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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

LTP mega gain

hey folks

repairing an amp and have gone down a long hole and need a sense check! The remaining problem is the LTP in this 1990 custom vibrolux reverb seems to be going mad.

Here is a cut of the schematic and I've annotated it with measured voltages. I've swapped the valve base as it looks damaged, I've tried different valves. Each of the resistors measures to spec. I tried replacing C18 with no effect. I've already replaced all filter caps as there was an appaling hum which has now gone, gvien eveything a clean etc.

schematic annotated.jpg

The gain of the LTP is almost double what it should be and I cannot figure out why that 71V is there when it should be 17.4. R36 is 1M resistor and doesn't seem to be dropping the signal to the 5.6Vp-p that is should, I'm getign 16Vp-p

I feel likes its really obvious or am I going mad?!

Guys and girls, I'd appreciate your thoughts. many, many thanks

Jan - Have measured that 71Vdc a few times as it seems odd. It would go through 39k + 820r to ground so I get 1.8mA? Theres 1.5Vdc across R39 and the rest over the 39K

Sorry - the 71Vdc is cathode to ground voltage, the schematic suggests it should be 17.4

12AX7 is the tube
ha, interesting, yes if the 17.4 is a typo then I'm OK. Still can't figure out why I measure 16Vpp on the far side of R36 (1Meg)

Based on the schematic with 5.2Vpp entering the LTP and 33.5 Vrms (46.9Vpp) ie 9 times. I've measuring 61Vrms (85 Vpp), but the valve is in full clipping. So it seems the LTP is working fine? Just done't know why the input is so high, especially when coming out of V4B (previous stage I'm getting the correct (roughly) 10vrms
Sorry, re the 61Vrms - I was just pointing out that it was so high teh 12AX7 was clipping so I could never have measured the full p-p.

I'm wondering if I just need to lower the gain of the LTP. Following the signal through the various stages its seems all to spec until the LTP. It might be that everythings OK but the gain is too hot (I've no idea why it would be as everything is as per the schematic).

as an example - putting into an 8 ohm dummy load I'm getting 55W output power with the volume set at 1. On the schematic, into R36 is 10.7Vrms (based on putting 83mVrms into the front end and setting the volume to 5), HOWEVER, with the volume at 1 (roughly where the LTP starts to clip) I measure 1.73 Vrms into R36.

AX tech editor
Joined 2002
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Well, first things first. He doubts whether there's something wrong with the amp. You can't check that while it is clipping.
So turn it down so it no longer clips.
Then check if the gain of the amp is as advertised.
If yes, take the next step and check the gain structure.
If not, debug the amp.
Guys, this is basic trouble shooting. Divide and conquer.

Joined 2011
How do you know that your input signal is not too high?
Post a clear readable copy of the complete schematic.

Have you measured the input signal, and does it meet spec?
If not, how do you know the input is not too high?
Last edited:
I once borrowed out my amplifier. My friend fed his tape output into the moving coil input. Yes it clipped. My HF speakrs (filtered 6dB) crashed along.
Sure it all aounded ‘different’. Sure, the dial was below ‘1’ or 10% or how you might describe it.
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