West Mini IR rebuild

[shanx]

Hi Simon,
The Mesa Boogie EL34s I think are pretty decent. I paid a pretty penny for them, but also bought in because a fellow musician has a Mesa Boogie Stiletto running the same tubes. No problems and a stunning sound from them. He can run them as ''triode'' connected, and pentode. I could probably check and see who builds them.
What is a bit strange is some of the web reviews gave the 12AX7EH from Electo-Harmonix (Russian sourced) a better noise rating than the Sovtek. What I think may have happened is Sovtek did in fact upgrade that line including the -WA version, they are stamped both 12AX7-WA/7025 and the construction looks a bit different.

The Ultra/Lin sound is probably the most focused and accurate of all modes. It sounds really good with my archtop's humbuckers. You get a warm note with a bit of singing overtones but retain a tight controlled sound. With single coils, Mark Knopfler or Steve Cropper like tone..Guess that is why it is the preferred in Hi-Fi amps.

I like your idea of the 3 way switch. I am not too worried about the OT handling it. It has gone through a lot of worse abuse.
Dave West seems to be getting his company off the ground again, He has released a new mini head, with promises of some reissues soon. I wish him luck.

PS If you have checked out Trainwreck schems by Ken Fisher it is quite similar to the Avalon (and also early Marshalls).

[shanx]

Running the preamp tubes now with the 12AX7WA/7025 and they are quiet.

That allowed me to try a tube swap on the PI from 12AT7 to the now spare 12AX7.
I left the 12AT7 on the reverb drive circuit.

With the gain increase at the PI , I am able to get into a nice harmonic distortion level with lower levels of guitar volume. The sound is slightly ''edgy'' ( need to roll off the highs a bit). Not a ''growling'' amp sound but definitely showing its teeth!

I still think the master volume would be nice adjustment to balance between a bit of preamp distortion and PI / Power tube overdrive. I will probably modify the NFB with a presence control pot (and some filtering to roll the highs off)

[Simon B]

Nice idea upping the PI gain.

Will be interested to hear what works for you with the prescence control / nfb - keep us posted.

[shanx]

The PI and Power tube section, I think from what I see, are equally important to the contribution of output overdrive.The NFB & adding filtering should (I hope) re-correct the voicing of the amp. To me there seems to be more high frequency emphasis, with the higher PI gain. Makes sense the 12 AX7 has different transconductance, and I think the capacitance is different too ( didn't check though). Obvious I want to keep the harmonics..just don't want to be on the edge of oscillation (it is really close right now).

[shanx]

I was looking at the VVR3 Kit from Hall Amplification, possibly as an alternate to a hot plate attenuator. Has anyone on DIY used it and if so, your comments?
Any other variable power supply types such as Power Scaling (London) etc..

[Simon B]

Haven't used one I'm afraid, but it's a very interesting idea.

You've got a couple of variables to consider here:

Do you reduce B+ to:

Output stage only (adjusting bias of course),
Output stage and PI,
Output stage, PI and pre-amp.

Also, do you want to feed the reduced voltage via a mosfet, which means you've got a stabilised supply, or do you want to stick with RC filtering, keeping some ripple in?

Just in case you aren't aware of it, there's opinion and evidence that ripple, and the way it dirties the ouptut signal is an important part of good rock guitar sound, which a stabilised supply would remove.

Let us know how you get on with this.

[shanx]

Hi Simon,
I think the VVR Kit from hall is a power mosfet type, controling the power tube supply/grid bias.
I would like to keep reasonable amount of headroom at V1 and V2..and the 12AX7 on the PI has a lot more gain now. A master volume pre-PI is probably going in, I like a small amount of preamp drive control, and I can always leave it at max if needed (!)
I am just finishing up an effects pedal build for a friend right now, and I intend to do a bit more fiddling with the amp shortly.

[shanx]

After comparing my notes and some fender and marshall master volumed amps, I decided to go for it. I have not yet changed any filtering, or NFB.

The mod was a big payoff, she sounds ''right'' now. The edge of oscillation and high frequency over-emphasis is gone, is running dead quiet at reasonable volumes. It has that bit of preamp warm tone, actually sounds like a tube amp should now..it was too cold/clean before. WIth the 12AX7 in the PI position it was almost too much gain and getting tinny. The MV seemed to calm it down and move the voicing back to normal.

I know some people may say they don't like the MV added on some amps. I wired it in and it only took total 1/2 hour to do and it worked for this amp. Not saying a solution for every amp, but it worked really well this time. If it hadn't worked out, I would have just taken another 15 minutes to pull it off circuit.

Using a 500K Lin sealed clarostat type, instead of the fender 1M. Because this ihas a reverb unit, I put the pot right before the PI. You get MV control with both reverb and dry. Top of pot to previous stage and the bottom doesn't go to ground: goes to the top of the 100 ohm resistor of the PI tail (where the NFB goes back to ) Fender did that on their '73 fender twin. But I did not add that filter capacitor that they put on the wiper. Wiper direct to the 0.01uF entry to PI as usual.

Simon, I am going to add the presence circuit, just need a little time now. Life gets in the way and I got loaded with a bunch of stuff at work...but I am hoping the electrics will be all in order for next shoowtime. It has not been publicly played in something like 20 years!!

I am going to email Dave West some of these minor mods I have done so far on this. Wish list is to put in some power tube control now.

[shanx]

PS Just to clarify, the 12AX7 at the PI socket is in fact staying in, it is now much more controllable by the MV.

[Simon B]

Good, sounds serendipitous, the 12AX7 PI working so nicely with the MV - or did you think it would?!

Keep us posted, a revised schematic would be poured over eagerly.....