Tube Amps / Music Electronics
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|11/30/1999 4:03 AM|
|Mike||Deluxe Reverb Cathode Bias?|
Is it safe to cathode bias a Deluxe Reverb? There is 410 volts on the plates of the 6V6's.
|11/30/1999 5:27 PM|
It will be somewhat less than 410V after you convert to cathode bias
It would work as long as you were sure to not exceed the maximum plate dissipation rating of the output tubes. The primary impedance of the Deluxe output transformer is a little low for a pair of cathode-biased 6V6GTs, but this might not make a big difference in actual fact. Maybe just a little more distortion.
For any cathode-biased arrangement of more than a couple of watts of output, I recommend a good almuminum-finned, chassis-mounted wirewound cathode resistor and a 105 degree C cathode bias cap rated for 200V or more. Keep the cap away from the resistor, to avoid heating it.
|12/3/1999 1:17 AM|
Gave it a shot tonight. I used 280ohms & 47uF bypass cap, but the bias (using the shunt method)is reading 46ma on both sides of the transformer! The voltage across the 280ohm resistor is around 29V. The plate voltage is now 405V. Have I done something wrong? Or do I just need a larger resistor?
|12/3/1999 1:45 AM|
That's way hot for a 6V6, IMO. Yes, try a larger value resistor. Maybe 390 ohms??
|12/3/1999 11:55 PM|
|Mike||Re: Deluxe Reverb Cathode Bias? (long)|
I did 2 things. First I used Randy Aiken's Zener sub to drop the B+ a bit. Second, I increased the cathode resistor to 645ohms. These changes gave 420V on the plates and 24.9mA bias with the shunt method. I'm concerned because every schematic I've seen with 6V6's cathode biased has a 250ohm cathode resistor. I should mention I left the 47uF cap across the 280ohm resistor and put a 365ohm in series with the network. I figure it's still a little hot @ 10.46W, not to metion 420Vs on the plates. Have I done something wrong? Or is the 420V on the plates the reason for the 645ohm resistor?
|12/4/1999 2:13 PM|
On a cathode biased 5e3 deluxe with a 250 ohm cathode resistor the screen supply is dropped 45v or so by a 5k resistor. that drop also helps reduce the plate current (10% or so).
Hey smart people, isn't 10.46W OK for cathode bias?
I remember reading that cathode bias could run up to
90% of rated power. right?
By the way i have a 5e3 clone with 395 on the pltes
that currently uses a 250 ohm and it's running
too hot. I will have to change it to 330 or so.
Maybe my transformer puts out a higher b+ than those
|12/4/1999 3:31 PM|
Mike, first of all, fixed bias will give you more head room, but if you want to cathode bias, John was correct, it will take anywhere from a 330-390 ohm resistor to bias those tubes. I personally would bias those tubes to about 30-32 ma (12-14 watts) which will also pull down your B+ little more. High voltage on those 6V6 is what you need to look out for. Try to keep them below 420 volts and you should be ok.
Bob @ DynaQamps
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