Tube Amp Archive

Smokebreak3 wrote: ↑Tue Oct 14, 2025 8:29 pm

Helmholtz wrote: ↑Tue Oct 07, 2025 10:01 pm What is the voltage at TP13 (V BIAS)?
What is the voltage "V LTP"?

Helmholtz, both points are -97VDC, which is also the same as I have at the grids of the EL34s

Then either R212 is shorted or TR4 doesn't conduct.

still, it will reduce voltage and current somewhat? Like any other screen node dropper or choke it lowers the the screen supply voltage when the average screen current increases - with a bit of delay due to the filter cap. A real screen resistor immediately reacts to screen current peaks by increas...

Yes, those 100R resistors are screen stoppers. Because of their low value they don't do much for screen current limiting. From tube literature (e.g. RDH4) it is known that directly paralleling power tubes invites RF oscillation. The screen stoppers provide some tube separation. The 400R in the power...

I think I may be noticing that the T config of bias power filter is used when it is tapped off an outer leg of the HT coil, and the simpler "L" config is use if there is a separate say, 60 or 70 vac bias tap, off the HT coil. When the bias voltage is derived from a high voltage PT secondary, large ...

So given the numbers, this winding really shouldn’t really make an audible difference then?

That's what I think.

Paralelling those two secondaries will lower the DC resistance of the secondary

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Yes, but only by 15% given the DCR numbers.
As total OT losses should be around 10% and around half of that is due to primary DCR, we're speaking of a power increase of less than 1%.

What is the voltage at TP13 (V BIAS)?
What is the voltage "V LTP"?

..idling each tube at 31 to 34 ma...

The range I suggested above is for cathode current of which 15% is screen current.
Plate idle current alone should not be below 30mA IME.

The plate resistor value of V2A has no influence on power tube bias. Power tube grid bias should not be measured at the grids (because the meter will pull down the voltage depending on meter input resistance). Rather it should be measured at point XF, i.e. between the 220k grid leaks. With EL34s I'd...

Have you got any thoughts on a more accurate way of describing / identifying it? I'd describe the principle as cathode autobias with a fixed, elevated grid voltage. (Cathode voltage is grid voltage + bias voltage required for the current Vk/Rk.) Same as with a DC coupled cathode follower or LTPI (l...

The original pi circuit used bootstrap biasing. The Paul C mod simply converts to fixed biasing. I don't know how the sound is affected. Your mod looks to be done correctly. The Paul C mod in the pic on the right uses the full 56k cathode resistor for autobias. It's not fixed bias as only the grid ...

. There is a tiny range of transconductance ("Mu") available, and all the tube stage gains involve mu as the major parameter. Mu is the ratio of plate current change divided by the change in grid voltage. Pretty much everything else falls out as a result of using resistors around the tube to use th...

How does the hum look at the speaker output?

As of post #1 the OP aready tried powering the heaters from a DC supply with positive results.

The scope pic looks like a typical AC heater supply voltage itself.
Wondering where this was scoped and if a x1 probe was used.

What harms the sound is a screen that connected to a saggy power supply that dips a lot when the screen current goes up. I tend to disagree. The stiffer the screen supply, the more pentodes/tetrodes sound like triodes. Being a blues/rock player I love the screen compression of vintage amps (think J...