DHT OTL Linestage - Tram 2

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.... Im getting Morten's Russian rectifiers:

5c3s_57_1.jpg


,,,,Morten did you get the older 1950's/1960's black plate type as reviewed here?
Eventually Peter and I might be able to compare the U52 vs the Russian.

A data sheet for the U52 states its a "half wave rectifier" while the 5U4G is a "full wave". I wonder what that means and if it would make any difference to the sound?
 
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Hugh and Morten, the Russian 5U4 rectifiers pictured, sound very average compared to the GEC U52 tube. The U52 tube was extremely quiet and extremely musical. I did not believe it until I heard it myself. There must be other musical rectifier tubes. If anyone has tried the U52 and other 5U4 equivalent tubes, please speak up. The GZ37 and GZ33 tubes are interesting as promising alternates.

Peter
 
Hugh and Morten, the Russian 5U4 rectifiers pictured, sound very average compared to the GEC U52 tube. The U52 tube was extremely quiet and extremely musical. I did not believe it until I heard it myself. There must be other musical rectifier tubes. If anyone has tried the U52 and other 5U4 equivalent tubes, please speak up. The GZ37 and GZ33 tubes are interesting as promising alternates.

Peter

I wonder how the U52 will sound with the Coleman Regulators and 2A3 tubes?
 
The GEC labelled U52 brown base, black plate, 'D' Getter with manufacturers 'KB/Z' rectifiers has a fast dynamic sound which Steve Garland who has heard my Tram2 said this tube would be a very good combination in my system. The Osram labelled U52 with brown base, black plate, 'D' Getter without the 'KB/Z' labeled have a lush, full bodied and slower sound. It all depends on your taste of how you like your music to sound. I will ask him on his pick of non U52 tubes that would suit the tram2 pre-amp. More later.

Peter
 
Has anyone had experience with the A1834 (6AS7G or CV2523) GEC or similar tube. They come in straight or curved brown base. See attached pictures for details.

Peter
 

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I made a small tweak recently...

I don't know about your Tram II's but mine is a pain when it comes to adjusting the 120V anode voltage on the CCS modules. I get it set to around 120V dc, all is good, but then after some weeks the anode voltage start drifting. This is heard as noise when the voltage drops below a certain point. Last time I opened it to re-calibrate the anode voltage it was down at around 90V dc in the left channel. (I should mention, than when adjusting the anode voltage I always let it stabilize for at least half an hour so I'm sure it's stable).

The problem is, that just a change of a few ohms on the trimmer will mean a big change in anode voltage, and it seems that these trimmers can drift a bit over time...

I have now installed resistors instead of the trimmers. In one channel I have an 82 ohm resistor, in the other channel I have 82 ohm // 2k2. The anode voltage appears to be rock solid (at least for now) and there is ZERO noise through the speakers..!!!! With the anode voltage being totally stable at the correct 120 - 125V and the Rod Coleman DHT regulators, the Tram II is now one of the quietest tube preamps I have experienced... Impressive for a DHT running 2A3 tubes :cool:

The downside is that it will of course be more work when changing tubes, because fixed resistors needs to be replaced, but since I'm done tube-rolling on the triodes, this is not a problem for me...
 
The trimmer is a good quality multiturn trimmer, so the ones in the preamp are as good as they get...

The problem is, that the whole resistor is in the trimmer, so even a microscopic change means a big voltage change on the anode. What can be done is a small tweak of the circuit where (for instance) a 75 ohm reistor is used in series with a 10 ohm multiturn trimmer. With this set-up just a little of the resistor is in the trimmer. It would be possible to adjust, the adjustment would be much easier because each turn on the trimmer is much less change in resistance. Any drift would also be much less drift in resistance meaning that the anode voltage is less likely to drop out of ''the zone''.

This little mod means replacing the existing multiturn trimmers with similar ones, just 10 ohm and then making a small mod, cutting a track on the small circuit board to get the 75 ohm reistor in series.

For now I'm happy with the fixed resistor solution, maybe some day I will make the modification mentioned here.
 
...This little mod means replacing the existing multiturn trimmers with similar ones, just 10 ohm and then making a small mod, cutting a track on the small circuit board to get the 75 ohm reistor in series...

That's brilliant.

This might be a silly question but do you think a 10ohn trimmer will have enough range to change from 45's to 2A3's OK?

Is it possible to mount the trimmer's so the adjustment can be done from the outside without needing to unplug the Tram2?

Cheers
 
Don't know if it's enough range for 45's, but since I don't use them I don't need range for them... The point is, that the smaller part of the total resistance value is in the trimmer, the easier the adjustment will be, and the less it's likely to drift. So if someone needs bigger range then just use slightly bigger trimmer and slightly smaller fixed resitor in series...

If you install multiturn trimmers facing up, drill some small holes in the top of the chassis, and install a voltmeter measuring the anode voltage then you can adjust without opening the chassis... But what is the point? The small mod is to aviod drift and to avoid having to make adjustments...

If you wan't it for easy tube rolling you can of course install this.
 
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