Newcomb M-5

[Guitarzan]

Hiya!
I just scored a Newcomb M-5 on fleabay and was wondering if anyone else has had any experience with one of these. According to radiomuseum.org, it's 4.5 watts which should (in my mind) put it in the same realm as my 5F1 Champ but I tried it out and it doesn't get anywhere near as loud as the Champ. It's cute as a bug's ear tho. Here's the link to it on radiomuseum...http://www.radiomuseum.org/r/newcomb_guitar_amp_m_5.html.
I was planning on going thru it and replacing caps and resistors as needed but if anyone has any ideas for what to mod on it to get the volume up there closer to the 5F1 I'd really appreciate it. I hope I got the schemo attached properly.

[terminalgs]

put a boost pedal, or any pedal that can give like +12dB or more of clean gain.  See what you get.  the 6SQ7 is most likely not giving you enough gain.   If more out front gain wakes up the amp, you might put a bottle in front of it,, or swap it out for a 6SL7 and use both triodes for gain....

Depending on the secondary winding that might be a lot of NFB to keep it clean and stable. disconnect the NFB temporarily,  see what it does.

[Guitarzan]

put a boost pedal, or any pedal that can give like +12dB or more of clean gain.  See what you get.  the 6SQ7 is most likely not giving you enough gain.   If more out front gain wakes up the amp, you might put a bottle in front of it,, or swap it out for a 6SL7 and use both triodes for gain....

Depending on the secondary winding that might be a lot of NFB to keep it clean and stable. disconnect the NFB temporarily,  see what it does.

I checked on that but the 6SL7 won't sub for the 6SQ7 which is a triode/dual diode.
I have a tube screamer I could try. Not sure of how much clean boost it'll provide tho.
Also, I should prolly mention that I have the harp player across the street in mind for this one. He bought one of my 5F1 builds and loves it. He calls it his luchbox & has gigged with it about 50-60 times over the past year. So I figured if he loves the tweed champ in part for it's size, then he'll flip over this thing since it's much smaller and lighter than a champ. It just doesn't seem like it's putting out anything like 4-1/2 watts.

[DummyLoad]

tone stack & filter at input has significant loss, NFB adds some loss.


disconnect tone-stack & disconnect NFB.


see attached schema for suggestions. 


--pete

[Guitarzan]

DummyLoad,
I will try that. Thanks very much for the drawing. I'll post the results back here as soon as I get this accomplished. I think it'll be Tuesday though. My son is coming home from hospital tomorrow & I don't think I'm gonna be able to work on anything until late tomorrow or more likely Tueday morning. I knew the tone stack could cause some loss but I had no idea it could be that pronounced. This thing wide open is like my champ on about vol-2.
Thanks again!

[PRR]

This is a phonograph amp, not a guitar amp. (I think RadioMuseum is wrong.)

Geetar is weaker than (old-style) phono pickups. You need one more stage of gain.

> same realm as my 5F1 Champ

You note that triode-preamp Champs have *two* triode gain-stages (or a hi-gain Pentode) before the 6V6.

You aren't boosting the guitar enough to SMACK the full several-Watts out of the 6V6.

We can estimate input sensitivity from hoped-for output and NFB network values. 4W in 4 Ohms is 4 Volts. 2700/100 is gain of 28. 4V/28 means 0.14V or 140mV needed at the input to get 4W output. Guitar amps are usually shimmed to need only 20mV input for full output. 50mV sensitivity (very early amps, Swing era) won't rock-out well (OK for polite rhythm work).

Like my backhoe- it has 20HP of yank, but the control lever linkage is loaded so I can't pull hard enough to get full hydraulic yank when the hoe is far off-side.

That tone-network isn't doing much loss when full-up.

The actual sensitivity is less than calculated above (the NFB loop isn't doing much). 14.5V peak 10V RMS at 6V6 grid, gain of 50 in 6SQ7, means 200mV input needed. You gonna need a HOT pickup and a handful of rage-pills to strum your axe near 200mV more than a few moments. That's working too hard. That's what amplifiers are supposed to do for us.... you need more low-level amplification.

Short answer: put a boost pedal in front. LBP is a classic. Mild fuzz turned-down out of gross fuzz would do.

Long answer: re-wire the first hole for a 2-stage 12SL7 6SL7 preamp.

