AB165 Bassman crackle in V1 and V3 stages??

[lespaulnmarshall]

Hi All,

Perhaps someone can help me out here. I've been working on my AB165 Bassman. There seems to be a crackle in both V1 and V3. I've replaced all the plate resistors twice now. It currently has big carbon comp 1 watters (brand new), but it still crackles. The crackle isn't loud but it's enough to annoy me. It seems to be doing it with the volume down or up. None of my other amps with CC plate resistors have this issue. It also gets louder when turned up, but it's there either way. It's quiet if I pull V2 and lift the 470k feedback resistor. With the resistor in place the crackle comes back even when V2 is pulled. It's also quiet if I pull V1, V2 and V3. Put any of them back and the crackle is there again. I've also lifted the 220k resistors on one enf going to the plates of each tube. Only quiet if both are lifted so it's really in both channels. I've tried nearly everything at this point. Does anyone have an idea what could be the issue here? I'm thinking maybe grounding, but my ground runners seems to be alright. Could it be the brass grounding plate not making good contact with the chassis maybe?

Thanks!

[pompeiisneaks]

Have you tried cleaning the tube sockets? Does chopsticking any specific components make it worse/better?

~Phil

[lespaulnmarshall]

Have you tried cleaning the tube sockets? Does chopsticking any specific components make it worse/better?

~Phil

Hi Phil,

Thanks for the reply,

Yes, couldn't find anything with chopsticking it.
I've also cleaned the tube sockets several times with very generous amounts of deoxit. Retensioned the pins as well... Nothing helped unfortunately...

[lespaulnmarshall]

Just spent the morning and most of the afternoon removing the brass grounding plate, cleaning everything and putting it back as well as a few other things. So far, nothing has worked..

[pompeiisneaks]

clarifying question. I'm looking at that schematic and they don't mark V1/V2 or V3 directly. Is this the schematic.

http://www.prowessamplifiers.com/schema ... _schem.pdf

Is your naming along the lines of

Bass V2
Normal V2
1/2 of the triode before the PI V3?
PI V4
?

Also if V3 is the middle half triode and it's there when you put it back, maybe it's on that one?

the removal of negative feedback fixing it sounds a little like what a problem someone here just had and it ended up being the OT, do you have a known good you can test with?

~Phil

[sluckey]

Look at any Fender layout. V1 is on the right, and they count up as you move to the left.

[lespaulnmarshall]

clarifying question. I'm looking at that schematic and they don't mark V1/V2 or V3 directly. Is this the schematic.

http://www.prowessamplifiers.com/schema ... _schem.pdf

Is your naming along the lines of

Bass V2
Normal V2
1/2 of the triode before the PI V3?
PI V4
?

Also if V3 is the middle half triode and it's there when you put it back, maybe it's on that one?

the removal of negative feedback fixing it sounds a little like what a problem someone here just had and it ended up being the OT, do you have a known good you can test with?

~Phil

Hi Phil,

Sorry should have clarified that already. I name the tubes V1 to V4 based on their location on the chassis (right to left when seen from the backside of the amp mounted in the head cabinet). V1 is the bass channel preamp tube. V2 is the 'feedback' stage that both the normal and bass channels are fed into. I call this the feedback stage as there is a 470k resistor between the plate and the coupling cap going back into that same stage's grid. V3 is the preamp tube for the normal channel. V4 is the PI. At this stage I'm convinced it's happening before the feedback stage (V2), as the amp is totally quiet when the two 220k resistors going into V2 are lifted. However, it should be noted that both have to be lifted in order to get rid of the crackle.

I have not touched the NFB between the OT secondary and the PI at all, only the feedback resistor for the V2 stage.

Thanks again!

[pompeiisneaks]

ok gotcha, now I'm following.

Since it comes only with BOTH 220k mixer's out, that leads me to believe something in what couples them (V1 and V3) together may be a problem, either the grounding plane like you tried, i.e. fixing that brass groudning plate, or the 8uF filter cap... is the cap newer? Is the ground not awesome on it? or maybe even the 27k 1W dropper? those are the components I see as 'linked' to both tubes.

Just a couple areas to think of.

Also when you cleaned the brass grounding plane, did you also clean the jacks connections around there? Maybe the jacks are all experiencing enough crusty stuff that they're making poorer connections? I know they still have the direct ground, but current still can and does flow through all connections, even if slight, and it may be enough to cause that crackle?

Just spitballing, though, to try and rule out things it can't be, and focus on what it might be.

Edited for clarity

~Phil

[lespaulnmarshall]

ok gotcha, now I'm following.

