Master volume mod

[Swampertech]

Has anyone implemented the master volume mod which replaces the 2 1M resistors in the LTPI with a dual concentric pot? Is this the BEST MV to use which doesn't change the amp tone? Thanks for any help can provide.

[Papa Jim]

I have, and my buddy who is a total tone guru freak has. I know it worked for me, and more importantly he (somebody that I really consider an expert on it) swears by it. I can probably hook you up to talk with him about it.

[Swampertech]

Papa Jim, is that the Kevin O'Conner mod that's shown in the archives on this site?

[Papa Jim]

Post the link and I will see if it is the same as how we modded our amps with.

[Swampertech]

Hey PJ, the schematic is the last one on this page.

http://el34world.com/Forum/index.php?topic=3496.0#lastPost

Thanks for confirming this.

[Papa Jim]

Use this

[Swampertech]

PJ, is this what you mean? I have one question though. What keeps the DC off pins 1 of the MV controls?

[Papa Jim]

Yes that is it. I don't know about the pin 1 question.  I am more or less a great builder and follower of schematics like I did on this one. I do know it worked though. Maybe someone more technical than myself will answer that one for us. I just built an Deluxe Reverb for a friend and put this MV in it for him. Also a presence control that was a Robrobinette mod design.

[Swampertech]

One other thing. I read somewhere that a 1M pot used as a MV before the PI had some type of tonal affect. Ever hear that? Do you know if this dual pot MV drastically changes the tone? Maybe 2deaf, PRR, or sluckey have had experience with this type control. Most MV's I've seen come after the tone stack, off the wiper of the treble control.

[Papa Jim]

Great point and that jogged my memory. My tonal freak friend who I built that Deluxe Reverb for had me use a 500ka gang pot instead. I wondered why but never got around to asking him. Just did what he wanted. Lol I will edit the schematic.

[Swampertech]

And the 10K grid stoppers?

[Papa Jim]

We used 10k grid stoppers.

[pdf64]

If the amp has an NFB loop, then such as the master vol is turned down the NFB will become disabled, same as with types 1-3 master vols.

[Papa Jim]

If the amp has an NFB loop, then such as the master vol is turned down the NFB will become disabled, same as with types 1-3 master vols.

That's a good thing isn't it? I hope.   

[sluckey]

PJ, is this what you mean? I have one question though. What keeps the DC off pins 1 of the MV controls?

There will be dc voltage on all three legs of the pot. But there is no current flow or voltage drop, so there will be no static when you turn the pot.

What is your intended purpose for Cx? As drawn it blocks the normal path for cathode current and forces the cathode to find a ground path through one of the higher value NFB resistors. Not gonna sound good! And if you turn off the NFB switch, you will totally shut down the PI tube.

If Cx was intended to block dc from the presence pot, you need to look at the Marshall 2204 to see how to properly do that.

[Swampertech]

So pdf64, is there a MV which can be used in a LTP PI, either before, within, or after, which doesn't affect the FB or tone? I'm inclined to use the dual 1M post PI MV, however sluckey uses the pre PI MV in his Hoffman/sluckey PR with a 100K pot. Steve is a design guru, and I'd expect his work to be well thought out. What are the types 1-3 MV's you reference, and do they run in LTP PI circuits that have FB?

PJ, did your tech run any tests on tone changes when he specked the MV using the dual 500K pots in the LTPI?

sluckey, on your Hoffman/sluckey PR no tremolo w/mid and MV, did you notice any tonal irregularities when adjusting the MV up or down? I have that 1 triode between the TMB and PI where I wobble it's output with the Tremor-lator. If I inserted the MV pot on the output of that triode (between two coupling caps), do you reckon there would be any tone problems or tremolo problems? I'll redraw the corrections and resubmit to the forum.

Just read this from sluckey:
What is your intended purpose for Cx? As drawn it blocks the normal path for cathode current and forces the cathode to find a ground path through one of the higher value NFB resistors. Not gonna sound good! And if you turn off the NFB switch, you will totally shut down the PI tube.

