Gulbranson Organ Tube Amp

[dpm309]

I am helping someone who wants to convert an old Gulbranson Organ Amp into a guitar amp.  I have attached the schematic for the amp, Gulbranson APS-5 chassis.  The amp looks like a typical push-pull circuit using 2 6L6s in the power section and 2 12AU7 in the pre-amp section.  It also has a 5U4GB rectifier tube and an 8-ohm output transformer.  Does anyone have any experience converting one of these or have any other suggestions.  Could this be as simple as rewiring the input and output jacks, installing a grounded 3-prong plug, and disconnecting anything that is not related to the amplifier circuit?

Thanks,

Dan

[kagliostro]

I'm not an expert but I will change the input to a standard guitar input

so eliminate all the existing circuit from jack to grid of V1 and replace coping a standard guitar arrangement (with 1M and 68k resistors)

I also will change the plate resistor, the cathode resistor and the bypass cap according to a "guitar amp standard"

looking to the schematic there is half a 12au7 that isn't in use

so you can look to some guitar schematic

this as an example

http://www.el34world.com/charts/Schematics/files/matchless/matchless_clubman.pdf

and try to use this half unused tube in parallel with the used half tube

May be you are happy with the 12au7 as V1, may be you will like more one other tube, like 12ax7 or other

(I'm not able to understand well the control that is there - the "expression" control)

I'll try also a 1MA pot instead of the 1M resistor from grid of V2 and ground (the pot extremity connected as the resistor and the grid of V2 connected to the wiper of the pot)

A more readable schematic

http://www.jimmyandsharonwilliams.com/gulbransen/schematics/APS5.gif

Other members of the forum will give you more councils than those I can give you

Kagliostro

[jojokeo]

At first glance you have a great base here to work with. I'd remove those first two 12au gain stages for a 12ax7 in those positions, remove the input pot for the initial vol control, then insert a tone stack w/ gain control between the first two stages instead. Then put a MV after the 2nd stage before going into the pi section or you could put in a post phase inverter MV - what ever you want but the first one is easier & more common to do.

This is all very basic & easy enough to do but it will require drilling some holes maybe & rewiring some things and changing or adding some components. I won't go further because if you don't understand these things then you should get someone to do what you want for you instead unless someone want's to put in all the time to re-do the schematic and guide you through all of the steps? Unfortunately that's a lot of time & work to invest that I don't have at the moment. There's a lot of good qualified people here that may consider doing this for you?

[DummyLoad]

...convert an old Gulbranson Organ Amp into a guitar amp.

hi-gain, middle of the road, or clean playing?

assume there are two 9pin sockets for 12Ax7 tubes? - only three triodes shown in schematic.

5E5 fender pro or the like seems like would fit nice. scrap everything from the SLPI back - including the SLPI if you want gain.

http://www.el34world.com/charts/Schematics/files/fender/PRO_5E5.pdf

insane gain   > preamp - tone/vol - preamp - mv - preamp - SLPI - P-P

hi-gain   > preamp - tone/vol - preamp - LTPI - P-P

middle of the road  > preamp - tone/vol - preamp - SLPI - P-P

clean  > preamp - multiband lossey tone/vol - preamp - SLPI - P-P

ground the PT CT for the 5U4 ckt. - if you must have sby. switch, switch the B+ before OT.

scrap all the selenium rectifiers and the low volt chokes. be careful with the selenium - the dust is toxic if taken in quantity.

[RicharD]

try this (see attached)

>ground the PT CT for the 5U4 ckt. - if you must have sby. switch, switch the B+ before OT.

It's a pretty cool arrangement they have.  If the speaker is unplugged, B+ is effectively disconnected.

[jojokeo]

Nice Buttery. All it needs is a coupling cap after the first 12au now.

[DummyLoad]

Nice Buttery. All it needs is a coupling cap after the first 12au now.

no, not needed - it's direct coupled. Va of the 1st 12AU sets the bias point for grid of SLPI (2nd 1/2 of 12AU7). study the 5F6A bassman and marshall cathode follower ckts. - same arrangement the second stage in the 5F6A ckt is a cathode follower biased by the preceding gain stage. the SLPI is similar to cathode follower. the bias scheme here is the same as with bassman ckt.

[jojokeo]

  I see the split load better after looking back at it more thoroughly. Thanks ISO

[kagliostro]

And now that the answer to the question was given

can someone try to explain how works these part of the circuit ?

