Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: On The Rocks on February 07, 2020, 09:48:16 am
-
So I have this SGH VSH120 Tube Amp, from the very little info I can find it is a jcm800 clone made by same company as Smarvo and Fame amps.
The issue is after about 20 minutes the volume drops to almost nothing. It is an instant drop, no weird noise or anything. It will continue at low volume until I cycle the standby switch. just a quick off/on and back up to regular volume. Then it will last 10 more minutes and do it again.
I have had this thing looked at by amp tech multiple times. He says has re-soldered pretty much everything, and I cant see it being a heat related issue because just have to cycle the standby switch and it works again. When it cuts out the tubes all glow like normal, I have tried running with a jumper cord in the effects loop, tubes were changed and biased about a month before issue started.
Does anyone have any ideas? This thing sounds too good to trash it but I cant rely on it to gig with :help:
-
Take it to a different tech.
There is no reason this should be a mystery. After swapping all tubes as possible culprits you would move on to standard troubleshooting procedures. But the fact that you had the tubes switched out right before the problem started happening, would lead me to suspect a "new" tube or socket issue.
- Use a dual trace scope on the output to SEE what's actually happening
- Once the volume drops out change the location of your scope lead to find the stage where it's dropping out
- Divide and conquer
This sounds like one of those quirky problems that you will see pop up in a PC board amp that might make it more difficult to find, but generally speaking, a "JCM800" is a fairly simple circuit.
-
I do solidly trust the tech that worked on it, he has a great reputation in our area. Other techcs I have taken it to say they can't make it act up, they say don't have a way tp "play guitar or get somebody to do it to test". The worst part is I have nobody in my area to work on tube amps. it is a 4 hour drive to take it in. I just find it funny that a quick off and back on of the standby switch brings it back to life. on a side note not sure if could have caused issues but at one time the switch on back got switched from 4ohm to 8ohm, so was running 8 ohm into 4 ohm cabinet but only for about 45 minute set. The worst part is you can not find any info anywhere about this amp other than a few pictures on the net, so can only use educated guesses when it comes to schematics
-
Nearly impossible to even guess with the little info we have. Some pics of the guts might help. Voltage readings for every pin of every tube while it's dead would help. If it's made on a pcb gonna need a schematic. Do you have any electronics knowledge and/or experience?
-
Hi,
I don't know if this could help but I got rhe same issue wth a Gretsch amplifiet which the plate pf one of the power tubes was not soldered properly. Contact would come on and off.
Good luck
-
Another update. I played until it went quiet then figured "fuck it" and kept playing with it like that until something failed and would be obvious where problem was. It just kept going, so figured would crank the volume and bang! Instantly came back to life but at ear blowing volume. Turned volume back down and after couple minutes it faded again. Turned volume knob snd back she came. There is no noise or scratching when adjusting volume so dont believe its the volume pot, and all the tubes tested good. I even swapped out pre-amp tubes to be sure. Power tubes keep glowing like they should, from what i can see, all the solder points look good.
So any ideas what would fail but repair itself by moving volume or cycling standby switch
-
Bad connection is still very likely. Could be almost anything. Get a wooden chopstick or something similar. An small artist paint brush handle works well. Start poking and prodding every wire connection and every component lead, especially where the lead goes into the component body. Might get lucky.