Newcomb Pathfinder E-10B

[SpareRibs]

Hello,
      I have this PA I would like to convert to a guitar amp. Is there a guitar amp with a similar configuration ?
 Thank you in advance for any information provided.

   

[eleventeen]

Ahhhh....Newcomb, the predecessor to Audiotronics, a co I worked at in the early 80's in North Hollywood, CA.

When you say "similar configuration", that opens up lots of areas, mostly "how similar is similar"? And how much electronics skill/experience do you have? It's a push-pull dual 6V6 amp with tube rectifier but with a rather unusual phase inverter (the tube/tubes immediately before the dual-6V6 output tubes) With smallish effort & cost, that could be converted to a Fender 5E3 Tweed Deluxe, a simple, direct, and proven design. Though the B+ (high voltage) is definitely on the low side, it would work. That's probably the most direct thing that most folks would suggest.

One evaluates:
How old and unused is the thing? Most amps from the 50's beg for electrolytic cap replacement, for safety and low-hum reasons. That would be so whether you wish to operate on it or to leave it essentially as is.
How much do you want to spend on it?
Do you know how to work on old amps and such?
Does the type of tone from an old Deluxe appeal to you?

Most people here would probably suggest trying to modify it very slightly, using it essentially as is. Of course you don't have tone controls....
Folks here could make valuable suggestions as to how to convert it to a 5E3. Generally speaking, amps like that were designed for clean sound and accurate reproduction within the limits of their power delivery; Guitar amps were designed (or it just turned out that way) to break up some as the volume is advanced.

Think about what you'd like to end up with.

[1rebmem]

That rectifier tube is only rated for 70mA. Would be OK for very low wattage amp.
What amp to convert it into depends on how much of the PA (parts) you wanted to reuse.

I'm thinking Fender Tweed 5E3. It takes 2 6V6's, 1 12AX7, 1 12AY7 and a rectifier.   Schematic:  http://www.prowessamplifiers.com/schematics/fender/Deluxe_5e3-Schematic.html
You could use your PA's two 6V6's and one 12AX7 tubes. Remove the 6AU6 and socket. Get a 12AY7. The 6X4 would be too small current output wise, replace with 5Y3. 
Your schematic shows the power transformer getting 117vac and you'll be giving it 120 something so your high volts and heater volts will be higher than the schematic shows. HV might end up being be good for a Tweed (350-360 ish). Heater can be reduced by .7 vac with 2 diodes installed in heater string. That output trans along with the 6V6's would make a nice tweed.

Having converted 3 old PA's into guitar amps, here's my humble opinion:
Replace all tube sockets (they're cheap), I've had trouble with old ones.
Strip away all the resistors and caps (they're cheap). You can reuse them but verify values. Carbon comp resistors drift high with age.
Remove the speaker impedance jack and put a plate in front or behind the hole and mount an impedance selector switch.
The old iron can sound real good.

[eleventeen]

If you look at this thread, you'll see one forum member's use of a parts board to convert a similar thing to a 5E3. He used 6L6 output tubes but otherwise the routine would be identical. http://www.el34world.com/Forum/index.php?topic=16358.0

From the comments on that thread, a number of folks, myself included, have commented that the physical placement of the board relative to the other parts is kind of "asking for trouble" in terms of hum and stability. You can go ahead and learn those things on your own via hard experience or take advantage of the folks here who have done these things before. I'm strongly recommending the latter course. You'll get plenty of help if you ask questions. 

It would IMO be to your benefit to take some pix of the thing and post them. Converting such an amp has various other considerations as to where your input jack(s) is/are/will be and various other layout type pointers that will prevent you from putting a lot of work into a configuration that invites hum and noise. One assumes you would also like to add a tone control as well. Those kinds of things don't show up on a schematic diagram but most folks here have been down the "conversion" road a good number of times and no doubt you could probably save yourself some grief. Building up an amp and having it be an unusable hum monster is a highly frustrating experience and there are some things that cannot be fixed. Better not to wander down those roads at all if you can avoid them.

