Type 4 Master Volume

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joe6v6
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Type 4 Master Volume

Post by joe6v6 » Mon Oct 23, 2017 3:41 am

Does anyone have any experience with the type 4 master volume in and Express Amp? Ive searched around and can find lots of info on 1,2&3 but very little on #4. I know its easy enough to put one in and try but I would like to see if anyone has tried it and what their thoughts are. .
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romberg
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Re: Type 4 Master Volume

Post by romberg » Mon Oct 23, 2017 7:43 am

I have experimented with the type 3 and type 2 masters in an express circuit. The type 3 (1m pot across PI output) sounded very unpleasant to me. The type 2 (LarMar) I still use. I've not tried a type 4 in an express but I may have some useful information. I have spent some time with my scope tracing a sine wave as it makes it's way through the express circuit.

The very first stage to distort is the power tubes. When the power tubes start to clip every other stage is clean (signal is still sinusoidal). As the gain goes up the next stage to distort is the phase inverter. When the gain is finally "cranked" the third stage will distort slightly (bottom half df wave is distorted but not clipped). At no point does stage 1 or 2 distort. It is also interesting to note that the signal of the clipping power tubes looks pretty similar to the clipping phase inverter on the scope.

A type 4 master is just adding a 1m volume pot after the treble pot (or tone stack). On an express the tone stack is right after stage 1. So, when you turn down this type 4 master you will have a "clean" amp. I've not actually built this but I'm fairly sure you will not be able to get an overdriven sound at lower volume with an express and a type 4 unless the type 4 is cranked up. This is because stage 1 never distorts (unless I guess you smash it with a pedal).

However a type 2 (LarMar) is after the phase inverter. When the type 2 is full up it is out of the circuit. When it is turned down you loose the power tube clipping. But you can still get the phase inverter overdrive. It does not of course, sound exactly the same as power tube clipping. But it also is not peeling paint off the walls. A reasonable trade off to me.

Mike

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norburybrook
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Re: Type 4 Master Volume

Post by norburybrook » Mon Oct 23, 2017 3:03 pm

I have tried type 3 and 2 and agree with MIke's findings. My type 2 is still on my express. It does beg the question though; could you not have both? ; type 2 and type 4 as they both do different things.


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joe6v6
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Re: Type 4 Master Volume

Post by joe6v6 » Mon Oct 23, 2017 3:17 pm

Thanks for the info Mike, my experience with the type 3 in other amps has been similar to yours, and others, from unpleasant sounding to just not very useful. I have used the LaMar style before and like you said there is a trade off there between the power tube distortion & volume. I understand your reasoning about the type 4 and in the Train Wreck Pages it pretty much says that #4 is to be implemented with and additional gain stage, so even though its so simple to try I think I will leave it alone for now. .
I have tried type 3 and 2 and agree with MIke's findings. My type 2 is still on my express. It does beg the question though; could you not have both? ; type 2 and type 4 as they both do different things.
Interesting , I just may try something like this one day. With some control over different areas there may be some possibilities there.
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Re: Type 4 Master Volume

Post by romberg » Mon Oct 23, 2017 8:17 pm

joe6v6 wrote:
Mon Oct 23, 2017 3:17 pm
It does beg the question though; could you not have both? ; type 2 and type 4 as they both do different things.
Interesting , I just may try something like this one day. With some control over different areas there may be some possibilities there.
Absolutely you can have both. And I really like having a master before the PI and after. The pre phase inverter master can control the amount of preamp overdrive you get. It also controls how hard you will overdrive the phase inverter. And then the post master sets the overall volume and has some control over power tube overdrive. The combination allows for a lot of flexibility.

I've implemented the pre/post PI master setup in my marshall and hiwatt builds. And I'm going to do the same in the ODS build I'm about to start. It might even be interesting to try it out in an express. I would place the pre master in an express after the third gain stage and before the phase inverter. Because of the gain structure in an express (It is ALL about power amp crunch), I would not expect one would be able to get good preamp gain tones out of this setup. However, I would expect that lower pre master settings would allow for the amp to clean up quite a bit more. It would not be difficult to wire up a simple 1m volume pot right before the phase inverter and try it out. I might try this the next tme I have an expess opened up.

Mike

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norburybrook
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Re: Type 4 Master Volume

Post by norburybrook » Tue Oct 24, 2017 12:48 pm

I'm going to give this a go today :D I'll post back.


I like my express but it's only usable for rock so not that usable for me generally. It will be an amp that gets used once or twice a year. If I could get some clean head room out of it then it would be a more usable amp and I might think about keeping it despite it not being 'traditional' express anymore.


TBC


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norburybrook
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Re: Type 4 Master Volume

Post by norburybrook » Tue Oct 24, 2017 5:02 pm

OK I've wired it up like this. but you've just got two volume controls together surely?
Trainwreck_MV_Type4.png

shouldn't the center of the new master volume pot go to the cap that connects to the 1M at the phase inverter?

M
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Re: Type 4 Master Volume

Post by romberg » Wed Oct 25, 2017 3:27 am

norburybrook wrote:
Tue Oct 24, 2017 5:02 pm
OK I've wired it up like this. but you've just got two volume controls together surely?
Yea. Adding another volume pot there on an express is kinda pointless as you can see. There is already a volume pot in that exact same spot. I think this type 4 diagram is meant for topologies where the tone stack comes right before the phase inverter (marshall, bassman, etc). If you wanna knock down the gain then you might try replacing the 150k before the 3rd stage with a pot and a parallel resistor. If you used a 1m pot with around 178k in parallel with it the 2nd stage would still see about 150k. So, replace the 150k with 178k and add the 1m volume there.

