Interstage transformer vs. conventional AC coupled stages

[David Root]

I was looking over some old schematics recently, Gibson EH-150 from 1936-42, and I am wondering if there might be any tonal differences between the transformer and the modern AC coupled method.

Like most people I don't have a lot of experience with the methods of the Ancients. I do have a '47 Gibson BR-9, which uses a transformer PI, but I had to change V1 from a 6SN7 to a 6SL7 to get any grind out of it at all.

Also, these old amps generally had field coil speakers with the coil also acting as a power supplychoke. Any tonal implications with that? I can see possibilities of it given that the coil is simultaneously in two different parts of the circuit.

[PRR]

> I am wondering if there might be any tonal differences between the transformer and the modern AC coupled method.

YES.

Garnet built tranny-coupled amps in the dawn of Rock and switched to R-C coupled (not "AC coupled").

There are many ways to knock your socks, but transformer drive won't grid-block the way R-C coupled will when you try to force 90 Watts from a 50W amp. Extreme grid-block is bad, but light grid drop is good because it softens the harshness of grid-clipping.

Fender's 300 Watt monster did use transformer drive. And to get near 300W it drove the power tube grids very positive, large grid current. I guess with 300W on tap, users didn't push far-far past its clipping point. (Actually, it is possible to put some grid-droop in a tranny-driven amp.) Fender used a 6V6 driver with a transformer much much larger than a souped-up Champ would run.

A more accessible and cheezy tranny-coupled amp is the Fender BassMaster. Push-pull AB2 6AQ5, and one 12AX7 preamp/driver. I find BassMasters in odd corners. An eBay hunt will probably turn one up at fair price for a vintage all-tube amp. Change the speaker, half some of the tone caps, it should be a groovy 15W guitar amp.

Aside from the way R-C grid-blocks.... since about 1938, the extra tube needed for push-pull R-C coupled drive has been cheaper/lighter than the transformer. Since 1958, the fad for NFB pushed the idea behind the bench, since it is very tough to wrap significant NFB around two transformers. Fender 300 did it, but not a lot of NFB (and an overkill driver transformer). Macintosh did it for a year, but extreme overkill transformers and some fussy design; they moved to R-C coupling and those are the Macs most folks know. The BastMasser had zero NFB, it was too cheap to spare any gain. (After a Champ, it may be the ultimate minimalist guitar amp.)

> a '47 Gibson BR-9... change V1 from a 6SN7 to a 6SL7 to get any grind out of it at all.

In those days, you did NOT distort. It would be like farting on stage (and in those pre-electric days, you could hear a fart). Amps were tuned to make it almost impossible to make rude noises.

Note that this also means the antique speaker was NOT expected to make rude noises, and continued grind could ruin an irreplaceable artifact. Tap the voice coil circuit so you can run a modern speaker for grind-gigs.

(The idea that guitarists didn't get rude is part myth. You clearly hear some gitar grind on old Les Paul and Texas Playboys recordings. But far less than in the 1960s, not to mention 1990s. Sax players had to blow hard to get big honk, why shouldn't the guitarist work hard for grind? And of course Les knew how to mod, and surely other 1940s-punks could find a sharp radio-guy to hot-up an amp. And they could buy a new-replacement fieldcoil speaker too, even on a Tulsa Saturday Night.)

> coil is simultaneously in two different parts of the circuit.

The field coil should be fed pure DC. Normally some ripple slips into the field and the 6F6-OT circuit, but they are wired to semi-cancel this. This could also semi-cancel any audio which got past the very small filter caps into the field.

The field current "should" be constant. Increase of current raises midrange sensitivity, by comparison the bass resonance does not stand out so much, and the coil overheats; conversely for lower current. Normal wall variation won't make a big difference. A fix-bias amp which has a large rise of current from idle to grind would really upset it; all FC chassis which I have met ran self-bias power bottles and often large bleeder current too. (Oh, large theater systems had separate FC power supplies.)

So to ignore the small variations: the field is equivalent to a permanent magnet except it eats power and runs hot.

