Mystery John Meck Industries/Old Kraftsman amp/tracing help?

[Diverted]

Hi all,

A friend of a friend told me he had an old amp that he bought at a yard sale years ago, and would like to get it playing. He brought it by, and it's a cool one but in a sorry state. It's marked "Old Kraftsman" on the front, with no other IDs except for a paper label inside from John Meck Industries. Apparantly JM made amps for Kay and/or Valco. Anyway, I have not been able to find any schematics for this amp (tube lineup 6SC7/6SC7/6V6/6V6/5Y3) but I have found photos of it online.

Considering that it looks like it's been modified I wanted to try to get a look at the circuit before I did ANYTHING and have spent the last day or so trying to trace it out on DIY PRO (great program!). The field coil and power transformer all test OK with winding continuity. The output transformer looks to be a replacement universal transformer that's been there many years.

This was my first attempt at tracing out a schematic and I'm wondering if anyone here can go over it with me, possibly fix any problems etc... I would like to give him the schematic with the amp when I'm reasonably confident that everything is in order, and also want to answer some questions. Some things look weird but I'm pretty sure the schematic is drawn correctly.
The amp looks to have two mods:

1. The square box screwed to the inside of the cab, with a pot marked "record." I'm assuming this is a preamp out. In any case, if it is it doesn't work because there's no connection to the main unit's heater circuit. The 6SJ7 first grid in the unit goes to an old style microphone screw jack.

2. A second pot was added to the back of the panel (I think). It's not on the same horizontal plane as the centered pot, the hole was crudely drilled and in other pix of this amp I found online this pot is not shown. Plus, most of the tube socket pins were painted yellow at the factory. Where the new pot connects, the paint is disturbed. Also, the high end of this pot is running into one half of the 6SC7 heater (pin 7)???

3. A second jack looks to have been added to panel. It's an odd one, with two shorting leads, not one. I had a hard time showing how the connections go to this one.

4. It looks like someone recapped it years ago (filter caps). THey used wonky values (10/40/80). Would the originals be something more like 20/10/10? And when it comes time to recap should I go with 20/20/20 or so?

Three photos are enclosed, plus my schematic. I also have the DIY and PDF files for this. I left out some stuff for clarity; secondary of output transformer, heaters, primary of power transformer etc.

I would appreciate any help going through this and if anyone can offer guidance I would appreciate it!

Ted

NOTE: Sorry, I originally attached an earlier version of the schematic. The one attached now is the most updated one I have.

[Diverted]

Followup, if anyone has ideas:

Hypothetically, to return it to stock (looks like just a single input/one volume pot) how would I remove the second 125K pot and record unit from the circuit?
I'm trying to trace out the signal flow minus those components, but I'm having a hard time following how it would work. Does this amp have a paraphrase inverter?

Thank you!

[j_bruce]

I think it's a karaoke machine!

 

No, seriously it looks to my (very novice eyes) like the "record" is where you input the record player and use the knob to adjust the input. I see that the plate of the 6sj7 is going to the pot and then into the second pre-amp stage so that is why I think it's for playback. When you see that the volumes are labeled for mics then a record player input also makes sense. I guess Karaoke was not a Japanese invention after all!

Now someone with way more knowledge than me will come along soon and give you the correct answers.

I am glad that you are saving it, very cool cabinet!

Cheers,
Jerry

[Diverted]

Ha! Thanks Jerry. I read "record" the other way! Thanks. Looking forward to trying to restore this. Definitely need some help to figure it all out but it'll be really fun.

[ShoemanGB]

That cabinet is fantastic!  A+!

[shooter]

how would I remove the second 125K pot

I'd be inclined to leave it and use as a TS?  looks like a close enough "match"?  the record module could go in the "for next build pile" 

[Diverted]

That's what I was thinking too.

So that would leave a single volume, single tone, with two inputs (one high, one low?)
If I did that and used the first triode for both inputs, what then would I do with the other triode?
Or would it be better to use each triode for one input (high on one, high on the other), both sharing tone control and then running into the phase inverter?
Just trying to wrap my head around what I'm seeing around that V1. It all looks unfamiliar to me and doesn't look anything what I would think I'd see. Anyway, thanks for the suggestion!

