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Lafayette KT-550 Amplifier OPT's

I've got an interesting conundrum. A while back (a few years ago) I acquired a NOS Lafayette KT-550. That is brand new, in the box, never built. Seems like green bananas are my kind of project, and maybe this one's time has come. Well I've just discovered the sad fact that the OPT's are in carton B which I do not posses. What would you do?

My first thought was to obtain output transformers suitable for PP 7027s. They like to see about 5000 ohms plate to plate at 25 watts or more. They're available from places like Edcor and others as well as eBay. I personally don't like the lopsided paraphase inverter they used, but that's just my taste. Depending on just what the chassis looks like where the OPTs go, you may have to fashion plates to cover a large opening if any. My other thought was to sell it on eBay. People seem to pay ridiculous money for unbuilt kits like that. Even without the trannies.
link to schematic
I just realized that you may not have the power transformer either, in which case fixing it is problematic.
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Interesting KT-550 schematic.

Now I see why the Cit II and KT-550 has the shunt plate to grid N Fdbks on the 12BY7/6CL6 driver stages (beside the outer local N Fdbks). The 6CL6 has an Rp of 150,000 Ohms. That needed to be lowered to drive the outputs. Apparently the idea got carried over to the Cit II, 93K Rp for the 12BY7 alone. A 6197 (fits 6CL6 socket) in place of the 6CL6 would lower the Rp down to 90K. A 12HL7 would get it down to 55K or lower with reduced plate load resistors (and the extra gm to drive those) (even fits the 12BY7 socket, with a re-bias).

[Aside:] This probably explains the weird RCA 50 Watt schematic too, with the 6CB6 driver stage. 6CB6 has a 280K Rp. The high Rp was ideal for the shunt "Schade" drive and that then lowered the final effective driver output Z to the output tube grids.

The 6BR8A paraphase front end is rather surprising. Bad tube curves, Bad impedance match (for driving the low Z inputs of the multiplie shunt "Schaded" input 6CL6 drivers), Bad unmatched splitting. Maybe the 2nd harmonic dist. was wanted? Hmmm.

my Fixups:
Use the lower Rp driver tubes (12HL7 with reduced load resistors best) and drop the inner shunt "Schades".
Move the outer "local" N Fdbks from the driver grids to the driver cathodes. (an impedance change, un-cross the Fdbk pathes, with split cathode resistors from the tail R or CCS)
Use a 6F12P or something else, maybe 6BN11, for the front end. Could be pentode to concertina splitter, LTP splitter, or even a Bailey-Radford splitter. (driver stage input grids hi-Z now)


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alternative fix:
Using say a 6BN11 differential splitter front end would allow keeping the driver stage outer "local" N Fdbks the same (to the driver stage grids, crossed) Equal splitter output Z's going to the shunt "Schaded' driver grids then. Would be curious to find out if there is any "sound" difference between the two versions.
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What would you do?

Unless you manage to found an exact replacement, I would use an easier to find quality transformer that was also available in the late 60s. ie. Acrosound TO-300 or Stancor A3851. With the TO-300 you can modify the amp to Ultra Linear late on. With the Stancor you can modify it to Triode output with cathode feedback. You get two fun for the price of one as a consolation price. I would avoid modern transformer for this rare NOS classic.
I would avoid modern transformer for this rare NOS classic.

That is a serious consideration....Back in late 1968 I built the 50W RCA version (as another reader mentioned) which had 6CB6 tubes with also those introvert feedback loops with all those irritable small pF's, and found it unstable with a clone o/p tranny. Stuck, even though I got it going, it was unstable with certain LS crossovers; something of an anathema to the tube amps of the day which liked feeding into a single order 6dB oct crossover. Dumbfounded, I basically rebuilt it to a simplified LTP for the same power.

Anyone care to give an opinion on value of what I've got here sans transformers?

Here's photos of all the parts, including the chassis, tube cage, control plate, knobs, PCB's, tubes, etc. All in original factory packaging.


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more photos

Photo's continued.


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