Fender Bassman Blackface '64 restore

[Shannon]

Having achieved some confidence in my soldering skills and with a LOT of assistance from the experts on this forum (see http://music-electronics-forum.com/t14055/ ), I've decided to tackle my crackling and popping Bassman 50 head and open it up to check the state of the caps (afterall it is old).

The amp has started to make quite a few more crackles and pops, even when the guitar is plugged in and idle. It gets worse when you attack the strings.

I bought this amp from a music teacher and it had been recovered, new tolex equivalent (feels like leather touch) and new original Fender grille cloth and there is a sticker saying that it was booked in for a service in 2000. However, the internals look untouched and seem to have all the original components. Tubes seem to be fairly new according to previous owner.

The amp came with a 4 x 10 cab that had also been recovered with new grill cloth etc.

The concerns are as follows:

1) buzzy noise with crackling and popping noises on both channels. Gets worse when playing - especially enthusiastic playing. Gotten worse over the last few months.

2) Power transformer wires look that they are burnt (discoloured and close to black - see attached picture)

3) pilot light comes on when it feels like - often after a long time in standby, I return to find it lit. Most of the time it does not come on.

4) Should I replace all caps, including power filter caps. How can you tell that these are leaking - visually they look fine.

5) What are those blue caps? There is no discernible value - unless its on the underside and I need to desolder it to see.

Like my Silverface, this amp is hardly played. It seems they may have had hard lives before, but are literally played once a week if that.

Any assistance is appreciated.

[TD_Madden]

Personally, I'd replace ALL electolytic caps in that amp, and leave the rest of the caps alone.

The pilot light situation sounds sort of like the socket itself is bad or the bulb should be replaced; I'd replace the bulb as a matter of course..they're pretty cheap.

The 100K ohm resistors in the preamp section tend to get noisy (snap, crackle, and pop) over time, I'd suggest swapping those out as well.

Don't know much about transformers...maybe someone else guide you there. Have you biased the output tubes?

Good luck!

[Shannon]

Thanks for the input.

The 100K ohm resistors in the preamp section tend to get noisy (snap, crackle, and pop) over time, I'd suggest swapping those out as well.

I've replaced the resistors with metal film 2watts (oddly I can't source 1 watt metal film locally - the SA market is primarily using 1/4 watt metal films) and some were off the rated resistance by more than 20%. Most were just within the 20% range. Had to use 2 watt carbon film ones where the metal film ones were not available eg 1k5, 820, 82k.

Personally, I'd replace ALL electolytic caps in that amp, and leave the rest of the caps alone.

I also changed all the electrolytics in the chassis from 25uF 25V originals to 22uF 63V equivalents. Getting axial values close to the ones required proved impossible.

There is a cap next to the transformer in the circuit and the diodes that was a 25uF 50V which I replaced with 22uF 250V axial (metal blue can).

I am busy with the power filter capacitors, but these a tricky to arrange as I am replacing them with 2 x 47uF 350V radial in series for each Mallory 20uF 550V. The original 70uF 350V is being replaced by a 68uF 400V radial.

The only concern is the ceramic disc caps. I worry that my soldering may have inadvertently damaged them. The cap tester gives some odd readings on one of them. Do I have to stick to the 3nF values?

Also the circuit diagram I have is unclear on the other values of the ceramic caps....

[Shannon]

Finished the replacement of components tonight. Amp no longer crackles and fizzes. For the first time there is a distinct difference between channel 1 and 2.

However there still is some residual hum that hasn't gone away.

I did not replace the resistors on the pins of the output valves (470 ohm carbon comp) and resistors in the filter cap tray (didn't have the values handy in 2watt metal film) and the resistors on the input jacks of channel 1 and 2.

Should I change these as well? Would they make the difference?

[TD_Madden]

that electrolytic next to the transformer is in the bias circuit, I'd actually use a larger value (I went to a 100mf/100v in my Vibrolux as I recall).

A good bias adjustment may minimize the hum as well...if the tubes aren't closely matched, that might give you some humming. If those are the original tubes, you might consider replacing them and rebiasing.

Any of those carbon-composition resistors receiving high voltage should probably be replaced (such as on the tube sockets and in the cap-pan.)

[Gtr_tech]

Thanks for the input.


I am busy with the power filter capacitors, but these a tricky to arrange as I am replacing them with 2 x 47uF 350V radial in series for each Mallory 20uF 550V. The original 70uF 350V is being replaced by a 68uF 400V radial.

Did you add parallel resistors on the caps you stacked in series as subs for the 20/500v caps?

[Shannon]

Did you add parallel resistors on the caps you stacked in series as subs for the 20/500v caps?

No. Is it necessary/required? What is the danger, if any, of not having them in?
I've already played the amp for a few hours...

[Gtr_tech]

You really should. Notice how the main plate supply filters (70u 350v) have the 220k 1w across them.....that keeps them balanced so one doesn't "see" more voltage than the other in the stack. Look at the schematic.

[Shannon]

You really should. Notice how the main plate supply filters (70u 350v) have the 220k 1w across them.....that keeps them balanced so one doesn't "see" more voltage than the other in the stack. Look at the schematic.

I get it. Thanks. I'll implement the same scheme for the 47uF in series branches.

Thanks again.

[Dave Curtis, dB AudioTech]

I also changed all the electrolytics in the chassis from 25uF 25V originals to 22uF 63V equivalents. Getting axial values close to the ones required proved impossible.

You can get 'em:
Price List and Order Form for CAPACITORS for Antique Tube Radios
I haven't had any problems with them.

.22 is fine, though, you can't tell the difference.

I am busy with the power filter capacitors, but these are tricky to arrange as I am replacing them with 2 x 47uF 350V radial in series for each Mallory 20uF 550V. The original 70uF 350V is being replaced by a 68uF 400V radial.

As Gtr tech mentioned, use balancing resistors on your stacked caps. They're also called "bleeder resistors" as they drain the voltage off to safe levels after power down within a short period of time (which is an NEC requirement nowadays, BTW).