Heyboer PT Issues

Cjsinla

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I dont’t see a lot of Bassman build threads here but I recently bought the Mojotone tweed Bassman kit. The plan was to build it with a cascaded V1 like a Marshall 2204. I did use Cliff jacks ala JCM-style Marshalls. I kept the Fender style board but Marshall values for the coupling caps. This kit uses the stock Mojotone/Heyboer transformers.

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I put Marshall knobs on the preamp section as a joke or homage.

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I used Weber’s recommended speakers for Bassman but had to switch the position of the speakers after this pic was taken as the ceramic speakers were too big for that top position. The dust caps had to be removed from the alnico speakers as well in order to fit.

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Cjsinla

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But, part of the purpose of this thread is to discuss a problem. When I first fired up the amp on the lightbulb limiter it worked fine. Off the limiter I had a fuse blow and arcing in the rectifier. I put in a new fuse and known good rectifier and power tubes. Everything ran fine for a few days after that with the exception of some odd buzzes or clicks on a few startups.

At the end of the first week I turned the amp on, took it off standby and the clicking noise started again immediately. I turned off the high voltage and looked in the back. The heaters looked brighter than I remembered. I turned on the high voltage and start wiggling the guitar cable thinking I had a dirty jack. few seconds later one of the filter cap blew and smoke poured out. I replaced the cap and it smoked on the bench.

i checked voltage without the rectifier in and found that the PT primary was now putting out 460v instead of the 355v it was rated for. The heaters were 4v, high as well. I put the amp on a variac and slowly brought up the voltage to about 98v. That’s when the heaters read 3.25v and primary voltage was 355. At that point the amp worked perfectly.

The problem turned out to be a short in the PT, specifically with the 100v tap. The lead wires were crushed inside the bell cap and the insulation on the 100v tap had worn through.

The 100v tap is the second wire from the right, black/blue.

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Note how badly the heater wires were crushed, it’s surprising they worked at all.

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Here’s what it looked like after I pried all the wires apart. You can see the bare wire on the 100v tap.

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Scorching on the inside of the cover at the hole.

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Cjsinla

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Happy ending. Mojotone told me that they were sending me a new PT before I even took the old one apart after I described my unstable voltage issues. I took apart the old PT while I wait for the new one to arrive. I sent some of these pics of the damage back to Mojotone, maybe Heyboer can make some manufacturing changes, we’ll see. I repaired the damaged PT and put it back in and the amp now works perfectly and sounds great. I kept the bell housing off. Heater voltage is 3.7, a little high but doesn’t seem to be a problem at this point. I’ll keep the new power transformer as a spare.

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This is what the finished amp looked like before I pulled out the PT. I did not use the 4 RCA jacks for the speakers. I opted to wire the speakers in parallel to a single 1/4” jack as I’ve had problems getting good solder connections on RCA jacks in the past.

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Cjsinla

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PT arrived today and it’s in much better shape than the first one. It’s the MOJO752 as opposed to the other model which is the MOJO752EX I believe. This new PT only has the 120v tap. There must be more room inside as these leads are barely crushed at all. The PT also has shielding.

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el84ster

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Yikes! That could have been even worse, but bad enough.
I just built a tweed bassman for a friend, man what an amp! Each tone control is in a perfected range, no matter how you set things, it sounds great!
I also used vintage Jensen alnico speakers, that was a major upgrade.
 

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