[bmccowan]

Did you see this listing? https://www.ebay.com/itm/283628427113
The only Meck schematics I've seen are offered for sale by Amplified Parts.
But there are plenty of 2x6SC7 and 2x6V6 (or6L6) amp schematics from this era available, and they are all pretty similar with grid leak or cathode preamp bias being the biggest difference. And given that it seems that somebody really mucked around with this amp, I'd find a similar circuit and rewire this amp. I have followed the Fender 5B6 preamp schematic when converting old PAs to guitar amps, but there are many others; early Deluxe, Valcos, Gibsons, etc.

[Diverted]

Thanks. I'm thinking I may go that route. I was looking around and think I may go with the preamp circuit from the Fender Pro 5C5. It uses two 6SC7 preamps; mine only has one so I am going to delete one channel and use hole the previous owner used for the extra pot as a tone control.
I think I will leave everything starting at the phase inverter through output stage intact, as long as I can confirm the PI wiring looks OK. Can anyone take a look at the phase inverter in the schematic I posted and give me an idea if it's OK? I have crossed referenced it with other paraphrase schematics and it looks correct, but I am not sure. Also, just wondering if I should have grid leak resistors on both output tubes? I don't think so, but just wanted to verify. Thank you!

[sluckey]

Your PI, power amp, and power supply schematic looks believable and should work...BUT... Move the 500K resistor to the grid of the top 6V6. As drawn, the top 6V6 grid has no DC path to ground. I suspect the amp is actually wired correctly and this is just a drawing error. Double check to be sure.

The rest of the schematic is all messed up and if the amp is really wired like that it will not work properly. I would strip out all the preamp circuitry and rebuild the preamp that you want. 

[Diverted]

Thanks Sluckey.
Thanks. I think I may go that route:
1. Ditch accessory unit
2. Rewire V1 based on Fender 5C5 circuit (attached). This circuit has two preamp tubes; mine only has one so I'm only going to do the one. I'll use the second potentiometer previous owner used for a tone control, and leave it at that.
3. I don't think I'll touch anything after the preamp. Going to leave paraphrase PI wiring intact noting your changes.

I'm also wondering about the placement of both the output transformer center tap and the 6V6 screen grids on the same power supply node. In my limited experience I've seen these one node down from the output transformer node. Is this correct?

Thank you!

Ted

[sluckey]

I'm also wondering about the placement of both the output transformer center tap and the 6V6 screen grids on the same power supply node. In my limited experience I've seen these one node down from the output transformer node. Is this correct?

A lot of old technology amps only use one node for plates and screens. It works, but using a separate node for screens works better.

I don't care for grid leak biased preamps. I much prefer cathode biased preamps, like the 5D5.

[shooter]

Not sure you'll get as much umph in your example pre;     maybe something closer to tweed, gain-TS-gain-PI.you can still do the hi/lo jacks, or go 1 jack and switchable cathode bypass cap switch in empty hole

[Diverted]

Thanks. if I were to go gain>TS>gain>pi can I still use a 6SC7 with its shared cathode? The chassis is marked 6SC7 adjacent to the socket and I would like to use the original tube lineup so it can look as original as possible, and so as not to confuse anyone in the future.

[sluckey]

Thanks. if I were to go gain>TS>gain>pi can I still use a 6SC7 with its shared cathode?

no

[Diverted]

I don't care for grid leak biased preamps. I much prefer cathode biased preamps, like the 5D5.

Thanks for this tip. The 5D5 preamp is a 12AY7. If I wanted to use this preamp design with a 6SC7, would any other changes need to be made given the different gains between these tubes?

[PRR]

> both the output transformer center tap and the 6V6 screen grids on the same power supply node.

With the field coil in the B+ filtering, this can be clean enough. But since this is going to be a major refresh, I hate to assume the heavy irreplaceable field coil will be good forever.

[Diverted]

So, would you recommend putting the screens one node down from the OPT?

Thanks!

[sluckey]

Thanks for this tip. The 5D5 preamp is a 12AY7. If I wanted to use this preamp design with a 6SC7, would any other changes need to be made given the different gains between these tubes?

