Bought a Mojotone kit off Craigslist...

[AlNewman]

Does anyone have a schematic?
 

[thetragichero]

i too love to turn old organs (obtained for the cost of "i will remove this from your property and not charge you a disposal fee") into guitar amps!

[sluckey]

I have a schematic for that little 3 tube chassis in the lower left of your pic.

Radiomuseum.org can probably help with some of the radios.

[thetragichero]

is that the hammond separate reverb chassis with the 6gw8? i turned one of those into a mean little combo with an added 12ax7 and way too much (in hindsight) gain

[sluckey]

is that the hammond separate reverb chassis with the 6gw8? i turned one of those into a mean little combo with an added 12ax7 and way too much (in hindsight) gain

Yes. AO-44...

     http://sluckeyamps.com/PeeWee/peewee.htm

[AlNewman]

Nice, that'd be a neat conversion to play around with.  Most of the chassis aren't great for reuse, but I was eyeballing a few of them.   I have all the amps for that hammond organ, including the leslie speaker amp.  I think there's a few transformers that could be used for different builds.  Quite a few of the tubes have been picked through, but there's some 6v6's left, and some 12a--'s left, quite a few rectifiers and lots of the 6b type tubes.  There's a couple more amps in cabinets that I didn't have room for and need another trip.

But carbon comp resistors, sprague and good-all caps in good values, tag strips, potentiometers, fuse holders, switches, etc, etc, etc.  I'll have lots to keep me out of trouble for a long while.

I wish it was free, but for what I paid it wouldn't have even covered the gas to collect it all, so close enough.

[jordan86]

Any 6K6 push pull amps? I’d be interested in one.

[g-man]

Deleted

[thetragichero]

Looks like a Hammond AO-29 in the chair to the right?

i really hate those ao29 because alls i can do is strip it for parts and build in a new chassis. ao43 can much more easily be turned into a geetar amp

[bmccowan]

Damn, that looks like my basement! I have about a dozen organ amps and maybe 20 old PAs. And that's in addition to many old guitar amps. Many are now working guitar amps, but many are still projects in waiting. Sometimes I just look around and mutter to myself, "you are friggin crazy." Other times my wife comes downstairs and says, "you are friggin crazy."

[kagliostro]

Does anyone have a schematic?

Schematics .. schematics is a big word

If you want that also not old foxes try to help you, please post some sigles, numbers, names, models, all what can help to identify those (appetizing 😋😋😋) chassis

Franco

[AlNewman]

Looks like a Hammond AO-29 in the chair to the right?

Yes, that's an AO-29.  Apparently was in working order when he removed it.  One of the few that has all the tubes still.

Any 6K6 push pull amps? I’d be interested in one.

I might, I know I have an assortment of 6K6, but haven't deep dived yet.  I do have an RCA victor 10-S which looks to be a 6f6 push pull.

Damn, that looks like my basement! I have about a dozen organ amps and maybe 20 old PAs. And that's in addition to many old guitar amps. Many are now working guitar amps, but many are still projects in waiting. Sometimes I just look around and mutter to myself, "you are friggin crazy." Other times my wife comes downstairs and says, "you are friggin crazy."

Unfortunately I don't have a wife to keep me in line.  I got tired of being told I was friggen crazy. 

Does anyone have a schematic?

Schematics .. schematics is a big word

If you want that also not old foxes try to help you, please post some sigles, numbers, names, models, all what can help to identify those (appetizing 😋😋😋) chassis

Franco

Lol, I'll post some pics and model numbers as I get into it.  For now, I'm scavenging a couple of the organ pre amp boards.  It's a humbling experience disassembling these products, so much work and time put into each part.  As well as design, each triode has it's own choke and independant capacitor values, there was thought put into every part.

[Platefire]

   That's a hunk of old iron! When I first got into amp building I was on the hunt continuously. Use to dream of finding an old closed electronic shop full of tube stuff and buying out all the old amps, tubes, parts and tools on the cheap. I did buy out all the NOS tubes left over from a closed Trade School Electronics Shop for $10 once. Only problem was most of them were radio and TV. As it's already been said, there is a ton of work in stripping down these old chassis and re-building them into a working amp. It's a great cheap way of learning how to build them. It's just a lot easier to build new from scratch, just more expensive:>)

[tdvt]

It's a humbling experience disassembling these products, so much work and time put into each part.  As well as design, each triode has it's own choke and independant capacitor values, there was thought put into every part.

I recently stripped out an AO-29 chassis like yours and had the same thought, & even some guilt in not saving too much of it. 