[1rebmem]

+1 eleventeen. great comments!

[SpareRibs]

Hello,
       OK, I got the amp from a person that acquired it from a church. They were cleaning out a storage room and it was destined for the junk pile. It was not because of malfunction ,but because no one knew how to install or operate it. It is in pristine condition. I hooked a guitar up to it and it is really loud. I installed the jack in the phono position as the mike position is a rubics cube of wiring. I replaced all of the caps and, when I turned it on I thought it wasn't working as it was absolutely silent.
       If I have to add some tone controls I am capable of that, so therefore I would like to do as few changes as possible. I am not capable of gutting the whole thing and starting from scratch. I have a 12", 16ohm speaker to use, I was told using the highest rating was a  good thing.
       When I inquired about it before I was told it should have some type of a fender tone stack. I am not sure about which kind or where would be the best place to put it. So that is where I am. Also in my forum search in recent days someone commented on the resistor right past the 6AU6 as being excessive at 560k, possibly causing excessive gain.
       So any help anyone would be willing to offer would be greatly appreciated. Thank you in advance.

                                                                                                                                                                                                                                                                                          
      
  

[eleventeen]

Well the short answer is, if you're happy with it...don't mess with it. To install a tone stack, it's probable that you'd have to add another gain stage ( = another tube) and that sounds like it's more than you're wanting to do. Over time, maybe you'll become dissatisfied with the tonality of the thing and that might motivate you to do more. Or not. There are some kind of unusual circuit features in that design...a mere 40 volts on the 6AU6 plate; the phase inverter (in guitar amps, that usually occurs in a dual triode like a 12aX7 or 12AT7 arranged in sort of a symmetrical arrangement; in this circuit, you're inserting one half of a 12AX7 to flip the phase to generate the two out-of-phase signals) I apologize, I overlooked the treble control, so you have some degree of tone control.

If the gain is too high, you could also replace the 1 Meg input control with a 500K or just place a 500K or 330K resistor across the existing control. You say it's already quiet. It might get a touch quieter if you disconnected the mic input. Just cut the wire going to the center of the mic volume control.

Use it for a while as it is before you perform surgery. That's what I'd say.


 

[DummyLoad]

attached is the approach i'd follow initially.

--pete

[SpareRibs]

Hello,
      Eleventeen, wouldn't the tone stack like the Tweed Deluxe go between the plate of the 12AX7 and to the grid of the 6AU6, that is the way it is
set up now for the volume. Isn't the the tone part of the volume in the Deluxe. It seems if I drop the value of the volume control and eliminate the  mic volume control that would let the circuit support a tone control. I really don't know, I just do not want to tear the whole thing apart to start from scratch.
      You guys may think I am being difficult about this, however all of this is not as easy for me as it would be for you. If one of you take something
apart you see a lot of possibilities, I will just see a lot of problems. Sorry but as I said previously I am just not ready to jump in over my head at this point. I think for now I will have to think more about the various options.  Thank You for your input and assistance.

[SpareRibs]

Hello,
      DummyLoad, I think for now I am going to go with the option you have suggested. It seems the best for now, as I am not as well versed in
electronics enough to be able to gut and rebuild things at this point. Thank You.

[DummyLoad]

I just do not want to tear the whole thing apart to start from scratch.

then don't.  make minor changes. expand on those if you must.

edited schema w/ 1 more change - plate R of 1st (MIC) stage.

Thank You.

you're welcome. i'll bet that thing sounds really nice when your finished. 

--pete

[SpareRibs]

Hello DummyLoad,
      I did the mods you suggested. I think halving the value of the resistor was just a little to much. I just added the same value in parallel. So I will just keep changing it to a  lesser value until I find a medium that works.  I am thinking it is putting to much voltage through, as it started motor boating, at least there were no clouds of smoke. It's still a work in progress.
      Also I did not have a 1M resistor but I used a 1,5M that is quiet a step down from 18M so I am sure that helped a bit. Someday it will be a guitar amp.
                                                                                 

[DummyLoad]

did you replace the power supply capacitors? usually a device that old with original capacitors, the capacitors are weak and could be the source of the motor-boating.

