VVR in JTM45: Need some guidance

[MHProd]

I installed a VVR in a 18W 1974 clone (with original RS OT!) a while back. In this amp, the VVR scales the whole circuit, which works fine but there is a significant loss of brightness and drive at lower settings which I am assuming comes from the low voltage in the preamp.

I am now rebuilding a clone of one of my original JTM45s and I would like to use the VVR for just the PA or PA+PI. In order to implement it properly, I'd need some guidance on the following questions:

In a JTM45, the B+ is dropped by an 8K2 resistor just before going to the PI and then another 10K in the preamp.

http://users.atw.hu/schfrenki/jtm45.jpg

If I scale just the PA+PI, should I replace the 10K R in the preamp with an 18K to get the right voltage drop?

The B+ will be split at the standby switch: one way to the VVR (then out to OT CT and choke) while the other way will go straight to the preamp.

Now if I want to scale the PA only, should I connect the split original B+ to the 8K2 R ? In this case, I would disconnect the side of the 8K2 closer to the tubesand connect it to the non VVRed B+, is this correct?

MV: The JTM45 doesn't have much distortion going in the preamp, do I really need to install a MV for lower VVR settings or could I just turn down the amp's volume to avoid the extra distortion?

If this isn't possible, would a simple Crossline MV (like a Cut control but without the cap) do the job?

THANKS for your help!!

[MHProd]

Anyone??

[Reeltarded]

I thought I replied to this.. oh dear..

Couple things to consider that I'm not sure you have. The driven PI is a big part of the Marshall thing. Havong never used VVR I can only imagine that running it PA only and in this case using a PPIMV might be the best way to go, maybe someone can confirm that idea or not?

8k2 or 10k are common values for JTM dropping on that node. I kind of like them better with the later early JMP 18k2 string there.

I didn't help at all but now I have created just a bit of controversy. Here comes a good answer.

[bal704]

Are you installing a VVM for volume control purposes? I've only done a VVM on a global level. However, I recently did a LARMAR PPIMV on a build, and it sounds amazing. Great volume control and doesn't really impact the tone much.

I would highly recommend you try the LARMAR. However, I'm interested in hearing ideas on how to implement the VVM on must the PA or PI+PA.

[MHProd]

Tried the LarMar but the tone becomes fizzy fairly quickly...

[Lindz]

I think the question should really be how much do you need to drop the volume?

I have a few different vvr's installed in a few builds, mostly Marshally stuff since that is my bag. I prefer to scale the whole amp though others will have varying opinions. I think it greatly depends on the type of amp and where the distortion is generated - preamp, PI or power amp as to the best choice in terms of where to scale it

I think scaling is very effective and transparent to a point, after which tone and drive do start to change - unlike some of the claims out there.

Measuring voltages I seem to be able to drop up to 1/3 of the total with very transparent results, by 2/3 drop the tone is still pretty close with seemingly a little treble rolloff and a little less drive, but below that it seems to be rapidly diminishing returns and sounds pretty weak.

Voltage drops along these lines result in greatly reduced volume though I do not have a db meter to say how much, but I do have to say that in my condo its still fu@%#ng loud with a 4x12 - definitely not neighbor friendly volume.

To address that I find that on some amps (say a hotrodded 50w plexi with Larmar type MV) I can knock off enough with the vvr that backing down the Larmar after that to get to a neighbor friendly volume is not too fizzy and it works great - whereas the same volume using just the MV to knock it back sounds quite fizzy as you load up the PI. My guess is a JTM 45 would be similar though you might get away with just a PPIMV because you are not pushing the preamp/PI in that circuit as much as say a Jose or other modded Plexi

But on an 18 watt where I want to push the preamp but also really crank the el84's, the VVR by itself does not really work at really reduced volumes because I like the power tube distortion in that amp, and it is still too loud even when I turn the vvr down to where it still sounds good - it sounds like crap if I turn the vvr down to a Neighbor friendly volume.

So, I use the vvr AND an attenuator on it and a few other amps where I push the power tubes. Still a little tone suck (mostlly treble rolloff) but it sounds pretty good. Some of the really expensive attenuators where they try and better mimic the speaker load may work better than the resistive and or Weber Mass I use so you might do even better with those but I cannot speak to that (and dropping several hundred on an attenuator may be out of the question) but even a cheapo "load dump" from Weber along with the VVR sounds pretty good on an 18 watter

Like most volume reduction techniques I find that attenuators tend to work reasonably transparently to knock a little off the top but tend to be tone suckers as you knock more off - so moderation is the key

I find that that using the vvr AND a bit of attenuation sounds better at the same volume than more use of either by itself -

my 2 cents

[MHProd]

Thanks Lindz.

I think I might scale the whole amp and add a LarMar.

I've been able to improve my build to sound more like one of my originals and after chaning a few things, the LarMar sounds better.

I'll go back and forth playing the build vs the original for another while and eventually post the specs.

