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Strange filament behavour

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Ralph

Ralph

Member
Joined 2002
Some strange thinks occur with my 300B filament supply. This is the schema: http://www.homepages.hetnet.nl/~rjonkers/psu.gif (one from the bottom).

1) Startposition: If I adjust R11 to deliver exactly 5.0vDC the circuit draws 1.41A. The 300B filament is grounded via the '-' of the DC supply and a 10R resistor (fixed bias circuit: http://www.homepages.hetnet.nl/~rjonkers/amp.gif ).

2) If I connect the 300B filament to ground with two 100R series resistors, the circuit only delivers 4.7V so I have to re-adjust R11 to get it back to 5V. Voltage drop over R12 is 1.5A, so for some strange reason the 300B's start drawing more current. How is this possible?

3) Back to startposition. If I don't connect the B+ of the 300B's right away in order to heat them up first, the circuit only delivers some 4.6V and never gets to 5.0. If I connect the B+ the normal 5.0V is delivered. Now the main issue, if I use a LM350 as a current source and set it to 1.41A why doesn't it deliver 5.0V without the B+?

Ralph
 
Ralph

Ralph

Member
Joined 2002
Thought there were some sonic benefits so I use a current source for both the 5687 and the 300B. Are BOTH those tubes not 'specified' for that?

Don't know about floating, I think they are. 5687 supply is only grouned via pin 8 of the 5687 and the 300B supply only via the '-' pin of the 300B's supply. But please remember that you are 1000 steps ahead of me ;) so I may not understand you correctly.
 
Ralph

Ralph

Member
Joined 2002
John,

I'll say there's a problem. They won't work! The filament in a DHT, since it is the cathode, cannot be isolated. It must be referenced to B-, C+, and possibly chassis "ground" if it's the same as the other two.
Click to expand...

I don't understand this. At this moment I use (just measured it, all relative to ground):

5687: +6.17v and -6.39v
300B: +6.34v and +1.31v

now I want to elevate both circuits approx 40V. Is it OK to try?

Ralph
 
dhaen

dhaen

Ex-Moderator
Joined 2002
keystrobe.co.uk/
Ralph,

In the case of the KR300, it is already elevated by the cathode bias.

On the 5687, you can if you want, but I see no reason to.
In the unlikely event of emission from the heater to the cathode, you'd only have to raise the heaters a few volts to stop it.

I really think your time and effort would be better spent in perfecting the layout and earthing scheme of this magnificent amplifier.

Cheers,

_____________________________

Frank,

"Messing" implies making unnecessary changes to an already working system;)

Strange, your understanding of English nuances is usually better than mine....

Cheers,
 
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