Heathkit SD-1 OT question also 6DZ7 = 2 x EL84 in one bottle..neat!

[Toxophilite]

Hi
A friend gave me this cool little heathkit SD-1 stereo hifi amp
i have not been successful finding a schematic


A Neat configuration using 3 12AT7 preamp tubes , a 6DZ7 power tube , and an SS rectifier.
The 6DZ7 is essentially 2 El84s in one large octal bottle, Cool! However as it is very rare at one time in the distant past someone built a 2, 9pin conversion into a defunct octal socket and used 2 6CM6 tubes which is very similar to a 6V6 except in a 9 pin bottle except cheaper. also cool!
I think it has a stereo OT which is kind of interesting too


Anyway my question is in regarding to the extra transformer . I though it was a choke and perhaps it is, though it has 4 wires coming off it
It's sitting between the two larger ones with bell caps
All the transformers are made by Chicago Standard with Heathkit codes PT 54-82  OT 51-41  ?T - 50-41


It really looks like a mono OT. however the red wire on one side hooks up to B+ whilst the blkish wire from the same side joins to the red wire coming from the primary of the stereo OT (which makes me think choke)
But then there's the two wires from the other side (green and Black)
The green wire hooks to the common ground of the speaker outputs (one common, a left, and a right screw pole)
The black wire hooks to what looks like the ground for the stereo OT secondary AND the inside wire of a coax coming from the input selector switch (stereo tuner, mono tuner, stereo phono, mono phono) The ground of the coax goes to the common ground of the speakers mentioned earlier


Any idears??

[DummyLoad]

fairly  certain that's  a mono amp. i have some of those tubes. cool tubes.

--pete

[Toxophilite]

Hi Pete
Thanks for your response


Though I have to respectfully disagree


It does say "Heathkit Stereo Amplifier" right on the front


There are separate left, right and common screw taps for the speaker wires


and as I stated in my posting there is a selector switch for stereo and mono (though that in itself isn't conclusive)


However more telling (to me) is the way the OT has 3 wires on the secondary. yellow going to left, green going to right and a black wire going to ground


Also here is the Heathkit label on the OT


They are interesting tubes!
What makes you sure this is a mono amp??
I'm curious


Also any other takers on the 3rd transformer question ?

[Toxophilite]

Oh and even more telling I was playing "Bim Bom" by Joao Gilberto through it and though it's a little distorted there's very definite stereo. Being an older stereo recording on headphones this song has the hi-hat very definitely on one side and the guitar on the other.
Oh and Mea Culpa! I probably confused the issue by saying the 6DZ7 was used in pushpull..wrong!

[DummyLoad]

ok i see now. that OT is a first for me - it's two SE amps with a single 6DZ7 and they use a singe OT with 2 winding pairs that are a separate winding for each channel. that things gotta have some channel cross-talk.


love the rig to emulate a 6DZ7 with 2 6BQ5. buy the correct tube and do away with the shock hazard?


http://www.ebay.com/itm/Vacuum-Tubes-6DZ7-MOTOROLA-1-NIB-NOS-3-available-made-by-GE-/360865068150?pt=LH_DefaultDomain_0&hash=item54053bf476



--pete

[Toxophilite]

It is an unusual setup.
That's one reason I posted it
I also wanted to find out what the other transformer was doing, maybe it's doing double duty too
At this point I won't buy a $30 tube . I have a line into a local guy with reasonable prices, he might have one.We'll see, it would be nice to have one, though.


I had also contemplated converting it for guitar (mono with a different OT) or making it into another reverb unit
It seems a shame as it'd be a cool little stereo if it were recapped and refurbished

[Toxophilite]

Nobody can tell me what the 3rd transformer is for?

[sluckey]

Nobody can tell me what the 3rd transformer is for?

Give us some resistance readings for the windings. Draw a schematic. Put some voltage readings on it.

[DummyLoad]

i think this is how the PS is wired - please see schematic attached... at least that's what i see.

i believe that this is a delon FW doubler and that the capacitor "C1" has an internal link from C1a (-) to C1b (+) - at least i think that's what the wire exiting the tab hole and disappearing under the phenolic wafer is for.

odd wiring of the secondary common via the inductor: perhaps it's some sort of hum bucking technique?

an odd little thing for sure. at least, not mainstream in configuration of the physical parts.

