DHT OTL Linestage - Tram 2

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Hi,

Hi Thorsten
i have tried it 2 times with my heater---where the sound died---first a little under 2,4 dc and then 2,6dc-----first with 2.7 it got stable in sound---is it the regulator that goes to zero with that low volt ?

Just to be clear, there was 2.4V across the heater (at the tube socket), the heater where glowing, the other voltages (anode, grid, bias across 12 Ohm to ground) where all OK but you lost sound completely?

I will try to replicate this.

Ciao T
 
Hi,

the 2.4 vdc was messured without any tubes---so is the 2.7vdc---it is the first thing to do before installing the tubes

The 2.7V setting should be in the manual.

There is a 0.1 Ohm resistor on the Filament supply, with a 45 it will drop 0.15V, with a 2A3 0.25V. There is probably more than 0.1 Ohm in total losses in the wiring to the socket as well.

So yes, at 2.4V without tube you are likely below being able to operate the Pre.

Best is to measure the heater voltage directly at the tube pins (this way even the socket resistance gets excluded) and set this to precisely 2.%V with your actual tubes in place. The setting without tube is meant to get you to the ballpark, so it is safe to plug the tube in.

Ciao T
 
Hi,

thanks for this advice, i will set my heater to 2.96v

Please do not do that.

Take the knobs from the Tram and place it on a piece of foam, face down. It will stand quite safely that way, resting on the transformer and face.

Then you have access to the underside with tubes fitted and you can adjust precisely.

If not, stick to the 2.7V, I worked these backwards with several different sets of tubes (both 45's and 2A3's) for the best compromise.

Ciao T
 
Hi,

i have now reset my heater and the anode volt---
one channel was 122 vdc---the other was 100vdc

This was probably okay, normal tube variation can account for this and it is okay.

not so easy to ajust precice on 120 vdc

I think manual states +/-5V or even more, it is not neccesarry to be a absolutely precise.

maybe a better 10turns variable resistor will be easyer to ajust

It has more to do with the time constant in the bias system.

I personally set the voltage once, very slowly and then monitor over the next ten minutes or so and adjust the voltage in small steps as it drifts.

Ciao T
 
Hi,

my variable resitor is not easy to turn ,without to much variation--even a tiny turn give to much---there for i think a 10 turns will be a little more easy

The resistor on the CCS is an 18 Turn one and as long as you try to get 120V +/-5V the action is quite okay. If you try to get 120.00V exactly, then yes, it may be too sensitive, but such accuracy is not really needed.

Ciao T
 
I finally nailed down the problem. Firstly to apologise to everyone for causing confusion regarding the heater voltage. The voltage was not a problem. I caused a short between the heater supply GND and the box (mains) GND by using wave washers under the PCB screws. They were making an intermittent contact with the PCB ground plane, through the silk printing, causing all those bizarre things that I explained earlier. As soon as I removed them everything went back to normal. I set the heater voltage to exactly 2.5V (measured at the valve socket). The bias was easily set to around 120V and remained stable. An increased level of buzz disappeared. However, I did use Loctite on those screws due to thermal cycling which will help to keep them in place.

My God that amp sounds good. Thanks for all the help.

PS. Another Tram is on it way to be assembled - I am sure I will have far less problems with it considering the lessons learned from the first one.
 
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Hi Thorsten,

With the TRAM2 in my system I noticed that the left channel has a high freq noise occurring from time to time which sounds like some sort of oscillation. Each time the noise continue for 5~10 seconds and then stop, and then come back at a random time later. The level is very low (~20mV at the spk terminals but nonetheless audible).

Using a scope connected to the speaker terminals I can see the high freq noise has freq from tens of Khz to over 100Khz. To isolate the noise I have so far tried the followings:

1. To rule out interference from nearby power supplies I have switched off the TV, STB, PS3. No effect.
2. To rule out the source I disconnected the only source (CD player) from the TRAM2. No effect either.
3. To rule out the power amp I swapped the left and right amps (mono block). The noise follows the left channel output so the amp is not the cause.
4. To rule out interference from the TRAM2's second output connected to the subwoofers I disconnected them. No effect.
5. Tried different tubes for the output tube, rectifier, and 6AS7. No effect either.

The high-freq whining noise occurs regardless of whether the source has signal, and is independent of the vol control level (even at zero).

Any suggestions are most appreciated. Thanks.

Jack
 
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