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Discussion in 'Glowing Bottle Tube Amp Forum' started by mojo2001, Feb 7, 2015.
oops- double post.
Hi robrob, I don't know if it's bias-based or not. I never noticed the bias being modulated but I'll have to take a closer look! And technically it's vibrato, not tremolo, so I'm not sure if that can be achieved with bias modulation but this thing is a bit of a mystery. I started reading the patent last night but didn't get far before sleep set in
Agreed about the temps inside the amp. I used the heat gun just to see if the problem changed with temperature (kind of the opposite of using chiller to isolate a faulty component). When it did effect a change (no pun) I thought maybe the caps had drifted over time. Of course, trying to focus the heat on a single component was not so easy, so it was hard to tell exactly which component(s) were most affected by the heat. Anyway, checked a few of the caps that appeared to be in the line of fire of the heat and they both tested pretty much right on the money (.0047).
This made me question the transistors since the beta would be affected by temperature. I had tested all the transistors before and they were all functional, but maybe they were marginal? Replaced one of them that seemed to be most sensitive to heat and... YES! the intensity/depth of modulation jumped up! Replaced one other one and it improved even more! BTW, I used 2N4401s to replace the 2N3341s. P.S. BTW, the transistors are all 2N3341, even though they are color-coded with either a blue, red, or white dab of paint on top. I wonder if this was some sort of sorting process to find transistors with the right characteristics (hfe?) for the different parts of the vibrato circuit? Similar to how the varistor-based circuit required a lot of sorting to find varistors with the right characteristics?
sounds like these amps don't use the varistors.
does the vibrato still sound the same?
and what happens when you plug in extension speakers?
I really don't have experience with Magnatones, but I have two of the big Audio Guild 12/8 amps, and the vibrato is killer. Every one of them sounds different though. The two I have now are the second and third ones I've owned, all identical, and each one sounds different.
I have a Versatone Pan-o-Flex (Model 133) that needs some service.
I'm looking for a reliable and trustworthy place to get it serviced somewhere in the Los Angeles area.
It has a bad (read: loud/annoying) "ground hum", which I assume is due to something like a transistor, transformer, wiring, or perhaps a dying tube related issue.
I'm not an electrical engineer, so I really need a good amp doctor that can better diagnose (and hopefully fix it).
Any suggestions would be much appreciated.
Given how rare these amps are, and the lack of documentation of the circuit, unless it is something obvious like it needs new filter caps to reduce the hum, it may not be easy to find someone who can repair it without investing a fair amount of time investigating the circuit. I would suggest Rob the Amp Man in Monrovia, simply because he repaired an old Ampeg for me that three other techs couldn't figure out and he liked the challenge. I have two of these amps, plus the bass version (which is killer), so I am a big fan of Audio Guild amps.