herbert plate voltage

G

geetar_geek79

Member
hello guys. i was biasing my herbert yesterday loaded with 4x6550 and 2x6L6. I was trying to calculate the bias on my own and came to measure the plate voltage - meter set to Vdc, black to ground, red to pin 3. It reads 453Vdc on all of output the tubes. I did a search here and found that the herbert has a plate voltage of 480Vdc.

I was using 35W dissipation for the 6550 and 30W dissipation for the 6L6 and then biased to 60%.

using my reading as 453Vdc plate voltage, the bias should be at 93mA/pair 6550, 79mA/pair 6L6.

using the 480Vdc plate voltage as discussed here, the bias should be 88mA/pair 6550, 80mA/pair 6L6.

Which one do I follow? Did I do something wrong in measuring the plate voltage?

Thanks!
 
The Herbert has between 477-480 Plate volts. As your tubes draw current, the plate voltage goes down, which is normal.
I tend not to go over 90mA on 6550's regardless of plate voltage.
 
A quad of 6550's with a high natural plate current can suck some volt's too at 93mA.
My AC line runs 117V in the Summer and around 123V in the Winter. The state song
in the Summer is Hum of the Air Conditioner's.

Don't forget that Herbert needs to have each pair balanced. When you bias one pair of 6550's,
the bias set on the other may go up or down. The same is true for the 6L6's. You have to slowly
balance them all out to your target. Adjust, check, adjust, check, adjust and keep checking all three duets
until they are at your calculated MPD.
 
MARK2C":3qv6qy3k said:
The Herbert has between 477-480 Plate volts. As your tubes draw current, the plate voltage goes down, which is normal.
I tend not to go over 90mA on 6550's regardless of plate voltage.
Click to expand...

I agree. Bias at 90 mA per pair, and leave it at that.
 
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