Strange problem with C-lator...(Solved!)

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chikov
Posts: 271
Joined: Tue Nov 09, 2010 8:19 pm
Location: Houston, TX

Strange problem with C-lator...(Solved!)

Post by chikov »

About 3 years ago I built C-lator from a kit. It seemed to be working well, as it should. A couple of days ago I noticed that the unit was producing a very high pitch sound (with no guitar playing). I opened it up and pocked around with my bamboo chop stick and noticed that the noise stopped when i wiggled the 12ax7 tube. I replaced it and the noise stopped, but I wanted to know what caused the tube to go bad. I tested the plate voltages and noticed that on the pin 1 it was 347VDC (instead of 256V) and on pin 6 it had 223VD (instead on 368V). I checked the voltages on the node 1 (aka D) - it was 348VDC and node 2 (E) -346VDC...According to the schematic it should be 368 and 256 VDC... Does anybody know what might be wrong? When I check a voltage drop across 100K 3w resistor it gives me only 1 V(!)...
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Last edited by chikov on Tue Sep 05, 2017 1:53 am, edited 1 time in total.
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xtian
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Re: Strange problem with C-lator...

Post by xtian »

Right, so when you made those voltage measurements, the tube was not conducting, therefore, no voltage drop. I'm sure if you had measured the voltage across the cathode resistors, you would have seen no voltage drop there, either.

Now that you put in a good tube, do you get the correct voltages?
I build and repair tube amps. http://amps.monkeymatic.com
chikov
Posts: 271
Joined: Tue Nov 09, 2010 8:19 pm
Location: Houston, TX

Re: Strange problem with C-lator...

Post by chikov »

No, no correct voltages with a good tube either... The input triode that should have (according to a given schematic) 256V on its plate (pin 1) still has 350V (too much for 12AX7 I think).On the Cathode (pin 3) I have 322 (!) VDC instead of 31V... The output triode has 225V on pin 6 and 350VDC on node 1... The unit works, but I do not like this high plate voltage at all...
chikov
Posts: 271
Joined: Tue Nov 09, 2010 8:19 pm
Location: Houston, TX

Re: Strange problem with C-lator...(Solved!)

Post by chikov »

I just figured it out. I knew that the problem had something to do with the first stage, so I started tracing the voltages and lo and behold, the green wire that grounds Cathode resistor R27k did brake off the ground solder joint. As soon as I soldered back, the plate voltage dropped to about 245VDC and the cathode was about 28V. it is much better. I will leave the unit on for an hour or so and if high pitch tone will not come back, i am in business. Thank you for even small help - I felt I was not alone :-)))
Bombacaototal
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Re: Strange problem with C-lator...(Solved!)

Post by Bombacaototal »

I was wondering if a D'lator type of unit is necessary if you use studio grade gear in the lool (instead of pedal for example).
A couple of example of a studio gear would be a Bricasti M7 or a Fulltone TERC
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xtian
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Re: Strange problem with C-lator...(Solved!)

Post by xtian »

That was my point, and I’m sure you’ll recognize this in the future. If the plate voltage is at full B+, and the cathode voltage is 0, the tube is not conducting. You found the issue and solved it! Good job.
I build and repair tube amps. http://amps.monkeymatic.com
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