the volume is set fairly low comfortable livingroom level...This is the OD channel only no PAB and no mid boost... Guitar is a Les Paul loaded with Fralins going through a pair of G12-65's I did change out the screen and preamp filters to NOS TVA 20uf and didn't notice to much change there in the amps ability to run away..I will get around to doing a better quality clip in the future as this is just a quick demo showing how this thing searches for feedback!!..Pretty NUTS!!

http://www.youtube.com/watch?v=qyB4HedEOvs

Tony

[amplifiednation]

Tony that is crazy. It's like the amp has a mind of it's own!! Maybe I should reduce the filtering on my 102???

[NickC]

Yeah, that's crazy ........... but in a good way.

It's right on the cusp of being too much of a good thing. Not quite .... but almost too much.

[vibratoking]

Thanks for the clip, Tony. That thing is lively to say the least.

FWIW, I have lowered my B+ capacitance to 50uF with very little result as far as feedback is concerned. I reported in another thread that the reduction in capacitance made the amp too bright, but I realized that was due to the bright cap that I added at the same time. Duh!
My amp will take off like yours, but only with OD and PAB on and with the volume uncomfortably loud. I should try to experiment with the other node caps.

[talbany]

It's hard to tell on the crappy video that as the notes swell and bloom there is a noticeable swell in volume as the feedback takes over..I would imagine with a hollow body and the amp stretched out a bit you would need to be aware of the feedback taking over.. With the Les Paul it's fairly manageable..Step on the PAB and it's definitely off to the races on any note on all the guitars I own and at times hard to manage.. I am still not quite sure what exactly makes this thing go nuts perhaps some further testing will reveal something..Right now I am just enjoying it..It will be tough going back to something else..Perhaps I will bring this one to the NY amp show in the spring!!

FWIW, I have lowered my B+ capacitance to 50uF with very little result as far as feedback is concerned. My amp will take off like yours, but only with OD and PAB on and with the volume uncomfortably loud. I should try to experiment with the other node caps.

Vibro
I am not so sure this anomaly lies in the filter caps or cap values..If I were to guess it sounds more like A PI/ NFB thing..This is using my ears.. FWIW.. In clean mode no PAB I can hit an open string note and it too will not die out..Ring forever and go into feedback as well..In clean with the PAB I get the same feedback effect just not on every note..I will do a quick clip of this when I get a chance..Don't know what to tell you right now
FWIW..Preamp tube changes do alter the effect naturally but for the most part the amp still wants to feedback just not with the same intensity..The Amphrex's in there measure on the tester medium gain for an AX and low noise

Good Luck!!

Tony

[vibratoking]

I am not so sure this anomaly lies in the filter caps or cap values..If I were to guess it sounds more like A PI/ NFB thing..

Tony, I would agree that it is not in the filter caps. I have also rolled quite a few tubes in/out and haven't found any that contribute heavily to the level of feedback.

I have adjusted the PI balance trimmer according to the semi-recent thread regarding lowest THD, by ear, etc... methods. I have not experienced a big impact on the feedback level. BTW, I used a spectrum analyzer while doing PI adjustments and I am sure that it is adjusted properly.

I have not played much with the GNFB, if that is what you are referring to. Maybe I should be taking a closer look at that?

[briane]

I have not played much with the GNFB, if that is what you are referring to. Maybe I should be taking a closer look at that?

yes, typically. That is if you want to adjust the feedback.

[vibratoking]

yes, typically. That is if you want to adjust the feedback.

I am not sure what you are trying to say with your statement? We have been using two different meanings for the word feedback in this thread. I was talking about changing the GNFB to investigate the the impact on the feeback that is demonstrated in Tonys clip. Changing the GNFB to change the GNFB would be redundant. So, what are you trying to add to the thread?

[talbany]

Here is another one clean channel with the PAB on lightly over a few passes..

http://www.youtube.com/watch?v=N2UOD-gqWRw

Tony

[odourboy]

Jeezez - that is nuts! If you could bottle that and hand it out in small doses I'd be first in line!!

[Cliff Schecht]

That didn't sound so good in the second clip (probably IMO ). It sounds like the gain is increasing as a note sustains and it starts to go from clean to blatty. It sounds like that weird octaving effect I've experienced with transistor based differential amps when driven incorrectly. Are you sure there isn't some sort of an issue with the OD channel here? Is there a leaky coupling cap to the PI?

[talbany]

Cliff the 2nd clip is clean channel.. Probably the crappy mic/ limiter in the cheap camera..(Not using the mic sitting in front)..No leaky caps AFAIK..
Thanks for listening..

Tony

[passfan]

Are these clips with the same output tubes as before Tony , or did you try out the 6L6 bottles you got ? Will it do it without the dumbleator in the loop ?