--pete

[SpareRibs]

Hello,
      Yes. The first thing I did when I got it. I replaced even the ones in the can, I think those are the ones you mean by power supply. A long time ago I was making an amplifier out of a Magnavox record player. I could not figure out what was wrong with it. It was those power supply caps in a can. When I opened it it looked like a roll of soggy toilet paper. Since then I replace those the very first thing problems or not.
       Another thing that may be a problem, I was trying it with a different speaker I had never used with that amp. I will go over it again tomorrow.
It could be as simple as a cold solder joint. So all is not lost.
                                                                                        

[eleventeen]

" Eleventeen, wouldn't the tone stack like the Tweed Deluxe go between the plate of the 12AX7 and to the grid of the 6AU6, that is the way it is
set up now for the volume."

It might...and then again, it might not. Yes, that's the generic "positioning" of a tone stack, but without the resources to fiddle with the gain before the stack and the gain after the stack (the TS eats a fair amount of gain) it's not trivial to do so without true mathematical circuit analysis. Or certainly, a much better guesstimate than I can furnish. PRR or HBP or a few others here could do it. Because the topology of this circuit is kind of on the odd side (compared to Fender amp style) I'm reluctant to opine "you can just throw it in there. OTOH, I myself might physically build it (without doing any calcs) and throw it in there and see how it worked and then try to tweak it to my liking. I myself can't give you an algorithm such as "change R42 to 47K and R49 to 33K". With a cigar box full of parts, I might well try to actually *do* something kinda like that with the idea that if I backed myself into a corner I could rip it all out and restore things to stock.

OTOH, I myself would probably lean towards ripping the whole thing apart and building a 5E3 from scratch, including replacing the 6AU6 (a 7-pin tube) with another 12AX7 ( a 9-pin tube, implying a socket change, implying slightish metalwork) but also implying moving directly towards the known-to-work with a parts list that I could know the price of, buy, and shove in there with 90+% confidence that it would all work out at known cost and known time expenditure. I'd probably want to move to a solid state rectifier to crank the B+ up some, say 35-50 volts, as well. This happens because the voltage drop across the 6X4 tube rectifier goes away.

There's every reason to believe that with the 6AU6 pentode after a tone stack, one could recover the losses that the tone stack is expected to produce. There's also good reason to believe that both the first triode of the 12AX7 AND the 6AU6 would want to see higher plate voltages than 75 and 40 volts, respectively. In other words, the tube sections are there, but there would almost certainly be a requirement to fiddle with the pre and post tone stack gains, mostly by changin plate resistors. One would probably want to add/adjust cathode resistors. It doesn't sound like you're equipped to make those changes so I am kinda discouraging you from starting. These things are not difficult to do but it is essentially impossible to do without a pile of resistors and caps of various values you can throw in and see how it works....or not.

[PRR]

> power supply caps in a can. When I opened it it looked like a roll of soggy toilet paper.

That's what it's supposed to look like. (Doesn't prove it works.)

When it looks like dirty DRY toilet-paper, it's expired. Even "dry electrolytic" caps have a touch of dampness when fresh and happy.

The original question.... if going into PHONO works for you, or Pete's minor mod (which may be too much), then be happy. Pick a semi-bright speaker to get the treble-rise guitar sound usually needs. There is a LOT of good music in a no-frills amp. Makes you focus on your skills instead of letting the amp gaudy-up the sound for you.

The phase inverter was VERY common 1938 thru early 1950s.
http://www.valvewizard.co.uk/paraphase.html
You have drive for one side (here, the 6AU6). You want unity-gain inverted. Any gain-amp stage inverts. Put a gain-cut ahead of it to kill the gain. Here it uses a 560K:27K divider, a 22:1 drop. The 12AX7 with cathode bypass gives gain near 50, but half that without bypass, 25. So 25/22= 1.15, which looks a bit higher than 1.00. But maybe it is closer than it looks. And all tube-work is +/-20%.

