Clementine Ultra, Need help

sweet & juicy SE amp for 1626 Darlings and the 6L6 family

Clementine Ultra, Need help

Postby zippy1540 » Sat Jun 01, 2013 10:22 am

I have a PtoP clementine ultra i built a while ago. Yesterday i fired it up and got a horrible screeching / motor boating from one speaker. I switched power tubes with no effect. I changed input tube with no effect. All resistances appear fine. Voltage measurements come out almost right on except for pin 5 of the 6L6. On the bad channel there i measure about -20 VDC the other measures about +.03VDC. It's worked great until yesterday. When i pull the input 6SL7 the pin 5 voltages are at 0. These are all with the inputs shorted. What would cause this change?
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Re: Clementine Ultra, Need help

Postby zippy1540 » Sat Jun 01, 2013 11:49 am

Adding to the issue, When i power it up about 5 seconds into the warmup the one output transformer starts a sweeping buzz from a high freq to a low freq with only an 8ohm resistor connected to the output and the input shorted. These are new Edcor GSXE 5K-8ohm transformers. Did i get a bad output transformer from Edcor?
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Re: Clementine Ultra, Need help

Postby miker » Sun Jun 02, 2013 8:03 am

With the inputs shorted, pin 5 of the output tubes should be 0. Did you swap the output tubes from left to right to see if the problem follows the tube? You could also have a leaking coupling cap.
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Re: Clementine Ultra, Need help

Postby zippy1540 » Sun Jun 02, 2013 8:38 am

The problem did not follow the tube. I did try to change out the coupling cap on the offending side with no success. The one i used was NOS so maybe that one was bad also. I'll try to change both out this morning with some orange drops i have. Could this be a bad output transformer? What else would cause -20 VDC to show up on pin 5? Anything on the input stage? when i pull the input tube and power it up pin 5 of both tubes reads 0.
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Re: Clementine Ultra, Need help

Postby Shannon Parks » Sun Jun 02, 2013 10:20 am

With the power off, just measure pin 5 resistance to ground on both V2 & V3. I think your grid resistor may be the wrong value.

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Re: Clementine Ultra, Need help

Postby zippy1540 » Sun Jun 02, 2013 12:03 pm

Pin 5 to Ground on both tubes measures 218ohm. That was my thought also but it looks good. The thing is it was running fine for months. the only thing i changed was the output transformers about three weeks ago. They also ran fine until the other day. There was a little popping upon start up for a few days before that but it played fine. Coupling Caps????
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Re: Clementine Ultra, Need help

Postby miker » Sun Jun 02, 2013 1:15 pm

Did you mean 218K ohms? It should be around 220K.

Make sure all solder connections are sound. Try a different coupling cap.

Output transformers are not prone to failure but, I'm sure it happens on rare occasion. Can you substitute back in the original outputs to make sure it's not in the circuit somewhere?
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Re: Clementine Ultra, Need help

Postby zippy1540 » Sun Jun 02, 2013 2:05 pm

Your correct it was 220K. Dang fat fingers. I'll try changing out the coupling caps. I think i have some .1mF orange drops. That'll at least confirm if they are the problem or not.
Thanks,
Paul
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Re: Clementine Ultra, Need help

Postby zippy1540 » Sun Jun 02, 2013 7:50 pm

I can confirm, the coupling caps are not the issue. Replaced them and after about 10seconds of warmup the squeel comes out of the speakers. Next step is to change out the outputs. Could this be a power supply cap problem? None of them look bad but is it possible that would cause the problem? i noticed i scratched the end with a drill bit when i was drilling for the new output transformers. it is the third one in the series and feeding the B+ to the outputs.
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Re: Clementine Ultra, Need help

Postby miker » Mon Jun 03, 2013 9:45 am

I assume from your original post that the plate and cathode voltages read correct on both channels (pins 3 and 8).

If so, I doubt you have a power supply issue.

Did you check the screen grid voltage at pin 4?

Is the feedback circuit still connected properly after you switched transformers?
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Re: Clementine Ultra, Need help

Postby Shannon Parks » Tue Jun 04, 2013 4:01 am

Good idea, disconnect the feedback for clarity. But that grid voltage is whacked - home in on that. Is the socket bad and not making connection on pin 5?

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Re: Clementine Ultra, Need help

Postby zippy1540 » Tue Jun 04, 2013 6:55 am

i know the socket is good, new belkin that has been working fine till now. I'll pull the screen grids when i get home from work and see if that helps. It was tough to get a good voltage reading last night on the screens or plates since the voltage would not stabilize. they are both still around 375v though.
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Re: Clementine Ultra, Need help

Postby zippy1540 » Wed Jun 05, 2013 7:03 pm

Ok i found part of the problem. When i changed transformers i had the feedback connected to the negative side of one output. I fixed that and now it doesn't have that nasty speaker killing squeel. I'm reading 375 on both 6L6GC pin 3 and 381 on pin 4. With edcor transformers i have the blue wire connected to pin 3 and the white with blue stripe connected to pin 4. My pin 5 is reading about .012V and pin 1-8 reads 30V. Shouldn't the screen be lower voltage than the plate? did edcor get the wire color code switched?
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Re: Clementine Ultra, Need help

Postby miker » Thu Jun 06, 2013 8:31 am

Paul,

It's not a problem for the screen voltage to be slightly higher than the plate. Your readings look normal. Often a 1K resistor is used between the U/L tap and the screen to lower the voltage a bit. I can check my Ultra voltages later but, i think they are very similar to what you have.

Does it sound good now? Do you like the Edcore outputs better than the original OT's?
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Re: Clementine Ultra, Need help

Postby Thermion » Fri Jun 07, 2013 6:06 am

zippy1540 wrote:Adding to the issue, When i power it up about 5 seconds into the warmup the one output transformer starts a sweeping buzz from a high freq to a low freq with only an 8ohm resistor connected to the output and the input shorted. These are new Edcor GSXE 5K-8ohm transformers. Did i get a bad output transformer from Edcor?


I just received new transformers from Edcor last week. Including a pair of GXSE 5k-8s. On Monday I received an email from Phyllis at Edcor indicating there may be a problem with the transformers. I tested them, and sure enough, there was a problem. Edcor shipped my replacement yesterday at no charge. I recommend you test the transformers and see if they are OK.

They said that the primary windings are out of phase. They pass a simple DC resistance check, but have little to no output when driven by an ac signal.

JT
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