Howdy all!
I’m fixing up a Dynaco SCA-35. There seem to be two great options to modernize the amp’s power supply using a custom PCB. I’m wondering if anybody has tried both to compare?
Firstly the EFB update that is so highly regarded. Needs no introduction!
Daves Store: Dynaco SCA-35 ST-35 EFB Upgrades and Restoration
Secondly the SDS Labs capacitor board. Still allows modern caps, diodes and separate cathode bypass resistors but with no other circuit changes. I like that it doesn’t add an ongoing bias adjustment requirement for the output tubes: Dynaco SCA-35 Capacitor Board w/Parts
The question is partly about performance but I’m also thinking about selling the amp eventually. I know there’s no such thing as a zero maintenance vintage tube amp but... for somebody who might just want to roll in some different EL84s, is the need for bias adjustment on the EFB too technical for the this amp’ typical buyer? Or is EFB too good to miss?
I wasn’t going to mess with the tone controls or phono circuit as it’s otherwuse very original so I’m not chasing ultimate performance, just a nice reliable amp.
Thanks in advance y’all.
I’m fixing up a Dynaco SCA-35. There seem to be two great options to modernize the amp’s power supply using a custom PCB. I’m wondering if anybody has tried both to compare?
Firstly the EFB update that is so highly regarded. Needs no introduction!
Daves Store: Dynaco SCA-35 ST-35 EFB Upgrades and Restoration
Secondly the SDS Labs capacitor board. Still allows modern caps, diodes and separate cathode bypass resistors but with no other circuit changes. I like that it doesn’t add an ongoing bias adjustment requirement for the output tubes: Dynaco SCA-35 Capacitor Board w/Parts
The question is partly about performance but I’m also thinking about selling the amp eventually. I know there’s no such thing as a zero maintenance vintage tube amp but... for somebody who might just want to roll in some different EL84s, is the need for bias adjustment on the EFB too technical for the this amp’ typical buyer? Or is EFB too good to miss?
I wasn’t going to mess with the tone controls or phono circuit as it’s otherwuse very original so I’m not chasing ultimate performance, just a nice reliable amp.
Thanks in advance y’all.
FWIW, boards for Dyna ST35's using type 7247/12DW7 bolt right into the SCA35. You'll probably want to eliminate the "tone stack" (love that term) because sensitivity is lower.
All good fortune,
Chris
All good fortune,
Chris
Keep the boards, or if 7199 are in need of replacements get PC-10A from gillespie.
Keep tonecontrols, as any destruction of original will reduce value (even if reversible)
Exceptions is EFB where value should increase as these are documented solidly.
Keep tonecontrols, as any destruction of original will reduce value (even if reversible)
Exceptions is EFB where value should increase as these are documented solidly.
efb and st-35 boards in sca-35 chassis
I just (yesterday) finished converting from the SCA-35 boards to ST-35 boards. I eliminated the tone and balance controls and replaced the volume control with an ALPS. I also installed the EFB board.
I've done the same on a st-35 chassis. Built it stock and then moved to EFB.
No question, the Gillespie EFB is a significant improvement, if only to run the tubes cooler. the lower distortion is a bonus.
The option of using separate cathode resistors, per the Triode/Stokes board, is thoroughly discussed in Gillespie's article on the Tronola website. It is highly not recommended.
If your amp boards are original, you probably need to replace the tube sockets. And the coupling caps. And the out of spec resistors.
May as well replace the board and have done with it.
The ST-35 board probably will not have enough gain with the tone controls installed. Without the tone controls you will have an st-35 in an sca-35 chassis, with a volume control. And no 7199 replacement troubles.
Good luck with your project!
I just (yesterday) finished converting from the SCA-35 boards to ST-35 boards. I eliminated the tone and balance controls and replaced the volume control with an ALPS. I also installed the EFB board.
I've done the same on a st-35 chassis. Built it stock and then moved to EFB.
No question, the Gillespie EFB is a significant improvement, if only to run the tubes cooler. the lower distortion is a bonus.
The option of using separate cathode resistors, per the Triode/Stokes board, is thoroughly discussed in Gillespie's article on the Tronola website. It is highly not recommended.
If your amp boards are original, you probably need to replace the tube sockets. And the coupling caps. And the out of spec resistors.
May as well replace the board and have done with it.
The ST-35 board probably will not have enough gain with the tone controls installed. Without the tone controls you will have an st-35 in an sca-35 chassis, with a volume control. And no 7199 replacement troubles.
Good luck with your project!
I have a 3rd option available that puts a DC heater with transformer on the same PCB. Also has adjustable fixed bias and replaces the selenium diodes and multi-section cap.
Sorry ... confusing it with the ST amps. Regardless, my PCB has on-board ultrafast diodes.
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