Re-Living My Past (OTL Vacuum Tube Builds)

[cnpope]

I think we're on the same page about these safety issues. For personal use/experimentation, it is fine, since I control the wiring in this building and it is conformant to NEC standards.
My approach is also to use 3-prong grounded plug, with the fuse taking up the slack if an outlet is reverse-wired hot/neutral. Current would flow to earth ground and the fuse would open in the case of miswired outlet.

Input isolation was achieved by means of true differential input stage. The CMRR is excellent and it works.

I am having some issues with making Circlotron work with the transformerless power supply. I've worked around some of them by using a simpler rectifier design and voltage doubler circuits. I have four power supply rails per channel on this amp.

Here is an early photo from last fall when I was testing the filament power supply. I'm using a TRIAC to power the 6P45s and series string with ballast resistor for the low power tubes. I may implement a DC filament for the first preamp stage at some point, depending on noise levels.

I'm working toward a goal of end of March readiness for the amp, so I can show it at the next Basspig Audiophile Society Social meeting this coming spring. This is purely for fun.

I'm posting updates at my business page on Facebook: https://www.facebook.com/AmplifierExperts

Your amplifier looks very nice! I'll be interested to hear how it progresses.

I'd also be very interested to hear how you are managing the power supplies for the circlotron topology. I had reached the conclusion that with direct mains-connected power supplies it wasn't going to be possible, and so for that reason I made mine a totem-pole. But I'm beginning to think about a possible follow-up project, again direct mains connected, and if there is any way to get around the obstacle, I would be very interested...

Chris

 

[Mark (Basspig) Weiss]

I've finally gotten the rest of the parts (saving money for this takes time) for my OTL amplifier project.

I decided that if I were to shun the power transformers and make it line connected, this design would fall far short of its potential. I really needed full wave rectification and floating power supply busses.

The search for suitable power transformers proved frustrating. Then I found industrial control tranformers, 500VA each. 120V primary, 240V x 2 secondaries each. I figured that with a split bridge rectifier, I can get +/- 170V DC from each secondary. Or... I can do something radical, and have +/- 340V DC!. The idea hit me last month that I could use MOSFETs to switch the rails, based on program demands. The amp would idle and operate up to 1/4 power on 170 volt rails, then switch to the 340 volt rails when musical peaks would call for more than 125 watts per channel, giving +6dB of headroom, or 500 watts per channel program peak capability. That would be time limited by the plate thermal mass, which is substantial on the 6P45.

So I scraped the cash together and bought the used transformers on eBay, and got a killer deal on three 20A 6.3 v filament transformers-- three for $75. And saved a bit more and bought the chassis last month. More banks of capacitors.. 14,400 uF at 200 volts in all for the main rectifiers, with additional capacitors for the preamp/driver stages under the chassis.

This is what the power supply looks like so far. On the right. Two filament transformers and two industrial control transformers. The amp is a dual mono design--two independent filament supplies and plate supplies. Connections between chassis are via 21-pin Cinch-Jones connectors.




I am hoping to have this built, debugged and functional in time for this spring's B.A.S.S. Meet. I'm also reworking some 15" multiway speaker systems that I built in the early 1970s, for this demo.