--pete

[Toxophilite]

Nice!
Did you draw that up?
Thanks
I have illustrator on my other "music" computer but nothing specifically set up for schematics
THat's it all right
I have 370 vdc on the red lead connecting to C2A and the 'tru ohm' resistor
And 365vdc  on the blk lead that connects to the red lead of the OT
Between the red and the blk lead mentioned about I get 118 ohms resistance
The other 2 (green and black)seem to give me 2 ohms 9which could be 0)


oh and on the green lead attaching to the speaker common ground I get 2 mv

[jazbo8]

Amazingly, there does not appear to be a copy of the SD-1 schematic anywhere, not even at some of the Heathkit archives that I checked. If you can trace out the circuit and post the schematic, that will be great!

[kagliostro]

If you can trace out the circuit and post the schematic, that will be great!

ditto

If you have problem with CAD schematics software, draw it by pencil

I'll draw it with a CAD for you

K

[Toxophilite]

Yes I looked for the schematic
I can't get any of those programs to be happy on my mac though I
ve tried a few variations
I'll try to map it out by hand though there's a little spaghetti of blue and grey wires going to the input selector switch that gives me the willies
I'd hoped that the power section as drawn out by Pete would give an indication of what the extra transformer was doing, One part of it seems to a choke , or at least that's how it looks to me .

[DummyLoad]

TXo try the link below. a good app. somewhat of a learning curve, but has the common tubes in the symbols library.

http://www.kicad-pcb.org/display/KICAD/Mac+OS+X

jschem is similar to express sch. and will import express sch files - binaries for MacOS in link below.

http://dhost.info/jschem/

--pete

[kagliostro]

An easy CAD for schematics is also FidocadJ, MAC version is present (and have tubes on library)


http://fidocadj.sourceforge.net/download.html


K

[jazbo8]

While we wait for the scheamtic to be traced out, I think it is pretty clear from DL's drawing that the SD-1 uses a matrix stereo output with a main OPT and a "maxtrix output" OPT. A rather rare and interesting design for sure.

[Toxophilite]

I just got Jschem and I
I'm figuring it out, pretty handy with the library of tubes etc
Might take me an eve though as I've never used it before and the results might be a little funky
It seems to have mostly european designations(which I don't mind) but I'll try to fake up  stereo OT




Can you enlighten me regarding " uses a matrix stereo output with a main OPT and a "maxtrix output" OPT'


What is a "matrix output" OPT?
I'm curious about the matrix part

[jazbo8]

The maxtrix stereo design was popular in the late 50's, basically, it works like this - one channel of the stereo is put out of phase with the other, then both channels feed a differential amplifier, the L-R and the L+R signals are re-combined by the OPT and the matrix output transformer (which is the name of the 50-40 in Heathkit catalog), so that allowed the manufacturers to make/sell stereo amplifiers "for the same price" as the mono push-pull amps. One such example was from Westinghouse as shown below, which used the same chassis in numerous stereo consoles and radios back in the day.

https://dl.dropboxusercontent.com/u/1326040/WH_H-F1010A.jpg

[Toxophilite]

How strange and wonderful is that.
I don't really need a stereo amp unless it's a pretty decent one(to play mixes through normal stereo speakers rather than studio reveals)
Is this a less than ideal stereo amp then?
THere does seem to be a little 'crosstalk' but it could be a dirty mixing pot
As is it would need a recap and likely the odd resistor replaced to function without distortion
I generally don't like to hoop decent stereos but if I only have one mono amp I don't mind converting them into guitar amps .


For interests sake I found this neat article where a fellow made one of these units more efficient
http://diyaudioprojects.com/mirror/members.aol.com/sbench/1060amp.html

[jazbo8]

It's a great find alright, but there are better candidates for conversion into guitar amp imo. If you want to mess with it, then you can follow Bench's design in his article to upgrade it. But I think the amp might be worth more to a collector, so I would just restore it to the original 6DZ7 output and leave it as is.

