Silvertone Organ amp to guitar amp conversion project (more changes)

[1blueheron]

I picked up a little Silvertone tube amplified reed organ last Sat.  a free CL find.

It works and the amp is fairly quiet.  Doesn't appear to have any apparent gremlins.

The schematic at first glance appears to be identical to this. https://el34world.com/charts/Schematics/files/Silvertone/Sivertone_4751_manual_schemtic.pdf

The chassis model # is designated as 157.10000

All original tubes with Silvertone markings all in good shape.  Interesting little amp.  Considering it for use as a Harp amp.  Search function alluded to the idea that Dummyload may have done a conversion project with one of these but cant find the thread and I also saw a thread by PRR somewhere that looked like he was familiar with it.

Any experience, suggestions, etc as to what can be expected from this either as an amp for my harmonica or as a guitar conversion would be appreciated.

I have it out of the organ and cleaned up.  Plan to do some voltage checks soon.  The organ had some broken keys and we already have a Magnus Chord organ so that part went in the trash.

[RWood]

Back in 2012 I converted one of those, putting it into the case from an RCA tape recorder. I used firemedic's schematic for a SE 5879 - 12AX7 - 6AQ5.
Great small amp and I would build another one. The guy I built it for liked it a lot.

[1blueheron]

RWood,

Thanks for the feedback.  Found the thread for the 5879SE   https://el34world.com/Forum/index.php?topic=10801.0

Looks interesting and nice job on the tape recorder case re-purpose.  Looks nice.  I will put it on my candidate list.

[1blueheron]

I am leaning heavily right now towards building something close to a Kalamazoo Model 2 with this chassis as it has a very similar layout in tube number, position, and compliment.  Differences being the 6BQ5 vs 6AQ5 and the 5Y3 in place of the 6X4, (both which seem to be improvements?) everything else seems pretty straight forward in the schematic.  I also like the low parts count it has and the way it sounds.   Maybe I'll call it the KrazyKazoo.

[shooter]

KrazyKazoo.

I grew up there and live close now, trust me, we have enough crazy already 

[sluckey]

I did a layout for a K2 several years ago for a forum member. By request it is laid out on a stock turret board with only two rows of turrets. May be of some interest. Maybe not. 


     http://sluckeyamps.com/misc/K2.pdf

[1blueheron]

I did a layout for a K2 several years ago for a forum member. By request it is laid out on a stock turret board with only two rows of turrets. May be of some interest. Maybe not. 


     http://sluckeyamps.com/misc/K2.pdf

Sluckey,

Thanks a million for that.  I like it.  I guess I will need to make a few minor changes in values to accomodate my specific PT and the tube changes?

Took some voltage readings tonite.

Here is the outcome:

V1

Pin 1=104V, pin 2=0, pin 3=0, pin 6=171V, pin 7=0, pin 8=0

V2
Pin 1=112V, pin 2=0, pin 3=1V, pin 6=192V, pin 7=0, pin 8=2V

V3
Pin 2=14V, Pin 5=259V, Pin 6=250V

The old electrolytic multi-cap can seems to be OK?  Running nice and cool, no hum/hiss.

Here is a schema with the specified vs actual measurements.

[1blueheron]

Before I gutted this little guy, I thought it would be interesting to just cut the mic off and put a 1/4" jack on it and see what it sounded like with a guitar.  Well I was a little bit surprised.  This little thing would make a great bedroom practice amp.  It is very civil, and well behaved. At WOT it has a little break up and thump if you dig in hard on the pick.  Lightly picked, it has a door chime characteristic.  It won't run you out of the house.  Not very loud but loud enough.  This was playing through it's stock speaker  (8")  and with everything in stock configuration.  It is also dead quiet, nothing in the hiss n' hum department.

As for what could use improvement...

It lacks a little in the sparkle and high frequency harmonics department.
Would be nice to have one more gain stage.
Would be nice to have an output jack to run a slightly larger speaker.
Tremolo is interesting but nothing to write home about.

