Anyone want to help me fixing/modding my JCM800? Clip Added!

  • Thread starter Thread starter JakeAC5253
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SpiderWars":3qjsuoa4 said:
You can decrease the 27k to get more bias voltage (not current, * - see below) as long as it doesn't go over the bias cap voltage rating (the bias filter caps are probably rated in 150v-160v range). If you can get there that way then leaving the 47k is fine.

As you increase your idle current the plate voltage will decrease a little bit. So you have to keep re-measuring your plate voltage, re-doing the calculation, and adjusting again to get it dialed in. Also, consider that 70% max dissipation as a max and don't just blindly set it there. Listen to the amp at 70% then 50%, see if you like one better and go from there. Run it as cool as you can and still get the tone you like. You may like 55% or 60% better than 70%.

* - just to clarify: you are setting a bias voltage (at the grid) such that it controls the idle current through the tube (from plate to cathode). Under normal conditions, no real current flows in the bias circuit itself, the current is flowing from plate to cathode and the grid (bias) voltage just controls that current.
Click to expand...

The 10uF bias caps in my amp are only rated at 100V. I'm wondering if I should replace those with higher rated ones during the process of fixing this bias issue.

So, since you are setting the voltage to control the current, then higher not lower resistance between the 47k + 22k will cause a higher negative voltage on the Grid which should help to increase the idle current of the stages? It makes sense since when talking about voltage, more parallel resistance means higher voltage on the rail.
 
JakeAC5253":3akcw7ie said:
SpiderWars":3akcw7ie said:
You can decrease the 27k to get more bias voltage (not current, * - see below) as long as it doesn't go over the bias cap voltage rating (the bias filter caps are probably rated in 150v-160v range). If you can get there that way then leaving the 47k is fine.

As you increase your idle current the plate voltage will decrease a little bit. So you have to keep re-measuring your plate voltage, re-doing the calculation, and adjusting again to get it dialed in. Also, consider that 70% max dissipation as a max and don't just blindly set it there. Listen to the amp at 70% then 50%, see if you like one better and go from there. Run it as cool as you can and still get the tone you like. You may like 55% or 60% better than 70%.

* - just to clarify: you are setting a bias voltage (at the grid) such that it controls the idle current through the tube (from plate to cathode). Under normal conditions, no real current flows in the bias circuit itself, the current is flowing from plate to cathode and the grid (bias) voltage just controls that current.
Click to expand...

The 10uF bias caps in my amp are only rated at 100V. I'm wondering if I should replace those with higher rated ones during the process of fixing this bias issue.
Click to expand...
Only if the supply is over that which it shouldn't be. Measure it.
 
SpiderWars":3jcz6n3z said:
JakeAC5253":3jcz6n3z said:
SpiderWars":3jcz6n3z said:
You can decrease the 27k to get more bias voltage (not current, * - see below) as long as it doesn't go over the bias cap voltage rating (the bias filter caps are probably rated in 150v-160v range). If you can get there that way then leaving the 47k is fine.

As you increase your idle current the plate voltage will decrease a little bit. So you have to keep re-measuring your plate voltage, re-doing the calculation, and adjusting again to get it dialed in. Also, consider that 70% max dissipation as a max and don't just blindly set it there. Listen to the amp at 70% then 50%, see if you like one better and go from there. Run it as cool as you can and still get the tone you like. You may like 55% or 60% better than 70%.

* - just to clarify: you are setting a bias voltage (at the grid) such that it controls the idle current through the tube (from plate to cathode). Under normal conditions, no real current flows in the bias circuit itself, the current is flowing from plate to cathode and the grid (bias) voltage just controls that current.
Click to expand...

The 10uF bias caps in my amp are only rated at 100V. I'm wondering if I should replace those with higher rated ones during the process of fixing this bias issue.
Click to expand...
Only if the supply is over that which it shouldn't be. Measure it.
Click to expand...

I can measure this voltage at the negative terminal of one of the bias filter caps right?
 
Yes, the one closest to the diode. The other bias cap should be at your actual bias voltage but that first one is the supply side.
 
I think after a bunch of mods I tried, the best was going back to stock with maybe a part or two of different values. Bottom line, the stock circuit needs to be run at high volume for it to fill out. Great sounding amp and I agree with all others, a really good boost is the best mod. Say what you want, when you start changing the values too much, the amp loses it's signature sound and becomes something else.
 
I have a pack of these on hand. It's a 15k Metal Film 1/2W Resistor that I could use in place of the 27k one if 1/2W is enough to handle the load there. Let me know if I should go ahead and replace that resistor, cause if I have to order a new one, that's fine.

1/2W 15k
http://www.mouser.com/ProductDetail/KOA ... 8dQw2Sc%3d


I metered the voltage at both the first cap and the second. The first one had a voltage of -46V, and the second had a voltage of -38V.


I actually have a pack of these as well. 1/2W 10k
http://www.mouser.com/ProductDetail/KOA ... hjYQcvz2NQ
 
what exactly are you trying to achieve, besides more gain?
how far are you willing to change the stock JCM sound?
is your amp brighter or is it one of the bass heavy ones?

duesentriebs tips on the input grid resistor and v1b cathode resistor are good
 
ericsabbath":2bkjw2fm said:
what exactly are you trying to achieve, besides more gain?
how far are you willing to change the stock JCM sound?
is your amp brighter or is it one of the bass heavy ones?

duesentriebs tips on the input grid resistor and v1b cathode resistor are good
Click to expand...

