ayan wrote: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
Smitty wrote: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
Gil is right, I forgot about the 100R creating a voltage divider between the cathode resistor and bypass cap.
I suppose it's possible that because of the large gain of the stage, a small series resistance in the cap creates enough voltage drop to stimulate the input?
Bill
I've never tried it myself, but it has been done at least in a couple of actual SSS amps.
Gil
wjdunham wrote:
ayan wrote: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
Smitty wrote: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
Gil is right, I forgot about the 100R creating a voltage divider between the cathode resistor and bypass cap.
I suppose it's possible that because of the large gain of the stage, a small series resistance in the cap creates enough voltage drop to stimulate the input?
Bill
Tony mentioned that he didn't orient the outside foil on this build. I'm wondering if the V1B coupling cap is inducing signal onto the V1A cathode bypass cap (yes right through the grounded body) if it is the one closest to the front of the amp. I know this is not what is depicted in the layout diagrams and my X-Ray glasses are on the fritz, but based on the photo you could imagine that to be the case. Those guys look pretty close together.
That would make sense if the circuit were stable at idle and then started to take off with lots of signal...and perhaps with the wires so close...
Not sure how hard it is to inductively couple between those caps, though.
Smitty wrote:Tony mentioned that he didn't orient the outside foil on this build. I'm wondering if the V1B coupling cap is inducing signal onto the V1A cathode bypass cap
The original post here reminded me that years ago I had a Boogie SOB. Not my favorite amp or brand, but it would take off on feed back much like the amp posted here.
One day I read something about a lissajous (SP?) pattern on a scope. IIIRC this involved putting the inputs and outputs of an amp signal on separate inputs of a scope. If you got a perfect circle that meant the amp was in perfect phase from front to back. Perfectly in phase is not a condition one wants, but inspection showed the SOB was not far from being in phase.
It occurred to me that perhaps amps that respond like that demonstrated here, might have small phase changes thru the components, that in some cases, bring the amplifier output "closer" to in-phase, thereby making them more likely to feedback.
Smitty wrote:Tony mentioned that he didn't orient the outside foil on this build. I'm wondering if the V1B coupling cap is inducing signal onto the V1A cathode bypass cap
V1B coupler is next to V1B cathode
TM
I was half expecting to hear this response from Tony, but given that I don't know who has possesion of this amp at the moment I'd like to follow up and be sure I understand your response. Do you mean to say that you are certain (as in checked this amp with a VOM or looked at a pic of the board pre-installation) that this is the case? ...or that this is the way that most/all are supposed to be constructed?
talbany wrote: Have all the original Dumbles you have played have this ability to run away like 124?
Hey Max same question to you as well!!
Tony
Can't say the ones I tried did. However, I remember Brandon saying that LC's Dumble feedback like crazy on the clean channel...
Hi all,
can someone here perhaps help me to find some professional HQ studio or live recording(s) - distributed by Amazon etc. - of some real piece(s) of music done by (a) real band(s) and done with some original Dumble ODS amp(s), where such a kind of behavior - as showcased by these clips of Tony's "Feedback City" - is IYO intentionally evoked? I ask, because right now only Carlos Santana comes to my mind who could perhaps be looked upon as a professional Dumble player intentionally evoking and using such a kind of behavior in the context of the performances of the Santana band in the studio and live?
I read through this thread just now (whew!), after only following it casually to date. Based on what has been discussed, it seems most likely that the source of this almost-runaway-FB is V1. The socket was mentioned once, and maybe that's worth some further investigation to see if there isn't some kind of electro-mechanical coupling going on. Is the amp sitting on top of the speaker cab? Does it make any difference if it isn't? Is there any effect from damping vibration in V1 by holding on to it with gloved fingers?
talbany wrote: Have all the original Dumbles you have played have this ability to run away like 124?
Hey Max same question to you as well!!
Tony
Can't say the ones I tried did. However, I remember Brandon saying that LC's Dumble feedback like crazy on the clean channel...
Hi all,
can someone here perhaps help me to find some professional HQ studio or live recording(s) - distributed by Amazon etc. - of some real piece(s) of music done by (a) real band(s) and done with some original Dumble ODS amp(s), where such a kind of behavior - as showcased by these clips of Tony's "Feedback City" - is IYO intentionally evoked? I ask, because right now only Carlos Santana comes to my mind who could perhaps be looked upon as a professional Dumble player intentionally evoking and using such a kind of behavior in the context of the performances of the Santana band in the studio and live?
Thanks in advance and all the best,
Max
This is a good question. Best I can come up with personally is... aside from Santana's (Rob Thomas) "Smooth"... http://youtu.be/Lwt9eKDxbbM - at 3:25 - and not nearly the bloom you hear from Tony's amp. But he holds a note with the drive up. Very slight harmonics toward the end of the note...
Robben also flirts with the bloom in the intro "Peace on my Mind" (Truth, 2007).
I would be curious to hear the how the amp blows (woosh) through a speaker without guitar signal while dialing up the knobs. (See Kimock's post titled "Dialing Up a Dumble"). This would probably require and be more effective if shielding the top of the amp. I do find that this method of dialing up an amplifier gives you some ideas on what the multiple thresholds/ranges are for certain characteristics of the amplifier. It made me rethink my approach to the controls.... With my question being, is there a useful amp setting that does not cause the amp to run away on every note... making it somewhat less pronounced, more controlable, and useful when desired.
Hemtt/Martin
I thought that as well, so yesterday I put the amp in the cab and took it to VVT's Shop in Waldorf to show the amp..Plugged it into a 2X12 Avatar loaded Vintage 30's..I tried an SG another Strat and an Epiphone Hollow body.. The amp was on a bench off the cab so no mechanical coupling was possible..Results were pretty much the same in a larger room..I demo-ed the amp to my partner Jim Hill and he too was amazed at how the amp searched for feedback..FWIW the SG was by far the best sounding out of the bunch for me, bloom and feedback effortlessly.. Jim would be glad to tell you how the amp performed if you want to call him!! http://www.vvtamps.com/
I will really try and get a better quality clip up in the next few days..Studio's giving me fits
Tony
" The psychics on my bench is the same as Dumble'"
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.
Tom
FWIW.... When I played the amp in another room with the amp off the cab it sounded to me as though I was getting more of a variety of different harmonics being feedback than in my studio which is much smaller.. Although I can't say for sure if this was due to a different guitar or different room or both!!..Perhaps this is a clue!!
Tony
" The psychics on my bench is the same as Dumble'"