[DummyLoad]

why 12SL7? heater shows 6.3V winding?


--pete

[Guitarzan]

PRR,
Thanks a bunch for that eval. I have seen a bunch of your posts to other guys looking for advice and was hoping you'd chime in on this one.
I have never modded or built from scratch. Only done kits up to this point. So bear with me if I sound ignorant, it's only because I am.
So why do you specify a 12SL7? Wouldn't a 6SL7 be appropriate? The 6 or 12 in this discussion is the filament voltage isn't it?
Also, would I refer to a champ 5C1 for how to wire that tube socket? I'm not great with schematics, but the 5C1 looks to me to be almost exactly like the Newcomb if the tone stack was taken out like DummyLoad suggested.

[Guitarzan]

I did manage to get it opened up and had a look inside. Newcomb used a pot with a switch on the tone and volume. So if I did lose the tone stack, it would be a simple matter to move the power switch wiring over to the volume pot ala champ.

[terminalgs]

I checked on that but the 6SL7 won't sub for the 6SQ7 which is a triode/dual diode.

no it won't, that'll require rewiring the socket, and adding a few components to provide a 2nd triode.  I suggested a 6SL7 because its the highest gain twin-triode octal that you can round up for a couple of buck$.   The 6SQ7 is only a single triode,..  Its' like 1/2  a 12AX7. for guitar, you want a full 12AX7,, or a 6SL7 if you want to stick to octals.

[thermion]

we run 12a_7 all the time off of 6.3vac filament supplies...

[Guitarzan]

Hello to all.
I'm thinking that if I'm going to start rewiring stuff in there, I might as well go all the way and convert it to a 5C1-type of circuit. Any thoughts on that plan?
There is one other possibility that could change things tho. I saw a 12AX7 to 6SQ7 adapter on eBay and was wondering if anyone knows anything about these and whether or not I can actually just plug it into the amp, plug a 12AX7 into it, and be good to go. If not, then I think the 5C1 is gonna be the plan.


So...the next questions I'll need to get answered are:
#1 Will the current PT be able to handle the 5C1 tube lineup? I'm guessing the answer is yes since it can handle the existing 3 tubes, but figured I'd better ask to be sure.
#2 Can I use the existing cap can rather than trying to cram the 3 filter caps in that teeny chassis?
#3 If the cap can will work, do I have to adjust the value of the associated 2-Watt resistors?
#4 Will the existing OT still work with the 5C1 changes?


I have a set of ClassicTone 5F1 trannies (I think it's the same ones for 5C1) here so I could use one or both as needed but the existing PT is also tiny, just like everything else on this unit. However, I believe it could work, but it's gonna be a bit of a squeeze as it is. I don't want to have to try and fit a larger transformer on it if I can help it.

[Guitarzan]

The M-5 schemo didn't take. I tried to attach it with the 5C1 docs but it didn't go for some reason.
so here it is

[Guitarzan]

Actually on those resistors, it looks like most modern schematics (as well as the 5F1 kits I built) use a 5-watt and and a 1 or 2 watt and NOT 2-watt in the filter.

[DummyLoad]

#1 Will the current PT be able to handle the 5C1 tube lineup? I'm guessing the answer is yes since it can handle the existing 3...
yes, 6SQ7 draws 150mA and 6SJ7 draws 300mA. additional 150mA should't be a problem. 

#2 Can I use the existing cap can rather than trying to cram the 3 filter caps in that teeny chassis?
yes, assuming can cap filters are still good though...


#3 If the cap can will work, do I have to adjust the value of the associated 2-Watt resistors?
no, just change the 6SQ7 to 6SJ7: add, screen resistor and screen bypass cap, change cathode and plate resistors to 5C1 values (or close standard values). the newcomb shows 275V supply to triode and 5C1 shows 280V: the difference is negligible. you may need to adjust 12K resistor to get close to 280V supply to 6SJ7 as in the 5C1 and if you do, a higher wattage rating won't hurt.


the filaments move from 7 & 8 (6SQ7) to 2 & 7 (6SJ7). the schematic shows one side of the filament secondary grounded.


on the 6SQ7 socket you have either pin 7 or pin 8 tied to ground at the socket.


a) if pin 8 is grounded, then lift it off ground as pin 8 becomes the plate; now ground pin 2.
b) if pin 7 is grounded, then move the filament supply wire to pin 2.