Putting a volume after the third stage would probably require adding another coupling cap as there is DC on the signal out of stage three into the PI cap. If you goal is more clean headroom then I think knocking down the volume after stage two is probably a better option anyway. From my scope experiments stage one and two never really distort. So, reducing the signal there would have the greatest effect.

You may just find a setting you are always happy with using a pot after stage 2. Then just measure the resistance where the "magic" spot is and wire in a simple voltage divider consisting of two fixed resistors. Merlin describes this technique in his preamp book where he uses pots for all interstage dividers and cathode resistors. One can then tweak away with all the pots until one is happy. Then replace most of the pots with fixed resistors.

Mike

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Re: Type 4 Master Volume

Post by pdf64 » Wed Oct 25, 2017 2:18 pm

norburybrook wrote:
Tue Oct 24, 2017 5:02 pm
OK I've wired it up like this. but you've just got two volume controls together surely?...
Bear in mind that the context for that Trainwreck Pages diagram is 'master volume mods for regular Marshalls; 4 types'.

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norburybrook
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Re: Type 4 Master Volume

Post by norburybrook » Wed Oct 25, 2017 2:54 pm

pdf64 wrote:
Wed Oct 25, 2017 2:18 pm
norburybrook wrote:
Tue Oct 24, 2017 5:02 pm
OK I've wired it up like this. but you've just got two volume controls together surely?...
Bear in mind that the context for that Trainwreck Pages diagram is 'master volume mods for regular Marshalls; 4 types'.
Thanks PDF64, Indeed, as I was attaching the wires yesterday I was thinking ,what am I doing?? one volume into another???? however in my defence I'm laid up at the moment with a trapped sciatic nerve so I'm on diazepam , co-codamol and naproxen ..a heady cocktail......I've not been out of the house for 4 days so in a moment of pain relief got out the soldering iron :D


I've been having a play with the express again today and I'm not even convinced the LArMAr master volume is really that good either even though it's the best of the bunch.

The fast /slow switch is actually the best way to knock off some gain IMHO having tried all the ways now.


Marcus

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Re: Type 4 Master Volume

Post by pdf64 » Wed Oct 25, 2017 5:43 pm

norburybrook wrote:
Wed Oct 25, 2017 2:54 pm
...in a moment of pain relief got out the soldering iron :D...
If you start getting a nice bbq smell try turning the iron around :lol:
I get a trapped lumber nerve occasionally, so I can sympathise :cry:
Pete

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M Fowler
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Re: Type 4 Master Volume

Post by M Fowler » Wed Oct 25, 2017 6:13 pm

One of my Trainwreck Express combo customer is using the purple THD Hot Plate and that works well rather then a master volume.
Jeff Rutland.jpg
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Re: Type 4 Master Volume

Post by norburybrook » Thu Oct 26, 2017 11:34 am

M Fowler wrote:
Wed Oct 25, 2017 6:13 pm
One of my Trainwreck Express combo customer is using the purple THD Hot Plate and that works well rather then a master volume.

Jeff Rutland.jpg
Mark that looks lovely, do you have any more pictures of that combo. I 've been thinking about making mine a combo but was wondering about how to mount it etc. That looks really classy :D

Marcus

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M Fowler
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Re: Type 4 Master Volume

Post by M Fowler » Thu Oct 26, 2017 1:01 pm

If you build yours in the typical 17 x 8 x 2 then you would either have to mount the chassis like a JTM45 bluesbreaker style with narrower faceplate opening or with the faceplate/knobs coming out the front of the cabinet but again the opening for the faceplate must be narrower or use bigger faceplate I've done that too.

Two different Express combo builts but 3 different cabs shown in photo, all built in 18w style open channel chassis.
Last photo are JCM800 but shows example of how to mount your regular Express chassis.

The last photo is an example of how to mount your regular Express chassis like the JTM45 does but you will need a deeper board to cover your 8" chassis because the JTM45 chassis is now as deep maybe 6.5".

I have done the same thing to Rocket builds in regular 17x8x2 chassis mounted it JTM45 knobs up style but made sure the top opening fit the 2" faceplate and the mounting board was deep enough to cover the 8" chassis.
TW Express Combo.jpg
Express Combo Build 014.JPG
Express Combo No.2.jpg
Fowler Amps JCM800 50w combo's.jpg
Dumble Fender 02.JPG
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pdf64
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Re: Type 4 Master Volume

Post by pdf64 » Fri Oct 27, 2017 8:34 pm

romberg wrote:
Mon Oct 23, 2017 7:43 am
... It is also interesting to note that the signal of the clipping power tubes looks pretty similar to the clipping phase inverter on the scope...
Yes, I think that's because with AB1 tube amps (and any 'non A/B2' tube amp in which the driver stage prior to the power tubes can't source grid current) the main clipping stage is the power tube's g1-k diode which conducts when the driver stage tries to pull the control grid voltage above the potential power tube cathode.

Hence it may be seen that the power tubes don't actually overdrive / saturate as such, they just amplify the clipped signal at their control grids.
However, in an AB1 guitar amp (and especially the Express) that amplification is modulated by the signal level, ie as the power tube g1-k voltage increases towards (and just above) 0V, the lower the g2 voltage will sag, thereby lowering the amplification / power tube plate current.

I think it's been shown that (in prolonged heavy overdrive) the Express's g2 HT node sag is so pronounced that it causes the LTP HT node to drop so much that the LTP runs out of headroom at such a low level that the power tube g1-k diode no longer become the main clipping element.

So a type 2, 3 or 4 master volume might allow lower signal levels at the power tube grids to have a similar tone to power tube g1-k overdrive, but the resulting amp output will lack the dynamic response caused by the HT sag of the power tubes being driven at their normal level.

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