What's up with my Reverb Rocket reissue? (V4 screens)

[eddie25]

I have a 90’s ‘SLM’ Ampeg Reverb Rocket reissue with EL34’s that I can’t turn past 6 on the master before the V4 screen grid starts getting REALLY stressed out under signal conditions. You can see V5 pulsing, but nothing unusual. V4 will be way brighter and you can hear it screaming, way more than normal. The amp worked fine for a while even at these volume levels, but the tubes were crackling and microphonic (so I stupidly tossed them before realizing this issue).

The screens are only at about 415 when it’s biased properly, and it happens with cold bias as well (which raises the screens to 430+) . Never seen any side of red-plating. It already caused an EL34EH tube to start glowing out of control and smoking the screen resistor (I think… the back was on), which were 470 ohms. I switched the screen resistors to 1K5 and nothing really changed with the pair of Valve Art EL34’s in there now.

The only strange reading I can find is the resistance between the OT primary legs to center-tap. 104 ohms on the normal side (V5) and 73 ohms on V4, but according to the voltage across them at idle, they are drawing similar amounts of current. Voltages and bias are consistent between the tubes.

It’s a PCB socket mounted amp so I also went over the connections on the power tubes and that didn’t help. And of course I switched the tubes around and it stays at V4.

I’m scratching my head wondering if the previous set of tubes, Svetlana’s, were just stronger, or if maybe I have a partially shorted OT winding, or something else? What could cause just one of the screens to be under so much stress and not the other?

[Dr-Joned]

From what you are describing, it appears that the output tranny could be partially shorted. If you look at the "load" on each tube, it appears that V4 is very heavily loaded compared to V5 side. Can you remove V4 & V5 or unplug and do a low voltage check on the transformer to check the correct turns ratio? I think this would be a step in the right direction.

[Structo]

Could you have a problem with the diodes on V4?

[eddie25]

I put a wall-wart across the primary real quick. It measured a 14.3VAC output and I measured a steady .582VAC at the 8 ohm secondary. From the primary CT to each side I get 1.16VAC on the secondary for both.

14.3/.582 = 24.57 Turns Ratio

4.8K primary impedance.

Seems normal right?

[eddie25]

Could you have a problem with the diodes on V4?

I checked those plate diodes in circuit and they all measured how a 1N4007 always does with my meters diode test.

[eddie25]

Is it typical that the power tube on the out of phase side of the LTP phase inverter experiences more screen dissipation?