Little amp

[davidwpack]

I've had this little fella in the closet for a while and think I might try to do something with it. Anyone recommend something I can build in a 10.5"x3"x3" chassis? Pentode preamp, se, and tube rectifier. I really dig the box and want to build something in there. I've got 2 vibro champs so I'd like to deviate from that direction. Here's pictures.

[davidwpack]

2 more.

[shooter]

plexi pre with a SE 6V6 very abused spec's 
head only, or with speaker?

head only; same pre, SE EL34 sweating

[davidwpack]

I'm thinking I'd like to use a speaker in there. Probably not that speaker I'm thinking.

[Tony Bones]

I've had this little fella in the closet for a while and think I might try to do something with it. Anyone recommend something I can build in a 10.5"x3"x3" chassis? Pentode preamp, se, and tube rectifier. I really dig the box and want to build something in there. I've got 2 vibro champs so I'd like to deviate from that direction. Here's pictures.

What's wrong with it the way it is? Seriously, with only one preamp socket, most practical circuits are going to be similar to either a Champ (two triodes), or what you have now (a single pentode.) I suppose you could use a 7199 or similar, but that just brings me back to the question: what's wrong with it the way it is?

One thing that is wrong with it is that it runs straight  off the mains so has no PT. But, anything you do with this cab and chassis will require the addition of a PT no matter whether keeping the rest of the circuit untouched, modifying, or ripping it all up re-doing.

[davidwpack]

Yeah that why. I intend on adding a PT. I don't have a schematic and have no idea what the tubes are and I'm pretty sure it's probably an electrocution hazard. I've debated restoring it as original for years but...

[shooter]

I don't have a schematic

so, you're qualified now, create one, should take you a nice relaxed evening V1pin1 to .....pin2 to......
I would do it even if I decided to gut n build

[tubenit]

What about using a 6BM8 tube which has a triode and pentode in one tube.  The 6BM8 pentode sounds different then a 6V6, IMO.    This schematic is just an idea and is NOT a proven design.  I have no idea what your PT is, so take this as a draft concept where you'll have to work out the PT and B+ rail.

You said you wanted something that was not VibroChamp-ish.   

with respect, Tubenit

[davidwpack]

Yeah, I'm going to draw something up when I figure out what to build. As far as the existing amp goes, I wasn't able to figure out what the tube compliment was so I just never drew one up.
That looks really interesting Tubenit. I'll definitely consider that. Also, I'm wondering why 75k is printed on the speaker?

[sluckey]

That may be part of the Magnatone family. Could be worth some money.   

If you want something simple that's not just another champ. look at this. Simple schematic at bottom of page...

     http://sluckeyamps.com/smoky/smoky.htm

[davidwpack]

Yeah, I really like that Sluckey. I may use a tube rectifier since I have a hole for one...or I may just bend a new piece of metal for it and Just pull this old one out and keep it as it is. It's just a two-sided L-shaped piece of metal. I should be able to bend a new one pretty easily. Thanks! I might build that one.

[PRR]

It's got a 50L6. That's a fine power amp. Small for stage but ample for bedroom or recording.

Tuck an isolation transformer high in the cabinet. Restore what's there, it isn't much. I bet 95%+ of the parts are still good. I'd even keep the 59-cent OT and 89-cent speaker for now.

The general plan will be the early pentode-preamp Champ (and many others like my Kent) except working on 100VDC.

[kagliostro]

Only one thing, in SE amp you really don't need a tube rectifier, if you want you can do without

Franco


BTW you say nothing about the original schematic

[tubenit]

That may be part of the Magnatone family. Could be worth some money.   

Wondering IF it might be the Magnatone 107?  Or similar to it?

With respect, Tubenit

[davidwpack]

Thanks PRR. I think that's pretty close.
Kagliostro, there is no original schematic. The only writing on the amp reads Don Noble.
All the tubes in there are the exact same size including the preamp tube. (About the size of a 6V6.)
If I can restore a safe version of the original I would love to do that beginning with PRRs suggestion of an isolation transformer. Thanks guys!

