JCM900 2500 SL-X mods anyone?

[RickyLee]

Nice idea. Lots of work. It is not a SL-X any longer.

Besides, his nephew may change his likings as he gets older. Believe it or not, I did not like Metallica or Megadeth or Sevendust or BLS or Pantera or Chevelle or Disturbed or Godsmack up until about 10 years ago when I decided to try stuff like that. Now I can not do without stuff like that. See where I am going.

Another simple thing is to take one of the not needed direct out jacks on the back and 86 it. Turn it into an input that bypasses the first or first two gain stages. It works. I tried it the other day.



Very easy to do and no altering of the PCB. I used an ABY switch and put this new input with it's needed circuit - 68K grid and 1MEG to Ground - at V2A. Then used the ABY switch for a clean dirty switching. The cool part was running both inputs at once. Had a very interesting tone. And VR2 still had control of this new clean channel. But there was a slight loss of gain at the full/normal input due to this new loading.

I started tinkering on that as I was looking to possibly do something along the lines of channel switching using a relay.

That's enough for the night as I only got 4 hours left to get some sleep before getting up to that dreaded worst enemy - the Alarm Clock!

And so much for what my Gal did to me to help me sleep - as I ended up getting on the Forum after that . . .




Now I'm not sleepy again!

 

[mickeydg5]

I thought about bypassing V2 all together with a relay, last night. That would make a good two channel Rythm/Lead amp and double good with the switchable master volumes. Like a Jubilee without transistors, but better.

 

[mickeydg5]

Ok, since you guys want a nice lower gain tube that will work fine in V1 or V3, I'll give you one. I looked in my tube ref notes to bouble check and there is a tube not too many people mention. It is a 12AZ7, has a 200V h-c and is a 60u (amp factor), not quite as low as 12ay7 but lower than a 5751. You can get old stock for $10 a piece. Try them.

 

[Blacque Jacque]

Caveat; 12AZ7 has a significantly higher heater current requirement, 450mA for a parallel connection or 50% more than an ECC83 / 12AX7. Given that the SL-X is prone to frying BR3 as it runs so close to the limit, fitting such a valve is at risk of damaging unmodified amps. Just one will put the heater current over 1A.

The 5751 also needs more current than a stock, but only 350mA for parallel heaters, which technically keeps it within the 1A spec of BR3 if just one is used (950mA total is still pushing it though without doubt)

My SL-X already has a larger BR3 fitted, so it should be fine with any of those, I'll see if I can find a 12AZ7 in the UK.

 

[autumnleaves]

I'll have to try to digest all this, but no need to have a solution in place until end of Dec when the nephew comes to visit. Meanwhile I have PMd the Maillet mod notes to Rick and Mick. Your comments would be very welcome. Assuming the existing filter caps and power tubes are in working order, I am leaning toward something simple: a few cap changes and judicious combination of preamp tubes, with possible fallback to more radical surgery if that doesn't pan out. Maybe we can settle on a formula everyone agrees with.

 

[autumnleaves]

I was also wondering about KT-66s, which may or may not fit here. This amp is powered by 5881s.
Thanks

 

[mickeydg5]

Caveat; 12AZ7 has a significantly higher heater current requirement, 450mA for a parallel connection or 50% more than an ECC83 / 12AX7. Given that the SL-X is prone to frying BR3 as it runs so close to the limit, fitting such a valve is at risk of damaging unmodified amps. Just one will put the heater current over 1A.

The 5751 also needs more current than a stock, but only 350mA for parallel heaters, which technically keeps it within the 1A spec of BR3 if just one is used (950mA total is still pushing it though without doubt)

My SL-X already has a larger BR3 fitted, so it should be fine with any of those, I'll see if I can find a 12AZ7 in the UK.

Those piece of crap BR's. Ha, ha, ha.
Yes, that is the big drawback. Each tube would have 150mA more current draw. I would only try one 12AZ7 at a time for that reason, unless you upgrade the rectifier.

 

[mickeydg5]

I was also wondering about KT-66s, which may or may not fit here. This amp is powered by 5881s.
Thanks

So that amp is 5881s. The 1959 Bassmans were also 5881s.
KT66s are nice. Keep in mind that they will draw more heater current.

 

[autumnleaves]

I am very much obliged to all of you (Rick, Mick and Jacques) for your discussions and suggestions. Here I take the liberty of collating and compiling all the proposals into a compact set of notes for your final consideration. Bearing in mind that the aim is to tame the gain and harness the sweetness the tubes are capable of suffusing into the tone, let me know if this looks like a program. I would very much appreciate any suggestions as to what to do with the negative feedback (R17/C13 of 100K/22pF) and candidates for the power tubes if this is likely to move us in the right direction. I read in another thread that the TAD 6L6GC-STR is very well respected. so here it is:
Thanks,


Mick mods:
R5 ==> 100K
C1 ==> 22u or a little higher
R8 ==> add 1uF cap from C1 across the resistor (parallel?)
VR7: change to 1M
What about the Feedback Circuit?

Rick mods:
C3 ==> 0.0022uF
C14 ==> 0.022uF
C5 ==> 0.010uF
C7 ==> 0.022uF
C9, C10 ==> 0.047uF (adds low end)
C19, C20 ==> 0.022uF (Tonestack caps)
C13, C15, C18 ==> Silver Mica 470pF or Ceramic Disk (better)
R5 & R7 both ==> 100K
(Orange Drop caps: 225P series Polyester is good for adding midrange.)

