that's one sweet build... i don't doubt it sounds killer

Probably an output valve blown.

Try plugging a 16 ohm 4 x 12 into each output in turn (each output is driven by its own 50W output transformer and output valve pair). Remember, if you use two 16 ohm cabs, to leave each output's impedance selector set to 16 ohms because they are not in parallel for the reason given above. Do both outputs exhibit the volume drop or just one side?

REMEMBER: don't play the amp unless at least one output is connected to a speaker cab load!

I'd bet the issue is related to tubes, most likely power tubes.  That's far more likely than a transformer.

Thanks for the replies guys. It does appear to be an output tube. I popped the back and saw that power tube 3 (from left to right) was glowing a white type light from the bottom. Tube 4 was glowing a little bluish towards the top. I'm guessing that's probably due to the bad tube in 3.

Steve, I'm real careful about never powering up without a load. That's not say that it didnt happen a few times when it was sitting in the store, but hopefully not. As to your question about volume drop.... yup, only on the high treble channel. Normal channel was fine. I guess a tube replacement is in order. Should I be looking at a matched quartet, or would a matched duo work?

I have run it pretty hot while recording (up in the 8 or 9 range). I'd be willing to bet less than 2 or 3 hours,max. I'm hopeful that's there is no underlying issue that's causing the tubes to blow. Thanks again for the assistance and suggestions!

Steve

If the Normal channel is fine then the problem is not output valves but more than likely your V1 preamp valve. Specifically the side that is fed by the High Treble input, (it's a double triode remember). Before you do anything else, just replace V1 (the first preamp valve) and see if that cures your problem.

Thanks for the tip. That would be great if it was only the preamp tube. Thanks for all of the free tech support advice!! I really appreciate the assistance and hope I'm not becoming too much of a PITA.... I'll give that a shot and see how it goes.

Steve

I tried a few different things, but the problem persists. There was definitely a blown power tube in the 3 position. I also tried replacing preamp tubes 1, then 2 and 3 (and all 3 preamp tubes), but no luck . It appears to only be happening on Channel 1. When the volume goes past 8.5, it starts crackling and then cuts out entirely (9 and above). Anyone have any ideas? I guess KORG USA is the distributor here, and maybe I need to contact them directly. I contacted my dealer that I purchased it through, but they stated that I needed to contact Marshall for any warranty service. I'm hoping that parts aren't too hard to replace with such a limited run of this series. I could try to find someone local to take a look at it, but I'm afraid that would void the warranty.

Are you saying that when you plug into the 'High Treble' channel 1 input you get this problem but when you plug into the 'Normal' channel 2 input you don't?

If so, it could be the High Treble 'Loudness 1' pot needs replacing (or maybe just cleaning!) if swapping out V1 hasn't done the trick.

I'll give that a shot....thanks Steve!

-- Tue Dec 08, 2009 1:35 am --

Just an update on my issue. Power tube was fried, and there was a loose connection/solder on the presence pot that was intermittent. 4 weeks and $400 later, but I'm still a happy camper (with Gold Lion's). The pots were allegedly pretty dirty, but all is well in JTM 45/100 land. Thanks again for all of the suggestions and help throughout  the ordeal. I'm very happy to be up and recording with it again

I see quite a few pics with all of the inputs plugged up with short cables.  I've never owned a vertical input amp; so I'm wondering why it is done.

On the early vertical input amps there were 2 channels (not to be confused with modern 2 channel amps), one was bright and the other darker. By connecting the inputs you could blend the 2 channels (via their seperate preamp knobs) to get a wider variety of tones.

Hi All new member here i have a jtm45/100 40th setup. the problem is it hums a little nothing serious but still a little bit annoying the hum goes up and down with the bias ie higher bias settings hum cold bias setting less hum. i have other amps one which is similar and is dead quiet, tried replacing tubes does not change. the problem still persists when using one or two cabs, do these amps have some sort of gound loop going on as there is two output transformers.


any ideas would be much appreciated.

thanks in advance

Welcome to the forum captaincrunch.

There is no ground loop.

It could be something as simple as lead dressing.

Hi Steve thanks for the reply could you explain about the lead dressing and what to look for, if i could just get rid of the hum i would be well happy and would be  a lot better for recording this wonderful amp.

thanks

captain crunch

ps think you did an awesome job on this and the super 100jh which i also own

Always on handwired amps the position of the wires from turret board to valve bases or pots or wherever can be very sensitve and need to be dressed to minimise hum and crosstalk. Simply moving a wire slightly can get rid of an extraneous noise picked up from a neighbouring conductor. A common source of hum can be the heater wires if they have been disturbed and/or their twisting has been compromised.