• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

IS there any OTL pre amps out there?

Generally, preamps don't use output transformers. They use output coupling capacitors.
You could design a preamp without output coupling capacitors if you used bipolar voltages on the preamp. I'd be afraid of too much residual d.c. voltage on the output of the preamp as the tubes reach operating temperature and as they age. Readjustment of the bias would be necessary on a regular basis.
In general ... not a good idea. A tube failure could put a huge amount of d.c. on the output.
 
Did you have anything in mind besides a direct-coupled output?

Well, to be honest, I've been running my system (which I just sold off) with only a passive volume pot going into a Behringer 3400 then to three amps, each with a volume pot to balance the volume between drivers.

So, this whole preamp is new to me. Just before I got ride of everything, I put the Hogan line stage right in front of the 3400 and the system really came alive.

So in the new system I wanted to include a preamp/line stage.....if there is a difference.

I am building three amps at the moment. A 12HL7/GM70, a PP E-linar 807, and a Tim Mellow OTL.

I am also building a three way tube crossover.

My speakers are the big Heil on top, a large Tractrix horn with the TAD 4001 for mid, and a vented box with TD15M for midbass, and a pair of subs with Martin Logan Sub drivers.

I am using a passive volume pot/selector switch before the pre.

SO, I could use some advice as to what to include in the preamp.
 
Hi Mike, the thing is, I am a large metal sculptor and my system is one have audio and the other half art. The volume pot and selector switch are going into a Lionel transformer (see pic). the left lever is volume and the right lever is selection. I've already build the phono pre on a piece of blue marble. I used the Hagerman Octal Cornet scheem.
 

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Well, to be honest, I've been running my system (which I just sold off) with only a passive volume pot going into a Behringer 3400 then to three amps, each with a volume pot to balance the volume between drivers.

So, this whole preamp is new to me. Just before I got ride of everything, I put the Hogan line stage right in front of the 3400 and the system really came alive.

So in the new system I wanted to include a preamp/line stage.....if there is a difference.

I am building three amps at the moment. A 12HL7/GM70, a PP E-linar 807, and a Tim Mellow OTL.

I am also building a three way tube crossover.

My speakers are the big Heil on top, a large Tractrix horn with the TAD 4001 for mid, and a vented box with TD15M for midbass, and a pair of subs with Martin Logan Sub drivers.

I am using a passive volume pot/selector switch before the pre.

SO, I could use some advice as to what to include in the preamp.
Well if you are planning to use a tube crossover, my inclination is that the thing to do is build that crossover with slightly more gain than you were planning, and then put a master control in front of the whole thing. Sort of build the line section into the crossover...

The more building blocks you put between you and the speaker, the easier it is to screw things up. But since you are bi or tri-amplifying, doing it this way will keep things simple.
 
Hello djn,
Tubes are cool. The idea of a tube crossover is interesting. I think that in the bi-amped system you are speaking of the tube pre-amp is the easy part. Pick one. I like the Aikido Cathode Follower; it has the grunt to push the line level XO. The output capacitor can be part of the following XO network, no need for that Op-Amp servo.
You look in on the speaker threads there are recommendations of steep XO slopes. I have not read happy stories of tube XO’s doing the job. Lots of tubes, power supplies and related noise, I think. A 2SK170 JFET XO has my eye.
Most important Your creation should have a sculpted chassis.
DT
All Just for fun!
 
HI DT, If you asked me last year, I would have agreed with you about tube crossovers, but a couple guys around here built the Steve Bench using The Planet PCB and REALLY good PS and they sounded better than the other actives I've had. They are 12db sloops.

The chassis is very steam punk and I am looking forward to starting the build on that.
 
HI DT, If you asked me last year, I would have agreed with you about tube crossovers, but a couple guys around here built the Steve Bench using The Planet PCB and REALLY good PS and they sounded better than the other actives I've had. They are 12db sloops.

The chassis is very steam punk and I am looking forward to starting the build on that.
I am glad to hear your report about the tube crossover, please let us know how yours turns out.
Your chassis sounds like it should be front and center on your rack.
For years I made my living with a TIG torch in one hand and 316L wire in the other. One of my fellows does really nice welded art frogs, turtles and flowers out of pipe.
Your welded and painted creations with humps keep amplifiers fun looking.
DT
All Just For Fun!