Tubes:
V1: only X primary gain X only (possibly Z)
V2: 5751, T or Y secondary gain, cascaded from V1 - 5751 or T or Y
V3: tone stack feed - X or U
V4: phase inverter - T
What combination of tubes will draw proper heater current?
Alternatively, increase heater capacity by upgrading to larger BR3?
What about power tubes? TAD 6L6GC-STR

Rick's surgical solutions:
A) Put gain stages in parallel: use both triodes of V1 in parallel for the first stage and same thing with V2
V1 in parallel with V2 and each having a Gain control to blend gain
B) '59 Bassman/Marshall V3 and V4 driver config
C) Turn one of the direct outs into an input that bypasses the first or first two gain stages.
Add an ABY switch and put this new input with it's needed circuit - 68K grid and 1MEG to Ground
- at V2A. Then use the ABY switch for a clean/dirty switching. (slight loss of gain in normal input)

 

[mickeydg5]

That is a lot of work. Do a little first, see what you get and think. Paralled and switchable gain stages are lots more work.

If you want more of a Bassman feel, than no. You do not want to decrease coupling caps. You do not want to increase R7. That negates your efforts.

Look at my post again, I added stuff. C5 change to 27n (for Bassman feel).

Yes, R8 paralled by 1u (from C1).

C9 and C10 (driver to power tube), change those to .047u (Bassman uses .1u)
There are redundant callouts from page to page with these schematics.

Just to make sure we are on the same page, a .022uF = 22n. That applies to all the "nF" cap callouts. A 470p is basically a 500p, so why change? No need.

The only caps I would change for the sake of changing are ceramic. The other caps are of good quality and not worth changing to Orange Drop or anything else, unless it is a bad cap.

Heater current:
12ax7, 12ay7, 12at7 = .3A
5751 = .35A
12az7 = .45A
A 12au7 can also be tried in V4 slot. But don't put low gain tubes in everything.

I guess TAD 6L6GCs are decent. KT66 are good. Old Stock 5881 are nice. I can't say to much about current production 6L6 types. Read about and ask on the power tube thread.

Leave the driver configs as is. No need to change.

Feedback circuit should be left as is. If you want the output cleaner, then short out he 33k/4n7. (4n7 = 4700p = .0047u) Talk about that more once the other mods ar done.

Questions?

 

[autumnleaves]

Thanks for the further clarification. For the sake of simplicity, it would seem prudent to first change 1 tube at a time to see if this resolves the matter (after checking tube and filter cap integrity, of course).
Next level intervention would then be A, B and C below, one at a time, in that order, until a satisfactory point is reached. How does that look?

A:
R5 ==> 100K
C1 ==> 0.022uF
C5 ==> 0.027uF
C9, C10 ==> 0.047uF
R8 ==> add 1uF cap from C1 across the resistor

B:
VR7: change to 1M

C:
Short out 33k/4n7. (4n7 = 4700p = .0047u)

 

[autumnleaves]

PS. Should we do the tone stack caps as a fourth phase?

 

[RickyLee]

The good 'Ol 12AZ7A. I have quite a few of these myself and have tried them in many amp circuits.

I was also wondering about KT-66s, which may or may not fit here. This amp is powered by 5881s.
Thanks

I would say, my favorite power valve in the SL-X.

 

[RickyLee]

I really think you can get away with running some choice preamp and poweramp valves and back those Gains down and get the Master Volume up to at least 6. Some decent KT66's as well as a set of JJ 6L6GC/GT 6L6S will really open this thing up.

As for swapping components? The ones that IN MY OPINION made the best improvement, was getting rid of those lime green resistor looking 470pF caps. That would be a good place to start and as the others said before, don't go mod crazy all at once.

Also for me, lowering the very first gain stage cathode bypass cap C1 down to .47uF and doing the resonance circuit mod as well as installing a Mercury 10H choke, made a huge impact towards having less flab in the bottom end.

But remember, that is my preference and you are actually wanting a more '59 Bassman'ish type tone. Quite a big difference there.

 

[mickeydg5]

Thanks for the further clarification. For the sake of simplicity, it would seem prudent to first change 1 tube at a time to see if this resolves the matter (after checking tube and filter cap integrity, of course).
Next level intervention would then be A, B and C below, one at a time, in that order, until a satisfactory point is reached. How does that look?

A:
R5 ==> 100K
C1 ==> 0.022uF 22uF
C5 ==> 0.027uF
C9, C10 ==> 0.047uF
R8 ==> add 1uF cap from C1 across the resistor

B:
VR7: change to 1M

C:
Short out 33k/4n7. (4n7 = 4700p = .0047u)

Place your cleanest lowest gain tube in V1 first. then try others if need be.
C1 is cathode bypass cap at V1, replace 1u with 22u across R3.
C19 and C20 (tone stack) are 22n, 22n = .022u, We may revisit C20 later.
You can place a short across feedback 33k/4n7 or across 27k/220p to see what you think.
But not across both.

 

[Blacque Jacque]

The BR3 upgrade is ESSENTIAL to be honest, there are so many threads on here with people asking why their SL-X has died & most are along the lines of "my first 3 tubes don't light up".

I'd be wary of tube swapping without doing that first.

 

[mickeydg5]

The BR3 upgrade is ESSENTIAL to be honest, there are so many threads on here with people asking why their SL-X has died & most are along the lines of "my first 3 tubes don't light up".

I'd be wary of tube swapping without doing that first.

Yes, exactly. I was supposed to add that to my post and forgot. Thanks.

Well, Autumnleaves shuuld thank you.

 

[autumnleaves]

I do thank you Jacques, and will gladly thank you again if you tell me what this beast is and what its replacement should be.

 

[autumnleaves]

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

 

[Blacque Jacque]

Indeed: BR3 is a diode bridge supplying dc for the heaters of the first 3 tubes.
This thread is pretty much all you'll need to help you out:
http://www.marshallforum.com/workbench/28376-pre-amp-not-working-jcm900-sl-x.html