Re-Living My Past (OTL Vacuum Tube Builds)

[Mark (Basspig) Weiss]

Way back in the '60's I wanted to build a hi fidelity amplifier, but could not afford the output transformer of that quality. So I looked over several tubes that had low plate resistance and settled on the 8020/6AS7 back then and built an amp with 3 6AS7's. It put out 25W into 16Ω loads and was flat from 10-100,000cps, at least as best I could tell on my Heathkit oscilloscope of that era.

Well after repairing and upgrading a few Phase-Linear amplifiers for fellow audiophile clients, which pays well, I decided I still enjoy this hobby. I updated some of my test gear and have spectrum analyzers, distortion analyzers, all sorts of signal generators, meters, power supplies, frequency counters, oscilloscopes, etc.

So at this point, I have a strong desire to revisit my OTL vacuum tube amp design, update it a bit, increase the power.

So here's my thought. Using Circlotron topology (really just a fancy name for a push pull cathode follower output) and some ultra low plate resistance sweep tubes, a solid state power rectifier (power supply will be transformerless too, making this amp as dangerous as the 5 tube AC/DC radios of the 1950s) maybe opto-coupled input stage to isolate source components and a couple 12AT7s in the gain preamp stages. Similar to Futterman's designs. I'm looking at 6C33 tubes, but also find the 6P45 a very attractive alternative. Allen Kimmel claims he achieved 125W RMS into 8Ω with just four EL509 (6P45) beam tetrodes in the output stage. 200W peak (and then his power supply sagged). So four tubes per channel would give me a nice 125wpc with headroom to 200wpc.
EL509/6P45 can do 1400mA plate current at 140V plate potential, per tube. Plate resistance is 80Ω and can be forward biased to reduce that figure substantially. Kimmel's amp claims to have a .8Ω output impedance, for a damping factor of 10 into 8Ω speakers.
This will be the lightest tube amp of substance ever built, being not one piece of iron on the chassis. I even have ideas for powering the filaments (2 amperes a piece for the 6P45) without a transformer, if possible.
Another goal for this amp is to have all stages DC coupled. A lot of cathodyne drive circuits, cathodes directly feeding control grids of the next stage, etc. Only one cap at the input. The rest servo feedback balanced to keep the DC balance across the load.
It should be a fun project. Have to wait for some of my stuff to sell on ebay, then buy chassis, Magnovel tube sockets, 6P45 tubes (from Russia), some electrolytic caps for the power supply, etc. I have most of the rest of the tubes, resistors and other parts. Planning to run strips of 2oz copper PCB as ground bus all around the inside of the chassis for ground ties, terminated at the input jack.
This project is about having fun and meeting the 'no transformers' challenge I've set for myself, while producing a useable amplifier with decent power output.

[microstrip]

Way back in the '60's I wanted to build a hi fidelity amplifier, but could not afford the output transformer of that quality. (...) I'm looking at 6C33 tubes, but also find the 6P45 a very attractive alternative. Allen Kimmel claims he achieved 125W RMS into 8Ω with just four EL509 (6P45) beam tetrodes in the output stage. 200W peak (and then his power supply sagged). So four tubes per channel would give me a nice 125wpc with headroom to 200wpc.
EL509/6P45 can do 1400mA plate current at 140V plate potential, per tube. Plate resistance is 80Ω and can be forward biased to reduce that figure substantially (...) .

Nice to know I am not alone. I was exposed to a Futterman OTL3 and later got into a DIY copy of it in the early 70s and since then OTLs were part of my life. Due to time and heat constraints some years ago I parted with a pair of the big Atmasphere MA2, that I have upgraded myself along several versions using advice and parts from Ralph Karsteen - a nice feature of his tubed point to point wiring OTLs!

If you want to go the EL509 route, you have to source NOS EL509's , that can be very expensive - current versions do not have the 1400mA capability - they are reported to do just 500 mA!

[treitz3]

Hey Mark, where do you plan on acquiring the chassis from?

[Mark (Basspig) Weiss]

Hey Mark, where do you plan on acquiring the chassis from?

Antique radio has pretty good prices on Hammermill chassis. About $20 for the size I need to build a mono block.

As for the tubes, EL509s are expensive. I'd probably go with 6P45s, which are the Russian version, similar ratings, magnoval base.

[JackD201]

Mark, so great to hear you are using your talents to help other music lovers and that it pays well to boot Looks like you're going to have a lot of fun with this project.

When you say dangerous, how dangerous?

[microstrip]

Mark, so great to hear you are using your talents to help other music lovers and that it pays well to boot Looks like you're going to have a lot of fun with this project.

When you say dangerous, how dangerous?

Jack,
Using the mains supply in amplifiers without an insulation transformer means that your 0V line must floating - not grounded - and statically at neutral or phase voltage, or something between them. It is a very dangerous practice and strictly forbidden in Europe - you have a 50% probability of being electrocuted by the signal lines each time you connect the power cable, as power connectors are not neutral-phase labeled!

[JackD201]

In other words, LETHALLY dangerous? Hot damn.

[mep]

In other words, LETHALLY dangerous? Hot damn.

Could be if you use a non-polarized PC and plug it in so you are putting the hot side to the chassis.

[JackD201]

I'm 100% sure Basspig will make sure he doesn't do that. Wonder though if the 0V line is always where it's supposed to be. Imagine if some work was done on the house electrical system and it got reversed somehow. Ouch.

[treitz3]

No doubt.