Marshall Plexi in an 18 Watt Marshall package

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

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Chappy
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Post by Chappy »

Zaphod wrote

"That's a cool design. We should make this another official 18W TMB variant - the JMP 18W! However, I think we need to find some use for the spare triode - maybe an 18W Normal channel, or just parallel up the first preamp stage."



Zaphod

I just did that not too long ago. I rewired my 18W TMB such that the preamp circuit is the JMP design. Both triodes of V1 feed into the cathode follower tube via two 470Kohm mixer resistors. The tone pot from the normal channel, has been replaced with a presence pot for the feedback loop I added. I also wired up the feedback resistor in series with a switchable pot. The pot will get mounted to the bottom of my chassis beside V1 eventually. This feedback pot allows me to vary the amount of negative feedback. The switch in the pot allows me to switch off the feedback.

I may go back to having a normal 18W channel though, without a tone control. I'll mix the two channels on the left input of the PI. I'm thinking of using small switch(s) to change between the JMP bright and normal channel on the remaining input. I'm still toying with the idea.

Chappy
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rjgtr
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Post by rjgtr »

Chappy,

I compared it and the fixed biasing makes a difference in the low end punch. The NFB helps clean up the grit, but the amp still doesn't have the same punch as with fixed biasing, IMHO.
zaphod_phil wrote:That's a cool design. We should make this another official 18W TMB variant - the JMP 18W! However, I think we need to find some use for the spare triode - maybe an 18W Normal channel, or just parallel up the first preamp stage.
Actually, I have the pefect use for the other triode. I was at my friend's shop and he put some speakers in a retolexed Marshall cab for a customer. So he got the bug and pulled out his vintage plexis and early 70s 50s and SLPs. He had a mod done back in the day that cascaded the "normal" input triode into the "brilliant" input triode. Sort of like a two input JCM-800 but it sounds really good. One of them even still had shared cathodes. So use the left over triode to cascade and make it switchable, which shouldn't be hard at all. Use a JTM-45 style small box, except cut out for EL84s, and you're all set. It is a great rock tone and would be perfect for a JMP Plexi 18watt. It also cleans up really nicely too with a good Marshall clean tone.

I'm definitely going to do this to my 1987x reissue when I take it all PTP and replace the OT.
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Chappy
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Post by Chappy »

Rjgtr

Thats another mod I might do as well. If I can't find the room for it, then I am going to return my amp back to a stock 18WTMB giving me the perfect excuse to start building another head. Maybe a Plexi 6V6 or a Plexi 6AQ5. This will have to wait though. I want to build a P1 Chamelion next, with an additional 6SJ7 input.

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erndawg
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Post by erndawg »

Pickaguitar,
Thanks for the info.

S2,
Is that the stock BJ speaker cable assy?
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zaphod_phil
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Post by zaphod_phil »

rjgtr wrote:Actually, I have the pefect use for the other triode....He had a mod done back in the day that cascaded the "normal" input triode into the "brilliant" input triode. Sort of like a two input JCM-800 but it sounds really good....So use the left over triode to cascade and make it switchable, which shouldn't be hard at all. ... It is a great rock tone and would be perfect for a JMP Plexi 18watt. It also cleans up really nicely too with a good Marshall clean tone.
That's a great suggestion and would make a real butt-kicking 18W! Isn't that the famous "one wire mod", as shown here?

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Nature abhors a clean tube amp

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Post by V846 »

I built a head version something like that, basically a November but single channel, one input and ran the triodes paralleled, ugly lookin but a lil screamer it is :twisted:

David

http://putfile.com/pic.php?pic=5/13215492262.jpg&s=f5

http://putfile.com/pic.php?pic=5/13215512084.jpg&s=f5
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bigsbybender
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Post by bigsbybender »

A wolf in sheep's clothing. Dig it....and I just build a tweed deluxe for Marshall style combo cabinet.....wierd. :huh:


J.
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twango
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Post by twango »

twango wrote:I made some of my amps with the VERY cheap PT from engineer Jan Wüsten. He runs a business in Germany, you can see his stuff at www.die-wuestens.de He is very Kind and helpful and has some PT´s for about € 20,-. Great to start out with.
zaphod_phil wrote:I've used one of his PTs myself, for a slightly lower voltage amp. What AC and DC voltages are you getting from your PT?
Sorry, been off for a long time...

Phil, I have 295V DC with a EZ81 and 305-310 V DC with 2x 1N4007 Diodes.
Can´t remember the AC values, but will take a measure tomorrow...

The amp has noticeable less power and more compression than a second I have with the same circuit but a Shinrock 300-0-300V PT.
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Plexi
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Re:

Post by Plexi »

We should make this another official 18W TMB variant - the JMP 18W
For most that were not around back when the forum started on the Yahoo forum site.. and now i think all of that great info has been lost.A little history.. And the reason about when all the first variant/offshoots first started on the old Yahoo forum,the rule for the variants were any offshoot had to keep something of the original 18 watt Marshall circuit. Either the PI or a channel or something etc.. most all of the other circuit variants have been done over the years but may not have been listed here if it was a straight JMP or 2203 type circuit etc.. which was done on other forums..
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