4 x KT88 Bass amp

[Leo_Gnardo]

No luck on a 25K RA pot(at an acceptable price), but I'm not even sure why the presence would need to be reverse audio taper anyways.

If they used reverse taper pots it was to spread the action across the whole turn of the pot. 25K linears tend to not show much change until you reach the end of the turn, all the "presence" seems to show up suddenly at 8-9-10. Those manufacturers that used linears didn't want to go the extra expense of specially made reverse taper pots either.

[Smokebreak]

That all makes good sense! I had a revisit of the secret life of pots. been a while ..

[Smokebreak]

So for the mid inductor, can I use something like this ( 2 in series for 1H) : http://www.mouser.com/ProductDetail/Bou ... 9kgp5ds%3d

or would I need something like this : http://www.mouser.com/Search/ProductDet ... ey553-C24X

or does it even matter, as both options would provide 1H of inductance. Or does that even matter and could I use a single 500mH or say 2H, and what would the effect be with a change of inductance for this mid circuit?

[Leo_Gnardo]

Or does that even matter and could I use a single 500mH or say 2H, and what would the effect be with a change of inductance for this mid circuit?

You could compensate and keep the frequency/time constant as expected by altering the capacitor value. Advantage of smaller value inductor is - all else being equal - lower resistance so you can have a higher Q = narrower bandwidth for your mid control. If that's an advantage.

[Meat&Beer]

I have a few of these pulled from an organs tone oscillator, generator, somethingoranother....

Red is CT, from it to either leg reads 760mH, I can only assume from leg to leg is ~1.6, my L meter doesn't go that high. CT to black is 99Ohm, CT to blue is 126Ohm, leg to leg is 225Ohm.

Why am I telling you all this? Well, you're welcome to one or two if you'd like, and if they fit your needs. Just pay the shipping is all I ask. Not sure if it's worth it seeing as these are a pretty cheap item, but I just thought I'd throw it out there!

[Smokebreak]

Thanks man! I appreciate the offer. I've got an open frame type and a couple lil RF chokes on the way, so I'm covered. I'm gonna use a 500mH and double the capacitance to start.
Centertapped , huh? I googled the part # and apparently it's a Conn tuning coil, or as they called them "oscillator transformer". Part of the RLC circuit in the organ.

[martin manning]

I have to correct myself (with an old post of mine!) on the shunted pot as a RA substitute. Its exactly the opposite- you can't get a RA variable resistor that way. See:
http://ampgarage.com/forum/viewtopic.ph ... 516#144516

[Smokebreak]

Well that makes good sense since it's only a variable resistor. All one can really do is decrease the resistance eh?
I was reading a little about the use of a RA for presence, and found an ancient post that suggested using a regular audio pot, wiring it backwards, and calling it "absence" instead of presence Then you get that taper we're talking about

[Leo_Gnardo]

Well that makes good sense since it's only a variable resistor. All one can really do is decrease the resistance eh?
I was reading a little about the use of a RA for presence, and found an ancient post that suggested using a regular audio pot, wiring it backwards, and calling it "absence" instead of presence Then you get that taper we're talking about

Dr Z did similar on his MAZ 18jr, just worked on one. Took Vox's cut control & swapped it 'round backwards. Still does the hi frequency "cut" as Vox but Dr Z's spread across the control nicely and ccw = dull, cw = bright as opposed to Vox where all the cut is crammed into last cw 15% of the turn.

[Smokebreak]

Yowza this thing is large. The photo does no justice to the 25lbs of iron. This will be my general placement. Chassis gets cut tomorrow.

[Leo_Gnardo]

Yowza this thing is large. The photo does no justice to the 25lbs of iron. This will be my general placement. Chassis gets cut tomorrow.

If you haven't started drilling & cutting yet -

Might consider moving the OT to the other end 1) to get it out of the 60/120 Hz magnetic field 2) better balance so the amp doesn't always try to point one end to the center of the earth every time you pick it up.

[Smokebreak]

Leo I thought about the balance issue, but am skiddish about putting the OT down there by the preamp, probably just because I've never tried it on an amp with a traditional eyelet/turret layout

[Smokebreak]

Ok after going down that rabbit hole for a bit, it seems the folks that put the OT at the other end(Hiwatt, SLO, SVT) had the preamp tubes closer to the front of the amp, allowing OT pri and sec to be routed towards the back and completely away from any input action.
I'm thinking that in order to balance the chassis, I should have designed around that in the first place, which I did not.
Here's my layout :

[Garthhog]

Leo I thought about the balance issue, but am skiddish about putting the OT down there by the preamp, probably just because I've never tried it on an amp with a traditional eyelet/turret layout

Make sure your twist the primary leads tightly and ensure that they come through the chassis as far away from your preamp circuit as is practical. I went through this on a recent design.

-Ryan

[M Fowler]

Because this is going to one hell of a heavy amp with those transformers I would move the OT to the other side as suggested to balance out the weight there will be no problem with noise just twist the wires.

Mark