What I do not recall in a guitar-amp is using the pentode gain/driver with the paraphase inverter. Usually we used a twin-triode. By using the pentode they get from mixer-level, 0.1V, all the way to power-tube grid level (20V), in one stage. Other designs use two triode stages to make that leap. This also gives an intermediate point to hang tone-controls in. I think they got a bit ahead of themselves here because the treble-cut in-side the NFB loop acts funny. Small cut does not cut, but raises treble distortion. Heavy cut *may* give a very steep cut-off. Heavier cut may oscillate. Maybe it did just what they needed for church PA.

[1rebmem]

OTOH, I myself would probably lean towards ripping the whole thing apart and building a 5E3 from scratch, including replacing the 6AU6 (a 7-pin tube) with another 12AX7 ( a 9-pin tube, implying a socket change, implying slightish metalwork) but also implying moving directly towards the known-to-work with a parts list that I could know the price of, buy, and shove in there with 90+% confidence that it would all work out at known cost and known time expenditure. I'd probably want to move to a solid state rectifier to crank the B+ up some, say 35-50 volts, as well. This happens because the voltage drop across the 6X4 tube rectifier goes away.

 

Though the OP doesn't want to get into gutting and starting from scratch , I'm with eleventeen.
I do think his B+ will be high enough because that schematic shows 117vac on the primary and he will be giving it probably 122-123vac so his secondary will me about 300-0-300 unloaded. That 6X4 would need to be replaced and a nice 5Y3 would get him about the right B+ for a tweed and have a more authentic flavor.
Just my thought.....if he went this way with a Tweed.

[sluckey]

That 6X4 would need to be replaced and a nice 5Y3 would get him about the right B+ for a tweed and have a more authentic flavor.

EZ81 might be a better choice since you would not need to replace the PT or add a filament transformer.

I don't think SpareRibs is ready to do anything like this though.

[1rebmem]

That 6X4 would need to be replaced and a nice 5Y3 would get him about the right B+ for a tweed and have a more authentic flavor.

EZ81 might be a better choice since you would not need to replace the PT or add a filament transformer.

I don't think SpareRibs is ready to do anything like this though.

Good point about using an EZ81

[PRR]

The 6X4 has been fine in this amp for a very long time.

We aren't changing the load impedance or (significantly) raising the B+. The rectifier current is the same now as in 1953.

[SpareRibs]

Hello All,
      Well today I made some progress. I made some alterations with the resistor from input to the cathode of the 12AX7 it is 1.4M that is the lowest value I had. It was 18M. I also installed the right capacitors on the tone stack after some experimentation earlier in this conversion. I halved the resistor going to the plate of the 12AX7, from 560K to 270K or thereabouts. It sounds cleaner now. It still seems to lean towards the bass side.
       I installed the Fender Tweed Deluxe tone stack. I am not sure if that is the best choice but I found the layout for it and it seemed better than what was there. I just used the phono volume pot for tone. Anyway its much better now, but still room for improvement.
       As far as ripping it apart and building a 5e3 from scratch, I would rather not go there. I am having enough problems now just modding this
one. If I tried to build an amp from scratch I could get two of the wires crossed or something simple like that and never figure it out. also isn't
a Champ an SE amp. I saw the kits for those go for $ 2-300.00. I have a Magnavox record player that I got to work for an amp, It didn't work as well as I was hoping as its stereo.  If I was going to do any building, I would try to turn that into a Gibson Crestline. It has most of the tubes and the Crestline looks much simpler. But at this point I am a long way away from a scratch built amp.

[1rebmem]

I'm thinking you're closer to a scratch build than you think, based on what you've done on this one.
I'm impressed so far!

[SpareRibs]

Hello,
      Thanks, I just wish I had as much confidence in myself as you do in me.