Ampeg SVT-VR Bias control

[SimTech]

I have been working on an Ampeg SVT VR for a few weeks now and replaced the power tubes and all electrolytic caps and am waiting on the owner to buy new preamp/driver tubes also. After a complete teardown (and some muscle relaxers for my now wrenched back due to 85 pounds of beast on the bench) I did a quick power up. I had to go it on faith because my variac can't handle the current of this monster. Good news is, no smoke, bad news is I can't control the bias settings at all so my time powered on is as short as is possible as half the tubes start to red plate as the HV comes up. I'm hoping the new preamp/driver tubes will help.
Anyway, I also replaced the 220 ohm screen resistors and the 5 ohm resistors on the tube board. The 47.5k grid resistors all check good. Also I had to replace R24 on the preamp because it had changed to 9.3k.
When I powered it on first the Balance LED was lit and one Bias LED was lit. After a brief full power check and a quick sweep of the bias pots to try to kill the LEDs I noticed the ates of the new power tubes starting to heat up and I shut it down immediately. I haven't been able to leave it turned on for more than a few seconds since it tries to melt the tubes right away.
So then I got online and found this forum where I've found a possible treasure trove of information that I can't get to. I don't know if it's because I'm new here or what but throughout all the articles and forum posts there are countless links to documents and such but I can only access links to previous posts. While there are alot of similarities in the SVT-VR and CL they are not absolutely identical and I'm wary about variances in values and possible mods to fix things that make sense. For instance, the mod to insulate/isolate the 12v regulator. I just added 2 lock washers between each of the standoffs and the chassis. This lowers the heatsink away from the chassis so ground loops aren't possible. My experience is you never float anything especially a heatsink. Cutting foil runs and mica insulators and all that just gives you a piece of ungrounded metal inside and that's never good.
Anyway I'd love to write alot more but for now my main problem is the lack of documentation. The schematics found on bustedgear are incomplete and don't have the transformers, output board, or the ac-term board nor are there any layout drawings. It appears that nevetslab has posted numerous links to lots of pertinent information but none of them are accessible. Before I can really go any further and ask for help or whatever I'd like to have a look at the full schematics and drawings. Does anybody know when, if, or where I can get access to these documents as I have not been able to locate them anywhere else and there's links to the documents I'd like to look at here but I can't view or download them.

[Helmholtz]

Did you check grid voltages at sockets with power tubes pulled?

[g1]

I don't think there are any attachments that are not accessible, even to non-members not logged in.
Have you tried another browser? There were some attachments that were lost, but usually those re-direct to the main page, or give 'invalid attachment' message.

[Mars Amp Repair]

Here's the SVTVR schemos;

[Mars Amp Repair]

I see where C-10 off off the plate of predriver tube J5-A & C-6 off of cathode of predriver tube J3-A if shorted or leaky, will adversely affect the power tube bias via J6-a/b. I have found these caps shorted/leaky before. They're usually rectangular and red in color.
Although will not cause red-plating, be certain you do have a 12AU7 in the proper position. The 3 predriver tubes are marked on the chassis. I believe the 12AU7 tube is on the far left viewing from the rear, but don't recall exactly. The tube numbers are marked on the chassis.
The bias adj/bal detecting circuit is comprised of 4 comparator IC's that take they're inputs from the cathodes of the power tubes. that circuit would not affect the actual bias but if defective, would give a false indication of the actual bias on the tubes. Since you have red-plating, the bias is obviously way off.
I'd measure the bias on the grids of the power tubes as I'll bet it's wayyy off. See the voltages on the schematic. The source of the bias voltage is from the -150V supply. That should be verified, also.
glen

[SimTech]

Well I finally managed to get a few more schematics thanks to Glen and a computer that doesn't care about the security certificate for this site but one of the things that's holding me back from powering on this thing again is that I'm unable to confirm the wiring. The 16-412-xx is exactly what I need but it's for the SVT-AV and this is an SVT-VR. Does anyone have a schematic that says what color wire goes where that is known to be correct. I'm convinced that someone worked on this amp before me and they gave up and sold it to the guy who owns it now because I just looked back at my teardown pics and there's a blue/WHT wire connected to J35 on the AC-TERM board. That CANNOT BE right I just know it. Somebody please help. But please keep in mind that while all these big 300w SVTs may look similar they are NOT. The transformers are big enough to run a small arc welder so everything MUST be correct or I'm going to melt something for sure. If it helps the serial number begins with DWTDSA00.
Thanks,
Russ

[g1]

Here is a transformer diagram nevetslab drew up, maybe it will help.
Otherwise, not sure what the 16-412-xx drawing you referred to is. If you could point us to it, we will not exactly what you are looking for (but wanting VR version).

[nevetslab]

I'd start with the six power tubes removed. Mark on their metal base their position in the chassis. The position numbers are back row, left to right: V4, V2, V3 and front row: V5, V6, V1 as viewed from the rear. V7 is the 12AX7 input stage to the power Amp, V8 & V9 are the 12AU7 Driver Tubes. Pin 8 is the Bias Voltage being applied to the grids of the six power tubes. Nominal bias voltage is -52VDC, but this will depend on the tubes' transconductance. Final Bias Voltage for nominal 23mA Plate Current will depend on that. With the six power tubes removed, you can check the bias adjustment range without causing any harm, and is an aid in troubleshooting the bias circuit, armed with the schematics.

Before finally fabricating a clear plastic Bias Adjustment Shield for safe access to the plate circuits of the six tubes, I had made one from a hard fiber notebook cover using the metal cover plate as a guide, and calipers, machinist square, scribe to lay out the pattern. On the SVT-VR, you're measuring plate current at dangerous potentials (650VDC), which always made me VERY NERVOUS. Here's some images of the Service Plate I had fabricated:



Once you have the Bias Circuit figured out and working, you can test the power tubes in pairs as I have shown above. It was much easier after finally committing to laying out the hole pattern on the clear plastic sheet, pre-cut to size. With the Brown Fiber Test Shield, I did my best to cut the slots for the probes above the associated plate resistors and test points, though still a bit of fumbling while getting the long EZ-Hook probes to clip onto the resistor leads.