My dynaco sca 35 seems to hate RCA’s 7199 and 6GH8A.
When I bought my sca 35 on ebay, it came with one original dynaco 7199 and one RCA 7199. The circuitry is original. I did swap out some caps and all the power tubes.
Later on, I found it would loose one channel’s output only when it is hot, and I swapped 7199 of two channels, the issue follow the one channel with RCA 7199. Then, I ordered 2 presumably “new” 6GH8A tubes with adapters. Unfortunately, it came with one RCA and one GE. I found the RCA 6GH8A has the same issue with the old RCA 7199 tube.
It only happens when it is hot. I guess it is related to heater-cathode leakage.
What do you think.
When I bought my sca 35 on ebay, it came with one original dynaco 7199 and one RCA 7199. The circuitry is original. I did swap out some caps and all the power tubes.
Later on, I found it would loose one channel’s output only when it is hot, and I swapped 7199 of two channels, the issue follow the one channel with RCA 7199. Then, I ordered 2 presumably “new” 6GH8A tubes with adapters. Unfortunately, it came with one RCA and one GE. I found the RCA 6GH8A has the same issue with the old RCA 7199 tube.
It only happens when it is hot. I guess it is related to heater-cathode leakage.
What do you think.
Problem with the pc-10 board. Get new ones, preferably for 6gh8 tubes and sell the 7199 to recup the cost of the boards.
Just googled regarding your suggestion. Are you referring pc-10a?
Dynaco SCA-35 Revisited - 1973shovel - Tube DIY Asylum
That one is promising. Thank you.
I have replaced the original PCBs in my SCA-35 with Dave Gillespie’s boards and have been very pleased. If you could stretch the budget, also do the EFB power supply board; that really elevates the old SCA-35 to a new level.
See: Daves Store: Dynaco SCA-35 ST-35 EFB Upgrades and Restoration
See: Daves Store: Dynaco SCA-35 ST-35 EFB Upgrades and Restoration
Yes, thats the one.
Dave also has a power supply that extends the capability of the power tubes "EFB" . It also replaces the can caps.
If the 7199 filament is not elevated to a reasonable DC voltage, it may be the problem you describe. + 60V is a good value, use a 2 resistor divider off of the B+, and a bypass cap from the divider junction to ground.
The cathode of the triode (concertina stage) is at about 80v to 100V; requires an elevated filament.
In a similar circuit, the Dyna Stereo 70 floats both 6.3V filament windings (one for L and one for R channel tubes). The only connections to ground were ceramic bypass caps.
One 7199 triode section from my ST70 had leakage of 100k from filament to cathode when the tube was cold (who knows how low the resistance was when hot, I did not test that).
A pair of resistors and bypass cap to elevate the filaments kept the 7199 tube's filaments to cathodes from wearing out and becoming leaky.
The cathode of the triode (concertina stage) is at about 80v to 100V; requires an elevated filament.
In a similar circuit, the Dyna Stereo 70 floats both 6.3V filament windings (one for L and one for R channel tubes). The only connections to ground were ceramic bypass caps.
One 7199 triode section from my ST70 had leakage of 100k from filament to cathode when the tube was cold (who knows how low the resistance was when hot, I did not test that).
A pair of resistors and bypass cap to elevate the filaments kept the 7199 tube's filaments to cathodes from wearing out and becoming leaky.
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100K leakage probably is on verge of unusable. The output of phase inverter is not balanced in your case.
I measured mine. There is no measurable resistance for the one 7199 that I have issue.
Elevated filament makes sense. It should be done.
There are 2 major quirks for this original sca-35 board.
1. The pentode is biased with grid leak current. It biases through 4.7M (not 470K) to ground.
2. The pentode's screen grid is biased with the resistor to the cathode of triode side. Not all 7199/6GH8 tubes are happy about this setup.
The st-70 board does not have those issues mentioned above. It should be more generous to 7199/6GH8 switching.
Switching to the newer PC-10a board on my sca-35 should solve those issues as well. However, I plan just go with 6U8 for now, since they are working fine.
Caution:
If you are using a PCB, you will have to make an adapter; or you will have to cut traces on the PCB.
The pin-outs of the 7199 i unique. The pin-outs of the 6GH8 and 6U8 are different than the 7199.
ZZZZap!
You may already know that.
But there are new-bees that will read this thread, and they may not know that.
Grid Leak Bias is good for RF detectors.
Grid Leak Bias sometimes are good for some input tube types on guitar amplifiers, but not if the input jack is DC coupled to the input grid.
. . . Grid Leak Bias for Hi Fi / Stereo . . . Why???
If you are using a PCB, you will have to make an adapter; or you will have to cut traces on the PCB.
The pin-outs of the 7199 i unique. The pin-outs of the 6GH8 and 6U8 are different than the 7199.
ZZZZap!
You may already know that.
But there are new-bees that will read this thread, and they may not know that.
Grid Leak Bias is good for RF detectors.
Grid Leak Bias sometimes are good for some input tube types on guitar amplifiers, but not if the input jack is DC coupled to the input grid.
. . . Grid Leak Bias for Hi Fi / Stereo . . . Why???
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I did some mods since I started this thread. It can work with 6GH8 now.
Here are what I found.
1. One of my RCA 7199 is just bad. It doesn't work on my dynaco SCA-35. It doesn't work on my dynaco st-70, either.
2. 6GH8 seems not to work with the original circuitry of SCA-35. The problem is not the oscillation. The problem is 6GH8 could not establish bias properly. Based on SCA-35 manual, the screen voltage is 32v, the cathode voltage is 0.7v. My solution is to series an extra resistor to the cathode, to bias grid more negative relative to cathode, about -1.5v. Thus, the screen voltage will rise to about 50v. Also put a bypass cap on that resistor just added, so that AC path is not affected by this change.
3. Grid leak bias adds more variable to the bias point. It has to go. Simply short the grid DC blocking capacitor C17 with a wire.
Here are what I found.
1. One of my RCA 7199 is just bad. It doesn't work on my dynaco SCA-35. It doesn't work on my dynaco st-70, either.
2. 6GH8 seems not to work with the original circuitry of SCA-35. The problem is not the oscillation. The problem is 6GH8 could not establish bias properly. Based on SCA-35 manual, the screen voltage is 32v, the cathode voltage is 0.7v. My solution is to series an extra resistor to the cathode, to bias grid more negative relative to cathode, about -1.5v. Thus, the screen voltage will rise to about 50v. Also put a bypass cap on that resistor just added, so that AC path is not affected by this change.
3. Grid leak bias adds more variable to the bias point. It has to go. Simply short the grid DC blocking capacitor C17 with a wire.
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This tubes are not electrical equivalents, so for sure the circuit must be modified if you want optimum operation for hi-fi ... not just some sound .
Like swapping between 12AX7 12AU7 12AT7 family .
Like swapping between 12AX7 12AU7 12AT7 family .
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