Help me convert this Vintage Organ Amp for guitar?

[OwnGoal]

Hi Guys,
 
 I recently acquired this nice little amp from a 1961 Gulbransen G-3 organ - for the staggering price of completely free. (Naturally, it came with 300 lbs of organ, but that's another problem..)  It has a pretty attractive tube line-up, and like your typical organ, really heavy iron.   But unlike most organ makers, the Gulbransen company built discrete power amps that could literally be unplugged from the organ, unbolted, and pulled right out.
 
 So that's what I did:



First,  a schematic of the unmolested amp, and Second what I've done to it so far. Obviously, I'd rather use 12AX7's in the preamp.  The 12AU7's are too wimply.  Do I need to make mod's to accommodate 12AX7's, or can I just sub them in?  I've already played the amp with a pair of 12AX7's in place.  It sounds great, but I'm getting a sizzling sound at higher volumes, or if I have the tone pot turned halfway up. Haven't replaced the electrolytics yet so I'm not sure if that sizzle is due to bad caps, or to my bad design.

PLEASE NOTE:
The first schematic is provided for reference ONLY!  I have already removed the expression control circuit and the 500k volume pot at the input.  I've rewired the amp as shown in the hand drawn schematic.

Take note of the unused half of the second preamp tube.  I'm wondering if I should add another stage using it, and how to go about that if I do.  I'm sort of a noob, still running with scissors, but I'm beginning to get a general idea of how these things work.   
 







I'm using the amp to drive the Leslie unit from the organ, so I'm keeping the selenium rectifiers and all the weird stuff at the bottom of the schematic, because that's what the amp uses to power the Leslie, and the relay used to switch the Leslie motor on and off.  I just deleted those taps from my schematic for clarity.

I've cut all of the organ away from it's speaker cab, and the amp bolts to the top of what's left.



Drinking and modding:



Here's the obligatory gut shot, prior to modding:


 

[kagliostro]

Nice Stuff 

Long time ago I've seen those expression control circuit in one other organ amp schematic

But I'm not able to remember how it acts 

Please someone can re-explain it ?  

Only I remember that there are varistors and seems to me that the variation on voltage supply change the conduction varying the capacitance of the circuit and so the tone, I'm I wrong if I say a tele acted ToneControl ?

More, I don't remember if that is something we would like to experiment in a guitar amp because new varistors are different from old one and are very difficult to be find (see Magnatone Vibrato varistor continue research)


Do you think that a new varistor can be tried with tat circuit ?


(I like old circuit stuff  )

Franco

[terminalgs]

I'm not able to remember how it acts    Please someone can re-explain it ?     

my stab:


expression pedal varies the VDC that those VSRs see.  As VDC on the VSRs varies, their impedance varies. Thus, the network of caps and VSRs will vary the frequency dependent impedance that audio frequencies encounter as the pedal position varies.  some frequencies pass the network heavily attenuated, some not at all, which vary with movement of the pedal.


One big difference to realize between a guitar amplifier and a organ amplifier is the engineer of the organ amplifier has a defined set of frequencies that will arrive at the first triode.  The guitar amplifier will see all kinds of frequencies, depending on the musician's playing style (harmonics, feedback swells, etc).   Whether or not the guitar amp engineer takes (or took) that into account is irrelevant, we know it to be true. So, tone stacks, impedance devices, etc on organs may/may not work well for guitar.


Owngoal: pics are great! I dig the project. my guess is the sizzle is maybe a cap like you are thinking. you might try a parallel 10uf with alligator clips to different filter caps to narrow it down.  Sometime tubes cause those kind of sizzles too, so if you've got some spares, swap'm.

