Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on August 10, 2011, 08:26:51 am
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Any pointers on how to speed up the time between when you hit the foot switch and the tremolo kicks in?
This would be a typical BF Princeton type set up.
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New tube first.
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http://www.freewebs.com/valvewizard1/trem1.html (http://www.freewebs.com/valvewizard1/trem1.html)
Read the paragraph at the bottom of the page under footswitching
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And if a new tube doesn't get it, ask how old the 25uF bypass cap is on that tube.
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Replaced the cap(s) so far.
Going to try the other stuff asap.
Thanks guys for the tips!
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How slow? The Princeton (whatever "typical" might be) does not have the kick-start of the high-price amps, will always take a few cycles to build.
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This is on that Columbia amp I posted a week or so ago.
It appears they used a triode on one of the 6BM8s.
I replaced all the caps on the triode side of that tube.
I am too cheap to go out and buy a new pair of these tubes, plus they just sound great.
I'll try swapping the two existing 6BM8s.
Interesting tube BTW.
It takes about 2-3 seconds for the tremolo to start after the amp is heated up.
Probably just the way it is.
Not that big of a deal, just tweaking.
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It appears they used a triode on one of the 6BM8s
That's quite different from a Princeton. Do you have a schematic? It may be possible to switch the trem effect on/off but leave the oscillator running continuously, giving instant on/off.
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That's quite different from a Princeton. Do you have a schematic? It may be possible to switch the trem effect on/off but leave the oscillator running continuously, giving instant on/off.
Genius idea about the oscillator!
A simple mod?
I'd do it on all my amps if I knew how.
I did put a 0.47 in place of the original .1 cap on the tremolo section.
It seemed to add depth and it was sitting in the parts bin forever.
Gawd I wish I did have a schematic of this amp!
To add additional intrigue, it appears there is no reverb transformer.
I reassembled the amp because I have some other stuff I need to get on the bench.
(it a royal pita to take apart and put together)
I took some pics if that helps at all?
I also get some slight crackle when it's cranked with the bass turned all the way up.(almost like a bad speaker but I know this speaker is fine)
I wonder if I should just do a complete cap job on it?
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Genius idea about the oscillator!
A simple mod?
I'd do it on all my amps if I knew how.
Probably as simple as moving the on/off switch. But without seeing a schematic, anything I could say would be a guess and most likely be wrong. The oscillator is probably very typical, but the injection point could be any of several popular schemes. And those Japanese may have even come up with a different scheme. I'm guessing the amp is push/pull, but is it fixed or cathode biased? With only 4 tubes in that top chassis, I bet you could easily draw a schematic. Then we could give more specific suggestions.
Why do you think they used a 6BM8 triode for the trem? Trem oscillators can be finicky. Weak tube, caps (especially cathode bypass cap), low or poorly filtered B+ can all cause problems. If you can't draw a schematic, try changing filter caps and check resistors in the B+ line. Might just get lucky.
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If you leave the trem running, the amp thumps all the time. This may be negligible or annoying. It is interesting that nearly ALL amps disable the trem oscillator. (Not to save power: consumption is a hair higher when stalled.)
Trem start-up is always a _compromise_ between smoothness and long start-up (and of course cost).
> I did put a 0.47 in place of the original .1 cap on the tremolo section.
Yes, this might make more slow trem, but might also cause 5X longer start-up.
I'd wonder if the original designer picked 0.1u for this reason.
Since this is not "typical BF Princeton", not stock Columbia, not documented, all's we can do is throw spitballs in the dark. Perhaps you should try the 0.1u again, then live with it until you have time to trace what is really in there so it can be analyzed.
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>>I'm guessing the amp is push/pull, but is it fixed or cathode biased?
Yes, that's right.
>>With only 4 tubes in that top chassis, I bet you could easily draw a schematic.
When I get more time I'll give it a shot
>>Why do you think they used a 6BM8 triode for the trem?
I am not sure.
I'd guess;
6AQ8-reverb
12ax7-gain stages
one-6BM8 (triode)-PI
one-6BM8 (triode)-tremolo.
>>try changing filter caps and check resistors in the B+ line. Might just get lucky.
Thanks Steve for everything.
>>all's we can do is throw spitballs in the dark. Perhaps you should try the 0.1u again, then live with it until you have time to trace what is really in there so it can be analyzed.
I hear ya.
I'll have to put this aside for the moment until I can dig deeper and come up with something concrete and more pertinent.
Thanks for taking the time.