My thoughts right now are to hold of on turning it into a parts donor and experiment on it and see what I can make it do.

Worked a little today on making a small cabinet for it so I can get the speaker on a baffle.  Once I get it mounted, the next thing will be to remove the Flute/Strings switch.  Guessing that removing the filtering networks on these will bring back some of the high frequencies that are missing.  Will likely replace with a basic tone control.

If this shows promise, I will likely turn the Trem into a second gain stage.

As it is, I would say it sounds better than most amps under 250.00  will try to get a sound clip of it posted up.

[shooter]

Change C1 n C4 to .022, or maybe .02 .01
Before you scrap the trem;
Add a 4.7uF Ecap to R9 and a 25uF 50V to R16  (close counts:)
Find someone with a 15” JBL and wire it in (close counts :)

Then think about a gain stage
 

[1blueheron]

Shooter,

Thanks for the feedback!

I have drawn a new schematic.  It removes the flute/strings filters and replaces with conventional tone pot. (area shown in blue)  I also drew in the added caps at R9 and R16 you suggested shown in purple.  Does this look right?

If so I will proceed with mods one at a time and check results with each change.

[DummyLoad]

https://reverb.com/item/6026699-silvertone-sears-organ-amp-1960-s-conversion-project

this is model that i converted  - i think, it's been so long.


--pete

[PRR]

...I have drawn a new schematic....

V2A has no gridleak.

The tone control full-up just shorts a cap.

[shooter]

Like PRR said

look at the link Sluckey posted n just copy, solves the grid ref. AND uses a .01uf coupler 

[1blueheron]

OK, thanks guys, I see the mistake.  The question is how to best fix it without moving the volume control.

In the scheme posted by Sluckey, (Kalamazoo model 2) the volume control is moved into the preamp section between V1B and V1A (which translates to V1A and V2A in silvertone world). The Silvertone places the volume control between the preamp section and the power tube.  I am trying to keep the volume control in it original location as I think it is key to the voicing of the amp, while adding a tone pot similar to that of the tweed deluxe to give a little more range than just the two position switched filter it now has.

Here is a new schematic.  Is this better?   

[1blueheron]

https://reverb.com/item/6026699-silvertone-sears-organ-amp-1960-s-conversion-project

this is model that i converted  - i think, it's been so long.


--pete

OK, thanks Pete.  Looks like a different beast than the one I have.  Mine is all PTP, no printed circuit board and a much smaller chassis.

[shooter]

C4 should be in parallel with R9 NOT series

leave your drive pot (MV) alone, but add the gain pot like the Kzoo.  I'm not sure the 10k is a good value for grid ref ?

[sluckey]

I've always questioned the biasing for V1A. R1 seems too low to be very effective for grid leak bias. And grid leak bias usually has a blocking cap on the grid. So, is this a poor grid leak bias circuit? Or did the engineers leave off a cathode resistor on the drawing? Is pin 3 really connected to ground?
 

[shooter]

same thoughts I had when I gave my list of changes.  was gonna suggest "standard" config, but wasn't that confident 

[1blueheron]

C4 should be in parallel with R9 NOT series

leave your drive pot (MV) alone, but add the gain pot like the Kzoo.  I'm not sure the 10k is a good value for grid ref ?

Shooter,

Thank you.  Hadn't thought of  additional Vol. as a gain pot and leaving the original as MV.  I like that.

Working on a revised drawing.  Is the 25mfd at R16 correct in series or should that be parallel as well?

I used 10K as value as that is what the original shematic was, albeit there was 2 of them.  I guess that issue is eliminated by going with the 1M vol/tone pots though correct?

[1blueheron]

I've always questioned the biasing for V1A. R1 seems too low to be very effective for grid leak bias. And grid leak bias usually has a blocking cap on the grid. So, is this a poor grid leak bias circuit? Or did the engineers leave off a cathode resistor on the drawing? Is pin 3 really connected to ground?

