50-RD drop out when volume at 5+ ...!?

Hi All.
I sure hope someone can offer an explanation/solution to the problem I am having with my '81 50-RD (chassis 1650-RD-CAL). Here is a brief description of the symptoms:
The amp signal drops out completely when I turn the volume control up past 5. This happens on both channels, clean and dirty. The amp sounds good with volume 1-5, but past this point it kinda fizzles out and then..nothing from 6 on up.
I confess I haven't used it as much in the past few years and have just moved from Canada to Europe, where I have an a/c step-down transformer for my North American gear and more time to practice. I remember replacing the tubes in it (12ax7 and two 6l6gc's are posted inside as the tubes it uses) a few years ago and notice now that the output tubes that I put in it are Sovtek 6L6GB not C's.
Has anyone experienced this with their MM amp? I'd sure like to get it back working the way it should.
Thanks in advance, Cam.

It almost sounds like a broken potentiometer! If the resistive track of the pot is cracked somewhere about mid-rotation, the wiper (ouput leg) of the pot would suddenly find itself in an area with no connection to the audio signal from the preamp when turned past this crack!
Steve

Hi Steve.
Thanks for the reply. I was away from my computer for a couple of days.
I thought maybe this was the problem at first as well. I took the chassis out and and sprayed DeOxit on the guilty pots, but problem persisted. So I bought 2 new pots, replaced and.........same problem! Today I tried some new (actually, old Sylvania 6L6GC's) output tubes I just purchased to no avail as well. I'm stumped. It keeps happening on BOTH channels, clean and "dirty", and 5 seems to be the magic number on the clean channel. Rotate the clean volume pot past 5 and the sound completely stops!?! The gain channel seems pretty shaky all around with the sound being very choked (no sustain to notes, amp kinda 'craps out' after note is struck) and very intermittent. I have tried various combinations of volume and gain control settings on this channel and it will only produce very distorted, intermittent sound. Any other suggestions would be MUCH appreciated!
Cam.

Another possibility just crossed my mind... if you have a leaky or shorted interstage coupling cap ahead of the volume control, then you would be passing a DC voltage through the pot. This voltage would vary with the pot's position.
Perhaps when you get to "5" on the pot you have dialed-in enough extra bias voltage to cause the following stage to go into cut-off and stop passing audio altogether? Put a DC voltmeter at the output (wiper) of the pot and see if you can vary a DC voltage when the pot is turned.
I would check all the coupling caps starting from the input (but concentrating on the one just prior to the pots in question). On all coupling caps following an amplifier stage you should see some DC offset voltage at the input of the cap but none at the output. If you can see a DC voltage at the output of a coupling cap (you can lift the output leg from the board to be sure) then it should be replaced.
Steve

Hi Steve.
Thanks again for your ideas (and time) on this. I will print this out and take it to a tech. The next problem will be finding one here in Zurich. I moved here about 6 months ago and haven't needed one so far. Fortunately I have a few "friend of a friend"'s who are in the industry here and could probably help out referring someone. Otherwise, I will bring the chassis back with me to Vancouver next time I go. Man, this amp is going to be racking up the frequent flyer miles if I don't find someone here. Again, thank you for the suggestions and I will be sure to post what was repaired when it gets done.
Cam.