Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jonas on May 01, 2025, 10:25:32 am
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I'm planning on building a stand alone reverb unit based on the AMPEG G12 reverb (Capacitance driven).
The reverb circuit is V6a (reverb preamp) - - -> V6b (reverb send) - - -> V7a (reverb recovery)
My initial thoughts are to start the circuit at V6a without the guitar preamp (no V4 or V5). The 6CG7 only has a gain factor of 20
My question is would V6a (6CG7) have enough gain to drive the reverb unit or would it be advised to keep the guitar preamp?
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would V6a (6CG7) have enough gain?
Capable of 5.7W with both triodes wired in parallel but you’d want a fairly lowish plate resistor (10k) to drive the pan good (assuming B+ of 400, plate idle voltage of 250v, and bias voltage of 8V - which would mean between 470 and 560 ohm cathode resistor delivering 9mA current) assuming both triodes are in parallel
Alternatively, if you used the 6CG7 to drive a reverb transformer with a reflected load of 20k and a plate idle voltage of 400V with 16V bias voltage (with a 2k cathode resistor) you’d get 8mA out of the tube, but a step up in current from the transformer secondary to drive the pan’s input transducer) and more headroom (because of the high grid to cathode voltage). (But the tube would wear out faster)
Someone please feel free to check my math- it’s 4am here as I write this and I’m tanked up on tramadol recovering in hospital from knee replacement surgery the day before yesterday- so I haven’t had a lot of sleep.
And it uses a single 6.3VAC filament @ 600mA
https://frank.pocnet.net/sheets/127/6/6CG7.pdf (https://frank.pocnet.net/sheets/127/6/6CG7.pdf)
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Are you still intending to use the reverb with guitar, or is this a line-level studio device now? I think with V6 alone you will have barely enough gain for use with guitar, if you add cathode bypass caps. But for line level it should be fine.
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[Are you still intending to use the reverb with guitar, or is this a line-level studio device now?]
This will be for guitar signal levels. I was thinking the gain would be too low which is why I asked. Would it be preferrable to preamp with a cathode follower or plate coupled stage?
[I’m tanked up on tramadol recovering in hospital]
Thanks for post!!!! Hopefully I can keep you entertained with my questions!! I did not think of paralleling tube halves here. Maybe the beefier ECC99 would be a better fit to drive the tank?
Hope you get better soon!!![/quote][/quote]
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I think you should be able to set the 6CG7 up to get enough current to drive the reverb pan input transducer using just one triode. (You want about 8mA). Note you will need a decent power handling rating for the cathode and plate resistors because you want those resistances low enough to source a decent amount of current. (And amps x volts = watts)
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I'm planning on building a stand alone reverb unit based on the AMPEG G12 reverb (Capacitance driven).
The reverb circuit is V6a (reverb preamp) - - -> V6b (reverb send) - - -> V7a (reverb recovery)
My initial thoughts are to start the circuit at V6a without the guitar preamp (no V4 or V5). The 6CG7 only has a gain factor of 20
My question is would V6a (6CG7) have enough gain to drive the reverb unit or would it be advised to keep the guitar preamp?
What if you built the Reverberocket circuit (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_r12r_reverb_rocket.pdf) instead? Meaning, "the Amp that put 'reverb' in Ampeg".
- Copy all of V2 and V3.
- See 6SN7 = 12AU7 = 6CG7 near enough (assuming you have a preference for one of those tubes over another).
In the schematic:
- V2 Pins 1-3 are "Dry Signal Gain Stage."
- V3 Pins 1-3 are "Reverb Driver Stage."
- V3 Pins 4-6 are "Reverb Recovery Gain Stage."
- V2 Pins 4-6 are "Dry/Reverb Mixer Gain Stage."
- R15 into R16 is the path that couples Dry Signal from the first gain stage to the Mixer gain stage. The Reverb Signal enters the grid of that stage.
No fuss, no muss. If you need a low-impedance output, make life easy & insert a JFET source-follower (which looks exactly like a 12AX7 cathode follower) between the Dry/Reverb Mixer Gain Stage and the output jack.
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Someone please feel free to check my math- it’s 4am here as I write this and I’m tanked up on tramadol recovering in hospital from knee replacement surgery the day before yesterday- so I haven’t had a lot of sleep.
