SVT-VRU Repair

[Gingertube]

G'day from Oz,
I have a SVT-VRU S/N EFXUU60023 to repair for a guy who is about to leave Oz with his band for a tour of the UK.

I found schematics on this site for the SVT-VR.

What is the significance of the "U" suffix in the model number?

Fault is in the power section - I put the six Sovtek 6550E through my AVO MK3 Tube tester and found 1 tube totally dead and a 2nd that tests OK except for the fact that it has a heater cathode short.
I had 3 matched quads of 6550 on the shelf so have managed to put together 6 new matched output tubes.

Are there any known factory mods which I should be doing as I repair this unit?

Thanks,
Ian

[Gingertube]

Bias Adjust

About to fit the new set of output tubes.

Instructions say to set bias controls to get the LED lighted.

Do I turn the bias adjustments fully anti-clockwise before starting?

Cheers,
Ian

[Tom Phillips]

...Do I turn the bias adjustments fully anti-clockwise before starting?

I can't say for sure but before you install the power tubes you could measure the voltage at the power tube control grid and start with a setting that gives you the most negative voltage reading. That would also be a good chance to verify that each tube is receiving bias voltage.

[Gingertube]

Tom,
Good Advice for anyone doing a retube of any amp - identify potential distasters BEFORE they have a chance to happen.
Thanks,
Ian

[Gingertube]

What a pain this amp has turned out to be.
1st I found that there were no screen supplies to the output tubes - The 6K8 10W dropping resistor was open, when I removed it there was an area of charcoaled board underneath where it had been arcing. Replaced that (with 2 off 3K9 5w in series) and it still did'nt fire up thou' there were now screen volts. Looked further, found the 12V reg (7812) for preamp tube heater supply had blown short putting 18V on the heaters, put all the preamp tubes thru' the AVO MKIII tester, they seem to have survived. Still would'nt work. More checking, something is dragging down the +/-15V rails for all the fancy bias/balance monitoring. This is turning into an epic. Public Holliday here in Oz tomorrow (Australia Day) so may get to progress it further. Did I mention I'm not a big fan of these amps?

Oh! - I did find that max anti-clockwise on the bias controls is maximum negative bias / minimum idle current.

Cheers,
Ian