I've never seen something like the expression control , what it does ?

the stage directions say

Min Volume 175v (or .175v - don't able to read well - I think more 175v)

Max Volume 0v

the remote control seems to be a "special acted rehostat"

Looking to this circuit I can understand that 325v B+ are connected via a 100k 2W resistor and the plug of the socket to a 2200ohm resistor in series with another 100k resistor, an electrolytic cap is connected between ground and the junction point of the 2200ohm and the second 100k resistors, the external "special acted rehostat" connect the junction of the first 100k (2W) resistor with the 2200ohm resistor to ground with a variable level, changing in some way the charge of the electrolytic cap

But I'm not able to understand more about this part of the circuit neither to understand what it does

thanks for any explanation about

Kagliostro

[jojokeo]

An expression control is foot pedal operated - a variable voltage divider which can control volume, tone, and effects in general terms but in this organ's use it was for volume control.

[dpm309]

That brings me to the next question.  There is a 500K pot where the signal enters the first power tube and am wondering if this is gain or volume control.  If so, I can eliminate the expression pedal portion of the circuit.  I will also have to add a tone control somewhere in the circuit (thinking 5E3 type as suggested earlier). I think I have an idea of where I am going on this.  I have built and repaired several tube amps over the last 10 years and am confident that I can take project on.  I want to thank everyone who has provided their valuable input and will be asking more questions as I progress.

Thanks,

Dan

[RicharD]

The 500k pot is an input attenuator, just like the pot on your geetar.  Totally redundant for this application + as it sits now, there isn't enough gain for a guitar amp.  Do like ISO said:

Gut all the unused power stuff. 
Properly ground, fuse, and switch the line power.
recap power supply if necessary.
Gut V1 and replace with a 12AX7.  Copy your flavorite guitar amp front end and rebuild.

The PI should be good as is and yes V2 direct couples to V2b to set the bias.  you could rip out the PI and replace it with a long tail if you need more gain.  At a glance, I don't think it'll be necessary especially if you use the Stout front in that I drew.

I'd leave the power stage cathode biased just exactly how it already is.  Keep it simple.

[kagliostro]

If so, I can eliminate the expression pedal portion of the circuit.

yes you can, anyway you can do that it depends on how deeply you want to go with mod

I'd remove those first two 12au gain stages for a 12ax7 in those positions

if V1 is used only half you have no problems to put there a 12ax7 or 12ay7 or other

for V2 the situation is different because V2 b is the phase splitter and in this position a 12au7 may be a good choice also if there are amps were this function is done by a 12ax7

I agree with Butterylicious council

Kagliostro

[dpm309]

I finally gutted the organ amp and am getting ready to do conversion based on Jojokeo's suggestions.  I will put the gain control and tone stack between the first two stages and a master volume after the second stage.  For the tone stack, I was planning on bass and treble only.  I found a diagram for a typical tone stack for Fenders but it has a mid control.  How do I modify it for just bass and treble?  I have included a drawing of the stack.

Thanks,

Dan

[sluckey]

Replace the Mid control with a 6.8K 1/2W resistor. Look at the AB763 Deluxe Reverb schematic...

http://www.el34world.com/charts/Schematics/files/fender/deluxe_reverb_ab763_schem.pdf

[kagliostro]

May be you are interested in this free program

http://www.duncanamps.com/tsc/

Kagliostro

[dpm309]

I decided to go with the mid in the tone pot and might still modify the values using the Duncan Tone Stack program.  I also sketched in a possible place for the master volume.  Does this make sense?  One final question, there is a connection from the hot lead of the output transformer to the 4.7K resistor under the first half of V2.  What purpose does this serve and should I remove it or leave it in.  See attached drawing.

Dan

[RicharD]

That's the negative feedback loop.  It helps keep the amp stable by reducing gain.  Try it with 1 end disconnected, or adjust the resistor value to taste.  A standby switch isn't really necessary.  That tone stack has a lot of loss.  You may end up wanting a cathode bypass cap on the 2nd 12AX7 stage.

[jojokeo]

You probably would like to start w/ a 25K linear pot for the midrange instead of 10K (better control range). The schematic looks good for building it that way and then you can always tweak a few values to fine tune things while testing if needed (ie, you may want a larger bass cap and slightly smaller mid cap eventually, a bright cap across the MV pot, tweak coupling cap & bypass cap values, etc...) that's part of the fun and personalizes your final tone ranges based on guitar, speaker, & music style.

[dpm309]

Thanks again for all of the great suggestions to tweak the tone and sound of this project.  I just received all of the parts and will start drilling the chassis to accommodate the extra pots, etc.  I do have a couple of additional questions.  The Gulbransen schematic shows a 5-amp quick blow fuse.  I used a 2-amp slo-blow on my tweed deluxe and lightning 15 (both ~15 watt output) and a 3-amp slow-blow on my '59 bassman (40-watt).  Would a 3-amp slo-blow be OK on this build?  Another question I have is on the front end.  The diagram that Butterylicious drew shows a 22k resistor going to pin 2 of V1.  On most of my builds I use a 68k.  I also noticed on the Stout schematic without reverb, there is no resistor.  Should I stick with the 22k as suggested or try different values to tweak the tone.

