5w Screen Grid Resistors solder melt

[Chuck H]

I wasn't able to find any definitive info on the chokes used in those Bassmasters. Usually a large choke like that would be a of a higher inductance. A higher inductance most often translates into higher DCR though it also depends on the rating and construction as it relates to size so it's impossible to know without concrete figures from a known working model.

If the plates don't remain positive WRT the screens the screens can try to dissipate more of the current moving through the tube. In an amp with over 400V B+ that is run hard, possibly clipped, I wouldn't be surprised if there was a problem.

I suspect your choke is bad. Try subbing in the 250 ohm resistor for the choke just to see if things improve from where they are with the choke. That should tell us enough.

[Pryde]

OK I can try that.

Do I just unhook that black lead from the filter cap and tack on the resistor there then over to the other cap with the red/yellow twisted?

Do I need to unsolder the red/yellow from there as well?

[Jazz P Bass]

100 ohms is fine for a higher power amp.

Have you checked the Vac ripple voltage before & after the choke.

[Chuck H]

That should be fine. But do check DCR from the choke leads to the chassis. If there is any continuity you'll need to unhook the choke entirely. Otherwise you can just leave the black lead off and parallel the resistor where the choke was.

[Pryde]

100 ohms is fine for a higher power amp.

Have you checked the Vac ripple voltage before & after the choke.

Sorry guys I'm learning, going to have to hold my hand some more. How is this accomplished?

[Chuck H]

100 ohms is fine for a higher power amp.

Have you checked the Vac ripple voltage before & after the choke.

I know it was done in some Fender amps. But this is a large choke. I would expect more DCR. If it's shorted, and so acting as a much smaller choke, it may not be providing isolation between the nodes. The ripple check is a good call to see if that may be what's happening.

[g1]

That should be a Hammond choke, any numbers on it? They probably have a spec for DCR if you can find the model #.

The screens being 2V higher than plate is a lot better than the previous measured 13V. Another series resistor between the choke and the 1K's (probably 470 or 1K) might do the trick. You have an empty eyelet next to the 1K's, you could move the pair of 1K leads there and connect them back to the choke/5K junction with the new resistor.

[Pryde]

That should be a Hammond choke, any numbers on it? They probably have a spec for DCR if you can find the model #.

The screens being 2V higher than plate is a lot better than the previous measured 13V. Another series resistor between the choke and the 1K's (probably 470 or 1K) might do the trick. You have an empty eyelet next to the 1K's, you could move the pair of 1K leads there and connect them back to the choke/5K junction with the new resistor.

Thanks for the tip. I have a 1k 5w resistor here that I can use for that. I will also try to locate a serial# on that choke. I will report some voltage readings in a bit.

[Pryde]

OK maybe final update???

I placed a 1k 5watt series resistor per g-one's tip and it dropped the screen voltage ~2 volts below the plate voltage:

plate 415v
screen 412-413v

I played the amp near dimed for 15 minutes straight (head hurts) and the voltages stayed stable, all 5w resistors are room temp. Everything seams ok and the amp sounds fantastic.

Are we good? Is the 2-3v lower screen voltage acceptable?

Thanks