Hello

[redelephant]

I just found this site! Never heard of it, but it seems right up my alley I've been building tubeamps for almost 15years now and guitar dittos for about 10 of those.
I try to introduce something new in addition to the standard vintage types out there, and have been making tube amps without the output transformer. So called OTL amps. I guess I'll start my first post here with a simplified schematic of the head I demo in this nerdy utube vid: YouTube - OTL project

On the schem here I only show the output stage, the preamp on this head is a 6922 double triode, which gives only two stages of pretty low gain. The output crunches before the preamp. I also use a semi-hybrid phase inverter to the output tubes and direct couple the output tubes to the PI.



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[Steve Conner]

Hey, that's neat! You need to get a better recording so we can hear what it really sounds like.

Do you have a schematic of the preamp?

[Diablo]

Can you explain the circuit in more detail? I'm used to the conventional tube amp with transformers. I just checked my stash and I have 7 of those 6AS7 tubes.

[Merlinb]

Hi Semper, I like your use of current mirrors! (Say them on DIYaudio first). Looks like it does a decent volume for a single 6AS7!

[redelephant]

Man what a day. Had to go to work on a Saturday b/c a design is due on Monday and I'm way overdue. Instead of getting my **** in line I spend most of the time discussing tubes with nerds on the web. Now I sit up too late listening to LPs and frustrated my stash of red wine is empty, my single-malt reached slightly to the edge of the ice cubicle, and the only thing left is some bourbon!!! AAAAAAARRRRGGHH!!! And I got to get up tomorrow and work the work I didn't work today! Can't wait till Monday!

Anyways. Thanks for positive feedback. Merlin...the world isn't big enough. I finally naively though I'd found a new place to hang around...kinda fed up of those perfect HiFi folks. But I guess it's nice to get a friendly response from someone who hangs out at the same place as me Cheers mate!

Steve, yeah, the vidcam really compresses the sound. These amps are very dynamic, meaning soft touches are mellow, and hit the strings hard the sound explodes. Some guitarists dont like it, since it hide nothing. Some love it , b/c you can play the amp. It is super touch sensitive. I'll try and post the preamp schematic when I have time to draw it up on a computer that's online.

Devil, well, the schem shows most of it but the power supply. The signal path on these are really short and direct, but the multiple voltages needed makes these amps complicated after all. I guess u can argue which is better, simple supplies, or simple signal paths...My main goal was to make something not 'me too' and just give all those clones the finger. We all have different tastes, and as I said, some guitarists like them, some don't. If u have some 6AS7s, I'd say build one. Not much to lose.

Well, the bourbon is starting to warm me up Next record up. I'm done for now....G'nite.

[Steve Conner]

OTLs... Single malt... Are yew one a' them "audiophiles"?

I think OTL makes more sense for guitar than hi-fi. The reason being we make our own speaker cabinets and can control the impedance. I'd love to hear one of these babies blasting through a 4x12 wired for 64 ohms, I bet it would sound absolutely massive.

I'm a member of diyaudio too, but haven't posted in years. I'm currently working on a Douglas Self Blameless power amp.

[redelephant]

Absolutely Steve! I have yet to build an OTL for my HiFi, and I have about a hundred of 6AS7s laying around. 8ohms is just not high enough. But like u said, in guitar and bass cabs we can series connect several high impedance speakers, and voila, perfect for OTLs

I had a long period away from forums, just too busy trying to sell my wierd amps

Blameless amp eh? I don't want one of those since I'd have to find other things to blame (and tweak).

[Steve Conner]

Well, how about an "Almost-OTL" for hi-fi. The output impedance of your setup is low enough that you could use a toroidal mains transformer meant for a solid-state hi-fi amp as the OPT. I've measured these, and they have great bandwidth, they just don't come in high enough primary impedances for use as a "regular" OPT.

They also can't stand any DC, but again with your topology you can use a DC block capacitor.

Good luck selling those amps! You need some open-minded customers I'm an EE, but I do industrial electronics and embedded programming for a living. It might not be very artistic, but it's a lot easier to make money out of it.

[Enzo]

Though I repair them for a living, as a potential buyer of an amp, when I see some weird approach, my first question has to be, what advantage does this give me over normal stuff? It may sound great, I hope it does. What will your response be to a customer who asks:

About that 32 ohm speaker cab, how would I use it with any other head? Or what would you charge to rewire my existing 8 ohm 4x12 with switching?

YOU may have a lot of 6AS7 on hand, but where do I get replacements when MINE go out? And what will they cost.

How is this amp when faced with the absence of load and played loud?

How is this amp when faced with a shorted load or speaker cord?

No OT saves $$$ on parts cost, but the added complexity of the power supply might eat that up, and the PT won;t be off the shelf, will it?

What ranges of power output can we reasonably expect? Strictly a 10v or can we get 50w?

Not picking on you, but if this is a commercial product to be sold...

50-60 years ago, Ford asked people what they wanted to see in a new car. They then built a car with all those features. No one bought it. It was called the Edsel, and is famous for that reason.

[Steve Conner]

Good luck selling those amps! You need some open-minded customers

I rest my case... I guess Enzo won't be buying one any time soon

Do Svetlana or Sovtek make a 6AS7? I guess it would be easy enough to replace it with two power MOSFETs.

One good thing about the Edsel was, there is no recorded case of one ever being stolen.

[Merlinb]

and the PT won;t be off the shelf, will it?

Mains isolation transformer, I bet...

[redelephant]

Indeed u are right. These amps are never going to be big sellers. The few customers I've had are all into the experimental side of music, kinda little Zappa wannabes. But they are happy with the amps.

My motivation for building OTL instrument amps was to do something different than the mainstream clones. Basically I ended up inventing the wheel all over... Naively I thought musicians to be artists with a wish to be genuinely original and such an amp would be one way to get a unique tone seperate from all the mainstreams. Mohahaha, how dumb I was Come to find out musicians are one conservative bunch all wanting to look the same on stage

I still enjoy tubes enough to appreciate building vintage (typical) type of amps as well. So my world doesn't end with the OTL project.

B/c of the odd loads required for best output power, a combo is the best option, though 8ohm cabs have been surprisingly good as long as a few watts have been enough. When increasing tube quantities to more than four, the OTL can drive 8ohm cabs no problem, but even then a 16ohm is preffered.
The tubes were in production by Svetlana, but I'm not sure they are any more. I know u can get new China tubes, and according to Ralph at Atmasphere they are just as good as any Russian variant. Presently they cost around $25 I think.
I've only had one go bad on me in the 10years I've played around with them.

The power transformers are custom from a local manufacturer. Toroid types. They actually don't cost much more than say a standard Hammond for normal tube amps.

[Diablo]

You can get NOS 6AS7 tubes for $8. I wouldn't worry about new production tubes unless Fender copies this design and greatly increases the demand for tubes.