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Old 25th March 2013, 09:09 PM   #21
Ketje is offline Ketje  Belgium
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You can never get the Vfk within the max. 90V specs for the upper and lower triode at the same time.One cathode almost Gnd level and the other near 200V is to much.
Max.distance is 2x90V !
A solution could be separate tubes for up and down with the upper two (other channel) a heatersource of it's own,200V above Gnd.
Or another tube
Mona
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Old 25th March 2013, 09:22 PM   #22
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Quote:
Originally Posted by tinitus View Post
quote from Valve Wizard

Heater elevation: Because the upper cathode is at a high voltage it is usually necessary to elevate the heater supply to avoid exceeding the maximum rated heater-cathode voltage (Vhk). For the ECC81 this is 90V, so we would need to elevate the heater by at least 100V.

I have no idea what it means, but thats what it says
only found the figures to look different
or are you saying your measured heater-cathode voltage is 76V, and ok being below 90V
Using 100V to elevate the heater like the attached schematic, I measure:

Plate lower triode
Right 137,4V
Left 135,5V
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Old 25th March 2013, 09:29 PM   #23
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Replace the red LED by the 470R and 22 in one channel as shown in the first schematic for testing purposes. Check voltages again afterwards.

Rundmaus

EDIT: If everything else seems ok, the LED bias scheme is the only difference to the schematic with its design voltages. Change back the bias scheme to see if the problem occurs from that.

Last edited by Rundmaus; 25th March 2013 at 09:36 PM. Reason: clarification
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Old 25th March 2013, 09:29 PM   #24
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Quote:
Originally Posted by Ketje View Post
You can never get the Vfk within the max. 90V specs for the upper and lower triode at the same time.One cathode almost Gnd level and the other near 200V is to much.
Max.distance is 2x90V !
A solution could be separate tubes for up and down with the upper two (other channel) a heatersource of it's own,200V above Gnd.
Or another tube
Mona
With other tube the same measurement.
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Old 25th March 2013, 09:36 PM   #25
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Quote:
Originally Posted by Rundmaus View Post
Replace the red LED by the 470R and 22 in one channel as shown in the first schematic for testing purposes. Check voltages again afterwards.

Rundmaus
Lower plate 103.7V
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Old 25th March 2013, 09:40 PM   #26
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That is significantly closer to the design value than before. Though still different enough to make me wonder why.

You did not comment on the differences in the earlier measurements: Were the 175V a misreading?

Rundmaus
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Old 25th March 2013, 09:57 PM   #27
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First post 175V was a typo/mistake

Now heaters elevated to 100V, I guess also helps.

So have I to come back to cathode RC in place of LED?

TIA
Felipe

Last edited by merlin el mago; 25th March 2013 at 10:09 PM.
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Old 25th March 2013, 10:11 PM   #28
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New elevated heaters used schematic
Attached Images
File Type: jpg ECC81 mu-follower heater elevated.jpg (31.1 KB, 204 views)
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Old 25th March 2013, 10:18 PM   #29
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Quote:
Originally Posted by merlin el mago View Post
So have I to come back to cathode RC in place of LED?
Not sure about that. I'd have to pull my Morgan Jones copy from below a large stack of physics stuff and read up on mu follower design.

Though I am able to do some fault finding by logical and general circuit design and measurement principles, I have no experience in designing mu followers.

Maybe some of the tube forum masters of old can cut in here?

Rundmaus
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Old 25th March 2013, 10:24 PM   #30
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Lower plate both cathode RC
right channel 103.7V
left channel 102V

Thanks Rundmaus, how could I reduce to 85V?
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