Standel 25L15 clone PT recommendation

[xk49w]

I'm researching this amp for a clonish build and I am looking for some assistance in selecting a power transformer. I need twenty-five very clean Watts using 807 output tubes. This amplifier has large transformers. It has no global feedback from the OPT secondary (6.6k ohm primary Z), but uses an UL connection and the finals are cathode biased with no capacitive bypassing of the cathode resistance.

There is speculation about the schematics and there is apparently some secrecy about them from those that claim to have the information. (Sound familiar?) Ted Weber, RIP was going to release a kit. There is a schematic on his web site. These combos used the split pre/pwr model typical of the so-called hi-fi amps of the day (50s) and I would like to duplicate this architecture if it is reasonable to do so.

Bob Crooks was probably, if not the first, then one of the first boutique amplifier builders. Kind of a garage operation as far as I have been able to glean. I think he made about 65 of these. To me these amps seem pretty much like a 25W 6L6 Williamson type design with over-rated transformers and a JBL D130 speaker. I'm guessing most of the characteristic sound was from the 15" JBL driven from a very linear design.

Back to the original question, I am considering a power transformer whose secondary can supply something in the 600VCT to 660VCT range to use with a low drop tube regulator, current capability of 200 mA. I believe the original used 5U4. The weber design uses GZ34 type regulator.

Both Hammond and Edcor have reasonably priced stock transfomers in this class, my question is whether the voltage/current range I have tentatively selected is good for 25 super clean Watts with 807 type finals and 6.6k OPT pri impedance.

TIA,

-- bradley

[xk49w]

Standel 25L15 for auction. No affiliation - just a fan of the amp.

[Andy Le Blanc]

Thank you for the link to the scheme, I had no idea.
One old friend has said to me that he played one these back in the day and they were good.
I haven't seen one personally .

Don't worry about the power side yet. You might as well scab it in to a silver face. The no feedback, the 12at7 inverter
and 12au7 buffer makes the amp. Ive tried similar based other schematics.
The clean was very nice, but the dirt was not so good.
I think tried a dano scheme with 12 ax7, I'm very curios to try this.
Is there any way to validate the posted scheme?.

[dano-rator]

Probably not much help here but:
[img:640:480]http://farm5.static.flickr.com/4047/417 ... 9d6b_o.jpg[/img][img:640:480]http://farm3.static.flickr.com/2638/417 ... e1d2_o.jpg[/img][img:640:480]http://farm5.static.flickr.com/4001/417 ... 8013_o.jpg[/img]

[Andy Le Blanc]

That's cool, the power side is under the decorative metal?

If you have the means, try the pre with a known power side. Use something
that's very common.
Give your self a chance to noodle with the circuit before you commit to a
full scale project. No sense discovering issues half way through a project.
I like to try it with KT88 or KT90, Phat, clean. The input looks interesting.

807's.... Triode Electronics carries NOS Russian for less than $25, matched pair.
I think New Sensor carries a Chinese version. Have you sourced sockets?

[xk49w]

That's cool, the power side is under the decorative metal?

I think this is a clone. [img:800:600]http://lh6.ggpht.com/_jpUbIh2UclM/SwQWt ... e08003.jpg[/img]

If you have the means, try the pre with a known power side. Use something that's very common.
Give your self a chance to noodle with the circuit before you commit to a
full scale project.

I want to add reverb. Some Standels had it but I have no schematic for a version with it. I will have to breadboard the amp.

Have you sourced sockets?

807s use a UX5 socket. It's pretty easy to find these:
[img:195:250]http://lh5.ggpht.com/_jpUbIh2UclM/SyEzu ... amic_1.jpg[/img]

If I can though I'd like to look at a bottom mount.
[img:177:116]http://lh3.ggpht.com/_jpUbIh2UclM/SyEvj ... _mount.JPG[/img] It might be easier to swap sockets if I wanted to try KT66 or whatever.

More Standel pics.

