updated new build pics/schemo 6AU6

[Alexander]

Hey Gents-

I finished the 12AX7 "Gain-E-ack" channel and posted a schemo of it. Very fun responsive little circuit! a tweaker's paradise- have fun! My goal with this channel was to NOT scoop beatiful mids out!! If Leo tried for HiGain Tweed this might be what they came up with, who knows...but it's organic and alive!

http://www.retrodyne1.com/29Wamp.html

Rock on,
Alexander
Retrodyne Amplification

[Alf]

Hi Alexander,

I like your design very much and must say your soundsample is very convincing !!
Especially the use of the 6au6 tube and the Pi driver tube is a very good idea because these tubes are still plenty available.
I think you forgot to add the value of the first stage 12dw7 tube cathode resistor.
Could you post the value please ?
And what OT did you use ?
The pictures are great too. A terrific build .

Alf

[Alexander]

Hey Alf-

Thanks for the kind words- this has been a fun build despite the conventional box chassis...I promise to go back to my mailboxes soon!

the PI 12DW7 'AX' side cathode resistor is 2.2k. I'll probably lower it in the next round of tweaking but there's other experiments first.

The OT is a Triode Electronics 6.6k p-p, 25W 'Deluxe' replacement that has UL taps & 4,8 ohm outputs. The make a paper interleaved one but I'll likely get a batch of Heyboers made with a 16-ohm tap added. I built a UL version of this circuit and the distortion qualities are more determined by the PI distortion versus power tube distortion. Yep, the crossover notch dissappears, yet there it is, the same tone!! So I dumped the massive 150ma choke and built this instead. Maybe UL is better with LTP inverters? I dunno, I'm tired of LTP's and am having more fun with my Concertina Turners. (TM)
:-)

Rock on,
Alexander
with sister living in Denmark

[bob p]

Hi Alexander

one thing that i thought was interesting about your design was how you combined the tube sockets and turrets on your small boards. it looks like you mounted your tube sockets on your G10 boards, and mounted turrets circumferentially around them. then you components were wired directly to the tube socket at one end, and passing outward radially to the turrets at the other. this reduces the need for a lot of jumper wires, and it cuts your turrent count in half. brilliant! could you provide some extra photos that show the detail of this design?

also, am I correct in assuming that the sockets are mounted directly on the G10 and not on the chassis, so that the tubes pass through holes in the chassis to reach the sockets that are mounted on the G10?

thanks!

bob

[Alexander]

Hi Bob-

Thanks for the compliments! Yep, the tube sockets are directly mounted on G10 boards, on silicone rubber sandwiches, and the G10 boards are also mounted on silicon washers above the aluminum standoffs. And the tube shields poke through the chassis without touching. I deliberately left enough room for octals to fit on these boards and have several octal-based preamp and PI boards I swap in & out to R&D different combos. And avoiding dozens of little wires/antennas was a MAJOR reason for me to explore this style of building!

I use a common ground point on each board and then run an individual ground wire to a chassis-mounted lug, seperate from the filter cap ground. I also run a ground wire from each stage to its respective copper control shield and don't solder anything to the back of the pots. I hate firing up my ancient 60W RCA soldering iron!

Placing the turrets to fit a variety of part dimensions is a headache as is the work involving facing the sockets and arraying the passives for 'orderly' signal flow. plus you have to consider the heater wires & grounds on the backsides of the boards. I avoid having grounds even cross over the heaters to keep potential hums out.

I may post some details of the boards eventually, but I have 4 amps in progress now all 'past due'...I needed to build this one to work out a 6AU6A pentode channel because I had too many microphonic 6SJ7WGT's from Angela. No wonder they were only 5.00 each!!

The "Gain-E-ack" was just an experiment to add a higher-gain yet vintage-feeling non mid-scooping channel on the board where the 12DW7 reverb tube usually resides. That 2-tube board is my 'standard' preamp board; I just mix 'n match features/tubes for each amp.

Happy Soldering,
Alexander
Retrodyne Amplification

[Loogie]

shields

How do you ground the shields?

[Alexander]

Hey Loogie-

I assume you refer to the inner pin shields? I ground them to the same stage node which also includes the respective copper control shield, and I run a ground wire to a seperate chassis lug seperate from the power supply filtering. I don't attempt to ground the outer shields since they're mostly aluminum; plus the amp is already quiet enough that there's no real need to go any further.

HTH,
Alexander
Retrodyne Amplification