Since it comes only with BOTH 220k mixer's out, that leads me to believe something in what couples them (V1 and V3) together may be a problem, either the grounding plane like you tried, i.e. fixing that brass groudning plate, or the 8uF filter cap... is the cap newer? Is the ground not awesome on it? or maybe even the 27k 1W dropper? those are the components I see as 'linked' to both tubes.

Just a couple areas to think of.

Also when you cleaned the brass grounding plane, did you also clean the jacks connections around there? Maybe the jacks are all experiencing enough crusty stuff that they're making poorer connections? I know they still have the direct ground, but current still can and does flow through all connections, even if slight, and it may be enough to cause that crackle?

Just spitballing, though, to try and rule out things it can't be, and focus on what it might be.

Edited for clarity

~Phil

I have completely eliminated the 8uf cap (an F&T from may last year) and the 27k resistor with no success. I wired up the filter cap board like an AA864, just for a test.

I cleaned the entire plate and sprayed the input jacks with deoxit. I've also had the input jacks disconnected completely, with the girds of the first triodes of each channel connected to ground (although not both simultaneously).

I'm absolutely stumped with this amp to be honest...

Edit: I've also reflowed every solder joint on the board by now.

[Doug R.]

V1 is bass input, V2 follows V1....V3 normal input, V4 PI.

[lespaulnmarshall]

After another afternoon spent with the amp, I noticed that 100k metal film plate resistors did seem to help the noise levels.
I only had 2 metal films on hand, but have some more on the way to try out!

[pompeiisneaks]

Yeah those Carbon Composite resistors are known to cause more noise. It's possible one of them somewhere in the signal chain is causing extra noise. I use Carbon film almost everywhere, because the metal film can be too sterile as they have the lowest noise. Others swear by only ever using CC OR MF just due to the noise or lack thereof. It's a debatable subject . It is proven, though that the quietest are metal film and carbon film are slightly noisier, and CC are the noisiest.

It's odd how intentionally wanting some noise seems to actually help a guitar amp, but not too much. I think even in the dumbles he used some CC resistors in some places where he wanted the noise it introduced.

Even I am not 100% sure I can tell a difference between CF and MF but I've never heard a CF amp I thought that sounded bad so far

It may be worth it to replace any in the signal path with the CF and see if that resolves it.

~Phil

[lespaulnmarshall]

Yeah those Carbon Composite resistors are known to cause more noise. It's possible one of them somewhere in the signal chain is causing extra noise. I use Carbon film almost everywhere, because the metal film can be too sterile as they have the lowest noise. Others swear by only ever using CC OR MF just due to the noise or lack thereof. It's a debatable subject . It is proven, though that the quietest are metal film and carbon film are slightly noisier, and CC are the noisiest.

It's odd how intentionally wanting some noise seems to actually help a guitar amp, but not too much. I think even in the dumbles he used some CC resistors in some places where he wanted the noise it introduced.

Even I am not 100% sure I can tell a difference between CF and MF but I've never heard a CF amp I thought that sounded bad so far

It may be worth it to replace any in the signal path with the CF and see if that resolves it.

~Phil

Honestly I've never had this much noise from carbon comps.. My other amps all have CCs as well and they are less noisy. Perhaps it's the slightly different circuit architecture of the AB165 that's causing the amp to have a bit higher noise floor with CCs, as compared to AB763 circuits, which I have the most experience with. That said, it seems like only the V1-V3 are really affected, so I wonder why that would be.
I'll be trying the metal film resistors in my amp for now as I already have them on the way. I'd really like to see if it quiets down the amp, most importantly. I may very well switch to carbon film after...

I've heard about the guys from two rock inducing more noise in one of their test amps by getting the outside foils wrong on all the caps, and they also said that really changed the sound of the amp altogether, so I suppose the theory of noise affecting the tone of the amp really does make sense.

If the resistors don't do it, maybe it's conductive circuit board disease... although the issues aren't really that severe, and I couldn't get any voltage measurements with my meter from the board, so it does seem unlikely. Either way, the amp sounds great otherwise, so if the resistors don't fis the issue it, might as well enjoy it as is then.

[pompeiisneaks]

well as CC resistors age, they can get significantly worse, but some do it worse than others as well. Basically you can have some CC last for 50 years w/o any issues, and others have serious problems after 10 or 20, from what I gather. It's due to how they're constructed I guess.

~Phil

[Paul G.]

There is one more resistor common to both of those stages. It is a 470K at the top of the board, going from the cathode bias resistors and bypass caps to the top of the .01uf cap feeding V2 grid, giving local negative feedback. Also, that cap, if still the original ceramic disc, is a noise factory. I've had luck changing that resistor to carbon film, and using an orange drop in place of the ceramic.