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Old 5th May 2011, 05:19 PM   #121
Desmo is offline Desmo  Denmark
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Join Date: Jan 2007
Location: Aarhus
Hi Thorsten,

Thanks for the answers..!

What can be wrong regarding the resistors on the AC side of the DHT supplies? And/or the DHT supply modules? Again I should note that I'm using 2a3 tubes. I have tried 3 different set's of 2a3 but the problem (when the resistors are installed) is the same with all of the tubes, so I gave up on that.

So to get the C16 issue clear (Please see the picture below): So C16 should be installed with opposite polarity according to the silk screening, so the NEGATIVE is facing the transistor on the right side of the cap? And the 7 other electrolytic caps on the board should be installed according to the silk screening?

Click the image to open in full size.

Last edited by Desmo; 5th May 2011 at 05:22 PM.
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Old 5th May 2011, 06:01 PM   #122
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Hi,

Quote:
Originally Posted by Desmo View Post
What can be wrong regarding the resistors on the AC side of the DHT supplies?
0.22 Ohm should drop around 0.8 - 1V AC. This may or may not be too much in your context. However, given that you write you can only get 2.2V with 0.22R in the AC Lines I think something is out.

Quote:
Originally Posted by Desmo View Post
So to get the C16 issue clear (Please see the picture below): So C16 should be installed with opposite polarity according to the silk screening, so the NEGATIVE is facing the transistor on the right side of the cap? And the 7 other electrolytic caps on the board should be installed according to the silk screening?
Yes.

Ciao T
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Old 5th May 2011, 06:19 PM   #123
Desmo is offline Desmo  Denmark
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I should have been more precise Thorsten... I understand, that you think something must be out, and my question should be: What do you think can be out..!?

It's been a while since I worked with the resistors, so I don't remember the voltage drop over the resistors or the AC input voltage to the DHT supply modules. But as mentioned when using 2a3 tubes I could not turn the DC voltage higher than around 2,2V when the 2 x 0R22 were installed on each modules AC input. The situation was exactly the same for both channels...
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Old 5th May 2011, 06:22 PM   #124
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Hi,

Quote:
Originally Posted by Desmo View Post
I could not turn the DC voltage higher than around 2,2V when the 2 x 0R22 were installed on each modules AC input. The situation was exactly the same for both channels...
2 pcs of 0.22R, so 0.44R on each module? That will loose 2V AC and about 3V DC before the Regulator. Of course that is too much resistance...

Ciao T
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Old 5th May 2011, 06:32 PM   #125
Desmo is offline Desmo  Denmark
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Quote:
Originally Posted by ThorstenL View Post
Hi,



2 pcs of 0.22R, so 0.44R on each module? That will loose 2V AC and about 3V DC before the Regulator. Of course that is too much resistance...

Ciao T
Communication can be hard... Let's take it from the beginning...

This scematic (see below) and a total of four 0R47 resistors was delivered together with my Tram 2. The description is to install these if the heat sinks get very hot. I installed these four resistors as illustrated (2 resistors for each DHT supply module on the AC input side). The modules could not regulate anywhere near 2,5V DC on the output.

I then bought four 0R22 resistors and installed 2 resistors on the AC input on each DHT supply module. The output could still not be regulated high enough. In my experience the mentioned 1 - 1,5V over the modules is not nearly enough for the modules to work...

Actually I also tried installing just one resistor for each module. Then I could regulate the DC output to 2,5V DC, but there was an unacceptable level of hum through the speakers so I had to remove the resistors...

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Old 5th May 2011, 06:48 PM   #126
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Hi,

Quote:
Originally Posted by Desmo View Post
Communication can be hard... Let's take it from the beginning...
Not really. The way you installed the resistors they essentially appear in series. So they will drop a LOT of voltage at 2.5A DC out.

Try a single 0.22R Resistor, that alone will drop out over 1V DC at the reservoir capacitor and would reduce the dissipation in the regulation element by the better part of 2.5 - 3W.

So adding resistors works, one just need to get the approximate resistance right.