[eleventeen]

Nice project. If "sizzle" is your complaint, I would without hesitation blow the $1 necessary to replace the 2 qty 100K plate resistors on the first 12AU7 and probably the 22K plate resistor on the phase splitter with metal film oversized (3-watt) ones like Doug sells. (The 22K is probably not a problem but I'd replace it just on a shotgun basis)


Older carbon comp plate resistors are a perennial source of "hiss" in old Fenders, and the difference they can make is borderline miraculous. They will likely make less difference than they would in an old Fender, because you have fewer gain stages, but the cost:improvement ratio can't be beat.


I would consider trying a 12AY7 or 5751 or even a 12AT7 for your first tube just for giggles, for a little higher gain. All those tubes have the same base diagram so you can just throw them in and see if you like the results. You'll find very few amps that have a 12AU7 for an input stage, you see it here because the old organ tone generators are of course electronic and probably put out 10x or 20x the signal level of a guitar. So I am suggesting a painless way to see if you might want more first-stage gain. It wouldn't surprise me to have that work nicely. Maybe a 12AX7 (the highest gain sibling in that family) will be too hot, then again. maybe you'll like it. Free and easy to try, if you have some of those tubes laying around. 

[OwnGoal]

...I would consider trying a 12AY7 or 5751 or even a 12AT7 for your first tube just for giggles, for a little higher gain. All those tubes have the same base diagram so you can just throw them in and see if you like the results. You'll find very few amps that have a 12AU7 for an input stage, you see it here because the old organ tone generators are of course electronic and probably put out 10x or 20x the signal level of a guitar. So I am suggesting a painless way to see if you might want more first-stage gain. It wouldn't surprise me to have that work nicely. Maybe a 12AX7 (the highest gain sibling in that family) will be too hot, then again. maybe you'll like it. Free and easy to try, if you have some of those tubes laying around.

 

I've already tried a 12AT7, it's better than the 12AU7, but the amp really seems to want the 12AX7. (I've got loads of them here but not a single 12AY7 in the house!)

I'll swap in the resistors as you suggested.  The stock plate resistors are rated for 1/2 watt!

Should I leave the 12AU7 in the second socket?

Should I rework the circuit in order to use the un-used half of the second preamp tube?

[eleventeen]

Do the resistors, esp the 100Ks, don't overthink that one.


You can always try the 12A_7 roulette, no harm, no foul.


It's not entirely clear to me which of the two little tubes has "the unused half". If it's the phase splitter that is only using half of a 12AU7, more gain there could overdrive the output tubes (on a technical basis) and maybe you'll like that.


Ponder this table: http://www.diyguitarist.com/GuitarAmps/SD-Convertible/patgf1.htm


This compares the relative gains of tubes in that family. If you have no 5751 and no 12AY7, ain't gonna kill you. A 12AT7 is pretty close to either, but 12AY7s have a certain something to them that many people like. The important thing is that 12AU7s are by far the lowest gain items. The curse of the organ dismantler is that you generally end up with dozens of them and they are (to me) sluggy, and very few amps use them, though some folks use them to dial down the driver tube in Fender reverbs: An excellent use, usually results in being able to use the entire range of the "reverb" control, which most can not turn up over "4".  But I'd be on the lookout for one of those 12AY7 in your wanderings if you like to experiment.

[jjasilli]

How are you using this amp?  If it’s to get signal from another amp or preamp, it might be usable as-is to function as a slave or power amp for the Leslie. If a standalone amp it could use a tonestack, likely between the two gain stages – essentially Princeton topography. 


IMHO, the cathodyne PI is fine for clean tone –another Princeton-like feature -- but I’m personally not a fan of an overdriven cathodyne PI..  There seems to be an unused ½ of a 12au7 tube, so an LTPI is an alternative.


BTW:  This amp has high quality trannies.  Probably the OT is full-range, so it’s also a candidate for hi-fi use.  The speaker also may be wider range than a typical guitar speaker.  This might make hi-frequency hiss more noticeable.


The electrolytics look like they need replacement.  Note that the cardboard-covered cans are LIVE and dangerous if uninsulated.  Hence the cardbaord & mounting brackets.


