Tube Amp Archive

Thanks for the replies. Yes, the voltage was off at the junction of the two 4.7Ω resistors. And it was Q202 that was deficient. My buddy had bought the replacement transistors from a seller on eBay, and dumb me didn't test them before installing them. Of course, according to Mouser, the TIP29 and TI...

Hi! I am currently repairing a Lab Series L7 amplifier for a friend. The amp seems to be functioning well, with the exception of the reverb, which is not working. I have checked the reverb tank, and it appears to be in good condition. Furthermore, the return circuit seems to be functioning properly....

I did reverse them "for the fun of it" to no avail. No difference.

I put it all back as I found it and gave it back to my friend.

I guess the "gimmick" is the answer I will have to go with.

Thanks for y'all's input.

Curiouser and curiouser. So, I performed a few experiments, and here are my findings. First, I redid the mains neutral using a terminal strip and reflowed and added solder to some of the uglier eyelets on the board. Then I disconnected the coil and bypassed the last filter cap with a 39pf/1000V cap,...

Thanks for the answers. When you say "low-value capacitor," do you mean 10pf or ?? And why is this necessary as this node is already bypassed to ground by a 5uF filter capacitor? And since I still have it, I will redo the mains issue and pretty up some of the soldering. And the blue tape was me cove...

Hi Martin,
No, it is not on the screen supply node, it is on the next node. The node that supplies the preamp anodes.

Where the yellow highlighter is on the schematic.

I should mention that the "Knock" was present whether the tremolo was on or off. It was very audible in both channels and was louder when the volume was turned up. It responded to the speed control.

A friend brought me a homemade amp he had bought at a flea market so that I could adjust the bias. He had just put in a new pair of 6v6s. He told me that it was a clone of a Silvertone 1482. I looked up the schematic and it is very close to the 1482. Actually, the bias was pretty much perfect so I d...

Thank you!

I've ordered a new Q8. I'll post again when it's fixed.

UPDATE. I verified that there is no significant DC voltage on the speaker output. ( <0.2V) Then I lifted a lead on D16 and D17. No Change. I take that to mean I should focus my attention on the Delay/Driver portion of the circuit. Right? In checking the voltage on the positive side of C63 I watched ...

Thanks, Sluckey!

That was exactly what I was looking for. You are not only gifted in your knowledge, but also in your ability to share it. And we are all lucky for your willingness to share it.

I'll post the results of my troubleshooting!

Merry Christmas!

bb750 dc oFFSET.JPG I have an MB Big Block 750 that has stopped producing sound. I have traced the signal through the preamp and the power block and have determined it enters the relay immediately before the speaker jacks but never exits. I'm not so good with solid-state or logic circuits, and I'll...

I have ordered a pair of new JJ 6L6GC tubes. I will let you know if that makes a difference.

Here's a photo of the tubes.
Right now the four "Matched" tubes I bought are the only 6L6GC tubes I have. Actually, I have one used Ruby Tube 6L6GC that looks nothing like these. I do have a new set of Chinese 5881A tubes. Would they be worth trying?

statorvane,
Did you mean 470R / 5W screen resistors? 470K seems awfully high.