Sears 157 organ amp to guitar amp

[AmberB]

I was recently given the amp chassis for a Sears model 157.10000 organ.  I looked up what information I could find from this forum but there was no conclusion to the thread I found from a few years back, plus there didn't seem to be a schematic for this chassis anywhere.  A google search led me to a thread on the Telecaster Guitar forum with a bit more information.  I also found a hand drawn schematic for the chassis, but that schematic did not match the amp I have in some respects, mostly a few component values, and a couple of differences in the routing of circuit components.  I don't know if they were mistakes by the person who drew the schematic, or if his 157.10000 was just different from the one I have.  I suspect that the component routing differences were mistakes on his part.
Then I found an on line copy of the service manual with schematic for the 157.47510 chassis.  I printed out the schematic from the service manual, and also the hand drawn one I found, and compared them both to the chassis that I have.  That's how I found what I believe were mistakes or variations between the hand drawn schematic and my chassis.  I also found that the 157.47510 chassis schematic is identical to my 157.10000 chassis except that my chassis has a center tap on the heater coil of the PT, and the schematic for the 157.47510 chassis does not show a center tap on the heater coil.  That was the only difference I could find.

I've seen people write about building a Silvertone guitar amp using this chassis, but the amps that they build only have a tone control.  I plan on building an amp with a bass and treble control.  I'm not sure if I want to modify the existing tremolo circuit or change it to a more well known design.  There are options here...

The chassis is really clean, so it should be pretty easy to do the build without having to clean it up first.

[AmberB]

I can only post so many pictures per post, so I'm posting the inside shot here.  The focus on these is not perfect...

[AmberB]

I need to make a separate copy of the schematic before I can post it.  It's part of a 3 page PDF.

[sluckey]

I need to make a separate copy of the schematic before I can post it.  It's part of a 3 page PDF.

Why? If the file size is less than 2048KB just post the whole pdf.

[AmberB]

OK, posting the Sears organ amp PDF

[AmberB]

Another difference that I just noticed between the schematic and my 157.10000 chassis is that the schematic shows the filter cap can at 40 +40 uf at 400 volts, and my chassis has a 40 + 40 at 350 volts.  Not all that significant considering the B+ voltage in this amp.

[PRR]

The whole PDF will be useful to someone. But a simple GIF of just the fold-out can post in-thread.

I think the winding taps and cap ratings are just normal drawing mix-ups and production changes; especially to Sears' captive suppliers.

I don't see how a "more well known design" is necessarily better. I sure would use it AS-IS for a while. I see it will probably overload on well-strummed guitar; turn the axe down, also try bypass the first stage.

[AmberB]

I'm guessing that I have to do away with all the circuit parts directly involved with the "flute-string" switch, although it might be useful to put a "voltage divider" in the signal path between V1a and V2a, and probably a volume control where the swell pedal is located on the schematic at the input grid of the 6AQ5, unless there's a better location for it.
I figured that the tone control, or controls would go between V1a and V2a if I was going to put them somewhere.
Also, the vibrato control is not located in the chassis, so I need to figure out how to deal with that properly.

[sluckey]

When I look at that all I see is a Fender Vibro Champ with 6AQ5 rather than 6V6.

     https://el34world.com/charts/Schematics/files/Fender/Fender_champ_vibro_aa764_schem.pdf

[AmberB]

You know what, I think that might just be exactly what I want! 
It certainly seems easily do-able in this chassis.
Now I just need to drill a bunch of holes for mounting the controls, input jack, speaker jack, and a better location for the power cord.

[AmberB]

When I look at that all I see is a Fender Vibro Champ with 6AQ5 rather than 6V6.

Of course, with a couple of tweaks, you could also build a Vox Pathfinder V1...

[AmberB]

Here's a start on the amp, gotta make the holes in the chassis for all the pots...
The one pot that's mounted is the original volume (swell) pot that was on the chassis when I got it.  It's a .5meg pot, so I'll be changing it to a 1meg as per the schematic.

