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RalphH
15-08-2017, 01:10 PM
During the planning of a scratch built tube amp, I came across a 1960’s 16w Japanese mono hi fi amp for $100 on Trade Me. With 3 X 12AX7’s in the front end, a push-pull output stage with 2 X EL84’s and a 6CA4 rectifier tube, it had all the ingredients for a low cost, easy, re-purposing project.

I retained everything from the old school cathodyne phase inverter to output stage, and configured the two dual triodes in the preamp in a Mesa/Soldano style cascade with a three band tone stack at the end of the chain.
I configured the first tube as a pair of common cathode preamps; one half for low gain followed by a switchable gain “crunch” stage using the other half of the tube. The gain control site between the two stages. The other tube acts as a further gain stage and a cathode follower buffer for the tone stack.

Because of the point to point wiring, “tuning” the cathode bypass frequencies, gain and filters by a bit of trial and error to get the tone I wanted was easy. I also placed a switchable filter in the feedback loop from the output transformer to PI as a sort of presence control. I called this one “edge”.

While not a high gain monster or particularly loud, the little amp makes a good fist of everything from super clean to dirty rock crunch with quite a bit of sustain when wound up. It still has the original NOS EL84’s and rectifier, so has been a bargain to build- less than $250 including new preamp tubes.

To complement the amp, I knocked up the appropriate era-correct looking 2 X 10” cabinet with a pair of 20W Jensens.
Pays to trawl the swap meets, garage sales etc.. who knows what may be hiding out there!

wokkaboy
15-08-2017, 01:16 PM
nice job Ralph, 90% of that post went over my head.
So how does it sound ?

RalphH
15-08-2017, 02:01 PM
nice job Ralph, 90% of that post went over my head.
So how does it sound ?

Ha! Sorry about the geek speak..
It's a work in progress, but currently sounds like a Mesa Boogie Mk II on clean, and a Soldano on crunch when cranked.
It still needs a bit more low mid balls, and when I've sorted it, will post some sound bites.

FrankenWashie
15-08-2017, 02:15 PM
Ha! Sorry about the geek speak..
It's a work in progress, but currently sounds like a Mesa Boogie Mk II on clean, and a Soldano on crunch when cranked.
It still needs a bit more low mid balls, and when I've sorted it, will post some sound bites.

It's OK Ralph, Doc and Simon knew exactly what you were saying....

Chuck
15-08-2017, 03:07 PM
That is some serious building! Like Wok, I don't understand very much of it at all, but I do love the look and feel.

DrNomis_44
15-08-2017, 03:08 PM
Sounds interesting, you should record some kind of demo so we can hear what it sounds like.

It's surprising what you can do with these old Tube Monoblock Amplifiers, when I first got introduced to the internet (around the early 90's) I used to do lots of searching for guitar amp schematics and I noticed that there were lots of forums geared towards modding Tube Hi Fi amps into guitar amps, lots of great info, I'm pretty sure that a lot of those forums are still going.

wokkaboy
15-08-2017, 03:23 PM
no worries Ralph, can't wait to hear it after you done the final tweaks.

JB RETRO
15-08-2017, 03:54 PM
Yep definitely serious stuff. Means absolutely nothing to me, but as woks said how does it sound. I like your changes but I did like the original knobs etc on the front

Marcel
15-08-2017, 05:00 PM
Nice work...

You have convinced me to get my A into G and go to the local swap meets.. not that there are many in my part of the world...lol

RalphH
16-08-2017, 06:17 AM
Sounds interesting, you should record some kind of demo so we can hear what it sounds like.

It's surprising what you can do with these old Tube Monoblock Amplifiers, when I first got introduced to the internet (around the early 90's) I used to do lots of searching for guitar amp schematics and I noticed that there were lots of forums geared towards modding Tube Hi Fi amps into guitar amps, lots of great info, I'm pretty sure that a lot of those forums are still going.

Yep, some of the old stuff was great for experimentation.. Back in the late 70's I built my first tube guitar amp out of an old PYE sterogram chassis. Was a bit like a Fender Bassman in it's simplicity, but driving two separate 8W single ended output stages to individual speakers. The output tubes were 807's I think... Sounded so cool with my Roland BEEGEE fuzz box. :cool:

Will put some audio samples up when I finish tweaking..

dave.king1
16-08-2017, 06:35 AM
807s what a great old tube, I used them in my AC30 inspired build in the mid 60s

RalphH
16-08-2017, 07:23 AM
807s what a great old tube, I used them in my AC30 inspired build in the mid 60s

Yea, they sounded nice, but the anode on the top could be a bit scary when poking around after the caps deteriorated.

dave.king1
16-08-2017, 07:40 AM
Yea, they sounded nice, but the anode on the top could be a bit scary when poking around after the caps deteriorated.

Yep, my elbow put a hole in the fibro wall of the rhythm guitarists garage when I got a 385v nudge from one of the anode caps.

Marcel
16-08-2017, 11:13 AM
807s what a great old tube, I used them in my AC30 inspired build in the mid 60s

I have a pair of 807's that I'm trying to sell on Ebay. Not many show much interest in them these days....

RalphH
16-08-2017, 12:12 PM
I have a pair of 807's that I'm trying to sell on Ebay. Not many show much interest in them these days....

Good luck! The only people that may be interested are the Ham radio guys.. new ones are about $30 each..