Marshall Plexi in an 18 Watt Marshall package

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

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

dartanion
Extremely Active Poster!
Extremely Active Poster!
Posts: 3100
Joined: Tue 01/25/05 2:00 am
Location: RWC, CA

Post by dartanion »

Isn't that one of them Blues Jr. conversions?

Which EL84s are you keeping in this amp? I see both GTs and Sovteks in the photos.
0 x

User avatar
pickaguitar
Newbie
Newbie
Posts: 37
Joined: Wed 02/01/06 2:00 am
Location: Dallas, TX
Contact:

Post by pickaguitar »

Thanks zaphod_phil!


Yea it's one of them BJ conversions.
It currently has the stock GTs currently however I've got some others on da way
0 x

User avatar
Dynaflow_Donnie
Frequent poster
Frequent poster
Posts: 986
Joined: Sat 12/31/05 2:00 am
Location: Phoenix, AZ.
Contact:

Post by Dynaflow_Donnie »

Converted Blues Jr right? The power tube holders look vaguely familiar (and they hold them in, but care must be taken not to knock them sideways a bit, done that a few times on the B. Jr...) If not, I stand corrected... Be a nice amp I'm sure. Thats ok, tweed, I'm going the other way, if and when I ever get my tweed deluxe built (which might happen soon, I actually have Tubes and more open now for the parts!) I'm going to put it in a marshall style head in black tolex.. haha..

Regards,

Dyna
0 x

Chappy
Occasional poster
Occasional poster
Posts: 247
Joined: Sun 05/01/05 2:00 am
Location: Chicoutimi Canada

Post by Chappy »

rjgtr wrote:I've built a bunch of amps with the same preamp, that is somewhat Marshall based, but with 18 watt, 20 watt, 6V6, 6L6/KT66 and EL 34 output stages. So here's my take -

You can get close to the plexi thing with EL84s, but you have to use fixed biasing and negative feedback. This makes the amp more tight like the Plexi. Sure an EL84 doesn't have the same low end, but it has the crunch and a pleasing midrange when pushed. The 6V6 has the bottom, but not the same crunch. Especially not a NOS American or even the JJ 6V6. The NOS Brimar 6V6s do get a nicer midrange, sort of like a KT-66.

I would say go ahead and build the EL84 version. I have a friend who is a died in the wool SLP junky and he loves the amps I built with the 20 watt power amps.

Just be prepaired to do a little fine tuning on the preamp after it is built. It might not sound EXACTLY like a plexi, but you can get very close. And still be able to hear!
RJGTR

Have you done an A/B comparison between a fixed biased and cathode biased output with negative feedback applied? I got my switchable/variable negative feedback loop working and noticed that it does tighten up the sound a little bit on my TMB. Will changing the bias to fixed help tighten it up a bit more too?

Chappy
0 x

Plexi
Charter Member
Charter Member
Posts: 4305
Joined: Fri 03/14/03 2:00 am
Location: Ky. USA

Post by Plexi »

Will changing the bias to fixed help tighten it up a bit more too?
Yes.. and a bit more volume level,just a slight twinge.

The amp in the soundfiles, i experimented with this..and what you hear in the 'richie' files is an amp that is adjustable grid biased,not cathode biased.

Richie
Last edited by Plexi on Sun 04/30/06 9:36 am, edited 1 time in total.
0 x

twango
Occasional poster
Occasional poster
Posts: 120
Joined: Mon 09/06/04 2:00 am
Location: Hamburg, Germany

Post by twango »

I did a 18 Watt with SS recto and NFB with a JMP preamp, just the high treble channel, but with an added Master at the treble pot (Pre PI MV).

This one sounds like my JMP50 or 1959SLP but at some volume levels that do not scratch the paint off the walls...

With a P90-guitar its my favorite amp!
0 x

User avatar
s2
Senior Amp Tech
Senior Amp Tech
Posts: 3182
Joined: Thu 05/20/04 2:00 am
Location: Dallas, TX
Contact:

Post by s2 »

Coco wrote:If you were going to use 6V6s in an 18 watt, what output transformer impedance would be preferred for a B+ of 350V? 8K, 6K, 5K?

I am getting some transformers made up & would like to get a second primary tap set that would be optimized for a 6V6 version of the 18 watt.

8K for the EL84, ideally, what do you think for the 6V6s?

If you had a choice for a second primary impedance for a 20 watt tranny, what would you choose?
According to my RCA RC-30, 6V6s want 8K in push-pull. To anyone who doesn't own a copy of this book, you really need to buy it. AES has in in reprint for about $20 IIRC. The introductory chapters are filled with priceless info and the tube charts are comprehensive and in plain English:

6V6GTA
Push Pull Class A1 Amplifier
Effective Load Resistance (Plate to Plate) ... 8000 Ohms

It also shows that if you go down in resistance, your THD increases dramatically. Must be part of the key to the Matchless sound.

