Improving “back-bias” PP amp?

[duffy878]

I’ve got a 50’s Danelectro Challenger 6L6 PP amp. It’s one of the amps that uses the power tube cathode voltage to heat the preamp tubes, which I’ve read is a form of back biasing. I’m wondering if there would be improvement by adding a cathode cap on the power tubes, like a common cathode biased design. It’s running 2 12AX7’s in series off the 20V cathode, so I think that should be 160 ohms? Would adding a 50uf e.cap increase gain and keep that voltage more constant or am I thinking about this wrong?

[Helmholtz]

From your circuit description the bias voltage is -20V and the 6L6s are running in class A.
With a balanced pure class A PP amp there's no cathode signal and a bypass cap would make no difference.
Doesn't hurt to try, though.

Do you have a schematic?

[duffy878]

From your circuit description the bias voltage is -20V and the 6L6s are running in class A.
With a balanced pure class A PP amp there's no cathode signal and a bypass cap would make no difference.
Doesn't hurt to try, though.

Do you have a schematic?

Okay, interesting. I think that makes sense to me. I guess I should tack on a cap and see what happens.

Unfortunately I can’t find an exact schematic, but it is nearly identical to the Silvertone 1335- https://el34world.com/charts/Schemat...rtone_1335.pdf

[Helmholtz]

Let me know if you like some more explanation.

[duffy878]

Let me know if you like some more explanation.

Yes! Between the “back-bias” configuration and the 6SN7 diff amp after the PI, this seems like a pretty unique arrangement and I’d love to understand it better. There doesn’t seem to be much information out there, at least not that I have found. I also find it interesting that they chose to use a bias modulating trem circuit (a 6AU6 in the Challenger), which obviously modulates the preamp heater voltages…

[Helmholtz]

I can only comment on the Silvertone schematic you provided.

It's not a typical "back bias" circuit where a negative bias voltage is produced by a voltage dropping resistor in the HT ground wiring.
Don't ask me if the term is correct here.

The cathodyne PI has a gain just below 1 and cannnot provide enough grid drive for the 6L6s. So it's followed by a differential amp. Needs to be a differential amp because it has to amplify 2 opposite phase signals.
No idea why they didn't just use a LTPI instead - maybe they went for a somewhat better linearity and balance, i.e. more "hifi".

The tremolo signal wiggles the grid voltage of the 6L6s, thereby modulating their gain.
IMO, modulating preamp heater voltage wouldn't be very effective.

[duffy878]

I guess it’s not traditional back bias but I don’t know what else to call it. If you think of the heater filament resistance as the cathode resistor, it’s sort of like cathode bias, right?

Yes, it is odd they chose to use the diff amp and cathodyne PI instead of a LTPI.

Regarding the trem- by modulating the grid voltage, the cathode voltage also modulates. Since the cathode voltage is used to heat the preamp tubes, it seems like a bad design choice. Would it be worth it to try to get the cathode voltage higher, since it is heating two 12AX7’s? I could lower the grid resistors to do this, correct?

[Helmholtz]

I guess it’s not traditional back bias but I don’t know what else to call it. If you think of the heater filament resistance as the cathode resistor, it’s sort of like cathode bias, right?

Yes it's cathode bias. Why do you need to give it a name?

Regarding the trem- by modulating the grid voltage, the cathode voltage also modulates. Since the cathode voltage is used to heat the preamp tubes, it seems like a bad design choice. Would it be worth it to try to get the cathode voltage higher, since it is heating two 12AX7’s? I could lower the grid resistors to do this, correct?

Yes, if the grid voltage is modulated, the cathode voltage will follow to some extent (cathode follower effect), as it's a common mode signal.
Actually this is not desirable as it lowers the tremolo depth.
A large enough cathode bypass cap might actually increase the tremolo depth.

A higher cathode voltage would bias the power tubes colder.
I think the preamp tubes should be fine with the low heater voltage, might even increase heater life.
Lowering the grid (leak) resistors will not change the bias or the cathode voltage, as those resistors don't drop voltage.
Can't think of a good way to increase the cathode voltage.
Of course you could modify to a heater transformer.

I would check if the cathode/heater voltage increases at large output.
If the increase is more than 15%, operation shifts to class AB and a bypass cap makes sense.

What is wrong with the amp (except that it should be safety-grounded)?

[duffy878]

Yes it's cathode bias. Why do you need to give it a name?




Yes, if the grid voltage is modulated, the cathode voltage will follow to some extent (cathode follower effect), as it's a common mode signal.
Actually this is not desirable as it lowers the tremolo depth.
A large enough cathode bypass cap might actually increase the tremolo depth.

A higher cathode voltage would bias the power tubes colder.
I think the preamp tubes should be fine with the low heater voltage, might even increase heater life.
Lowering the grid (leak) resistors will not change the bias or the cathode voltage, as those resistors don't drop voltage.
Can't think of a good way to increase the cathode voltage.
Of course you could modify to a heater transformer.

I would check if the cathode/heater voltage increases at large output.
If the increase is more than 15%, operation shifts to class AB and a bypass cap makes sense.