[bmccowan]

I believe the Magnatone connection is right. Don Noble amps were contracted to Magna and maybe Danelectro, but that chassis sure looks like Magna. They became part of the Strum and Drum distribution empire of lower priced instruments. You have all Octal sockets, so if it were me, I'd keep it that way, using a preamp design from that period, of course with a PT or an isolation T. 6SL7; 6SC7; 6SN7 or Pentode 6SJ7 are all possible choices for a preamp.

[davidwpack]

Yeah, that pretty much exactly what I was thinking with the 6SN7. There's hardly any info on Don Noble amps but I think you guys are right about the Magnatone connection. One guy was saying that they were made by Magnatone, Danelectro, and Ampeg.

[jjasilli]

Maybe this will help

[davidwpack]

YES! Thank you! I was about to ask how to hook up an isolation transformer. I'll try to go through the amp this week and verify component values. Thanks again!

[kagliostro]

The guy on this link

https://www.worthpoint.com/worthopedia/mots-vintage-noble-tube-amplifier-176981826

say he is not sure if the tubes are as per the original schematic

but on his amp there are 35Z3 - 12SL7 - 50L6

so seems that preamp use a triode

EDIT: Compared, the two circuit are very close, also if in the photo of the Noble of the other guy I see a cap between preamp tube and power tube that I can't see  in your amp

Your's


Other's


Franco

[davidwpack]

Which cap? I'll look tomorrow when I get home from work tomorrow and compare. I'll try to get a better lighted picture too. If there were any mods done they are very old mods. Everything looks original to me. Thanks for the info!

[shooter]

Which cap?

[kagliostro]

I think your amp is stock, the other may be "revised"

If you take close photo for the whole circuit and for each tube we can draw a schematic

--

No prints on tubes ?

Franco

[davidwpack]

No all the writing has faded off the glass. If I remember correctly I believe one may have had a serial or some kind of batch number on the bottom but nothing indicating the tube types.

[shooter]

all the writing has faded off the glass

a good spot light and "fog" the tube, sometimes revels hints of what it mighta been

[Tony Bones]

My guess is that it's close to the Magnatone 107 as suggested by Tubenit. That drawing is missing a grid leak resistor on the input tube. It also looks like the input tube is grid leak biased, unlike the schematic. I've made those changes, and added pin numbers to the drawing just to get you started. Now it's up to you to do the leg work and verify and correct the schematic to match your amp.

You can find a table of resistor color codes on the web. Tube datasheets, if you want them, are at https://frank.pocnet.net/.

We could try to draw a schematic from photos, but I think you should do it yourself. It'll be a good learning experience for you. 

[kagliostro]

I tried to draw what I can see (assuming the tubes are 12SL7-50L6-35Z3), but is very difficult with the actual photos



May be Tony Bones got it

Franco

[davidwpack]

Yeah I intend on drawing one up. I just never did because I didn't think it would matter not knowing what the tube types were. Now that I have some idea I'll get one drawn up. Thanks!

[sluckey]

Drawing the circuit may very well show you the tube types. It's a simple amp. Shouldn't be too challenging with the amp in front of you. I would start by assuming the 'other' amp tube types are correct, except for the rectifier and that's no big deal.

[kagliostro]

If you draw a schematic, please share it

Thanks

Franco

[davidwpack]

I will. I'll try to get a start on it when I get home this morning. Thanks for the help and picture Kagliostro!

[davidwpack]

I followed Shooters advice and fogged them up and found writing on the tubes. They are: 12SL7, 50L6, and 35Z5. I'll try to get a schematic done this week. Thanks everybody.

[kagliostro]

 

Franco

[jjasilli]

Note: the filaments of the 3 tubes are probably wired in series: 12 + 35 + 50 = 97 VAC.  There are a number of small vintage amps like this with similar circuits overall.  Check where your amp's filament supply originates, because this varies.  E.g., in the Alamo Capri there is a 98V filament supply on the PT secondary. 


But on the Lectrolab schematic I posted, the filament supply is sourced from the AC wall supply, probably standard 110 VAC at the time.  So a series resistor of proper W rating was often used to drop 110 VAC to 97 VAC, under load (i.e., with tubes IN).  Today's wall supply is typically about 123 VAC, so you may need a larger value series resistor for a larger voltage drop from 123 to 97.

