Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on October 20, 2011, 04:47:21 pm
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I have a 1961 Super Amp that i am having a time with getting the Vibrato to work. I have replaced 6 caps in the Vibrato section
I have one resistor that is suppose to be 8.2M. It is reading 9.5M. Cathode and other voltages on the schematic are showing right on my DVM
I am using a 5U4GB recitifier tube instead of a GZ34 which this amp calls for. The voltages are a bit lower. It also shows 5881 on the tube chart
I have no oscillation. This should be working. I am over looking something. :help:
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Take a look at page 9 of my Scrapbook (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf) for some theory of operation.
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Interesting reading. I liike the idea that it does not breath when guitar signal is not applied. I have been using the method of turning up the volume to hear when i get breathing. So i cant do that with this one. It is a fairly long circuit? Well i will get back on it tommorrow. Thanks for that info Steve. I cant be too far off from getting it working
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Steve i am not sure what i am seeing for voltage on the schematic at the 470K cathode joining the 8.2M resistor
I think it is showing 175VDC but i am only getting 10VDC. All the other voltages on the schematic for the Vibrato check out right
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I found one major problem. Some one unsoldered the plate lead on V3 connecting from pin #1 over to pin #7
I jumpered pin 1 over to pin 7 V3 and i now have 160VDC on the cathode of V3B pin 8
Still no Vibrato. I am soldering the missing lead in place now
I solder wicked and pulled the 470K lead through and over to pin 7
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Well i soldered the 470K over to pin 7 and cleaned up my bench. Turned the amp on and BINGO! Really nice warm vibrato. Way different that what some of us are accustomed to hearing. Why would someone remove the 470K lead and kill the Vibrato on this? Trying to get more gain probabaly but i cant really hear any gain loss. Im deaf anyway
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Probably a case of "a little knowledge is dangerous."
On a different forum, someone commented on an amp that was just like a Princeton Reverb, but had no "tone-sucking trem." They were not aware that most of the larger blackface amps have the optoisolator trem circuit, but the Princeton uses a bias-vary trem. They assumed the removal of the trem circuit would somehow make the amp sound better.
Maybe someone made the same error with that Super, and looked for a way to kill the trem circuit.
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Well they sure sacrified the best part of the amp doing it. And for no benifit. It sounds amazing and actually usefiul. So quiet!
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The bias vary trem on my Vibro Champ does lower the gain of the amp a little even when off due to the 50K intensity pot going to ground. No need to disconnect the trem entirely as the previous owner of my amp did, though. Since my amp was not in collector's shape I installed a DPDT switch to activate/deactivate the trem. One side shorts the oscillator like the stock vibrato pedal would and the other side opens the path to that 50K pot. It's subtle but noticeable. Did the same thing on my old opto Super Reverb.
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The bias vary trem on my Vibro Champ does lower the gain of the amp a little even when off due to the 50K intensity pot going to ground.
The stock value of that pot is 25k, and is parallel with the 1.5k cathode resistor (well, 1547 ohms, after feedback resistor).
Since the pot is more than 10x the value of the cathode resistor, if you do the math you'll find it reduces the value of the cathode resistor less than the 10% tolerance of a typical part. So it can't have any significant effect on the operation of the signal path.