[vibratoking]

I have VVR scaling the whole amp in a 5E3 and an Express. Works good, really good, in both amps. No MV in either. The idea behind VVR is that you can get power tube distortion without the full volume. VVR sounds very good IMO until you reduce the volume to the point that the speaker basically stops pushing air. Then the whole thing stinks. It doesn't matter how you get there, VVR or MV or whatever, as soon as the speaker stops moving air the sound sucks. That point is subjective, but I've never played an amp that sounded good without moving significant air and that's usually fairly loud. I prefer to scale the whole amp with VVR over anything else FWIW. I use VVR to take the the edge plus a little more off when needed.

[MHProd]

OK, so I just finished my rebuild with the VVR scaling the whole amp and a LarMar PPIMV.

The VVR sounds good up until 2/3 of the way, then the drive and top end disappear. However I'm noticing a little bit of fizz as I turn it down. Could there be some crossover distortion going on?

The PPIMV sounds good too.

If I use both in conjunction however, the amp becomes kinda flat, losing its dynamics and I get a more noticeable increase in fizz. That's even with moderate settings like 2 o'clock on each controls.

I wonder if I should try to scale the power tubes only?

Also, the mofset gets very hot. Anyone could recommend a heatsink for this particular mofset?

Thanks!

[Scooter Thomas]

I bought a VVR for my JTM but never got around to using it, so I'm keenly interested in any further info here. I'm interested in heat sink recommendations too.

I use a VVR on my high power 5E3. I had it installed so not sure whether it's whole amp, just power etc. The tech was so trustworthy I just trusted him to do the right thing. Early on I was concerned with the mosfet heat, but it's going on two years with no problem. The aluminum JTM chassis should disperse the heat more effectively than a steel chassis.

[vibratoking]

You'll know if there is too much heat...the VVR knob won't do anything. This has been reported many times and I had that problem myself. As for a heat sink, I used what I had laying around. I used thermal paste and mounted the heat sink on the outside. My first attempt was an inside mounted sink and the FET cooked.

Also, take static precautions. You can zap the FET easily.

[Richie]

If your going to drop the voltage very low, be sure to add the cap and 1M resistor at the tube pin on the inputs.
You can use a terminal lug thing,and mount them on the tube socket mounting bolts. And use that for the extra parts.
If you go too low on the voltage, and without these installed, the amp can sound weak, thing or get splatty sounding at low voltage setting. The amp can't make signal. There should be some info posted here somewhere on it.
But Some miss it or don't know about it. I've also used a small heat sink, and put a small PC fan on the heat sink on a 50 watt marshall.

[MHProd]

Well I tried to scale the PA only and I don't like it at all... (Pretty sure I did it correctly). Very little effect up until 12 o'clock and then the preamp stage starts to overload the PA (increased distortion and noise). The PPIMV helps a bit but not impressed. I will revert back to the whole amp being scaled.

So far it seems the VVR + PPIMV combo is redundant in this amp. I feel the MV works better. Dropping the B+ seems to add way too much sag in the amp and all the punch is gone.

Oh well... At least at full tilt, the amp sounds very very close to my '65 block logo which took me 2 years to accomplish (trying different transformers, different types of caps and resistors and some value changes from the original schematic).

[renshen1957]

I installed a VVR in a 18W 1974 clone (with original RS OT!) a while back. In this amp, the VVR scales the whole circuit, which works fine but there is a significant loss of brightness and drive at lower settings which I am assuming comes from the low voltage in the preamp.

I am now rebuilding a clone of one of my original JTM45s and I would like to use the VVR for just the PA or PA+PI. In order to implement it properly, I'd need some guidance on the following questions:

In a JTM45, the B+ is dropped by an 8K2 resistor just before going to the PI and then another 10K in the preamp.

http://users.atw.hu/schfrenki/jtm45.jpg

If I scale just the PA+PI, should I replace the 10K R in the preamp with an 18K to get the right voltage drop?

The B+ will be split at the standby switch: one way to the VVR (then out to OT CT and choke) while the other way will go straight to the preamp.

Now if I want to scale the PA only, should I connect the split original B+ to the 8K2 R ? In this case, I would disconnect the side of the 8K2 closer to the tubesand connect it to the non VVRed B+, is this correct?

MV: The JTM45 doesn't have much distortion going in the preamp, do I really need to install a MV for lower VVR settings or could I just turn down the amp's volume to avoid the extra distortion?

If this isn't possible, would a simple Crossline MV (like a Cut control but without the cap) do the job?

THANKS for your help!!

Hi,

V V R works fine on Cathode Bias amps. However, implementation in a fixed bias amp is more complicated. If it was my amp (and has been) I would check out London Power, Power Scaling. Yes, it costs more money, Kevin O'Connor has been at this a lot longer and in the long run the circuits (he has several options) are more versatile. KOC DC power scaling is the most versatile, and quickest. (More information over at www.londonpower.com

I've implemented Power scaling in which only the power amp is scaled using a MV prior to the PI. This gave the option of clean preamp/distorted power amp, distorted preamp/clean power amp, etc., all at very low volumes (baby asleep in an apartment at 2:30 am wouldn't be awoken) or at other volumes. Bias issues, amp tone becoming cleaner when you dial down, using Military grade 2 watt pots aren't an issue with DC power scaling.

I've been happy with earlier versions of power scaling.

If you are into "speaker distortion" (speaker about to die from abuse) as part of your tone, neither VVR or Powerscaling are going to achieve this at very low volumes.

Best regards,

Steve