(100 watt amp) Two grid stoppers or four?

Matthews Guitars

Well-Known Member
Joined
Aug 17, 2019
Messages
6,154
Reaction score
9,764
I've noticed, as I was browsing the Amp Archives, that despite the fact that Marshall's schematics always show a '59 as having four 5.6K grid stoppers, Marshall apparently incorrectly installed just TWO in, basically, every original pre-1970 amp I saw photos of.

Starting in 1970 they put the grid stopper resistors on the tube sockets and linked them up
with short wire links for each outer pair. That matches the schematics.

But before that, they way they had it, only the outer two tubes actually got the benefit of the grid stoppers. The inner pair were direct connected. The outer pair were connected thru a 5.6K resistor via its uncut leads.

What's the deal on that? Was that actually a commonly repeated assembly error or did engineering tell them to "do it this way, not what's on the schematic"?

And how does that change materially affect the amp?

It seems to me that since this is on the audio input signal to the power tubes, the inner two tubes get a slighly hotter signal, with a higher chance of achieving blocking distortion in those two tubes as well.
 

TAZIN

Well-Known Member
Joined
Mar 18, 2013
Messages
719
Reaction score
430
I believe the use of just two 1K5 grid resistors started in late 1965 shortly after the initial 100w amps were introduced and ran until October 1969. Then they changed to using four 5K6 grid resistors. Not sure if Marshall ever produced a four 1K5 resistor version...
 

neikeel

Well-Known Member
Joined
Feb 12, 2014
Messages
7,107
Reaction score
6,541
As above for lead amps
SB continued with 1k5 x2 for longer.
Never seen 1k5 x4
 

shakti

Active Member
Joined
Jul 14, 2016
Messages
251
Reaction score
235
I’ve wondered the same. Seems a weird choice to have grid stoppers on just two tubes. I built my black flag JTM100 that way for originality (single 1k5 per side) but have been meaning to revert it to a 5k6 for each tube. Wonder if I can hear a difference...
 

yladrd61

Well-Known Member
Joined
Jun 20, 2010
Messages
2,401
Reaction score
1,854
Location
Tacoma-ish, WA
My 5F8A high power Tweed Twin clone howled like a pack of rabid wolves at high power settings with a single 1K5 per side, I went with 5K6 on each tube and now it stays tight all the way up to 12 ;)
 

neikeel

Well-Known Member
Joined
Feb 12, 2014
Messages
7,107
Reaction score
6,541
Often it is quality of lead-dress and layout that stops oscillation.
Grids stoppers are sometimes necessary but can be a source of unwanted noise of their own kind too.
 

william vogel

Well-Known Member
Gold Supporting Member
Joined
Dec 18, 2016
Messages
916
Reaction score
1,112
I’m building a serial number 7026 45/100 clone right now and it’s getting the two outside 1.5k. I’ve never heard the difference in a properly operating amp between having the grid stoppers and none. If it has an issue, the grid stoppers straighten it out.
 

Matthews Guitars

Well-Known Member
Joined
Aug 17, 2019
Messages
6,154
Reaction score
9,764
From what I'm seeing via Amparchives photos and others, it looks like Marshall switched to using four grid stoppers somwhere in 1970. Every 1971 and later Superlead I've seen (unless it was given a retro mod) has four grid stoppers. And some 1970s.
 

neikeel

Well-Known Member
Joined
Feb 12, 2014
Messages
7,107
Reaction score
6,541
I believe the use of just two 1K5 grid resistors started in late 1965 shortly after the initial 100w amps were introduced and ran until October 1969. Then they changed to using four 5K6 grid resistors.
 

cooljuk

Active Member
Joined
Mar 11, 2009
Messages
305
Reaction score
90
The 1959HW has the two 1.5k grid stoppers again.
True.

i.php
 

cooljuk

Active Member
Joined
Mar 11, 2009
Messages
305
Reaction score
90
The one consistent thing about 1960s and 1970s Marshalls is that they were never consistent!

:hmm:

I say the same thing about McCarty-era Gibson to customers about 5 times a day!

It helps keep things interesting and keeps us on our toes.
 

danfrank

Well-Known Member
Joined
Mar 5, 2015
Messages
1,024
Reaction score
579
More bad Marshall engineering. EVERY power tube should have a grid stopper. This is the one place where CC resistors work well, and they need to be right at pin 5 of the tube socket to be most effective. 1K5 is kind of a small value especially for EL34s, will still cause blocking distortion. Marshall got it right when they switched to 5K6 resistors on each output tube.
 

cooljuk

Active Member
Joined
Mar 11, 2009
Messages
305
Reaction score
90
More bad Marshall engineering. EVERY power tube should have a grid stopper. This is the one place where CC resistors work well, and they need to be right at pin 5 of the tube socket to be most effective. 1K5 is kind of a small value especially for EL34s, will still cause blocking distortion. Marshall got it right when they switched to 5K6 resistors on each output tube.


What would be the difference in sound / performance if I converted either of my two amps above to four 5.6k, rather than the factory two 1.5k, grid stoppers?
 

neikeel

Well-Known Member
Joined
Feb 12, 2014
Messages
7,107
Reaction score
6,541
What would be the difference in sound / performance if I converted either of my two amps above to four 5.6k, rather than the factory two 1.5k, grid stoppers?

If lead dress is good and you are not getting oscillation now there will be no clear benefit, sometimes they can add noise and if too big trim off some highs, in general they can be a good idea http://www.valvewizard.co.uk/gridstopper.html.
 

yladrd61

Well-Known Member
Joined
Jun 20, 2010
Messages
2,401
Reaction score
1,854
Location
Tacoma-ish, WA
If lead dress is good and you are not getting oscillation now there will be no clear benefit, sometimes they can add noise and if too big trim off some highs, in general they can be a good idea http://www.valvewizard.co.uk/gridstopper.html.
The 5F8A also has much less real estate inside the chassis with the added effect of 80-90 watts and 2 12" alnico speakers pummeling the upside down tubes ;)
 

cooljuk

Active Member
Joined
Mar 11, 2009
Messages
305
Reaction score
90
If lead dress is good and you are not getting oscillation now there will be no clear benefit, sometimes they can add noise and if too big trim off some highs, in general they can be a good idea http://www.valvewizard.co.uk/gridstopper.html.

Thanks. Those are the only two 1959s I've had with only two grid stoppers.

That 1972 SL above gets just a little fussy on the bench out of the box but behaves well when back in the head box with the shielding plate. I haven't yet put that one on a scope to look for anything funny in the waveform, but it sounds great, overall. I hear a little crossover distortion but it's also biased cold at the moment, needing some tubes that can better handle the 530v plate/screen.
 

Latest posts



Top