Leslie 102 amplifier transformers?

[SSLtech]

So I've just acquired a Leslie 102 which is in bad shape... I don't think it's worth fixing. All of the tubes are missing or broken, one of the two can capacitors is absent, the other is almost certainly bad. One of the circuit boards is completely broken also.

It seems to be a 4-channel chassis, and the transformers are there... one of them is larger than the rest; this I presume is for the bass channel.

I'm wondering if any of these speaker transformers would be any use in a Marshall 18-Watt type circuit? The Masher uses EL34's, whereas the Leslie uses7189's. I think they're similar, but the 7189's ran at about a hundred volts higher, if I understand correctly.

Whaddayathink?

Masher schematic
Lezzer schematic

 

[lassoharp]

. . . I'll be by in a few days to remove the unsightly junk from your premises  ;D


Ok,  On the 18W Marshall clone OT Q,  I have such a beast and it uses a Hammond OT 'designed for EL34s' and runs a pair of EL84s instead and it's the bee's knees.

An 'EL84 transformer'  would usually have a little higher primary impedance but your 'EL34 OT' should work fine with EL84s if wish to use those.  You'll already be under wattage with a pair of 84s so no problem there.  The lower impedance probably makes the tubes strain a little more but nothing outrageous.

In case you don't already have an 18W circuit picked out, here's my recommendation:

http://mhuss.com/18watt/schematics/18minwatt.gif

 

[mad.ax]

Keith, both your links points to the same Leslie schemo...

The Marshall 18W use EL84. 7189 are the American version of EL84, and usually a bit more robust, hence the higher max plate voltage.
Those Leslie OT should work perfectly with a 18W ckt. If it were me, I would try both the big and the small core... As sometimes, a more 'hifi' sound is not neccessarilly what we look for in a 'Masher'

Axel

 

[SSLtech]

oops... -thanks for the tipoff regarding the link. -Fixed.

Yes I MEANT to write EL84, -sorry, I've been short on sleep for the last week; I'm dog-sitting for my in-laws, and I can't seem to sleep properly or for very long.

Yummy! -I might try both types of transformer, but I'll for sure begin with the larger one. -It's the same physical size as the Hammond 269JX, which is used for a lot of LA-2a clones, and ALMOST the same size as the power transformer which I've already got for the Masher! (I think it's a Hammond 270, but I don't recall the suffix)

I'll try to look into the output impedance of the transformer as used in the Leslie 102.

Keith

 

[jackies]

I have one of these, the transformers are for 16 ohm.
I think anyway...

 

[PRR]

EL84 and 7189 are the same basic tube.

Simple self-bias designs will use 250V-300V for 12W-18W out, so the EL84 is rated 300V.

Goosed to 400V you can get 24W but for reasonable load impedance you must go fixed-bias. You were supposed to buy 7189 for this service.

Either way you load with 8K or so. Not very critical. 

Since millions of geetar amps run EL84 at 350V-400V, that's the main market, I'm sure EL84s from any reputable guitar-amp tube vendor will give reasonable service life at 400V. I didn't even think about it when I stuck Sovtek EL84 in my 415V 7189 bassmate. I did check and finesse the idle dissipation, but 25mA wasn't much different from 10mA. (These tubes come alive at 50mA-70mA but the plate-glow is distressing.)

You really would like to hold G2 voltage down near 300V. G2 current varies a LOT, so a dropping resisor won't work. The Fisher wasted near as much heat in a screen-divider as in the tubes (it seemed). This Hammond appears to have been a 300V 6V6/EL84 amp because it has 300V from the FWB plus another 100V from a tapped winding to get the 400V. If you are really afraid of over-volting, take the 300V point.

> output impedance of the transformer as used in the Leslie 102.

Plan shows 16 ohms on three channels.

> I don't think it's worth fixing

It's a real pink elephant. The PT is good for any 96 Watt 400V amp, but the OTs don't really suit one big or two medium channels. The OTs would work in small gitar amps, but may be "too good", and who needs three 18W amps? (Plus an 18W bass-amp.) And you'd need the 3 or 4 smaller PTs.

The highest/best use might be TV sound. Left, right, center, and woof. Assuming you can find 16 ohm speakers for these functions. And knowing that a high-level woofer for 24 Watts will be BIG, bigger than the CRT set you just got rid of.

 

[Bluto]

Hi,

i'm new to this forum... i really appreciate the great info on this thread.  i've had a 102 amp sitting in the garage for years and i've wanted to do a guitar amp project with it for some time now.  it's clear to me now that i can make use of the OTs in an 18 watt project which is really cool.