Well, since I have a pretty unmolested factory version of this, I will trace it out and tell you if the schematic is wrong, or it is actually attached directly to ground.  I will also post a few close up high res pictures of the PTP wiring on this before I start tearing anything up.   I am really amazed at how clean this thing is.  Apart from some minor dust, it is immaculate.  No pitting or discoloration.  Multi-cap is running cool as a cucumber.  PT is barely warm after being played through.  Started checking resistor voltages and a few have drifted up a little but nothing alarming.  Voltage at pin 1 on V2A seems a little on the low side though.  Going to check R10 again. 

[sluckey]

I suspect there is a LOT of circuitry between the keyboard and the input to that amp. Seeing the rest of the circuitry would probably explain my concern. But regardless, that amp ain't guitar friendly in its present condition. If I was doing this, I'd rip out everything except the power supply and power amp and build a guitar preamp to feed the poweramp. Ain't no guitar mojo in that amp until you put it there. 

[1blueheron]

I suspect there is a LOT of circuitry between the keyboard and the input to that amp. Seeing the rest of the circuitry would probably explain my concern. But regardless, that amp ain't guitar friendly in its present condition. If I was doing this, I'd rip out everything except the power supply and power amp and build a guitar preamp to feed the poweramp. Ain't no guitar mojo in that amp until you put it there. 

Slucky,

That is the surprising part.  There was absolutely nothing in the way of circuitry between the keyboard and the amp except for a little microphone and one resistor soldered on the microphone. 

The keyboard was not electronic, it was reeds and a fan. 

[PRR]

> questioned the biasing for V1A. R1 seems too low to be very effective for grid leak bias.

Some of this might not make sense, until we try it, with an eye to selling at $29.98. The chamber mike may not have enough output to overdrive the unbiased input tube. If it did, Sears could go back to the mike-maker and ask for something *less* hot (at less cost), to avoid the few pennies for bias. The maximum level is totally limited by the blower... no fear of a hopped-up player mixing pep-pills and vitamins and wielding a titanium pick. Anyway if the $29 model is not totally perfect, Sears had $59 and $199 models to try.

> volume control between the preamp section and the power tube.  I am trying to keep the volume control in it original location as I think it is key to the voicing of the amp

On *guitar*, volume pot before all amplification is prone to hiss at full-down. Vol-pot too late in the chain is prone to overload on that pep-pill player before it gets to a turn-down pot. The most generally satisfactory place for VOL is between 1st and 2nd stages. However many other ways have been done, and sold, and played. And you know where your soldering iron is. Just try it.

[shooter]

parallel as well

[1blueheron]

OK,

Here is another stab at this.  It reflects the tone/gain circuit in place of the Flute strings switch.  It also shows bypass caps on V2A and V3 as recommended by shooter.

I will leave the bias resistor off the schematic until I  A) verify it does not have one or B) verify it needs one.

My mission for tonight is to verify everything is according to schematic, make a pre-mod sound recording and take pics of everything as is.  I might also try the mic on a harmonica as is and record it.  I cant help but think it is pretty similar as they were both reed instruments, albeit I put out a bit more volume than the keyboard did...

If the schematic is finally right according to the Braintrust here, I should begin the desoldering/soldering fun tomorrow night.

[shooter]

looks like it's hot solder time!

I might also try the mic on a harmonica as is and record it.

the couple harp guys I fixed/tuned for both had the original Kzo as their "base" amp, that was in my no notes, just wing it times so no data points 
they did like 'em. 

[sluckey]

If you intend to plug a guitar into the input, then put a 1.5K resistor between V1A cathode and ground. And change R1 to 1M. Maybe put a 33K between the input and V1A pin 2.

[PRR]

> harmonica... I put out a bit more volume than the keyboard did...

Put your ear INside the organ chamber. It is a lot louder in there than in the room.

Sluckey's advice to use a 1.5k cathode resistor is spot-on conventional, and WILL work, though other circuit changes may be suggested (like a volume control after this stage to adapt to the much larger dynamic range of pop/rock guitar versus $29 parlor organ).