Operationally it should be OK to swap-in 12ax7's.  But tonewise the two gain stages have starvation plate voltages for a 12ax7.  This results in low gain and dark tone for 12ax7's. 

[OwnGoal]

...It's not entirely clear to me which of the two little tubes has "the unused half". If it's the phase splitter that is only using half of a 12AU7, more gain there could overdrive the output tubes (on a technical basis) and maybe you'll like that...

The second tube has the unused half.

 Frankly, I don't know which is the phase splitter of phase inverter.  I get the concept, sort of, but I can't ID it in a circuit yet.

[OwnGoal]

Operationally it should be OK to swap-in 12ax7's.  But tonewise the two gain stages have starvation plate voltages for a 12ax7.  This results in low gain and dark tone for 12ax7's.

Take a look at the plate voltages on my hand drawn schematic.  They have increased radically since I cut out the expression control circuit, and the 500k volume pot at the input.

Plate one is at 184v.  Plate two is 205v.

[kagliostro]

I don't know which is the phase splitter of phase inverter.  I get the concept, sort of, but I can't ID it in a circuit yet.

look to the schematic, the 12au7 triode that preceed the power tubes is the PI in this amp the PI is what is called a Concertina or Split Load

a LTPI (Long Tail Phase Inverter - Look to a Vox AC30 schematic) is a kind of PI that uses a pair of triodes instead of one triode

Franco


p.s.: I have not established exactly if they are usable, but don't waste the two vactrols, there is the possibility they can be used in a Vibrato circuit a la Magnatone (I'm not sure but may be)

EDIT: many thanks for the explanation Terminalgs

[OwnGoal]

How are you using this amp?  If it’s to get signal from another amp or preamp, it might be usable as-is to function as a slave or power amp for the Leslie. If a standalone amp it could use a tonestack, likely between the two gain stages – essentially Princeton topography. 

Using it as a stand alone amp.  I've already put in a tone stack.  See the hand drawn schematic in the OP.  I used the simple design from the 5e3 Deluxe. Considered the Princeton circuit, but I didn't have the materials on hand. 

IMHO, the cathodyne PI is fine for clean tone –another Princeton-like feature -- but I’m personally not a fan of an overdriven cathodyne PI..  There seems to be an unused ½ of a 12au7 tube, so an LTPI is an alternative.

I'm a noob.  I was hoping someone could sketch in a LTPI for me using that unused 12AU7 half.  I'm not sure how to do it..

BTW:  This amp has high quality trannies.  Probably the OT is full-range, so it’s also a candidate for hi-fi use.  The speaker also may be wider range than a typical guitar speaker.  This might make hi-frequency hiss more noticeable.

Curious: How can you spot the tranny quality? The speaker is a 12" CTS.  I think it's an instrument speaker, but can't be sure. How to tell?

The electrolytics look like they need replacement.  Note that the cardboard-covered cans are LIVE and dangerous if uninsulated.  Hence the cardbaord & mounting brackets.

Yeah those big caps in the organ control circuit pack some zaps.  I've been religious about draining them and they are MISTER SPARKY. I will be replacing the signal electrolytics asap.

I'm still using that huge organ control circuit to operate the little relay that switches the Leslie motor.  I'd really love to simplify it, but it uses a pair of selenium transistors, and I don't have a clue what those even are.  I suspect I could get rid of most of that stuff but I don't know how to do it.  I need to keep the -22v connection at (pin #5) for the switch. There's also a 17 VDC brake that slows the Leslie when you turn it off.

[eleventeen]

" Frankly, I don't know which is the phase splitter of phase inverter."


It's the one which directly feeds the output tubes. The one not connected to the tone control(s). It's perfectly logical that the preamp tube (which you've correctly identified) which uses the two halves of [the same] 12A_7 would be in one tube and the phase splitter/inverter is in another one. 


