DHT OTL Linestage - Tram 2
What do DHT (direct heated triodes) REALLY sound like? Yes we've heard VT25, 45, 2A3, 205d, 300B etc in many an amplifier, enshrouded by driver tubes and output transformers. And we can plainly hear the differences between DHT types in that context, tainted though the sound must be with so many extraneous elements.
The only way to really hear the sound though is to go spud. And what better place to do that than right at the beginning - the linestage. Many will say that the linestage contributes more than the amplifier, so...
UX4 sockets: this would allow 10/10Y/VT25, 26, 45, 71A, 2A3, 801A/VT62, 300B, 101D, 205D if we can design in enough control to supply the variety of filament, and B+ voltages needed. Used spud we'd have a friendly gain structure of between 9 and 18db. Most have lowish anode impedance; no transformer needed as in the range ideally suited for preamp duty. So...OTL it is!
With adjustable CCS for the anode we can control the anode current. Easy done.
Since this is an all-out attempt to hear the real DHT, and all of it, we don't want un-necessary caps in the signal path. So let's go fixed bias, thus avoiding the cathode bypass cap. To make this easy we have our ABS module feeding the grid which can be set with one resistor for the desired operating point.
Power supply: to stay with the theme lets use a DH rectifier tube.
We don't want ANY electrolytic caps blowing smog into our sonic landscape so we will let the all film-cap FCUPS module provide the P:USH power we need. This has the added advantage of supplying the bias voltage needed by the ABS and bonus: jumper setting for virtual battery or virtual choke operation. And it is super quiet.
The filament supply also needs to be quiet, be a brick wall to the signal (DHT, remember?) and be adjustable: DIYHFS filament supply just happens to be on our shelf.
Transformer noise will be an issue so let's order a potted (3mm steel) torroid which has the advantage of radiating EMF axially thus avoiding noise in the chassis. Our goal is to do this all on one chassis. We'll mount tx forward, to keep it away from the signal end of the chassis.
We'll need one pair of 2.2uf/250v coupling caps for output and 1 pair 0.1uf on the input grid.
Chassis will be the Projekt Box, also at hand.
Layout will look something like the attached.
Here we go!
how did the project go?
DHT OTL Linestage - Tram 2 - Update
I'm getting quite a few emails asking for updates on the Tram2:
We're pretty well there. The current design will grow one more tube, a 6AS7. This will allow us to expand the range of DHT that can be used without major circuit changes. As seen here we can change 2A3 and 45 on the fly.
Microphonics was a big problem early on but this has been solved using floppy tube socket mounting and selectable output gain, 12, 3 and -6db gain, as well as a few other tricks. I think most tubes can be used. The first build could only use a pair of NOS 45's we have. With all others it was the Gong Show. All the new manufacture 2a3/45's have much larger glass bodies than NOS and support the filament with spring tension and so ring like Big Ben at noon. The Shuguang 2A3 were the worst - looks like pen springs used. But now we are able to use every tube we've tried - about 8 different types including Shuguang, TJ, and NOS types.
Also selectable ouput gain means linestage gain can be matched to speaker efficiency.
You can see we've made good use of the Technology Bites Series of modules we sell.
Noise was solved using a very low noise filament supply implementation.
We've used the ABS (auto bias system) to safely exploit the benefits of fixed bias. This allows getting rid of the cathode capacitor and using a single low value, high quality resistor on the cathode. At the linestage level, these design touches are very noticeable, more so than down the signal chain.
The FCUPS (Film Cap Universal Power Supply) supplies the DHT B+ which also uses a CCS for the anode load. This might seem like overkill for a linestage but if we want a big sound we need big tubes and big power supply. The FCUPS does everything the original Tram regulated power supply did -- and more. The DIYHFS theme song is: No Electrolytics! so we use film caps as much as possible.
The for sale version of the Tram2 will have an option for the UVC (Universal Remote Control) with 126 steps and 6 inputs (if we can find enough space on the back panel).
So what are we waiting for?
Transformers. Soon as we get them in stock we will do the build, shoot the pics for the manual and list them. Price to be fixed later when all costs are in.
Oh yeah, the sound: This is No Sacrifice Sound. The last 7 years I've gone back and forth between TVC and active; each has its merits. But the Tram2 does it all. Huge dynamics with all the inner detail and transparency of TVC.
This is The One!
Wham! Bam! it's the Tram 2 - DHT OTL Linestage
The guided tour:
Back: 2 tubes are the Direct Heated Triodes - 2A3 or 45 plug and playable without any circuit changes.
Middle: left tube is the Super Mu 6AS7 and right is the 5U4 rectifier
The tube sockets are mounted in a subplate which is compliantly mounted to the chassis. Rapping on the chassis produces no audible feedback.
Front: the power transformer. From our 98db speakers we can hear just a little tube rush from the 2A3. No hum.
Front panel: Remote controlled selector then power switch left, 120 step volume control on the right. Selector has 5 inputs (also additional inputs for REC on back panel)
Back panel: 6 prs RCA inputs and 3 prs outputs, +6, 0, -6db (this can be changed internally) for system matching which is absolutely necessary to marry DHT and high efficiency speakers.
Inside the chassis: Will post pics next report but power supply is FCUPS and all film cap (big L small c). Ultra Low noise Filament Supplies on the 2A3/45. Auto bias Supply controls the DHT. Etc Etc.
On the bench we get extremely linear and low distortion test results.
OK more later.
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!
On the Bench
Frequency response: better than ±0.5dB 20Hz - 20KHz
S/N Ratio: > 93dBA
THD & N: < 0.2%
All measurements referenced to +18dBu (6V), +10dBu (2.45V) and +2dBu (1V) for the respective outputs
All measurements taken with Sovtek 2A3 and vary with tubes employed
Hi I have been reading your updates and am thinking about building this. I use bastanis open baffles and run the sub woofer off my current pre out - would this be possible with this amp i.e. can I take a parallel output.
The Tram 2 can comfortably drive loads down to 2kohm
Have you tried driving the subamp from the speaker out of the amp through a dropping resistor into the low level inputs of the subamp> When we had the Prometheus we found it gave the best sound. Also that way the subs see exactly the same signal as the main speakers, and impedance is no longer an issue.
I have not tried driving the subamps that way - i will think aout it. My preamp (home made) has 2 outputs, one essentially an anode and one a cathode follower - this seems to drive the speakers well.
thanks for the reply.
Grand Slam for the Tram2
Enjoythemusic has just posted the world's first review of the Tram2.
This was a very well conducted review. The reviewer understood exactly the point of a DHT linestage its care and feeding. Please note the 'Fettling Guide'
Have a read here
We've finally sourced a compact remote control which we will program and make available as an option.
Some notes about tubes: NOS sound rich but difficult to get a pair quiet enough for linestage use. With the Herbie guitar dampers, Tj and other modern production 2A3/45 can be used. Note re EML that tube pins 2/3 are oversized compared to NOS spec and will damage teflon tube socket pins grippers. We have both EML 2a3 and 45 here and that is our experience.
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