Reducing noise in Carlsbro valve PA head

[Martin Beer]

I have a late 60s Carlsbro CS100/8 head which I am servicing with a view to using it as a bass head. I have already replaced all of the filter caps, replaced the power valves with EH 6CA7s and replaced a bad resistor in the bias section, and the amp seems to be pretty healthy, except for some background noise which I would like to get rid of. The noise consists of fairly high frequency hiss with a random crackly element to it, which comes and goes intermittently. It is noise based, rather than sounding like any sort of oscillation. It is independent of the channel volume and tone controls and master volume.
So, where is a good place to start looking? I figure that cleaning the valve sockets and swapping out the phase inverter ECC83 are options, but are there more things I should look at? The amp uses carbon comp resistors throughout which I have read can be noisy - is it worth replacing these in some positions?
The schematic of the amp can be found here: Carlsbro CS 100/8 P.A. Amp Schematic

[Alex R]

There isn't too in there after the MV. I'd start with renewing the 100K and 91K PI plate resistors.

Does it have enough gain for your bass?

Hey check the OT - is it an RS?

[Martin Beer]

I'll try changing the plate resistors and see how it goes. The gain seems reasonable, as I have no problem getting the volume I need, but it's a very clean amp. I've changed the tone stack in one channel for a Fender type, just for a bit of variety. I might try cascading two of the input stages (losing the tone stack on one) to make a drive channel though. There are no markings on the OT, so I don't know who the maker was, but it looks identical to this one: Servicing a Carlsbro CS100/4 PA Amplifier

[Alex R]

Oh right. No there was a very old Carlsbro PA amp that used the RS Deluxe OT, but that's not one of those.

[Rhodesplyr]

I would start with the ECC83. Tubes can be noisy, especially if the pins are very dirty.

A simple bad solder joint is also quite capable of generating that kind of noise. If you know what you're doing, you can poke around with something non-conductive, and you'll know if you move the bad connection. Years of heating cycles on tube socket terminals can cause some cracking of the solder joints because the heat expansion of the terminal is slightly different from that of the solder itself.

There's something about the way the negative feedback circuit is drawn that I don't get. It seems like an error to me.

64uf capacitors? Weird.

[Steve Conner]

Edit: I reread the above posts and deleted a bunch of my comments. If the noise is independent of all the other controls, it probably is the PI tube or its plate resistors. I had these plate resistors go noisy in my old Selmer T'n'B 50.

These amps were supposed to blast out at club volumes, so they can seem pretty noisy when you listen to them at home. The noise would be drowned out by the babble of drunken punters.

[Martin Beer]

Swapping the PI tube didn't alter the noise, so I'll try the plate resistors next. I'm not sure where a 91k resistor can be obtained, but a 1M in parallel with a 100k should get me there (that may even be what's in there - I don't have the amp in front of me right now).

[Martin Beer]

I've just revisited this amp and tried swapping valves, cleaning the sockets, poking around the PI circuit for bad joints (and re-soldering a couple which looked dull), as well as replacing the PI plate resistors and a drifted 22k resistor in the power supply with metal film types. The noise is still present - should I look at the other resistors on the PI next, or just accept it as the nature of the amp? Because the level of the noise is independent of the master volume, I still get a usable signal/noise ratio at the kind of volumes I'd gig with.

[Guitarist]

A couple days ago, I replace the first pair of filter caps on an old Univox. Cost was around $12. V1 is also a good idea, any size. I really noticed the difference + you can hear the subtleties of the guitar. Thereabouts, also changed the plate Rs to metal-films too. Also went 5uF on the cathode caps. So roughly the same approach.
Keep pouring on the TLC, that bass amp will come around.
Is it just me, or do you guys also work hours and hours on an amp?

[Martin Beer]

Hmm, yeah, I do have a good few hours sunk in to this amp now. I've replaced all the filter caps, put a fender tonestack in one channel (only to find I prefer the James stacks in the stock channels most of the time), put in new 6CA7s and renewed some drifted resistors to get the bias right. The ECC83s are all old Brimars, possibly original, so perhaps there'd be less noise with new ones. It's just a matter of deciding how fastidious I want to be about reducing noise. I took the amp to a recording session today, and the noise level was no problem with the master up at about 10-11 o'clock - the engineer didn't comment on it and the amp sounded great. So maybe I shouldn't stress too much about getting it quiet at home practice levels, and just keep it how it is now.

[Giaime]

What I would try:
- elevating the heater reference from ground
- replacing every R & C in the amp (even plastic capacitors) - extreme solution but if you plan to gig the amp, it's worth it.

[Martin Beer]

Fixed it! Eventually...

I'm pleased to say I've finally got it sorted. The amp is really quiet now - with the master volume down it's practically silent and any preamp hiss isn't audible until the amp is running extremely loud. It was much simpler than I'd expected - one of my EH 6CA7s was on the way out. I'd just assumed that as they were fairly new, they must be Ok! Unfortunately changing the power valves was the last thing I tried, as I didn't have any spares around. Before I tried that, I'd already installed new ceramic octal sockets in case it was an arcing issue, and replaced a load of resistors with metal film. So, it was definitely a learning experience, as I've done a lot of work that didn't really need doing for what was actually a very simple problem. I'm not too bothered about that though, as the inside of the amp is looking very tidy now, it should be dependable for a while to come, and I feel like I've learned a bit in the process.