Being lazy, I would definitely exhaust the possibilities of swapping preamp tube types around before doing much deeper work.


I guess you could always try to get another gain stage in there....meaning, another triode section (half a tube) wired up with R's and C's and fed B+, etc; The idea of an unused triode is of course almost intolerable to anyone on this board!


This could be a teeny bit problematical in the sense that, if for some reason you can't use a 12A_7 (where 12A_7 is one of those tubes OTHER THAN a 12AU7) as the phase splitter because it overdrives the output tubes in a way you dislike....then you become stuck using the other half of that same 12AU7 as your extra gain stage and it's a mediocre tube for that application. In that case, you might change to one of the higher gain tubes and have to change a part or two in the phase splitter to now REDUCE its gain while using the higher gain tube. I am sure we could help you do that.

[OwnGoal]

I guess you could always try to get another gain stage in there....meaning, another triode section (half a tube) wired up with R's and C's and fed B+, etc; The idea of an unused triode is of course almost intolerable to anyone on this board!


... I am sure we could help you do that.

Yes, please.  And are you talking about converting to a long tail pair?

[kagliostro]

Or you can arrange an oscillator for tremolo


but, just a moment, you want to use the amp with the leslie, so ..... a tremolo isn't a good choice


K

[PRR]

> cathodyne PI is fine for clean tone

Ampeg VT-40 is cathodyne. Get a couple of these in a garage with some teenagers and some under-age beer, I don't think it can be called "clean".

Hard to generalize. The cathodyne can do odd things in OVER-drive, the long-tail looks better on the 'scope, but not necessarily to the ear.

[eleventeen]

I wasn't advocating one way or another changing the type of phase splitter to LTP (requiring TWO triodes....."pair")...I would probably leave what you have as it is, knowing the 6BQ5s do not require the amount of drive as 6L6's. I was suggesting maybe using the unused half of the second 12AU7 as an extra gain stage...and to do that, requiring only a small number of R's and C's, it is THAT [newly built] section, that add'l gain stage, that may want to be changed to 12AT7. That gain stage would be inserted into the circuit BEFORE the present phase splitter. It's possible that could overdrive the phase splitter you have, and that may or may not work out for you. It would possibly offer a chance to have a "post" gain control and maybe even a master volume control which could be nice. But see if just jamming higher gain tubes in there with no circuit changes appeals to you.


What I would do in your place is to simply try 12AX7 for the first tube (seems like you like that) and the second to a 12AT7/12AX7, changing nothing else, and see if you like that. And, when you buy the new 100K plate resistors, buy an extra or two just in case.

[TIMBO]

OwnGoal, You have kicked a winner with this score. These Leslie's are great. Here are some ideas  http://el34world.com/Forum/index.php?topic=17277.0
http://el34world.com/Forum/index.php?topic=17572.0
I pulled a similar amp from a Hammond and plan to mod it to an AC15 or similar as it has a CATHODYNE PI.

To save some headaches I would be inclined to replace ALL Ecaps similar to the originals in the power supply chain and the smaller Ecaps on the cathode bypass increase to either 10uf or 22uf this will give a bit of GAIN to the 12ax7s in the preamp positions. This can be subbed with other 12_X7s if things get a bit fuzzy
The unused TRIODE may have to be just that, if you are wanting this amp to power the Leslie, it will need to be as loud and clean as you can get it.
If the hiss can be narrowed down to the resistors then replace as suggested but I would NOT be too concerned at this stage as HUMS and HISSES don't really project like a normal speaker cab. These units are quite DEAD when the rotor is not spinning and when they are...........
Stick to the cathodyne PI for the moment as converting would mean a lot of cutting and pasting and if you are prepared to do this you may as well gut the lot.
You may tweak the cathode resistor on the ELs to get some more horse power and lower the Ecap to keep it CLEAN
The tremolo is not going to be a useful FX.
This is a great project and well worth the time and effort, good luck. 