For usable stuff, I ended up with the PT & the tubes, as the OT was no good. Also saved the populated terminal strips but honestly can't see taking the time to save the parts but couldn't throw them away just yet (the guilt-part)

The rest is already gone, other than the "Expression Control" (aluminum box) which looks kind of interesting.

Internally, it looks like an old radio tuning capacitor (with the blades), read that it was an interactive combo volume/tone control. Not sure if anyone has experimented with one of these for guitar.

[bmccowan]

When I first saw your post, I thought you were just joking about another current thread.
Anyways looks like some nice iron - certainly the AO-44 and AO-29. Too bad no AO-39 in the mix, I have used that iron and sometimes the chassis too for a number amps.
What I have found for some iron like some of the other chassis you picked up, is that they are overly hot for a simple guitar amp, and I need to figure out how to tame the B+ voltage. That was usually chassis that had one or more multiple lead plugs that led to some other component. Some of these massive PTs are boat anchors gathering dust.
Besides the typical Fender and Marshall circuits, there are many Valco and Gibson circuits that lend themselves well to old organ/radio/PA iron. I've had lots of fun building old iron into those circuits.

[Platefire]

   One more thing I'll say about old P to P amps. In tearing down these old amps and reusing a lot of the terminal strips, tube sockets and other Misc parts, the thing that stood out to me was the mechanical connections of the wire. They were so tight and solid with such a big blob of solder that it was a real job to de-solder them and disconnect the wire from the terminals. I would dare say that a lot of those old amps would probably work without any solder because the mechanical joints were so tight and solid. I guess you could say in many cases back then, it was over done but truly solid builds in most cases

[AlNewman]

I think it's worth it, not everything will be reusable.  A lot of the caps are only rated for 100-150 volts, even though they are good values.  Not great for coupling caps on most guitar amps, but good for certain applications.  Most of the resistors have fairly useable leads, and they are about 50 cents a piece, for 1/4 watt, and that's if you buy 500 at a time.  The good thing about tag strips is you can set your leads as long as you want.  I'll probably be able to use the chokes and definitely the sockets, as in it will probably be years before i need to buy another socket.

I did one board so far, ended up with about 200 odds and ends, took a couple hours.  Time is relative, I have spent hours going through suppliers websites placing orders, and then still had a large bill, plus waiting for it to be shipped.  I live in the boonies, so there are no options for this stuff other than online.  I will say one thing though, they can shove their rivets right where the sun don't shine.

[AlNewman]

Was just checking online, and it looks like what I thought were chokes are actually maybe frequency dividers?  So probably of no use to me after all.

[bmccowan]

In tearing down these old amps and reusing a lot of the terminal strips, tube sockets and other Misc parts, the thing that stood out to me was the mechanical connections of the wire. They were so tight and solid with such a big blob of solder that it was a real job to de-solder them and disconnect the wire from the terminals.

So true - two things - a de-soldering iron like the Hakko FR-301 works great for sucking out that solder. Expensive but worth it. And, you will be sniffing a lot of nasty stuff, so a ventilation fan at least, or one of the filter fans helps. The same one that Hakko puts their name on can be found for about 50% of their price - various trade names. And I like the nice 5&10W resistors that are found in these old amps - so much nicer than the ceramic bricks made today.

[AlNewman]

So I was tearing down one of the hammond boards, and got to this point, and realized this would be a good foundation to make a test amp to try different tube and voicing combinations, as well as a test bench for the numerous tubes I have in unknown conditions. 

It came complete with 3 different transformers, one was connected to the 2 6v6, and the others were on the solo circuit, so maybe they can be used for reverb drivers.  It has 2 7 way switches, that I can use to make capacitor and resistor farms to jumper in for testing. 

It also comes with a remote power supply, A0-27-1 with numerous high and low positive and negative voltage taps.

[AlNewman]

The rectifier

[Platefire]

  You may have something there as a amp test chassis. That is a big chassis. I tell you what, that old Hammond equipment is a treasure trove of parts. Over the years i've picked up two Hammond PR-40 Organ Tone Cabinets for real cheap. It had loads of tubes, transformers, speakers, tube sockets, terminal strips and on and on. I even convertedone of the PR-40 for guitar. It is to huge to carry around but I left set up at my house for about a year and finally parted it out. Platefire

[AlNewman]

  You may have something there as a amp test chassis. That is a big chassis. I tell you what, that old Hammond equipment is a treasure trove of parts. Over the years i've picked up two Hammond PR-40 Organ Tone Cabinets for real cheap. It had loads of tubes, transformers, speakers, tube sockets, terminal strips and on and on. I even convertedone of the PR-40 for guitar. It is to huge to carry around but I left set up at my house for about a year and finally parted it out. Platefire

One side is just a plate with brackets, so I think once I decide how much room I need I can pare it down.  I think maybe I'll just start by running heater wires.

