Single-ended-series amp with a "1 winding" transformer

[tubee]

Hello

I was busy in my garage and found a sketch again of a 20 Watt tube poweramp. I redraw it and made a photo of it to show here, comments welcome.

The idea is to make a simple power amp with the tubes, chokes and transformers i have here at hand. Have a lot of EF86's penthodes, four EL36 power penthode with anode cap on top.(are slightly smaller then the well known EL34)

Input stage is simple of in triode operating penthodes, second EF86 is for phase reversal.
The virtues of the single ended endtube design is the simplicity, and the rather low impedance, one kathode drives the output transformer and one anode. This low impedance gives a chance of using a simple transformer of about 500 ohms The transformers i have here are with a single winding, this is better for high frequency coupling to the speaker. But on the contrary it needs an output cap to the transformer, otherwise the core is saturated due to dc current.
Each EL36 is in triode and top and bottom tube has to be biased with its own negative supply, automatic bias with a resistor is not easy (nor possible?) with series endtubes.

Well, comments welcome!

[Tubes4e4]

Hello Tubee,

interesting circuit, but for better performance, the upper tube should get about 3...4 times the driving swing the lower tubes gets. (Unloaded mu of trioded EL36 is about 5 times.)

The easiest method to get such a drive ratio would be to follow up the concertina with a differential stage, or replace it by a LTP splitter, with one of the both plate resistors split into two serialized ones giving the desired ratio from the tap between them.

If of interest to you, you can find the triode strapped plate curves for EL36
here at my site.

Regards, Tom

[ilimzn]

...also, it may be difficult to get 20W out of two EL36 in triode... it will have to run class AB.

[tubee]

Thanks for replys Tubes4e4 and ilimzn

Interesting thought yes the different driving of the endtubes, didn't thought of that! I have somewhere a good phase inverter schematic which is able to drive unequal, i think it's a paraphase, not sure how it's called. Have to get into tubes again a little, did a lot of other stuff lately.

The stated 20 watt is a little overated yes, realised it when i closed the post. A single EL34 in class A in triode delivers about 8 Watt, so this circuit max 16 watt.

The coupling cap on the end can be a computer grade lytic, or a black gate (expensive!) First find some stuff out of a box and see if the schematic works. The end transformers did i find in big trashbin at street, two identicals.

[poynton]

It looks like an OTL design with a matching transformer

Andy

[planet10]

It looks like the Philips SRPP amplifier used to drive their 800 ohm speakers.

dave

[Tubes4e4]

Hi Dave,

Originally posted by planet10
It looks like the Philips SRPP amplifier used to drive their 800 ohm speakers.

Nah, that Philips one is quite different; SRPP indeed, single drive (lower tube only), pentode mode, no OPT.

Tom

[planet10]

The ones i've had thru here all had output chokes (like this one)

dave

[poynton]

Originally posted by poynton
It looks like an OTL design with a matching transformer

Andy

What I meant was that, regardless of the matching transformer, the usual criticisms of OTLs can be made eg cathode follower/anode follower etc.. (post 2)

Andy

[tubetvr]

the usual criticisms of OTLs can be made eg cathode follower/anode follower etc.. (post 2)

Yes and of course the same methods as used in OTL to achieve balance can be used, e.g Futterman or other methods. BTW the method suggetsed in post 2, (changing the gain of the signal to the lower tube dosen't work as the gain of the lower tube changes with different load, therefore static gain balancing only work for one load impedance value, the Futterman method achieve full balance regardless of output impedance.

Regards Hans