Ampeg SVT CL Rg2 values?

[tubeswell]

I've got one of these in for repair - customer did a self tube change and attempted to bias the amp with the red and green diodes - amp fizzled and is now deadski - haven't opened it up yet. Will get around to it later today.

But I noticed that the http://www.bustedgear.com/images/sch...peg_SVT-CL.pdf schematic shows Rg2 = 22R. Seems pointlessly low. Should that be mean't to be 220R?

[nsubulysses]

220R 1W from all that I've read. I'll be doing one of these up tomorrow with a bunch of burnt screen grid resistors I'll let you know how it goes. Unfortunately you must do a pretty major disassembly to reach these resistors.

Not as nice to work on as the old SVTs

[g1]

Those values should be changed, here is the update:

[Gregg]

There's a technical bulletin from Ampeg which says those resistors should be replaced and more.
I just saw that g1 already attached it.

[tubeswell]

Well I believe I found something. D22, D23, D24, D25 are all shorted (which probably means AC voltage has been dumped onto C24, C25 C26 and C27 methinks - although the caps don't measure as short circuits). D27 and D28 (15V 1W zeners) both measure 1.6V one way and 0.6V the other way on my meter (diode test function). Thermistors CB1 and CB2 both test at about 1R when cold. This is the rectifier and filter section from the filament transformer that supplies the +15/-15V supply (for the bias sensor circuit? and the relay control to the terminal board?). The fault light isn't working. The red and green bias LEDs test okay with a 9VDC battery, All the other diodes in the bias sensor circuit (and everywhere else in the circuit - except for the D55 and D56 in the pre-amp) test okay on my fancy pants meter. All the resistors test okay.

Re the power supply - One of the two white wires from the filament transformer going to the output tube board was an unhealthy blacky-brown/carbon colour near the terminal for all the Pin 7s of the 6550s (like it has been subject to quite a bit of heat). Looks like someone tried to clean up a mess there sometime in the past. I haven't AC-tested any of the power transformers yet. There don't appear to be any DC shorts between the secondary windings or between any of the primary wires and the secondaries. Oddly the mains fuses weren't blown. Yet somehow I wonder about the filament transformer. I don't have a schematic of the primary side for the power supply section. Has anyone got one handy?

None of the screen resistors looked problematic - so I don't think it was an output tube problem at this stage. They were all new tubes - apparently put in after he heard funny noises from the old set. The screen clamping diodes had all been removed and the 22R Rgs had been replaced with 220R. Nevertheless, when I get it resurrected I will check the current draw for each tube to try and get each side of the output stage within the ballpark of each other.

[Enzo]

This?:

http://bmamps.com/Schematics/ampeg/A..._Schematic.pdf

[tubeswell]

Thanks Enzo :-)