[Platefire]

On my Plexi, pictured below. The power transformer and choke came out of a Hammond PR-40 Tone Cabinet.

[AlNewman]

That's pretty sweet.

[Platefire]

DummyLoad had a pretty elaborate amp test set up. If your serious about making that chassis into an amp testingchassis, I would recommend you touching base with him

[AlNewman]

DummyLoad had a pretty elaborate amp test set up. If your serious about making that chassis into an amp testingchassis, I would recommend you touching base with him

I wonder if there's a thread here on it?
Got the heater wires in.  Anybody see anything that might cause issues so far?  Also, all the sockets were grounded together, is that necessary, or even recommended?

[scstill]

On my Plexi, pictured below. The power transformer and choke came out of a Hammond PR-40 Tone Cabinet.

I learned in this forum that it is good practice to mount transformers so their coils are not in the same plane.
Was there any noisy (or other) issues with the way these transformers are aligned?

[Platefire]

scstill---no noise problems with the Plexi

[HotBluePlates]

... What I have found for some iron like some of the other chassis you picked up, is that they are overly hot for a simple guitar amp, and I need to figure out how to tame the B+ voltage. That was usually chassis that had one or more multiple lead plugs that led to some other component. Some of these massive PTs are boat anchors gathering dust. ...

... came complete with 3 different transformers ... maybe they can be used for reverb drivers. ...

I think about this differently.

Rather than, "I can pull a bunch of parts to build an amp cheap," I see it as, "These parts are right-size to build ______, so is ____ pretty close to being a guitar amp already?"

Conversions from whatever are more successful when you have a power supply & output section (including phase inverter) that can be left as-is, and you only need to graft a preamp with the desired features.

[bmccowan]

Conversions from whatever are more successful when you have a power supply & output section (including phase inverter) that can be left as-is, and you only need to graft a preamp with the desired features.

I think that is true. That's why some organ amps and most old tube PAs work so well as guitar amp projects.

[AlNewman]

... What I have found for some iron like some of the other chassis you picked up, is that they are overly hot for a simple guitar amp, and I need to figure out how to tame the B+ voltage. That was usually chassis that had one or more multiple lead plugs that led to some other component. Some of these massive PTs are boat anchors gathering dust. ...

... came complete with 3 different transformers ... maybe they can be used for reverb drivers. ...

I think about this differently.

Rather than, "I can pull a bunch of parts to build an am cheap," I see it as, "These parts are right-size to build ______, so is ____ pretty close to being a guitar amp already?"

Conversions from whatever are more successful when you have a power supply & output section (including phase inverter) that can be left as-is, and you only need to graft a preamp with the desired features.

I missed that particular statement by Bcmcowan. 

Here's a link to the service manual on this line of organs... 

https://ia600301.us.archive.org/12/items/HammondChordOrganServiceManualModelsSS-1S-4/HammondChordOrganServiceManual-ModelsSS1S4_text.pdf

Pages 53-54 of the pdf show the power source, power section, and the solo driver section (where the other transformers are.)

I'm not familiar with this particular phase inverter, it looks similar to a bassman?  There look to be multiple different power sources, based on the version, but they seem to range in B+ voltages from 285-330v.  I'm not sure which one I have, I need to replace the power cord and test it, I figured because it was built with 6v6, I should be able to use it again?  I wouldn't mind using the PI as is, although I'm not familiar with it, I was thinking of converting it to a LTP, which I am more familiar with.

As much as I am trying to build an amp on the cheap, I am also trying to repurpose something that was otherwise destined to a landfill.  In retrospect, I probably should have left some of the wiring intact on this chassis, but I did not have an idea to use it otherwise until I already had it stripped basically. 

[PRR]

...complete with 3 different transformers, one was connected to the 2 6v6, and the others were on the solo circuit, so maybe they can be used for reverb drivers.

The two 6V6 OT is fine (tho impedance is not marked?). The 'solo' iron is not reverb-suitable, it is a variable gain amplifier. Could be tremolo, could be compressor. Probably very excellent for those uses. But a full push-pull variable-gain amplifier is a LOT to add to a stage amp. Complexity, weight, power.

