Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Grantorino on June 13, 2026, 11:44:49 am
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Hello again EL34 World.
Working on a Marshall 1987X Plexi 50W head.
Plate voltage around 460V. Bias set to approximately 35-36mA per tube at idle, as judged with a bias probe into a DMM. Probe sits between tube and tube socket.
Owner runs the amp cranked into a Fryette Power Station reactive attenuator.
When injecting a 400Hz 0.1V signal, V5 peaks at 133mA and V4 peaks at 160mA.
During guitar play-testing, V5 peaks around 160mA and V4 peaks around 180-200mA+ during sustained chord playing.
Both tubes recover quickly to idle current when playing stops. No red plating observed during short play-test.
How should these readings be interpreted? The asymmetry between the two sockets is consistent and does not change if the power tubes are switched between the sockets.
Are consistent peaks around 180-200mA inherently a problem?
Is the asymmetry in current between the V4 and V5 socket within normal parameters?
Work done:
The amplifier has new Belton power tube sockets, the PI coupling caps were replaced, as were the 5W screen resistors. All board-mounted resistors are within tolerance, the output transformer primary halves are balanced within 0.9 ohms, the power and pre-amp tubes were swapped out with no major change to asymmetry or high current peaks on the V4 socket.
At idle, the power tube bias point is stable. Is this behaviour under signal typical for an EL34-driven amplifier?
I would be interested to hear any thoughts.
Thank you.
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it's "normal" for an amp to hit 2X power dissipation when driven deep into AB(2)
are you numbers "normal", not sure. the way I test, I set my meter to Record Min/Max/Avg when driving hard, then do some quicky math to see how far "outta spec" it gets at Max n Avg, then re-adjust idle bias if it's running to hot outta spec using the AVG value math.
what Resistance is on your screens?
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It's all normal.
AB amps pass more current with signal than at idle, overdriven fixed bias it can be loads more.
The asymmetry is normal with Marshall.
I don't think any regular valve guitar amps operate in AB2?
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AB2
I was in farmer mode when replying :laugh:
wasn't sure if this combo; Owner runs the amp cranked into a Fryette Power Station reactive attenuator.
would send one send it into unknown territory
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AB2
I was in farmer mode when replying :laugh:
wasn't sure if this combo; Owner runs the amp cranked into a Fryette Power Station reactive attenuator.
would send one send it into unknown territory
The uppermost anode characteristics plot usable in AB1 is Vg1=0. Above that, the control grid becomes forward biased and draws current from the signal source, clipping off the upper signal peaks unless the preceding driver stage can provide a high current signal.
Whereas in AB2, the driver stage is itself a power amp, that can provide lcurrent to pull the grid positive without the signal voltage becoming clipped.
Hence a key fundemental difference between AB1&2 is the driver stage, immediately preceding the output valves.
So however the anode is loaded, I can't see how it could affect whether AB2 operation is achievable?
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How should these readings be interpreted? The asymmetry between the two sockets is consistent and does not change if the power tubes are switched between the sockets.
The LTP is not perfectly balanced, especially during overdrive, so one of the power tubes gets driven a bit harder than the other. It's normal.
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... Marshall 1987X Plexi 50W head.
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When injecting a 400Hz 0.1V signal, V5 peaks at 133mA and V4 peaks at 160mA.
During guitar play-testing, V5 peaks around 160mA and V4 peaks around 180-200mA+ during sustained chord playing.
...
How should these readings be interpreted? ...
Are consistent peaks around 180-200mA inherently a problem?
...
A 50w Marshall's output transformer likely has a primary impedance around 3.4kΩ.
Power = Volts x Current, and
Volts = Current x Impedance, so
Power = (Current x Impedance) x Current = Current2 x Impedance, so
RMS Current = √(Power / Impedance) = √(50w / 3.4kΩ) = 121.3mA RMS
The Peak Value of a (RMS) Sine Wave is Current RMS x √2, so
121.3mA RMS x 1.414 = ~172mA peak
Your 50w 1987X should manage 50 watts clean (and perhaps even a bit more), and will approach something like twice this much when distorted (2x sine-wave output power is the limit, which happens when the signal is distorted to symmetrical square-waves).
Your observations seem to be perfectly in-line with "clean signal test" when applying the 100mV 400Hz test signal, and "somewhat distorted signal" with your play-test.