O.K. I'm Back

[unhappybassman]

I have a bassman that has pops and crackles when at idle and during playing. No popping when in standby, no popping when the driver tube is removed.Still popping even when the preamp tubes are removed one by one. Its a silverface with new tubes now(Just replaced to see if bad tubes). Fender tubes. I have looked inside to see if anything looks burnt or discolored, nothing. I have tapped the parts to see if anything causes it to pop more...it just seems to pop on its own. Any ideas? Getting frustrated with it. Had to step away for a week or two. It popps right from startup. Please any ideas to get me on track would be appreciated.

Unhappybassman

[drewl]

Loose solder joint or noisy plate resistor?

[TD_Madden]

bad driver tube?

[unhappybassman]

Bad Resister

Alright,
I have a two 220k resistors in the driver circuit going to the bias. One of these resistors measures 220k and the other (going to the adjustable bias pot) has drifted to 58k. I have measured some of the other resistors in the driver circuit and they have drifted quite a ways off. I have a brand new Driver tube...Would all this drifting in the driver resistors be making one of the other resistors in the driver circuit short out occaisionally? I will post any new findings when they arise...Let me know what you think.

[Paleo Pete]

The same thing was happening with my Super Reverb (also silverface) when I first got it working right. It turned out to be a bad preamp tube, and it was a brand new one. The tube works fine, amp sounds fine, but it pops. Swap it for a different preamp tube, no pop. The point though, it was brand new, less than a month old when I found out what was making it pop, so new tubes don't automagically mean good ones.

I would also wonder about the resistors too though, I also had several 1 watt resistors that needed replacing, the amp would hiss at idle. New 1 watt resistors cured that. I don't know if they could also cause it to pop, but I wonder...

[Enzo]

I would find it highly unlikely a 220k resistor would "drift" to 58k. That sounds more like a parallel current path to me. Disconnect one end of that resistor and measure it that way.

Besides, you said when you removed the PI tube the noise stopped. Well those resistors are still in the power tube circuit even when teh PI is removed, so they are not involved.

Noisy plate load resistors will not look any different visually.

DO any of the controls affect the noise? if the volume control will quiet the noise or if the tine controls will affect teh sound of it, then they are AFTER the source of the noise.

[Jag]

+1 what Enzo said.

There were a few other suggestions, in your original thread, that I don't recall if you tried. Maybe revisit that thread and let us know if you've tried those other things.

I'm still betting on plate load resistors. I recall that someone (Stokes?) suggested the chopstick test in relation to this. I've had many noisy plate load resistors that did not react with pops and crackles when tapped.

[unhappybassman]

Chopsticks

Yeh I have tried the chopstick test. The tapping didnt seem to make anything pop any more than it is already. I have tried to move all of the knobs and no change.I had checked the resistor again. I know It sounds strange. I always heard of it drifting upward, but never down in resistace. I had read another article where the individual had mentioned that he had a plate resister that was shorting internally and after it was changed it stopped popping. Is there a normal resistance value for the plate/bios resistor. It is measuring 58K. Strange. Has anyone ever bought the book on tube amps by Dan Torres. I have diagrams of the driver circuit and some changes that can be made to them. Anyone have a simple diagram of driver circuits for a CBS type Bassman 100. The diagrams at Fender dont seem to be that accurate for what I have. Does anyone know a good place online to purchase good resistors for amps?
As always I appreciate any suggestions....

unhappybassman

[Enzo]

Does that resistor still measure 58k when DISCONNECTED from everything?

REmember, that part of the circuit does not make the noise when the PI is removed, so I don't think THAT is the noisy resistor.

[mac1amps]

It sounds to me like you have a noisy plate resistor problem, which for some reason is more common to Bassman amps than any other model Fender. Change the 100K plate load (and voltage divider) resistors on all the preamp stages, and I'm willing to wager that this will alleviate your static issue.

[TD_Madden]

mac, if he has the preamp-tubes pulled, wouldn't the popping go away even if the preamp plate resistors are the problem?

[Euthymia]

Replace the plate load resistors on the PI.

While you're at it, you don't say what flavor of Bassman you have, but if those resistors are 47K, it's a good time to blackface them back to 82K/100K.

[Jag]

I would fix it before modding it. I think he implied that he has a CBS Bassman 100. Although I'm not sure about his reference to two 220K resistors in the driver circuit. I could be looking at the wrong Bassman 100 schematic.

[mac1amps]

Depending on which stage the noisey plate-load resistors are in, the static can remain in the sound even if the pre-tubes are pulled. I've run into this issue a number of times [in my 34+ years of pro-amp servicing], and new 100K (I choose to go with ½ watters) resistors have fixed the problem in most all situations (once I've verified that the problem ISN'T with the pre-tubes).

[unhappybassman]

It took some time to find....

O.K. I'm changing my user name to happy bassman. I have changed and replaced 4 resistors in the driver circuit...Wa la...No more popping or bacon noices...Surprise...I expected to hear the popping when it was trned on and I couldn't even hear the amp running...OH no I thought..That did it I killed the amp totally. No the bypass switch was still on and I flipped the switch and the amp was so quiet, I couldn't even tell it was on. It looks like this amp was modded some time ago in the driver section. I replaced the resistor goin to the bias and the plate resistors in the curcuit. Works perfectly. Thank you for all your suggestions. It definately helped in my troubleshooting. Now that I got it running...any ideas on a good driver mod or preamp mod to date it back a little...????

Thanks again
happy bassman