DHT OTL Linestage - Tram 2

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Hi Graham,

Thanks Clive. I have a pair of Y-cables I can use in the short term.

If the Amplifiers have significantly different sensitivities and at least one set has level controls you can also use the different gain outputs (e.g. an SE Amp with low sensitivity might be best off on the +6dB Gain output, while a Solid State Amp with very high sensitivity may be best of on the -12dB output).

You can mix'n'match that way.

Ciao T
 
Hi,

Glad you added that Thorsten! I was going to say something similar (I deleted what I wrote) but I wasn't totally sure whether amps connected at different attenuation points would be ok. It is, so thanks for the confirmation.

Well, there are limits of course (no 600 Ohm or even 200 Ohm input impedance Pro Studio Amp's please), but anything above 10K input impedance should no issue...

Ciao T
 
I've just checked - the Trends TA10.1 I'll use in the beginning has an input impedance of 50k when used as a power amplifier. The use of the Trends is a case of 'use what's available', but it should be more than adequate for now.

Not sure what impedance my 2A3 monos (with ECC81/12AT7WA) will be (I could work it out) but I suspect it will need to be something quite awkward and unusual to present any problems to a Tram.
 
Just a note to Tram users: You can clip this line stage with high-output sources. If you run into this problem I might suggest some type of high-quality inline attenuators (Rothwell). For example, my Herron phono stage with 66 dB of gain is pretty high-level with my .5 mV Ortofon cart and causes the preamp to clip on transients - only on records recorded at a rather high level.

I solved this issue with the Rothwells.
 
Paul,

Just a note to Tram users: You can clip this line stage with high-output sources. If you run into this problem I might suggest some type of high-quality inline attenuators (Rothwell).

There is an easier way. You can internally replace the wirelink from the RCA Socket to the input board by a high quality 47K or 100K resistor (raises the overload margin by 3dB and 6dB respectively) or if you have a unit build by diyhifisupply you can ask to have this included (generic resistors free, Takman or Tantalum at extra cost).

I really need to chase Simon to update the website with these options.

Ciao T
 
Hey Thorsten, why does the preamp seem to have such harsh clipping behavior? It's really nasty when clipped. I mean, this is in practice a non-issue because few sources will clip it and you can easily get around this; I'm just curious why it sounds so 'nasty' when it does clip. I thought my cartridge was busted at first...
 
Paul,

Hey Thorsten, why does the preamp seem to have such harsh clipping behavior? It's really nasty when clipped. I mean, this is in practice a non-issue because few sources will clip it and you can easily get around this; I'm just curious why it sounds so 'nasty' when it does clip. I thought my cartridge was busted at first...

This is the price to be paid for using the electronic volume control.

It is VERY GOOD (I would say better than most affordable stepped attenuators, you need a ToKo Stepper or a TVC to do significantly better) but it means if the signal becomes greater than the Volume Controls supply rail nasty stuff happens, not just clipping, but protection circuitry kicks in as well...

We run it as standard without series resistors in the Tram, as it sounds best that way. That said, maybe I'mm make an attenuated input with a Takman 100K standard for Phono, though we only had this issue two time so far, once with Clive and now with you.

Ciao T
 
Paul,



This is the price to be paid for using the electronic volume control.

It is VERY GOOD (I would say better than most affordable stepped attenuators, you need a ToKo Stepper or a TVC to do significantly better) but it means if the signal becomes greater than the Volume Controls supply rail nasty stuff happens, not just clipping, but protection circuitry kicks in as well...

We run it as standard without series resistors in the Tram, as it sounds best that way. That said, maybe I'mm make an attenuated input with a Takman 100K standard for Phono, though we only had this issue two time so far, once with Clive and now with you.

Ciao T

I agree that the ''standard'' configuration of the volume control is best sounding with the shunt resistor installed. But when upgrading the shunt resistors (I use Vishay naked Z foil) IMHO the shunt configuration sounds better.

And since there is no limit / clipping in shunt mode then this might also be an option for you Paul.
 
I’m new to the forum with the recent acquisition of a Tram. I’m having a problem maintaining heater power on the right side. Both sides play fine upon start up, and after a few minutes, the right channel fades out (no more orange glow).

For diagnostics, there is 5.9 VAC going into each filament supply, and when all the tubes are in, I get ~66V (2A3 ground and anode check points) at immediate turn on, but then, the right side drifts down to zero. The left side stays steady at 38v but can't make any adjustment that affects either anode bias voltage (in attempting to get to 120V balanced per instructions – can’t do it)

Prior to putting in the 2A3s. I did test the ground to grid voltage and instructions say it should read -65V. I got 2.73VDC there for both sides out. Mainly, why does the heater voltage drop to zero out on the right side and why can't I bias even close to the 120V recommended? Appreciate any help or direction in diagnosing and repairing this.

Thanks, Forch
 
Paul,



This is the price to be paid for using the electronic volume control.

It is VERY GOOD (I would say better than most affordable stepped attenuators, you need a ToKo Stepper or a TVC to do significantly better) but it means if the signal becomes greater than the Volume Controls supply rail nasty stuff happens, not just clipping, but protection circuitry kicks in as well...

We run it as standard without series resistors in the Tram, as it sounds best that way. That said, maybe I'mm make an attenuated input with a Takman 100K standard for Phono, though we only had this issue two time so far, once with Clive and now with you.

Ciao T

I agree it is a small price. I actually had the problem with another phono pre (Tom Evans Microgroove+), but again the gain was too high for the cart. Both setups are very high-gain.
 
I agree it is a small price. I actually had the problem with another phono pre (Tom Evans Microgroove+), but again the gain was too high for the cart. Both setups are very high-gain.

Why not give the shunt volume a try? It's so nice that the design is made, so both solutions can be applied. All it cost you to try shunt volume at it's best is to buy two 100 K resistors in the best possible quality (I use Vishay naked Z foil).

With shunt volume you have no clipping limits on the inputs, and personally I prefer the sound with the shunt with Vishays. With the standard resistors supplied I agree with Thorsten that the normal volume connection is better.
 
Need for AC heater input resistors?

Hello, TRAM MK2 Question: I measured the input to the filament heater supply at 5.8VAC and adjusted to 2.7VDC out (US 120v using 2A3 tubes). Is this too much of a voltage drop that could cause overheating or failure?

I have the supplied resistors to reduce the AC voltage, but would like to know if this "needs" to be done respective to the voltages I measured? If yes, based on your experience, what is the desired value (ohms) to insert?

Thank you. I am using the new/improved version of the filament supply.

Jeff
 
Hi,

Hello, TRAM MK2 Question: I measured the input to the filament heater supply at 5.8VAC and adjusted to 2.7VDC out (US 120v using 2A3 tubes). Is this too much of a voltage drop that could cause overheating or failure?

I have the supplied resistors to reduce the AC voltage, but would like to know if this "needs" to be done respective to the voltages I measured? If yes, based on your experience, what is the desired value (ohms) to insert?

Try two 0.33R resistors in parallel. If this drops too much voltage you will have loads of hum, in which case just bypass the resistors with a piece of wire...

Ciao T
 
ThorstenL, thank you for the recommendation. In the filament literature, it said I could reduce noise by reducing the voltage drop across the filament heater. I am experiencing some noise now (not 60hz hum, but higher order buzz) in each channel and was thinking that by inserting the resistors to knock down the incoming AC may also reduce noise I am experiencing. You mention hum could be higher with the resistors. The hum you describe - is this "true" 60hz hum - I don't hear that specifically through the speakers - just a higher frequency buzz. I'm using the -12db outputs but running up against a gain issue.

Best Regards, Jeff
 
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