Bassman RI troubleshooting

[Bluefinger]

Hi,

I could use some ideas. I am trying to debug an older Bassman RI (pre LTD) that belongs to a friend on mine. It makes a loud crackling noise sometimes (like bacon frying). Sometimes it is there as soon as I switch it on, sometimes it appears after a while and soemtimes it's quiet for hours. If it does crackle it sometimes goes away by switching the amp on and off. The power and standby switch seem to be ok tho. After switching the amp on, there are always a couple of loud cracks and pops. The volume pots do not affect the volume of the crackle but the tone stack does change the sound So I assume that the crackle comes from somewhere between the volume pots and the tone stack ... or maybe from the power rail (filter caps?). The solder spots look solid but I have resoldered most of them anyway. Tapping on the components does not affect the crackling but most areas of the PCB are very microphonic. I have also cleaned and tested the contacts on the input jacks and cleaned the tube socket contacts. The tubes have been replaced as well.

How would you proceed? Do the symptoms sound familiar? Replacing a component on the PCB correctly would require disassembling the whole internals because everything is soldered on the back of the board (who's idea was that?) so I don't want to do this over and over again. I'm not very fond of cutting the legs and soldering the new component to it.

Looking inside that amp makes my cringe anyway. I'm used to my hand wired amps so all I see is inferior components, tiny wires with isolations that melt away easily and a very cluttered layout. If it was my amp, I would tear everything out and handwire it but since it isn't I wan't to keep the cost low.

I thought about tearing it apart once and replacing all the resistors, tone caps and filter caps with quality components. I'd like to replace those cheap tube sockets as well but the holes in the chassis are too small for the ceramic sockets I like to use. Any other ideas?

Thanks and all the best!

[pdf64]

Does the noise ever appear if V1 is removed? If yes, then how about with V2 removed?
If removing V1 or V2 stops the noise, the the plate resistors (R8, R9, R14, see schematic below) are the likely suspects, ASSUMING that V1 and V2 are good.
http://www.schematicheaven.com/fende...man_manual.pdf

[Bluefinger]

Hi,

Thanks for the response! I have shortened out the signal from V1 and the noise still was there. With V2 disabled, it was gone. This might mean that the source of the crackle lies before V2 or that the noise was temporarely gone while I have done this. The resistors look like they are carbon film so I guess I will simply replace them all with metal films while I have the PCB disassembled. They cost next to nothing and should improve the amp's future reliability. I might even replace the caps and electrolytics as well while that amp is open.

All tubes are good. I replaced them all and the crackle still has there.

[pdf64]

If fitting metal film resistors, it's best to check their voltage rating - many are only 250V, some 350V, few are 500V.
The 350V ones might be ok for plate resistors in your amp, but 500V would be preferable.
Carbon comp and carbon film are generally 500V.
For the plate resistors, I'd fit turrets, mount carbon comps on them, and just replace them every 15 or 20 years, with the electrolytics. Geofex indicates that they add measurable 2nd harmonic when used sensibly.
http://www.geofex.com/Article_Folder...carboncomp.htm

[Chuck H]

The early Bassman RI used 1/4 watt plate resistors. I don't know about the later ones. These have been known to go bad and crackle. Since you are replacing resistors anyway I would suggest not only checking the voltage rating on the new resistors (as suggested) but also upgrade the preamp plate resistors to 1/2 watt rating.

As for the board being microphonic, it has to be a tube or component. The board itself is not microphonic.

Chuck

[Bluefinger]

I'll be using .6 Watts resistors. I'd have to check, but they are rated at 450V IIRC. Usually I prefer 1 Watters throughout the whole amp because they are bulletproof but the thicker legs don't fit through the PCB. I never had a problem with thoem so I guess I'll be using those.
I have to admit that I sometimes use carbon comps as plate resistors myself because of the qualities they are supposed to have but to be honest, I can't hear a difference. Maybe I'm just deaf

[Bluefinger]

Yes, of course it's not the PCB itself but there must be a couple of components across the board that are extremely microphonic. Since I have to disassemble the whole amp anyway to get to the back of the board I'll use the opportunity to replace everything in there. That should rest the case for the next ten years or so. At least I hope so ...

[Chuck H]

Ceramic disc caps are probably the most prone to microphonics of any component. Be sure to tap the tubes. A microphonic tube will make a ping or pong almost no matter where you strike the amp.

[Enzo]

Especially the Chinese tubes. The Chinese are particularly good at ping pong.









Thank you thank you, I'll be here all week. Try the veal.

[Bluefinger]

Tubes have been checked and are ok. There aren't any chinese tubes in there anymore BTW. I'll check the disc caps. I have already replaced the 100pF bright cap because it caused some hiss but I'll replace the others as well just to make sure.

Actually it was the V2 plate resistor that caused the crackle. I's a 100k resistor and it's value has already drifted to 269k so this one has died for sure.

Thanks everybody!

[Billy R aka DynaFreak]

I replace a lot of plate resistors on PCB amps. I really like the Dale CMF60 series. They are small for 1W resistors, rated to 500V, leads are small enough in diameter to replace 1/4 W components, relatively inexpensive, and made in the USA! Available from Mouser.

[Bluefinger]

Thanks Billy, I'll check those out!

[Rhodesplyr]

I second the Vishay-Dale RN60/CMF60/CMF55 series resistors: very low noise and temperature coefficient, rugged, non-magnetic, and endorsed by audiophiles for Hi-Fi amps. Great value for the money.

If you want some carbon in your sound, Mouser also carries KOA Speer SPR series carbon film resistors, but you have to know to look under 'Power Resistors' or search directly. In 1% tolerance, these are rebranded "Kiwame" and sold for higher prices by boutique parts dealers. They're popular as plate resistors, especially if higher power ratings are needed. They come in 1, 2, 3, and 5W ranges.

[Bluefinger]

Oh man, that plate resistor sure had his best days behind him but it still wasn't the reson for the crackling. It came back a day later. Fortunately I have not given it back to it's owner yet. I finally did what I should have done earlier. I ripped out all the internal components and replaced every resistor and cap with metal films, Orange Drops and TAD electrolytics. The wobbly plastic tube sockets have been replaced with ceramic ones and the thin wires (some of them already started to break near the solder joints) were replaced with solid cloth covered pushback wire which allowed me to run the wires close to the chassis. I used a more vintage style filtering (4 22µF in the cap can and 3 8µF on the board). The stock EH preamp tubes sounded thin and sterile so I went with a Tung Sol 12AX7 in V1 and JJs in V2 and V3. The Sovtek 5881 were replaced with TAD 6L6WGCs and the solid state rectifyer was replaced with a tube. This is a whole different amp now. It used to have a pretty harsh, aggressive tone and now it's fat, creamy and punchy. Due to the lead dress and higher quality resistors the hiss and hum is at least cut in hals and the most important thing of them all is that the crackling is gone. I will never know what the actual reason was but I spent less time with the complete rebuild than with the troubleshooting before. The only thing left to do is hoping that the amp's owner will like it as much as I do ...

[garyf9]

Did you ever find a solution to this problem? I have had the same with my 410 Bassman. It was an intermittent crackling, but now it has advanced into consistent weak, scratchy output.