[kagliostro]

Very interesting explanation Jazbo8

Many thanks

Franco

[Toxophilite]

Funny
My friend took it out of the small mahogany veneer console cabinet as he wasn't about to use it. it was built by a ham operator in the small town of Barhead(yes barhead!) Alberta for my friend's Wife's Dad.
The little  stereo amp was running into a heathkit crossover and then both channels were run into a single Jensen P8-QF woofer!!(Dated 43 week 1959)
NO tweeter, no other cab apparent (or if so it got lost or thrown out)

I guess I could restore it for them but the tone control circuit has these multi combo resistor cap components that are baffling unless you have the schematic, 10-1 it's a baxandall anyway


A plan for a conversion would involve a different OT (Of which I have a few pushpull EL84/6V6 types) and a rewiring of course


The other option was a revibe or another 6G15


hmmm

[kagliostro]

If you convert it (in a guitar amp) why don't use the original OT ?


Franco

[Toxophilite]

I was somehow under the (perhaps mistaken)impression it was a stereo OPT
But this talk of the matrix stereo leads me to believe it's just a conventional pushpull OPT used in a non conventional manner? yes no?.it does have an extra tap on the secondary side. Do I just ignored what seems to be a center tap (hooked to the other OT) and use the wires for left and right?
Sorry if this question seems a little pedantic


Curiously
The OPT on this one is bigger than the PT
I get 190 VAC coming off the PT and 380 VDC off the rectifier diodes  which doesn't seem like tonnes

[jjasilli]

Fascinating thread!  Never heard of matrix stereo.  As a hi-fi amp this puppy will likely have limited frequency response, and cross-talk.  A rebuild may defeat cross-talk, but there's still the frequency response issue for these pre-60's amps.  OTOH as an As-Is stereo amp for el guitar, these hi-fi limitations should not matter.

[jazbo8]

I was somehow under the (perhaps mistaken)impression it was a stereo OPT
But this talk of the matrix stereo leads me to believe it's just a conventional pushpull OPT used in a non conventional manner? yes no?.it does have an extra tap on the secondary side. Do I just ignored what seems to be a center tap (hooked to the other OT) and use the wires for left and right?

No, you are correct, there is nothing special about the main OPT, you can use it for normal output, but you first need to modify the other parts of the circuit to make it work as a mono amp/guitar amp. The center tap of OPT can be used as the 4R tap (assuming the full seconary was for a 8R load). If you really want to modify it, then you need to trace out the circuit so you know exactly what it is currently.

[Toxophilite]

Cool
I've done a few successful conversions and a couple strip and complete rebuilds too
THanks

[DummyLoad]

http://tubedata.milbert.com/sheets/127/5/50FY8.pdf

--pete

[PRR]

> Never heard of matrix stereo.

It was not-popular for about a year and then vanished.

IIRC, Columbia had the idea. Their stereo cartridge could be wired L+R or M+S. On the records of the day, the M (mono sum) signal was usually larger than the S (side differences). So they ran two tubes push-pull for the M signal, the same two tubes SE for the S signal, and extracted L+R with clever secondary connection. Musta saved a few bucks on a complete Console Stereo Phonograph.

Heathkit took a license and this is the model. Problem is that if you must take conventional L+R signals then you need to phase-invert one of them to get the M+S signals. That takes much of the savings out.

"The single chassis amplifier separates the individual stereo channels by utilizing the sum and difference of the total signal and directing the sound to the appropriate right and left channels, reproduced by the stereo wing speakers. The centrally located woofer reproduces the non-directional bass frequencies. The result of this modern stereo reproduction is a breathtaking experience of sound coming to you with depth and direction seldom achieved by conventional stereophonic methods..."

Also the art of stereo grooves was improving and you could get quite large S signals which would overload the SE path in a strange way.

Channel separation on the HeathKit was claimed better than 30dB over most of the upper audio band. Since phono needles of the day struggled to get 20dB, this was (and still is) ample.

You should have got a whole console and spinner for $180 in 1959:

[jjasilli]

Fascinating stuff.  Those Heathkit wing speakers look like a precursor to later Bose designs!


Problem is that if you must take conventional L+R signals then you need to phase-invert one of them to get the M+S signals. That takes much of the savings out.


I was wondering about that.

[ke4mcl]

Joined up just because of this thread. I recently bought a heath sd1 off eBay merely for its novelty. I have a big collection of tube amps but this thing has got to be the oddest. As weird as it is, its surprisingly fun on a pair of altec VOTTs. The power TX runs quite hot so I have a fan on it.

Being a sucker for weird amps i'm now on the lookout for a second one. That OPT is unobtanium and it would be a big bummer if it quit. Its not like I can drop an edcor in there.