[tictac]

is there any way to validate the posted scheme?.

I saw a real 25L15 schematic years ago; the Weber circuit is not the same as the one I recall. The PI was like a Tweed Deluxe but with a 12AU7. The preamp was unique, unlike anything I'd ever seen before. I believe it used a 12AX7 for V1 and a 12AT7 for V2

[Andy Le Blanc]

You might have to go as far as to yank and photograph the power unit from the bottom
of the cabinet. A couple closer shots of the top unit too.

The amp is one of those that still holds a little mystery. Ain't no fender, thankfully.
The scheme presented from the Weber site looks like an early 50's HI-FI,
with a modded front end. One of the shot's of the ebay picts looks for the
world like something out of an old Sam's Photofact.

[FYL]

I saw a real 25L15 schematic years ago; the Weber circuit is not the same as the one I recall.

Yup. The JP25 is a Weber interpretation of the 25L15 with many differences - different preamp, fixed bias, etc.

The PI was like a Tweed Deluxe but with a 12AU7. The preamp was unique, unlike anything I'd ever seen before. I believe it used a 12AX7 for V1 and a 12AT7 for V2

Here are some schematics collected from the web. No warranties etc.

[xk49w]

Yup. The JP25 is a Weber interpretation of the 25L15 with many differences - different preamp, fixed bias, etc.

Here are some schematics collected from the web. No warranties etc.

Fascinating. The Vin_Pre_Amp_25L15 dated 1953 has the look of authenticity anyway. It shows the push-pull drive connector on the right coming off of the last 12AU7 stage. It is also interesting that none of the PAs show an UL connection.

Other signals brought in on a 6-pin connector. Probably a Hubble like Leslies use. DC filament voltages?

The tone control is definitely not a Fender. It looks like feedback over two stages, similar to Langford-Smith page 646, except this one uses a cathode follower to couple the feedback to the cathode of the 12AT7 first stage. And then there is the treble control... I might try a spice model.

Somewhere I was reading that there were four power amp versions for this amp. The pictures of old ones I have seen show two; one has the 807s extending to the sides and then, like the one on ebay, extending toward the front. That doesn't really say anything about the actual circuit topology.

That's really interesting, thanks for posting.

-- brad

[FYL]

The tone control is definitely not a Fender. .../... I might try a spice model.

You may begin with a standard Baxandall topology.

Somewhere I was reading that there were four power amp versions for this amp.

They were boutique amps built one at a time, highly customized for customers, so I guess that each amp was pretty much unique.

[tictac]

Yea,

That's the one I saw; looks like some kind of active tone controls going on.

You wouldn't happen to have those pics (especially the last one) in another format would you?

I keep getting a prompt to buy Quicktime player when I try to view...

TT

[FYL]

You wouldn't happen to have those pics (especially the last one) in another format would you?

They are in png format, a lossless open standard that all current browsers are able to read. You'll find gif versions attached to this post.

I keep getting a prompt to buy Quicktime player when I try to view...

You've installed some Apple software which has hijacked file associations in the registry, now your computer believes that Quicktime is required in order to read standard files and Apple wants you to believe that you should buy QT Pro...

(Microsoft and all of the others behave as badly).

[Andy Le Blanc]

very happy to see more thank you...

The structure through the pre seems to be consistent so far.

12ax7, 12at7, 12au7.... smacks of a hi fi approach, the power side looks like
there was a similar thought process. Balance, ac and dc and at least with
the Weber scheme, ultralinear OPT. That pre might be fairly unique for a
musical instrument amp for the time, a standout, with the market full of
wards/airline, gibsons, fender etc... a tight clean with head room vs. the
more usual amp with more distortion the more you push it. The what
appears to active tone control is a real stand out for the time too.
Makes the more familiar TMB, or vol. /tone look primitive, no global feedback to boot.

[FYL]

12ax7, 12at7, 12au7.... smacks of a hi fi approach, the power side looks like there was a similar thought process.

Exactly.