#4 Will the existing OT still work with the 5C1 changes?
irrelevant to the changes being made.

[terminalgs]

I saw a 12AX7 to 6SQ7 adapter on eBay and was wondering if anyone knows anything about these and whether or not I can actually just plug it into the amp, plug a 12AX7 into it, and be good to go.

I'm not what they might be doing with such a connector other than using only a single triode of the 12ax7 wired to the 6sq7's triode (and ignoring the 6sq7's diodes).  It the end,  your circuit is wired for a single triode.  both the 6sq7's triode and a single triode of a 12ax7 has a max Mu of 100, so .... it'll do nothing really...

If not, then I think the 5C1 is gonna be the plan.

I second (or third!) the motion to "just re-wiring the 6sq7 socket to accept a 6sj7"   

follow Dummyload's rewiring, also ground  pin1 (the shield).  make sure you have a coupling cap between the grid and the guitar and you should be good to go.

I'd use the transformers as-is.


After that, see where your at gain/volume wise,, and then plan the next move (if needed).

#2 Can I use the existing cap can rather than trying to cram the 3 filter caps in that teeny chassis?
#3 If the cap can will work, do I have to adjust the value of the associated 2-Watt resistors?

I'd start AS-IS.  once running, you can measure voltage drop across resistors to see how close to the resistor's wattage rating you happen to be.. (or over..).

[DummyLoad]

6sj7 in 5C1 is drawing ~650uA.


looking at the schematic, i think 90V is stated at the plate of 6SQ7 with 275V supply.
6SQ7 is drawing 185V/470K or ~390uA


45V drop across 12K is 3.75mA @ 168mW; that's screen current + 6SQ7 load of 390uA. so screens are pulling 3.75mA-390uA=3.36mA


new load is 3.36mA+650uA=4.01mA @ 185mW


45V/4.01mA=10.9K   replace 12K with a 10K 1/2w -or- just leave it and live with a slightly lower supply voltage to 6SJ7. 


--pete

[Guitarzan]

#2 Can I use the existing cap can rather than trying to cram the 3 filter caps in that teeny chassis?
yes, assuming can cap filters are still good though...

In addition to it being quiet due to low gain, it's really very quiet as it pertains to hum. No hum at all really. In fact it sounds quite good save for the way too low gain situation. The clean sounds chimey and open until about half way up on the volume pot and then it distorts nicely the rest of the way up. It would make a good bedroom amp. Really bedroom...not like the 5F1 which is way too loud (IMO) for bedroom volume when you want it to give up that tube distortion.

[Guitarzan]

I want to thank all of you guys who've replied to this thread. i truly appreciate your input and will let you know what my results are when I make the changes.
that cap can also has the 25/50 cap for the 6V6 in it. I'm assuming I can also reuse that. I do have caps of that flavor in my boxes of stuff, but again, space is going to be an issue if I start trying to cram "normal" sixed parts in there. As I mentioned, the thing is loaded with mini-components. Evidently, that was how they managed to make the finished product about the size of a Batman lunchbox.

[Guitarzan]

I'm getting some 6SL7s from a guy who wants to know if I have a preference for brand, black or grey plates, siver dome, etc. I have no experience with this tube so I don't have a clue how to answer those questions.
So what do you guys like?
If there are common drop-in subs that can experiment with, what would they be?

[DummyLoad]

RCA red base 5691; sylvania chrome top with black round or triangular plates; tung-sol shorty's with the smoke glass.


--pete

[PRR]

"12SL7" was a typo. Sorry.

> we run 12a_7 all the time off of 6.3vac filament supplies...

12AX7 and kin have a tapped heater which can run on 6V -or- 12V.

Two separate triodes plus a tapped heater means 9 pins.

This chassis has Octal (8-hole) sockets. And 6V heat supply. He wants the 6 Volt version of a two hi-gain triode. This would generally be 6SL7.

(6SC7 seems suitable BUT both cathodes are tied together. This works fine for many things, but not for sequential gain stages.)

He can of course fill the Octal hole and punch it for a 9-mini socket, use the familiar 12AX7.

