Biasing a Fender 5E5 Clone

[ked]

Just got done rebuilding a 5E5 clone that I had sitting around. I'm using Tung Sol 5881 RI tubes in the amp. According to the tube specs, it is a 23 watt tube. This amp is cathode biased using a 250 ohm, 10 watt resistor. According to my calculations, the tubes are running at around 17 watts, which is about 75% dissipation. Aren't cathode biased amp usually set to run around 100% dissipation at idle? Would it be a bad idea to reduce the resistor to try to get closer to 100% dissipation?

Ken

[Prattacaster]

In my experience cathode bias amps sound best and operate great close to 100%. Try a 220 or 200ohm resistor.

[ked]

Thanks, I'll see what I can round up in the shop.


Ken

[Patrick620]

Bump. Just wondering if you swapped out that resistor and the results.

[ked]

Using what resistors I had on hand, I put a 200 ohm resistor in the amp and got the plate dissipation up to around 20 watts (87% plate dissipation). I'm using Tung Sol 5881 RI tubes that are rated at 23 watts max. The amp sounds better and no red plating. May try to go a little higher when I get some resistors.

Ken

[Patrick620]

Thanks, man. I am thinking about doing the same thing. Were there any changes in how early the amp breaks up? More or less headroom?

[ked]

The amp does break up earlier and compresses more. I'm running single coils and that is what I was looking for.

Ken

[Patrick620]

Ked, did you keep the cathode bypass cap value the same at 25uf 50v when you changed the value of the resistor?

[Ron]

Just got done rebuilding a 5E5 clone that I had sitting around. I'm using Tung Sol 5881 RI tubes in the amp. According to the tube specs, it is a 23 watt tube. This amp is cathode biased using a 250 ohm, 10 watt resistor. According to my calculations, the tubes are running at around 17 watts, which is about 75% dissipation. Aren't cathode biased amp usually set to run around 100% dissipation at idle? Would it be a bad idea to reduce the resistor to try to get closer to 100% dissipation?

Ken
Depends on how long you want to play your amp with the same pair of output tubes.

It's a bad idea if you expect them to have a reasonably long service life, if you don't push air across the glass to cool the tubes. Plate dissipation ratings are done under specific circumstances - the tube is in open air of a specific temperature. In an amp, both the chassis and the air around the tubes get hot, which reduces the actual tube plate dissipation capability.

Cathode bias them so they are at about 70% of rated dissipation with no signal. Or, run air across the output tubes and the chassis at the base of the tubes.

Increasing the cathode bypass capacitance tends to increase the amplification of the lower frequencies.

BTW, the calculated plate dissipation is (idle plate current/tube) x (plate-to-cathode voltage). Some people don't subtract the cathode to ground voltage and overestimate the plate dissipation.

[Patrick620]

Depends on how long you want to play your amp with the same pair of output tubes.

Dont mean to jump in out of turn. Im at about 87% right now. Maybe I will just leave well enough alone.

[ked]

Ked, did you keep the cathode bypass cap value the same at 25uf 50v when you changed the value of the resistor?

I didn't change the bypass cap.


Thanks for the information Ron.

Ken