Marshall 68 50 watt build nos parts

beaulieu · Post by **beaulieu** » Sun Sep 01, 2013 6:29 pm

Heres an amp I just finished.

e-0d4a-4911-9383-3c93c6026dfd_zps75176d0d

neikeel · Post by **neikeel** » Mon Sep 02, 2013 5:49 am

Tell us more?
Whose trannies?

I see you used late split cathode with mustard byasses on the cathodes, but an earlier 56k slope resistor (presumably with 250pF cap and early spec 0.1uF PI outputs?

Did you try it as shared cathode first? That would be mor in keeping with the JTM front plate, thought about a tube rectifier -that spec is quite sweet (late black flag)

beaulieu · Post by **beaulieu** » Mon Sep 02, 2013 10:14 am

Merr.n Audio Trannies. Didn't try with shared cathode. Also didn't think of tube rectifier ( I sold the 2 gz34 Mullards I had) Never had a tube rectified amp yet. The 15k that's coming is for this amp

Amp sounds really good though. HAd a problem with no sound when finished. Used a 1 meg under the 68 on v2 DUH the 820 I had was to short and thought I used a 1 k. Dumb mistake.

neikeel · Post by **neikeel** » Mon Sep 02, 2013 11:51 am

beaulieu wrote:Merr.n Audio Trannies. Didn't try with shared cathode. Also didn't think of tube rectifier ( I sold the 2 gz34 Mullards I had) Never had a tube rectified amp yet. The 15k that's coming is for this amp Amp sounds really good though. HAd a problem with no sound when finished. Used a 1 meg under the 68 on v2 DUH the 820 I had was to short and thought I used a 1 k. Dumb mistake.

You should try the shared cathode. What you have now is quite a 'blend' of specs, of course if you like it cool but it is not like anything that ever left the factory!

beaulieu · Post by **beaulieu** » Mon Sep 02, 2013 12:01 pm

so I should try shared ...what about the the 68 on v2? remove it?

neikeel · Post by **neikeel** » Mon Sep 02, 2013 12:40 pm

beaulieu wrote:so I should try shared ...what about the the 68 on v2? remove it?

Just for now

beaulieu · Post by **beaulieu** » Tue Sep 03, 2013 1:35 pm

10-4! Guessing I should also remove the 4800 on vol 1 pot as well.

neikeel · Post by **neikeel** » Tue Sep 03, 2013 1:44 pm

beaulieu wrote:10-4! Guessing I should also remove the 4800 on vol 1 pot as well.

Same! Just use a ceramic 100pF instead.

If you have the right spec and a good guitar with decent pots, caps and pups these are the plexi amps that you pretty much dime on the front panel and then control the volume and dirt with the guitar controls.

beaulieu · Post by **beaulieu** » Thu Sep 05, 2013 9:11 pm

Her ya go Neil. Tthe cans on top are 50/50. What did these amps have? 32uf? I have a dual 32 on the board. I didn't put the 15k in yet!

Tazin · Post by **Tazin** » Mon Sep 09, 2013 6:50 pm

The vast majority of the 50w JTM's ran a GZ34 rectifier. These amps ran a single element 32uF/450v filter cap for the mains which was mounted top side next to the valve rectifier. The inside filter cap (mounted on the side of the chassis) is a dual element 32uF + 16uF/450v for the screens and PI. The pre-amp filter (on the tag board) is a 32uF + 32uF/350v dual element cap. The solid state rectifier version (diode rectification) of the 50w JTM seemed to show up at the very end of the JTM line before they switched to the JMP panels. There doesn't seem to be all that many of them from what I've seen. These amps run a single element 32uF/450v filter cap for the mains filtering which was mounted top side where the GZ34 use to be mounted. The screens and PI filter cap is a dual element 32uF + 16uF/450v mounted top side next to the 32uF/450v mains filter cap. The pre-amp filter (on the tag board) is a 32uF + 32uF/350v dual element cap.

neikeel · Post by **neikeel** » Tue Sep 10, 2013 2:43 am

For the BF 50s I have seen Tazin is correct.

I find uppin the mains filtering beneficial so would leave that at 100uF( 50+50 in parallel). If you want a greasy feel to the amp 16uF screens is best and PI at 32 although I am not sure Marshall ever made a 50 that way - AFAIK 16 screens and 32 PI more normal?

The way Mark has it now is mid 68 JMP spec (my 68 lead has two 50/50 up top and 32/32 on the board with diodes mounted on the PT)

Tazin · Post by **Tazin** » Tue Sep 10, 2013 9:27 am

From what I've seen Marshall did wire the amps both ways; 16uF for the screens and 32uF for the PI; also 32uF for the screens and 16 for the PI. I think the desending way (32uF screens, 16uF PI) is more common and perhaps the correct way.

rgorke · Post by **rgorke** » Wed Sep 25, 2013 10:17 pm

Looks great Mark. Makes me want to finish up collecting the parts for mine. How do you like the shared vs the split cathode?

I am planning on a tube rectified "model" as well.

Neil, is there another "option" other than Mustards for the bypass caps? WIMA?

neikeel · Post by **neikeel** » Thu Sep 26, 2013 4:55 am

rgorke wrote:Looks great Mark. Makes me want to finish up collecting the parts for mine. How do you like the shared vs the split cathode?

I am planning on a tube rectified "model" as well.

Neil, is there another "option" other than Mustards for the bypass caps? WIMA?

The correct 400v green WIMAs are even harder to find!

rgorke · Post by **rgorke** » Thu Sep 26, 2013 10:49 am

Is there any noticeable difference in tone between WIMA and Mustard caps in the bypass positions?

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