help me put humpty together again(Sound City Content)

[truth_hz]

Hey all,

I'm working on a sound city L100W. Poor thing looks like a monkey with a soldering iron got into it. I have a very strong suspicion that it is this very same amp from this thread:

http://music-electronics-forum.com/t17257/

anyways, burnt up screen resistors, terrible tube socket wiring... The OT tap to one set of EL34's was not even soldered, and making intermittent contact. Bias supply was all wonky. 10 amp(!!) mains fuse. Chopped up vacuum cleaner power cord, etc. There looks like there was a bunch of work done, and NOT done well.

I thought it was funny that someone spent a bunch of money on a shiny new Heyboer PT, and then did such a terrible job on all the other stuff.

Anyways, if you read the thread above, you'll note that this circuit isn't stock, and I would like to return it to somewhat original shape. For reference, here's the schematic:

http://www.soundcitysite.com/sc_webpages/B100_schem.jpg

My question pertains to V2....

1) Is that really a 10k anode resistor? I know it is shared between the 2 halves of the tube, but it seems way too low for there. I have dug around without a clear answer, but I did find a few pics here:

J-Flats.com: Click image to close this window

showing a 220k (which is what is in mine). I looked for other gut shots, but they were all to small to see clearly.

2) Is the resistor on the last B+ tap 68k(the one feeding the input tube stage anodes through the 220k's)? Mine has a 10k in there now, again way off.

Anyways, with all that stuff wrong, it still sounded decent before I unbuttoned it to look. As soon as I pulled the chassis, my eyes popped.... Slightly scorched EL34 socket, replaced bias pots, and all the other goofy "mods".

I'd be so appreciative of anyone with a L100w that could help me out by letting me know the value of those 2 resistors in their amp.

thanks much
-Morgan

[booj]

Considering you have 210 volts at HT+3 and 190 volts on the plates of V2, I'd say it's likely that the plate resistor is indeed 10k.

[truth_hz]

Considering you have 210 volts at HT+3 and 190 volts on the plates of V2, I'd say it's likely that the plate resistor is indeed 10k.

I guess I'm just second guessing myself. I did get that impression looking at the voltages, I'm only hesitating, because I have seen examples of both. another gut shot, this one actually seems to match the schematic:

Amp Archives/Sound City/69 Sound City L100W SN M3239/inside3

It just seemed weird, that's all. Been staring at it too long, I just need to trust the schematic. I have heard there were different versions of this amp, so I was curious if anyone else had one that way.

Thanks for taking the time to reply.
-m

[truth_hz]

Update!

Whew, it was a mess in there. I think I finally have it sorted though. After all was said and done:

-All Electrolytic replaced, the originals were dry as a bone
-All new bias circuitry and 50k bias pots to compensate for the higher voltage bias winding on the new PT
-New Screen resistors
-New Power tube sockets
-PI Anode R's and Coupling caps
-Rebuilt V2 back to stock, including the goofy trimpot Cathode R's
-New Mains and Standby Switches
-Elevated the heaters(70v-ish)
-Some new B+ dropping R's

This got it to playable condition, biased it @ 38ma/tube. Sounding good now, but there is still a bit of noise(hiss, not so much hum) after 5-6 on the volume dial. By that time, it's freakin loud as hell.

I'm probably going to go back and use shielded wire from the PI to the power tube grids(it's about an 8" run). I'll probably check the rest of the coupling caps for DC as well.

If anyone has any ideas what parts of this amp might be more prone to noise, I'm all ears!!

Thanks Ampage!
-Morgan

[truth_hz]

another update....

I've been having some noise/rf issues, and after staring at the schematic all night I noticed something:



If you plug into brilliant #2, or normal #1 (the higher gain inputs) it leaves the 68k resistor thats tied to the Low inputs floating in a loop on the input tube grids.

I've dug up a bunch of Fender and Marshall Schematics, and this definitely seems off.

Shouldn't the NC shunt on the brilliant #1 and normal #2 jacks be connected to the Tip lugs of the brilliant #2, and normal #1 respectively?

This seems to be how everyone else does it. It seems a little weird to me that apparently it's wrong on every Sound City schematic I've seen.

If anyone can confirm, or tell me I'm nuts, I'd appreciate it.

-Morgan

[Ronsonic]

British schematics always make my eyes hurt. I think you're right, is that actually what's happening in the amp?

[truth_hz]

British schematics always make my eyes hurt. I think you're right, is that actually what's happening in the amp?

I'm picking up a bit of radio in the amp, mostly as I turn up, but I know its there at the lower levels. It's also way to hummy with single coils. If I use a hum-cancelling position on my strat, it's greatly improved. I figured the input tube grids are a pretty lousy place to get hum, since it'll ride along through the whole amp. And even when you plug into the Low input of one channel, the other channels grid still has that funky loop around it.


The way the schematic looks, the (low)inputs are -always- floating with that 68k in a loop at its grid. I verified that's how the amp is wired too:


All the fender/marshall style setups are this way. The NC shunt on the Low input goes the Tip of the High. So with nothing plugged in both clannels are muted.




I posted because I wasn't sure if thousands of Sound city amps could be wired wrong, or me just being dumb.

Thanks for replying!!
-morgan