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A client brought me a 60s Ampeg Reverbrocket II for some refurbishing and updating: Filter caps, cathode electrolytics, replacement of badly drifted resistors, tubes tested and replaced as necessary, bias checked and set, 3-prong AC cord, and a specific complaint about the reverb. The reverb in Ampegs is an acquired taste; it's not my cup of tea but I decided to keep an open mind. The client wanted his amp to have a very clean sound with a strong reverb sound for an instrument other than guitar. The reverb in this amp was very weak and after all the other work was completed, it didn't improve too much (I left the reverb-tweaking to the end). The reverb sounded very small and the reverb (echo) control had to be advanced alot to get even a moderate amount of reverb - suffice it to say it was not the voice of God. After examining the schematic, it seemed to me that the reverb signal was overly manipulated and filtered and could coaxed into a more natural sound - alot of signal was being dumped to ground.
First, I made sure there was a strong tube in both the drive and recovery sections of the reverb circuit. Next, I inspected and measured the reverb pan to make sure it was working optimally as were the interconnecting cables. Then I went to the component level.
Ampeg reverbs are usually capacitively coupled as opposed to Fender reverbs which are coupled to the reverb tank by a transformer. It made sense to mount a new high quality .47uf cap between the plate of V4-B (6U10, pin 10) and the input of the reverb tank in this important spot. But also from pin 10 but going to ground is a .02 cap. I lifted this cap from the plate side and found that it made a small but noticeable difference: the reverb sound was fuller and slightly less nasal. I heat-shrinked off the plate-side lead of this cap, left the other end connected to ground and tucked it under some parts on the circuitboard. The other thing I considered in the drive circuit was raising the value of the grid load resistor of V4-B (6U10, pin 9) from 220K to 470K but ended up not doing this, at least for now.
Now on to the recovery circuit. There were several caps on this side of the reverb pan that called for examination and experimentation. There is a .005 cap mounted directly on the 6U10 (V4-C) tube socket between the grid and the cathode (pins 7 to 6) but again I left this for later. Also, there is a .005 cap between the plate and the cathode of V4-C (6U10, pins 5 to 6). On the schematic that I had this was designated as a .0039uf cap but this small variation in value from the cap actually used is fairly typical with Ampeg. At any rate, I removed this cap and it made a big difference; alot more signal and alot more hi-fi - the reverb really opened up. Next, I paid attention to the cathode components of V4-C (pin 6). I replaced the 3.3K cathode resistor (which had drifted out of spec) with a 2.2K device and changed the .1uf bypass cap with a high quality .47uf cap. This raised the gain of the recovery circuit, moved the tone in the right direction and gave me more than enough reverb sound so now I was able to use the reverb control in the 9 to 11 o'clock range for a normal amount of verb. My last change was to change out the .002uf cap coming from the output of the reverb control (500K-Right lug) to a .0068uf cap. This put the finishing and smoothing touch on the sound and the job was complete; a much improved reverb sound with plenty of 'verb to spare. It could now do anything from normal reverb accompaniment to over-the-top surfs up!
This is what I thought my client was after, from our discussions we had. If this tweaking would have been for me personally, I would have taken it a bit farther. I would have experimented with a small-value bypass cap across the reverb control (somewhere between 68pf~24pf). Also, downstream of the aforementioned .002uf cap (now changed to .0068uf) is the 150K summing resistor. I would consider bypassing this resistor with a very small cap. Either or both of these ideas will add some air/top end to the reverb sound, something I'm really fond of in reverb, although it might make the reverb more fendery.
All of these changes were derived by trial and error. I usually clipped in several values of a new component and conducted copious listening tests until it was right to my ears. Of course, this is all subjective and YMMV. However, there is no doubt that if you're dissatisfied with your Reverbrocket reverb sound, this might just be the ticket for you!
Bob M.
Here's a summary:
Reverb Changes
Drive Circuit - 6U10 - V4-B
1) Replace .47uf cap between the plate, pin 10 and the input of the reverb tank.
2) Lift or remove .02uf cap (plate side) from the plate, pin 10 to ground.
3) Raise value of the grid load resistor, pin 9 to ground from 220K to 470K - no action taken now.
Recovery Circuit - 6U10 - V4-C
4) Remove or lessen .005uf cap mounted directly on the tube socket between the grid and the cathode (pins 7 to 6) - no action taken now.
5) Remove .005uf cap between the plate and the cathode (pins 5 to 6). On the schematic this was designated as a .0039uf cap.