[ayan]

Hi Tony.

Incredible feedback machine you have there. Since the behaviour is displayed in both in clean and OD modes, I think it's safe to say that V1 seems to be a player; V2 might or might not be. My next question would be whether that paritcular amp's power section has anything to do with it or not. Could you take the ouput from that amp via the Pre Out, feed the signal into any other amp and see what you get? If you got the same results, the whole thing resides within the preamp section alone. If you got nothing, I think you could narrow the "effect" to something happening in the PI/power section, or some some interactions between the PI/powe section and V1, which could be some in phase coupling somewhere or power supply modulation (I think that'd be unlikely). Go figure out that puzzle... but if you crack it, you will have solved a lot of the mystique surrounding Dumble.

I have always been fascinated by how some amps seem to want to feedback reglardless of anything (tubes, choice of particular components, etc.) and some just don't sound as lively. I have looked for some in-phase coupling within the amp, but never got anywhere with that theory when applied Dumbler layout since it seems to be pretty stable-- amps build into like a Fender chassis, on the other hand, were a lot more unstable because of the weird layout and wires runiing back and forth, resulting in lots of coupling.

Gil

Here is a quick vid of the 102 showing how this thing wants to take off on virtually any note..The D-lator is hooked up and the volume is set fairly low comfortable livingroom level...This is the OD channel only no PAB and no mid boost... Guitar is a Les Paul loaded with Fralins going through a pair of G12-65's I did change out the screen and preamp filters to NOS TVA 20uf and didn't notice to much change there in the amps ability to run away..I will get around to doing a better quality clip in the future as this is just a quick demo showing how this thing searches for feedback!!..Pretty NUTS!!

http://www.youtube.com/watch?v=qyB4HedEOvs

Tony

[talbany]

Are these clips with the same output tubes as before Tony , or did you try out the 6L6 bottles you got ? Will it do it without the dumbleator in the loop ?

Pass
I started out with some old 7581 pulls that were pretty worn and not well matched..As per my review the amp took off with these tubes..I then scored some newer STR 387's that were in better shape and more closely matched and the amp took off.. The newer 387's sound a bit better and had more headroom but the change in feedback didn't seem to be affected..Still takes off.. Actually the amp seems to want to go into feedback blooms a bit better without the loop.. Perhaps because the amp is brighter without it..Not exactly sure though? The loop so far is not IMO a contributing factor..

Hi Tony.

Incredible feedback machine you have there. Since the behaviour is displayed in both in clean and OD modes, I think it's safe to say that V1 seems to be a player; V2 might or might not be. My next question would be whether that paritcular amp's power section has anything to do with it or not. Could you take the ouput from that amp via the Pre Out, feed the signal into any other amp and see what you get? If you got the same results, the whole thing resides within the preamp section alone. If you got nothing, I think you could narrow the "effect" to something happening in the PI/power section, or some some interactions between the PI/powe section and V1, which could be some in phase coupling somewhere or power supply modulation (I think that'd be unlikely). Go figure out that puzzle... but if you crack it, you will have solved a lot of the mystique surrounding Dumble.

I have always been fascinated by how some amps seem to want to feedback reglardless of anything (tubes, choice of particular components, etc.) and some just don't sound as lively. I have looked for some in-phase coupling within the amp, but never got anywhere with that theory when applied Dumbler layout since it seems to be pretty stable-- amps build into like a Fender chassis, on the other hand, were a lot more unstable because of the weird layout and wires runiing back and forth, resulting in lots of coupling.

Gil

Here is a quick vid of the 102 showing how this thing wants to take off on virtually any note..The D-lator is hooked up and the volume is set fairly low comfortable livingroom level...This is the OD channel only no PAB and no mid boost... Guitar is a Les Paul loaded with Fralins going through a pair of G12-65's I did change out the screen and preamp filters to NOS TVA 20uf and didn't notice to much change there in the amps ability to run away..I will get around to doing a better quality clip in the future as this is just a quick demo showing how this thing searches for feedback!!..Pretty NUTS!!

http://www.youtube.com/watch?v=qyB4HedEOvs

Tony

Gil
Thanks for checking out the clips and joining the discussion..This amp has totally changed my view on having to use certain tubes dialing in the sweet spot on the trimmer and that type of compulsive behavior we sometimes obsess ourselves with.. If the fundamental property exists the rest is icing on an already sweet cake.. Wonderful idea on the loop inputs to determine where and hopefully what is contributing to this amps ability to take off.. I can say with some certainty that most of the feedback that is occurring is happening inside the amp and not so much as one being acoustically driven like most amps.. I have to go out of town a few days but will do the experiment and will report my results to you..Yes I will crack this if it KILLS ME!!

All The Best...

Tony