PRR... you sound really knowledgeable about the PT in the 102.  It's probably way overkill, but can i use the 102 PT to run a 5AR4 and (2) 6V6's with a standard 18watt pre amp section using the 102 OTs without any major problems??  the amp was given to me so i have nothing invested in the PT and i don't mind under-using it.

Also...  does anyone know how the wiring shakes out on the PT?  i can find 300v, 100v, 40v, 8o+-v.... but nothing close to a good voltage for the heaters...

Any Help here would be really appreciated!

 

[PRR]

> you sound really knowledgeable about the PT in the 102

All I know is what I see on the plan.

> run a 5AR4

NO. Look at it. The main 300V winding is NOT center-tapped, will not work with a 2-diode (common bottled rectifier) plan. You could half-wave but that's high ripple and hard on the rectifier. You can full-wave with three bottles (double loss) or a bottle and two 1N4007 to form a full-wave Bridge.

And there is no 5V winding to heat standard rectifier bottles.

The kicker winding for the 400V stands on the 300V and will give 100VDC at best, less with bottle losses.

Use it like Leslie did. Silly-state FWB on the 220VAC winding to give 300V. If you like to over-volt the 6V6 (many gitar amps do), then add two 1N4007 and another cap just like Leslie did.

> (2) 6V6's

It's clearly wound for four 20-24W channels, 90-some watt output.

It is "stupidly" costly and heavy for a 6V6 team at 10-20W output.

If the cost has been forgotten, and it will never move, WTH, use it.

I am reminded of my smog-era ThunderBurd. For various reasons, it had the heavy engine casting of a Large V8 with the meek power of a light Six.

With the under-loading the voltages will run high. Probably not a lot. With 6V6, start with the "300V" winding. If it runs 330V that will be fine. People do run 6V6 at 440, but if you don't fix-bias then you either melt the tubes or lose in current what you think you gained in voltage. Use 6L6 for 400+V amps.

> nothing close to a good voltage for the heaters...

The plan shows a 12VAC to ground off the PT. And the 6V/12V small-tubes are wired 12V. Oddly the 6V-only power tubes seem to be wired 6V parallel. I'm guessing that if the PT is really 12V, then some draftsman copied 6V heater-detail from another plan. No problem working a power amp with 12V heat and it does save a bit of wiring around the many sockets. However the low-level preamp stages have to be wired just-so to give low hum (by guitar standards) with 6V heat, and 12V does not make it easier, especially if you have not a lot of trouble-hacking experience. 

Do note that right-size Fender DeLuxe and Marshall 18W iron is VERY readily available. It's not easy to spend money in this economy, but a "right" part does get you to the making-music point quicker, and part of economic recovery is to move money to important things (amplifier part suppliers) rather than housing bubbles.

 

[Bluto]

Ok PRR,

Those are some major issues and your points are well taken.  Sounds best to save the OTs for a stand alone project using a more appropriate PT.

All things considered,  what Guitar amp design would be best suited for use with this PT?

Bluto

 

[PRR]

> what Guitar amp design would be best suited for use with this PT?

I can get, cheap, the engine from a 5-ton truck. What sports-car is best suited?

I'm not just being confrontational. The 427 Cobra is a truck engine in a sleek light roadster. Originally scaled for fours and the BMW/Bristol Six, the Ford 260/289 V-8 was a good fit and a good (for 1964) hot sportscar. But the Ferrari had the edge. Ford also had the 360/390/427 engine IMHO best suited for trucks but "the result was scary". That led to MAJOR redesign of the entire running gear and was a commercial failure (though now very valuable and emulated) machine.

So while there may be a good plan, it will take a LOT of redesign.

Off-hand, free ideas: large current at lower voltage suggests rows of smaller tubes. Eight or ten 6V6 or EL84, 60W-90W output. Load per pair is 8K, so an 8-pack could use the 2KCT OT from a four-6L6/EL34 Fender or Marshall. At 330V you can self-bias the tubes and use the 500K/tube grid-leak, ~~100K-200K input to the power array. The larger quad-tube amps' driver will work.

What you do in front, preamp and tone controls, is irrelevant. It's like: the 427 Cobra was available with or without a glovebox.

The 12V heater winding is annoying. Maybe feed the big tubes in pairs. Then I would ground one end and build a voltage doubler to make ~~34V DC (+/-17V DC), filter again for ~~30VDC, and run preamp heaters in series.

All of which is a lot of custom design, which you will have to work out on your own.

 

[Bluto]

Great info, Thanks.

It might not end up being the best amp possible but i'm sure to get a bit of an education out of it.  my hacking skills are more in the area of electro-mechanical circuits. Building a circuit (power amp) from scratch with this PT will be a good exercise in developing a better understanding of this level of electronics.  It seems that using 4 big bottles is the best approach here. 

Thanks again...

Bluto