Stewart push-pull 50L6 amp

[Steve_P]

In preparation for a Weber reverb tank kit that I have on order, I need to take care of this bench queen. It's a cheezee Stewart 50L6 push-pull amp with 2x12AX7 preamps and 2x35Z5 rectifiers with one feeding the other. It has two Oak 6x9 speakers for a 4 ohm total load.
The build quality is bad...





But it looks pretty cool.





Not having ever heard a 50L6 amp, is there any reason I shouldn't gut this and put in a Harvard?

[stingray_65]

not heard one either.

if it has a unique sound that isn't awful, keep it.

If it sound like crud put a fender cicuit and weber speaker in it :)

[P Batty]

Very cool looking amp- love the swirls in the grill cloth.  The inside is a nightmare, I wouldn't hesitate in replacing that junk with a 6G2 circuit , but I'd leave the externals. If the grill cloth isn't glued, I'd carefully unstaple it and rebuild the speaker baffle with one good 10 inch speaker. 

[PRR]

The 50L6 is a fine tube. It isn't a little 6L6 (though they let you think that). Still this could be a fine amp.

The two 35Z5 are surely set up as a voltage doubler to get near the 200V rating of 50L6. At this point there may be 8 whopping watts of output. In a good (light cone large box) speaker, that's a lot of bark for the price.

BUT.... that tube lineup almost HAS to be no power transformer!!! Which means you are holding on to one hole in the wall outlet. Which hole? This is 50:50 Russian Roulette. Probably not THAT bad; there is probably a "ground cap" (which is old and potentially shorted). Test it with dry sneakers on dry wood floor and NO other gear or wiring in reach.

I'd vote to "restore", even though that means a major re-wiring and adding proper lug-strips. And adding a proper power transformer! It is wired to take 117VAC directly. You can get 115V:115V "isolation" transformers. This large (for its class) amp will need 40-50VA transformer. Float the power to the old line-cord terminals. Use a proper 3-pin cord to the wall and nail the ground wire to the chassis; now it is safe.

[Steve_P]

I have an iso-tranny for this thing and there is no ground cap.  :shocked: Needless to say, it's sat, while other things have had my attention. It seems to me that the cathode of one 35z5 is wired directly to the plate of the next.

[sluckey]

It seems to me that the cathode of one 35z5 is wired directly to the plate of the next.

Kinda like one of these voltage doublers?

[Steve_P]

No, it looks like voltage ===> recto plate==> cathode plus filtercap==> plate of recto #2==> cathode #2 and filtercap. I have to draw out a schemo for it.

[Steve_P]

I take it that a triad N-51X would be too spindly for this amp?

[PRR]

> I have an iso-tranny for this thing

Smart man. Once in a while someone finds a hot-chassis amp and isn't aware of the danger.

> triad N-51X would be too spindly

"Where" are you? 117V land or 230V land?

ASSuming 117V: we can't stack the heaters 35V+35V+50V+50V+12V+12V. There's a couple ways to do it, but they seem to be variations of two series strings, each at 0.150A. That's 0.3A. Plate current for one hard-working 50L6 is near 50mA, so 100mA or 0.1A for two, add a bit for AC-DC conversion and small bottles, say 0.15A of AC needed for plates.

That's 0.45A. The N-51X is rated 0.3A. It will run, saggy, hot, maybe HOT. Perhaps not a good long-term choice.

_I_ think it is a cool old cheap amp. I'm even thinking 25Z5 25L6 so the heaters can all run on a 24V 1.5A transformer. Gild the lily with DC for the 12AX7s. But it is your amp, and even if you could part with it, I'm up to my ears in home repair and can't take on another problem/project. And yeah, it's a hasty build in sad shape, with the rat-nest wiring and the green socket lugs and all those wax caps. 

Yeah, chisel the chassis and build yet another classic Fender.

[billcreller]

  There's that word again.......Fender!

[Steve_P]

> I have an iso-tranny for this thing

Smart man. Once in a while someone finds a hot-chassis amp and isn't aware of the danger.

> triad N-51X would be too spindly

"Where" are you? 117V land or 230V land?

ASSuming 117V: we can't stack the heaters 35V+35V+50V+50V+12V+12V. There's a couple ways to do it, but they seem to be variations of two series strings, each at 0.150A. That's 0.3A. Plate current for one hard-working 50L6 is near 50mA, so 100mA or 0.1A for two, add a bit for AC-DC conversion and small bottles, say 0.15A of AC needed for plates.

That's 0.45A. The N-51X is rated 0.3A. It will run, saggy, hot, maybe HOT. Perhaps not a good long-term choice.

_I_ think it is a cool old cheap amp. I'm even thinking 25Z5 25L6 so the heaters can all run on a 24V 1.5A transformer. Gild the lily with DC for the 12AX7s. But it is your amp, and even if you could part with it, I'm up to my ears in home repair and can't take on another problem/project. And yeah, it's a hasty build in sad shape, with the rat-nest wiring and the green socket lugs and all those wax caps.  

Yeah, chisel the chassis and build yet another classic Fender.

It's primitive pcboard, flat sockets on fiberboard. Thinking the best way to solve the heater string problem would be to run diodes instead of 35z5s, then you'd have 50/50/6/6 for 112v. Yes, more effort than it's worth. Isn't that what makes it fun?  
N-76u for an upgraded isotranny.