[AmberB]

I'm pretty far along in this build now.  Last night, I decided to change the cathode resistor for the 6AQ5 output tube to a mor robust resistor.  Manufacturers tend to put undersized cathode resistors in little amps, apparently they don't calculate on the amp being run wide open all the time...
Anyway, the cathode resistor was a 390 ohm, 2 watt, which is probably marginally enough wattage.  I wanted to replace it with a 5 watt resistor.  Also, the cathode is not bypasses in this little amp, so I wanted to put in a bypass cap.  I decided to go with a 50uf at 50 volts, mostly because I have several of them.
I did some research on the net and found a few 6AQ5 amps with various cathode resistor ohms ratings, everything from 270 ohms to 470 ohms.  I don't have a 5 watt 390 ohm resistor, but I do have a 5 watt 350 ohm resistor, so I went with that.
Unsoldering the connector tabs for the 7 pin socket proved to be a bit too much of a challenge for the socket, the way it was assembled with all the parts leads through the slot and then wrapped around the tab before soldering tends to be difficult to unassemble, and, of course, I broke the tab off the socket in the process.  I wasn't planning on replacing any sockets in this little chassis, but that made it necessary because the connector broke off close to the base of the socket.
So that was my latest adventure with this little project.
 Also, since the heater winding has a center tap, I moved it from direct ground to the cathode of the 6AQ7, to help with potential heater hum.

[sluckey]

I used a 470Ω 3W and 47µF on the cathode of my SE 6AQ5 Smoky. There's 20V across the resistor so only .85W actual dissipation.  3 watts is plenty of safety margin for my amp.

[AmberB]

I have the wiring mostly done on this little amp now.  With point to point wiring, it's the usual rat's nest of wires and parts.  Somewhere in the process of working on the amp, I decided to change the input triode to the same tube as the output triode.  The original configuration had the input triode on the same tube as the tremolo LFO.  The change will allow me to use the second triode of the tremolo tube for the cathode follower of the Fender type tremolo if I need to do that.  I'm going to try the tremolo setup that was in the Sears chassis first to see if it works without the cathode follower  If I don't need the cathode follower, there's no point in wiring it up.  I also decided to add another filter section for the preamp tubes.  The original Sears circuit only has 2 power supply filter sections.
I still need to drill 1 more hole in the back of the chassis for the tremolo footswitch jack.  I forgot about that when I was laying out all the other holes.  Today was the fun job of making the double flat side hole in the back of the chassis for the cord clamp/strain relief for the power cord.  Normally I install the IEC power cord jack on my amp builds, but this chassis is rather small and not so much space available, plus I didn't want to take half the parts out of the chassis to make the hole for the jack without breaking stuff.
This picture is from before I decided to use the power cord clamp/strain relief.

[AmberB]

I finally got around to testing this thing this evening.  It works, and it doesn't buzz any more than you would expect from a SE amplifier, so that's good.  It doesn't get very loud, kind of what you might expect from a single 6AQ5 into a 12 inch speaker, but it seems that perhaps it doesn't get as loud as it should.  Also, with the volume all the way up, it's just starting to distort.  I may not be getting enough drive from the preamp tube...gotta write down the voltages. 
What I remember is that the plate of the 6AQ5 is getting just over 250 volts from the 5Y3, the screen is about 10 volts less, and I think there was about 12 volts on the cathode.
Another interesting thing is that if I turn the speed and depth controls up too much on the tremolo, the sound disappears.  Also, there's a point where I can stop the oscillator from working while adjusting the speed and depth controls.  I wasn't sure if the original tremolo configuration would work properly with the changes I made to the amp, so I need to look into that also.
The tremolo works on the cathode of the second triode of the 12AX7, similar to the AA-764 VibroChamp, but it's not the same setup.  I may have to change the tremolo circuit to the AA-764 style, using both triodes of the 12AX7 that is now the oscillator tube with the other triode unused.  Not difficult changes to make, and might also give me more output on the second preamp triode...

[AmberB]

I got a bit more work done on this little amp recently, and a bit more testing.  I changed the preamp tube circuit a bit on the second triode.  The Sears circuit had the cathode going through a 3Kohm pot for the tremolo control, and then a 390 ohm resistor to ground.  That setup probably was why the 6AQ5 wasn't getting driven very hard.  I changed the cathode circuit to the standard Fender setup of a 1.5Kohm resistor in parallel with a 22uf cap to ground.  That now runs in parallel with the tremolo circuit.  Also, the feedback circuit from the speaker output goes through a 15Kohm resistor to the cathode of the tube.  I left it that way instead of changing it to the way the AA764 Vibro Champ does it.  Also, I have an 82Kohm plate resistor on the triode.
Of course, now I have too much drive to the 6AQ5, it distorts too early, before it gets very loud, as much as a SE 6AQ5 gets loud...
So now, I need to figure out how to reduce the output from the triode to the 6AQ5 grid.  I have a 1.5Kohm grid stopper on the grid of the 6AQ5, and a 220Kohm grid bias resistor to ground.
So, do I increase the feedback from the output, do I remove the cathode bypass cap from the triode, or is there another way to lower the output of the triode, but not too much.

Another thing, the tremolo barely works right now, but I'm not going to worry about that until after I get the rest of the amp working properly...