I would build the preamp like this schematic. You may want to experiment with the component values shown in red...

[Diverted]

Great! So just bypass the cathodes, add a resistor and drop the dropping resistor down. That I can do. When I get to this over the next week or so I will post voltages etc. Many thanks.

[Diverted]

Just tried re-drawing the schematic based on everyone's advice. How does it look? See any errors?

I moved the 6V6 screens to the third filter cap. They were on the second originally.

Thank you!

[sluckey]

As long as you are improving things may I suggest this... Separate plate B+ node, separate screens B+ node, separate PI B+ node, and separate preamp B+ node. See attached pic...

Your volume and tone controls are wired backwards. Look at the 5C3 layout to see what I mean.

[Diverted]

Wow, two additional filter caps. I can do that! If it makes for a safer cleaner more solid power supply I can drop those in.

Related question. This amp had been recapped and the size of the caps was all wonky... 10uf for first, 40 for second I think and 80 for third.
What would be your recommendation for sizing from first to last? Thanks.

I see what you mean on the pots. Thank you.

[shooter]

the "1st" cap is limited by the PS tube, lookup the datasheet for "recommended".  in PP amps 47uF for each node is probably overkill, but is ~ the same $'s as 20uF, so do you want saggy sound or crisper

[Diverted]

I finally got around to working on the amp. Rewired the front end, replaced all caps/resistors that had drifted (basically all of them), traced circuit out twice to look for errors, fired it up and hoped for the best. There's good news, bad news and some questions.

The amp is playing but the voltages seem very low.
At 110VAC wall voltage, I'm getting 314VDC at the rectifier. After the field coil (1.5K ohms approx) the B voltage drops way down to 243.
6V6 plates are at 236.5 average, with 13.5 on the cathodes.  Very cold! Bias resistor is 297 ohms.
Not surprisingly the amp is pretty low in volume.
After that, the voltages fall right in line down the B+ rail with about 110VDC on the V1 plates.
My main question for now is about the field coil. A voltage drop of 71 volts seems super high. Any ideas why this would be happening?
The output transformer is a replacement Stancor universal. It's tiny, smaller than a single ended Champ transformer.
I built the amp with five power supply nodes as suggested by Sluckey. The schematic is below .. disregard the backward wiring on the volume/tone pots .. I corrected that when wiring and they are wired properly.

Thank you!

My

[shooter]

 

Bias resistor is 297 ohms.

you have that made special!


250 is a closer value, I have a pp 6v6 250 R ~ 20vdc with 330vdc at plate
so my guess is your FC is dragging it some

[Diverted]

Ha! Actually a 300-ohm rated resistor.  All I had and I figured a little bigger would not hurt for now. The big dogbone 250 ohm was pretty cooked.

[PRR]

...field coil. A voltage drop of 71 volts seems super high. ...

No. Until we get free superconductors, an electromagnet MUST waste voltage. In older/bigger gear, sometimes as much as the audio tube's power. 20%-25% does not seem wrong.

[Diverted]

Interesting. Good to know.

So then I guess this amp is just made to run at lower voltages than I’m used to seeing. I’m going to drop the bias resistor value and get the 6v6s running hotter.
Pretty rewarding to reverse engineer this amp and get it playing again. I have learned a ton, and thanks to everyone who chimed in so far..

[shooter]

it's a point to point amp, take 4 1k 2W in ||, walla 250ohm 8W 

[Diverted]

Here's the voltages. Everything looks OK but still running very cold, around 55 percent or so.
I'm going to leave it as is unless someone spots something? I added a thyristor to the AC line to reduce inrush ... without it the amp was hitting about 510VDC before the voltages started dropping down. It keeps the peak DC on startup to around 490.

AC voltage: 123
Rectifier B+: 374

Node A: 277
Node B: 258
Node C: 216
Node D: 198

6V6A plate: 267
6V6B plate: 272
Cathodes: 15.6
Bias resistor: 247 ohms

V1 plates: 114
V1 cathode: 1.25

V2 plate1: 105
V2 plate2: 99
V2 cathode: 1.1