The Amp Garage

The original RS deLuxe OT was rated for 15/7.5/3.75 Ohm secondary impedances.
When used with standard speaker impedances the 6.6k primary impedance increases to just above 7k.
My Marstran RS clone measures 7030 Ohm without and 7550 Ohm with transformer resistance included at the "6.6k" primary.

To me a "JTM45" with EL34s and a 3.4k OT is not a JTM45. When Marshall changed to EL34s with a different OT, the amp was renamed JTM50 and later JMP50. So if you want a JTM45 sound, use KT66s with a 7k to 8k OT. EL34s will give more power (around 50W) while the original amp with KT66s and 8k OT only...

Just as with plate voltage the screen voltage limit applies between screen and cathode and not to ground.
A tube can't know what its pin voltages are wrt ground.

An MOV (metal oxide varistor) is a bidirectional voltage dependent resistor used to clamp and absorb voltage spikes above its threshold voltage.
It cannot be used to limit an inrush current like an NTC (thermistor).

Interesting.
So the culprit is the LTPI.
Your scope pic clearly shows a shifted duty cycle, so one tube conducts longer than the other.

OT phase is correct when disconnecting the feedback increases gain/output signal.

I have input sine amplitude set to produce 100W output into a resistive load, then just remove the two tubes on one side and rerun. Thanks. I'm surprized by the huge increase in PD. Grid signal, bias and plate load should essentially be the same. Leaves less plate and screen voltage sag, but I didn...

Here you can see the E shaped laminations with the center leg, which is in parallel with the shorter side:
http://jamestransformer.com/uploads/all ... nation.jpg

martin manning wrote: ↑Mon May 19, 2025 10:08 pm Plate dissipation went from 21.8 (72%) to 36.8W (123%) per tube..

Interesting.
I assume this is not at idle?

pottedplant wrote: ↑Mon May 19, 2025 10:20 pm I guess I'm just really dumb because I do not understand what you mean by shorter sides

Shorter side is the one with the dimension F in this picture:
http://jamestransformer.com/uploads/all ... mer-3d.jpg

The amp in the photo has the transformers with their shorter sides in parallel. Means that the coils' axes are in parallel.
That's not optimal.
Would better with the OT rotated by 90°.

Which one has the axis' in parallel according to you? I tried finding what you were talking about but I'm unsure. In the images above it all seems like they're perpendicular but you see one that isn't so I am very curious to learn which one to expand my knowledge I was referring to the pictures sho...

Apologies, I forgot to mention I am using a PT for a Tremolux/Vibrolux so seems voltages will be a little lower... Measurements from my original 1970 Vibrolux Rev. (with GZ34): - PT HT winding: 340V - 0 - 340V in Standby, - B+: 422V at idle, - Bias voltage: -41V @ 38mA idle cathode current, - Clean...

PT HT voltage (from your thread on the TGP) and B+ are considerably lower than original.
So will be the screen voltage.
The lower the screen voltage, the higher (i.e. less negative) the grid voltage needs to be for the same idle current.

For least coupling the magnetic axes of the PT and the OT should be at right angles. The magnetic axis is defined by the coil. As the coil winds around the center leg of the core, you want the center legs of the transformers to be at right angles for least coupling. So you need to guess or know the ...