DHT OTL Linestage - Tram 2

[ThorstenL]

Hi,

Originally Posted by DECKY999

No problems - the tubes are NOS National Union 45s from ebay. Can't really claim warranty on them but they are easy to replace. I will suggest Emission Labs 45 to the owner.

Shame to such a lovely tube die, but it was well past retirement age...

Ciao T

[forch]

T, Forch here, with some unexpected bad news. The left filament heater just crapped out. This was newly replaced and included the recommended dropping resistors (0.16ohm)). I thought all as OK as it played well for a month.

Is there anyway to bypass those things or take another approach that is more reliable, even if passive and simple?

thanks, Jeff

[ThorstenL]

Hi,

First, while I can try to help, this really needs to go through the support at diyhifisupply.

Originally Posted by forch

T, Forch here, with some unexpected bad news. The left filament heater just crapped out. This was newly replaced and included the recommended dropping resistors (0.16ohm)).

There is no such thing as "recommended dropping resistors of 0.16 Ohm".

What is recommended is the correct resistor value that will reduce excessive input voltage. You should measure with the tube in place and adjust for 5.5V AC at the input to the filament supply.

You may also want to check if the tubes you use draw excessive heater current. I recently came across a Tram that kept blowing heater supplies. Turns out it was used with "super 2A3's" that draw (contrary to their official specification) a lot more heater current than 2.5A. Such tubes will not work in the Tram II.

Originally Posted by forch

Is there anyway to bypass those things or take another approach that is more reliable, even if passive and simple?

Not really. To be honest, if the heater voltages are set correctly and the tubes comply with reasonable specifications and tolerances, there really is no problem.

Ciao T

[forch]

Thanks T. I'm just a little frustrated, that's all. It's a super pre and when I went back to my standby, there's just not the same transparency, dynamics and detail.

[forch]

T, received the filament supplies and have the Tram back up and running. Voltage to supplies is 5.8VAC and 2.7VDC out. Biased to 120V. Tried two manufactures of 2A3 (VA and Shanguang) and each set appeared to run similarily, so I think tubes are not pulling extra current - settings were rock solid on the supplies. I will monitor it more closely to make sure I am in recommended range. Any significance between my measured 5.8VAC and your 5.5VAC target - is it close enough? Thanks, Jeff

[ThorstenL]

Hi,

Originally Posted by forch

Any significance between my measured 5.8VAC and your 5.5VAC target - is it close enough? Thanks, Jeff

Should be close enough, though 5.3V would be even better. 5.8V AC creates 1W more heat than 5.5V and 5.3V creates less than that...

You can try taking one of the three 0.47 Ohm resistors in series with the AC line to the Filament supply out.also check if the Regulator chip has an isolation washer or not. If it has either Mica or silicone pads fitted remove them. In the tram this does not create any problems with high voltages on the heatsinks etc.

Ciao T

[forch]

T, I checked by using the supplied 0.47ohm, but I could not get the voltage regulated out to stablize. I got to 5.8VAC using two 0.33ohm resistors in parallel (equiv 0.16ohm). To drop it down to 5.3, what resistance should I target? (If you could give me a value, I can start there.) Thanks, Jeff

[ThorstenL]

Hi,

Originally Posted by forch

T, I checked by using the supplied 0.47ohm, but I could not get the voltage regulated out to stablize. I got to 5.8VAC using two 0.33ohm resistors in parallel (equiv 0.16ohm). To drop it down to 5.3, what resistance should I target? (If you could give me a value, I can start there.)

You should have had 3pcs 0.47, which in parallel give 0.16 Ohm (rounded).

In your case try 1pcs 0.33 and 1pcs 0.47.

Ciao T

[forch]

T, your resistor recommendation dropped the incoming AC to 5.3V, as you anticipated. Also noticed that the pre got quieter as well - always a good thing!

Thanks for your guidance. Jeff

[hkl]

Hello

Unfortunately, the right channel of my Tram2 has kicked the bucket again. I bought the TRAM2 as an assembled item from DIYHIFI about 6 months back. I had the same problem about 3 months ago with the same symptoms - i.e. the right channel 2A3 tube wasn't heating up and hence no music. Last time round DIYHIFI informed me the problem was with the heater supply overheating. I'm guessing it is the same problem this time as the symptoms are the same. I've read over the previous posts on this so I'm wondering if anyone else has come across this problem lately, and what solution you've employed to get around this? I think Decky has tried a different type of heatsink and drilled additional holes in the chassis to improve airflow. I'm not keen on installing additional fans or putting a laptop-style pad underneath to blow air into the chassis.. I'm looking for a more elegant solution that solves the problem neatly and thoroughly.

In any case I'll be taking the Tram2 back to DIYHIFI for them to look at. Fortunately I live in Hong Kong, but its still a hassle. The Tram2 is really good when its working so I'm going to miss the music very much!

Incidentally, I'm using VA 2A3 tubes with a 274 rectifier. Have also tried the 5U4 previously but this doesn't seem to make a difference, as I was using the 5U4 when the problem occurred the first time. AC mains voltage in HK is 230V and hence should be spot-on within specification of the pre-amp.

Cheers
HKL