Another strange beastie..but is it.suitable for conversion?

[devilsindetails]

OK...Hard as it may seem to believe just had another wickedly strange amplifier dropped on my doorstep. That German amp of some months ago seems pretty ordinary compared to this IMO

In this case it is a Minshall Mono Organ Amplifier

Tube complement is as follows:
5V4G Rectifier
2X 6L6 Outpu tubes
2X 6SF5
1X 6SN7
and one ???? tube which appears to be a dual triode as well (just not sure exactly what designation)
And a 6E5 tuning Eye tube used as an "Overload Indicator"

Huge Hammond Trafo Iron on-board.

I understand this company later was bought out by Estey Organs.

I am contemplating conversion to a guitar head. Currently has the typical Microphone and Phono inputs as well as one for "chimes".

Looked for schematics everywhere but no luck. May have to draw it out myself.

Haven't dared power this up...but will be checking the Transformer etc and then try and bring it up slowly to about 100 VAC over the course of a few hours.

Anyone familiar with this type of beast? Suggestions..other than the usual replace caps, resistors etc etc...?

[Firestorm]

You could have some serious fun with that. Screw the schematics, spec the transformers and start from scratch. Bonus points if you can make the eye tube do something cool.

[EtherealWidow]

Oh my gosh dude. I'm just waiting to save up enough allowance to violate an old organ off of craigslist. That looks wonderful. Please keep us updated.

[Structo]

Yep that is pretty cool.

Vintage points = coolness factor.

[overtone]

Impressive.
As Firestorm said, spec the iron, see what ratios the OT offers you to see what is possible.
Looks like there are huge possibilities.

[devilsindetails]

I am inclined to try and modify this rather than strip down....converted an old Bell 2122a mono amp for a friend who owns a Tiny Terror and a Dr. Z M12 and just loved the unique sound of the old Bell feeding his 1X12.

Will keep everyone posted how this old organ amp does.or does not develop. Iron tests no shorts with a dim bulb tester so I'll open er up and see the innards and how the guts look and how exactly this thing is laid out...will post those pics shortly.

[EtherealWidow]

Maybe you could do something like
Beam hexode gain stage
Beam hexode strapped as triode (maybe CF if it's safe?)
Tone stack
Dual triode long tail pair
6L6's

I dunno. Maybe that's boring though.

[devilsindetails]

Boring is fine as long as it's stable and reliable. Many "out there" builds but usually unstable as hell...

What I've noticed of late, is a lot of guitarists (blues, jazz and R&B anyways) seem to be looking for something with a distinct tonal character without a lot of superfluous stuff (amp circuit design) between the pickups and the speaker...preferring to massage the guitar tone pots and the hot and cold channels of the amp.

Recent case in point was a little Bogen CHB-10 PA Amp with a hot microphone input and cold phono input.

This little sleeper is really versatile not because of a profusion of pre-amp-tone-stack stages (it only uses one 7025) but simply in the manner it reacts to the slightest guitar tone and gain variations.

The Phono channel is super clean and won't break up at any level while the Mic input gets nice and gnarly even with out prodigious amounts of guitar pot gain.

The guitarist who bought this off me simply loved it as is because it was so different!

I am hoping this great big beastie may have its own distinct and mysterious sonic signature.

[EtherealWidow]

Yeah. I think this could make a really cool amp. I don't know if a beam hexode strapped as a triode in a CF position would be stable... Haha. I don't know though. Two of those stages would seem a little much. Would definitely have some unique mojo though.

[gingertube]

6SN7 - pull that and run a 6SL7 with a few component changes as the phase splitter. Use 12AX7 component values - they will work fine with a 6SL7.
See the Junkbox Trainwreck circuits by myself and Overtone.

The 6SF5 is a single High mu triode, very much like 1/2 a 12AX7 (data is same as 6F5 if looking for pin out details etc.)

By subbing a 6SL7 into the 6SF5 socket and rewiring you can do a basic BF Fender Preamp. If you want an Express or similar preamp then you need another tube to get at least one more triode section.

I would trace teh existing schematic before makeing up my mind as to what is teh best to do with it but be assured that something lovely CAN be done.

Trace and post for our worth "what you pay for them" opinions. Actually or opinions are in general better than that.

Cheers,
Ian

[Luthierwnc]

That looks like a good candidate for a Gibson BR 1. Think Rock Around the Clock! Attached is a schematic I modded for use without a field-coil speaker. I've never gotten a definitive B+ but I think this was a 500 to 540 VDC power transformer. The choke/capacitor power section helps get the volts down. On an original I rebuilt I adjusted the variac down to 110 VAC and then at 120 modern voltage. It sounded better at the higher setting so YMMV.

As long as you are bringing up the amp on a variac, disconnect the OT leads and check the impedance ratios. European amps didn't always use 8 ohm speakers. I've got a mark on the dial at exactly 100VAC to make the math easier. Primary divided by secondary for the turns ratio, square that number for the impedance ratio per one ohm on the secondary.

Have fun! sh

[EtherealWidow]

That's really weird. Whenever I looked up the 6SF5 it said it was a shadow grid pentode... found the correct info. A simplified version of a 6G3 might be cool.

[devilsindetails]

As promised here are gut shots of the chassis.

Pleasantly surprised to find it in such nice shape sans the usual thermal deterioration found in most amps of the period ( no brittle wiring, melted wax caps etc)

Nicely laid out and implemented IMO...it will be relatively easy to work on.
The Trafos are OK too.

However, one of the two-stage filter can capacitors is definitely shot...(a neat socket plug-in type0 easy to remedy.

Will keep all posted on my progress and decided path for this one.

[Firestorm]

Wow. I love that prior art version of the "candohm" resistor across the back. A lot of radios used candohms and they all shorted eventually. That multitap wirewound could outlive us.

[Structo]

Yep, very cool!

I like the shield between the power tubes and preamp.

I may only alter the preamp to make it more guitar friendly.