The small variations may make some slight differences. The rise of current in most hard-worked amps would raise sensitivity and increase the loud parts, a sloppy expander. It can't do much expansion: it would overheat on loud or be under-fielded and weak when near idle.

The big difference isn't the field. Speaker motor and cone technology and detailing changed a lot through the 1930s. The MGM Movie Speaker pushed the pro-speaker art. Jim Lansing worked with that and also into non-movie applications, finding the D-130/120 curvilinear pulp cone around 1950. Utah advanced the cheap home speaker art. And while all these improvements were limping out of near-broke labs, the old-style speakers wrapped from scrapiron, butcher paper, and secret shellac-mix kept getting made and sold.

The arrival of Alnico at a price cheaper than a pound of thin wire and good DC power, and general post-WWII hoopla (ballpoint pen, nylon stockings), finally drove the old-style speakers off the market. The new ones sound different, but not so much for Alnico as for a lot of small coil and cone details coming together.

But in 1947, Alnico was still costly, most tech factories were re-starting pre-war tooling after 4 years of military contracts and pent-up consumer demand, so a 1947 would have pre-war speaker flavor just like most 1947 cars were 1939-1941 models. Would be a few years for post-war designs to come to market.

(Were there any 1947 US cars? Or was Detroit, the Price Board, and the UAW still in that 3-way snit?)

[gregarious]

quote="jelle"]PRR,

I really like your comments! Please keep them coming!

Jelle[/quote]

Roger that one!


+1 for the Texas Playboys - love that Bob - the twin guitars of Leon McAuliffe and Eldon Shamblin - Al Stricklin the 'ol piano pounder - Johnny Gimble & Leon Rausch ah now! Genre? Who needs genre?!!

[Bob-I]

Sax players had to blow hard to get big honk,

Not exactly. There's an old trick, you HUM while playing and the disonance creates that grind in the horn. It's really not that hard but personally I don't like that sound so I never play that way.

[David Root]

Thank you, PRR, for your encyclopedic review of my questions. You must have grown up reading Radiotron Designers Handbook instead of Marvel comics like the rest of us!

I was aware of the non-grid blocking property of the transformer in this application, but not that a little grid block smooths out clipping. So if you do push an interstage transformer real hard, it will do a little grid block smoothing? If so, is this a resistive effect, as I believe a 220K or so resistor immediately ahead of an overdriven PI will do the same thing?

I can see why high cost and NFB combined to kill off the interstage transformer commercially. However, I don't give a rat's tutu about cost in that respect, I'll do what it takes for the right tone.

Farting on stage would indeed not be proper in those days, but some of these old amps will fart mellifluously if you feed them the right beans! Hence my interest.

Besides Les Paul, another relevant "1940s punk" would be pro radio repair guy and immortal slide man Elmore James, who did tweak his own amps.

Thanx too for the detailed explanation of the field coil, especially its questionable power response characteritics, which I wasn't aware of. I didn't want to include this in a new build because it would have been a lot of work, taking the magnet off a modern speaker and modding it to field coil, probably not even feasible anyway unless you had one built from scratch.

Presumably a small alnico plug style magnet on a 1" voice coil with a smooth cone and no dope on the surround would be the way to go?

At this point I'm seriously considering an EH-150 build, based either on style 3 (with interstage PI transformer) or style 4 (6N7 PI). Now where did I put that old slide?

[RJ Guitars]

DR - Great questions and great answers from everybody else. I've been learning amp building from a guy named Jason Cox who has a keen mind for innovative amp designs... you can see his stuff at http://boozhoundlabs.com/
He is an unbelievably decent guy and a great teacher plus his amps are nothing short of awesome.

finally... he designed a low power amp for me that uses an interstage transformer as the phase inverter. I'm just putting the finishing touches on it and as soon as the UPS delivery makes it with the interstage transformer, I should be getting some sound out of it...

I think the idea behind this is to get a near perfect balance on each side of the phase inverter... I hope I can tell you it sounds great within a week or so..

rj