Standel 25L15 and dual PI

[Baguette]

Hey there folks,

I was browsing the Standel 25L15 schemo and realised this amp has tons of unusal features. Unique tone stack, 807 power tubes, UL OT, massive filtering and so on.
But the thing that really strikes me is the dual phase inverter in series. The signal out of the tone stack recovery gain stage is fed to a cathodyne PI. From there, each phases of the signal are fed into the inputs of a LTP.
Isn't it AMAZING?!
But what is the purpose of such a configuration?

Serisouly, I'd love to hear one of those. From the look of it, it's really UNIQUE.

Here is the schemo. http://taweber.powweb.com/store/jp25_schem.jpg

Feel free to chime in. Thanks!

{edit: overlarge inline image changed to link. --PRR}

[sluckey]

V2A V2B is the actual phase splitter/inverter. V3, LTP, is really a dual input, dual output, differential amplifier. And that's how it's being used here. Apparently the 807 needs a little more drive. I've seen this configuration in several of the high powered amps.

[kagliostro]

V2A is the actual phase splitter/inverter

Sorry, V2A or V2B ???

Kagliostro

[sluckey]

My bad. V2B is the phase inverter.   

[octal]

The PI and power amp is actually a modified "Williamson" circuit.  (This was a stock British circuit which was popular in '50s hi-fi on both sides of the pond) You can probably google "Williamson amplifier" and find all manner of schematics and explanations.

Nathan

[Baguette]

I see, the cathodyne splits the signal, and the 12AU7 pumps the 2 phases up.
Makes sense.

Thanks folks!

Any comment on the Standel tone is welcome though...

[HotBluePlates]

For what it's worth...

People often ask for pros/cons of split-load vs. long-tail. Which is better? Why is the split-load always used in low-powered amps?

In a nutshell, the split-load's drawback is you have to divide the supply voltage into 2 equally-big output signal plus voltage left across the tube (so it will actually function). That means that compared to the long-tail, it can output a smaller maximum signal with a given supply voltage. It is therefore more often used in smaller (guitar) amps, with smaller bias voltages and smaller required grid-drive voltage.

Hi-fi amps can do whatever works best. Split-load can output 2 signals that are essentially identical and opposite phase. That makes it an ideal inverter. But you might need more output than can be managed from split-load with the given supply. No prob... pump the signal up slightly with a differential amp (as done here).

For what it's worth, my McIntosh MC-30's use the exact same arrangement (split-load into a differential amplifier into output tube grids).

Any comment on the Standel tone is welcome though...

I'd like to hear any sound/video clips that are known cases of the Standel amp being used. I've heard more people talk about them (which is honestly not many folks) than any that have actually heard one.

I've listened to a LOT of Chet Atkins in the last couple years, and I haven't heard much that wasn't due to his hands (he played a lot of different guitars with different characteristics over the decades), or maybe the boing inherent in some DeArmond pickups in some of his Gretsch guitars. But he sounds pretty much like him no matter what he played.

I mention Chet because that's the guy I've most heard people mention in relation to Standel amps.

[PRR]

> cathodyne PI. From there, each phases of the signal are fed into the inputs of a LTP.
Isn't it AMAZING?!

And probably not what Standel did. The R22 22K tail resistor is not found on Williamson and probably not on Standel. It isn't needed, the cathodyne splits the phase, But Weber was so used to pairs having long tails that he put it in.

Rather than argue the theory, build it with a switch to short R22. You can switch "hot": it will thump but nothing will blow. See what sounds better.

R19 looks more like a 12AX7 value than a 12AU7 value? 1K-2K makes more sense.

Standel approximations are out there. The power section was typically near-Williamson, and that is well documented.

[HotBluePlates]

> cathodyne PI. From there, each phases of the signal are fed into the inputs of a LTP.
Isn't it AMAZING?!

And probably not what Standel did.

Come to find out, that not what Standel did, confirming PRR's guess. The info I base my comments on comes from a small group of folks who were given access to an actual early-50's Standel to reverse-engineer.

Standel approximations are out there. The power section was typically near-Williamson, and that is well documented.

My best info is that the link between Standel and Williamson comes from a comment Chet Atkins made to an interviewer a very long time ago.

However, the info I have shows the Standel has a couple of minor characteristics very reminiscent of the Williamson, but also has a whole lot else that doesn't match the Williamson. So we'd be lead astray to try to look at a Williamson schematic to guess at a Standel schematic.

The Weber schematic linked also has some major changes in what is otherwise an amp "in the spirit of" the Standel. It removes some of the Standel's quirkier features and adds a couple of things in their place.

[tdvt]

The info I base my comments on comes from a small group of folks who were given access to an actual early-50's Standel to reverse-engineer.

So we'd be lead astray to try to look at a Williamson schematic to guess at a Standel schematic.

It removes some of the Standel's quirkier features and adds a couple of things in their place.

I knew of or had heard of Standel over the years, but it sounds like there isn't concrete knowledge of what was in there.

Were they the Dumble of their time keeping the specifics of the circuits secret?

The Weber schematic is pretty interesting, but what were some of the features he changed out?

[HotBluePlates]

Were they the Dumble of their time keeping the specifics of the circuits secret?

Sorta. If you forget the hype surrounding Dumble amps, the key fact that makes them of value to some pros is that he'll custom build an amp to your exact desires, and fine tune the voicing of the amp to suit that particular player.

Standel initially made amps for pro musicians in the early 50's. He had a couple of standard models, but each was voiced and/or altered to suit the individual buyer. According to the "Butcher Paper List" there are about 66 amps made during this period.

Standel eventually grew to having standard models and larger production, and those amps from the 60's are somewhat less collectible. The ones from the 50's are exceedingly hard to come by, because so few were made, and diehard Chet Atkins, Merle Travis, Joe Maphis, and 50's steel fans see them as the holy grail.

Anyway, the amp I have documentation for is from Feb '54. According to my info, the phase inverter is split-load, the output tubes are not run ultralinear, the input stage is not parallel triodes, the tone circuit is incorporated differently (and has only Treble/Bass), and there are additional parts in the Weber re-think that simply aren't in the original.

I think Weber looked at a stock Standel and chose to do some things better/differently than the original. I don't have a problem with that. Some folks want a recreation exact down to the style of screws... they would have a problem with the changes, even if performance is unaffected.

Further complicating wide knowledge of these models is that there is a new company that now owns the name, and produces amps that are similar to the original, as well as some that are advancements. My source for info required a non-disclosure/non-production agreement from me, to avoid potential lawsuit for sharing what could be viewed as proprietary information. Building an amp for myself is not considered competing with the company; if I released a line of clones, clone circuit boards, etc, then my source felt he might be legally impacted.

[Dave]

I have an old Standel 4X12 straight (by straight I mean, all 4 speakers are in a straight line) cab. Its not an amp, but it might shed some light on the quality of Standel products.
It is as heavy and sturdy as any cab I have ever seen.
I paid 15 bucks for it (empty). Loaded it with 85 watt speakers and shoved it under my work bench. It works great as a test load. 340 watts will take a lot of abuse. Since its a straight cab, it fits nicely under by bench and you don't even know its there.

Dave

[PRR]

.

[Platefire]

Yall got my interest up on this so I did a little research and read the history. Very Interesting! On his first failure with Bigsby, he got a guitar and kept on tweaking until he had something. I can't help but wonder if he was a guitar player or did he just hook a guitar up to it and banged around to tell what kind of tone he was getting? Platefire

[tdvt]

It is all pretty interesting. Pretty sure I spotted two Standel amps on Austin City Limits last night (Wilco), Marshall head configuration.