> questioned the biasing for V1A. R1 seems too low to be very effective for grid leak bias.

I got wondering. By inspection, it runs real close to the zero-grid line. Approximately -0.2V Vg1. (Peak plate swing OTOO 10V.)

Do we get 0.2V from a 220k grid resistor? IS it a 220k? There is a microphone. What kind? In that day/price, either a crystal or a dynamic. The crystal IS a capacitor, avoiding your objection that gridleak needs a cap to not suck-off bias. A dynamic would surely be <1k, so grid really should be AT 0.0V. IMHO a crystal is more likely...

AH! "CM 390k microphone resistor". Makes no sense with dynamic. On a crystal, this (with 220k) sets a high-pass. You really want a high-pass on a mike inside a reed-organ chamber, there's lots of low-freq turbulence from the blower, enough to overload the amplifier with blower-rumble.

So the gridleak is 220k|390k= 141k. Apparent grid bias is 0.2V. 0.2V/141k is 1.4 microAmps. This is not an unlikely value for small triode at -0.2V. 12AX7 grid current varies a LOT between brands and vintages. In this zone, easily 10:1. Grid wire smoothness, alignment, coating, gas/ion content. I believe vintage 12AX7 controlled this approximately; tubes made after gridleak fell out of fashion may have a much wider range of grid current.

[1blueheron]

This is getting to be an interesting experiment!  Thanks PRR for the math/science of it.  What I can tell you apart from the math and science is it was well worth the gas money I spent to go pick it up. 

Upon getting home tonite, I soldered a 1/4" jack on the microphone lead.  Plugged it in and went to harping.  Cord wasn;t long enough to really crank it up to full volume without catching some nasty feedback but at lower volume, it served itself well with the stock mic.  The resistor soldered to the mic measured 441k.

pin 3 on V1A is definitely soldered directly to ground. The pin is bent over and the ground tab on the socket bent up and they are all soldered together.

Mic lead is soldered to pin 2 and to ground.

Once again, we plugged in a guitar as is.  No issues.  It plays well on the stock speaker but nothing spectacular.  Then I disconnected the stock 8" speaker and connected it directly to the 12" speaker in a Johnson JT50 combo.   Wow! this thing really came to life.  No shortage of volume but still gets into breakup territory before it wakes the neighbors.

The volume control on it is .5M and the taper is steep.  I think it should be changed out.

There is a slight high frequency resonance somewhere.  Needs tracking down but otherwise this thing has some Mojo.  I don't feel like I am polishing a turd at all.  Definitely some real potential here without a full gutting.  Does some things real nice.  My phone dies while recording.  Will see what I ended up with and try to post some sound clips.

[1blueheron]

More pics..

[1blueheron]

Before I go any further I will try Sluckey's input resistor & suggestions and play with some more speakers to see how it sounds.   Then proceed from there.

[1blueheron]

Here's some noodleing on guitar recorded impromptu with my cellphone.  No editing or processing.  Strat plugged directly into stock mic cord after cutting mic off.  Starts off with amp volume at 10.  Backed it off a little a later.  This is through the stock 8" speaker without a cab.  This is the worst this little thing will do without any mods.

https://soundcloud.com/user-331808262/20190819-2020101a

[1blueheron]

Conversion is coming along nicely.  I have most of the casework complete and a new panel built.  IEC plug connector for 3 wire conversion is in place.   A 1/4" jack is installed on the rear of the chassis for external speaker connection.  I have 2 questions:

1) I would like to replace the stock vibrato pot so that the shaft diameter/length and mounting is standard with the new alpha pots I am installing,  It's value is listed as 3K.  Is there a more common value someone can recommend as I can't seem to find a 3K?  Can I change it to 500K as in sluckeys drawings or will that require additional changes.

2) The stock unit was not fused so I am adding a fuse.  Would the  fuse value of 1A be appropriate for this amp?