Quote:
Originally Posted by Desmo View Post
Actually I also tried installing just one resistor for each module. Then I could regulate the DC output to 2,5V DC, but there was an unacceptable level of hum through the speakers so I had to remove the resistors...
Single 0.22 or single 0.47?

Ciao T
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Old 5th May 2011, 06:55 PM   #127
Desmo is offline Desmo  Denmark
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Well, I installed the resistors according to the instructions supplied to me by the manufacturer...

With a single 0R47 resistors, the modules could not regulate the output high enough (makes sence since this is basically the same scenario as the two 0R22 on each module).

With a single 0R22 resistor the modules could regulate the DC to 2,5V but there was hum.
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Old 5th May 2011, 07:00 PM   #128
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Hi,

Quote:
Originally Posted by Desmo View Post
With a single 0R47 resistors, the modules could not regulate the output high enough (makes sence since this is basically the same scenario as the two 0R22 on each module).

With a single 0R22 resistor the modules could regulate the DC to 2,5V but there was hum.
Then 0.15 Ohm should be just right, try it.

Ciao T

PS, the resistors and instruction come with all Filament supplies, I don't think they are supposed to be included with the Tram Kit...
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Old 6th May 2011, 03:14 AM   #129
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I should add my experience as well. I did the following mods:

- Bonded Zelman ZM-NB47J heat sink to the existing ones with some fin hacking. I used Arctic silver epoxy bond for this.

- Drilled 6x5mm holes in the line of the hetasink PCB cutouts right through the sub-chassis and the outer box to improve the airflow

- Installed 0.33R resistors in the heater AC line

- Used a step-down transformer 240 to 220

With all of this installed the AC heater line input is ~4.8VAC. Without the step down trafo the voltage is slightly above 5VAC. So this should be spot-on as per design.

Now with 2A3 valves the outcome was that regulators could not work at all. Under load and with 4.8VAC at the input there was barely 1V at the output.

With 45 valves situation was much better. With the step-down trafo the output voltage exhibited a slight sag being controllable up to ~2.45VDC. Without the trafo and 5VAC at the input the heater regulation was spot on and stable. The heat-sink temperature did not cross 56degC even after an hour or so of combined idling and normal operation.

This was all happening with LM1084 in operation - the replacement ones - the original BM1084 died earlier. So far I have only one thing I can think of - a sub-standard and/or fake and/or faulty regulator chips that I got. I ordered a new batch so I will report more after I replace the current ones.

However there is also a possibility, a small one, that these heater regulators a just too close call. A borderline design where statistics plays a huge role which in my mind is not an acceptable design. Most of the voltage regulators have very poor control over low voltages, especially if this is combined with the minimal voltage drop and high current load. I am simply not convinced that this amp can reliably work with 2A3 valves as it is.

Also a word of wisdom - it is nice to have thick PCBs for stiffness but they are PITA for re-work and de-soldering.

PS. I apologise for two accounts - happened long time ago and now I am stuck with them
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Last edited by DECKY999; 6th May 2011 at 03:18 AM.
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Old 6th May 2011, 03:48 AM   #130
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Hi,

Quote:
Originally Posted by DECKY999 View Post
Installed 0.33R resistors in the heater AC line
Try using two 0.33 Ohm in parallel in only one lead of the AC line to the module and no stepdown transformer.

We regularly run the Tram II with 230V AC, no resistors or special measures and 2A3's fitted and experience no problems, both with units build for customers and with our own long serving unit. And all that at over 30 degrees ambient.

If the Mains voltage is much above 230V then a small value series resistor may be needed to control the excess heat.

Due to the way power supplies (any powersupplies) work, with 2.5A DC drawn from output of the filament supply, the AC Draw from the transformer is around 3.6A RMS (and a quite spikey waveform at that).

So a 0.165 Ohm resistor (two 0.33R in parallel) will drop 0.6V AC, which become around 1V reduction in DC at the regulator input.

So an 0.165 Ohm Resistor will cut dissipation in the regulator by around 2.5W!

Using much higher values will limit regulation and may lead through hum breakthrough.

The only way to make a supply for 2.5V/2.5A that works over a really wide range of voltages is a switched mode supply or one with humungous heatsinks, if there is no adjusting to the local mains voltage.

Ciao T
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