Fender Silverface Bassman 10 isn't working. I need help fixing it!

[docSanchez]

Hello hello

My friend asked me to take a look at his Bassman. He said it blew the fuse. So, we replaced the fuse which got the status light on and the tubes warming, but no signal. We looked and found 2 components had burnt; a 470 ohm 1 watt coming off of pin 4 of one of the 6L6 power tubes, and a 2.2k ohm 5 watt connected to the Red cable from the output transformer and the Standby switch.

Well, I replaced those and the situation remained the same. Jewel turns on, tubes heating, but no signal at all; no crackling, no hum, no quiet noise, just dead. After a few seconds the amp starts smoking so I unplug it. The two components that I had just replaced had just burnt out again. However, the fuse had not blown.

Something worth noting is the fact that there are 2 of the 470 ohm resistors; one for each power tube. The first time his amp blew up (the reason he brought it to me) the left tube's resistor had fried. The second time the amp blew up (after I had just put in the new parts) the right tube's resistor fried. In both cases only one resistor blew up while the other one was left untouched. This leads me to suspect that one of the power tubes is somehow bad, and that over the course of my disassembly and reassembly of the amp I switched them.

So... thoughts? I am still relatively new at working with tube amps and am at a loss for what to do next.

Here is a link to a page with the schematic for the amp: Amp Guide » Fender*Bassman 10

Aaaand I have attached a schematic with the problem areas highlighted.

Thanks much for the help!

[Jazz P Bass]

The 470 ohm resistor is the 6L6 tube screen reistor.
Remove the output tubes, replace the burnt parts .
If all is o/k, with the amp cranked, you should have a nice healthy signal at pin 5 of each output tube socket.
Replace the bad 6L6 tubes.

[docSanchez]

So it is indeed the tubes then, yes?

[Amp Kat]

Obviously first things first be damn Carefull because you are messing with the high voltage source of the amp and they can be lethal for certain individuals. I like Jazz's idea of replacing the Screen resistors and without the tubes in make sure you get from ground you're high voltage DC on pins 3,4 & 6, pin 5 should be around -45 to -50, Pin 1 & 8=0, in between pins 2 & 7 should get 6.3 to 7 VAC. If you get that then, power down and install the tubes and hope you get the majic tone. If not post back and get ready for RG's troubleshooting page.

[Gibsonman63]

I like to use 2W metal film resistors for my screen resistors. Those tube sockets can get hot, expecially when the tubes are failing. The metal film resistors can handle the heat a little better and won't drift in value as much when heated.

[docSanchez]

@Gibsonman yeah, I put some 2W in there because I figured the socket was just running hot. I increased the wattage on all the parts I'm replacing into the amp.

@Ampkat well, I pulled the power tubes out, powered up, and took the measurements. Everything checked out; pin 3 to ground read 520v, pin 4 to ground read 495v, pin 5 to ground read -47v, pin 6 isn't used, pins 1 and 8 went to ground, and the heaters read 6.5vac.

So I plugged the power tubes back in and started it up. Same problem as before. Light turns on, tubes warm up, silence, and the screen resistor starts to smoke. (Which smells naaaasty, let me tell you)

At this point would it be safe to assume at least one of the power tubes is shot and is the problem?

P.S. I appreciate the concern for my safety; Caution is my middle name ;D

[docSanchez]

Sorry to double post so rapidly but it is one of the tubes for certain.

I marked one of the tubes with a bit of yellow spike tape (but not on the glass hah!) and watched as the screen resistor for the marked tube smoked. Then I powered down, switched the tubes, and what do you know the screen resistor for the marked tube started to smoke. So... problem solved.

Thank you all for the help though, I appreciate it!

[Jag]

I'm glad you solved the problem but, you really could have caused major damage testing that way. I'd suggest looking up how to build a lightbulb limiter for testing and test for bad tubes by swapping in known good tubes.

Just my $0.02.

[sleepingAwake]

Here I am reviving an old thread but I have a very similar problem.

I received an a 72 Bassman 10 with a blown 470 ohm resistor on one of the tube sockets, leaking filter caps and a 100 ohm blown resistor going from the indicator lamp to ground.
I replaced all of the above and swapped out a few of the resistors in the filter cap area as they were old and had gone up in resistance some.

I installed 2 new 6L6 tubes and powered it on. No sound. Tubes glow but no output at all.

[acorkos]

Here I am reviving an old thread but I have a very similar problem. I received an a 72 Bassman 10 with a blown 470 ohm resistor on one of the tube sockets, leaking filter caps and a 100 ohm blown resistor going from the indicator lamp to ground. I replaced all of the above and swapped out a few of the resistors in the filter cap area as they were old and had gone up in resistance some. I installed 2 new 6L6 tubes and powered it on. No sound. Tubes glow but no output at all.

follow Amp Kat's advice a few posts above...:

Obviously first things first be damn Carefull because you are messing with the high voltage source of the amp and they can be lethal for certain individuals. I like Jazz's idea of replacing the Screen resistors and without the tubes in make sure you get from ground you're high voltage DC on pins 3,4 & 6, pin 5 should be around -45 to -50, Pin 1 & 8=0, in between pins 2 & 7 should get 6.3 to 7 VAC. If you get that then, power down and install the tubes and hope you get the majic tone. If not post back and get ready for RG's troubleshooting page.

[Jazz P Bass]

Here I am reviving an old thread but I have a very similar problem.

I received an a 72 Bassman 10 with a blown 470 ohm resistor on one of the tube sockets, leaking filter caps and a 100 ohm blown resistor going from the indicator lamp to ground.
I replaced all of the above and swapped out a few of the resistors in the filter cap area as they were old and had gone up in resistance some.

I installed 2 new 6L6 tubes and powered it on. No sound. Tubes glow but no output at all.

Troubleshoot the amp.
Are all voltages of the plates & cathodes proper, per the schematic.
Remove the 6L6 tubes.
Pin 5 is the bias voltage.
Is there a minus voltage there (about -50Vdc)
Run a signal through the amp & check pin 5 for a Vac signal.
100mv signal in should produce a nice healthy signal at pin 5.
If there is no signal there, then work your way back.

[sleepingAwake]

I checked it out a little today and with the power tubes unplugged I'm not getting anything on pins 4 and 6 of the 6L6 sockets. Pin 3 had 513 volts.
Pin 5 had around -40 volts which seemed low.

[sleepingAwake]

I believe I found the problem.

There is a 2.2k ohm 7 watt resistor that appears to be open internally. Im reading infinite resistance across it. I stuck a 2.2k 1 watt resistor in there for a second and got some noise from the speakers.

Trouble is I cant find a suitable replacement locally. Would anyone care to comment on using a resistor of either 2k or 2.7k would cause a big problem?
I assume using a bigger value would err on the side of caution and lower the plate voltage instead of a smaller value increasing it.

[g1]

The power tube plates are before the resistor. That resistor will affect the voltage to everything after the power tube plates. 2K should be close enough.
Was the -40V you measured at the grids with the tubes out? I would think you should see closer to -50 there. The 2 caps in the bias circuit probably need to be replaced.

[sleepingAwake]

The power tube plates are before the resistor. That resistor will affect the voltage to everything after the power tube plates. 2K should be close enough.
Was the -40V you measured at the grids with the tubes out? I would think you should see closer to -50 there. The 2 caps in the bias circuit probably need to be replaced.

Yes that measurement was taken with the tubes out. I actually did replace those 2 caps along with all the filter caps. I used sprague caps of the same values.