Low volume, possible cathodyne issue, help needed.

[g1]

kept the 12dw7 in the socket while resoldering and I have burnt out the tube grid I think, maaaaaaaaaaaaaan.

I don't think I've ever removed a tube to resolder a socket, I very much doubt you could damage a tube with heat.

[Chuck H]

I don't think I've ever removed a tube to resolder a socket, I very much doubt you could damage a tube with heat.

+1
The only time untubed sockets get soldered around here is when I'm still building the amp

I did once have a weird thing happen though. I wasn't even being particularly clumsy but somehow all the solder I was melting onto a tube pin just got sucked into the pin hole by capillary action. Obviously it was the last pin to be soldered and obviously I didn't have a replacement on hand

It's possible the socket that was being soldered just had some goo or flux heat up and start interfering with pin contact. Clean the socket and see if it gets better.

[wozt]

I had already ordered a replacement 12dw7, installed it and there was no improvement. I have rewired / resoldered around the the 12ax7 side of the 12dw7 to no avail. I can connect V1a+b output straight into the 12au7 side of the dw7 and all is good. So it is the 12ax7 side causing problems.

How do I clean the tube socket.

[Chuck H]

Clean the socket with "contact cleaner". any brand will do. put (or spray) a little bit onto the tube pins and insert the tube into the socket. Now remove it with a gentle and subtle oscillating motion. Now do it again. Wait for the solvents in the cleaner to evaporate before power the amp up.

But this may not be the problem. Usually just plugging a new tube in will scrape or clear debris enough to show you some intermittent issue so it seems unlikely that dirty sockets are your problem if you experienced no change whatever.

If the 12ax7 side of that socket has a cathode bypass cap make sure it's in correct polarity. If it's not, it's possible the cap shorted and is causing a bias problem. If the cap is in correct polarity then...

You were soldering on that socket. Why? If you were making circuit changes make certain everything is wired correctly. It's possible to look at something multiple times and still miss a mistake. If everything seems right and all voltages measure correct try remelting the solder joints on that tube pin (and anything else you redid in that last operation) with fresh flux in case there is a cold connection. If you use rosin core solder just remelt a little fresh solder if there's room. If there's already ample solder on the pins you'll want to suck it out and add new.

[wozt]

Cleaning didn't work, I installed a new socket and again no change. I remeasured everything and there is no dc on the cathode of the dw7/ax7. There is 305 HT and 305 on the plate. Both dw7 tubes are showing this. The cathode resistor is tied to a ground point node shared by v1a+b, these are showing 1.5v and 1.9v.

[wozt]

Man I am p*****ed off. The board is getting shoddy looking with all this rewiring. I can not get dc on the cathode of the dw7/ax7, it is driving me mad.

[Chuck H]

Man I am p*****ed off. The board is getting shoddy looking with all this rewiring. I can not get dc on the cathode of the dw7/ax7, it is driving me mad.

Hey man... Cheer down

That's why you post these things here before replacing the whole socket. If you had told us you didn't have current through the tube we might have saved you the trouble. I'll bet the cathode ground isn't hooked up properly. Maybe there's a wire there for it, but something is amiss. There's not much to the circuit. A lead from the cathode tube pin to a series resistor (possibly with a capacitor parallel) then another lead to ground. You can test the resistor to see if it's open, you can test the lead to see if it's open and you can test the ground connection to see that it's grounded. The problem is almost certainly there.

[wozt]

Hey man... Cheer down

That's why you post these things here before replacing the whole socket. If you had told us you didn't have current through the tube we might have saved you the trouble. I'll bet the cathode ground isn't hooked up properly. Maybe there's a wire there for it, but something is amiss. There's not much to the circuit. A lead from the cathode tube pin to a series resistor (possibly with a capacitor parallel) then another lead to ground. You can test the resistor to see if it's open, you can test the lead to see if it's open and you can test the ground connection to see that it's grounded. The problem is almost certainly there.

I'm going to wire the cathode -resistor- ground to a separate ground point and see if that sorts it out.

[wozt]

Couldn't wait till tomorrow, so I wired the cathode resistor straight to a nearby hole on the chassis and it's got 2.2v on it. Of course it working now, I'll have to wait to turn it up but it's looking good. I have rewired, re-flowed these joint so many times I'm aghast they were causing the fault. Thanks for you help.

[wozt]

Well, next day the 2.2v bias disappeared but eventually traced it back to a dodgy pot / ground so the grid wasn't biased (grounded).
THE F**** THING IS WORKING and it is louder than before I started tinkering.

THANK YOU ALL.

I will be picking up on some of the stuff mentioned in the thread and hopefully you'll continue to educate me.