About four months ago I bought a late-70's Marshall JMP 1992 Super Bass MkII. I found it second-hand for a good price. It was just the amp head though; no accompanying cabinet.
Since then, I've been on the lookout for a decent 4×12 cabinet to play it through. Last week I found such a cab for a very good price. One reason I considered the price very good was that it included a Marshall JCM 800 2210.
My new purchase:
I picked up the amp + cab today, from a seller who let's say was clearly not a rock 'n' roll enthusiast. To let me try the gear before I bought, they had plugged the JCM 800 in and turned it on — but they had failed to plug the amp head into the cabinet at the back. Uh oh. I worried that the JCM 800's output transformer might have blown. I turned the amp off, plugged the head into the cabinet, turned it back on, turned the Standby on, and waited. The amp started to warm up. But it didn't reach the volume that I expected from this amp.
I explained this to the seller, and haggled a discount.
So my question is, how can I tell from the amp's sound if the output transformer has blown? What are the 'symptoms' of that? Or perhaps the power amp needs some new tubes... how can I tell whether any tubes need replacing? I assure you, dear reader, I will most likely take the head to someone who is comfortable messing around with the lethal voltages found in a tube amp (because that sure isn't me) to get a more complete diagnosis. But it would be nice to have a general overview of what's up.
(In the meantime, maybe I can plug the JMP 1992 into the cab instead, and then I only need finish a guitar's wiring to be able to record a demo of that stock Boss Blues Driver pedal I have, so as to record that pedal's original "before" sound before I modify it! Isn't there always something that needs doing before you can do what you really want?)
Post by thetragichero on Oct 11, 2020 10:38:17 GMT -5
as a matter of course I'll always change the power tubes when purchasing a used amp (and filter capacitors if it's more than 30 years old) you may also do well to check the bias. rob robinette has good instructions on his site (and a bias calculator i keep in my bookmarks) if the amp does not have bias test points/resistors between power tube cathodes and ground (i am familiar with the jcm800 topology but the actual Marshalls are out of my price range) 1w 1 ohm 1% metal film are pretty cheap (1w is overkill but I'd rather overkill than underkill ) i wouldn't suspect a blown transformer until those other things are taken care of (especially if it was a short time and you didn't notice any smells or anything)
Thanks trag'! I'm scared. I think I'll take this to an amp tech and let them put themselves in electricity's way. Glad to hear you don't think the output transformer is broken. I report no smells thus far.
Post by thetragichero on Oct 12, 2020 15:31:05 GMT -5
i like using the analogy of driving: operating scared is super dangerous; operating with respect of the power allows one to be careful and cautious without making mistakes out of fear by all means nothing to be afraid of as long as you learn safety so i guess read up!