diytube.com

by **guitarpsych** » Sat Mar 31, 2007 1:50 pm

Yes the tubes light up.

The original problem with this amp was that it would work fine and every now and then gradually start humming. The hum would get really loud and then the fuse would blow. That's happened 2 or 3 times in the last 3 years I've had the amp. But recently, it started happening more frequently, and I'd turn the amp off before the fuse blew. The last time that happened, I noticed one of the tubes got really red/orange. I switched the tubes around to see if it was a tube or an issue with that location and none of the tubes repeated the intense glowing, but one channel stopped working. It came back on for about a minute but then went out again and that's where I'm at now. I also tried flipping the wires from the preamp (ruled out a preamp issue), and also tried flipping the speaker wires (ruled out a speaker issue).

by **TomMcNally** » Sat Mar 31, 2007 2:01 pm

A tube turning bright red usually indicates no bias, or a bad tube. If it's happening intermittently, it usually means a bad connection someplace. The only thng to be concerned with now is anything involved with the output tubes and the lack of any voltage at the bias test point (across the 10 ohm resistors) Compare the good and bad channels very carefully.
You should have voltage across the 10 ohm cathode resistors regardless of anything else on the driver board, etc.

Honestly, your amp is a good candidate for a rebuilt, but you wanna get it working as is first, you'll learn a lot.

by **erichayes** » Sat Mar 31, 2007 2:02 pm

Hi All,

Dynaco relied on the chassis and lockwashers to provide a common ground--there was no star or bus grounding. Not so occaisionally, the kit builder would forget a lockwasher here and there, or would forget to torque down a screw or two, or forget to solder a connection. You have to keep in mind that 90% of these amps were not factory built. The builder's workbench was the kitchen table and his soldering iron was a filed down woodburning tool.

Look for mistakes.

by **TomMcNally** » Sat Mar 31, 2007 2:05 pm

Hey ! I build all my amps on the kitchen counter and solder them on the dining room table. I finally replaced the filed down woodburning tool with a Weller Soldering station in 1985. Yellow_Light_Colorz_PDT_06

by **guitarpsych** » Sat Mar 31, 2007 2:09 pm

I know. I have been looking for mistakes. But also, this amp has been working for a long time. It didn't arrive dead. I've tried tightening all the screws also.

by **TomMcNally** » Sat Mar 31, 2007 2:18 pm

If you don't make any progress, I would try tacking a ground lead from the can cap to each of the tube socket ground rings ... I've been thinking ground issues from the beginning, and Eric is thinking the same thing. Forget about the meter ... give it a try, it doesn't have to be perfect, just wire !

by **guitarpsych** » Sat Mar 31, 2007 3:02 pm

Ok I just did that (with the two tubes on the bad channel). I wired it from the resistor side that goes to ground over to the ground wire of the filter cap can (it is in between the bias potentiometer and the cap can).

The same thing is happening.

by **TomMcNally** » Sat Mar 31, 2007 3:06 pm

The same thing meaning "it doesn't work" ?? hmmm It doesn't make a whole lot of sense, if you have B+ on the plates, and the cathode resistor has a path to ground, and you have bias, the tubes should work. They aren't bright red are they ? You are measuring voltages with the tubes in the sockets right ? There will be no voltage without the tubes. Sorry this is taking so long ... but I'm stuck at work today, so I have time !

by **guitarpsych** » Sat Mar 31, 2007 5:29 pm

No problem, I appreciate the help.
Yes, by 'same thing is happening" I mean it is still not working. Yes, I was testing it with the tubes in. This is a frustrating one. Let me know if you think of anything else. Is there anything I should recheck, any volt readings?

by **TomMcNally** » Sat Mar 31, 2007 8:02 pm

I would put the voltmeter aside and do a very very thorough visual check. Something is broken or disconnected, dirty or loose contact. Compare good side to bad side, pin by pin. Then do the same thing with the voltmeter, with it off, check resistance from one side to the other and see what is different.

by **guitarpsych** » Tue May 01, 2007 9:48 am

After a troubleshooting session with EricHayes we finally tracked it down to bad tube socket(s). Thanks everyone for helping out. I've got it working now.

diytube.com

reading 0 V on pins 1 and 8

Who is online