JCM900 2500 SL-X mods anyone?

[autumnleaves]

Checked out your tip Jacques, and it appears that at least the BR and C8 will have to be upgraded/replaced. However, others recommend doing all electrolytics on a 10 yr old amp. Problem is, I can't read the spec on C8 on the schematic, and wonder which other caps would be eligible for replacement.

 

[mickeydg5]

Checked out your tip Jacques, and it appears that at least the BR and C8 will have to be upgraded/replaced. However, others recommend doing all electrolytics on a 10 yr old amp. Problem is, I can't read the spec on C8 on the schematic, and wonder which other caps would be eligible for replacement.

From what I can make out on the schematic, it looks like a 16V 10,000uF capacitor. What is in the amp?

 

[autumnleaves]

Thanks Mick. I won't see the amp until Christmas. Meanwhile, my nephew is away at school and doesn't have it with him.

 

[autumnleaves]

Rick, I checked Antique Electronics for chokes, but the only models they have are for JCM800. I don't see a Mercury choke specifically for the JCM900 either. Do you recall the model you used. Given the cost, I would not consider a change of OT and Power trannies at this time, or likely any time. It would double what I paid for the amp.

 

[autumnleaves]

Here is what we have thus far. First step is a change of preamp tube. We see what minimum swap can bring about the desired tonal modification. Swaps as follows:
V1: (primary gain) - X only (possibly Z)
V2: (secondary gain, cascaded from V1) - 5751 or T or Y
V3: (tone stack feed) - X or U
V4: (phase inverter) - T
Possible also swap of power tubes for TungSol or TAD.
Then if the results are not satisfactory, step 2:

R5 ==> 100K
C1 ==> 22uF (formerly 1uF)
C5 ==> 0.027uF
C9, C10 ==> 0.047uF (adds low end)
R8 ==> add 1uF cap from C1 across the resistor (parallel?)
C13, C15, C18 ==> Silver Mica 470pF or Ceramic Disk (better)
Replace BR3 with GBPC1210W 12A Bridge Rectifier (which allows more tube experimentation)
Replace C8 (10,000 uF 16V) What other electrolytics to replace?
(C19 and C20 (tone stack) are 22n, 22n = .022uF, Leave alone for now.)

 

[autumnleaves]

Jacques, with an upgraded BR (12A), can I assume a 5751 will work in any valve position? More than one at a time?

 

[autumnleaves]

Rick, does this look like the right ceramic disk?
<http://ca.mouser.com/ProductDetail/Vishay-Cera-Mite/440LT47-R/?qs=sGAEpiMZZMt1mVBmZSXTPPdlv9NyAbbXitG2vkLvrrU%3d>

 

[Blacque Jacque]

Yeah you shouldn't have any issues with heater current, 12AZ7 will be usable too.

 

[autumnleaves]

The assortment of caps and resistors on Mouser is mind boggling. Do the voltage and tolerance of the R or C matter here? Rs are of course 1/4 W.

 

[autumnleaves]

Thanks Jacques. How is everything at Cambridge? I have friends who studied there.

 

[mickeydg5]

Jacques, with an upgraded BR (12A), can I assume a 5751 will work in any valve position? More than one at a time?

No. The BR3 change is only for suppling heater voltage to the preamp tubes. That has nothing to do with heater-cathode maximum rating.

A 5751 can only handle 100V h-c. V1 and V3 are placing well over 100V on the cathode. Cannot use a 5751 there.

 

[mickeydg5]

Rick, I checked Antique Electronics for chokes, but the only models they have are for JCM800. I don't see a Mercury choke specifically for the JCM900 either. Do you recall the model you used. Given the cost, I would not consider a change of OT and Power trannies at this time, or likely any time. It would double what I paid for the amp.

A 3 to 5H choke at about 100 dcr will do.

If you want closer to the Bassman feel, you will need a 20H choke.
Check out Classic Tone.

Ricky placed a 10H choke I believe, thats in between. Mercury Magnetics.

Before you order anything from Antique Electronic Supply, make sure of the specifications. I do not see any on their site.

 

[autumnleaves]

Thanks Mick. So is there any benefit from replacing BR3 apart from avoiding failure? On the choke side, Classic Tone carries a 250mA DC, 3 Hy which is the same as the one they offer for the Plexis. I don't see anything with a rating in the 20Hy range.

