John Day's Big Blue

Hopefully someone wrote down all the things he told us, 5U4 rectifier, choked, PIO caps in PS, Hammond OPTs, Dual triode in the big bottle with some small Russian triodes for amplifiers. He did some fairly tricky stuff with using windings in the OPTs to buffer and I just got lost..............It was just way over my head! I don't have a clue as to how this works, but it sounded great, looked beautiful and was a high quality build. Lots of thought and effort went into it and a lot of it was not mainstream DIY. John is the Lonestar Bottlehead Audio Club's amp design/ builder big dog and he is lots of fun to be around.
Next meeting should unveil his TL with JBL D-123 2 way with Heil AMT speakers.
I will contact John and find out if he will describe how this beauty works.
 
Last edited:
He lost me about 1 sentence into his description.

I do recall he said he had 6.25wpc. And he configured it differently than most would, said he put more load on the front of the big tube, and that's why he needed the smaller Russians to....

Hell I don't know, I wish he'd pipe in, he did say the end result was MUCH higher damping factor.

Cheers,
Russ
 
"Blue Magoo" Amp

Wow,
David (Thatch Ear), how do you expect me to live up to that intro?
I'm going to sneak out the back now, thank you...
As for the amp it is a marriage between an Otis Elevator high current voltage regulator tube (Tung Sol 6336-A) and a MIG Foxbat fighter plane radar amplification tube (6C45PE).
And y'all thought gay marriage was controversial!
Q:Why would anyone do that? (And without a condom?)
A:The 6336 is a very low plate resistance tube, with excellent linearity,
and it has TWO 30 Watt triodes in it, almost like 2 X 300B, but without filament supply hum problems. It is a high quality tube, especially the A and B variants with zirconium coated milled graphite plates. Glowing dull orange is a way of life for these puppies. The grid is gold plated molybdenum. It is cool just to say that, so it's gotta be good. The sound may not be what you'd expect. It is very organic and natural. This is my second amp using this tube and the smoothe, warm, harmonically rich characteristic is there in both amps despite different drive arrangements, current, voltage and load arrangements. Detail is very good, but never etched or glaring. The heft and skin of drums comes through a little more than the rim whacks, though frequency response is flat across the audio band.
The 6C45PE is an inexpensive Russian copy of the highest transconductance tube ever made, the Western Electric 437A. It was astoundingly difficult to produce, and in both cases was used as a grounded-grid microwave (what radar is) amplification device. It can give more drive in one stage than any other tube I know of. However, it is a deal with the devil. (Even if you don't believe in a devil, please bear with me.) Grounded grid is a very stable configuration, and these little firecrackers want to run away at radio frequency when you ground their cathodes and drive their grids like we usually do in audio. So how do you make this marriage work when one of the partners totally freaks-out during intercourse?
Marital aids... In this case carbon comp resistors on both the grid and the plate, but suprisingly a 4 nanofarad cap had to be soldered between the grid pins of the heater supply of each tube because RF was leaking in and driving the cathode at 150 MHz. Joe Roberts clued me in to that fix; I'd never have known to try it without his help and advice.
That being cleared up, you may wonder about the topology.
The 6C45PE drives the 1/2 6336 per channel through a Lundahl LL1660 (10 mA)
transformer configured as a 1:2.25 step-up, with 20 mA running through the 6C45PE. (That is OK because the primary windings are in parallel and can handle twice the current that is specified when series wired.)
The 6336 is running 110 mA at 270 Volts plate to cathode. The transformer is a Hammond 2.5K SE design with minor surgery perfofmed to separate the primary windings 60:40 at the ultralinear tap. The 60% winding loads the plate of the 6336, and the 40% winding loads the cathode, providing part of the cathode bias resistance along the way. This provides a higher damping factor for the output stage and gives a strong, sure, competent bass sound instead of boom and bloat, or just weakness as is often the case with SET amps.
The down side is that even more drive is needed from the drive stage, really a lot, about 220Volts peak to peak without distortion. Now that little Russian vixen is starting to make sense, huh? She is a real swinger! (Just talking voltage here, any rising passions must be quelled with logic and reason at this point.) The drive stage is set at 175 Volts on the plate of the 6C45PE, and due to the step-up of the interstage transformer, it can drive a clean 300 Volts peak-to-peak.....
Gotta stop and breathe for a minute....
The power supply is pretty mundane employing a power transformer made for do-it-yourself TV construction in the 1950s, but never used, a 5 Henry @ 300 mA choke, with a 2uF/660VAC oil cap tuning the voltage up a hair and filtering the spikes between the 5U4GB rectifier and the choke. Four 30 uF @ 370 VAC oil caps are paralleled after that to feed the output stage. A 5K resistor per side ferries voltage to the driver tubes, where a 20 uF @ 600VDC solen cap provides final filtering. Cathode bypass caps are absolutely necessary to keep hum out of the 6C45PE cathodes, another lesson I learned... Th 91 Ohm resistors are bypassed with 2200 uF. Less will be noisier. Don't say I didn't warn you about this tube, she needs a firm hand ,to be sure...
The 600 Ohm cathode resistors on the 6336s also get bypassed, but that isn't critical. I used 220 uF @ 100Volts, all Sprague Extralytics, my favorites.
The final toll is: Too much money
57.5 pounds
6.25 Watts per channel before clipping on the scope 20-20kHz.
Looks pretty, glows orange, sounds good

In the immortal words of the delightfully mortal Red Skelton,
"Good night, and may God bless..."

John Day
 
Thanks John! You went through this tap dance at the meeting but there was no way I could absorb it let alone repeat it.
Needless to say it does sound very nice, John wouldn't loan it to me, and I asked. :D
 
I think this is the amp John had at the 2004 Bottlehead meet. I was able to view it in person. It was too impressive! The sad part I never got to hear it, damn! The speakers he had made playing in the front room were fantastic.
I talked to John for awhile but his knowledge and building experience are way beyond my laymen abilities.
Just beautiful John! Hope to hear it someday! :thmbsp:
 
Huh, this is truly interesting. It's rare to see an amp designed these days, to run in what appears to be very similar to "Quad-mode" (as in Quad the English manufacturer, electrostats and electronics and the like)... with the cathode winding in the transformer. The alternative to Ultra-Linear mode... a configuration very few were willing to tackle, up to now...

Regards,
Gordon.
 
Back
Top Bottom