Thanks,

Dan

[jojokeo]

2A slo-blo is fine, stick w/ Buttery's info using the 22k, a higher value 68k will "deaden" your sound more losing highs/clarity. You can always change things when done as is usually the case when you're tweaking the final results to taylor exactly what you want. An example would be trying out a bypass cap accross the lone 1500r after the tone stack and see if you end up w/ too much gain or it might be just what you need to really drive the 6L6s? Or you may feel you need a small bright cap accross the vol pot (lugs 1 & 2)? That's the fun part when you're finished is dialing everything in.

[RicharD]

Exactly!  I bet this amp is stable w/o any feedback.  It may be perfect or a bit harsh.  I'll often clip lead in a pot and dial in just a tiny bit of feedback to smooth thangs out.  Cathode bypass caps are a huge tweak.  Marshall used a .68uF, Fender used a 15uF.  Me I like 2.2uF most of the time.

[dpm309]

Well, it's alive now.  It has fairly good tone, though a bit thin, and the master volume control produces good distortion.  I tried with and without the 2.2K feedback resistor and could not tell too much difference except it squealed at high volumes when it was disconnected.  I will put a pot in there and try to dial in the best value.  I do have some hum which was not unexpected due to the nature of the point to point construction (no tag or turret board).  I replaced the lead from the volume pot to pin 7 on V1 with a shielded cable which reduced some of the hum.  The connection from the input jack resistor to pin2, V1 is also shielded.  The grounding scheme is not optimal with multiple grounding points.  I did solder a ground bus across the back of the pots but I don't know if this was necessary or could be adding to the problem.  The voltages I get are right on the money according to the original Gulbransen schematic.  This thing has a monster power transformer (it also powered a Leslie speaker as well as rest of the organ components) and I shrunk wrapped the ends of the 5 unused leads.  I don't know if this could be contributing to the hum problem.

The tone I am getting is a bit thin in my opinion so I will try tweaking the tone stack.  Any suggestions on these remaining issues would be appreciated.

Thanks,

Dan

[RicharD]

Is your hum 60 or 120 Hz?  If it's 60Hz, check how the filaments are grounded.  If it's 120Hz, you've got a ground loop somewhere.

>The tone I am getting is a bit thin

Try changing the cathode bypass cap of the 1st stage from the 5uF you posted to a 10 or 15uF.  If it's anemic, add a cathode bypass cap to the 2nd stage.  Obviously you can jack with the values in the tone stack.  Play with the mid if you do. 

[dpm309]

I think I'm getting a little of both 120 and 60 cycle hum.  I did ground the filaments using an artificial tap with 2 100 ohm resistors (the original amp did not have this) but do not hear much difference before or after.  I will revisit the grounding scheme and try to follow Doug's grounding plan.  I need to separate the power supply grounds from the preamp grounds to see if this helps.   

When tweaking the tone stack, should I try another value for the mid pot or change the value of the .022 cap?  I have already added a 4.7 uf cathode bypass cap to the 2nd stage as recommended earlier.  Should I try a different bypass cap?

Dan

[DummyLoad]

should I try another value for the mid pot or change the value of the .022 cap?

either or both. 

[dpm309]

I changed the mid pot cap to a .01 uF and the bass cap to .047 uF and this seemed to do the trick and fattened up the tone.  Other changes I made included a .022 uF cap between the tone stack and pin 1 on V1 and added a couple of 1.5K 1-watt resistors on the screens of the power tubes.

I separated the preamp grounds from the other grounds the best I could and this reduced the hum somewhat.  I also re-routed the shielded cable from the volume control to pin 7 on V1.  I am still getting some hum but it is almost in the acceptable range.  I am using shielded cables from the volume control and the input jack.  Are there any other places I could use shielded cables to reduce the hum further?  I suspect that the remaining hum is just a factor of the layout of this amp.  Other than that, the amp is sounding good.

Dan

[scrimpus]

dpm309, I have this same amp and I am wondering is maybe you have a final schematic of this conversion.

[dpm309]

Scrimpus,

Here is the final schematic.  I also e-mailed you a copy of this as well as a before and after picture of the point to point wiring.  The before picture is after I removed anything not related to the amplifier portion of the organ.  I had to drill some holes in the chassis for the new tone and volume controls.  This was a fun, but challenging project that I could not have done without the help and advice from this forum.

Good luck

Dan