Plexi Vintage 50W 1968 without screen grid resistors blowing tubes

[jftman]

I am selling my 50w plexi that has just been tuned up, caps, tubes, biased for matched set of EL34’s.

Here is what John Nau from Nau Engineering said about the grid resisters.
He basically says, adding the resistors will allow any tubes to be used, if they are matched and the amp is biased with expert care.

‘It dawned on me why the high stress is occurring on the output tubes: this amp does not have screen grid resistors. I think the best idea is to leave it the way it is, totally stock and original, but explain to any future owner that the amp would run better with these resistors installed. I could supply you with a pair of these to provided with the amp, as well as simple instructions to install them. I’m not sure why the designers decided to build it this way: almost every amp I ever see (Marshalls included) have screen resistors.’


I must post 5 threads before listing the head.
I will make these posts as replies to this thread.
Stay tuned!

I have 35 pics in this album, posting after 5 posts when links are allowed.!

 

[79 2203]

You’re probably better posting this in the vintage section, but it’s my understanding that the combination of wall voltages increasing combined with the poor quality of modern tubes that means that grid stoppers are necessary these days.
Anyway, that’s my dream amp. I’ve got a couple of 68 straight cabs with Pulsonics and I’m saving up to finally pair them up with a Plexi.

 

[Purgasound]

...grid stoppers are necessary these days...

"grid stoppers" are the 5.6K resistors tacked on to pin 5. It can be confusing because "screen" and "grid" are used interchangeably sometimes. There are three separate grids in an EL34 on pin 1, pin 4 and pin 5. Pin 1 is unused and tied to pin 8 which is the cathode and is grounded. There are many who can likely explain it better than I can, but the grid stopper is there to prevent blocking distortion and the screen grid resistor is there to prevent the grid from melting down in a power tube by controlling and stabilizing the current flowing through it.

It's not advantageous in anyway to run an amplifier like this without screen grid resistors. I prefer to use 1K 5W cement wirewound resistors here. The metal oxide or metal film ones over heat too much when the amp is dimed.

Cheers

To the OP, when it's time to sell, move the post over to the classifieds section and be sure to list your location. There are lots of members here from different parts of the world.

 

[Beryllium-9]

1968 JMP50? Hmm... Can you post pics of the internal chassis?

 

[jftman]

I am slowly posting the minimum of 5 threads then I can share a link to a google drive album of 36 pics.

In the vintage area I started the thread
Is a 1968 Super Lead and Bass nearly the same amp circuit?

Some pics of the internal are included there
Amp is not yet picked up after the cap job by John Nau , NAU ENGINEERING near Rochester, New York.

 

[RadioAD5GB]

I am selling my 50w plexi that has just been tuned up, caps, tubes, biased for matched set of EL34’s.

Here is what John Nau from Nau Engineering said about the grid resisters.
He basically says, adding the resistors will allow any tubes to be used, if they are matched and the amp is biased with expert care.

‘It dawned on me why the high stress is occurring on the output tubes: this amp does not have screen grid resistors. I think the best idea is to leave it the way it is, totally stock and original, but explain to any future owner that the amp would run better with these resistors installed. I could supply you with a pair of these to provided with the amp, as well as simple instructions to install them. I’m not sure why the designers decided to build it this way: almost every amp I ever see (Marshalls included) have screen resistors.’


I must post 5 threads before listing the head.
I will make these posts as replies to this thread.
Stay tuned!

I have 35 pics in this album, posting after 5 posts when links are allowed.!

WOW! That's insane. You simply CAN NOT run EL34s without screen resistors (on pin 4) unless they're strapped as triodes. No joke! They're "power pentodes" and even with those 1K5W resistors in place, they draw a heck of a lot of screen current! WOW! Just WOW!

 

[NewReligion]

"grid stoppers" are the 5.6K resistors tacked on to pin 5. It can be confusing because "screen" and "grid" are used interchangeably sometimes. There are three separate grids in an EL34 on pin 1, pin 4 and pin 5. Pin 1 is unused and tied to pin 8 which is the cathode and is grounded. There are many who can likely explain it better than I can, but the grid stopper is there to prevent blocking distortion and the screen grid resistor is there to prevent the grid from melting down in a power tube by controlling and stabilizing the current flowing through it.

It's not advantageous in anyway to run an amplifier like this without screen grid resistors. I prefer to use 1K 5W cement wirewound resistors here. The metal oxide or metal film ones over heat too much when the amp is dimed.

Cheers

To the OP, when it's time to sell, move the post over to the classifieds section and be sure to list your location. There are lots of members here from different parts of the world.

