when inserting a current limiting R is very difficult

[dai h.]

working on an old (around 1980 vintage I think) guyatone tube amp(basically looks like a silverface champ clone but a bit different). The construction in this chassis SUCKS royally in terms of providing easy repair and maintainance. In order to install a 100k current limiting R in series with the rect. out (I want to do this to try to ensure the filters are reformed if they can be--got the amp as "junk" condition which means basically some defect(or defects)) the construction is such that I have to unsolder a multitude of wires off the PCB for the main filters, power tube cathode R, K, plus some R,C for the reverb driver tube (to get under the PCB to insert the R at the output of the B+ bridge rect., or to replace filters, etc.). Is there any way I can put the resistor on the primary side of the PT(or something) to limit the current? And also(re: the repair/maintainance unfriendly construction), the spk out does not have it's own jack, so the spk. cable is hard wired to the inside and the crimp/spade type connectors to the spk. are on so tight I think they will break if I try to force them off(so the chassis is sitting on top of the cab and I guess I'll have to work on it like that). The soldering sucks ass as well. Balls, splatters, over application, etc. (and original piece of cut off insulation from a wire). Older chassis of Japanese amps I've seen on the web (old 60s Guyatone or whatever) showed much better workmanship(soldering). But happy camper I shall be if it works regardless.

[Steve Conner]

Hey Dai

It has a diode bridge rectifier, right? You can put the resistor in series with one of the HT wires from the PT to the rectifier.

Wouldn't recommend putting it in the primary, it has to supply the magnetizing current of the PT too, so it won't do anything until you make it much smaller than 100k, and then the caps will get reformed too fast.

[dai h.]

cool thanks!!! that will be so much easier. Just looked at the reverb section (solid state--looks like a couple discrete transistors--ooh I think I got it wrong above, the second of the two 12AX7s isn't for the reverb but for the driver--triode sections are wired in parallel maybe to drive the 6L6GC?) and anywayzZz I wanted to check out if I could do the same there for the supply (also a bridge) but (again) set up super difficult to access (but maybe I can get away with starting with a low input AC to the amp or something--I've read the higher voltage filters can be more in need of current limited reforming than the lower voltage alu electros on a (Nippon Chemi-con?) datasheet.

[dai h.]

got it hooked up now. Going up slowly (with the Variac). Seems to be reforming (voltage went up a bit after turning on and applying).

[dai h.]

small update:

over a number of hours, (using the series R in one leg of the rect. cct.) I gradually raised the AC voltage/B+ and watched for the voltage across the 100k to go low and went up to about 340V(350V max. for caps). Then, took the R out to achieve "normal operating mode" and tried the amp at really low volume early in the morning. I was very pleased. I liked it dry, as well as with the spring reverb. The next day, powered the amp up again in the evening this time (so I could raise the vol. higher than something above "whisper quiet"). Utterly different perception. MAJOR dissapointment as I encountered a significant buzz whenever I took my hand off any metal parts on the gtr.(strings, bridge, jack, etc.) and an obtrusive amount of hum as I raised the volume further.

So, I've been drawing the chassis out (the wiring layout with pencil and paper) and staring and staring at the open chassis to see if I could spot anything that appeared particularly problematic. It was hard to see how all the grounds are connected due to the way it's put together, but a lot seemed like it should be okay, such as the portions of the ground line immediately following the B+ rect. output being kept off the chassis. One thing I spotted that could be problematic was the way the spk. output loop physically included the PCB for the early filtering for the B+, power tube cathode circuitry and driver. Seemed it might be a source of instability(noise?) and in a pic of another GA-100, the wiring is different to mine.

Guitar/Amp

Anyway, I got the spk. wires disconnected without major melting and shrinkback(the original "lazy tack soldering" actually helped in that regard since I didn't need to unwrap anything from lugs, etc.), and it looks like there is enough slack and space to re-route the spk. wiring up through the grommet used to route the AC wiring for the reverb and heater ccts. (spk. wires were originally routed with the reverb. in/out cables to the tank which didn't seem ideal for stability). Something that looked like it could be dangerous with the re-routing was that the placement of the spk. cable would bring it closer to the envelope of the power tube, so at this point, I need to go buy a plastic wire retainer thing to ensure it stays clear and doesn't melt on the power tube. Also ripped a bit of the adhesive-backed alu sheeting on the top(inside) of the cab. that came off and was loose so have celotaped it for now to try to prevent it from getting worse while I figure out the most appropriate stuff to remove the old glue and stick it securely back on (not totally sure yet but after some websearching looks like acetone to remove old glue and synthetic rubber based adhesive for the glue). Think this could be a cool little black/silverface Fender Champ-type amp when working correctly, but still HATE the non-maintainance/repair friendly construction.

[Gtr_tech]

Passive p-ups in the guitar? The noise when you don't touch any metal parts on the guitar or amp chassis is normal.....don't stress over that. Only active p-ups are immune to this noise because you will disconnect the string gnd wire....its not needed with actives. And reduces the shock hazard with 2 wire equipment.

[dai h.]

well I'm less concerned with the fact there typically is greater noise (when not touching) than the *amount* of hum and buzz which to me is excessive. My 50W Marshall, for example (which has more gain) is far quieter.

[Enzo]

Turn the volume control on the guitar to zero. Now,listen to the amp. Whatever hum you hear now is the amp's fault. Anything else is just feeding into the amp from the guitar.

[dai h.]

thx Enzo. I'll try that when I get it re-assembled.