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Fender amp troubleshooting - help?

wallybill

wallybill

Apr 4, 2007
241
16
Tuscola, Il.
wallybill.com
#1
Hey guys,
I've got a '65 Bassman that was making a lot of noise. I replaced the electrolytic caps and cleaned all the pots. Now, at least the pots don't add to the problem. When I take it off standby, there is a 'pop' and then a sound akin to radio static. When I shut it off, it takes a few seconds for the static to go away. I'm figuring plate resistors and coupling caps next - but I've gotten great advice from this board before, so I'm hoping you might have some suggestions as to the cause? I've never heard one make a noise like that when it's powered off.
 
#4
Hi Johnk,
Yep, I switched out all the tubes and it still makes the noise. The tubes show OK on the Mighty Mite tester, but I don't know if it would catch a noisy tube. Also, adjusting the pots doesn't affect the static (anymore - the volume pots were really noisy before the deoxit). That leads me to guess that the problem would be in the power amp section - plate resistors and/or the cap after the volume control. I'll have to order the parts.
But, I wouldn't expect a bad resistor to keep making the noise after the power is shut off - just a guess. I wonder if a bad coupling cap would make static noises as it discharges? Could a bad cap keep making noises a couple of seconds after the power switch is turned off? I've never heard a bad coupling cap before. All the electrolytics had bubbles on the ends, so I started there.
I put on a new 3 prong power cord and removed the ground switch cap too.
Do ya'll think I'm on the right path (I'm learning!) or bumbling in the dark?
 
#5
It's probably a noisy resistor, but could be a bad tube socket contact. And yeah, after you kill the power, the tubes are still warm and the filter caps still have some voltage on them, so the amp continues to work for a few seconds afterwards.

Try wiggling the tubes in their sockets while the amp is on. Another thing about the old Fender amps is sometimes the board would actually become leaky. Not sure any way to fix that other than replacing the phenolic board with a new one.

Divide and conquer - Break the circuit in a safe way, like pull up one end of the coupling capacitor that feeds the power amp driver, then turn it on and see if the noise is gone. If it's gone, then the problem is up-stream. Put the cap back in place, then move back another stage and lift up a cap again, try it. Eventually you'll figure out what part is the source of the noise.

Chris
 
#6
even the best tube testers don't expose whether a tube is noisy or not. and most of them don't test the tube at real world voltages either.

coupling caps usually don't make noises after the amp is shut down, but anything is possible.

i would start by checking/replacing the two .1uf caps coming out of the phase inverter.

after repairing hundreds of 50's & 60's fenders over the years, i've never had one of their boards 'become leaky'.

i'd also check all of grounds in the amp, even if they 'appear' to be good.
 
#7
Hi guys,
Thanks so much for the advice - ya'll are great!! I played around with it some more this morning. This time I took out pre-amp tubes and just left them out. I took out the second preamp tube for channel one and the noise disappeared. I plugged the trusty bass into channel 2 and it sounds awesome! It is quieter than it has ever been. I tried a few different tubes in V1 & V2, but the noise always comes back. So now I believe I was wrong about the power section, I think it's in channel one and it should be after the volume control since adjusting the control doesn't affect the volume of the noise (my current guess).
To check the grounds, I hooked the meter to the ground on the new 3 prong plug and used the other end to check for resistance to the ends of the components that were connected to ground. Most were 0, but as I got farther from the power cable, it went up to the highest of .2 ohms. Does that sound right to you? I figured there would be a little resistance, but since it started showing a tiny resistance the farther I got from the power cable and channel 1 is the farthest from the power...... If you have a correct way of checking the grounds, I'd sure like to know your process.
To Throbbinnut - d'oh! I've heard my tube amps trail off after turning the power off while still playing. I don't know why I thought the noise would be any different - but I sure did. Man, I hate getting old. I shook the tubes a bit while the amp was on and it didn't seem to affect the noise. I'd like to try your idea of lifting a cap and then trying it. I was worried that might damage the amp. Should it only be run for just a few seconds like that, or does it matter?
Sorry for all the questions and thanks again for the advice.
 
#8
.2 ohms sounds okay for a ground in those amps. of course .0 ohms is best.

if the noise is not a preamp tube, you might want to replace the 100K plate resistors in channel one and/or the .22uf & .047uf caps as well as the dual 25/25 cap ( i usually use two separate 25uf/50V caps) as there's really not much else in there that can cause the noise that your describe.
 
#9
I changed out the dual 25/25 cap already - with two separate caps (I think I've seen you suggest that before on here). I'll have to order the resistors but I think I have the other caps already.
What's your thoughts on plate resistors - should they stay carbon comp or switch to metal film ones?
 
#11
The 1/2 watt carbon comps barely did the job, sounds like the plate resistor in channel one is grumpy.
Good suggestion from Paul 1 watts will due nicely.
Leaking coupling caps can make the amp weak/off sounding, grindy. If you lift the end going to the next stage, attach a volt meter (DC) and check for voltage leakage. Anything over 15mv change it now (Tubes removed during testing, amp set to operate).
 
#14
Hi all,
I finally got some time to mess with this amp again. I opened it up and applied the chinese chopstick test to it. It made the same noise when I took it off standy (kind of like it's trying to pull in a radio station, static isn't consistent). I first tapped on the 100k resistors but that didn't make any changes. Then I tried to tap a 470k resistor next to a .01 (coupling?) cap (disc). I managed to tap both of them at the same time and the noise stopped. Tapping some more doesn't bring the noise back. When I tap on the .01 cap, it's pretty microphonic. Does the cap being microphonic indicate that it's bad or is that normal?
 
#15
wallybill said:
Hi all,
I finally got some time to mess with this amp again. I opened it up and applied the chinese chopstick test to it. It made the same noise when I took it off standy (kind of like it's trying to pull in a radio station, static isn't consistent). I first tapped on the 100k resistors but that didn't make any changes. Then I tried to tap a 470k resistor next to a .01 (coupling?) cap (disc). I managed to tap both of them at the same time and the noise stopped. Tapping some more doesn't bring the noise back. When I tap on the .01 cap, it's pretty microphonic. Does the cap being microphonic indicate that it's bad or is that normal?
Click to expand...

That pretty much says replace me!
 

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