Clone of Marshall JTM 45 Fails Bulb Test

[timmyjoe]

Started up a new amp build and it blew a fuse after about 1/2 hour of playing. After searching around town I finally found an incandescent 60W bulb and made the load limiter. (No 100w available) Amp lit the bulb so I guess there is a short? I'm not seeing a problem, but maybe something came loose under the board?

*I used the Mojotone page to give me a layout. But, I colored the wires so I would be less apt to make a mistake. Plans I used are attached. Here is a link to Mojotone original plans.
*Changes from the Mojotone plans include removing two inputs, adding a second speaker jack, adding a master volume.
*Another thing that happened in the build is that I had to swap two wires on the negative feedback. It honked like a blow horn the first time I started up. The yellow and brown wire placements are opposite from the plans.

[sluckey]

Amp lit the bulb so I guess there is a short?

How bright? That amp should dimly light the bulb, but nowhere near full brightness.

[timmyjoe]

It was pretty bright, like close to normal bright for a 60 watt.

Also, I put in a new fuse that is fast acting, could find the slow acting one at store.

Can’t imagine that affecting anything though.

[pdf64]

A suitable next step is to remove all the valves and see how the light bulb limiter reacts.
If it goes dim, try fitting the rectifier valve.
If ok, try fitting the output valves.
If ok, try fitting the phase splitter valve in V3.
If ok, try fitting the valve in V2.
If ok, try fitting the valve in V1.

[timmyjoe]

Pdf64,
Tried rectifier and all good. No good with output tubes. Limiter light comes on.

[pdf64]

With the output valves not fitted, what V DC do you measure at each pin of each output valve socket?

[timmyjoe]

Pdf64,
Output valves thermals read following  without tubes:
1: 0
2: 0
3: 425
4: 425
5: -38
6: 425
7: 0
8: 0

[timmyjoe]

With the amp originally starting up ok, could this all just be bad luck and failed power tubes?

[pdf64]

Oops, sorry, I missed replying yesterday.
The voltages look ok, though does the amp have bias adjustment? If so, adjust it to increase the magnitude of the bias voltage on terminal 5, to its most negative voltage.
Then try the output valves again (via the limiter), but one at a time rather than both together.

[timmyjoe]

Pdf64,
I got the negative on terminal 5 to around -48. Even one power tube is lighting up the limiter.

And time you spend in helping me figure this out is appreciated.

[labb]

You have probably already done it but I would be double checking power tube wiring. For sure pin one and eight grounded. Assuming you have EL34 tubes. One and eight are normally jumpered together and then grounded.

[pdf64]

… one power tube is lighting up the limiter.
…

Is that the same for either of the output valves?
What valve type are they?

[timmyjoe]

Amp uses el34 tubes and it is properly grounded at 1 and 8 pins.

It lights up the limiter with either socket or tube. I tried flipping things too so not just one bad tube I guess.

The thing that gets me is that it worked for about half an hour. Then blew fuse later when I turned it on.

[pdf64]

Is your limiter bulb definitely incandescent or halogen, 60W?
Does the limiter work ok with a different amp or something?

[labb]

Something you might try. Disconnect the wire at the cap can that goes to the center wire of the OT. That will kill the high voltage to everything. Now do your light bulb test. If it lights up the problem is in the heater wiring. Your voltage readings on the tubes look good.

[timmyjoe]

Pdf64, you fixed it with the post before the last one.

I plugged in my 5E3 into the limiter and it lit the bulb up just like jtm. Then swapped out the bulb from 60 to 100w. The 100w is frosted so I was worried that I would miss the glow. But now I have a base line for the start up and then the normal glow on that bulb.

Tried jtm again with all tubes. It’s a go! My guess is that it only blew the fuse because I turned off and on in very rapid succession.

Could you look at the wiring diagram and tell me if I got the standby switch bypassed correctly?

Thanks a ton! Sorry this was such teeth-cutting mistake on the testing procedures. Is there a book I should pick up on learning to work on amps?

[pdf64]

…
Could you look at the wiring diagram and tell me if I got the standby switch bypassed correctly?
 …

From what I can tell it looks ok.
Is the reservoir cap 64uF? That’s quite a lot.

There’s loads of good info at the valve wizard, Aiken and Geofex sites. And Merlin’s books.

[timmyjoe]

Pdf64,

By 64 reservoirs I’m guessing that is the two 32 capacitors. I was just following the mojotone plans. I’m not finding a good set of plans for the original jtm45. Everyone just posts the bassman plans. Which does show only 20 uF capacitors used. Seems like fender didn’t follow those bassman plans and neither did Marshall. So I give up. The amp sounds good to me😄. Surprised how much I use the presence knob. And surprised anyone ever wanted it in a combo with how heavy it is!

[pdf64]

This JTM45 schematic is pretty good https://mhuss.com/amps/JTM45/JTM45Classic2.pdf
Note that although 8k2 is occasionally used as R32, it seems much more usual for it to be 1k 2W.