GRID LOAD RESISTORS

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heisty
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GRID LOAD RESISTORS

Post by heisty » Sun Sep 02, 2007 8:48 am

Hi i have a JMP50 plexi and not sure on how the 2 x 470k mix resistors work, I think one is the mix and the other with the bright cap is the grid load side?
Can someone explain and what happens if u lower values here???
Cheers

Trace
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Post by Trace » Sun Sep 02, 2007 2:36 pm

Hi i have a JMP50 plexi and not sure on how the 2 x 470k mix resistors work, I think one is the mix and the other with the bright cap is the grid load side? Can someone explain and what happens if u lower values here??? Cheers

I've seen JMP50's that are mostly Master Volumes but I've also seen JMP50's that were plexi based. As you've stated plexi in your description I will presume that it is a 1987 plexi circuit.

Without getting overly technical; The Volume pots are in essence the voltage dividers and the 470K's are the grid stopper/input resistors (some refer to these as "mix" resistors in a plexi). When first starting out most tend to think of a voltage divider like those found in the JCM800 2204/2203 (IE: 2=470K's in series with the output signal tapped at the junction, which then goes into the grid of V2A). Using a pot is essentially the same principle but with the advantage that you can adjust the pot to where ever you desire, therefore, you have a variable voltage divider verses a fixed voltage divider.

Naturally the 470K going into the grid adds resistance to the signal effectively reducing the amount of signal allowed to pass through to the grid while also attenuating frequency response as well. These can be used to color or fine tune the sound and/or to avoid unwanted grid blocking distortion (not very pleasant to the ears). The cap across the resistor (470pF) is used to bypass the resistor effectively attenuating top end/treble frequencies (IE: making the signal brighter).

Hope this makes sense, it's Saturday morning and I haven't had any coffee yet (ha, ha).


Have a great weekend
Trace

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UR12
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Post by UR12 » Sun Sep 02, 2007 2:40 pm

Trace wrote:
Hope this makes sense, it's Saturday morning and I haven't had any coffee yet (ha, ha).


Have a great weekend
Trace
Trace. I hate to tell you this but it is Sunday Morning :lol: :lol:

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statorvane
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Post by statorvane » Sun Sep 02, 2007 2:44 pm

Well, I think they are both mixed and feed the grid of V2A. The 5F6-A originally used 270K mixers here; no bright cap. The JTM45 used the same 270K and added a 500pF bright cap. I think you get a little extra gain with the 270K, but it is probably trivial.

Here is a good explanation: http://marshall.redpt.com/clay/mixer.html

Trace
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Post by Trace » Mon Sep 03, 2007 1:29 am

Trace. I hate to tell you this but it is Sunday Morning :lol: :lol:

Well if that's not a classic example I don't know what is (ha, ha). Sorry about that!


Note to self - drink coffee before posting (ha, ha)
Trace

heisty
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Post by heisty » Mon Sep 03, 2007 9:10 am

Thanks people, ive got it, now if u lower the value say from 470k to 270k u will get more drive or signal and will this produce to much blocking distortion?
Also would a good carbon film be better than a c/c in this position,should i use 1watt instead of 1/2 watt resistors?
Cheers

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statorvane
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Post by statorvane » Mon Sep 03, 2007 1:35 pm

I strictly use metal film resistors, unless I need a wirewound to really dissipate some energy. I'm sure other folks will have different opinions. 1/2 watt resistors are be more than adequate here.

As far a blocking distortion, I don't know. I would think something as venerable as the 5F6-A (270K mixers) doesn't necessarily exhibit excessive blocking distortion. Then again the first tube is a 12AY7...

I would stick with the 470K and 470pF as on the 1987. There is plenty of gain with that setup. If you need more, I don't think changing to lower value mixer resistors is really going to buy you much in the way of perceptible gain.

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mhuss
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Post by mhuss » Mon Sep 03, 2007 2:23 pm

(Again, assuming a 1987) lowering the big resistors won't buy you much in terms of gain. If you want a bit of a gain bump, bypass the second stage 820 ohm resistor with e.g. a 1uF cap. If that's not enough gain, you're going to need to rearrange things a la JCM800. But perhaps at that point it would be better to sell the 50 and buy something gain-ier!

--mark

heisty
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Post by heisty » Mon Sep 03, 2007 9:51 pm

Hi thanks men, mine has the split cathode on first pre,i am already running a 10uf + 820 and 2k7 + .68sozo , and a .68 on 2nd pre cathode
Cheers

heisty
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Post by heisty » Tue Sep 04, 2007 11:47 am

Also why has the plexi a 1k5 2watt resistor coming from the tail pair of 220s from PI to pin 5 on EL34 , and the JCM800 has 5K6? what is the difference?
Cheers

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statorvane
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Post by statorvane » Tue Sep 04, 2007 4:32 pm

I believe these are to prevent oscillation. Start here and scroll down to the control grid resistors (near the bottom). http://marshall.redpt.com/clay/marshall ... s_101.html There may be better on-line references, but this is the site I most often frequent.

Using Marstran schemos for reference here....

5F6-A - None
JTM-45 - None
1987 - None (Early) 5K6 (Late '73?)
1959 - 5K6
2204 - 5K6

heisty
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Post by heisty » Fri Sep 07, 2007 8:32 am

how does this work??? preamp main filter cap has a 10k resistor , ive heard that the optimum value that suits jcms is 4k7 ? how does the lower resistance effect tone or performance ???

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statorvane
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Post by statorvane » Sat Sep 08, 2007 12:52 am

Well, I am confused by your question. I thought you were interested in Plexi's. Seems like you are now interested in JCM800s. So, here's a link to some highly opinionated views of Marshall amps - 1987s and JCM800s. I used one of his preamp tweaks to get a little more beef out of a JCM800 clone I built for my son. It is a great amp IMHO (the real deal - clone ain't bad either).

http://tone-lizard.com/Marshall_Myths.htm

heisty
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Post by heisty » Sat Sep 08, 2007 9:06 am

Hi yes thanks , i am just using the jcm as an example , if i lower the 10k on the plexi this will raise preamp voltages how will this effect the tone or response of the amp??

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statorvane
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Post by statorvane » Sat Sep 08, 2007 12:55 pm

Okay, so your referring to the power rail voltages. Well, I am conjecturing the reason it is different is the 2204 preamp was built around the hot wire (or one wire) mod. This technique allowed 1987 preamps to chain the two triodes; Bright fed the Normal channel. Possibly when developing this into the 2204, Marshall felt higher rail voltages to the PI and preamp would improve the sound. It is relatively easy to try both values and evaluate the result.

FWIW, if you have ever tried to map out Marshall 50W PP amp voltages - they are all over the joint.

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