Doesn't get simpler than that!
0 x
--s2
Custom Amplifiers
Amp Parts & Accessories
www.s2amps.com
www.category5amps.com

Coco
Frequent poster
Frequent poster
Posts: 777
Joined: Sun 05/25/03 2:00 am
Location: Toronto, Canada
Contact:

Post by Coco »

Not the 30 but you can see the RCA RC-25 Receiving Tube Manual on-line at:


http://208.190.133.201/tubes/rc25/index.html
0 x
coco

ur12
Frequent poster
Frequent poster
Posts: 749
Joined: Sat 08/27/05 2:00 am

Post by ur12 »

s2 wrote:

It also shows that if you go down in resistance, your THD increases dramatically. Must be part of the key to the Matchless sound.

Doesn't get simpler than that!

Actually, I read somewhere that by going with a lower impedance than normal you can actually decrease 2nd and 3rd order harmonic distortion that will lower your overall THD. Some of the HiFi guys use this design. I'm sure carefull calculations come into play and I wouldn't just put any old transformer in there and expect less THD. Maybe Phil can shed some light on this.
0 x

User avatar
davemw
Occasional poster
Occasional poster
Posts: 143
Joined: Thu 04/28/05 2:00 am
Location: CA
Contact:

Post by davemw »

:lol: i like the look anyway :P
0 x

User avatar
JeffE
Newbie
Newbie
Posts: 86
Joined: Mon 04/10/06 2:00 am
Location: Norfolk, Nebraska

Post by JeffE »

twango wrote:I did a 18 Watt with SS recto and NFB with a JMP preamp, just the high treble channel, but with an added Master at the treble pot (Pre PI MV).

This one sounds like my JMP50 or 1959SLP but at some volume levels that do not scratch the paint off the walls...

With a P90-guitar its my favorite amp!
On the schematic it says to "ask Jan first" about the power tranny, so I'm asking... what did you use?
0 x

twango
Occasional poster
Occasional poster
Posts: 120
Joined: Mon 09/06/04 2:00 am
Location: Hamburg, Germany

Post by twango »

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. He calls his shop "ask Jan first"... That´s the story behind the note on the schem. BTW, I just did copy and paste with some existing drawings and then matched some parts values to my personal taste, nothing exciting with it.

(I think on many other schematics you can add the note "ask Phil first" :D)



Boooaaahhh, just saw my avatar, gllll, ugly, have to change it quick....
1 x

User avatar
erndawg
Newbie
Newbie
Posts: 81
Joined: Wed 01/04/06 2:00 am
Location: Right here

Post by erndawg »

Pickaguitar: What is that 1/4 inch plug and cable assembly you're using for the speaker connection? Do you know the manufacturer, guage, etc?
0 x

User avatar
pickaguitar
Newbie
Newbie
Posts: 37
Joined: Wed 02/01/06 2:00 am
Location: Dallas, TX
Contact:

Post by pickaguitar »

erndawg,
I believe the plug/cable might have come from the remaining parts from the Blues Jr. amp that this used to be. If not...then you'd have to ask S2 (Steven). (Maybe he used his own instead of the Blues Jr)

Jason
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

twango wrote:I did a 18 Watt with SS recto and NFB with a JMP preamp, just the high treble channel, but with an added Master at the treble pot (Pre PI MV). This one sounds like my JMP50 or 1959SLP but at some volume levels that do not scratch the paint off the walls...
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.
ur12 wrote:Actually, I read somewhere that by going with a lower impedance than normal you can actually decrease 2nd and 3rd order harmonic distortion that will lower your overall THD. Some of the HiFi guys use this design. I'm sure carefull calculations come into play and I wouldn't just put any old transformer in there and expect less THD.
There are a lot of variables involved and you can't generalise. I have found however that with 18W amps, lower impedance mismatches, cause the amp to break up sooner with a creamier overdrive tone, presumably due to tube saturation.
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.
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?
0 x
Nature abhors a clean tube amp

Chappy
Occasional poster
Occasional poster
Posts: 247
Joined: Sun 05/01/05 2:00 am
Location: Chicoutimi Canada

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
0 x

rjgtr
Builder, Admin
Builder, Admin
Posts: 6668
Joined: Wed 11/24/04 2:00 am
Location: Jax, FL
Contact:

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.
0 x

Chappy
Occasional poster
Occasional poster
Posts: 247
Joined: Sun 05/01/05 2:00 am
Location: Chicoutimi Canada

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.

Chappy
0 x

User avatar
erndawg
Newbie
Newbie
Posts: 81
Joined: Wed 01/04/06 2:00 am
Location: Right here

Post by erndawg »

Pickaguitar,
Thanks for the info.

S2,
Is that the stock BJ speaker cable assy?
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

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?

Image
0 x
Nature abhors a clean tube amp

Post Reply