Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on March 17, 2018, 10:10:55 am
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I have a Carvin Legacy on the bench with a PI issue.
I have no Vdrop across the Anode load resistor on the V5A "top triode of PI".
I get normal voltages when I pull the NFB coupling cap to that stages grid. In the original schematic it appears to be a 10uF non polar.
I then swapped that 10uF cap and was back to no VDrop across the anode resistor. okaaaaaayyyyyyyy???????
And, to boot I had DC voltage after that cap headed to the NFB connection on the OT side.
Strangely, I only see the voltage (approx. 31VDC) on the side headed to the NFB, here is my test points along that part of the circuit.
NFB side of 10uF > ribbon cable pins H5A I/J > H5B I/J > stopping on the top side of the R91/C70 connection on the NFB board
Again when I lift the 10uF at the PI grid, phantom voltages toward the NFB seem to go away and the PI exhibits "normal vdrop from both triodes in V5.
Any ideas here? I have swapped the PI tube to known working tube, no change.
Thanks
-joe
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Measure the voltage on the grid of V5B. Then replace the 10µF with a .1µF. What is the voltage on V5B grid now?
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Hey Sluckey!
Yeah it blocked DC from the NFB side. And I have Vdrop across the V5A anode. Yay!!!!! I have 80 volts at the grid. So can I go back to the 10uF or stick with the .1uf @250V?
So why did the 10uF non polar replacement not jive??? And if I needed to go polar would the cathode side of the cap be towards the NFB????
I don't understand why your recommendation worked, but I'm sure glad it did!
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Based on what you said, the 10µF was leaking and upsetting the bias on that LTP PI. You can use a polar e-cap with the positive end connected to the grid (because of the positive voltage). I would use at least a 150V rating.
Using a .1µF cap will change the NFB. Don't know if that's good or bad. I would rather use the same value of cap that was originally in the amp. What was the voltage rating of the original?
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Yeah that what I thought. It's a 10uF non polar @ 50v stock. The weird part is I swapped it originally with a 10uF non polar at 100V, and it had the same issue. But the .1uf "test cap" was fine.
Fool me once, fool me twice, ask Sluckey to bail me out. :)
Now that I know that's the issue, I'll go back to the 10uF till it's up and going so as not to fool with the response of the NFB.
Thanks
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50v rating is insufficient in my mind. You measured 80V on the grid with a non leaky cap. The actual voltage is even higher than that. Your meter loads the voltage down.
Now that I know that's the issue, I'll go back to the 10uF till it's up and going so as not to fool with the response of the NFB.
I would approach that differently. That 10µF is making your LTP PI tube misbehave and that will probably mask other problems you may have. I would put the .1 cap in and fix any other issues then come back and address the cap issue when the amp is otherwise working properly.
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Great advice, but with the PI biased correctly everything else seems cool. I'll leave it cook for a bit and check back later.
Again, thank you.
-joe
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The non-polar electrolytic leaks too much for this circuit. It "may" be OK now, if you wait to charge, but is sure to get leakier over time.
Non-polar also makes no sense: there's steady known DC, polar could be used, if that made sense.
10uFd here makes NO sense. That against the 360K is flat to 0.05Hz-- pointless for guitar.
Neither does 10pFd which doesn't do anything until 50KHz.
I think the drafting program defaults to a "10" and nobody put the right value in. Maybe it was quitting time, and didn't get picked-up the next morning.
Simple audio math suggests 360K wants a 0.03uFd cap to work over the audio band. 0.05u and 0.1u are reasonable values. 100X (10uFd) is NOT reasonable.
I calculate 88V here. This echoes your observations, which were meter-loaded. Prudence suggests 200V rating. Surely you have 0.05u-0.1u 400V film caps in stock?
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Well the amp was fine with a .1uF in that position. As soon as I add an electrolytic. (10uF @150V) anode toward grid of PI, it has the same problem.
HMMMMMMMmmmmm?????????
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Perfect PRR. Those calculations are a bit over my head as far as frequency response with regard to the NFB.
I sure do like the sound of a .03-.1uF in that position to cover useful guitar freq's.
Thanks, I'm gonna let it cool down and swap a 400V poly cap in that position.