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Hi, I'm new to this forum-- but think I have some valuable info for the list.
Had an amp in for repair-- single owner prior to my client's purchase. I found not just a legible 98RT schematic, but the bonus Reverb Converter schematic (on a fold).
Please note that this schematic seems to have some errors
1) The plate resistor for the reverb driver was 2.2k on the amp.
2) Missing some filter caps. Compare with a same vintage Ampeg R12R, it is missing 20uF filter caps on nodes S and T. Without these the reverb control will act as a gain knob since the resistor on the reverb driver isn't a high value. I also found that hum dropped when I added another filter cap to node U. As the amp had been worked on before me, I can't confirm the stock arrangement.
Since the input jacks are grounded to chassis-- care must be taken to avoid ground loops if you replace the can cap with individual caps. I converted to start ground. More specifically, what I did was remove the can cap, install a hole plug and mount a terminal strip on the screw from that cap. I removed the bus wire that is under the tag board (it was nice and rusty). All the power supply resistors that were cracked and failing on the amp were mounted on that creating room for 3 filter caps on the tag board. 2 more were fit on the board near the preamp stages by moving components around a bit. The final "bonus" cap was installed by with a zip tie to a ground wire bundle to avoid more extensive tag board changes. Grounds from each preamp stage (including pot) are run to the star point (ground of first filter cap. The backs of the pot are not grounded.
After that work and replacement of the usual suspects, the amp sounds great-- I wasn't happy returning it! It is interesting to note that amp arrived with a Amperex 5AR4 and some Sylvania 6L6GCs. The amp had been running that way for a least a few decades and that setup had a lot more clarity than a 5Y3/JJ 6V6s combo. In comparison, the later was compressed sounding which wasn't the voicing my client wanted.
I definitely prefer the reverb tone to a transformer coupled design. I was sad to give it up, but I have a late 50's 66-J. Alas, no octal preamp tubes...
Had an amp in for repair-- single owner prior to my client's purchase. I found not just a legible 98RT schematic, but the bonus Reverb Converter schematic (on a fold).
Please note that this schematic seems to have some errors
1) The plate resistor for the reverb driver was 2.2k on the amp.
2) Missing some filter caps. Compare with a same vintage Ampeg R12R, it is missing 20uF filter caps on nodes S and T. Without these the reverb control will act as a gain knob since the resistor on the reverb driver isn't a high value. I also found that hum dropped when I added another filter cap to node U. As the amp had been worked on before me, I can't confirm the stock arrangement.
Since the input jacks are grounded to chassis-- care must be taken to avoid ground loops if you replace the can cap with individual caps. I converted to start ground. More specifically, what I did was remove the can cap, install a hole plug and mount a terminal strip on the screw from that cap. I removed the bus wire that is under the tag board (it was nice and rusty). All the power supply resistors that were cracked and failing on the amp were mounted on that creating room for 3 filter caps on the tag board. 2 more were fit on the board near the preamp stages by moving components around a bit. The final "bonus" cap was installed by with a zip tie to a ground wire bundle to avoid more extensive tag board changes. Grounds from each preamp stage (including pot) are run to the star point (ground of first filter cap. The backs of the pot are not grounded.
After that work and replacement of the usual suspects, the amp sounds great-- I wasn't happy returning it! It is interesting to note that amp arrived with a Amperex 5AR4 and some Sylvania 6L6GCs. The amp had been running that way for a least a few decades and that setup had a lot more clarity than a 5Y3/JJ 6V6s combo. In comparison, the later was compressed sounding which wasn't the voicing my client wanted.
I definitely prefer the reverb tone to a transformer coupled design. I was sad to give it up, but I have a late 50's 66-J. Alas, no octal preamp tubes...