SRV Twinmaster

[tubenit]

One of the forum guys built this amp. The soundclips are very nice, so I wanted to post some schematics.

Soundbits:   http://www.soundclick.com/bands/default.cfm?bandID=942167

He used a PPMIV that I don't recommend, so I redrew the SCH schematic to use a Bruce Collins type design. One could also use a on/off 1M crossline Mstr Vol.  I included an option #2 that might be the approach I would take if building this amp.

The GIF schematic is the original.

With respect, Tubenit

You can find most of these schematics in the "Broken Links" file.  Write down the name of what you are looking for and then click on to this Broken Links file list:

http://el34world.com/Forum/index.php?topic=17420.0

[FYL]

You may also consider using a PI bootstrap MV, which gives IMO the best results in amps using a cathodyne PI.

The 500KA pot can be replaced by a 250KA or 100KA version as it's bootstrapped and will show >1M reflected R. The 10K maintains a sensible R even with the pot at 0K and also acts as a small grid stopper.

(Edit: corrected typos)

[jhadhar65]

FYL-

I've got the same MV in a design of mine, except I used a 1M as a direct grid-leak replacement.  Why use half the value?  And if I stay with the 1M, would the reflected resistance look like >2M then?  The >2M issue may be answering my own question, but I don't see how that would look different from the stock fixed resistor.  BTW, I nabbed my PI from the 5F4 Super.

I hadn't thought of the grid stop.

[FYL]

I've got the same MV in a design of mine, except I used a 1M as a direct grid-leak replacement.  Why use half the value?

The bootstrap effect multiplies apparent Z :
Zin = Rg / (1 - Av * (Rl / Rl + Rb))
and
RI = 56K, Rb = 1K5, Av = app 0.95

Zin = Rg / (1 - (.9*(56000/56000+1500)) = Rg / 0.0942

Rg = 100K => Zin = 1M06; a 10.6 factor

We're in the Mohm range even with a relatively small Rg... And the lower Rg the better when it comes to noise and long-term stability.

I hadn't thought of the grid stop.

Grid stops are a must nowadays with all the RF crap around. Small value R's suppress a lot of parasitics and don't hurt the tone, why take any risks?

[jhadhar65]

Great math!  ...and thanks for that tutorial.  So 1M for Rg = 10M6.  Hmmm, now I have more questions...

Zin=10M6 for either the stock 1M fixed or substituted MV.  I understand that more resistance equals more noise, but in this case Rg is part of the load resistance for the preceding stage and not a series resistance in any case.  Is the reduced noise floor due to reduced resistance (with 500k or 100k or whatever) or just less apparent drive voltage?  I don't know why I'm even asking that now that I've typed it, since it really makes no difference, but I'll let it stand and see what you say.  Since I did ask, do you also advocate swapping the stock 1M fixed for a 470k or 100k fixed if one wasn't inclined to make a bootstrap MV?

>Grid stops... why take any risks?

Good point... as always.

[StevieRayVehkakoski]

I guess I might as well join in as this was my project that I finally finished..

The master-volume before the PI and the preceding gain-stage was something I just had to do because the bassman-style gain-stage+cathode-follower just killed the Deluxe-style PI. So this isn't really a MV that I actively use, it's more like a built-in attenuator; with no feedback I have to cut the signal by about 50% to get decent sounds and with the feedback loop switched I can raise it to about 70%.

I'm really interested in any ideas on making the amp better. Right now I've frozen the design as I've spent way too many nights experimenting all the variants there are on the circuit but I guess in a few weeks I'm back in business  :P

[FYL]

in this case Rg is part of the load resistance for the preceding stage and not a series resistance in any case.

It's in a feedforward path - the signal from the cathode is reinjected in phase thru the grid. This isn't a simple grid leak circuit, but a bootstrap, with the 1M referencing the grid as well as the junction between Rb and Ri.

Since I did ask, do you also advocate swapping the stock 1M fixed for a 470k or 100k fixed if one wasn't inclined to make a bootstrap MV?

1M is OK. Heck, who am I to argue with Leo and the Old Ones?

Anyway, I would use lower values as 1M is the specified max for grid R in fixed bias according to most US datasheets (Mullard specs 2M2 FB and 22M GC).

BTW, you could also consider lifting the heaters by at least 40V DC as heater to cathode voltage is close to 140V. 12AX7's are spec'ed at 100V max DC.

[jhadhar65]

>...the signal from the cathode is reinjected in phase thru the grid.

Of course... my mistake in ignoring that.

>...you could also consider lifting the heaters...

True.  I'm thinking that's not quite as hazardous as the specs caution or there'd be a lot of melted cathode followers out there.  Then again, God looks after drunks, fools, and haphazard amp designers.  At least that's how I think the saying goes.  Thanks for the insight and math, FYL!!  ...and thanks to SRV and Tubenit for giving us something else to think about!!