OTL, quick `n dirty

[Fuling]

Hi!

Once again I´ve managed to pile up a lot of junk that just has to be transformed into something useful, like an OTL amp.
How about this:

1 big power transformer, 1kVA with 2x115V secondaries
1 filament transformer, 6,3V 10A
4 25mH 3A C-core chokes
20+ 6C19 output triodes
Various high voltage transformers and preamp/driver tubes.

All I need to buy is PSU caps, tube sockets and some of the small stuff (caps and resistors)


Rosenblitz T16 amp seems interesting, but I can´t find the schematic online.

BTW: Since I´m using fullrange drivers in backloaded horns I don´t need much power or low output impedance, I´d be perfectly happy with 3W and 10-15 ohms out.
Maybe I can get away with 4 x 6C19 per channel?

Any ideas of what i can put together with these parts are very welcome!
I´m completely fresh to OTL designing...

Cheers
/Daniel

[Sch3mat1c]

Lemme see, what's a 6C19... (6S19P?)...

11W 140mA 250V, Rp(typ) = 400 ohms.

The curves don't show anything above -5Vg or 140mA (figures, as a regulator it may experience those continuously so no need to know data outside of permissible ratings... nuts to them!).

Well let's see. Start with something like class AB1, say +/-140V supply, you can go up to 80mA per to stay within ratings but running power to a speaker is going to put a whippin' on them so keep it like 40mA (-43V bias, about). Peak might be well, with 10A of filament power you can run five in parallel for each half (assuming an SEPP topology), so an 8 ohm speaker looks like 40 ohms to each individual tube out of class A. That means uh... 25mA per volt. So for say, 500mA peak = 60W = 5 times rated dissipation (so about 2 x rated when averaged over a sinewave), you get 20V peak = 14.14Vrms across the load = 25W into 8 ohms. It's better, about double the power, into 16 ohms. It'll keep doubling until voltage starts dropping towards 50V peak (on the tube's loadline), at which point you have to increase supply voltage to get more power output.

Such an arrangement, for posterity's sake, would probably give 100-200W into a proper transformerized load...that's why OTL's really well and truely suck. Not to mention more filament than OUTPUT power.

Tim

[fdegrove]

Hi,

that's why OTL's really well and truely suck. Not to mention more filament than OUTPUT power.

True....
But then we don't ask you to pay our electricity bills either...

Running OTLs in class A and single ended would not make matters better in that department either.

Cheers,

[etalon90]

Why nobody want to build a otl in his optimal condition?

The best OTL/loudsepaker system may be:

A line array with 8-9 driver wired in serial. A beautifull 64-70ohm load.

Build an otl for this kind of load will not require a load of tube in parallel.

A typical ciclotron using 2 pair of 6c33 without feedback will provide 24ohm Zout and give 40Watts at 2% thd and 3.5 Damping factor!!! (in 68ohm load)
add a little 6Db of fedback and you have a 12ohm Zout and 7X damping factor.


Even better, a ciclotron using a single pair of 6c333, biased hard (95V, 300ma) without feedback, will give you a beautifull 6.5watt of pure class A at 0.9% thd and 25ohm Zout...

Linearray and otl sound very good for me.

but for 8ohm load, otl are not the best (too much feedback/parallel tube are required)

My 2 cent.

Martin

[Sch3mat1c]

Originally posted by etalon90
The best OTL/loudsepaker system may be:

A line array with 8-9 driver wired in serial. A beautifull 64-70ohm load.

Build an otl for this kind of load will not require a load of tube in parallel.

Well, not if you want the same conditions as a typical loudspeaker (4 to 16 ohms) into a standard parallel setup.

For optimal load matching you need a series array AND paralleled tubes. Then you can approach proper efficiencies, with the added advantage of, since you have so many tubes, you can run a good amount of power output - 100 to 300W perhaps.

I think it was once said you need 625 6V6's to match to an 8 ohm speaker... this requires 300V at 37 amperes (this includes the 5.85 amps required for heaters, if arranged in 18 series strings), but does output 2.5kW SE.

Tim