Fitting a MOSFET VVR JTM45

[chopperferrari]

Hi Guys,

I've currently got the PPIMV volume mod on my JTM45, I've been looking at fitting a MOSFET VVR instead. Any thoughts on this? Good idea?

Thanks,
Ben

[chopperferrari]

I'm going to take the plunge. I'd like to connect it only to the poweramp section. Has anyone done this already? Not sure exactly how to do it, some guidance would be great. Thanks Ben

[Haze13]

What this thing does? Lowers a B+?

[chopperferrari]

Yes, It lowers the B+.

[Haze13]

Here is a problem: The guitar speaker is 100dB at 1watt of power. So, if the 8 ohm guitar speaker has 2.83VRMS across it it makes 100dB of sound. It's very loud. I heave an Single Ended amp with a 6973 tube (Sort of EL84 but smaller in size and some say it's better). The max power of that amp is 3.5 watts. Less then 2 watts of undistorted output. In other words - this little thing makes more that 100dB with a guitar speaker. That's what you'll have to deal with, no matter will you lower the B+ (Yes it will be less loud). Don't forget when you lower the Voltage the whole amp will be affected. The operating points will change of every stage. The presence control will be effected because the PI will have less gain, the power amp will have less gain and it will not work as it should.
In other words it's better to make small amp to sound as you want than to make a 50 or 100 watt amp to work at full power and to produce only couple of volts.
The EVH did lower the B+, but it still was very loud.
So if you want to make an amp to work inside your home and work at Bedroom Levels - waste of time.
I have JCM 800 50 watt that I built, and now I work on that Single Ended amp and one output tube can be more efficient at lower volumes than modded 50watt head.

[chopperferrari]

Thanks Haze. It is actually for giging, the JTM45 is closer to 38watts and it just needs a little bit of volume reduction for most gigs so the VVR should be fine, i've got a PPMIV in at the moment, but I don't really like it.

[neikeel]

An easier solution (and a similar tone) will be achieved with a pair of yellow jackets and EL84s, IMO, but it sounds like you need the journey of the project to come to that decision yourself!

[Haze13]

50 watt amp is only 3dB louder than a 25 watt amp. So 38 or 50 - is the same thing for the ears.
VVR will give you sooner breakup too and less headroom.
Hope it will work for you! Good Luck!
P.S. Niekeel is right:) El84 can sound very good! 6V6's can work too, they'll be cleaner.

[chopperferrari]

Thank you guys. I've tried amps with EL84 in before, for the most part, they are a little bit too quiet, it's somewhere in the middle I want to be... Do 6v6s have a bit more volume in a JTM45 than EL84s?

[Carbia]

With the 6v6GT you'll get more power than with the El84's.
Around 22w (like a Fender Deluxe Reverb) vs the 15-18w of the pair of El84's cathode biased with the yellow jackest.

in addition, 6v6GT sounds really fine in a JTM45.

Use the JJ 6v6s for safety. JJ's can manage higher plate voltages than any other 6V6

[Haze13]

Don't forget that different tubes have different voltage for driving them. Example:
For a PP El34 guitar amp bias is 55-65 in volts on the control grid. Hotter bias - less voltage -> more cathode current. The meaning of this is that (Lets say that bias is 60 volts) the PI section have to produce this voltage for a MAX output CLEAN power . Bias is 60 volts in RMS so the output voltage of PI is 120 Volts Peak-to-Peak. Now, you change the tubes for El84 and the bias for these tubes is less than 16 volts (Class AB, 290v B+, 9 volts on the control grid) but your PI still produces 120 Vp-p (may-be less because of different grid-leak resistors and feedback was effected too, since tubes are different and their amplification is different) and tubes saturates more. You'll get more distortion, but not the Wattage - amplitude is smaller...
SO! If you want to make your amp sound quieter with lower wattage tubes, these are the things that you have to look for - The Bias, the PI voltage, feedback.

[chopperferrari]

Sorry for the delay, been getting married.

I'd like to try 6v6s. So i'm guessing you that the JJ6v6s are a good idea because of they are a little tougher than some of the others. Do I need to do anything else? Like swap the rectifier vlave?

Thanks,
Ben

[Haze13]

Well... There can be a small problem with JJ6v6's. They are very look-a-like and work-a-like 6l6, so they are just lower power 6l6s. JJ make very good 12ax7 and el84, and may be some other tubes, but 6v6 are not so good. It's and american tube, so I think a Thung-Sol will sound better (even though these are russian made today, their 5881 are very close to the originals and 6v6,5881,6l6,6550 are the same family). Actually you can use 5881 too... Look at the Soldano 25 hot rod amp. It uses two of these and it's less than a 25 watt amp. B+ around 310 volts + hot bias (tubes just love current, but not too mutch) and it will sound good. + the bias voltage of 5881 is higher, so it will not distort as 6v6.
You could try to use El34 in triode mode, they will produce less power than a pentode, so you will have to tweak a negative feedback too.
Anyway, there is always more than just replacing the tubes. Best sounding amps are those that were designed for a specific output tube. How many people do you know that use El34 or KT66 in their fender twin reverbs? Maybe 6v6 in-the-VOX-users?
Rock On!