jcm800 piggyback resistor and harshness

[bennieusmaximus]

Hey there.I just got a JCM 800 2203, horizantal input,5 cap cans,date wrote on the chassis tag is 4/6/84.I have some questions.The first is the amp seems a little harsh,no matter what cab I use(75w celest 2x12 or a greenback 4x12).Should I clip the bright cap?It also does not sound better as the volume goes up.I think it even may loose gain.First I will check the bias.
So I go in to check the bias and see a resistor piggybacked on R26.R26 is a 4.7k resistor and the resistor piggybacked on it is a 68k resistor.Should I remove this piggyback resistor?What function does R26 have?I am also thinking about recaping the amp since the caps are original,but I dont want to spent the cash on this amp If the harshness wont go away.The amp has new jj E34L's in it and Sheild logo old mullard 12ax7's.I have rolled power and preamp tubes.(thats when I saw the piggyback resistor).Im playing old stock les pauls threw this.

 

[tresmarshallz]

I am not a tech and not qualified to verify this data, but when I researching my 50W 2204 circuit I ran across this on a web site regarding the functions, I don't know if it is the same on your 2203.

r26 works in conjuntion with bias pot to set the adjustable range.

r30 sets the available voltage for the bias supply ( lower value gives more negative voltage )

r24/25 decides how hard you hit the power tubes ( 150k will shunt more signal to ground than a 220k & be easier on the output tubes )


And this website has an awesome calculator to add up the value of piggybacked resistors...
http://www.1728.org/resistrs.htm

 

[bennieusmaximus]

Thanks for your help,I will be looking into this.Mabey the previous owner had some kind of tubes in it Other than el34's.

 

[Alabama Thunderpussy]

r24/25 decides how hard you hit the power tubes ( 150k will shunt more signal to ground than a 220k & be easier on the output tubes )

This is an inaccurate explanation of what those resistors do, imo. They attenuate the signal (and only to a debatable degree) but they do not provide a chassis "ground" reference to the power tube grids, or to shunt signal to. The reference is to the negative voltage from the bias supply, which is obviously less than chassis potential. The size of the grid swampers will have more effect on the frequency response than the bias feed resistors.

 

[tresmarshallz]

thanks, that is interesting and good to know. I think I adequately stated my position of not being a tech and warning that it was information that I ran across online while researching that I could not verify

 

[bennieusmaximus]

So for what reason is there another resistor on r26?

 

[Alabama Thunderpussy]

They don't shunt signal to ground. They load down the PI output, but that's not their main purpose. Their main purpose is to drain away leakage currents and keep the power tube grids at a stable dc reference and prevent thermal issues.

I'm not sure why there is a resistor piggybacked there. Someone apparently thought they needed 4.3k as opposed to 4.7k. are you sure it's a 4.7k and not the 47k.. hmm...

in which case the total resistance would be 27k.

 

[tresmarshallz]

So for what reason is there another resistor on r26?

To make changes in the adjustable range of the bias? Maybe someone was messing around with a different tube type or trying to fine tune the bias to a set of tubes that ran hotter/colder than normal. I have done this on my jcm800 to try and get it in the range for 6550 tubes. I don't recommend trying this if you are like me and not a tech, it is risky business!

 

[bennieusmaximus]

Ya Alabama your right.my total resistance is 29.7k.My eyes must me going!.The new reisister is covering the old resistor pretty good,couldent tell red from orange.I adjusted the bias and alot of the harshness went away.I had a '79 jmp 2203 and this seems like it has a tad more gain and more high end.I heard the circuits were prettty much the same,but there is a definate differance between the two years as far as gain and tone goes.

 

[plexifier]

R26 (should be 47k stock), along with the bias pot, determine how much negative bias voltage is sent to ground.

Someone apparently had tubes that wouldn't bias hot enough so they added the parallel 68k to reduce neg bias voltage sent to the tubes. Just leave the 68k in until you see how the tubes bias. If you can't get the bias cold enough (i.e. it is always above 70%), remove the piggyback 68k and see where it's at.

If by "harshness" you mean it's too bright, there are several easy mods you can choose from if you want to go that route.

 

[bennieusmaximus]

Ya plexi I think youre right with the word bright.Ya got any suggestions?

 

[Alabama Thunderpussy]

If you can't get the bias cold enough (i.e. always above 70%), remove the piggyback 68k and see where it's at.

i'm curious, what do you mean by always above 70%

Let's do the smart thing and start from the bottom. No one has asked our OP what his EQ settings are.

Treble and Presence settings, mainly.

 

[plexifier]

i'm curious, what do you mean by always above 70%

I just meant that the bias pot might not have enough range to get the bias at or below 70% because of the parallel resistor that was added to the bias circuit.

bennieusmaximus,
I agree that we should know how you're using the amp before considering mods. What do you typically set the preamp gain, treble and presence at? The 75w Celestions you mentioned can be very bright and thin (mid scoop) so I wouldn't suggest modding the amp to fit those unless they are going to be the main speakers you'll use the amp with.

If you absolutely can't dial out the brightness then we can look at a few simple mods.

 

[bennieusmaximus]

Good call plexi,I usually run threw my greenback 4x12 jcm 800 slant cab.pressence-0, bass 6.5, mid 3. high 5. master vol at 7 amp volume varries.I play with a '78 and a '81 les paul customs with stock pickups.

 

[plexifier]

Turn up the mids and lower the treble!

If the amp mainly seems too bright with the preamp gain below about 7 then removing the bright cap will help a lot. However, if it's always too bright, even with the gain above 7, then it won't help as much.

In case you don't know, the bright cap has more effect as you turn down the preamp gain. That's why it can sound thin or bright at lower settings. The stock cap is 1000pF, which can let some harsh upper-mids through at lower gain settings.

 

[bennieusmaximus]

Thanks for youre help plexi.

 

[Alabama Thunderpussy]

 

[bennieusmaximus]

You too ATP.I love that band.