I updated 1968 100 watt Plexi. It had 4 Marconi dual 42uf Cap cans on the power board. I removed them & installed 4 100uf Axials.

I also changed out the 10uf 100 V bias caps as if they go bad it could be a catastrophic.

I installed 1.5k 5 watt replacements on pins 4 to 6.

Each side of the OT primary had a single 5.6k stopper.

Below you can hear it domed through a Boss Waza TAE. I’m using a Boss GE-10 in front.

The component boards can be seen at 3:29 in the video.

David

 

[LoudStroud]

Yeah, I’m a little confused too. The screen grid resistors across pins 4 and 6 have always been there in any vintage Marshall I’ve seen. It’s the 5.6k grid stopper resistors on pin 5 that were always missing on the early amps. Are you saying your Plexi has neither?

Your Plexi is also the first 1968 year I’ve seen with an upright mounted 1202 164 power transformer. Every other I’ve seen had a laydown style 1202 118. I have a friend’s ‘69 50 watt Lead head here at the house with the same PT, but it reflects the much higher voltages on the plates at around 440vdc. Curious if yours is the same.

Back to the screen grid resistors, a gut shot of the chassis would clear up any confusion in this conversation.

For the sake of selling your amp, you could sell it as is and let somebody else deal with it. If it is missing the 5.6K grid stoppers, that seems to be a universally accepted modification, since nowadays it is the only way for the amps to be stable.

 

[Browneyesound]

I have not seen myself, but have read that some early 50W amps did not have screen grid resistors.

A testament to the old mullards that they could even get away with that.

 

[Derrick111]

Yeah, I’m a little confused too. The screen grid resistors across pins 4 and 6 have always been there in any vintage Marshall I’ve seen. It’s the 5.6k grid stopper resistors on pin 5 that were always missing on the early amps. Are you saying your Plexi has neither?

Your Plexi is also the first 1968 year I’ve seen with an upright mounted 1202 164 power transformer. Every other I’ve seen had a laydown style 1202 118. I have a friend’s ‘69 50 watt Lead head here at the house with the same PT, but it reflects the much higher voltages on the plates at around 440vdc. Curious if yours is the same.

Back to the screen grid resistors, a gut shot of the chassis would clear up any confusion in this conversation.

For the sake of selling your amp, you could sell it as is and let somebody else deal with it. If it is missing the 5.6K grid stoppers, that seems to be a universally accepted modification, since nowadays it is the only way for the amps to be stable.

I don't think this is quite right. If I recall correctly, when Marshall changed from JTM-45s to JTM-50s and JMPs in the 60s, the 100 watters had 1k5w screen grid resistors and 5.6k grid stopper resistors from day one. However, the 50 watters never had screen grid resistors or grid stoppers initially. Screen grid resistors did begin to be added around 1969 on 50 watters, but Marshall continued to omit the 5.6k grid stoppers on any of the 50 watters through the point to point wired era into the 70s. The OPs amp is a 1968 50 watter so it likely wouldn't have had either 1k5w screen grid resistors or 5.6k grid stopper resistors as it had left the factory. I am a stickler for originality, but I install 1k5w screen grid resistors in a heartbeat on any of my 60s 50 watt plexis. I find that doing this, along with keeping the supply voltage in check (by verifying 6.3v +/- 10% on the heaters) makes for happy and the best sounding vintage 50w plexis.

 

[jftman]

Ok.!
John Nau has the amp and did the work.
What he indicated is the first batch of amps that were being built to be exported to USA were recommended to use 6550 power tubes.

He and I agreed that if the amp sold for more than 5K it would ship with NOS Mullard tubes. So he found matching tubes and biased the amp. He ‘dimed’ the volume and in a short amount of time, (I was not there), he cooked three vintage Mullards before wasting more.

The head as is runs stable with the Svetlana
EL34’s. The amp will ship with the schematic instructions John provided and two 1K resistors. The original build is preserved.

My sense says this will be a little project amp for the experienced, not for one to throw in some ‘fav’ and crazy expensive tubes and dime the amp wanting to be immersed in EVH glory.
But that is sweet just thinking of it.
The point? Tubes, resistors, no-resistors a VARIAC , a scope, and have at it.

 

[jftman]

Yeah, I’m a little confused too. The screen grid resistors across pins 4 and 6 have always been there in any vintage Marshall I’ve seen. It’s the 5.6k grid stopper resistors on pin 5 that were always missing on the early amps. Are you saying your Plexi has neither?

