Tweed Bassman RI trouble

[xtian]

No output, loud pops and snaps with two sets of tubes. With all tubes pulled, voltages seem OK, ca. 470 on the plates. No hiss, so maybe trouble with PI? Anyway, can't test with tubes in place because of loud pops and snaps. Nothing looks burnt. Time to pull the board and touch up the solder? What else can I test with no tubes before I pull the dang circuit board?

[Milkmansound]

probably a bad solder joint somewhere - its a very common problem for the reissue amps. You will see a whole bunch of them when you open the amp up - check any joints on tube sockets, or on moving parts (pots, input jacks, etc) where they solder to circuit boards.

[xtian]

By testing voltages I identified two 100K plate resistors that had gone open or nearly open (after I pulled them out, they tested 500L and 10M). So I replaced them, and other plate resistors shown in yellow. I did not replace the one shown in red, because for some reason, it's a 2-watt resistor, while all the others are 1/2 watt.

So now the amp is working nicely, but I still hear some bacon frying, and it's coming from V1 or V2, I think, because it stops if I pull these tubes. I replaced V1 and V2 with new JJs, and it's been on for 15 minutes with no sizzling at all.

All good.


So, anyone want to guess why the 100K resistor marked in red is 2W?

[mr_hankey]

Guessing: is it mounted closer to heat-producing components than the other resistors are? Does the PCB layout require a physically larger resistor in that spot?

I really don't see why you couldn't use a 1/2W resistor in that spot. However, I would use 1W (or 2W for carbon comp) on the plates, because of noise concerns.

[guitardude57]

Bacon fry sound... another solder joint that needs a touch up. Keep looking. Also look for any arcing on tube sockets.

That resistor doesn't need to be that large. Sometimes a slightly larger wattage resistor is a little quieter...

Probably, they were short on the right part, when building.

[xtian]

Bacon fry sound... another solder joint that needs a touch up. Keep looking. Also look for any arcing on tube sockets.

That resistor doesn't need to be that large. Sometimes a slightly larger wattage resistor is a little quieter...

Probably, they were short on the right part, when building.

Thanks, Mike. The last of the sizzling sounds stopped when I replaced the preamp tubes. I have already retensioned and applied DeOxIt to the tube pins; they all looked clean.

Thanks, everyone.

[pdf64]

The RI scheme shows 215V across the cathode follower, which works out to 0.46W.
http://www.webphix.com/schematic%20heav ... manual.pdf
To improve reliability (5 year warranty on these), it's good practice to derate resistors by at least 50%.
Pete