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04-27-2023, 04:48 PM
Hi Guys
Re: TUT Fig.6-11 right side:
R1+R2 can only be used as a grid-leak for the output tube if the output stage is cathode-biased. My comments above are looking at the splitter in isolation. With fixed-bias, there would be a coupling cap into each power tube grid.
In the context of cathode-bias, the description above applies.
If you want the net grid-circuit resistance to be 220k, start with a 250kA pot (very common value) and make R1,2 around 2M5. This would likely be a 2M2 for R1 and then something between 22k and 240k depending on the gain of the second triode. You might want to use a 1M trimmer for R2 just to dial in the closest output balance.
For either bias method, this addendum applies
POP shows other splitter variations that have cathode-coupling between the triodes (Fig.4-20), forcing one to use two pot sections at the output or one pot section at the input for the single-pot PS solution.
Regarding old amps: The resistors used might be E24 20% cracked carbon if very old, carbon film if not so ancient. In any case, 20% resistor values around the tubes do not control things too tightly. In your new builds, you likely have 5% values, and even using 1% will not make the tubes work exactly as you might hope. Since the left-side circuit has free-running stages, balance of output is at the mercy of the tube gain spread (do not waste money on "matched sections") and on the circuit tolerances. Presumably, using such a splitter today means you are looking for some of that imbalance to add fatness to the tone?
When you look at the right-hand circuit and those from POP, you would expect excellent output matching, but that's not what you get. Despite throwing away all of the tube's voltage gain to have unity-inverting, the gain will be a bit lower because the amount of gain sacrificed is still relatively small compared to what ideal feedback control demands. This might be okay.
The input resistor becomes a load to the free-running input stage for the PA, along with the grid-leak circuit of the first power tube. The free-running stage may not exhibit its maximum or desired gain, but at least it is running free. The see-saw stage is supposed to provide exactly -1 gain and you might have to tweek one of the feedback Rs to get there.
In any case, because the signal goes through an extra tube to get to one half of the output stage, that signal will be a bit more distorted, or "harmonically rich" compared to the drive into the other side. This will produce a warmer tone without there being noticeable imbalance between the stages.
*As far as I know, the triode sections cannot be tested until they are in a vacuum, so once they are within the full glass envelope the deed is done.
Re: TUT Fig.6-11 right side:
R1+R2 can only be used as a grid-leak for the output tube if the output stage is cathode-biased. My comments above are looking at the splitter in isolation. With fixed-bias, there would be a coupling cap into each power tube grid.
In the context of cathode-bias, the description above applies.
If you want the net grid-circuit resistance to be 220k, start with a 250kA pot (very common value) and make R1,2 around 2M5. This would likely be a 2M2 for R1 and then something between 22k and 240k depending on the gain of the second triode. You might want to use a 1M trimmer for R2 just to dial in the closest output balance.
For either bias method, this addendum applies
POP shows other splitter variations that have cathode-coupling between the triodes (Fig.4-20), forcing one to use two pot sections at the output or one pot section at the input for the single-pot PS solution.
Regarding old amps: The resistors used might be E24 20% cracked carbon if very old, carbon film if not so ancient. In any case, 20% resistor values around the tubes do not control things too tightly. In your new builds, you likely have 5% values, and even using 1% will not make the tubes work exactly as you might hope. Since the left-side circuit has free-running stages, balance of output is at the mercy of the tube gain spread (do not waste money on "matched sections") and on the circuit tolerances. Presumably, using such a splitter today means you are looking for some of that imbalance to add fatness to the tone?
When you look at the right-hand circuit and those from POP, you would expect excellent output matching, but that's not what you get. Despite throwing away all of the tube's voltage gain to have unity-inverting, the gain will be a bit lower because the amount of gain sacrificed is still relatively small compared to what ideal feedback control demands. This might be okay.
The input resistor becomes a load to the free-running input stage for the PA, along with the grid-leak circuit of the first power tube. The free-running stage may not exhibit its maximum or desired gain, but at least it is running free. The see-saw stage is supposed to provide exactly -1 gain and you might have to tweek one of the feedback Rs to get there.
In any case, because the signal goes through an extra tube to get to one half of the output stage, that signal will be a bit more distorted, or "harmonically rich" compared to the drive into the other side. This will produce a warmer tone without there being noticeable imbalance between the stages.
*As far as I know, the triode sections cannot be tested until they are in a vacuum, so once they are within the full glass envelope the deed is done.
