bandmaster reverb not well

bandmaster188

was just doing a quick comparison of my 2 bandmasters today and the 76 master volume one seems a bit poorly. i've not used it in a while but this is what i'm hearing.

there is some low pitched crackle coming through which i think went away when the reverb was turned right off. the other thing was the bass frequency just dropped out as i was playing, i flicked the stanby on and off and it came back but then went away again. i only had time to try different valves in v1 and v2 but nothing changed. i'm hoping it is only gonna be valve related.

a bit of amp history..... i had a new circuit board fitted a couple of years ago as the old one had become conductive but alot of the original components were put back on. its been blackfaced. just thinking about it, the last time the amp was properly used it blew a output valve (one of those philips 6l6wgb's mentioned in another dicussion) i had a new set of svetlanas put in and haven't really used it since. that low pitched crackle/rumble is a very similar noise it was making when the circuit board went conductive. (it can't be that though as the new board is a different type to the old)

any ideas folks

 

ICBM

Low-pitched crackle/rumble is usually a faulty power valve. Or it could be a broken reverb return transducer that's making intermittent contact, if it goes away when you turn the reverb off.

When you say it 'blew' a valve, did it short (ie probably) blow the fuse? If so, were the screen and grid resistors checked and preferably upgraded? A cracked screen resistor will cause the power valve to cut out, possibly intermittently - which will cut the bass response - it's common if a valve shorts. A cracked grid resistor will take the bias off the valve and could have been the cause of the old one blowing rather than the other way round. Fender used carbon-comps for both these, which is not ideal - I would use wirewound for the screens and carbon film or metal film for the grids, which are much more reliable. Or it could even be knackered valve sockets, which could do either of these things.

bandmaster188

I don't think any of the resistors were changed as I took it to the amp tech and just asked him to rebias the amp with the new tubes in it. It didn't blow a fuse when the valve died. Shall I post some internal pics to shed some more light on it.

after another quick test, the noise from the reverb seems to be a seperate thing from the crackle. reverb noise is a bit like holding a shell to your ear and gets louder as the reverb is turned up and goes when turn right down. the crackling sound is very intermitent and happens with no instrument plugged in, as does the reverb noise. i'm gonna take it apart a get some pics to put up.

ICBM

bandmaster188 said:

I don't think any of the resistors were changed as I took it to the amp tech and just asked him to rebias the amp with the new tubes in it. It didn't blow a fuse when the valve died. Shall I post some internal pics to shed some more light on it.

Could be useful - also try wiggling the valves a bit while looking at the resistors, that will usually reveal any cracks.

For what it's worth the resistors should really be changed any time an old Fender is being serviced - they can just degrade and crack over time with the heat from the valves, even if one doesn't blow. I admit I don't always though, if they're intact (and stay that way when prodded) and haven't drifted out of value.

bandmaster188

not sure which are the screen resistors. the ones between 4&8 and 5&3 all appear to be intact as do the ones that come from 7 and go to ground. does seem to be a good few components that look like they  need replacing ie some of those ceramic disc caps.

http://i1231.photobucket.com/albums/ee512/bandmaster188/image_zps799f2fb8.jpg http://i1231.photobucket.com/albums/ee512/bandmaster188/image_zpse0a1aa3a.jpg http://i1231.photobucket.com/albums/ee512/bandmaster188/image_zps43461334.jpg http://i1231.photobucket.com/albums/ee512/bandmaster188/image_zps1f761b39.jpg http://i1231.photobucket.com/albums/ee512/bandmaster188/image_zpsea3b86d1.jpg

ICBM

The resistors have all been replaced… although not well - the green ones are the screen resistors and are the right value, but I would have used a slightly higher power rating to make them more robust (those look like 2W, I would prefer to use 5W), and the beige ones are the grid stoppers and are the wrong value (1K, should be 1.5K) although I doubt it makes a real difference.

There also appear to have been 1-ohm bias test resistors fitted, but bypassed - why, I'm not sure! Again I would have used a slightly bigger power rating for these, to save them blowing if a valve shorts. (Or maybe that's why they're bypassed - but I can't see any sign of them being blown.)

If you're going to get the grid resistors replaced with the correct 1.5Ks, have the bias supply resistor of the same value changed as well - just to be extra safe, it can very occasionally fail or at least drift out of value and reduce the bias voltage. The cap has been done already, which is good.

The brown caps on the power valve sockets are the infamous CBS 'tone suck' caps and are best removed really - I would unsolder them at the socket end but leave them in situ attached to the chassis in case anyone ever wants to put them back. (The chassis joints are very difficult to cleanly unsolder or resolder to.) The rest of the ceramic caps are probably fine, they very rarely fail.

I would also remove the ground switch (which doesn't do anything) and replace it with a HT fuse holder - an HT fuse will save further damage if a valve does short, much more effectively than the mains fuse. The reason a valve short does sometimes take out the screen resistor is exactly because the mains fuse often doesn't go quickly enough.

But I can't actually see any obvious cause of the problem, so you're going to need to investigate hands-on… sorry!

bandmaster188

cheers icbm. i think i will have to get it sorted out by a pro really. i've just been experimenting with some other 6l6's incase one of the svets was faulty. i came across the philips 6l6wgb's which i'd kept for some reason(probably cause they were expensive and i couldn't bare to bin them) and put them in. it appears that they are both working fine. i'm pretty sure that at a rehearsal when the amp was playing up that one of them was not lit up or warm so i just assumed it was blown???

the crackling noise is still there though with different power tubes in place.

now to try and find an amp tech in the south east as my one has gone awol. i don't suppose you'd be swinging by biggin hill any time soon.

[-O<

ICBM

bandmaster188 said:

cheers icbm. i think i will have to get it sorted out by a pro really. i've just been experimenting with some other 6l6's incase one of the svets was faulty. i came across the philips 6l6wgb's which i'd kept for some reason(probably cause they were expensive and i couldn't bare to bin them) and put them in. it appears that they are both working fine. i'm pretty sure that at a rehearsal when the amp was playing up that one of them was not lit up or warm so i just assumed it was blown???

Very good news if the valve is OK! Great that you didn't chuck them out.

That's likely to be a power valve socket that needs re-tensioning - a poor contact to the valve pin can do that. In fact I would say it's essential, since if the socket is loose it's Russian roulette for the valve - filament pin, screen pin or cathode pin and the valve just stops working; grid pin or plate pin and the valve blows.

This is something you can do yourself. First, make sure the caps are drained - the easiest way is to remove the rectifier valve, then power the amp up again (fully on including standby), and wait about two minutes. The caps will discharge through the valves and without the rectifier, can't recharge. Then unplug the amp!

Turn the chassis socket side up and get a good light so you can see down inside the sockets. Using a small flat-blade screwdriver, very carefully force the metal 'tubes' closed a bit by levering between the side of the socket and the metal. Don't overdo it or you won't get the valves back in. Do the power valve and rectifier sockets.

Preamp valve sockets and some power valve sockets with 'pitchfork' contacts are a bit trickier, you need something like a large needle to fit into the tiny gap behind the fork prongs.

bandmaster188 said:

i don't suppose you'd be swinging by biggin hill any time soon.

Sadly not...

bandmaster188

cheers fella, always good advice!