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JTM45 impedance query

9/24/1998 4:45 AM
JTM45 impedance query
I've just bought a JTM 45 reissue head and can honestly say that this is the nicest sounding amp I have ever heard. I'm playing it fairly loudly through a 2x12 16 ohm cab, with some mild overdrive. I would, however like to drive those 6L6's a little harder (best sounds at high vol.?), but will not, in principal, shell out £200 (approx. $350) for a powerbrake. Can I change the amps output impedance from 16 to 8 ohms to cut power to the speakers or am I likely to fry the output tubes and transformer?! Opinions please. Thanks.
9/24/1998 5:35 AM
Lars Kohkoinen

I don't know if it's a good idea to change the output impedance.  
I was in the same situation and did'nt have the money for a powerbrake.  
I have a Bluesbreaker (it's a JTM45 2*12' combo with tremolo), and I installed a master Volume after the drive tube (ECC83) just before the power Tube's. It's a simple job to do and the result is excelent, The price for the Volume Pot and two cap's is about 120 SKR (approx $15). I placed the new master Volume in Channel one's Low input place. Now i can have this soft distortion at low volume.  
Try this  
Book Of The Day The Ultimate Tone, Volume III by Kevin O'Connor
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9/24/1998 2:17 PM

Chris: I would definitly NOT try this with any marshall. They are very sensitive to impedence changes.If this is a new amp I would think it would also tell you this in the owners manuel.I think Lars has a good idea with the master volume,I am not a believer in brakes and all the other stuff hooked on the back to reduce volume.What you like, and are hearing when turning up your amp, is preamp and power tubes working together, to get that sound. The preamp master volume will just crank up the preamp,[which is not bad]but will not give the same sound as the power tubes kicking in.Most all old marshalls[and the one you have modeled after an old one] were made to be played loud,and unless you get somewhere to play where you can turn it up,it just won't sound as good.But you may get by with using Lars suggestion, with the master vol.I kinda look at it like having a dragster in a school zone,you know the thing will fly,but have to creep through real slow.You have a loud amp but can't use it like it supposed to be used. [Richie]
9/25/1998 1:50 AM
Steve A.

    There are several mv designs going around- which one did you use in your JTM45:  
1. The Ken Fischer one that replaces the two 220k (or whatever) bias feed resistors with a dual-ganged 100k to 250k pot. If this design, did the wiper go to the grids of the output tubes or to the coupling caps from the PI.  
2. The Dan Torres/Craig Anderton one that replaces each coupling cap with a pair of them and puts half of the dual-ganged 500k pot between each pair. If this design, was the wiper connected to the coupling cap from the PI or to the coupling cap going to the output tube grids.  
3. The "Spitfire" one that is a single 100k (or whatever) pot that just shorts out the two p-p signals after the stock coupling caps.  
    Any details on what exactly you did would be appreciated!  
Steve Ahola
9/25/1998 3:45 AM
Lars Kohkoinen

I did use the Dan Torres/Craig Anderton one.  
I used a dual 1 M pot and two new couplig cap (0,15-0,22 uF). (LH Musik in Stockholm helped my out).  
1M ohm  
new Cap _____  
From ECC83--
9/25/1998 9:19 AM

The best sounding MV in a Marshall, to me, is the JCM-800 type where the pot is connected between the tone stack and phase inverter. This gives an excellent crunchy sound without becoming mushy at higher volumes.
9/25/1998 9:32 PM
Steve A.

    I think I've gotten more consistent results in all of my amps using the pre-PI mv you recommend. One suggestion from Kevin O'Connor passed on to us by Doc here is to insert something like a 100k resistor on the wiper of a 1M pot (assuming that your preamp puts out enough juice to handle the 10% loss in signal). I've tried them both ways and it does seem to smooth out the response of the control.  
    Post-PI mv's seem to be touchier (the two sections of the pot must be well matched, etc.) and they don't work well with some circuits, but when they do work you get the distortion from the driver/PI at low settings, which some people prefer.  
Steve Ahola
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