JTM50 with full wave bridge rectifier

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sonicmojo
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JTM50 with full wave bridge rectifier

Post by sonicmojo »

I recently built a JTM50 out of extra parts. I used a set of transformers from a Fender Super 60 which seemed about right. The part that I had go away from stock on was there were no center taps on the PT secondaries. I implemented a full wave bridge rectifier and nixed the standard 220K link to the bias supply and used the extra available PT secondaries to power bias instead. I had to adjust down the stock 47K bias resistor before the trim pot to 10K to get EL34s to be in range. The amp sounds good but is there anything else I should consider doing with this rectifier architecture?

I also added an artificial center tap on the heaters to prevent hum.
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Roe
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Re: JTM50 with full wave bridge rectifier

Post by Roe »

sounds good to me. Check the 100w schematics for something similar
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sonicmojo
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Re: JTM50 with full wave bridge rectifier

Post by sonicmojo »

Ah yes, I see the 100s use the bridge rectifier. Thanks!

I've been kicking around these Super 60 transformers for years and I'm glad I finally found a use for them. Here are pics...I'm now working on a head cab for it.

[img:1500:593]http://www.sonicmojo.com/jtm50/jtm50-1.jpg[/img]
[img:1500:730]http://www.sonicmojo.com/jtm50/jtm50-2.jpg[/img]
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Re: JTM50 with full wave bridge rectifier

Post by dcribbs1412 »

Looks nice sonic!
Love the look of those knobs ?

D
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sonicmojo
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Re: JTM50 with full wave bridge rectifier

Post by sonicmojo »

Thanks! I found those knobs at a local shop for cheap. No idea what they are.

Upon burning in, I'm getting the V4 power tube heating up to slight red and blowing the internal fuse after about 5 minutes of pumping sound through it. I hope I don't have a going bad OT. I tried different tubes. Bias checked Ok on both tubes. Maybe I just have a bad solder joint that is heating up.....

I can't think my slightly modified bias supply would affect just one side...
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ToneMerc
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Re: JTM50 with full wave bridge rectifier

Post by ToneMerc »

sonicmojo wrote:Thanks! I found those knobs at a local shop for cheap. No idea what they are.

Upon burning in, I'm getting the V4 power tube heating up to slight red and blowing the internal fuse after about 5 minutes of pumping sound through it. I hope I don't have a going bad OT. I tried different tubes. Bias checked Ok on both tubes. Maybe I just have a bad solder joint that is heating up.....

I can't think my slightly modified bias supply would affect just one side...
What value grid stoppers do you have on the power tubes? Parasitics can cause red plating as well so it could be lead dress.Worse case, you might have to put grid stoppers on the Pi as well.

TM
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sonicmojo
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Re: JTM50 with full wave bridge rectifier

Post by sonicmojo »

ToneMerc wrote:
What value grid stoppers do you have on the power tubes? Parasitics can cause red plating as well so it could be lead dress.

TM
5.6K. The wiring is pretty crammed in that spot. What could I try to move?
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ToneMerc
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Re: JTM50 with full wave bridge rectifier

Post by ToneMerc »

sonicmojo wrote:
ToneMerc wrote:
What value grid stoppers do you have on the power tubes? Parasitics can cause red plating as well so it could be lead dress.

TM
5.6K. The wiring is pretty crammed in that spot. What could I try to move?
Look at the PI and v4 input wires. The PI wires from your board are very short, but for grins do you have another 1/2w 47k that you tack on the PI input pins, one for each side would be even better.

TM
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sonicmojo
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Re: JTM50 with full wave bridge rectifier

Post by sonicmojo »

Thanks for the tips. I'll try a couple of things in a few days when I get back to it.
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Re: JTM50 with full wave bridge rectifier

Post by pdf64 »

I suggest to twist the balanced lines (LTP plates, LTP output, OT plate wires) to reduce EM radiation / susceptibility.
Route the wire to the presence pot away from the other tone control wiring, as it's a completely different signal, and coupling between them is unlikely to be beneficial.
If it still red plates, maybe it's the tube; try swapping them over.
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Post by Stevem »

The knobs are off a hi fi stereo pre!
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sonicmojo
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Re: JTM50 with full wave bridge rectifier

Post by sonicmojo »

I had a few minutes and I implemented some of the suggestions. What seemed to solve it is replacing the bias caps. I'm not convinced I don't still have a problem with wiring somewhere. I can't get volume to completely shut off on either channel when the knobs are fully turned, so I'm perplexed at the moment with that so I need to keep looking. It is not super loud, but significantly there. I think the pots are okay and I can't find any grounding problems as yet.
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Re: JTM50 with full wave bridge rectifier

Post by Bob S »

Can't see in the pics - is the FWBR ground point good?
I built a 2204 & forgot to ground one of the B nodes. Confused me for a while.
Good Luck!
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sonicmojo
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Re: JTM50 with full wave bridge rectifier

Post by sonicmojo »

Bob S wrote:Can't see in the pics - is the FWBR ground point good?
I built a 2204 & forgot to ground one of the B nodes. Confused me for a while.
Good Luck!
Pretty sure. Grounded to the standoff on a PT nut. The 500pf mica going to vol1 is adding a lot of the extra volume. I changed that to a .02 ceramic for kicks and it cut it way down, about to the level of my JTM45 build which does the same thing. Here's the really weird thing though, if I pull tube 1 AND lift the 68k pair from the "1" input jacks, I still hear some bleed through. Something wacky is going on. At least I'm not blowing fuses any longer. I think v4 is still getting too hot but I may have compromised that tube in prior testing. Will try another. One thing that really concerns me is the OT has been putting off some lacquer smell which may mean it is failing, or not....
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sonicmojo
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Re: JTM50 with full wave bridge rectifier

Post by sonicmojo »

The '69 Super Bass I have does the same thing with the volume not going all the way off with the knobs. Is this a feature of these old Marshall circuits? I don't quite understand why this would be. Is there some sort of buss bar ground bleed back occurring in this design? There has to be a physics explanation or I need to rethink my grounding scheme.
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