102 Feedback City

[Smitty]

Tony,

I've been staring at your lead dress and was wondering if you would consider moving the V1A cathode lead away from the V1B plate lead to see if that changes the response of the clean channel. I'm guessing that because those two leads are in phase they could be coupling and causing positive feedback. They look like they run very close for a substantial portion of the way.

I wonder what would happen if one were to twist these two leads together?

Smitty

[wjdunham]

Tony,

I've been staring at your lead dress and was wondering if you would consider moving the V1A cathode lead away from the V1B plate lead to see if that changes the response of the clean channel. I'm guessing that because those two leads are in phase they could be coupling and causing positive feedback. They look like they run very close for a substantial portion of the way.

I wonder what would happen if one were to twist these two leads together?

Smitty

I had a similar thought on this. The Cathode is actually another input to the stage, it is an inverting input so with the plate wire being inverted already, you have a positive feedback loop (Dumble used the cathode as an input for the "local" negative feedback loops on some of the early SSS amps, although the signal came from elsewhere). If what you say is indeed the case, that the resonance is coming from the preamp, and that it's coming from V1 then this is one of the likely sources. Would be very interesting to see if moving the plate and cathode leads apart has any effect.
Bill

[Max]

Have all the original Dumbles you have played have this ability to run away like 124?

Tony, let’s perhaps concentrate on Dumble ODS amps. Even then your question is a complicated one for me to answer:

My personal impression is this:

What I did find in all the ODS amps I've played, from the oldest to the newest, is an ability to "support" a player that after some time has learned how they react. Sometimes this will need 5 minutes, sometimes 5 months, sometimes it might need 5 years.

After understanding the individual character of a specimen of these amps, IMO they all start to get kind of "interactive": They kind of work with your input and may even point you to a possible direction you could perhaps take. They all add IMO a bit of their own and ask you: "Did you like how I responded to this picking nuance? Do you want to have more of this?" IMO they kind of show you where you could perhaps go together if you should chose to do so.

But - as I said – this needs some time of "living together", sometimes more time, sometimes less time. I even know of Overdrive Specials that I personally never had the luck to completely understand while I had the luck to be able to use them. But some other musicians just plugged their guitars into the very same ODS and suddenly it did everything they asked this amp to do. Perhaps the amount of time needed to understand a specimen of the ODS family depends on how close the personal playing style of some player is to the personal playing style of the player who ordered this amp from Alexander Dumble? This is of course just a speculation.

What I’m talking about here is IMO exactly what SRV talked about when he once said concerning Dumble Overdrive Specials in general:

"Some Dumbles - like the Overdrive Special - you 've got to know what you 're doing with them, because they'll get away from you and take you with 'em."

source: http://web.archive.org/web/200802091812 ... ble-e.html

So what is sometimes discussed here by using metaphors like "blooming" or "note flipping" etc., IMO perhaps isn’t depending that much on a certain kind of circuit version of some special specimen of an ODS in the sense of "ODS #xyz does this, but ODS #zyx doesn’t". IMO this perhaps depends more on the level and quality of a player's understanding of a single specimen of an ODS and the level and quality of interaction depending on this level on understanding.

What IMO will always stay different to some extent is of course the "timbre" or "voice" of a player, when he plays e.g. a 1st generation ODS and a 5th generation ODS. But IMO they all have the ability to support a player who is patient enough to let the chemistry develop.

Having said this and considering what I hear in these clips of your "Feedback City" amp, this kind of behaviour seems in my personal perception to be perhaps a bit more "out of control" as I know this from playing the original ODS amps I’m familiar with.

But I don’t know the reason why this kind of behaviour that is showcased in these clips of "Feedback City" seems to be a bit "out of control" – at least in my perception. IMO the reason might be some possible general differences between the behaviour of "Feedback City" and the behaviour of the original ODS amps I’ve played, or the reason might perhaps just be, that you just need some more time to develop the best possible grade of interaction with this amp? Please understand that this is hard to tell apart for me just based on these clips and without playing "Feedback City".

I hope that the rather metaphoric way I wrote about this topic here was perhaps nonetheless a bit informative for you.

Have fun!

Max

[CHIP]

I wonder if the cloth wire, as opposed to teflon, is having an effect on transconductance of the signal from the cathode of V1a to the plate of V1b?

