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This is something I haven't tried yet, it's one of many AX84 user contributions, but it's similar to the Standel concept. I had an original 25L15 (gone now) still have my '55 15L12 which doesn't exist, and is the coolest vintage guitar amp I've ever owned, and I've had most of the greatest hits. There's something about this feedback/gain control that makes a nice compression while NOT being power tube saturation or PS sag. Compression while clean is a challenging proposition especially if you prefer organic simple circuits for "tone honesty". Sure, you could just use an outboard compressor, but...
I think if you're a Fender lover, you'd be pleased by this. It's NOT a Fender sound but IS a great country rockabilly Luther Perkins thing, even jazz, and yes, I know he played a dual rectifier Bassman and then a Vibrosonic. But, IMHO, more user friendly, not tending to have your G or B string "jump out" without excellent right hand control. Fender's all have this problem/feature, how ever you view it. Ya gotta remember, Speedy West, one of the greatest pedal steel players ever, played through a 25L15, Chet Atkins too for a while. Speedy used a JBL d130 and Atkins used a 130A.
My latest project described below, get's close (while not being remotely close schematically) and I think it's for several reasons. The main one is the intentional lack of gain AFTER the first two obligatory stages and tone stack, which is why I used the no gain inverter I did. The 6J5 probably won't clip easily, as it's grid sits at 8 volts or so, where as a 12AX7 would, which would be normally preferred with a different goal. The others may be the PS, but probably a larger contributor is the cathode biased 6L6's (6BG6GA) which are Less gainy and jumpy than even cathode biased 6V6's....So you just turn it up more, have less head room, but still enough, try it! One last tidbit is the .001 (or .002?) cap paralleled across a 220K/220K voltage divider between the first two stages which I just copied verbatim from the original AX84. This is also something not found in a Fender, but could easily be added to a Fenderoid circuit, I just did.
Follow Ups:
I've also been toying with the idea of using 25L15 circuitry for a clean guitar amp. From the schematics I've looked at no one can seem to agree on atleast two critical points. GSH, did your 25L15 have a negative supply or was it cathode biased, and finally, was the 25L15's output transformer wired in ultra-linear?
I believe all "original" Standel 25L15's are cathode bias, and NOT run UL.
The OPT's were 9K Triads, which makes sense for class A-AB cathode bias 6L6's or 807's. The current "reissues" are NOT the same as the original 1950's amps, what the differences are, I don't know nor care.
Some early Standels used DC heaters on the preamp tubes only, derived
from the HT supply and NOT a separate dedicated filtered supply. I don't advise either.
I am a big fan of the Standel sound. My latest project is a 5E8A, aka 57 Twin using Dynaco ST70 transformers. I have 2 extra switches, fixed vs cathod bias, and pentode vs UL connection
I noticed that the 25L15 Bob Crooks schematic has the output end from the inverter on is very similar to the Mid 50's Fenders with cathodine invertor and cathode bias. Do you have any suggestions how to make my Twin more Standel-ish sound? I understand the 6L6 is a close cousin of the 807. Sounding close to the Standel is probably good enough for me.
Thanks in advance.
Thank you GSH. If you'ld indulge me with another question or four? Are there two chokes in the power supply of the original 25L15? If so, was the first choke before or after the first capacitor? Which choke fed the 807 screen? How did the original design keep the screens safely below or near 300VDC?
PS of 1953 25L15
Thanks for the great read. I just finished three Fender tweed clones (5D4, 5C8, 5B6?) and looking for another. The Standel looks like it might be the next project.
looking for some jazz and a little libations - js
Bob Crooks (Standel) had some pretty specific ideas about what he wanted a guitar amp to sound like. His tube amps were pretty incredible, and yes, very different from the Fender concept. Clarity and voicing were everything!
I worked with Bob when BBE was being founded (BBE was his invention, conceived during his days with Barcus Berry). At one point, Rose Maddox sent in her 25L15 to Bob for a rebuild, and I had the pleasure of hearing some amazing guitar work through this amp. We had a guitar tech named Johnny Davis who was a genius at the rockabillly/Merle Travis thing. Incredible sound came out of that amp!
I may just have to build up a preamp from this schematic!
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Wow....long time since I've seen that.
Any chance you would be willing to post it as a PDF? That is just barely **not** legible on my screen.
thanks
Kind of a Weber's Kit cousin (or so I've been told) to the 25L15. Should really have a unique tone with those 807's. I've never seen or heard this amp with 807's. Just 6L6GC'sIf you scroll down to bottom of thread, you'll see the Standel 25L15 schemo's.
Edits: 01/23/12
Interesting ideas. You ever do input stages with 12AY7/6072 instead of 12AX7?Anyhow... guess everyone has their own ideas of tone magic (Fenderosity in Leo's terms). I'm more an early BF or brownface tone addict. But, that's just me.
Don't want to delve too deeply into guitar amp strategies, as the Audio Folks will tell us to re-boot to a guitar amp Forum.
Still a few photos (esp of the "guts"-- I have a fetish for those shots--- wouldn't hurt.
8^)
Edits: 01/20/12
But here's my friend, at the time with the English Beat, the bassman clone head is the Jetsen's space lunchbox thing above the "Getty". It was 1/4" aluminum sides with a clear plexiglass cylindrical handle (barely visible).
It had old mil-spec potted iron with clamp type tube "seatbelts" so it really was tough, made to travel, be dropped etc...I have no gut's shots, too bad this isn't a good picture of the top, which was kinda cool.
That went onto form FYC?
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