Marshall Plexi in an 18 Watt Marshall package

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bignicky
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Marshall Plexi in an 18 Watt Marshall package

Post by bignicky »

Has anyone designed an 18 watter with a plexi or JTM 45 preamp section utilizing either a 1974X or 2061X output section? A mini plexi with four inputs, a Bright Volume, and a Bass Volume, with Treble, Middle, Bass, and Presence? That would have to be the ultimate in cool tone!
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Post by brownnote »

I believe that's pretty close to what you get with the TMB
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Re: Marshall Plexi in an 18 Watt Marshall package

Post by str82hvn »

bignicky wrote:Has anyone designed an 18 watter with a plexi or JTM 45 preamp section utilizing either a 1974X or 2061X output section? A mini plexi with four inputs, a Bright Volume, and a Bass Volume, with Treble, Middle, Bass, and Presence? That would have to be the ultimate in cool tone!
I agree with Brownnote from the preamp side but an EL84 just sounds way different than a EL34 and most of an 18 watts amps "Sound" comes from overdriving the power tubes.

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Re: Marshall Plexi in an 18 Watt Marshall package

Post by zaphod_phil »

brownnote wrote:I believe that's pretty close to what you get with the TMB
Yes, the 18W TMB gets close to that idea, although it's not a direct copy of the 1987 Plexi preamp with the Bright & Dark channels. Rather, it's a Plexi-like TMB channel, and a regular 18W preamp channel, which can be jumpered together.
str82hvn wrote:... but an EL84 just sounds way different than a EL34 and most of an 18 watts amps "Sound" comes from overdriving the power tubes.
With the TMB version a lot of the distortion also comes from the preamp, with the cathode follower. However, it's true that EL84s are brighter sounding than EL34s and don't have the same warm throatiness of the EL34s. Therefore, you may do better building one of these amps, but using 6V6s for the power tubes. It's an easy swap, with a couple or so resistor value changes, and 6V6s sound golden in Marshall Plexi style amps.

Now, if you want an exact clone of the Plexi preamp with the Dark and Bright channels, your best bet may be to build a 6V6 Plexi, as designed by Mark Huss - http://mhuss.com/Plexi6V6/index.html
Or the October, derived by Cliff Chappel from Mark Huss's design, with a few extra neat features - http://users3.ev1.net/~cliffchappell/am ... 09.28.html
Both of these are awesome sounding amps, with a tone to die for!
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Post by bignicky »

I love the sound of 6V6's ala a Princeton, Champ, or Deluxe, and a Plexi preamp w/2-6V6's does sound inviting. I still think that one could possibly modify to bring out the best qualities of the "cathode follower circuit" ; "the plexi preamp circuit" ; and the 18 watt output section of the 1974X Marshall. A choice between 6V6 and EL84 Plexis would be just what the doctor ordered.
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Post by bignicky »

I love the sound of 6V6's ala a Princeton, Champ, or Deluxe, and a Plexi preamp w/2-6V6's does sound inviting. I still think that one could possibly modify to bring out the best qualities of the "cathode follower circuit" ; "the plexi preamp circuit" ; and the 18 watt output section of the 1974X Marshall. A choice between 6V6 and EL84 Plexis would be just what the doctor ordered.
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Post by Coco »

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?
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Post by Dynaflow_Donnie »

I thought for 6V6's like in a deluxe it was also 8k...

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Post by rickp »

I am looking at building a Trinity TMB. How difficult might it be to mod it for 6v6s and how might that change the tone?
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Post by Coco »

rickp wrote:I am looking at building a Trinity TMB. How difficult might it be to mod it for 6v6s and how might that change the tone?
Rick,

Pretty simple actually. Change the hole to suit 6V6 (1" dia) and pretty well pop them in. We're woking on this concept to and there is a new Output transformer in the works to give you more options in doing what you want to do. Go and listen to mark huss 6V6 plexi clips

http://mhuss.com/Plexi6V6/page4.html
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Post by rjgtr »

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!
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Post by Coco »

rgtr,

What impedance transformer did you use for the 6V6, 6L6/KT66 and EL34 tubes? I'm looking for a suggestion for a second primary on some new transformers.
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Post by dartanion »

8K should work just fine with 6V6s. Although the Deluxe Reverb uses a 6.6K OT.

I have been working on a prototype amp that is using a plexi style preamp channel and variation of the 18watt PI that is now more Vox AC30 like and a pair of 6973s that are cathode biased. This amp has a much darker and thicker tone than EL84s and has a nice woody tone similar to 6550s, but at about 20watts output. And this is with a 125E OT 8O
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Post by pickaguitar »

First off...I'm not a builder. Just an admirer/player :D

Here's some pics of my brand new & first 18 watt micro plexi from S2:
I don't have any clips online to post right now but to me it sounds completely amazing.
I'm really digging it by itself or with my sun face fuzz and vibe-unit pedals. Please excuse because the pics are kind of big...

Image
Image
Image
Image
Image

Jason
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Post by zaphod_phil »

One of the cleanest looking builds I've ever seen! congrats

But a Plexi in a tweed cab...??? :? :roll:
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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.
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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
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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..

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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
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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.
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