One Pentode "can" give the required gain. However that much up-front gain can overload before the volume control. All the one-pentode-preamp amplifiers faded for twin-triode plans.... they handle much wider range of input levels. However pentode preamps do have fans.

[Guitarzan]

Here's a shot of the innards for anyone who might be interested...

[Guitarzan]

He can of course fill the Octal hole and punch it for a 9-mini socket, use the familiar 12AX7.

Is there an "official" piece of hardware to do this? Or would I just have to put a "patch" over the octal hole and punch it for the 9-pin mini?


I'm starting to think about just going for the 12AX7 since I have a bunch of them already. I have a bunch of sockets, & tube shields too.

[sluckey]

Weber has an adapter for this. Will it work with your size octal hole? I don't know.

I would probably stay with the octal socket and rewire it for a 6SL7. Look at some old Ampeg schematics for some good sounding circuit options.

[DummyLoad]

cool looking little chassis...

my vote gets the 6SL7GT: no machining involved.


if it were mine: i'd remove the pot closest to the chassis edge and replace it with an input jack: the hole size is the same. leave center pot and AC switching alone.


attached is a simple champ circuit i'm fond of...  whatever you decide, keep us in the loop.


--pete

[Guitarzan]

Pete,
I will definitely keep you guys posted on this. Thanks again (and again) for all of the advice and schemos and everything.
I do like the idea of going with the octal preamp tube for the simple fact that the socket is already there. I've been browsing all sorts of web pages regarding this stuff and found quite a few folks reprorting octal preamps tubes having a tendency to going microphonic. That was why I began leaning toward the 12AX7. I've run into some 12AX7s that were microphonic tho, so I doubt it's anything unique to the octal tube. Only I have a box full of 12AX7s and not a single 6S*7 in the house. Not the end of the world tho. Then I read that the 6X5 tends to be a problematic rectifier so I started thinking of going with a 5Y3** which I also have a slew of in here. However, that little tranny doesn't have any 5v secondary so it would mean (ugh) even more machining to make a champ tranny fit.
 
So I guess I'm going to have to get hold of a few 6Sl7s or equivalent and just do it that way. It seems to be the concensus on this board as well.
 
BTW, I couldn't open the attachment with the .sch ext. Do you have it in any other foramt?

[Willabe]

BTW, I couldn't open the attachment with the .sch ext. Do you have it in any other format?

Here's a link to download the free .sch program.

http://el34world.com/Forum/index.php?board=12.0 


              Brad     

[John]

Guitarzan, just 2 quick things from a guy not all that experienced. First, I love the 6SL7. I think they sound very sweet, and the sockets are easier to wire cuz they're bigger.    Nothing wrong with the 12ax7 though, they don't sell a million amps with them because they're not any good. And don't worry about installing a filament transformer, just use a couple diodes, the 1N4007s will be fine, in place of the rect. tube. If you want to have the sag of the tube, but want some protection just wire the diodes in series with it. Just my 2 cents.

[Guitarzan]

cool looking little chassis...

my vote gets the 6SL7GT: no machining involved.


if it were mine: i'd remove the pot closest to the chassis edge and replace it with an input jack: the hole size is the same. leave center pot and AC switching alone.


attached is a simple champ circuit i'm fond of...  whatever you decide, keep us in the loop.


--pete

Hey Pete...


This Newcomb mods pdf looks simple enough for even me. I like it. I'm going for it.
I thought about leaving just a volume/power switch per your vote, but I think I'll keep the tone control too. I have some appropriate silver mica caps that I can use there and I was thinking along the lines of a 5F2a type since from what I've read all over the place, it is a low loss setup and I just want something different from a champ type of amp for this project.
Sure, it's still basically a 5C1 with tone stack, but...y'know...


So my very first idea for this thing was to add an input jack on the other side from the pots. It'll go right next to the preamp tube. You have to open the back to use it anyway and the little face-plate on the front says tone and volume so that'll keep that part of it stock. I found a Jensen P69R for 25 bux delivered. The stock speaker has a torn surround anyway so out it goes.


I don't doubt that as PRR said, it's likely a record player amp, but I think Newcomb took an amp that they already had on the shelf and made this setup out of it. The patch cable has 1/4" phone plug on the one end, and is hardwired in the chassis, plus it shows the 1/4" plug on the schemo. So whether they had guitar amp in mind, or just some a powered ext speaker for something I don't know. But I have no doubt it came from the factory just the way I got it.