If Cx was intended to block dc from the presence pot, you need to look at the Marshall 2204 to see how to properly do that.

Thanks Steve. I wondered about the .1uF being below the presence pot in the Marshall 2204, but now I see that it blocks DC from the presence pot. And yes, Cx was added to block DC from the presence pot. My bad!

[Swampertech]

Steve, question. First, your choice of 100K MV pot in the Hoffman/sluckey PR no term, mid and MV added vs. the 1M pot used in the Dual 50. Then the 22K used in the cathode bias of the PI instead of the 10K. I know this will change the hot/cold biasing of the PI.

Here's the schematic redrawn. Will it fly?

[Papa Jim]

PJ, did your tech run any tests on tone changes when he specked the MV using the dual 500K pots in the LTPI?


 No from the get go he used 500K. I will get you and answer as to why he did today.
He has been on a 5 or more year quest for a certain tone, and is very, very happy with what he is getting on that amp at the moment.

[Papa Jim]

PM sent Swampertech. I might get the guy to call and you can listen possibly

[sluckey]

Steve, question. First, your choice of 100K MV pot in the Hoffman/sluckey PR no term, mid and MV added vs. the 1M pot used in the Dual 50. Then the 22K used in the cathode bias of the PI instead of the 10K. I know this will change the hot/cold biasing of the PI.

Here's the schematic redrawn. Will it fly?

I don't know what a Hoffman/sluckey PR is. I don't have such an amp. Can you link to it?

The 1M MV in the Dual 50 is the value that Marshall used in the JCM-800 2204.

It will fly.

[Swampertech]

Steve, this one!

[sluckey]

I vaguely remember that. A member (don't remember who) wanted to convert his PR cathodyne to a LTP PI and also remove the tremolo, and add a mid and MV. The amp is really closer to a single channel DR now. I used a 100K MV more to simulate the missing 50K INT from a real DR. 1M will work too but the amp will sound more gainy than a real DR.

[Swampertech]

PJ, sluckey, and all, here is a corrected PI with MV added and presence control fixed.

[pdf64]

If the amp has an NFB loop, then such as the master vol is turned down the NFB will become disabled, same as with types 1-3 master vols.

That's a good thing isn't it? I hope.   

If the goal is for the master volume not to change the frequency response as it's rotated over its range of control, then I don't think it is a good thing for the power amp's NFB loop ratio to change along with the master volume control setting (as it will do with the dual pot arrangement proposed above, in post 5 and 6 of this thread).

...What are the types 1-3 MV's...

Ken Fisher provided a nomenclature for master vols, see p32 of the Trainwreck Pages https://robrobinette.com/The_Trainwreck_Pages.htm

If there's a NFB loop, then altering the amp's open loop gain (as with master vol types 1-3) or altering the NFB ratio (as with the dual pot control put forward in this thread) it will inevitably alter the amp's freq response and speaker damping.

So if the goal is not to alter tone, then a type 4 master should be used.
If the feed circuit can accommodate a lower value master volume pot than 1M then do so; the higher the pot value, then given the inevitable parasitic effects (following stage's Miller capacitance being perhaps the most significant), the worse the frequency response performance over the range of control. You'll be unlikely to see a 1M vol control in a hifi tube amp due to this; a 100k pot is likely to be much better.

HT fusing is highly beneficial, but it needs to be done correctly. Regarding the above schematic, it's bad to combine a HT winding tapped bias supply with a fuse or switch that interrupts the HT CT 0V path. If your F1 blows, the bias caps will be subjected to extreme overvoltage.
Either fuse each HT winding output, or take the bias supply feed from the full HT winding out (obviously a higher value dropping resistor will be needed) not the '~50V' bias tap.
Some more fusing info http://www.valvewizard.co.uk/fuses.html

[Swampertech]

Thanks pdf64, looks like it's 2 fuses on the PT secondary outputs, and probably the 1M MV pot connected between the tone stack treble out and the PI in. Thanks again!