Something is wrong. (Solved!)

for the DIY ST35, the Dynakit and every other PP EL84

Something is wrong. (Solved!)

Postby corndog71 » Sun Jun 12, 2016 5:16 pm

I recently decided to rebuild my ST35 into a smaller enclosure. It uses Shannon's driver circuit with EFB and just one bias point per channel. Unfortunately, I got overconfident and screwed up some wiring. Upon powering up the amp I just got a really loud tone. I had missed a wire connecting the high voltage at cap 2 to the 360K resistor in the EFB circuit. As far as I can tell I didn't blow up any resistors or caps though I suspect the LM337 may have been damaged. I fixed the miswire and quadruple checked everything else. The most obvious problem I'm now encountering is the high voltage seems higher than it should be.

With 117vac going into the amp I get the following measurements. This is with no tubes as I didn't want to damage them.

PT secondary - 641vac

1st cap - 418vdc

2nd cap - 418vdc

3rd cap - 418vdc

V1 plates - 405vdc

V2 plates - 418vdc

LM337 - out - 2.08vdc, adj - 8.92vdc

It seems weird that the B+ doesn't really drop after the 6.8K resistor. I replaced it to make sure and got the same results. I'm suspicious of the LM337 so I went ahead and put in a new one. I haven't powered it up yet but I thought I would post this for any possible help.
Last edited by corndog71 on Thu Jun 16, 2016 11:48 pm, edited 1 time in total.
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Re: Something is wrong.

Postby battradio » Sun Jun 12, 2016 10:55 pm

Disconnect the feedback loop and see if the tone goes away ,
you may have to swap the blue and brown leads of the output transfomers along with the ultra linear leads
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Re: Something is wrong.

Postby TomMcNally » Mon Jun 13, 2016 7:27 pm

Rob -

>>> It seems weird that the B+ doesn't really drop after the 6.8K resistor.

None of your DC voltages will make any sense with the tubes removed, as there is no current being drawn in the circuit. A resistor doesn't "resist" without a load.
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Re: Something is wrong.

Postby corndog71 » Wed Jun 15, 2016 7:52 am

TomMcNally wrote:Rob -

>>> It seems weird that the B+ doesn't really drop after the 6.8K resistor.

None of your DC voltages will make any sense with the tubes removed, as there is no current being drawn in the circuit. A resistor doesn't "resist" without a load.



Right. Duh. That makes sense. I guess I'll just cross my fingers and try it again.
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Re: Something is wrong.

Postby corndog71 » Thu Jun 16, 2016 11:47 pm

Whew! I figured it out. The feedback was off. I may have fried the old regulator. Also, I needed to drop the AC to 116vac. Thankfully my tubes survived. It sounds amazing and is dead quiet with shorted inputs. This is my 3rd time rebuilding this amp.

http://i576.photobucket.com/albums/ss20 ... dtzvl9.jpg

http://i576.photobucket.com/albums/ss20 ... vnt60k.jpg

http://i576.photobucket.com/albums/ss20 ... es5nne.jpg

http://i576.photobucket.com/albums/ss20 ... uuzivy.jpg
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