A grounded power cord will also be in the plan. Whether or not I put one of those IEC connectors in there, I still haven't decided.


So thanks again for everything. I knew I'd get good answers to my questions here. As I have researched other projects along the way, I'm consistently directed to this board by Google and have always found the members to be great in the way they make really good suggestions, and make 'em understandable for guys like me. Very friendly bunch here.   


I'll take some pix along the way and post 'em with progress reports. It's the least I can do in return for the help. You should be kept in it as it's really your project to in a way.


TTYL
-d {8^)

[DummyLoad]

GZ, happy to help.


if you're going to drill another hole, suggest moving both pots closer to the center and use the hole at the edge (where the tone pot is now) for the input jack. the input jack should be as close as possible to V1.


leave the AC switching on the tone pot: switch neutral. take the neutral wire from IEC to switch on the tone pot then to PT. the hot goes from IEC to fuse, out of the fuse to PT. if you decide not to cut for an IEC, then just enlarge the strain-relief hole to accept and 18/3 power cord and 18/3 size strain-relief.


i updated the schematic.


good luck! 


--pete

[Guitarzan]

Yeah! That's pretty close to the way they wired it.
I'm not used to seeing the neutral switched. I used to wire homes for a living and you never switch the neutral in that world.
BTW, Where I was talking about drilling for the input jack is actually closer to v1 than putting it in the pot hole.
I appreciate that you have my back tho.
Thanks for the mains tip and the updated docs!
Just for your review, here's a gut shot with the phone plug right in the neighborhood where I'm talking about putting it. v1 is in the upper right corner.
I tried to get a shot of this weird little wafer thingy (cap?)
I've never seen one like that before. It has about 6 wires coming out of it.

[Guitarzan]

I goofed up the attachments again and only got 1 of 'em.
Here's the one with the proposed input jack loc.

[Willabe]

I tried to get a shot of this weird little wafer thingy (cap?)
I've never seen one like that before. It has about 6 wires coming out of it.

It's a pre made circuit. Ampeg used them in the tone stack in some of their amps.

They were made for company's to wire up amps and other things faster, less solder joints.


                Brad     

[Guitarzan]

I tried to get a shot of this weird little wafer thingy (cap?)
I've never seen one like that before. It has about 6 wires coming out of it.

It's a pre made circuit. Ampeg used them in the tone stack in some of their amps.

They were made for company's to wire up amps and other things faster, less solder joints.


                Brad     

no kiddin' hmph. I'll have to figure out what part of the schemo represents the thing.
I thought it was probably an OEM sort of thing cuz it only has the numbers 1-8 on it. I No mfd or ohms symol or anything that might identify it otherwise.
I think there are 7 wires. I think it skips a number so there's like, 1-5 then 7 & 8 or something like that. I'll get a better look at it when I start de-soldering tomorrow

[Willabe]

I tried to post a highlighted pic of an Ampeg B-15's tone stack circuit but I'm unable to do it.   

Look in Doug's schematic library of an B-15 and the part of the tone stack that is in the dotted lines (caps/resistors) is like you have.

Here's a link;

http://el34world.com/charts/Schematics/files/ampeg/Ampeg_B15N.pdf


                   Brad     

[P Batty]

I bet it's a multi-section capacitor. Recently I got a couple included with a bunch of TV parts, they were set up in just like that.  Test it with capacitor checker, or you may find a part number on it, you should be able to figure it out.

[Guitarzan]

I tried to post a highlighted pic of an Ampeg B-15's tone stack circuit but I'm unable to do it.   

Look in Doug's schematic library of an B-15 and the part of the tone stack that is in the dotted lines (caps/resistors) is like you have.

Here's a link;

http://el34world.com/charts/Schematics/files/ampeg/Ampeg_B15N.pdf


                   Brad     

The part of mt schematic that's in the dotted rectangle is definitely the cap can. It says CE-36 (I think) under it.
The pots have individual caps and resistors soldered on and that chip thingy is soldered to V1 and V2 so I don't think it's part of the tone stack.
I dunno...I s'pose I'll have to get it nailed down tho.

[Guitarzan]

I bet it's a multi-section capacitor. Recently I got a couple included with a bunch of TV parts, they were set up in just like that.  Test it with capacitor checker, or you may find a part number on it, you should be able to figure it out.

