KT88 Standel

[Andy Le Blanc]

My bone yard has yielded forth..... a beat old Peavey to start on the next go round.
The power transformer checks out, 385 to 390v after a bridge rectifier.
It was a 4 x 6l6 thing, "vintage", a transistorized 100w bassman, damned loud.
The stack on the PT looks like it might have plenty-o-current.
I haven't tested the turns ratio of the OPT yet, but 4 6l6 should have around
a 2k primary, I should be able to acheive around 4k for the KT88 with load choice.

The data for the tube type says that I should be able to get 30w out of a
a cathode biased, push-pull circuit with a 4k load, triode connection.

That sounds fun, simple to dress too. The Standel pre. doesn't look like it
was ment to be over driven, so triode might be a very pleasing choice.
I've heard from a few sources, the tone of these thing described as the
ultimate lounge tone, clean and tight but with a warmth.

I'm not after a clone in any way, more the exploration of what looks like
a nicely unique circuit

[Andy Le Blanc]

1v ac applied to the secondary of the opt produced approximately 22v ac at the secondary.

So that's a ratio of 484 : 1, so the KT88 should see a plate load of around
3.9k when loaded with 8 ohm's, which is in the ball park with my early assumptions.

The scanked out old peavey was so rusted that all but one of the bolts that
held the PT seared. jeez........

should be fun, tomorrow will fab the chassis, the next day should see the
dress complete or nearly so, I have two other project needing cabinets and
a few repairs to turn over and three xmas shows to run FOH for , busy....

[bcmatt]

Very cool! I'm excited to see the progress.

[xk49w]

The Standel pre. doesn't look like it
was ment to be over driven, so triode might be a very pleasing choice.
I've heard from a few sources, the tone of these thing described as the
ultimate lounge tone, clean and tight but with a warmth.

I'm suspicious that the feedback tone control would react badly to overload/clipping. Other steps could be taken.

In my spice modeling of the tone shaping part of the standel I have noticed that the Tee section notch filter could be a good place to employ a PAB type ground lift. Some modification to this thing could tame that rising high end if necessary.

[Andy Le Blanc]

I'm curious too, I'm not too worried about the first stage however, It looks like just
about anything will work there. It looks like there will be a fair amount of
degenerative feedback over the cathode resistor with both the 12at7 and the
driver for the inverter. The response of a 15" driver will also attenuate the
top some as well. Without global feed back the places to get a little usable feedback will
be the cathode resistor(s) of the power tubes and the grid resistor at the inverter.

I want to hear what the over all gain structure thru the entire pre sounds like.
I've built and gigged a silvertone circuit with a buffer stage after the inverter.
Great tone clean, until it started to clip, then it was nasty. My hunch here is
that there isn't a lot of gain realistically, after the first stage, so you'll have
a clean undistorted range. A way to get more current than voltage gain in the signal.

Unless your cloning I don't think the input or power side matters too much.
Its the basic concept of the gain structure and a simple active tone control
that will make the project.

[Andy Le Blanc]

cleaned up parts....

The donor used a bridge rectifier, I have intentions toward dressing the kt88 triode.
The PS filtering might be an issue, ripple might be very evident with the triode dress.

I've been playing a bit with tremolo, have a notion to include some in the project.
Big ,clean and wiggly.

[Structo]

What did you use to clean the transformers?

[Andy Le Blanc]

A wire brush wheel to remove rust/paint on the bells, very lightly with 000 wool on the lamination.
Valspar anti-rust. crazy.... more to keep it from getting worse.
Their for a one off, prototype, in no way a restoration or production. Once the lamination
is corrupted, its all over and can't be trusted.

[Structo]

Yeah but, Mercury Magnetics say rust is a good thing now.

[JD0x0]

Yeah but, Mercury Magnetics say rust is a good thing now.

lol you beat me to it

[xk49w]

A wire brush wheel to remove rust/paint on the bells, very lightly with 000 wool on the lamination.
Valspar anti-rust. crazy.... more to keep it from getting worse.
Their for a one off, prototype, in no way a restoration or production. Once the lamination is corrupted, its all over and can't be trusted.

I've got a set of Leslie 122 transformers and choke I was going to use for a KT66 version of the Standel output stage. The Leslie uses 6550s so heater current should be no problem. Voltage is a bit high, about 420 from solid state rectifier. Maybe I'll go with a tube here, try to lower that some.

They were locked up in someone's storage shed and are in about the same shape as your Peaveys were.

[Andy Le Blanc]

I had do a parts order (mouser) for caps and resistors. Bone yard is thin on
PS caps and I didn't want to fart around too much with cathode resistors this time.
Watch out for old iron. I had one bite me a little while back.
I was thinking to scab up a tranny no load, with a light bulb limiter, and measure, from the case or lamination, to a ground to see if the insulation was still ok.

kt66 are fun, I've got a PS dilemma too, stuck with a bridge, thinking of maybe using a modified bridge.
Tube for the positive legs, got a couple 6ca4 but the heaters draw 1 amp 6.3v.
still wrangeling that.

[selloutrr]

here is a cool way to do a rectifier

[Structo]

I understand the caps but what are the resistors for? Phasing?

[xk49w]

I understand the caps but what are the resistors for? Phasing?

The radio amateur's handbook, 1967:
[img:423:406]http://lh3.ggpht.com/_jpUbIh2UclM/S00cU ... ig12-2.jpg[/img]