[kagliostro]

I think that large resistor Is connected to the rectifier socket in your amp

You can see It on the pictures

Franco

[davidwpack]

Yes it is. Also that cap that you couldn't see is there. It was just bent over. I'm getting a little of it hand-drawn. I'll get it transferred over with the J Schem when I get it completed. So far it's checking out to be pretty much what Toney Bones posted. I tested one resistor that read 56k but should be 4.7M according to the color code but I am testing them in-circuit so it's probably off. The printing on the caps has long since worn off, if I have to I'll disconnect them and use a capacitor checker. Might be a few days before I get it completed because of my work schedule but I will get it finished and will post it here when I get it sorted out.

[davidwpack]

I don't know man. I'm slowly drawing stuff out but I'm having to draw pin numbers on the schematic that aren't normally included. For instance, a 4.7k resistor connects pin 6 to pin 8 on the 12SL7 (cathode to heater) and I'm not sure how to do that on a schematic. On the 50L6 pin 1 is connected to pin 8 via a __ resistor. Pin 8 of the 12SL7 is also connected to the on switch. Here's what I drew so far. I'll get back on it my next day off.

[PRR]

...in SE amp you really don't need a tube rectifier...

I think you already knew this: *Here* the several tube heaters are in series across the 110V wall voltage. A 5 tube radio heater string adds-up near 110V. This amplifier omits two tubes so will have a power resistor for the ~~13V excess. (Actually 5-tube radios aimed at 121V, two more 12V heaters.) Omitting the rectifier makes an awkward 12V+50V heater string.

It would be reasonable to re-wire to parallel heaters and get a 12L6. They are not so common, but tubedepot has 'em for $4.

However a 2-tube amplifier just looks bare inside. Keep the rectifier for "cheer".

[davidwpack]

Oh ok. I wouldn't have thought of that ...but yeah I plan on keeping it in there.

[PRR]

> resistor connects pin 6 to pin 8 on the 12SL7 (cathode to heater) and I'm not sure how to do that on a schematic.

It may go to heater pin, but sure *both* go to "ground". Actually "switched power return".

Yes, the super-simple construction is confusing, but there are few variations possible. The plans posted by Tony and others WILL be 98% same as yours. Print one of those, then trace your amp against it, noting different values (occasionally missing/added parts).

[davidwpack]

Will do. Thanks for the tips PRR!

[kagliostro]

Ciao PRR

...in SE amp you really don't need a tube rectifier...

I was saying that only because I was thinking he want to gut the original circuit and build a different circuit

in that eventuality the choice for a PT will not be obliged to something that can supply also a Tube Rectifier

I like much more the idea of a refurbishment of the original

--

Those old resistor can easily have drifted, measure them one by one and replace if necessary

Franco

[DummyLoad]

better pics? more light, in focus, & closer shots. 


--pete

[davidwpack]

I'll get some new pics tomorrow and try to get more drawn up when I get home.

[davidwpack]

Ey! Here's pics for those that wanted them. I'm trying to do this one by meself so hold off on posting yer versions for a bit if you would...although I think it's going to be remarkably similar to the one Tubenit and Toney Bones posted. I have more off-time next week so I should have something by then.

[davidwpack]

Check out the power supply upgrade!

[davidwpack]

Here's what I've done. Don't know if it's right but I believe all the parts are there. Like I said I put extra pin numbers that aren't normally included on schematics so I guess it's kinda like a layout-schematic. A schematical layout? Anyways, if no one sees anything terribly wrong I'll draw it up on the computer my next day off.

[Tony Bones]

You're definitely getting there!

I didn't go over the whole drawing with a red pen or anything, but a few things jump out at me. One, I can't tell B+ from GND. Lines are crossing each other, but I can't tell which are supposed to be connected electrically and which aren't. You might have gotten yourself mixed up too. 

I think that the wire between 12SL7 p8 and 50L6 p1 should be GND.

There's a grid leak resistor missing on the 50L6 (p5 to GND.)

Maybe other errors too, but get that straightened out and we'll go from there. 

[davidwpack]

Ok. Thanks for looking! Probably be my next day off but I'll get it. Thanks!

[Eulipian]

I think this may be a Dickerson, made by Magnatone circa 1948, per this site: http://magnatoneamps.com/dickerson.html