The power-amp driver is a fix-bias long-tail. The main input is super high impedance only to suit the "Expression Control", a different sort of wah-pedal, which I suppose you didn't get? R289 can be 1Meg.

The power supply NEEDS the field-coil speakers' field-coil (or equivalent power resistor) to make decent voltages. Without, you'll have >450V instead of 360-ish. Also to get the several negative biases. This is "why" it would not have made sense to "keep the power sections stock", which is other usually good advice (just tinker the NFB, don't rebuild).

[tdvt]

The main input is super high impedance only to suit the "Expression Control", a different sort of wah-pedal, which I suppose you didn't get?

PRR, have you ever seen one of the Expression Control units repurposed or adapted to guitar use?
That was one of the pieces I DID keep while parting out am AO-29 a month or so ago.

[PRR]

> have you ever seen one of the Expression Control units repurposed or adapted

Nope. Only glanced at it. Who knew the unobtanium part would become a glut on the market?

The actual control doesn't have electrical connection. It must catch signal by capacitive proximity. It cross-fades from LOUD direct connect to SOFT a nest of tangled Rs and Cs, obviously a tone shaper. perhaps in the direction of the '5F6a' tone control at customary knob settings: some mid-dip to put the tone "back", with bass and/or treble bump to stay in the picture? Someone with more curiosity can throw it in SPICE. Remember that solo organ is musically different than guitar in a band, so it may be "not-right" in Bakersfield or Memphis.

EDIT: lost image reappears in post #37 below.

[AlNewman]

Thanks PRR, all good info.  I may already be hornswaggled though, I hooked up a new power cord to the supply and am getting some strange readings.  I couldn't get any readings in dc, the meter couldn't lock in, I thought maybe it's because there's no filtering, or the rectifier is bad, although it did heat up, and the 5v supply is good.  The AC readings I got were strange though, Pin 4 reads 73 volts, pin 6 reads 501 volts.  Which does add up to 285/side, but I'm wondering if I have a short in the windings, or am I missing something?

I do have the expression control, I also have another on the AO-29 chassis, it looks like it might be neat to play with if I ever get anything working.  I'll do some research on the PI, thanks for clarifying the type.  I've got a good tap now I can measure the output transformer and figure out the impedence.  So if I wanted to lower impedence on the PI, I could reduce the value of the grid leak resistor to something like 1M rather than 5M to make it suitable without the expression control?

Edit:  I was able to get ac and dc measurements measuring to the center tap.  Good steady 330v dc from B+ to B-. 

[tdvt]

This is one of the descriptions I found out there regarding the "Expression Control" at Benton Electronics


"In the expression control circuit, you will see there are two plates shown after V4(a), and a third movable plate shown going into the next stage of V4b. The higher plate is connected directly to the output of V4a, while the lower plate has some RC (resistive/capacitive) circuits shown. If the movable plate is closer to the higher stationary plate, the volume is louder. As it moves to the lower stationary plate, it gets softer. Because of the RC network, the signal also gets a tone change. In other words, the high frequencies are attenuated faster than the lower frequencies. This means at lower volumes, the bass will actually remain more present than the highs. Since we hear different frequencies differently at different volumes, (see fletcher-munson curves) this acts like a loudness curve in a home stereo."



There were one or two more, but I didn't bookmark them when I was looking at all this stuff weeks back

[PRR]

Ah, my planned image musta got stuck in a snow bank.

[AlNewman]

Ah, my planned image musta got stuck in a snow bank.

You guys have winter down there?  We had snow for about a week through Christmas, but the winter is still young.

So I guess the 5M resistor gets bridged with the 1000pf cap?  I didn't even realize the pink area was the schematic for the expression unit.... or at least I think it is, now that you've outlined it.  It looks a lot like Fender's later LTP set up, if you just flip it on it's side.  So is this PI setup fairly similar to the Princeton?

I'll still be a while before I can set it up, I'm trying to figure out a grounding layout.  I think I'll run 2 bus lines at the top and bottom of the chassis, and have them meet at the front of the preamp.

[sluckey]

So is this PI setup fairly similar to the Princeton?

It's nothing like the Princeton PI. The Princeton has a cathodyne phase inverter, AKA the 'split load' or 'concertina inverter'. This PI is a fixed bias LTP phase inverter more similar to the bootstrapped LTP found in AB763 amps.

