Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Sonny ReVerb on October 21, 2018, 12:08:48 pm
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I built my own version of a Revibe. Since I like my Reverberocket R12-R build so much, I decided to use an Ampeg circuit (instead of Fender) for the reverb side.
The vibrato side works great. The reverb, however, is somewhat weak. Also, when I turn the vibrato level to zero, there is very little output from the reverb side. You can hear reverb spring jangle, but that's about it. Seems like I should have decent output from the reverb side regardless of the vibrato level. I might try an audio probe to see what kind of signal I get along the reverb stages. Any glaring errors in the schematic? Suggestions?
[edit - removed partial schematic]
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Show us the ENTIRE circuit.
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Show us the ENTIRE circuit.
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Nice work. Let me try again. Show us the entire schematic. :wink:
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Nice work. Let me try again. Show us the entire schematic. :wink:
removed
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what's the tube compliment?
tank may be loaded too much. try 100k-220k for grid leak of reverb recovery stage - V3a. if V2a-b is a 6FQ7 then the 10k/100k plate divider is not helping. bypass R6 and/or R10 to compensate that's easy enough to do and non-invasive test if yous some clip-leads.
--pete
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Thanks, man. Tubes: V1,V3,V4 - 12AX7; V2 - 6CG7(equivalent to 6FQ7)
The split load of V1-A only passes ~9% of the signal, correct? I guess with the 6CG7 only having an amplification factor of 20 it would need more signal passed to it?
Ampeg used 22k vs Fender's 220k to ground at the output of the tank (grid leak of V3-A?). I thought that might dump too much signal to ground for this application?
The reverb circuit was taken from the Ampeg G12:
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Duh, finally got it. Sorry. Sounds like you're on the right track. I would swap places with R2 and R3 and change R11 to 220K. Any better?
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After looking at the G12 schematic I think your main problem is not enough drive to the reverb tank. The G12 has two gain stages in front of the 6CG7 circuit. But you only have a single triode before and you're only using about 10% of it's signal. In addition to the other suggestions, try replacing R8 with a 220K or even a 470K. Also try a 10µF to 25µF cap parallel to R6.
All of these things together may put you in surf reverb territory. So you may need to juggle 'til you're happy.
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I would swap places with R2 and R3 and change R11 to 220K. Any better?
Yup, yup, that seems to be the ticket. I tried both of these suggestions and it's definitely in the ballpark now. It may be on the edge of too much, so I may try to adjust the split load resistors. I tested it on the normal channel of a Vibrolux Reverb and A/B'd it with the Fender reverb, which is a bit brighter. Thanks, Steve!
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another thing to note - the dry signal path has gain whereas the 6g15 uses a cathode follower so no gain in the dry path - that coupled with the lower gain of the tank driver is likely why the apparent weak reverb.
you could try reconfiguring V1-b as a cathode follower - you'll need to bootstrap bias or add a bias resistor voltage divider pair with an input cap.
--pete