[PRR]

> run diodes instead of 35z5s

Yeah, though as a 117VAC doubler that would head toward 320VDC, way over what 50L6 is rated; the inefficiency of the hollow rectifier puts it right around 200V-220VDC, to make the most out of what were once very low-price power pentodes.

Non-doubler gives 160V, which is good power, but not all that a 50L6 can give.

To drop the 100V at 100mA is a full 10 Watts heat. 1K 20W part, with ample air, but protected from fingers. Do-able, but awkward.

> 50/50/6/6 for 112v

50L6 takes 50V 0.15A; 12AX7 can be wired 6V 0.3A or 12V 0.15A. In series with the 50L6 we seem to want the 12V 0.15A connection. This adds to 124V, which is probably fine for modern wall outlets.

And what do you do with the extra sockets??

thetuestore.com has 50L6 at $10, 25L6 at $7. Same tube, different heater (25V 0.3A). A quad of 25L6 would eat 100V, then 2*6V for little bottles is 112V 0.3A. If one will make 3.8W at 10% THD, then a push-pull quad should be good for 16W at well under 10%, maybe a 20W guitar rating. This would shred the original speaker, but there's lotsa good loud speakers rated 25W guitar.

How big? 15" tall and 20" wide? That's not a small box.

> more effort than it's worth. Isn't that what makes it fun?

Yup.

[Steve_P]

Wonder whether the 50L6s were stock.
35+35= 70
12+12= 24
12+12= 24
          118 volts

[PRR]

> Wonder whether the 50L6s were stock.

You have the amp there? You can carefully measure the voltage across each heater. If all are nearly nominal, it is probably as-designed. Factory-design or later re-design; a 1960s re-build will probably show in different workmanship at heater pins.

It's just a cross-word puzzle. Or Soduku.

You need two rectifiers, two power bottles, two twin-triodes.

Series-string tubes (before TV) came in 300mA and 150mA types.

Rect and power bottle take nearly the same power, +/-50%. Twin-triode will take 1/3rd or 1/4 the power of the big bottles. Or conversely, the big bottles take 3 or 4 times the heat of each tt bottle.

3R+3R+4P+4P+1tt+1TT= equivalent 16 small heaters. At 117V that suggests 7.3V per tt, 22V per Rect, 29V per Power pentode. These lower voltages suggest the 300mA series. 6.3V+6.3V, then two 25L6 then two 25Z6... 112.6V

However 25Z6 is wired quite different than the 35Z5 you find in it.

There is a 12L6 but it sucks 600mA, which makes the rest of the string dubious.

[Steve_P]

Suduku, yeah that's it!

Rectos are wired with their own circuit using a big@$$ resistor. Grid leak first stage, then into the tone control. Drew out the output stage, 255 ohm cathode resistor, 50 mf bypass cap. The chassis is bent, don't know how THAT happened, because the cab is not exactly roadworthy. Need to get a chassis made. The output of one recto is run to a 80 mf cap and then into the plate of the second 35z5. Hard to tell with the lateness of the hour and the rat's nest, but the 112 heater string looks like it might be dc too.

[PRR]

> heater string looks like it might be dc too.

I've seen some weird stuff, but that's unlikely....

1) it takes a BIG bottle to rectify heater current, even "just" 150mA

2) if the rectifiers are DC heat, how do they get started?

> chassis is bent... Need to get a chassis made.

I wonder if it is worth it. Then I look back at that funky tweed and the very April 1959 grille-cloth (though probably bought on close-out in the early 1960s).

[Steve_P]

my bad for posting late at night. The rectos are not dc filtered, the pres and power might be (they are wired in series for 112 volts), haven't finished with the rat's nest wiring yet. Board is off the chassis, chassis is punched for 6 octals.

At this point, it's a science experiment.

[Steve_P]

A little thread necrophilia!

Run 6w6's using 6 volt heaters?
http://www.edcorusa.com/products/632-xpwr039_120.aspx

[DummyLoad]

run with it as is...

hammond 169RS (60VA/435mA) or 169SS (90VA/650mA) isolation tranny.

169RD - 51 clams from mouser.
169SS - 55  "       "       "

--DL

[Tiny_Daddy]

I would use an isolation tranny, 6.3v filament xform and solid-state voltage doubler and a pair of 6V6. Toss the pair of 35w4 and have more room for transformers.

[PRR]

> Run 6w6's using 6 volt heaters?

Yes, 6W6 is an up-rated 6-volt 50L6.

XPWR039-120/240 seems way over-size. We need 6V at 3A and 200VDC at 0.12ADC. The Edcor has 8 Amps of 6V and 0.5 Amps AC of 110VAC.

I'm still inclined to keep it "stock" except isolated. You really don't want to re-run ALL the heater wires in addition to the other work (wax caps).

FYI: I found a lesser death-trap and have not done anything with it yet.

[Steve_P]

Haven't been around in a while. Thanks very much!

[eleventeen]

I don't know how good or bad they might sound, but 6W6's are a VERY rugged tube and they are for sale CHEEP. I have a Lambda power supply that uses 5 or 6 qty 6W6 and I could buy them for $4-5 each on ebay NOS. The ones in the Lambda ps run HOTT.

I have to say that I would tend towards buying and adapting a tweed-era Fender Deluxe or Princeton metal chassis if there was any way to do so and to rebuild the guts of that from zero.