Steve Bench OTL

[Klimon]

A quick question about Steve Bench' inverted triode otl: http://members.aol.com/sbench102/PowerAmps/se_otl.gif

Can anyone explain what the weird looking driver stage is? I've never seen an amp with - / + voltage rails nor does my brain get any grip on it's functioning (still talking about the driver stage). Could I simply replace it with a one tube grounded cathode stage, RC-coupled to the ouput tubes or is the weird topology a must for this amp?

Thanks a lot

Simon

[Sheldon]

All is explained here: http://members.aol.com/sbench102/inverted.html

[Klimon]

No cigar; I've read that article several times already and it doesn't seem to contain any motivation about the weird driver stage, correct me if I'm wrong (and meanwhile explain why the driver looks that way )

[Sheldon]

Originally posted by Klimon
No cigar; I've read that article several times already and it doesn't seem to contain any motivation about the weird driver stage, correct me if I'm wrong (and meanwhile explain why the driver looks that way )

I'm not sure what's weird about it. It looks like a pretty straightforward two stage amp to get to the voltage swing needed for the output tubes. A negative rail is required to bias the output anyway, so it looks like Steve just incorporated into the driver too. That gives a high B+, which allows large plate resistors. I guess you could just have an amp with a single B+ rail, but you'd also need 650-700V rated coupling caps, if you wanted to go much higher than 350V. Might me difficult to get the voltage amplification and current needed with a single stage.

The key might be this sentence: This section is merely a drive amplifier for the 5687 output stage. The input stage amplifies the signal from 2 volts to 28 volts p-p. This drives the second stage which amplifies the signal from 28V p-p to 400v p-p. The power supply is +/- 350 volts (700 volt supply), and about 330 volts is placed across the 6CG7, keeping the device within its ratings. The output impedance of this stage is about 7k. The frequency response extends from about 2 Hz to about 200 kHz. The 400V p-p output does include the effects of the 470k load of the output stage.

Sheldon

Sorry for all the edits

[phn]

I don't get the +/- either. But that has to do with my limited knowledge. There are many ECC88 buffer/line stages, for example, floating around with +/- supplies. And the ECC88 isn't even an inverted triode. The only thing I know about inverted triodes is that the ECC230/6080/6AS7G makes for a first-rate output triode. Superior damping. Ripe for a SET with SET and PP qualities.

Steve Bench is clearly one person that knows what he's doing, though I can't understand why you would want to waste ten first-rate driver triodes like this. And I can't begin to understand how much the OTL guys must hate the amps of Ken Shindo and Susumu Sakuma. Actually, I can. They probably hate Shindo and Sakuma's amps as much as I hate OTL amps. That's what makes audio fun and interesting. If everybody had the same taste, there would only be one amplifier.

[Alain Dupont]

By the author...

>> The input stage amplifies the signal from 2 volts to 28 volts p-p.
>> This drives the second stage which amplifies the signal from 28V p-p to 400v p-p

1st triode works as Grounded cathode
{The tric is the heaters and screen are at 0 volts}

1 st triode gain about *14
input 2 AC volts ; output 14*2 = 28 AC Volts

and is connected with the 0.47 mF to the grid of the
second triode {Tric here: make a vertical mirror of this triode}.
Also in Grounded cathode
2 nd triode about same gain.. so 28 *14 = 392 AC Volts

400v p-p for the final...

And it's why there is a -+ 350 DC suppply otherwise no way to get 400v p-p
on the plate of the second triode. {+200 / -200} from a single 6CG7!!!
Or use a 700 V Dc PS! And the 6CG7 cannot pass over 330 plate voltage...

Not possible with my little knowledge :
Replace with only 1 Tube with a gain of 200... (find one!)

Alain.

[Klimon]

Thanks to you all for shedding a light on the circuit, Alain in particular for untangling the driver circuit. I'll probably go for e55l driver (around 30 gain) and 12b4 preamp (6 gain).

Phn; you're begging the question: what makes you hate otl's? Prejudice or bad experiences with their sonics? I haven't heard any yet but I think Steve's circuit would make a great low budget try-out, 5687 are very cheap (tons of nos for computer duties) so an obvious candidate (and I happen to have ten 5687wb philips ecg, impressing denomination but a lot more distortion than the normal 5687, hope it won't show as much in this atypical circuit) . Plus: I definitely prefer se to pp and the designer seems quite enthousiastic about the sound + he's got a few amps to compare with...

I've got a hunch it'll be well worth the effort

Simon

[Alain Dupont]

Klimon,

I had the privilege to listen to the

Joule Electra VZN-100 Mk. III Marquis a 100 Watts per amplifier, Class A OTL design
{6C33C-B x 6}

and the

Tenor Audio 75WI Monoblocks also OTL {four 6C33C-B output tubes}

both with 6C33C-B tubes ; wouahhhh

it was 2 years ago at: Festival du Son in Montreal

I have to tell, it has been a great experience in both case!

Great sound, wide image, and very good definition.

Regards.

Alain.

[dshortt9]

I'm using a Transcendent OTL with one pair of 6C33C-B per channel and it is is in my top 3 best amps ever heard.

[Klimon]

wouahhhh

it is is in my top 3 best amps ever heard

Well, I'll let you know in a few months how steve Bench' design sounds. Also planning to build an older design from Bruce Rozenblit: http://web.tiscali.it/ampli/otl.htm if I can collect the huge load of iron from some cheap (free) source...

Simon