4 x KT88 Bass amp

[brewdude]

As fOr balancing the weight, why not just install the handle at the center of gravity-- that is, towards the heavy end so that when you pick it up it stays balanced.

[Leo_Gnardo]

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

You're right to consider bringing the output near the input can cause some freaky oscillations. A Fargen I had to fix some years ago, s/n 007 by coincidence, had the OT leads sprouting up right under the most sensitive node in the circuit, the sum point for dry & reverb, of course it was bedlam as you might expect. A shield made of copper foil with a ground lead, sandwiched between layers of fish paper & applied under the circuit board where that node was, solved that, but you're going to be smarter about where the OT leads come thru, right? Some hi gain Mesa's I've had to do similarly, put a copper foil barrier between OT leads and input territory so they wouldn't "see" each other, then no more ultrasonic feedback follies.

If you can keep all leads from the OT tucked all the way to the rear of the chassis I expect you can win with the OT at the other end for balance.

And you can put the OT where you planned, offset the handle to the center of gravity instead... was gonna mention that but Mr Fowler beat me to it. And I wouldn't think any less of you if it turned out that way. Decisions, decisions...

[rp]

The TA is the easy wire secondaries @ 4/8/16. I stared at the A for a bit, had it figured out, then was like where am I gonna get that switch?!?

Good thing Hammond did that, the other way was mind boggling, why they ever did that the hard way?

The 1600 line is for Hifi. Making the secondaries in a way that you are forced to use a complete winding supposedly sounds better. Sowter does something like this too. I think the switch was an after thought when unsuspecting users complained, and then the "simple wiring" option appeared when they kept complaining. When was the last time 16ohm hifi speakers were popular, 1955? And, Hammond probably figured how often will someone change hifi speakers, once a decade? I haven't changed the speakers or impedance in 25 years

It's nice that Hammond chose to offer both secondary types. Lends credence to the OTs sound best on the 16ohm tap theory.

[pdf64]

..It's nice that Hammond chose to offer both secondary types. Lends credence to the OTs sound best on the 16ohm tap theory.

I don't think there any debate about this for the hifi market that these are intended for.
Hifi transducers should be expected to cover the full 20-20 bandwidth, and so the amps should not compromise that, whereas I doubt there's much 20kHz signal content from a guitar or a guitar type speaker.
Maintaining frequency / phase linearity into ultrasonics is therefore highly desirable for hifi, so it would be perverse to needlessly compromise OT primary-secondary coupling by leaving chunks of the secondary unused.
Bear in mind that some hifi designs use way more global NFB than MI amps and so phase shift within the loop may reduce their margin of stability.
Hence OT performance way above audio can be a dealbreaker.

[Smokebreak]

Ok, 2 things :

For those of you that have successfully put an OT directly in between the input jack and the first triode, I'd like to see some pics!

Concerning the UL OT, is there any reason why I shouldn't run the screens with the UL taps?

I've had another project supercede this one, so as I had mentioned this build goes on the backburner for a bit...

[Leo_Gnardo]

I don't think there any debate about this for the hifi market that these are intended for.
Hifi transducers should be expected to cover the full 20-20 bandwidth, and so the amps should not compromise that, whereas I doubt there's much 20kHz signal content from a guitar or a guitar type speaker.
Maintaining frequency / phase linearity into ultrasonics is therefore highly desirable for hifi, so it would be perverse to needlessly compromise OT primary-secondary coupling by leaving chunks of the secondary unused.
Bear in mind that some hifi designs use way more global NFB than MI amps and so phase shift within the loop may reduce their margin of stability.
Hence OT performance way above audio can be a dealbreaker.

FWIW I have used the same OT in its earlier separate output windings version in a bass amp, no instability, lots o' power, sounds great even when the owner uses the 4 ohm output leaving the 4 to 8 coil unloaded. 19 years in use so far, no complaints.

It would be a hit I'm sure to build hi fi with these. Please excuse my laziness, I have a couple of perfectly serviceable Dynaco and McIntosh amps for that. Just call me Lucky Leo.

[M Fowler]

I've put the PT on one end and OT on the other in several builds one being a JCM800 50w combo see photo.

Also Henry from Red Plate amps has does this see photo. Henry runs the OT wires on the out side of the chassis.