[Lectroid]

i really hate those ao29 because alls i can do is strip it for parts and build in a new chassis. ao43 can much more easily be turned into a geetar amp

I second that.  The PT can put out 440V with aSS rectifier and the OT is huge.  In the house, my A0-43 DR clone is just too damn LOUD anywhere past 3.

[tdvt]

The Hammond "Expression Control" unveiled

[AlNewman]

The Hammond "Expression Control" unveiled

Haha, I just took mine apart too.  Not much to it.

[AlNewman]

So is this PI setup fairly similar to the Princeton?

It's nothing like the Princeton PI. The Princeton has a cathodyne phase inverter, AKA the 'split load' or 'concertina inverter'. This PI is a fixed bias LTP phase inverter more similar to the bootstrapped LTP found in AB763 amps.

Good to know.

[PRR]

The Hammond "Expression Control" unveiled

AH! Not what I pictured. There can't be 50pFd in that box. It is a differential capacitor from some VHF radio test gear. It with box might even have been commodity, or overstock, surplus from military or industrial.

> You guys have winter down there?

I may be north of you.
2nd athletic facility dome in Maine collapses under weight of wet snow

[AlNewman]

Wow PRR, luckily nobody was injured.  I'm still a little north, and a long ways west. 

So I took a break from Frankenstein today and pulled this guy off the pile.  Westinghouse RA-513 from the 50's.  Plugged it into my current limiter, and didn't light up the house, so I went into mains and got some decent readings.  B+ was 288, Plate of 6k6 was 258.  Cathode resistor had drifted from 300 ohms to 400 ohms, so between that and the 70 year old caps, it was pulling 34 mA.  Not sure what the screen would reduce that by, but as it sits, it was around 9.5 watts, or just a wee bit hot.  The impedence ratio of the OT came in at 2300 ohms, so I should be able to run a 4 ohm speaker at about 3-4 watts.

Looks like a good contender for a champ style build to me.  Maybe I can even use up some of my 6v preamp tubes.

[AlNewman]

.

[bmccowan]

"Also on Thursday, the Mahaney Dome at the University of Maine in Orono collapsed under the weight of the snow and rain that was dumped over the state on Thursday. It is at least the fourth time that the athletic facility has collapsed since it was installed at the university."
Yikes - I'm almost embarrassed to admit that I graduated from U Maine - the thing has collapsed 4 times?! slow learners.
I live a few miles from the Topsham facility. That snow/rain was brutal. I want the old Maine winters back - snow from November till April. No damn rain and slush.
All octal 6 volt champ type - should be fun. Good examples out there with 6SC7, 6SL7, 6SJ7 etc. The Valco and Gibson circuits are sweet.

[scstill]

...The impedance ratio of the OT came in at 2300 ohms, so I should be able to run a 4 ohm speaker at about 3-4 watts.

The 6k6 spec sheet says 7600ohms load at 250v as a pentode
Maybe a 8 or 16ohm speaker would be better

But if you are running as triode it says 2500ohms

[AlNewman]

"Also on Thursday, the Mahaney Dome at the University of Maine in Orono collapsed under the weight of the snow and rain that was dumped over the state on Thursday. It is at least the fourth time that the athletic facility has collapsed since it was installed at the university."
Yikes - I'm almost embarrassed to admit that I graduated from U Maine - the thing has collapsed 4 times?! slow learners.
I live a few miles from the Topsham facility. That snow/rain was brutal. I want the old Maine winters back - snow from November till April. No damn rain and slush.
All octal 6 volt champ type - should be fun. Good examples out there with 6SC7, 6SL7, 6SJ7 etc. The Valco and Gibson circuits are sweet.

Betcha it looked cool though.  They probably saved a few bucks on an engineer by having a competition for the students to design the facility.  In Canada building codes are the same through the entire country for snow load.  Then they designed a suspension bridge in Vancouver that bombs vehicles with falling ice every year.  Looks cool though.  Progress.

I wasn't even thinking of using the octal sockets.  Good point though, I could.  I stripped down the chassis, I might be able to use it still, but I'd want to plate over the top.  I probably won't use can capacitors, and won't need as many sockets, also there's radio tuners sticking out as well, it's like swiss cheese....so I'll probably just redesign.  I was looking more into the miniature 7 pin sockets maybe, I have a bunch of 6ac4, 6av6, which I think could maybe work.  I could just maybe use a 12ax7 like the original too.  There's all sorts of 6be6, 6ba6, 6au6, etc.  I doubt I can find a use for all of them.  The Gibson/Valco idea is interesting, I'll have to look at some of the schematics.