DHT OTL Linestage - Tram 2

[ThorstenL]

Hi Desmo,

Originally Posted by Desmo

1). C16 on the autobias module blows up! I don't have a circuit diagram, so I have no idea what this 100uF, 25V elco is doing, but after a few days it blev up (metal housing was shot of the cap). I hoped it was just a bad cap, so I have replaced it but the new one is beginning to ''lift it's top'' as you can see in one of the attached close up pics. Can you help me with a solution to this..!?

This Cap sits only in the turn on delay circuit, it does not really do much. I think you can take it out quite safely.

Originally Posted by Desmo

2) HEAT... I run 2a3 tubes and the heat sinks on the DHT supply modules get so hot, that the supplies are getting unstable after +5 hour. Unstable = noise heard through the speakers and low frequency pulsing (woofers flapping) that indicate some sort of low frequency oscillation. I should mention, that I have the preamp located on an open shelf with lot's of ventilation around it. Also worth mentioning is that I have tried with both 0.47 ohm resistors and 0.22 ohm resistors on the input of the DHT heater supply modules, but the modules can't regulate the output to 2,5V dc with the resistors installed.

Please check that regulator chips are firmly screwed down to the Heatsink, it is possible that the supplier of these module had some problems there.

We have been testing the Tram with 2A3's and the same Modules during the middle of Hong Kong Summer (near 40 degrees ambient) and there where no issues. They run hot (unavoiably), but solid.

Do you have enough airflow from under the preamp?

Originally Posted by Desmo

BTW: Wonderful sounding preamp...! The best I have ever had in my system..! I have done a few upgrades:

Nice work. These Duelund Cap's look nice.

Ciao T

[Desmo]

Hi Thorsten,

Thank you so much for your answers.

Very strange with C16 on the autobias module. I will try to remove it and see if the module still operates OK...

I will check if the regulator chips are tight to the heat sinks / check if the surface is smooth/even for best heat transfer. If they can run hot, but solid in HK, then I agree with you, that it should also be possible here..!

Thanks, I like to do my DIY projects well, and I'm quite satisfied with the way this one turned out. The Duelund caps are indeed very good, please try them sometime when you get the chance.

Regarding ventilation I have taken some pictures of the set-up, and I don't see any way that I can improve ventilation. As you can see the Tram is on an open shelf, and I even lifted it to get more air under the bottom. It's hard to see from the pictures, but there is more than 20 cm up to the turntable shelf from the Tram...

Thanks,
Morten

[Desmo]

Originally Posted by ThorstenL

Some recent batches of TJ 45 pick up hum from the rectifier. TJ changed the tube structure over older versions and somehow these hum like crazy in the tram, especially the tube that is closer to the rectifier. You can shield the 45's, the Rectifier or not use TJ's from that batch.

Other 45's generally are quiet, as are 2A3's, however it can depend on the rectifier. Some rectifiers have a very strong magnetic or electrostatic field around them and then any tube in the Tram picks up that hum. For example the Valve Art 2A3 are dead quiet in the Tram.

I have found that the Valve Art 274A are the best rectifiers in terms of being quiet, followed by any coke bottle rectifier 5U4G. The short, stubby 5U4's etc and GZ34 produce hum. The Valve Art 274A may be acceptable even with the "bad batch" TJ 45's.

Ciao T

Thanks again Thorsten,

I have both the Valve Art 274A and some NOS Svetlana coke bottle shaped rectifiers. The Valve Arts were suggested by DIY hifi supply as the best when using TJ45 (to minimize hum), but Peter from Acoustic Dimension in Holland (where I bought my Tram) still thought that the level of hum was too much with the TJ45. From your description it seems that the TJ45 they tested with might have been from the bad batch..!

Your words on this for sure gives me confidence in trying 45 tubes also.

[JoshK]

I just ordered one. Can't wait to try it out!

[ktuuri]

JoshK have you got your Tram yet??
I have been saving up and my Piggy bank is almost full, I'm pertty sure I will be pulling the trigger soon and getting one as well.

Desmo have you sorted out the problems you were having?
Nice work, by the way! Beautiful looking Duelund Cap's in there, it looks like the value is 0.1uf/ 200v, is that correct?

I'm getting excited just thinking about the Trams in my system.

Is everyone still happy with the Tram, has there been any issues that have might come up.

Ken

[Desmo]

@ktuuri... I still have the issues, nothing new to report on that. However with the larger heat sinks on the DHT suppy modules the supplies are stable, also with 2a3 tubes. So if you should experience something similar this is an easy fix. I have also bought some small fans that I will try to install in the bottom of the Tram to help move the air (they will be connected so they are just running slowly/silent). I have just not had the time to work with it lately...

Yes, the Duelund CAST input caps are 0.1uF / 200 V

The sound of the Tram 2 is really something special. I have tried many preamps during the years, also some that are many times more expensive, but the Tram 2 is one of the best I have ever tried in my system. For the money it's a steal...!

BTW: I'm using Treasure 2a3 tubes now, and they sound very good in the Tram. I have also bought a pair of NOS RCA 2a3 black plates, but there is quite a lot of hum with these tubes, even when I use the Valve Art 274 rectifiers. Too bad because they sound good...

[JoshK]

Mine is to be shipped in the next few days.

[ktuuri]

Thanks for the reply Desmo + JoshK.


It sounds like it can run on the hot side for sure, I can have my system on for 10-12hrs sometimes.
Judging from the pics I have seen, I wonder if drilling a few extra holes in the chassis, (before building) both top and bottom might help. It looks like there is some room to do this.
I have access to a good drill press, maybe making a nice circle pattern with a few holes in the circle on top might help. As long as it can be done proper manner, it should help.

I have a few 6SA7G tubes and a few 5U4G + V/A274B as well.
Guess I would just need some 2A3 tubes and the Preamp ofcourse! :O)

look forward to JoshK getting his and posting his thoughts.

Desmo, that sure is a nice looking setup you have, nice pics you have posted!

[decky]

Just to add one question in the discussion - regarding the 2A3 heater regulated supplies. I just had one of the regulator chips die on a practically brand new Tram. I replaced it with a LM1084 which behaves a bit different then BM1084 that is used on the original board but does the job nonetheless. However after replacing them I was monitoring voltages and temperatures and recorded 104degC on the LM1084 heatsinks while the heater voltage was stable at ~2.5V and anode bias was set at 120V as per manual.


I would not call 104C at the heat-sink particularly healthy especially considering that I measured this with the chassis bottom open and amp upside down. In a normal operating position this is probably even higher. LM1084 has max junction temperature at 150C so I doubt that they can work under this load for extended period of time. Did anyone else had issues similar to this or there is something else wrong with my amp. The main valves used are TJ 2A3s.

[ThorstenL]

Hi,

Originally Posted by decky

Just to add one question in the discussion - regarding the 2A3 heater regulated supplies.

This should not be the case. The temperature is way too high. Which heater supplies (version number) are fitted on your Tram and what is your local mains voltage?

Ciao T