[Smokebreak]

Thanks Mark! That's what I needed to see.

[eddie25]

Is it an 8" chassis? I do my OT's on the input side, but just put them in the back. The primary's just go straight to the back and run along the back edge and up to the sockets. All wires remain a good distance from any input circuitry. On my Model T, I used terminal strips and the put the socket right by the jacks so the OT wires are a mile away.

Are you going to raise the 20uF caps? I imagine they used those because they were the easiest/cheapest/only available at 600V, but that's a wild guess. I used 'em because this amp is ultimately intended for that type of sound and because I also wanted to use 600V caps for layout purposes. I probably would have considered 30 or 40uF at least if those were available in 600V.

[Smokebreak]

Hey Eddie, yep my chassis is 8". Sockets by the input and term strips is a good idea. So much prep is already done on this amp that it'd kinda be a hacky job if I started rearranging things at this point, I'm realizing. I checked with my buddy and he said he was so used to using Fenders all these years, lopsidedness didn't bother him a bit. He also said the amp will hardly move from home. That kinda sealed the deal for me . I'll do some experimentation on OT placement on the next one

Yep I'm using 2 sections of 220uf/350V in series. I've attached my layout. If anybody happens to want it full scale PM me I can send it.
Just radio has 100u 600V caps if you ever want to add more without mucking with your layout too muchhttp://www.justradios.com/cart.html

[Smokebreak]

Gettin a board together. For some reason I thought the 278CX had a bias tap but lo and behold it does not! So , I had to make due with the space and eyelets I'd alotted for a single cap style supply. I'll start with a 220K dropper, and fiddle with it from there.
I used a .5H inductor for the mid switch, instead of the stock 1H, so I doubled up on cap values for the LC circuits. Not the most exciting looking board I've ever seen

[Smokebreak]

I started to wire up the input jacks. I'm not doing any "outs", just 3 jacks - hi, lo, and both.
I noticed the 1M input resistors are after the 68K stoppers. Now this will cause a slight attenuation. So I was going to do them "normally", and then I realized that perhaps this was designed, as if unless you wire them as a divider, the 2 1Ms end up in parallel when running both inputs.
Have I got this right? Which do you guys think is worse? A 500K input resistor or a 68K/1M divider at the input?

[Leo_Gnardo]

Which do you guys think is worse? A 500K input resistor or a 68K/1M divider at the input?

I doubt you would notice the 68K/1M over a 500K input R. Also very unlikely to notice either if there's any buffered effects between guitar and amp, or active pickup/ line drive within the guitar. Bright sounding pickups would be affected the most, single coils. Also, it's a bass amp. Unless you really need a lot of ping in the top end for a Stanley Clarke sound, not much use worrying about response over 5-6KHz. As the owner of my build says "it's a bass, not a treble." Hold down the low end, let the hi-hat have its space. Even a 500K input load won't turn a Precision or Jazz bass into mud.

[Smokebreak]

Cool. Thanks, Leo. Yea, therein lies a quandry in the back of my mind - tweaking the thing when it comes time. I don't even own a bass, much less have any experience with bass player tone talk. Should be new and interesting.
Customer says he wants lots o headroom, but the ability to dial in some growl. I'm gonna put a bypass on a switch for the 2nd stage Rk. maybe even put a 100K or so pot underneath it and call it "growl".
On an unrelated note, AC wire management has always been kinda a pet peeve. On this one I've got an added filament transformer, and a 120V lamp, so I'm trying out this method with a turret strip. I feel better already

[Smokebreak]

Leo, do you remember the 6.3V transformer you used?
Eddie, did you just use the 278CX winding for the heaters?
I got this thing fired up today, and everything seems fine, but everything is so hot!
The PT is fine(not even using the fil taps), but my fil transformer is really hot, and it got hot pretty quickly. After about 30min of being on, I could leave my hand on the metal bracket for 3-4 sec then it hurts.
Granted, I've never built an amp that has a transformer being taxed for 7-8 Amps, so I'm wondering if this is just normal for these big boys. Those KT88s get hella hot too, my goodness.
Here's what I used : http://www.mouser.com/ProductDetail/Ham ... iq4u5ZA%3d