Lead boost idea(s)?

[dennyg]

My main gigging amp is an SE (the SEL from AX84) running an EL34 (which confounds sound engineers that it's as loud as it is).  I run the MV at about half and would like to mod it to basically dime the MV for leads.  It's already a 2-channel (clean/crunch) using separate MV's and not enough room to add a third pot & additional relay circuit.  Have an idea wanted to post before I try it: 
* add a 1M grid leak in parallel with MV (1M)
* run the MV ground to footswitch so signal bypasses MV with ground open

Would this work and yield similar output as MV maxed?
Other ideas?

[labb]

Don't see much about the AX84 SEL but that is one heck of a single ended amp. Don't know about their new version with the building blocks of pre/power modules but the original is great amp. One I have is set up to run a 6L6. I think that I like it better with a 6V6 but the guy playing it likes the 6L6. Tried it once with a KT 88. Now that will really get loud. Just did not want to change the OT to match it. Did not answer you question but just had to praise that amp.

[dennyg]

Yeah I love that forum too.  I'm coming up on my 3rd anniversary of amp building and it all started over there - i didn't even know how read a schematic back then so you can imagine the stupid questions those guys endured.  I got a thread going with Merlin - printed and framed it! The SEL was my first although I've done quite a few mods on it since.  I built the Uber (used the SEL PA, not the ultralinear PA per design) and loaned it to my other guitarist in the band - he sold his Mesa Dual Rect and bought my Uber so now our band is "powered by AX84".  We just recorded an EP and he's redoing all of his parts with the Uber.  He's also a recording engineer with his own studio, and his clients love that amp and I've got several requests to build both the Uber and the SEL. Heck I'm still an amateur but if I keep learning more maybe I'll turn this hobby into some money on the side!  My parts just came in for Tubenit's TOS - my first PP build - and stoked to get that one started. 
Thanks for the AX84 endorsement - between that forum and this one, any builder can get all the support they need.
Denny

[92Volts]

You can probably skip the grid leak, you've got a ground reference through the tonestack and adding more would increase the load on the tonestack (reducing volume and/or changing tone behavior). Now, I guess removing the MV from ground with no added grid leak decreases the load, but you're going for max volume so that might be a step in the right direction.

If you wanted to get fancy you could use a relay (to avoid picking up noise or stray capacitance or whatever outside the amp). To get really fancy you could use a dual-pole relay (or dual pole switch in the footswitch) to switch the signal from the wiper of the pot to the top of the pot, therefore keeping it in place and not messing with the grid leak arrangement. With this said, you're temporarily removing the grid from any connection/reference so I expect this is more prone to popping and other noise than less-direct changes like grounding.

Edit: On second thought, it's probably not the "best idea" to ever remove the grid/bias reference from the power tube, even temporarily.

But you could use a dual-pole switch and/or relay to remove the ground of the MV *and connect a parallel 1M grid leak in its place*. This way, the grid leak loads the circuit back to regular configuration in the lead setting, but is unconnected in the normal setting.

The best part of this, compared to my previous suggestion, is if these connections fail and neither is connected at all, you still get a ground reference through the tonestack for safe/consistent bias.

[dennyg]

Yep - I once watched a 5W cathode resistor cook the paint right off of it because I'd turned the amp on w/o connecting the g1 lead to the MV.  But lessons learned the hard way are the ones that stick with you! If my thinking is correct, adding the 1M grid leak will yield a 500K load in parallel with the tone stack vs 1M w/o it - so may have to notch up the MV a bit to attain same volume as I use now, but when switch out the MV ground, it should send comparable signal to the grid as with MV dimed and my bandmates will stop complaining that they can't hear my leads.
I'll give it a try and report back. 

[cliffs_trip]

I just wanted to chime in and say my second build was an SEL and I love it! 

I keep buying broken amps and repairing them but am not as satisfied with the tone and they are usually sent back out the door in favor of one of the amps I've built!

I was thinking about building the uber preamp an sticking it into a 50 watt poweramp, or I have an old UL 135Watt transformer from a 70's Twin I could use!

I wonder what a 135 Watts of 6V6s would sound like...

[dennyg]

So I ended up lifting the tone stack ground and it provides just the right level of boost.  It also works well at all MV settings so I can use it at rehearsals .   

[92Volts]

I just wanted to chime in and say my second build was an SEL and I love it! 

I keep buying broken amps and repairing them but am not as satisfied with the tone and they are usually sent back out the door in favor of one of the amps I've built!

I was thinking about building the uber preamp an sticking it into a 50 watt poweramp, or I have an old UL 135Watt transformer from a 70's Twin I could use!

I wonder what a 135 Watts of 6V6s would sound like...

I built the Lead 2 thinking it would clean up better than the Uber. It has all the gain I'd hoped for, but doesn't clean up well at all. I'd say the Uber is likely a simpler way to achieve the same thing, though I'm also happy with the Lead 2 for high gain use.

[jjasilli]

Maybe try a fixed resistor lifting the MV pot from ground.  A SW (possibly relay operated) would short the resistor. 


*  with the SW closed, the resistor is shorted: the pot will work normally
*  with the SW open, the resistor lifts the pot from ground, for a boost.


To determine the value of the fixed resistor, set the MV pot where you normally use it about 1/2 way.  Measure its resistance.  Use a fixed resistor value + ~1/2 MV pot value = 1M.



CONS:  if the MV pot is already dimed + fixed resistor, the result may not be desirable.

[dennyg]

I built the Lead 2 thinking it would clean up better than the Uber. It has all the gain I'd hoped for, but doesn't clean up well at all. I'd say the Uber is likely a simpler way to achieve the same thing, though I'm also happy with the Lead 2 for high gain use.

Cool - i actually built the simpler version of the lead II (Brian's) and agree it's a great high gain circuit.  Also built Merlin's UHG preamp from his book - very smooth Soldano-like gain.  The Uber is more mesa-like - without that annoying high-end buzzy thing (i guess some like that tho).  The Uber pre-amp with an SE PA is it's own unique amazing gain-monster tone. 

[2deaf]

So I ended up lifting the tone stack ground and it provides just the right level of boost.  It also works well at all MV settings so I can use it at rehearsals .

Do you mean that you disconnected the tone stack from ground altogether or that you lifted the tone stack up by inserting a resistor between the stack and ground, like jjasilli's suggestion only on the stack instead of the MV?

Lifting the tone stack also alters the EQ, which I consider to be desirable for leads.