I hope so. I think I will.
It's the right color to be one of those little disk-type caps. That was my 1st thought. That it was a multi-cap of some sort. Sort of like a disk version of a cap can?

[P Batty]

Here's a picture with the application sheet, I'm sure yours is a little different but probably similar:

[sluckey]

It's a resistor/capacitor pack. They were very popular during the '60s for consumer electronics. They ain't all the same. But does it really matter what's inside yours? Don't you plan to replace that tube with a 6SL7 dual triode and build a totally different circuit?

[Guitarzan]

It's a resistor/capacitor pack. They were very popular during the '60s for consumer electronics. They ain't all the same. But does it really matter what's inside yours? Don't you plan to replace that tube with a 6SL7 dual triode and build a totally different circuit?

Yes, that's the plan.
So I guess I don't need to worry about any of the functions it performs.

[Guitarzan]

Here's a picture with the application sheet, I'm sure yours is a little different but probably similar:

Well it looks almost exactly like that. Yeah, it probably has different values than this one but that's definitely the idea.
Slukey makes an excellent point but it's still good to know what it is/was.
thanks.

[Guitarzan]

I have a question that's sort of related but not exactly. Anyway, here goes...
On a champ turret board the 1st 2 filter caps have the grounds tied together but the 3rd one is grounded separately from those 1st two.
Why aren't the 3 filter caps tied together on the neg side?

[Guitarzan]

I removed all of the wires from the V1 socket and am I ever frustrated! The person who put it together was fond of wrapping the wire around the lug 2 or 3 times in addition to going thru the lug with it. Wow. It really makes it a pain to disassemble.

[terminalgs]

I removed all of the wires from the V1 socket and am I ever frustrated! The person who put it together was fond of wrapping the wire around the lug 2 or 3 times in addition to going thru the lug with it. Wow. It really makes it a pain to disassemble.

yyup.  a small flat-blade screw driver is helpful to pry the wire off the lug while the solder is hot... snip... heat,, suck, heat, pry, snip...

[DummyLoad]

I removed all of the wires from the V1 socket and am I ever frustrated! The person who put it together was fond of wrapping the wire around the lug 2 or 3 times in addition to going thru the lug with it. Wow. It really makes it a pain to disassemble.

yyup.  a small flat-blade screw driver is helpful to pry the wire off the lug while the solder is hot... snip... heat,, suck, heat, pry, snip...

i use an x-acto knife and soldapullit, and of course, small cutter and needle-nose pliers. on octal sockets you can heat and flick the terminal with your iron and the solder splatters off: mind your eyes and face (wear safety goggles hombre!). kind of messy but effective.


soldapullit


GZ, have you decided what's going in for a preamp?


--pete

[sluckey]

On a champ turret board the 1st 2 filter caps have the grounds tied together but the 3rd one is grounded separately from those 1st two.
Why aren't the 3 filter caps tied together on the neg side?

The first two caps supply the power tube plate and screen. They are considered the 'power' ground.

The third cap feeds the preamp tube and is considered the 'preamp' ground. It's good practice to keep the two separated so the preamp current does not flow thru the same path as the high current power ground.

Those two grounds are electrically connected together thru the chassis, but the currents flowing thru each ground circuit is kept separate.

[DummyLoad]

just doodling... based on princeton 5F2-A preamp. drawing generated with visio 2010. if you'd like to have it in native visio format, PM an email address.


hoping others here will help check for errors.    


happy rosin smokin'. 


--pete

[Guitarzan]

On a champ turret board the 1st 2 filter caps have the grounds tied together but the 3rd one is grounded separately from those 1st two.
Why aren't the 3 filter caps tied together on the neg side?

The first two caps supply the power tube plate and screen. They are considered the 'power' ground.

The third cap feeds the preamp tube and is considered the 'preamp' ground. It's good practice to keep the two separated so the preamp current does not flow thru the same path as the high current power ground.

Those two grounds are electrically connected together thru the chassis, but the currents flowing thru each ground circuit is kept separate.

Wow! I really can't get my head around how they're separate, when they ultimately go to the same PT post an inch away.
I do get that the 1st two are power and the 3rd is pre tho.
Thanks for the explanation.