270k & 470k mixer resistors.

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270k & 470k mixer resistors.

Postby outlier on Tue Feb 24, 2004 1:20 am

Can someone clarify when the transition was made?

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Re: 270k & 470k mixer resistors.

Postby clay_finley on Tue Feb 24, 2004 2:47 am

Marshall changed from 270k to 470k resistors in mid '66 (even though 270k's did creep in after that date occassionally). The presence of 470k mixer resistors will date the amp to be post mid '66 (you can't really draw conclusive dating info from the presence of the 270k's). The exception is that 4 channel PA heads normally came with 470k mixer resistors.

Ted B. told me this. It's on my web page (somewhere):
http://www.lastwordsmusic.com/clay/marshall_circuits_101.html

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Re: 270k & 470k mixer resistors.

Postby Ned B on Tue Feb 24, 2004 3:44 am

A '67 black flag SB I once owned deffinately still had the 270Ks. Of interest to me is when did the SLs started appearing with 470Ks. My SL black flag has non-original 470Ks, so it is no help. I find these to be commonly changed out in old Marshalls. I'm think that it probably wasn't an abrupt change, but I think the lead amps were the first to exibit the change over and it probably happened during the end of the Drake period (1203-8- PT 1202-119/132 OTs). There was a Drake tranny '67 briefly on ebay last week and the mixer resistors (470K) looked to be about the only changed parts on the whole board. Maybe that is evidence there were 270ks there originally.
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Re: 270k & 470k mixer resistors.

Postby Hotfreaks on Tue Feb 24, 2004 11:55 am

Nickcha had pics of a black flag jtm50('67) that had 270k's....FWIW.

I'd say there was quite a lot of spillover until sometime in 67 when the JMP's started rolling out.
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Re: 270k & 470k mixer resistors.

Postby Wannatone on Tue Feb 24, 2004 7:37 pm

Clay,

my aluminum end block JTM45 w/ KT66´s has 270k.
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Re: 270k & 470k mixer resistors.

Postby outlier on Tue Feb 24, 2004 8:03 pm

It's a good sound. Maybe I've missed it, but I don't really see too many people using 270k mixer resistors for mods.
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Re: 270k & 470k mixer resistors.

Postby bschart73 on Wed Feb 25, 2004 3:33 am

Anyone care to comment on the tone differences between the two? My '66 JTM100 Super PA has the 470K mixers for all 4 channels (seems to be the stock PA head value). I have CH1 converted to SL specs (500pF CH1 mixer bypass cap and 100pF brite cap on CH1 vol pot). Was wondering if it would make much difference if I changed the CH1/CH2 mixers to 270K? BTW, I have CH3/CH4 disconnected from the circuit to avoid some signal loss thru CH3/CH4 pots to ground. The amp seems to have more gain this way? Maybe I'm hearing things? :D

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Re: 270k & 470k mixer resistors.

Postby Marshall Maniac on Wed Feb 25, 2004 4:34 am

My JMP 50w w/ tube rectifier has 270k. That's probably 1967, just after the flag logo.

As far as tone is concerned, the 270k resistors allow a bit more loading from one channel to the other. You can hear it when you plug into Ch1 and turn the volume of Ch2. So, you loose some gain and some highs, depending on where you put the other channel's volume control.
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Re: 270k & 470k mixer resistors.

Postby JeffWest on Wed Feb 25, 2004 11:53 am

270Ks seem to have shown up stock occasionally at least as late as '69. Another sonic difference is that the impact of any associated ~500pf mixer bypass cap is changed w/the 270K compared to the 470K.
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Re: 270k & 470k mixer resistors.

Postby drewleslie on Wed Feb 25, 2004 1:14 pm

Interesting. I got a board for my bluesbreakerRI from RayD with 470Ks on it ... he also sent along some 270Ks to experiment with. Guess now's the time.

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Re: 270k & 470k mixer resistors.

Postby bschart73 on Wed Feb 25, 2004 6:25 pm

What about 'mixed' mixer resistors? I was thinking maybe use 270K on CH1 to pass more signal (more gain) on CH1 and 470K on CH2 to keep the signal loss to CH2 vol pot to a minimum. Has anyone tried this?

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Re: 270k & 470k mixer resistors.

Postby cerrem on Wed Feb 25, 2004 7:42 pm

These two resistors which make up the "mixer" resistors is really a VOLTAGE DIVIDER.....
The sum of the two resistors is in PARALLEL with the 100K plate resistor and is also in parallel with the 1M pot....So with 270K resistors the loading the 12AX7 sees is 100K//(270K+270K)//1M = 77.8K load and a Gain of 56.4 before the mixer..than after the mixer it would be 28...

With 470K mixer resistors you get 100K//(470K+470K)//1M = 83K load and a Gain of 58 before the mixer and after the mixer it would be 29....
So the gain difference linearly is small but the harmonic content is a bit bigger with the 470K resistors...so you get a "thicker" sound with the 470K resistors since it has less loading affects on the 12AX7 gain stage..
The 500pF cap place on the 270K mixer resistors provides enhanced treble...BUT when changing to 470K resistors you would need to move the cap value down to 330pF to maintain the "same" treble boost as the older circuit..

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Re: 270k & 470k mixer resistors.

Postby JeffWest on Wed Feb 25, 2004 11:00 pm

But of course Marshall typically didn't reduce the value of the 500pf bypass much or at all when they went to 470K (I know yours with the 330pf tan tube is an exception, Chris), so you typically get a little more perceived high end bite on that deal alone.

Another thing worth mentioning about the summing resistors and any bypass is that when the vol knob on that channel is at zero and you're using another channel, you've got that bypass cap on the other channel draining signal right to ground (in conjunction with the resistor itself). That's definitely one of the reasons that ChII on a split cathode Lead is often so stubbornly dull. Becomes especially pronounced in situations like a Super PA where you may have added bypass caps on several channels' mixers. One easy partial fix/experiment- turn the volume up some on the channel(s) you're not using. You'll get a little extra power and edge on the channel in use, along with more noise unfortunately. But on some older amps especially with less high end gain on the front end so noise is less of a problem this is like an added fine tune tone control.

Drewleslie- what I recommend initially if you want to hear the difference between 270K and 470K mixers holding everything else constant is not to swap the resistors but to connect with a jumper a 680K or 750K or 820K resistor in parallel with the installed 470K, so you can "convert" to about 270K instantaneously on and off for a real comparison to hear the difference. I wouldn't do it at extreme volume, but you can really tell what the difference is and isn't in the way you hear it that way, with a particular amp and tubes, etc.

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Re: 270k & 470k mixer resistors.

Postby drewleslie on Fri Feb 27, 2004 1:10 pm

Thanx Jeff, neat idea, but I already switched to 270Ks ... easy to do on smallbox's turret board.

As Chris' calcs showed, I didn't hear increase in gain. Tonewise seems a bit more clarity, maybe thinner tonewise but really fattens up when jumper the inputs. I'll have to season my ears on this new setup for a while to see if it's a keeper. Made some clean and dirty digital clips that should help with the a-b.

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