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'67 Bassman down, photos inside

scott1568

Member
Messages
408
Well, I left my amp on all night on accident Thursday night. I woke up the next day and shut it down and went about my business. That evening I powered it up and the output was severely diminished. It sounded like a dying fuzz pedal. I shut it down and went through the steps:

cables
different guitar
fuse
checked speakers
swapped tubes on by one

and still the same symptoms

***DISCLAIMER*** I know the procedure for draining the filter caps to avoid electrocution

The filter caps were new Sprague Atoms installed in 2000. However the amp sat for 6 years mostly unused. It has been great for the last 7 years (except I had to replace the diodes in the rectifier section last year).

The Spragues show no leakage or bulging at the ends.....so

I looked inside for any scorched components and the screen grid resistors look like this:

Right


Left


 

scott1568

Member
Messages
408
I am assuming these are fried? What would cause this?

Also, as a precaution, do I need to replace the filter caps?
 

Blue Strat

Member
Messages
30,075
Measure them with an ohm meter. If they check out ok, then turn the amp on, with a load, and measure voltages around the power tube sockets and compare them to the schematic.

If any of this seems difficult you may need assistance.
 

soldersucker

Member
Messages
2,583
1.5 K screen resistor looks cooked in first pic.
Likely a bad power tube replace the resistor
and try some known good tubes.
 

scott1568

Member
Messages
408
Left is at 1.5
Right is at 1.444

I just off the phone with my friend who is an amp tech. We are going to swap those and go from there. He said they might measure OK cold, but it could be a different story at temperature.


Thanks for your replies all!
 

Blue Strat

Member
Messages
30,075
Another test is to check voltage across the 1 ohm resistors that have been installed for checking bias current.
 

scott1568

Member
Messages
408
OK Mike,

Both resistors on 4 and 6 are 464 ohms.

The resistors to ground off pin 8 of each socket read "OL" on my meter at its max setting of 2M. The AB165 layout does not show a value.


I'm not sure where to go on the bias currrent resistors because the bias adjustment was modded years ago. I had a bias pot added to the stock hum balance pot - gives me the option of using unmatched tubes if I want to.
 

Weezykid

Member
Messages
53
When you check the voltage ACROSS the 1 ohm bias resistor to ground.... Put the red lead on the socket side of the resistors then the black side of the multimeter on the chassis side of the resistor (just paste the resistor body on each side). The multimeter needs to be set for volts dc (millivolts)..

The 470ohm resistor on 4/6 seem to measure ok cold..
 

Prattacaster

Member
Messages
1,369
The resistors to ground off pin 8 of each socket read "OL" on my meter at its max setting of 2M. The AB165 layout does not show a value.
It should read 0.00 ohms on the 2M setting. Maybe you had your meter setting wrong/bad connection or maybe they are both open, which would be quite a coincidence or they are cold joints that have no connection, also unlikely but possible.

Don't rush anything when your in there with a live amp, make sure you have a load, and dont press to hard with your probes or they might slip which could result in damaged goods. Watch out for the pwr and stby switch they've plenty of voltage on them.
 

phsyconoodler

Member
Messages
4,315
The resistors to ground off pin 8 of each socket read "OL" on my meter at its max setting of 2M. The AB165 layout does not show a value.
The AB165 never had those 1 ohm resistors to check bias;those were added by someone rather recently considering they are modern looking metal film resistors.
Your meter indeed will show OL because you can't read a 1 ohm resistor on the 2M setting.Try the 200 ohm setting or lower if you have it.
The 470 ohm resistors are 'screen grid' resistors.The 1.5k are 'control grid' resistors.
The control grid is the negative bias voltage to pin 5 of the power tubes
 

AR-305

Member
Messages
976
The AB165 never had those 1 ohm resistors to check bias;those were added by someone rather recently considering they are modern looking metal film resistors.
Your meter indeed will show OL because you can't read a 1 ohm resistor on the 2M setting.Try the 200 ohm setting or lower if you have it.
The 470 ohm resistors are 'screen grid' resistors.The 1.5k are 'control grid' resistors.
The control grid is the negative bias voltage to pin 5 of the power tubes
Glad somebody finally cleared this up.
 

scott1568

Member
Messages
408
Well, 4 months had passed and I just couldn't take it anymore, I got it fixed with a 30 cent 100K resistor.

I had replaced what looked like fried screen resistors, but that didn't cure the problem.

Last week I found a video on YouTube that showed how to check voltages (relatively safely) in a live amp. It turns out the 100k resistor feeding pin 6 on the PI was bad. So I went to Radio Shack, got a pack of 100k 1/2 watt resistors and had it fixed in about 15 minutes!

I really missed this amp. It's an AB165 modified to AA864 in the output section. I had borrowed a friends Mesa F40 for which I am greatly appreciative, but for me there is just no comparison (no offense to anyone here).

Thanks all for your help!!
 

Blue Strat

Member
Messages
30,075
Good work. Changing the heat stressed, even under the best conditions, resistors on the tube sockets is considered proper 30+ year maintenance so that's good.

If the electrolytic caps are original they should be replaced. Forget about what they look like, 75% of the bad ones I've replaced showed no visible evidence of problems.
 

tonalvision

Member
Messages
160
Congrats,

Satisfying when you fix it yourself. You are lucky with your radio shack. 1/2 watt resistors are nowhere to be found in the store near me. Of course I will take e-bay over radio shack any day.

Regards
 

scott1568

Member
Messages
408
Hey Mike,

The filter caps are about 10 years old and seem to be fine for now. No ripple noises or flabby output. Thanks!
 




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