2 x EL84 power amp

[Leevi]

I'm looking for a non-VOX circuit for one EL84 pair power amp (/preamp)
that would produce the following sound:

/Leevi

[mresistor]

If you look closely in the video there is an Xits x10 amp that is exactly what you are looking for....   
http://www.xits-sound.com/details.php?id=13

I like the trainwreck type amp this guy is making.... rjguitars
a trainwreck circuit might be what you're looking for....

[Leevi]

Yes, thanks for the link.
It would be nice to know how the V1's two first amplification stages are wired.
Do you think the PI and power amp are from VOX?
/Leevi

[Fresh_Start]

I'm really curious about the topology and tone stack of that amp.  The comment "treble pot also adds gain" intrigues me.

From the product description, my first guess for the preamp would be:
gain stage
gain stage
preamp volume
gain stage/cathode follower
tone stack
PI
PPIMV
power tubes

However, another alternative which fits the description might be:

gain stage
preamp volume
gain stage
tone stack
parallel triode gain stage
PI
PPIMV
power tubes

I imagine that the phase inverter is a long-tailed PI.  Clean and low gain.  Makes sense with a PPIMV.  Vox, Marshal 18-Watt, something else... who knows?  Getting 15 watts out of the EL-84s makes me think it's probably cathode biased but that's just a guess.  The Fender Blues Jr. uses two EL-84s and has a pretty hot fixed bias circuit.  Also, with three preamp gain stages there's plenty of room for distortion in the preamp.  Might be a relatively clean fixed bias power amp, especially with the PPIMV included.

2CW

Chip

[tubenit]

I'm really curious about the topology and tone stack of that amp.  The comment "treble pot also adds gain" intrigues me.

This tone stack has low insertion loss and the treble definitely adds some gain. I used this in my original Carolina Blues Special that I built maybe 5+ yrs ago.  I got the idea from Kevin O'Connor's amp design called the Soma 18 which was his answer to the 18w Marshall.

With respect, Tubenit

[tubenit]

Just an idea that borrows/steals from some of the Trainwreck stuff. I have NO idea how this would sound but thought I'd just toss out the idea.  Not sure about the dropping resistors in the preamp?  Trainwreck shows values as low as 56k-150k between the second and third gain stages.  Maybe also use a 500k  volume pot between gain stage 1 & 2?
 

With respect, Tubenit

[darkbluemurder]

I'm really curious about the topology and tone stack of that amp.  The comment "treble pot also adds gain" intrigues me.

Chip

Hi Chip,

On www.ampgarage.com in the Trainwreck section there is a long thread about the Xits X-10 in which I also participated. We were guessing the schem from the gutshots available and trying to make some educated guesses about what the circuit might be.

Cheers Stephan

[VMS]

Hi,

here is some gutshots and info about the amp:

http://www.fatsoundguitars.com/store/amps/xits-amplifiers/xits-amps.php

[tubenit]

Looks like 3 12Ax7 triodes into a LTPI and cathode biased (per gut shot chassis).

Looks like maybe .1 cap going into the volume pot?

It also looks like there is a typical type of Fender/Marshall/Vox tone stack possibly between 2nd and 3rd gain stage.  And looks like maybe a snubbing cap across the LTPI ? 

With respect, Tubenit

[Leevi]

One option I'll probably try:

1. stage: 1/2 12AX7 + capacitor switch + VOL1
2. stage: 1/2 12AX7 + fixed cap + VOL2
3. stage: 12AX7 (like VOX, Marshall...) + two pot tone stack (e.g. Baxandall)
PI: still open (not the VOX stack)
MASTER
Cathode biased power amp (as hot as possible)

/Leevi

[phsyconoodler]

A non-cathode follower tone stack like a Tweed 5F2A adds gain as you turn up the treble control,simply because it lessens the resistance to ground.
  An 18 watt lite circuit with a PPIMV would sound very similar and would need one less preamp tube.A 2204 preamp mated to an EL84 output section would also do killer tones.The post phase inverter master is a great way to throttle the EL84's.
  Those clips were recorded very well.
   

[Fresh_Start]

Stephan - thanks so much for the link to the Amp Garage discussion.  That's one hell of an amp IHMO.

phsyco - good observations except that he said you added gain as the Treble pot is turned counterclockwise.  IOW turning Treble down adds gain.  

I haven't ever played with a Vox (either guitar pick or soldering iron) so wasn't familiar with the interaction of the Bass & Treble controls.  Just tried it with Duncan's Tone Stack Calculator - with Bass @ 5, the full range of the Treble control with a Fender TMB changes the bass end of the curve 2db.  For the Vox, it changes the bass end of the curve 7db and there is a much larger shift to the right (into low-mids) as you turn Treble down.

The PPIMV seems to work really well here too.  Neat circuit! (despite having an un-used triode)

Chip