[jjasilli]

> cathodyne PI is fine for clean tone

Ampeg VT-40 is cathodyne. Get a couple of these in a garage with some teenagers and some under-age beer, I don't think it can be called "clean".

Hard to generalize. The cathodyne can do odd things in OVER-drive, the long-tail looks better on the 'scope, but not necessarily to the ear.

The VT-40 schematics seem to be LTP, but your point is well-taken.  I'm speaking within the context of simple, Princeton topology, where the only "stage" likely to overdrive is the PI itself, and maybe the tone recovery stage.  This will produce a lopsided curve shape, and to my ear, a "ratty" overdrive tone.  This is subjective and many people might like it.  More complex preamps with cascading gain stages can shape overdrive tone on both sides of the X-axis of the curve for smoother overdrive tone, before it reaches the PI.   

[PRR]

> VT-40 ...seem to be LTP

Sorry, I forgot. The same model-number covers several quite different products.

The late front-panel VT-40 with "Distortion" knob is solidly cathodyne.

. . . . OK, I found it in Doug's library, under several model-numbers.

Ampeg_V2_V4_VT22_VT40_RevD.pdf - 168 Kb   - replaced now

Ampeg-V2-V4-VT22-VT40-1976.pdf

[Ritchie200]

With just two EL84's, its not like it has to swing a lot of current.  He should be able to swap PI tubes to taste?

Sorry, the half driver tube?

Jim

[terminalgs]

I thought all ampegs were cathodyne, without exception.

[OwnGoal]

Guys, if you would, I'd love it if we could stay on topic. 

Thanks

[terminalgs]

Guys, if you would, I'd love it if we could stay on topic. 

stick with the cathodyne 

[DummyLoad]

just graft in the unused triode as a gain stage. replace the 12AU7 tubes with 12AX7 tubes.

--pete

[OwnGoal]

Thanks guys. 

Getting ready to order filter caps, coupling caps, resistors, tone caps etc.  This thing sounds pretty good with the 12AU7's, but I am still getting a lot of sizzling and hum when I put in the AX7's.

Probably going to try an additional gain stage first.  I'm treating this thing as a learning laboratory. 

Decided it's best to put the project on hold til I swap out the electrolytics.  I let you know when I get 'em.

Thanks for all the ideas.

[eleventeen]

Sizzle = old carbon comp plate resistors. I don't expect you to believe me until you replace them w/the metal films. Then you will be a convert for 60 cents!


I had a very old blkface Pro Reverb, #00138, which was all the way stock. I owned it for 35 years and it was in storage for the last ten years. It made so many rat noises it was ridiculous, comical. Plus  sizzle and pops and siren kinda noises. 6 qty 100Ks, replace the electrolytics, quiet like a brand new amp. It was stunning.

[OwnGoal]

Oh, I am definitely doing the plate resistors.  You already talked me into the metal films.
Thanks

[OwnGoal]

I understand you want to keep the selenium diode.  I strongly recommend you replace it with a silicon diode, the selenium diode can present a health risk.

Thanks.  I went back through the manual associated with the schematic, and in fact, these are silicon rectifiers.  An earlier version uses selenium,  but not mine.

[OwnGoal]

I understand you want to keep the selenium diode.  I strongly recommend you replace it with a silicon diode, the selenium diode can present a health risk.

Thanks.  I went back through the manual associated with the schematic, and in fact, these are silicon rectifiers.  An earlier version uses selenium,  but not mine.

I think so anyway..
Someone tell me if I'm wrong

[eleventeen]

Those are silicon, they are fine as they are. Seleniums have a very distinct look, piled up squares (mostly square, sometimes round) of metal, usually painted grey or green or orange or blue. Google images "selenium rectifier" if you are curious. Yes, it is a good idea to replace those as they produce toxic gases if they flame. (I suspected you had seleniums because they were kind of hidden in your pix) No worries.