Repairing Fender 1969 Bassman AB165 questions

[Fuzzy Slippers]

My Bassman died about a year ago and has sadly stared at me wondering why I never bothered to open her up and see what's going on.

I was playing her cranked thru a THD 4ohm hot plate when something popped and a smell of something "burny" tickled my nose.

This week I finally opened her up and started trouble shooting using R.G. Keen's amp debugging guide: Tube Amplifier Debugging Page

I have discovered that the fuse has blown. I removed the power tubes and replaced the fuse. It's been powered up for an hour now (with out power tubes) and the fuse hasn't blown.

I have moved on to the next section of his debugging guide for fuse blowing: "Power tube shorted". Power Tube Shorted

In this he says to check the values of the screen resistors. Through searching this site and looking at this layout



i have assumed that that are the 470ohm resistors connected to pins 4 and 6 on the 6l6gc sockets.

Here's the interesting part: one socket measures at 520 and one measures in at 840.

Here's a picture of the power tubes:

I'm about 75% sure the one of the left, with the discoloration in the screen printing was in the socket with the high value screen resistor.

Does this discoloration come from high heat? Could this be the cause of my blown fuse? Could the 850ohm screen resistor cause this?

Do I need to replace that resistor? The schematic says its a 1W resistor. Will any 1W resistor do? or do i need to find an NOS one for any reason.

Speaking of I found one:
Allen Bradley Carbon Comp Resistor 470 ohmRCR 1W 5% - eBay (item 400003595887 end time Sep-11-10 19:24:25 PDT)

i'm going to post more questions as i go thru the check list. figured i get this started for now.

[Chuck H]

I was playing her cranked thru a THD 4ohm hot plate when something popped and a smell of something "burny" tickled my nose.

Just want to say... Try not to blame the THD hot plate. Attenuators arent bad for amps other than the fact that they make them easier to crank ALL THE TIME. That's hard on amps running through speakers too. Just wanted to say that. Now...

one socket measures at 520 and one measures in at 840.

Yup. Those resistors drift with heat and age. One may even be measuring VERY differently with voltage on it while it's actually doing work. Best to replace both resistors with the correct value/rating spec'd in the schem. But don't use carbon composition resistors, use metal oxide or wire wound. These resistors aren't in the signal path so the amp doesn't care and you won't hear any difference in the resistor type.

I'm about 75% sure the one of the left, with the discoloration in the screen printing was in the socket with the high value screen resistor.

Does this discoloration come from high heat?

I can't quite make out if those are Groove Tubes or not, but I recognize the orange printing as the same material. As with Groove Tubes many tube distributors use printing that is designed to discolor if the tubes get too hot as an aid to diagnosis. So yes, I believe the discoloration is from high heat. I've had amps that run hot where all the power tubes were slightly discolored, normal. The fact that your amp discolored ONE tube is telling.

Could this be the cause of my blown fuse?

Yup. But it's more likely that the tube shorted and stressed that resistor. Either way you will need to replace both those screen resistors and both of your power tubes.

And Kudos to you for doing your research and testing before posting. You found a great site for reference.

Your case looks simple. Check the tube sockets for char. If they look good then replace the screen resistors and check voltages on the power tube sockets. They will read high of the schem because there are no tubes drawing current yet. If everything looks high but consistant and OK plug in the power tubes. You'll want to bias the amp now. Lots of good info on line for that. I would bet 6 beer (imported or American) that's all there is too it. With one caveate...

If the filter caps in that amp have never been replaced then it's time. Those caps (the big aluminum caps in the power supply) have a working life of about twenty years and go bad more quickly when they are allowed to sit uncharged for long periods of time. So with your circumstances, if those caps are original, they absolutely should be replaced or you can expect the amp to hum and fail again shortly after powering it up.

Chuck

[Fuzzy Slippers]

Thanks for the help. Switched out the power tubes and everything is fine.

wondering about the mod:

take a look at the 2 .1uF blue caps. they connect to a 220k reistor and pin 6 of a 7025. the red and orange wires go to the 1meg pot mounted in the speaker jack.

what does this do?

i have tried to figure out how it makes the amp sound and it's never be a gradual change. any position seems to do nothing or change things in a way i can't describe.

my guess is it's some how bridging the two channels?

[Chuck H]

Oh yea, Someones sure been in there. I think that may have been an attempt to cascade the two channels. There seems to be some other shinanagins too.

Did you replace the screen grid resistors? If not, you should. Did you re bias? You should.

How does the amp sound? You say the "mod" changes the tone in a way that's hard to describe... Please try.

Chuck