Best sounding master volume?

[Rp3703]

So I’m planning to put together a stereo 50 watt plexi and wanted to get some opinions on the best(least tone degrading) Master volume. So the simplest would be just a plain 1M master volume pot following the treble pot. The second would be a PPIMV. The third would be a Trainwreck MV and the fourth possible option would be to install an L-pad attenuator as a volume control. Please let me know if there is anything I didn’t think of.

[TIMBO]

1M master volume with ppimv

[dunner84]

In my experience it's amp specific. My modded 5e3 was happy with a simple ppimv, but my ab763/soldano combo sounded best with the type 2 master volume. It is the most transparent that I have tried so far.

[pdf64]

Decide whether you are happy to lose power amp negative feedback / presence control efficacy as the master is turned down; if not, there's only one option.

Ken Fisher categorised and provided a useful nomenclature for the various types of master vol in The Trainwreck Pages p41-44 https://robrobinette.com/images/Guitar/Trainwreck/The_Trainwreck_Pages_searchable.pdf

[MFowler]

You could implement both types of master volume controls in your amp just make the PPIMV the Master Volume and the ( type 4) (1m after treble pot) as preamp Master volume.  One for each channel if you like.


Mark

[Ed_Chambley]

No master volume Plexi will sound like a Plexi.  The reason is the tone from a non-master plexi 1987 Circuit, 2 tubes 50 watter gets its clipping/ tube saturation at the power tubes.  Which if you are acustomed to hearing this coming from EL34 Pentodes, when clippinng every note is still articulate, but since the output is compressed the resut is gobs of sustain.


So, if I understand corrrectly you are preplanning your amps to be too loud and need a way to control the output volume.  This is the forever ongoing topic, everyone wants to get a big bottle tone at less volume, but it is the speakers and cabinet doing this and if oyu add more wattage the lows get tighter.


I have  JMP 50 with 2 EL34's.  THey call it a 50 watt amp, but I have watched if produce 74 watts continuously.  There are 2 approaches to amp buiding really.  One is to make a powerful clean output section that stays ljean like a Fender AB763 Twin, or really any Fender Blackface with a 12At7 Phase inverter and 2 to 4 6L6Gc tubes.  Then build a addditional gainstage/overdrive that will allow you to adjust the characteristics and wil be relible to you tone settings or you can build to Ouput power distortion.


This is usuallly lower wattage, an not a speaker will not sound the same in a 10 watt amp as it will in a 50 watt amp.


You can actually build a 25 watt, Push Pull EL34 Plexi, but the voltage has to get aroud 320 Vdc.  The preamp kept at the same voltages will keep the signal hot.  Your EL34's willl begin to breakup easily.  I built one.  Cathode Biased and fixed bias, switchable.  Never really could get the grunt from the mids I expect from a non-master Plexi.


Changed the tubes to 6V6 and the amp really sounds a lot better.

[Big_Mike]

Look up the PPIMV used by George Metropolous on his 50 watt plexi build.  You can find it at the valvestorm website.  Also, check out the Youtube Video he did with the PEC dual ganged potentiometer and his PPIMV.  Sounds awesome.  I am thinking of building a 50 watt plexi myself and I would use that master volume circuit in a heartbeat.

[shooter]

if I understand correctly you are preplanning your amps to be too loud

I think it's the difference between musicians that build and electronic folks that build
at the end of day though, if it floats your boat, play

[Ed_Chambley]

The LARMAR Master was made during a long thread at the metro forum.  I have attached it here. It is one of the best in mantaining a lower volume, but not much lower.  The problem with this design is it changes the Presence control and makes it inactive.

PEC dual pots track more consistent.  For instance, an Alpha dual 250K linear if perfect will be 125K at mid point.  Make a reading of both pots and I will bet you they are not within 25K of resistance.  PEC pots will be better, but cost 5 times.

Check with Steve at apexjr.com and ask him if he has any NOS Allen Bradley dual 250K or a little more precision pot.  Still, you will only be able to reduce volume some since it changes the amps tone a lot when below 120K.

For preamp distortion, I prefer the JCM800 master and or a hot switch.

[dennyg]

"...The preamp kept at the same voltages will keep the signal hot.  Your EL34's will begin to breakup easily."

Ed - how's that work? 

[Rp3703]

So this project is more of a retrofit than a new build. I plan to take a Marshall 9000 dual 50 watt tube power amp, alter it a bit and screw a Hoffman stereo preamp I built to the top of it, only this time I am going to remove the PSU from the stereo pre and use the 9000 to power it.

So nobody likes using an attenuator with their Plexi? This video gives you an idea of any tonal changes at decreased volume and I cannot hear a difference.

. I just figured it couldn’t be too difficult to build two internally to use as a master.

[ginger]

Try a VVR , or THIS Dumble.... i have not built this , but some guys on this forum have.... i just don't want another box to lug around , although you CAN build it into your amp

[Ed_Chambley]

"...The preamp kept at the same voltages will keep the signal hot.  Your EL34's will begin to breakup easily."

Ed - how's that work?

Your transformers will dictate power.  For instance, The Matchless Clubman 35 a Push Pull EL34, 2 tubes, Cathode Biased (each drawing 85ma idle, or a total of 170ma, so 250 ma would be my choice. with 400 Plate Volts and a 4K:8 ohm Output Transformer will provide 35 sort of clean watts.

Use the Power Trans, https://edcorusa.com/xpwr007 (550V(275-0-275)@200mA, 6.3V@6A & 5V@3A) and it will be under 30 watts if you use a Bassman 4K:8 ohm, but you may want to use a 6.6K:8 ohms or even higher like 7.7K:8 ohms.  The clubman turns ratio is 15.9k:1 at 16 ohms.  The output transformers for the Clubman 35 are 4045 reflected at 16 ohms and basically the same at 8 ohms as well.

You would just adjust the dropping resistors to keep preamp tubes voltages up.  The less voltage the plate receives, the less it will attract electrons.  If the signal swing is kept high in the preamp, and the power tube voltage is low, the tube will compress as the lower voltage tubes have less headroom.

[Rp3703]

Let me know if I should just start another thread for this but since we were on the subject, this is what I will be using for the power amp. Notice the B+ voltages. I’m guessing the really low B+ at the PI was done to accommodate the 12AX7 power tube but not sure.  Do the voltages on the EL34’s look right?

[Rp3703]

Anyhow, my plan is to turn it into this and eliminate the 12AX7 power tube section. Do you think the usual Marshall 20K-PI, 10K—V2, 10K-V1 resistor configuration supply the correct B+ voltage to the PI and preamp tubes?

[PRR]

> I’m guessing the really low B+ at the PI was done to...

V1 only has to drive V2. V2 stage eats 338V. Assume gain of 50 in V2 stage. Then if V1 was built the same, it would only need 338V/50= 7V supply! Actually it has a Long Tail, so needs more like 10V. And we really do not like to run 12AX7 at few-Volt because of uncertain contact potential. In fact the old-old rule was to try for a B+ greater than Mu (masks a contact-potential variation of about a Volt). Mu is 100, B+ here is 132V, job done.

Your revised/snipped power-amp plan looks OK at a glance. (The 3000px image is either too big or too small on my screen.)