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Rumbling tube preamp has had me baffled for months

nickdinatale

nickdinatale

Active Member
I'm just about at my wits end with this one. Finally decided to come here for help.

Somewhere over a year ago, I got my pair of McIntosh C8 and C8s mono preamps up and running with a homemade power supply. I also went through and fully recapped, changed resistors, cleaned pots, etc., etc. Before any of this, the C8s had a "rumble" problem, even when I was running it directly off of a McIntosh power amp. Anyway, once I got everything redone, I figured I would be all set and the rumble would have gone away. Wrong! It always comes back, even when it appears to have subsided for a while.

Today I resoldered a number of connections, just trying to make sure I didn't have any cold joints. I also retensioned the tube and power sockets, and cleaned all pots and switches again. I played about two records after getting everything hooked back up and after that amount of time, the rumble returned once again. I'll do my best to describe it - it sounds like a full range static, and it responds to the volume, tone controls, EQ switches, aural comp, and rumble filter. It is also only present in the phono and tape head channels, so that seems to narrow down where it could possibly be coming from.

If anyone has any advice, I would love to hear it. Maybe I'm just missing something obvious, but I think I've covered all of my bases. I will note that there were a few caps that didn't get changed, but they are all ceramic and (I think) mylar. C8s schematic is attached if anyone wants to have a look. Screen Shot 2020-03-25 at 6.38.09 PM.png
 
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No electronics expert here, but if this is a tube preamp, is it possible your tubes in that section are bad, or very microphonic? Just a guess, since you’ve done so much work already.
 
different tubes is the easiest thing to try. The other likely thing I'd have suggested is resistors but you already changed those. Mostly it would be V1, the half of V2 in that section is just a cathode follower. Not so likely to be introducing noise. Quick and dirty, swap V1 and V3 to see if anything changes. Or borrow them from the preamp that doesn't make noise.

does it do this on the mic input too? Thats the other one that uses the low level signal input.

I don't see a tape head input on this one. It appears to be a line level input. Looks like a pair of phono inputs though.
 
Did you put a (preferably an analog) scope on it? Can be as simple as some parasitic oscillation due to some wiring not being routed properly. Can happen with driver tubes due to slight differences in internal capacitances (in the tubes). Do not forget that there are unscrupulous dealers that relabel some Chinese or Russian tubes and they may just be slightly different enough to cause this.

A grid stopper resistor, as close as possible to the tube socket may help here. It can also be due to coupling capacitors leaking or not being just right - sometimes they are far too big for the role they've got to play and replacing them with an aftermarket capacitor can upset the apple cart. I've known amplifiers requiring just that exact coupling capacitor....

Frustrating when it happens.

Hope you get to the bottom of it soon.
 
Right now it's behaving pretty well, but as soon as I have a chance I'm going to swap out the set of tubes I have in it. If that doesn't work I'll be digging into it further. Sort of wondering if I rerouted a wire slightly that could be causing an issue. Thank you for all of the suggestions - I will report back.
 
Finally had some more time to really spend some more time with this issue. Tried swapping tubes (switching positions, as well as swapping out for new ones) with no luck. Upon running it over the past few days, I want to say that just having powered up for a period of time doesn't cause the rumbling. When it has signal passing through it over an extended period of time (usually about 40-60 minutes), that's when it seems to start. I was able to keep it powered up after the rumbling started and scoped the output, then started working my way back. After 10 or so minutes without an input signal, the rumbling disappeared from the scope. Does this make ANY sense?

In response to @gadget73, yep it does it on the mic/tape head input (early C8's were labeled "Mic" and later models were "Tape Head"), so on all low level inputs.

@AmadeusMozart I did put a Heathkit analog scope on it, though I'm no expert when it comes to scopes and signal tracing.
 
something must be getting temperature sensitive. Cold it works and when it heat soaks it works, but in between not so much?
 
You could have a microphonic cap or resistor. Even part of the circuit board or tracings could be noisy. Try tapping the circuit components with a wooden chopstick, while unit is turned-on. Listen for a response. I like using headphones for this test.

Good luck! :thumbsup:
 
With my Scotts and Fisher tube stuff it was always bad tubes and capacitors. MY C-11, 22, MA 230, and Mac tube amps I never had the problem. Now I know C-20's had rectifier problems when first introduced and was one of the reasons for the starts of Mcintosh Amplifier and pre-amplifier clinics. I always found early. pre-amps to be to noisy for me, but most of the time it was thermal noise and hiss. I wasn't happy till I got my C-29. I wish you good luck and hope you reveal to us what your solution was.
 
