Reducing Gain in Phase Splitter - JTM45
« on: June 09, 2007, 10:50:19 PM »
I have a JTM45 (guitar amp) which sounds really sweet, but I don't like the way it breaks up when you drive the input gain beyond about 5. I want the break up to start at about 8 so I have better volume control range when it is clean.

Have checked with a CRO, the distortion (squaring off) is introduced at the phase splitter, so have tried lower MU tubes to replace the high MU 12AX7, tried a 12AT7 and a 12AV7, they both kinda improve it a little, but the higer freq tone and definition just dissapper, making the amp sound quite flat.

Have searched many forums but can't find anyone who has addressed this. Any ideas??

Does anyone know how or if you can adjust the gain in a phase splitter? Also how can I increase the headroom?

Link to schematic, have linked the Fender 5F6 as the 45 schematics are really hard to read, same circuit except 45 has KT66 instead of 5881 tubes.




Reducing Gain in Phase Splitter - JTM45
« Reply #1 on: June 09, 2007, 11:12:11 PM »
You could change the caps just prior to the phase splitter..... the .02 and the .1uf.  Try  .002 or .01 and  .01/.02/.047uf. This will roll off the lows ..... and it's probable that the distortion is occurring in the lows first.  You could also play with the 10k resistor and see if reducing that helped..... although 10k seems OK there ..... other circuits went up to 22k I think.

Just thoughts ... Hopefully someone smarter will come along.


Reducing Gain in Phase Splitter - JTM45
« Reply #2 on: June 09, 2007, 11:36:00 PM »
Try adding some neg feedback from stage 3 to stage 2.


Reducing Gain in Phase Splitter - JTM45
« Reply #3 on: June 10, 2007, 01:22:21 AM »
Yes, Try 22k where the 10k is, and .02 where the .1 is. Need to increase the headroom, maybe add a 4k7 resistor before the presence pot. I think that would reduce the current resulting in more voltage/ more headroom. although I have a Park 75 lead schematic with the 4k7 parallel to the pres. pot.
Blown like a fuse


Reducing Gain in Phase Splitter - JTM45
« Reply #4 on: June 11, 2007, 08:05:45 AM »
Thanks guys for your suggestions.

What I have done is inserted a voltage divider consisting of a 500K pot and switched resistive network, in series totalling 1M between the treble pot wiper to ground and output to the .02U cap. This allows me to reduce the level into the phase splitter with the pot so that the amp stays clean and then the swicthed resistors allow me to drive the phase splitter harder if I really want the cranked 45 distortion. It's kinda like a Master Volume with an extra distortion boost switching.

This is working really well and maintains the tone of the original amp.



Reducing Gain in Phase Splitter - JTM45
« Reply #5 on: June 11, 2007, 05:12:27 PM »
This has got to be a first: Modifying a Marshall for less gain.

How 'bout putting some resistance between V-1's cathode RC and ground?


Reducing Gain in Phase Splitter - JTM45
« Reply #6 on: June 13, 2007, 05:04:17 PM »
On the other hand, if you want More crunch, put in one of those dual pot master volumes,  after the splitter.
After trying a million master circuits, this one is hard to beat.
You will need an extra set of coupling caps.

Either that, or buy a Fender.  :razz:
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


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