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.
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
Pat
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.
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
Pat
Either the tube(s) are too hot, too much current, or the cathode R has grown too large. Did you check the resistor value?
What's the DC grid bias, is that high too? You list the AC voltage but the workingn point is set by the DC.
Jan
What's the DC grid bias, is that high too? You list the AC voltage but the workingn point is set by the DC.
Jan
so the 71Vdc is DC to ground. The bias on both sides is -1.4Vdc
sorry, as mentioned, all resistors measure fine
sorry, as mentioned, all resistors measure fine
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Did you measure with input shorted? 71VDC across something less than 1k is more than 70mA for two tubes.
The psu should collapse with that. How sure are you of that 71VDC?
What tube type is that?
Jan
The psu should collapse with that. How sure are you of that 71VDC?
What tube type is that?
Jan
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
Sorry - the 71Vdc is cathode to ground voltage, the schematic suggests it should be 17.4
12AX7 is the tube
I couldn't read the schematic I thought that was 390 ohms. OK, I get it, yes much less current.
But then 1.8mA through 39k + 820R is 71.7V which jives with your measured 71. So maybe the schematic annotation is off? It's hard to read anyway.
Jan
But then 1.8mA through 39k + 820R is 71.7V which jives with your measured 71. So maybe the schematic annotation is off? It's hard to read anyway.
Jan
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
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
First of all, stay out of clipping, that throws all bets off. Your RMS meter is also unreliable with a non-sinusoidal wave.
Did you measure the output voltage single ended or between the two outputs?
That's a factor of 2 difference.
Jan
Did you measure the output voltage single ended or between the two outputs?
That's a factor of 2 difference.
Jan
Vpp usually means peak- to - peak
so 33.5 Vrms corresponds to 94.7 Vpp for a sinusiodal signal
and 61 Vrms = 172.5 Vpp
Yes, you mean the peak voltage (from zero).
So 1V rms on a sine wave is 1V x 1.414 = 1.414V peak.
And twice that is 2.828V peak to peak (literally from the lowest point to the highest point).
So 1V rms on a sine wave is 1V x 1.414 = 1.414V peak.
And twice that is 2.828V peak to peak (literally from the lowest point to the highest point).
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.
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.
P
It's clipping with the volume at 1. 0 to 55W from 0 to 1 on the volume knob. I think the guy might want more range than that!
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.
jan
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.
jan
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?
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?
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