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Vintage threads from the first ten years

Re: Vibrolux meets Bassman

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9/18/1997 9:07 AM
J Epstein	Re: Vibrolux meets Bassman On my homebrew amp I installed a switch that is an DPDT On-Off-On as follows : bypass caps (25uF) on one, zero or two gain stages. Handy as hell. I thought I'd just flip it occaisonally so I put it under the chassis, near the tubes in question, but it really belongs on the control plate, I use it all the time. -j

9/19/1997 7:53 PM

It sounds like you fixed your oscillation problem, but for a relatively low gain design, like what you mentioned, it shouldn't be necessary to bypass plate loads. It's more likely to be layout induced as Mark Cameron mentioned in an earlier post. You might want to check input and output layout for the stages and use some shielded cable on the low level inputs. Having said that, I'll also say that I have heard some amps where there was some stray capacitive coupling between circuit points that seemed to actually improve the sound. Layout does have a bearing on the sound so I don't advocate "shielding everything" as standard practice, but only as a means of stopping oscillation, and otherwise where it makes sense. Does anyone else have an opinion on this?
Regards,
Gary

9/20/1997 10:07 AM
R.G.	You might also want to dig through the suggestions for stopping oscillation that are presented in the Tube Amp Debugging Page.

9/30/1997 8:41 AM

I think I finally solved the puzzle to my satisfaction, I went back and took off the plate load bypass cap and reduced the treble cap in the tone stack to a slightly higher 300pf. this helped,but still went into oscillation with treb and presence controls at about 8. What seemed to stop it was adding a 220k resister at the junction of the normal (bassman) channel and the Vibrato channel. There was a resistor on the on and just the output of the treble control on the other and I think it was coupling somewhat with out that resister. At this point I also was able to raise the phase inv. cap to .01uf and the amp sounds great. There is still some oscillation when everything is dimed, but I don't ever play there anyway. I'd rather have the tone it gets now then squash it . Alos I have to run a fair amount of shielding , becaues the first stage tube is the #5 tube (vibrato) so there is shielding to it and back to the nor. vol control then after the tone stack back to the phase inverter junction. Thanks for all the suggestions and ideas, I thinks this is a great place for ideas (and amps) to develop!

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