I'd tend to agree, usually tubes, maybe caps or resistors. If all of it has been changed and the problem remains, it has to be something else. Not that there is much else. Other than maybe some remaining crusty solder, a bad tube socket, or some contamination on the switch causing some sort of signal crossing I don't know.

If you're up for some stage isolation, try lifting the plate resistor from V1A and let us know if the noise continues. If it doesn't, lift it from V1B. If it doesn't go away, it must be in V2.
 
Spent some more time today going through the problem unit. Tapped every single component with no luck. Then while reading some voltages, I managed to short one of the power supply caps (the 20uf / 400v off of the plate on V3b) to the chassis. After I jumped, I realized that the rumble had IMMEDIATELY stopped. I went ahead with changing that cap out and was able to listen to two full records without any rumbling. Hoping that this keeps up.
 
Your diagnosis is probably right. In my experience, power supply electrolytics closest to the rectifier get beat on by the rectifier and seem to die the soonest.
From the schematic that looks like the "first" cap, even though the rectifier is on another chassis. I think you found your problem...
 
hm, if it was new but bad I guess it was just a dud. Happens, but you wouldn't think the old cap and the new cap would both be bad in the same way.

How is the voltage across that cap vs it's rating, particularly at startup? If the voltage swings past the cap's rating it might need a bump.
 
gadget73 said:
hm, if it was new but bad I guess it was just a dud. Happens, but you wouldn't think the old cap and the new cap would both be bad in the same way.

How is the voltage across that cap vs it's rating, particularly at startup? If the voltage swings past the cap's rating it might need a bump.
Click to expand...
When I first started using these, I'm pretty sure I rebuilt the internal power supplies even before trying them out just in order to get the selenium rectifiers out. So if my memory is serving me correctly, it has been the same cap all along. Definitely sort of strange if that turns out to have been the problem component, especially considering the caps I've been using are rated for 450v instead of the 400v it calls for. Crossing my fingers that everything goes smoothly when I listen again today. With that said, I'll check the voltage on startup - not a bad idea either way.
 
nickdinatale said:
Spent some more time today going through the problem unit. Tapped every single component with no luck. Then while reading some voltages, I managed to short one of the power supply caps (the 20uf / 400v off of the plate on V3b) to the chassis. After I jumped, I realized that the rumble had IMMEDIATELY stopped. I went ahead with changing that cap out and was able to listen to two full records without any rumbling. Hoping that this keeps up.
Click to expand...
Tryt LIGHTLY touching the soldering iron to each component on the effected side. See if you can illicit the sound or even make it worse by bringing heat to the component.

OR - buy a couple of cans of freeze spray and spray each component on the effected side to see if you can stop the rumble.

A rumble sound, it's hard to say if that's a transistor. But you never know. Try all components, resistors, caps, even connection points and all.
 
So after a few days, the rumble is still present, though not as loud. As @gadget73 suggested, I measured the voltage on startup to see if it was spiking higher than the cap was spec'd for. It got up to about 425vdc, before coming back down into the 300s. HOWEVER! I tried giving a little volume while having my meter on the plate of V3. With just a little bit of movement from the lead, I was able to reproduce the exact rumbling sound. That particular pin on the tube socket has some movement to it. Could this be the problem?
 
could be. Poor contact at the plate would result in random noises. Might be able to just re-tension the pin if you're careful.
 
It would be unusual if the rumble is caused by a bad contact to a pin of the tube. I wonder if there is instability in the amplifier somewhere, "motor boating" can be caused by a bad choice of power supply setup, some caps that have drifted in value and /or some resistors that have drifted in value. Carbon resistors can have a different resistance value at a different current which is something to be aware off.

It is frustrating when this happens.

I have taken another look at the circuit diagram. You did swap the tubes around but do you use new 12AX7 or original 12AX7 or NOS? I once build an Audio Note clone and it had non-bypassed cathode resistors and it had trouble with hum.Tried different brands of tubes which made a difference but did not remove it. There are big differences in recommended operating points for the 12AX7. The European ECC83 is a better tube (although on paper the same). Raising the filament voltage a lot above ground made the amplifier dead quiet (rather than having the tap on the 6.3V winding going to ground I raised it to 50V DC - stops the cathode-filament diode action. A well known builder (name escapes me at the moment, memory no longer what it was) likes to use 75 VDC.

good luck.
 
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