Hey bud. I was mainly interested in modding it because it didn't sound or feel good, didn't really have that Marshall sound and wasn't fun to play. About halfway through the thread I realized it was bc the circuit wasn't stock but also not modded well, and the power tubes weren't biasing properly. Now I'm in the process of fixing that, but I'm still taking mod advice, because I'm going to hook up some mod switches once the circuit is fixed ;) Def going to have a switch for the 10k to 2k7+680nF change, and a few others.
 
I fixed it!!! The 15k resistor didn't fix it by itself, but on a hunch, I decided to try holding resistors in parallel with the 47k resistor until I could achieve a bias of at least 41mA, and the value that did it was 47k. 2x47k parallel is 23.5k, so I threw the next best thing in there, a 22k that I had on hand. Well, it's biasing exceptionally well now, reaching 41mA at only about 60% sweep on the trimpot. Plate voltage is still firm at 425V though, which is surprising, I expected it to change. I can't play it yet, but hopefully soon I'll get to test it out and see what a Marshall actually sounds like for once, instead of a local-tech-special lol. I'm going to play with the bias and see how it sounds at different ranges, but just the fact that it can reach 70% dissipation now is really exciting to me. Thanks to everyone who has helped me so far, its awesome to see such a great classic amp working at 100% again.

https://dl.dropboxusercontent.com/u/883 ... 224858.jpg
 
Sweet! Yeah, you needed to go higher on that 27k. Higher bias voltage = lower idle current. But what you have works. :rock:
 
SpiderWars":2jx0j9l0 said:
Sweet! Yeah, you needed to go higher on that 27k. Higher bias voltage = lower idle current. But what you have works. :rock:
Click to expand...

Is what I have fine, or is the way I did it going to cause stress to something?
 
Kapo_Polenton":1je52kst said:
I think after a bunch of mods I tried, the best was going back to stock with maybe a part or two of different values. Bottom line, the stock circuit needs to be run at high volume for it to fill out. Great sounding amp and I agree with all others, a really good boost is the best mod. Say what you want, when you start changing the values too much, the amp loses it's signature sound and becomes something else.
Click to expand...

Yes, and do this to address most of what you describe for a few bucks and little time:

Just add a 100k resistor to the wiper of the stock MV. Thank me later. Courtesy of Kevin O'Conner.
 
Update. The amp sounds pretty fucking killer now. WAY better than it ever has since being in my posession, like going from solid state to tubes for the first time. I'm actually somewhat shocked at how good it sounds, but then again, that's why these amps are legend :lol: :LOL: In the way of mods, it definitely doesn't need much tbh. Little bit of extra gain is all that jumps out at me immediately. Sometimes when you hit a chord it sounds a bit splatchy. That could also be because I didn't have the chance to really let her rip yet. Just other end of the house loud, not other end of the street loud lol. Lows are full, highs are not harsh, mids are perfect.

Can anyone confirm whether the Cathode biasing components on one of the gain stages can be put on a switch? I seem to think that Mesa does that in at least one of their amps. I know how to do it, but just want to make sure that it's not going to cause damage when the switch is flipped in either direction. I can almost guarantee that there will at least be an audible pop sound.
 
Bump for answer? Or a link to a forum that would know?

As for the parts that will be replaced in the signal path, could I just use metal film resistors, or is there a good enough reason to use carbon composition resistors? Polypropylene film capacitors a good choice? WIMA? 1/2W for the resistors? What voltage rating for the cathode bypass cap when there is a 2k7 resistor there? The parts I'm going to change are the input grid resistor, the V1B cathode bias components, a 1M resistor to go across the gain pot, and possibly play around with a resistor after the master volume.
 
Look at a Bogner XTC schematic and look for the Structure switch for a good example of how to do that.
 
SpiderWars":33xmeiki said:
Look at a Bogner XTC schematic and look for the Structure switch for a good example of how to do that.
Click to expand...

I see. So you have to switch in a parallel load, as opposed to having two that are distinct and independent? That's what Mesa does in the Recto too. Either way, cool cool, I'll just use a 3k6 resistor parallel to the 10k instead of a 2k7 and that should get me close enough, .680uF cap value should stay the same.
 
Just play around with values for the resistor and cap. Start with a 10k in parallel for the 10k and so on. 1/4 watt for them is fine, voltage rating of the cathode caps: 25V min. I prefer to use WIMA FKP or Tantalum here.
If you increase the gain on V1B heavily and you get too much mud, decrease the value of the gainpot. 500k or even 250k log. Or decrease the value of the coupling cap from 22nF to 10nF to 4n7 (500-630V rated) and so on . . .
 
duesentrieb":32cstta5 said:
Just play around with values for the resistor and cap. Start with a 10k in parallel for the 10k and so on. 1/4 watt for them is fine, voltage rating of the cathode caps: 25V min. I prefer to use WIMA FKP or Tantalum here.
If you increase the gain on V1B heavily and you get too much mud, decrease the value of the gainpot. 500k or even 250k log. Or decrease the value of the coupling cap from 22nF to 10nF to 4n7 (500-630V rated) and so on . . .
Click to expand...

Awesome, liking every bit of advice.

I'm currently sourcing parts. The only thing I've not liked about WIMA in the past is that the component leads are very short, makes them a pain to deal with. I may get an ON-OFF-ON switch just for the V1B cathode parts with stock and two mod options, and hard modify the other things. The other things I'm looking to do are subtle enough that I don't think I would mind hard modifying them. No point in having them on a switch if it's a subtle difference. Only thing I'm worried about is the input grid resistor mod. Is 1k enough to not make it sound like shit if I want to run a fuzz pedal like the Big Muff into it? Or would I have to use the low input for that? I'm going to just put a 1M resistor across the gain pot, lot easier than changing the whole pot to a different value, and more easily un-done if need be.
 
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