Your Plexi is also the first 1968 year I’ve seen with an upright mounted 1202 164 power transformer. Every other I’ve seen had a laydown style 1202 118. I have a friend’s ‘69 50 watt Lead head here at the house with the same PT, but it reflects the much higher voltages on the plates at around 440vdc. Curious if yours is the same.

Back to the screen grid resistors, a gut shot of the chassis would clear up any confusion in this conversation.

For the sake of selling your amp, you could sell it as is and let somebody else deal with it. If it is missing the 5.6K grid stoppers, that seems to be a universally accepted modification, since nowadays it is the only way for the amps to be stable.

LoudStroud I asked John to measure the voltage, when available I will post it here!

 

[Trapland]

Yeah, I’m a little confused too. The screen grid resistors across pins 4 and 6 have always been there in any vintage Marshall I’ve seen. It’s the 5.6k grid stopper resistors on pin 5 that were always missing on the early amps. Are you saying your Plexi has neither?

Your Plexi is also the first 1968 year I’ve seen with an upright mounted 1202 164 power transformer. Every other I’ve seen had a laydown style 1202 118. I have a friend’s ‘69 50 watt Lead head here at the house with the same PT, but it reflects the much higher voltages on the plates at around 440vdc. Curious if yours is the same.

Back to the screen grid resistors, a gut shot of the chassis would clear up any confusion in this conversation.

For the sake of selling your amp, you could sell it as is and let somebody else deal with it. If it is missing the 5.6K grid stoppers, that seems to be a universally accepted modification, since nowadays it is the only way for the amps to be stable.

My 1968 50 watt plexi did NOT come with screen grid resistors. It looks just like the OP amp. It also came with the NFB on the selector rather than a tap so the nfb varies with impedance setting.

 

[jftman]

Yeah, I’m a little confused too. The screen grid resistors across pins 4 and 6 have always been there in any vintage Marshall I’ve seen. It’s the 5.6k grid stopper resistors on pin 5 that were always missing on the early amps. Are you saying your Plexi has neither?

Your Plexi is also the first 1968 year I’ve seen with an upright mounted 1202 164 power transformer. Every other I’ve seen had a laydown style 1202 118. I have a friend’s ‘69 50 watt Lead head here at the house with the same PT, but it reflects the much higher voltages on the plates at around 440vdc. Curious if yours is the same.

Back to the screen grid resistors, a gut shot of the chassis would clear up any confusion in this conversation.

For the sake of selling your amp, you could sell it as is and let somebody else deal with it. If it is missing the 5.6K grid stoppers, that seems to be a universally accepted modification, since nowadays it is the only way for the amps to be stable.

 

[TAZIN]

Negative for a 1968 build. I'd say it's a 1970 (Jan.) build based on board type, wire colors, and component type. I believe this is a Music Ground amp which would explain the plexi panels...

 

[Ivan H]

There are three separate grids in an EL34 on pin 1, pin 4 and pin 5. Pin 1 is unused and tied to pin 8 which is the cathode and is grounded.

This is partially correct & partially incorrect. Pin 1 is the "suppressor grid & it is used, though not as efficiently as it could be. It's job is to reduce secondary emission (electrons bouncing off the anode). Running the suppressor grid at a small negative potential is more efficient.
I also have a 1968 JMP50 (serial No. S/ 11225) that has never had either screen grid or grid stopper resistors fitted. When I got it, it was fitted with a duet of Telefunken "winged plate" EL34's. They handled not having screen grid resistors. It now has Blackburn Mullard dual halo getter xf2 EL34's fitted that also handle no screen resistors. I don't "dime" the amp though, however I do run it with the bright channel's volume up around 6 1/2 ~ 7, which seems to be the sweet spot. Cheers

 

[jftman]

From the information I have gotten on line regarding the circuit board, standup transformer, and 10K series serial numbers, I don’t really know with certainty all that much. And the experienced who can appreciate transition periods can justify the lay down to standup happening in 1969. Obviously I except the board being not fully perforated makes it NOT early or more likely 1969, not 1968.

1968 plexis such as EVH sport a serial number in the 12K series. So, there is something that shows numbers and the year of manufacturing is not a good guide.

I will addup the two more empirical pieces, the board and the transformer and change the year of this amp as 1969-1970 PLEXI as a transitional plexi amp ahead of the metal panel variety.

I have not tried dating the amp from pot codes. When I pick it up I will check. John Nau thinks it is a 1969. I on the other hand put my estimation of 1968 from the serial number of the plexi panel.

Who can say anything about the chassis and the cab being never swapped. The telling lip having a curved edge also shares being late 68 and 69.

Here is the google drive link of 36 pics.

