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The concept couldn't be more simple: Use direct heated triodes for linestage duty. Put some signal on the grid, heat the cathode, add B+ to the anode and voila! What could be easier? Works for DHT amplifiers, and parts count is low.

But put a DHT into a linestage where signal levels are in the mv range and we crash into noise, microphonics, noise, possible impedance issues and more noise.

Noise can come from power transformer, filament supply, B+, poor grounding, bad layout, not to mention proximity to outside sources of noise.

The Tram 2 starts with good layout. We wanted a single chassis solution together with power transformer. So transformer at the front, triodes at the back close to input/output connections. Tube rectifier and 6AS7 closest to the transformer.
B+ is supplied from the ultra low noise film cap power supply module and the cathode supply comes from our filament supply module with added capacitance.

Microphonics must be the middle name of the triode family. We already encounter this in amplifier position, but in a linestage -- talk to your triodes and maybe your speaker talks to you! All slight to jarring level vibrations will mess up the clarity of the sound. Everything around us resonates and thus vibrates. The Tram 2 mounts the triodes on the 3mm tube subplate which is in turn mounted to the chassis with compliant isomount standoffs. Knuckle rap on the chassis doesn't transfer so we've done well.
The other factor is system gain. High sensitivity amplifiers and speakers will feedback through the tubes and exacerbate microphonics. So we have provisioned 3 outputs: +6db, -3db and -12db. The user needs to select in system but in general: amplifiers with input sensitivity of 2 to 6v would match +6db outputs; 0.7v - 2v amps: -3db outputs; below 0.7v amps: -12db outputs.

Of course this impacts on impedance so it requires finesse. The Tram 2 can comfortably handle amplifier loads down to 2k ohms.

Result: through our in-house Crescendo speaker system, 98db/w (100db/2.83v/1m in room) we get no hum, just a little tube rush.

We'd originally planned to use a choke to load the anode of the triode but that would push us to a 2 chassis linestage and potentially more noise pickup. So we instead used a 6AS7 together with CCS to load the anode. Result is very low output impedance and very high anode load. The effect is as a super mu follower.

Why DHT OTL linestage?
DHT sound better. On the test bench we get very low noise and very low distortion. What little distortion is there, is of the single ended type which many feel to be consonant with the sound of music.
The Tram 2 as presented can use either 45 or 2a3 types. With some tweeking to the circuit 26, 71, 27 and 56 (non-DHT) can also be used.
One thing to note: as with a formula class racing car, with a DHT linestage we are going for ultimate performance and so give up a few niceties. Care needs to be taken to place the Tram 2 away from noise sources from other gear such as big torroids. Tube selection is important. NOS and ST types generally have less microphonics. We've tried with NOS 45/2A3, TJ meshplate 45/2a3 and carbon/meshplate plate globe types and Valve Art 2A3 single and double plate. Overall the ST types were less noisy. The big globes tend to be more microphonic; ditto for some of the new production types with coil spring tensioners for the grid wire.

What you get is very low distortion, linear output with all the tonality offered by direct heated triodes as close to the source as possible.
DIY - Done Right!



Edits: 04/11/10

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