6) Pin 6 - Replace 3.3K cathode resistor with 2.2K resistor. Replace .1uf bypass cap with .47uf cap.
7) Replace .002uf cap, coming from output of the reverb control (500K-Right lug) with .0068uf cap.
8) Add small-value bypass cap across the reverb control (somewhere between 68pf~24pf) - no action taken now.
9) Add small-value bypass cap across 150K summing resistor, downstream of .002uf reverb output cap (now changed to .0068uf) - no action taken now.
First, I made sure there was a strong tube in both the drive and recovery sections of the reverb circuit. Next, I inspected and measured the reverb pan to make sure it was working optimally as were the interconnecting cables. Then I went to the component level.
Ampeg reverbs are usually capacitively coupled as opposed to Fender reverbs which are coupled to the reverb tank by a transformer. It made sense to mount a new high quality .47uf cap between the plate of V4-B (6U10, pin 10) and the input of the reverb tank in this important spot. But also from pin 10 but going to ground is a .02 cap. I lifted this cap from the plate side and found that it made a small but noticeable difference: the reverb sound was fuller and slightly less nasal. I heat-shrinked off the plate-side lead of this cap, left the other end connected to ground and tucked it under some parts on the circuitboard. The other thing I considered in the drive circuit was raising the value of the grid load resistor of V4-B (6U10, pin 9) from 220K to 470K but ended up not doing this, at least for now.
Now on to the recovery circuit. There were several caps on this side of the reverb pan that called for examination and experimentation. There is a .005 cap mounted directly on the 6U10 (V4-C) tube socket between the grid and the cathode (pins 7 to 6) but again I left this for later. Also, there is a .005 cap between the plate and the cathode of V4-C (6U10, pins 5 to 6). On the schematic that I had this was designated as a .0039uf cap but this small variation in value from the cap actually used is fairly typical with Ampeg. At any rate, I removed this cap and it made a big difference; alot more signal and alot more hi-fi - the reverb really opened up. Next, I paid attention to the cathode components of V4-C (pin 6). I replaced the 3.3K cathode resistor (which had drifted out of spec) with a 2.2K device and changed the .1uf bypass cap with a high quality .47uf cap. This raised the gain of the recovery circuit, moved the tone in the right direction and gave me more than enough reverb sound so now I was able to use the reverb control in the 9 to 11 o'clock range for a normal amount of verb. My last change was to change out the .002uf cap coming from the output of the reverb control (500K-Right lug) to a .0068uf cap. This put the finishing and smoothing touch on the sound and the job was complete; a much improved reverb sound with plenty of 'verb to spare. It could now do anything from normal reverb accompaniment to over-the-top surfs up!
This is what I thought my client was after, from our discussions we had. If this tweaking would have been for me personally, I would have taken it a bit farther. I would have experimented with a small-value bypass cap across the reverb control (somewhere between 68pf~24pf). Also, downstream of the aforementioned .002uf cap (now changed to .0068uf) is the 150K summing resistor. I would consider bypassing this resistor with a very small cap. Either or both of these ideas will add some air/top end to the reverb sound, something I'm really fond of in reverb, although it might make the reverb more fendery.
All of these changes were derived by trial and error. I usually clipped in several values of a new component and conducted copious listening tests until it was right to my ears. Of course, this is all subjective and YMMV. However, there is no doubt that if you're dissatisfied with your Reverbrocket reverb sound, this might just be the ticket for you!
Bob M.
Here's a summary:
Reverb Changes
Drive Circuit - 6U10 - V4-B
1) Replace .47uf cap between the plate, pin 10 and the input of the reverb tank.
2) Lift or remove .02uf cap (plate side) from the plate, pin 10 to ground.
3) Raise value of the grid load resistor, pin 9 to ground from 220K to 470K - no action taken now.
Recovery Circuit - 6U10 - V4-C
4) Remove or lessen .005uf cap mounted directly on the tube socket between the grid and the cathode (pins 7 to 6) - no action taken now.
5) Remove .005uf cap between the plate and the cathode (pins 5 to 6). On the schematic this was designated as a .0039uf cap.
6) Pin 6 - Replace 3.3K cathode resistor with 2.2K resistor. Replace .1uf bypass cap with .47uf cap.
7) Replace .002uf cap, coming from output of the reverb control (500K-Right lug) with .0068uf cap.
8) Add small-value bypass cap across the reverb control (somewhere between 68pf~24pf) - no action taken now.
9) Add small-value bypass cap across 150K summing resistor, downstream of .002uf reverb output cap (now changed to .0068uf) - no action taken now.