Marshall Plexi - Google Drive

drive.google.com

 

[LoudStroud]

The experts here have jogged my memory on the circuit timeline with screen grids and grid stoppers. I bought my first '68 Plexi 50 in '88, before I knew anything about the amps internally. The amp had been biased with a set of Chinese EL34's. A friend of mine and I fired it up and I hit one deflating chord when my friend witnessed a flared tube and the fuse blew. From there forward began my journey of having the necessary mods made to make em reliable, which on that amp, must've been my tech adding the screen grid resistors and grid stoppers. I eventually learned to do those mods myself and didn't waste any time adding the grid stoppers to any future early heads I picked up, which from that point were all '69 or later.

I guess my point being...if you've already sold the head, yes, good idea giving the new owner a heads up about installing the screen grids. I'd also suggest the grid stoppers. Along with running the amp at least close to its proper voltage (that one may possibly be 115VAC in the U.S.), it'll make it bulletproof when dimed.

I see that at least one of the power supply cap cans have been replaced in its lifetime, along with the electrolytics in the bias circuit. IMO, might also suggest replacing the other cap cans. That would serve as extra insurance for stability when dimed for long periods.

Heck, even Hendrix, Blackmore and Duane Allman had their amps modified for stability back in the day. Unless the amp is a museum piece virgin untouched circuit, might as well make these necessary improvements.

 

[jftman]

This amp is available for sale. I want to cooperate and provide detailed pics trying to pin down it as a plexi, from the last of the plexi amps.
The rolled lip too has ‘footing’ in 68 into 69 if one can believe what is out there on the timelines for plexi cabs!

Speaking of footing, those grey low profile feet are sweet…..
I intend to list this in the member Classified section.

My thought is what can be determined to support a Plexi having a stand up trans ? Are there other Plexi owners with stand up transformers?

The serial number is clear in the photo album. If the amp is a metal panel faked to be a PLEXI, what happened to the 1969-1970 panels? The chassis can’t be played with much for deception and sport plexiglass panels. Evidence?

What if it is a 1967 10K series chassis?. Somebody would have completely gutted the perforated board of an old Marshall to transplant a nonPlexi board into this chassis? Is that even a strategy of Music Ground?

Can we honestly find this to be the Summer/Autumn 1969 amp by inspection as a Plexi?

 

[Derrick111]

John Nau has the amp and did the work.
What he indicated is the first batch of amps that were being built to be exported to USA were recommended to use 6550 power tubes.

So he found matching tubes and biased the amp. He ‘dimed’ the volume and in a short amount of time, (I was not there), he cooked three vintage Mullards before wasting more.

This is a red flag for me... your "tech" claims the first batch of Marshalls in the USA were shipped from the factory with 6550s? Then he cooked three vintage Mullards after biasing your amp? If I am understandig you correctly, this doesn't seem like someone who should be touching amps let alone vintage ones of any value.

This amp is available for sale. I want to cooperate and provide detailed pics trying to pin down it as a plexi, from the last of the plexi amps.
The rolled lip too has ‘footing’ in 68 into 69 if one can believe what is out there on the timelines for plexi cabs!

Speaking of footing, those grey low profile feet are sweet…..
I intend to list this in the member Classified section.

My thought is what can be determined to support a Plexi having a stand up trans ? Are there other Plexi owners with stand up transformers?

The serial number is clear in the photo album. If the amp is a metal panel faked to be a PLEXI, what happened to the 1969-1970 panels? The chassis can’t be played with much for deception and sport plexiglass panels. Evidence?

What if it is a 1967 10K series chassis?. Somebody would have completely gutted the perforated board of an old Marshall to transplant a nonPlexi board into this chassis? Is that even a strategy of Music Ground?

Can we honestly find this to be the Summer/Autumn 1969 amp by inspection as a Plexi?

It is widely accepted that the last plexis rolled out of the Marshall factory in the summer of 1969, so no such thing as a fall 1969 plexi nor has a legitimate one one been seen. My Spring 1969 JMP-50 plexi is one of the latest plexis I've seen (they had dates on them by this point) and it still has a perforated board. @TAZIN has weighed in on this, and I find him to be of the more knowledgeable people anywhere regarding when Marshall made changes, what's original or changed, and in many cases down to the month of dating an amp. He will likely be the first to tell you he doesn't know it all and is human, but I think his assessment of your amp seems spot on from my moderately extensive knowledge on them. Music Ground did (does?) some terribly unscrupulous things to amps and to their customers. You can look up the leagle troubles and claims agains them online if you have a few weeks to kill.