Edit #2... the voltages on the 6AQ5 are as follows- plate-240 volts, screen grid-229 volts, cathode-12.2 volts.

[bmccowan]

By now, you may have resolved this. If not, a couple ideas. The easiest way to reduce gain a bit is to swap in a different preamp tube 5751, 12Ay7, etc. Also, I (and maybe others) have a hard time following your description of changes. It would help to sketch out what you now have. It sounds like you have 3 gain stages feeding the 6AQ5. If so, you could consider cutting back to 2 stages.

[AmberB]

By now, you may have resolved this. If not, a couple ideas. The easiest way to reduce gain a bit is to swap in a different preamp tube 5751, 12Ay7, etc. Also, I (and maybe others) have a hard time following your description of changes. It would help to sketch out what you now have. It sounds like you have 3 gain stages feeding the 6AQ5. If so, you could consider cutting back to 2 stages.

I used to have a 12AY7, but I haven't been abler to find it to try it out.  I really should order another one.
As for the gain stages, there's just 2, the amp resembles a Vibro Champ in the preamp circuit.  The other 12AX7 is just the vibrato tube.
I tried removing the cathode bypass cap on the second gain stage to lower the gain to ther 6AQ5, and that did help control the overdrive somewhat.  It's not too bad now, but I still want to try a 12AY7 just to see how that performs.  I was also thinking about trying a 12AT7 to see how that sounds.

[AlNewman]

Without looking at an updated schematic, it sounds like you've paralleled the 3.4k bias on v3 with 1.5k.  Which reduces the bias of that tube to 1k.  That's fairly significant.  I'm not sure how that relates to how the tremolo functions, but I imagine it matters a bit.  You've also reduced the NFB to 1/3, which would probably be a significant gain increase as well.

You could try reverting back to original design, and perhaps increasing the plate resistor of V3, and/or reducing the 15k NFB loop resistor to taste to dial in gain on that stage.  I'm not sure you would want mess with the relationship of the tremolo circuit and the cathode of V3, but I think you could play with NFB.

You could try different tubes, but truly, each one needs to bias differently to work efficiently and sound good.  So you'd be chasing your tail.  You may want to look at reducing gain at an earlier stage.

[AmberB]

It's been a while since I've worked on this amp, until today.  I've gotten the gain to the 6AQ5 somewhat near what I wanted basically by removing the bypass cap from the cathode of V2.  The plate resistor on V2 is 100K and the cathode resistor is 1.5K.  I had tried an 82K plate resistor, but the amp seemed a bit anemic that way, so I put the 100K resistor back on the plate.
Another issue that I've had with this amp is "thumping" from the tremolo.  I did a search through past threads on here to see what I could find about the subject.  The Vibro Champ circuit seems to be a fairly rare topic for tremolo problems, but I did find a few suggestions for dealing with it.  The 3 main ones were to decouple the LFO plate voltage from the preamp voltage, to move the tremolo wires away from the preamp wires, and there was 1post where someone put an .022uf cap across the plate and the cathode of the LFO triode.
The only way to decouple the LFO B+ from the preamp B+ on this amp was to use the screen grid node for the power tube to power the LFO, that just made the thumping a bit louder and more prominent.  Moving the LFO wires in this little chassis, and even rerouting the LFO grounds away from the preamp made no difference that I could tell.  However, putting the .022uf cap between the plate and the cathode of the LFO cut the thumping in half, maybe more.  At one point, I could hear the thumping with the volume all the way down and the depth up, now that's gone.  I think the tremolo effect is perhaps not quite as strong now, but the thumping is almost gone, so I'll call that a win.
It would seem that SE amps just don't do as well with tremolo as PP amps do.
In case you're wondering, I built this amp like a Vibro Champ with lower B+ voltage and a 6AQ5 power tube.

[bmccowan]

This is my go to thread on tremolo: https://el34world.com/Forum/index.php?topic=20727.0 It includes PRR's controversial dog in a dryer theories
Nice project - I have a similar circuit in an Ampro or Ampex (I forget which) tape monitor that is waiting attention. Did you sketch out the final schematic?
BTW - I have a Vibro Champ and a Valco SE with trem. I like the Valco trem better. But I like trem slow and subtle - hamster in a dryer.

[AmberB]

I didn't bother to sketch out a schematic of the final circuit design, it's very much like the Vibro Champ circuits with only a couple of minor changes.  The power supply section was left in the original Sears amp configuration, with the addition of 1 more node for the preamp tube.  I used a 10uf cap for the filter for that last node, it's what I had on hand.  I would have preferred a 20uf but I would have had to order one

[sluckey]

I didn't bother to sketch out a schematic of the final circuit design

Many times the devil is in the details.