AB165 Bassman crackle in V1 and V3 stages??

[lespaulnmarshall]

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.

That is really great advice there! I have a 470k metal film on the way as well, although carbon film might be even better, tonally. Lifting the resistor indeed takes care of the problem if the preamp tubes are pulled. Tons of noise of course with the preamp tubes in place if the resistor is lifted, but that is to be expected. Will try the metal film in that spot. The coupling cap there is a TAD PIO currently.

[angelodp]

Have you tried replacing all the tubes in order with known tubes? I have the same amp and cathode caps and tubes did the trick.

Ange

[lespaulnmarshall]

Have you tried replacing all the tubes in order with known tubes? I have the same amp and cathode caps and tubes did the trick.

Ange

Hey Ange,

Thanks! Yes, all cathode caps replaced with brand new Spragues. I've even tested the new caps to be sure they were ok. Have tried multiple sets of known good tubes as well. Currently hoping the plate resistors and 470k will do the trick !!

[Doug R.]

Look to purchase an inexpensive scope...that will make your life a lot simpler when tracking down noise and other issues. Changing resistors is a good idea, but if that is not the source of your problem....what next ? Just my 2 cents..

[lespaulnmarshall]

Look to purchase an inexpensive scope...that will make your life a lot simpler when tracking down noise and other issues. Changing resistors is a good idea, but if that is not the source of your problem....what next ? Just my 2 cents..

I have one actually, but it hasn't helped me much with this one... I don't have an isolation transformer at the moment so I'm not willing to take risks hooking it up in high voltage areas of the circuit. Haven't had much luck using it in other places with this amp. Have also already eliminated pretty much all other possible problems already..

[chrisoldman]

What happens if you lift one end of the 220k mixer resistor between the Bass channel and the grid of V3

[chrisoldman]

It maybe the case that the eyelet board has become conductive. Had this problem with a Twin . Had to mod as follows actually lift the tone stack cct off the board

[lespaulnmarshall]

What happens if you lift one end of the 220k mixer resistor between the Bass channel and the grid of V3

It was noisy unless those resistors were lifted for both channels. The issue is now solved, more info in the post below.

[lespaulnmarshall]

The issue is now solved! I had metal films on the way but decided to go the carbon film route in the end so it took me a little longer to get the amp fixed. Paul's post below was the golden ticket! That 470k is what created the noise in my amp as it was a carbon comp. I've never heard about that specific resistor causing noise before, there doesn't seem to be any talk about it anywhere on the internet, so hopefully this thread will help some people. Paul, I'm not quite sure how to thank you!!

While I was at it, I also replaced the 220k mixer resistors and all the plate resistors with carbon film ones. When I flipped the standby switch I still had some crackle that would fade away in the first minute of it being on, dead quiet operation after that. I figured that wouldn't be a big issue, and my first instinct was it being a bad preamp tube, and indeed it was. I had gone back to the original tubes that were in the amp when I got it as known good ones hadn't solved anything before. The reason the tube issue was now noticeable was that the amp now operates so quietly. With the level of noise/crackle it had before the tube problem would have just gone unnoticed.

Anyway, super happy to have this problem worked out, who would have thought one single resistor could cause so much trouble (literally days of troubleshooting)?

Thanks everyone!!!

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.

[pompeiisneaks]

I don't know about online, but I do know it's a chapter in either the tube preamps for guitars book or the hifi tube book by the valve wizard merlin blencowe that covers the kinds of noise that each of the different components create etc.

I highly recommend both books.

~Phil

[lespaulnmarshall]

I don't know about online, but I do know it's a chapter in either the tube preamps for guitars book or the hifi tube book by the valve wizard merlin blencowe that covers the kinds of noise that each of the different components create etc.

I highly recommend both books.

~Phil

Cool! To be honest I have not yet read enough books when it comes to amps. Will look into both of these. Thanks for your recommendation!

[martin manning]

In a nutshell, noise-wise, MF are almost perfect, followed by CF, which are much better than CC. This CC you replaced had clearly gone bad.

[lespaulnmarshall]

In a nutshell, noise-wise, MF are almost perfect, followed by CF, which are much better than CC. This CC you replaced had clearly gone bad.

This is the interesting part, I thought so too, but I've tried two different CCs in that position before (both good) and both were noisy...

[martin manning]

Were these new CC's or old ones?

[lespaulnmarshall]

Were these new CC's or old ones?

Brand new.