6CD6GA in Enhanced Triode Mode for SE Amp

[wrenchone]

I partially fixed hum on Miz Piggie using an RC post filter on the 400V B+ going to the output anodes. I used 2 X 270 ohm 3W resistors in parallel followed by a 220uF, 450V cap. This quieted things down to an acceptable level. I had tried an LC post filter, but I got some weird crackly noise at the amp outputs, and I didn't want to take the time to get to the root cause.

I took the pig with me to Burning Amp, and she did well with the higher efficiency speakers. People couldn't beleive such nice sounds were coming out of cheapo tubes and Hammond budget series output iron. I'll probably press her into service at home after putting some holes in the bottom plate for better interior ventilation.

Next up is Mister Piggy - I have some EL509s that would work a treat and no doubt blow off 40W dissipation without working up a sweat. I see no need to change the basic schematic, though I may use a better screen regulator for the 6JC6 input stage. Going to a higher output stage bias/output power ( I'm using 70 mA, ~400V now) means either turning up the plate voltage or ratcheting down the output transformer impedance to 3k. I'll have to ponder the implications of these alternate moves. Any comments form those who might have already been there?

[wrenchone]

I decided to keep the XFMR impedance at 5K and raise the output stage plate voltage. I've been buying up old Wurlitzer organ amps on Fleabay for salvage, and the power transformers are truly massive, probably as they are sized to power the rest of the organ as well. I dragged one to work to measure it up, and it has a nice 375-0-375V seconadry that should deliver over 500VDC even when loaded. It also has a 6.3-0-6.3V winding, a 5V winding, and a HV bias winding with a tap. If the size is any indication, it should have no problem powering what I have in mind. The output tubes in this case were 7868s. The output transformer measured to be 3.5K, which is kinda low, especially with such a high B+. It was also a bit on the small side. Strange design philosophy... I may use the output transformers in a small screen-driven sweep tube push-pull.

At any rate, I have a spare 16 X 8 X 2 chassis, which looks perfect for the underpinning for Mr. Piggy. Miz Piggy was a little too small, and the wiring is pretty crowded.

[wrenchone]

I was thinking (dangerous practice), and if the organ had a 16 ohm speaker, the reflected impedance at the transformder primary would be 7000 ohms, which makes a lot more sense considering the tubes and the plate voltages, and would allow a relatively wimpy output XFMR, which is what I've got. any comments from folks who've parted out Wurlitzers?

[wrenchone]

Onr caveat for those who might be thinking of building something like this. Take a look a the picture of Miz Piggy a few posts back - see that? Use a bigger chassis and place the iron a little farther away from the glass. The output tubes in the missus blow almost 30W apiece, and in extended listening tests (very satisfactory, if you must know), the tubes heat up both the output and power transformers. It's not a show-stopper - the power XFMR is probably at around 40C on the side facing the tubes, but it can be avoided with more generous spacing. It may be a problem with transformers that are painted black and run naturally hotter, so best to space things out and avoid some grief. Some holes in the chassis around the output tubes to provide ventilation would help, too.

[wrenchone]

For the sake of completeness, attached is a file showing the power supply schematic for Miz Piggy. It was dictated in part by the unique nature of the power transformer I happened to have lying around - a one-of-a-kind surplus buy. I used a second PC-mount transformer for the bias voltages. All the power supply stuff is on a piece of perf board that is attached to the transformer mounting bolts via spacers. If I had used a less cramped chassis space, things would have been laid out a bit differently. Folks wanting to duplicate this more or less with available iron would need a 540-560 VCT, 200 mA or greater transformer with a 6.3V filament winding with at least 6A capacity. Hammond has one. You would need to use a dropper resistor and a filter cap to power the source follower. A separate small transformer would still be needed for the bias voltages. A 24-0-24V transformer would work - these are readily available.

Unless there are other questions, that's it for this thread. When I have sufficient progress on "Mister Piggy" to warrent mention, I'll start a new thread.