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Old 8th January 2003, 03:55 PM   #1
Ralph is offline Ralph  Netherlands
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Default I really would like your help on my 300B project

Hi all,

During the last 2 months I've built my first poweramp. I finished it today. I only know the basics about tubes and a friend of mine drew the schema. I used PSU Designer for the PSU and enjoyed building and soldering very much.

Before building the parts into nice enclosures, I today plugged in the amp and the voltages etc were more or less as expected (nothing blew up)

This is my workbench with the amp, to give you an impression of what I built:
http://www.homepages.hetnet.nl/~rjonkers/parts.jpg

Schematics for the PSU and amp:
http://www.homepages.hetnet.nl/~rjonkers/amp.jpg
http://www.homepages.hetnet.nl/~rjonkers/psu.jpg

I started with the biassetting on -95V and messured the following:

- Anode 300B approx 500V
- Voltage drop over 300B cathode (10 ohm) approx 1,57V. This gives 157mA.
- 500V x 0,157mA = 78,5 Watt. This is way too high even for my KR300bXLS tubes.

I would really like to operate it at 450V @ 130mA (58,5 Watt) or 500V @ 115mA. But how to I get to these values? How do I lower the current?

I played a little with the fixed bias setting. I can change it from approx -80V to -110V. At -110V the current for the 300B was lower (I measured 1,32V = 132mA) but the anode voltage raised to 525V.

I also connected a preamp and cdplayer and the sound was very quiet with no bass and no treble whatsoever. There's little hum however :-)

Can someone point me in a good direction? Help would be very much appreciated!

Ralph
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Old 8th January 2003, 04:06 PM   #2
dhaen is offline dhaen  Europe
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Hi Ralph,

I'm sure between us we can help.

Before we start, I see you also got your transformers from Wil at Automatic Electric Europe :
http://www.ae-europe.nl/
They look beautiful, just like mine! I'm very pleased with the results.

The symptoms you are comlaining of, do not seem to me to be directly related to the over-running of the KR's.
However, what idle current did you ask Wil to "gap" the output and interstage transformers to?
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Old 8th January 2003, 04:24 PM   #3
Ralph is offline Ralph  Netherlands
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Hi John,

Yep, the iron parts are from AE-Europe . The powertrafo is specified as 500-0-500 200mA, the interstage as 6K:6K 30mA and the OPT is 2K5 140mA. The 5687 driver runs at approx 200V @ 14mA. Does this help? I'm a little afraid to turn it on because I don't like blowing up the 300B's the first day.

Ralph
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Old 8th January 2003, 04:48 PM   #4
dhaen is offline dhaen  Europe
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Ralph,

The main thing is to keep the current down. Perhaps re-bias for a value of 100mA, until you've got things sorted out.
Slight overvoltage won't hurt. It's the power dissapation that needs to be controlled.

Alternatively, you could disconnect C1 2uF temporarily. This will reduce the voltage, but the 5H choke might buzz. (It won't hurt it).

Whatever happens, don't overrun the valves for any amount of time.
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Old 8th January 2003, 04:54 PM   #5
Ralph is offline Ralph  Netherlands
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John,

I like to lower the current before trying to reduce the anode voltage, I guess.

>> Perhaps re-bias for a value of 100mA

Can you please instruct me how to? My biasrange is now limited from -80 to -110V. How do I calculate the correct value?
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Old 8th January 2003, 05:11 PM   #6
dhaen is offline dhaen  Europe
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Ralph,

Ah, I see that you are limited to that as it's almost the full bias supply voltage.
You can get some more negative volts by (temporarily) bypassing R3 with a lower value resistor 500R ~ 2K if you have one. No harm should occur, as long as you turn the bias pot R5 slowly There will be a little bit of hum injected into the grid, but it'll be OK for tests.
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Old 8th January 2003, 05:21 PM   #7
dhaen is offline dhaen  Europe
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Ralph,

I don't think I answered your last question properly.
You were asking how to calculate the bias value?
First check the data sheet to get a "ball park" value.
Use this initially, and check the current. It will be a little different than predicted.
Adjust the bias voltage to get the desired current.
Your current will be higher than expected if the +B is higher.
You will require more negative bias voltage to "tame" it.

If you cannot get enough bias volts...there are ways..
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Old 8th January 2003, 06:05 PM   #8
dhaen is offline dhaen  Europe
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Ralf,

Can you do a screen capture of your PSU designer, or post the *.psu file?
The reason is, that I get different results than you.

Presumably you have grounded the transformer centre taps? They're not drawn on the diagram.

Cheers,
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Old 8th January 2003, 09:35 PM   #9
Ralph is offline Ralph  Netherlands
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John,

Thanks for helping me in this So I understand that to lower the current I have to 'insert' more negative bias voltage into the circuit. In order to do so, I need to lower R3 to somewhere in the 450 Ohm - 1700 Ohm range by paralleling a 500R - 2K resistor. I'll have to buy one tomorrow and will investigate and report.

My max biassetting is now approx -120V. This gives me 1,32V across the cathode resistor of 10 Ohms (132 mA). B+ raises to 525-530. So I guess I need maybe -130V bias or something.

All the centertaps are grounded yes. And my PSU Designer also gives other (lower) figures on the B+ voltage .... This is the PSU file http://www.homepages.hetnet.nl/~rjonkers/300B_500V.psu

Another question comes to mind, you say adding more bias introduces more hum and is a temporary solution, am I right? Is there something fundamentaly wrong with my scheme that may require other things to change for the definitive fix?

Ralph
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Old 8th January 2003, 10:21 PM   #10
dhaen is offline dhaen  Europe
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Ralph,

Firstly, let's get it safe to operate.
For now, buy the resistors to adjust the bias, so that the anode (plate) dissapation can be reduced. Then we can fault find.

When it's safe to run, I'd like you to measure the AC voltage on the secondary of the 500volt transformer.

Have a look at the graph on this page:http://www.audiolamp.com/page524.html from it, you can gauge the grid voltage required for an anode current at a particular anode voltage. As I said, each valve will differ a bit.

Your eventual aim must be to reduce the anode voltage below 500 volts. When you've achieved that, you may find that the bias circuit you have is adequate.
And yes, the probable hum is only temporary.

There is nothing fundamentaly wrong with the design.
You have been sensible to "bread-board" it frst. It's easier to tweak in this form.

Oh, and your *.psu file was the same as mine. My mistake.


Cheers,
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