[vibratoking]

I've been staring at your lead dress and was wondering if you would consider moving the V1A cathode lead away from the V1B plate lead to see if that changes the response of the clean channel. I'm guessing that because those two leads are in phase they could be coupling and causing positive feedback. They look like they run very close for a substantial portion of the way.

I think most Dstyle builds follow this lead dress with V1A cathode and V1B plate running parallel this way. This is well known and is also done on V2. This would be easy for Tony to test with his amp, but this type of lead dress does not change the amount of feedback in my amp. It does make my amp sound smoother by reducing some of the high end harshness. YMMV. I'd be interested to see if this effects the feedback character of Tony's amp.

[ayan]

All the cathodes are bypassed -- to increasae gain and prevent degenerative feedback -- and therefore, theoretically, at AC ground. If the cathodes were not bypassed, I'd say there could be something there, but in this case I don't think so. This is typical Dumble lead dress, as we all know.

Cheers,

Gil

Tony,

I've been staring at your lead dress and was wondering if you would consider moving the V1A cathode lead away from the V1B plate lead to see if that changes the response of the clean channel. I'm guessing that because those two leads are in phase they could be coupling and causing positive feedback. They look like they run very close for a substantial portion of the way.

I wonder what would happen if one were to twist these two leads together?

Smitty

[bluesfendermanblues]

How about the V1 tube...will swapping V1 for another tube create similar results? maybe this particular V1 has a very high gain factor?

[llemtt]

@Gil +1

a wrong assumption here made by most is reasoning about sustain and other "envelope" related properties in terms of "signal" feedback (in phase or not) which affect only steady-state amplitude/frequency-response (considering an amp a non-resonant system on his own...)

Just an idea: v1a grid bias?

cheers
teo

[talbany]

Blues..
The tubes in there are Phillips Mullards measure a strong low noise medium gain for an AX (don't remember the exact #).. I am not at the shop today but will get back tommorrow on the tubes exact Mu/trans conductance and noise!!..

No real change in bloom and feedback with the plate/cathode wires separated!!

Keep Thinking!!

Tony

[Smitty]

Gil,

I was thinking that the series resistance of the cathode bypass cap might be coming into play. Yet another reason I want to get a current probe.

Tony,

Thanks for indulging me.

Smit

[ayan]

I hear you, some audiophiles bypass byapass electrolytics with plastic caps to overcome some of that, I believe. Still, I think it'd be a long strentch to get something in through the cathode when it is bypassed.

Somer SSS are known to V1B plate to V1A cathode feedback. Off V1B's plate: .1uF, 1 Meg resistor, to ground end of V1A's 1.5K//5uF cap, but in this case, that network is floated above ground via 100R. I can see that perhaps working, but not the plate and cathode wires running together.

Gil

Gil,

I was thinking that the series resistance of the cathode bypass cap might be coming into play. Yet another reason I want to get a current probe.

Tony,

Thanks for indulging me.

Smit

[ayan]

@Gil +1

a wrong assumption here made by most is reasoning about sustain and other "envelope" related properties in terms of "signal" feedback (in phase or not) which affect only steady-state amplitude/frequency-response (considering an amp a non-resonant system on his own...)

Just an idea: v1a grid bias?

cheers
teo

I'd buy changing the "bias," whether it be done at the grid or cathode... since Vgc is what actually matters.

Cheers,

Gil

[martin manning]

Keep Thinking!!

Tony, it doesn't have snubbers on V2, right? What if you tack some on there and see if that kills the (glorious) feedback?

[Structo]

Just so we're clear when we say V1a, we are talking first stage right?

Because most Dumble amps are wired with the "b" side first.

I am puzzled by the frequency of the feedback Tony gets.
It doesn't seem like a harmonic feedback as I notice the feedback frequency or note doesn't necessarily correlate with the note being played.

Several times it resolves to the same feedback frequency although the played note is different.
This almost seems like an acoustic feedback verses an internal harmonic being generated.

At least that is what I think I'm hearing.

[Smitty]

Structo,

I paused when I wrote the post for the same reason. I'm hoping folks took it the way you expressed it. V1A as input stage.

Gil,

Hmm...positive feedback divider/injector from stage two plate to bottom of stage one cathode. Hmm... Wonder how much positive feedback it could handle?

Smit