What is wrong with the amp (except that it should be safety-grounded)?

I don’t need it named, it’s just easier to talk about if it was to have one.

I already did the 3-prog, filter caps, and drifted resistors. I also found a wiring error in the diff amp (missing the 15k leg). It’s running well now and much quieter than when I started. I’m really just looking to learn something and see if there is any improvement to be made. I know these were budget amps at the time and corners were cut.

With the trem all the way up, the cathode voltage swings between 18 and 25 volts. When it is off it is roughly 20 volts. I don’t use the trem much, but 25 is over 15% so I think the cap makes sense. I do not have time to crank it and measure today, but what value cap do you think would suffice? You said a large value would deepen the trem. If each series 12AX7 heater winding is 80ish ohms, that would be 160ish ohms total, so 100u? 220u? Thanks again!

[Helmholtz]

With the trem all the way up, the cathode voltage swings between 18 and 25 volts. When it is off it is roughly 20 volts. I don’t use the trem much, but 25 is over 15% so I think the cap makes sense. I do not have time to crank it and measure today, but what value cap do you think would suffice? You said a large value would deepen the trem. If each series 12AX7 heater winding is 80ish ohms, that would be 160ish ohms total, so 100u? 220u? Thanks again!

The above 15% increase of cathode voltage I mentioned above refers to large output with normal signal - not tremolo.

Regarding the bypass cap for increasing tremolo depth, capacitance needs to be large because the tremolo frequency is very low.
I would try 220µ up to 1000µ, maybe even more.

[SoulFetish]

This was a common configuration in some Silvertone Amps of a certain era (the 1474 comes to mind). I remember the first time I saw it, I thought someone had made a grave wiring mistake.
I had never seen anything like it before, but, after I had become familiar with the amp and the whole line of Silvertone amps, I grew to appreciate all the nuanced and unique quality of their designs. This particular circuit is a clever design Silvertone used bias their 6L6 amps and create a fairly stable 12V DC filament supply for the 12AX7 preamp tubes. It's efficient, quiet, inexpensive, and cuts down on the parts count where available chassis space was at a premium.
In the models I've worked on, I've observed that for whatever reason, the heaters run a bit under voltage at about 20-21V for the pair of tubes in series, in spite of subtle changes I've made to try and alter the cathode voltage. The other thing is that these models seems to take a bit longer to warm up and start conducting, but neither of these things seem to cause any observable negative effects to the performance of the tubes or amp.
To comment on a question you posted earlier; since the heater/cathode bias supply is DC, there is no problem with adding a bypass capacitor from cathode to ground.
Interestingly, in the Silvertone 1474, they run that filament circuit in parallel with a resistor(s) because of the additional current demand of a quad of 6L6s instead of the pair.

[duffy878]

This was a common configuration in some Silvertone Amps of a certain era (the 1474 comes to mind). I remember the first time I saw it, I thought someone had made a grave wiring mistake.
I had never seen anything like it before, but, after I had become familiar with the amp and the whole line of Silvertone amps, I grew to appreciate all the nuanced and unique quality of their designs. This particular circuit is a clever design Silvertone used bias their 6L6 amps and create a fairly stable 12V DC filament supply for the 12AX7 preamp tubes. It's efficient, quiet, inexpensive, and cuts down on the parts count where available chassis space was at a premium.
In the models I've worked on, I've observed that for whatever reason, the heaters run a bit under voltage at about 20-21V for the pair of tubes in series, in spite of subtle changes I've made to try and alter the cathode voltage. The other thing is that these models seems to take a bit longer to warm up and start conducting, but neither of these things seem to cause any observable negative effects to the performance of the tubes or amp.
To comment on a question you posted earlier; since the heater/cathode bias supply is DC, there is no problem with adding a bypass capacitor from cathode to ground.
Interestingly, in the Silvertone 1474, they run that filament circuit in parallel with a resistor(s) because of the additional current demand of a quad of 6L6s instead of the pair.

The amp is surprisingly quiet now that I’ve fixed the 6SN7 diff amp!

I tried to tweak some values and also could not get the cathode voltage to change much. Is there potentially some sort of current limiting happening? Either the 5Y3 being undersized, or through the draw of the 2x 12AX7 heaters on the cathode?

Do you think a cathode cap would help? I wonder if at high volumes and the power tubes are really working, if a cap would help keep things steady for the heaters.

[Helmholtz]

Consider this:
If at large output the cathode voltage rises by e.g. 30% (like with a Vox AC30), the heater voltage will be perfect.
Now if cathode voltage already was perfect at idle, it would get dangerously high for the heaters at large output.

[duffy878]

Consider this:
If at large output the cathode voltage rises by e.g. 30% (like with a Vox AC30), the heater voltage will be perfect.
Now if cathode voltage already was perfect at idle, it would get dangerously high for the heaters at large output.

Okay, understood! I was thinking the increased current draw would lower the voltage for some reason, but that isn’t how it works. I will try this out when I have time to open it up.

[mozz]

It's a good use for all those 6au6's. Now if I could find a use for 1B3's id be set. Could always irradite old bottles and make them purple.