Thanks!

[shooter]

500k will not work so well, 5k is pretty standard, might get away with a 10k pot in parallel with a 10k fixed R.

1A is a good start, if the amp burns up lower it 

[sluckey]

That 3K pot also has a "C" taper. I would use it if it's not damaged.If the shaft is just too long you can cut it to length with a hacksaw and a vise.

[1blueheron]

It works fine.  Will see if it can be hacked to work.  It was ganged with a power on/off switch and not a typical threaded mount. Was solder/ bent tang mounted.

[1blueheron]

After closer look, I think I can take the threaded brass bushing portion of the  Flute/Strings selector switch and use it to modify the trem pot making it work.  Getting some parts ordered today.

[sluckey]

Will you post a pic of this pot? May inspire some other ideas.

[1blueheron]

Will do when I get home.

[1blueheron]

Made some progress last night and took a few pics.

Here is the 3k pot with on/off switch in its original long split shaft form.

[1blueheron]

I attempted to use the threaded bushing/plat off the selector switch seen here.

[1blueheron]

It worked but was ugly and would still have needed to shorten the threads.  Then I realized I had a 250k pot with short threaded bushing and plate that would fit perfect, so I just transferred that over to the 3k, cut off the split end to make it a short solid shaft and I think it came out decent.  Hopefully it still works ok.

[1blueheron]

Here is a pic of the three wire IEC cord mod with fuse holder.  IEC cord receptacle was salvaged out of a Dell server PSU.  Worked nicely but a pain to cut out the hole.

[1blueheron]

This is the unfinished cabinet that will be the amps new home.  Still have some work to do on it.  Have to buy some SS screws and washers today to get it screwed together.  Still need to cut holes in the back panel for power cord, fuse, speaker jack and ventilation. 

The end pieces are some maple butcher block scraps I salvaged and the front top and back are wormy spalted maple I cut from a tree in the back yard about 15 years ago.  I thought leaving the brand on the butcher block piece was interesting.  Knobs are on order and I have some interesting feet to put on it.  Face place was some scrap aluminum sheet from the garage. I cleaned it up on the belt sander.

[1blueheron]

Almost done.  Everything in its location and have all the parts now. Got my tone pots, knobs and feet.  Still need to put an oil finish on it and finish wiring up the tone pots.  Ideas for labelling pots welcome.  I could print them out and put under some plexiglass?  Engraved plexiglass backlit?  Waterslides?

[1blueheron]

Still could use a little finish sanding as well.

[PRR]

> Ideas for labelling

With just 4 knobs you can use single letters.

Wood lettering:
https://www.etsy.com/listing/645578917/unpainted-wizard-wood-letters-baltic
Scrabble(R) tiles:
https://www.etsy.com/listing/462500184/wood-letters-for-wall-crossword-name

[1blueheron]

Hey, that's a cool idea.  I'll play with it. Thanks!

[1blueheron]

Had a little time this past weekend to fool with the little Silvertone.  Fired it up last night with the new gain control and tone control in place of the Flute/Strings circuit and with the newly modified Vibrato pot.

I have not yet installed the bypass caps.  I don't have any 400V caps in those values. Need to go hunting for them.

As is, it sounds great for rythym guitar, has early breakup without blowing you out of the room but will still cleanup. It is still very quiet but sounds a little dead and lifeless.  It doesn't reproduce harmonics very well.  Still need to do some tweaking.  Hoping the bypass caps will help.

[1blueheron]

Spent some time chasing tone on the little silvertone tonight.  With the suggested cathode caps installed, I got scratchy pots and some funky oscillation.  Had very pronounced pick attack but very rapid decay and no high frequency sustain.

Pulled cathode caps back out and made the following input stage mods.

installed 33k grid stopper
installed 1M input resistor,
changed coupling cap to .022
installed 1.5K cathode resistor

 Things sound a little better.  Think I am headed in a positive direction.

Question... What does C6 do and what happens if I remove it.