Get well soon & best regards.
--Pete
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What if you built the Reverberocket circuit instead?
Yes, I like this idea, thanks!!!!
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ok starting to move forward following the suggestion of HotBluePlates Ampeg Reverberocket V2 AND V3 with a few questions...
1) can anyone help me identify the original reverberocket voltage at nodes D, E, and F?
2) will the Hammond 261G6 be a good choice? (250VCT at 130mA with 2A heater winding) I plan to have some relays and potentially add a tube preamp so the 2A winding may be needed especially if I use the original 6CG7 at 0.45A each
3) Should I follow the original Ampeg power supply with all the filtering stages and drop down resistors (prior to nodes D, E, and F)
4) For grounding, do I recommend I separate the first filter stage immediately after the rectifier?
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If its a 6V6 powered model, a safe bet would be a PT that goes with a Princeton Reverb (to give you a B+ +/- 400V) i.e. with an HT winding around 340-0-340 or 350-0-350. Its not that critical YMMV
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This will be a stand alone reverb unit with no power section no tube rectifier. Basically V2 and V3 from the reverberocket
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Then supply voltage is not critical . I’d built that with a 350V B+
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ok thanks I will shot for B+ of 350
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Then supply voltage is not critical. I’d built that with a 350V B+
Isn't the B+ going to run high with a small load of only 2 @ 12_ _7's?
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Then supply voltage is not critical. I’d built that with a 350V B+
Isn't the B+ going to run high with a small load of only 2 @ 12_ _7's?
As I said, it’s not critical. BF Fender preamp is around 400v supply. I think 350 is good for a clean preamp. Got options. YMMV
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ok starting to move forward following the suggestion of HotBluePlates Ampeg Reverberocket V2 AND V3 with a few questions...
1) can anyone help me identify the original reverberocket voltage at nodes D, E, and F?
2) will the Hammond 261G6 be a good choice? (250VCT at 130mA with 2A heater winding) I plan to have some relays and potentially add a tube preamp so the 2A winding may be needed especially if I use the original 6CG7 at 0.45A each
This post claims (https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/page-2#post-11430809) to have voltages from an Ampeg Service Manual for the R-12-R Reverberocket.
Shoot low for voltage: more like a 300v supply rather than a 400+ volt supply.
Using the post & the R-12-R schematic, node voltages appear to be about:
D: 240v
E: 202v
F: 157v
You probably would like a stage or so of filtering before "Node D" so something around 300v DC would be plenty. I see you ask about the Hammond that has a 250v CT secondary, so I'm guessing you're thinking of a bridge rectifier? The Hammond 261D6 (https://www.hammfg.com/files/parts/pdf/261D6.pdf) is massive overkill for the high voltage winding, so I'd pick it even though a pair of 6CG7s will pull slightly more heater current than it claims (overall, I think the transformer will still be under-loaded).
Since this PT choice will output around 350v DC at the first filter cap, you will need a resistor before the "Node D" filter. Total current is about 16mA, so you would need about 6.8kΩ 5w to drop from the 1st filter cap if "Node D" is your 2nd filter cap. Otherwise, you can split the resistancee (and heat) across more resistors.
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looking for advice on reverb tank / reverb jack grounding.... I have a type 'C' tank input isolated output grounded.
do i want to float the both send and return RCA jacks at the chassis, or should one of them be grounded?
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looking for advice on reverb tank / reverb jack grounding.... I have a type 'C' tank input isolated output grounded.
do i want to float the both send and return RCA jacks at the chassis, or should one of them be grounded?
In that case you would only want to have your send and return connected to the circuit? And the reverb tank chassis will be grounded to the output of the reverb tank.
Also EF80 is a good tube for cap driven reverb.
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looking for advice on reverb tank / reverb jack grounding.... I have a type 'C' tank input isolated output grounded.
do i want to float the both send and return RCA jacks at the chassis, or should one of them be grounded?
Fender had the both RCA jacks at the amp grounded to the chassis. To break the ground-loop that would result, they used a tank like what you have: Input isolated (from the tank-chassis), Output grounded (to the tank-chassis).
The Output was grounded because it is the weakest signal (weaker than a guitar pickup), so the shielding afforded by the tank-chassis, through the shielded output cable, to the amp's RCA jack/chassis was good for reducing noise.