 

[Joey Voltage]

Thanks Mick. So is there any benefit from replacing BR3 apart from avoiding failure? On the choke side, Classic Tone carries a 250mA DC, 3 Hy which is the same as the one they offer for the Plexis. I don't see anything with a rating in the 20Hy range.

It's over in the JTM-45 Section, however in my personal experience I rarely go over 5hy, 3hy being my go-to. Large inductance chokes first of all I feel unnecessary for a guitar amp, and second super smoothed stiff screen nodes do something really icky IMHO.

 

[mickeydg5]

Thanks Mick. So is there any benefit from replacing BR3 apart from avoiding failure? On the choke side, Classic Tone carries a 250mA DC, 3 Hy which is the same as the one they offer for the Plexis. I don't see anything with a rating in the 20Hy range.

The BR upgrade is only to avoid failure.

The choke, your thinking JCM 900, nope. Plexi amps were steering away.
Think Bassman, Marshall copy of, think JTM45, you will see the 20H.

 

[autumnleaves]

Ahh! I see. There is a JTM45 choke from Classic (70mA DC, 20 Hy), and for that matter Mercury makes a 10Hy 200mA and even a 25Hy (no voltage given). You really think the high henry is the way to go? For that matter I could probably get something made to spec from <http://hammond-transformers.com/>.

 

[mickeydg5]

Ahh! I see. There is a JTM45 choke from Classic (70mA DC, 20 Hy), and for that matter Mercury makes a 10Hy 200mA and even a 25Hy (no voltage given). You really think the high henry is the way to go? For that matter I could probably get something made to spec from <http://hammond-transformers.com/>.

You did notice the $500 minimum for a custom order ???

What is up with that Hammond 40H? damn?

The reactance of the 20H will come closer to the Bassman.
Like Joey said too, a good 5H will do nicely.

 

[autumnleaves]

That one got by me. Here's the fallback position then <http://heyboertransformers.com/tubeamps.shtml>

So the voltages are of no consequence in a choke?

 

[RickyLee]

Rick, I notice that in the schematic I uploaded, the anonymous modder changed C13, C15 and C18 to 500pF. Is that likely to make any difference, or should these simply be upgraded to Silver Mica or Ceramic Disk? Tx

C13 & C15 are from the two separate treble peak pairings, and the caps that might be those funky resistor looking lime green ones I said to change out. Raising the value from 470pF to 500pF will not be a huge change in brightness but you might notice the 500pF giving you a bit more top end.

C18 is in the tonestack and raising the value from 470pF to 500pF will give you a slight bump up in the upper mids.

Ceramic disk is what you would see in the older amps and that would be a great choice. Some say ceramic is better for a dirtier style amp as they add a bit of graininess to the tone while Silver Mica is supposedly a bit cleaner. I have not compared them myself, but I would like to see if I can actually hear any difference.

Rick, I checked Antique Electronics for chokes, but the only models they have are for JCM800. I don't see a Mercury choke specifically for the JCM900 either. Do you recall the model you used. Given the cost, I would not consider a change of OT and Power trannies at this time, or likely any time. It would double what I paid for the amp.

I went with a Mercury Magnetics 10 Henry MC10H model in my SL-X. But the majority of people around these parts prefer the 3 Henry choke. Mercury will set you back $30 or more. Our member here, jcmjmp, deals in transformers and you could PM him. Or there's one of our sponsors here, ClassicTone. Much cheaper than Mercury and their products are excellent. I have the ClassicTone 3 Henry choke 40-18058, which also has a nice robust 250mA rating, installed in my Jubilee Clone. If you are wondering about choke Henry values here is this to look at or you can Google it:

http://www.marshallforum.com/workbench/24598-mercury-choke-specs-best-dsl100.html

EDIT: I would not worry about adding in a choke right away. To be honest, I did not hear any drastic change when I put that 10H in my SL-X. If you are going to be pushing this amp, cranking it up quite loud, then you will hear and feel a bit more of the change. BUT, some people actually prefer NOT having a choke.

 

[autumnleaves]

Hi Rick, I'll leave the choke out of the mix for the time being, although I recently put one in a Peavey Delta Blues with moderately pleasant results. I am very grateful to all for your patience and advice. I suppose that there is no single formula for coming up with a mod as so much of the result is subjective. Still, it is much better to have some landmarks to aid navigation through this electronic jungle. Now I must try to lay hold of the necessary components so the modfest can start over Christmas.