A bit of EL84 tube talk

Re: A bit of EL84 tube talk

Lazarus1140 said:
How can one (me be one) tell if an amp is cathode biased?
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On fixed bias, the cathodes are generally connected straight to ground, and bias voltage is fed to the screens along with the input signal. With cathode bias, the cathodes are generally elevated from ground by a resistor and capacitor in parallel. Changing the values there can adjust how "hot" the power tubes are biased.
 
Re: A bit of EL84 tube talk

trevorus said:
On fixed bias, the cathodes are generally connected straight to ground, and bias voltage is fed to the screens along with the input signal. With cathode bias, the cathodes are generally elevated from ground by a resistor and capacitor in parallel. Changing the values there can adjust how "hot" the power tubes are biased.
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I appreciate the excellent technical info, but it's not quite what I was looking for. I am in research mode for a new amp and based upon my limited experience with EL84s I'm not a big fan. Now that I've read that they have a rather short life in a cathode biased amp that is another strike against them .... at least in that particular design.

If it is even possible, how can I determine from looking at manufacturers' amp specifications if a certain amp is a cathode biased design?

I am primarily looking at Dr. Z, Matchless, and Category 5 amps ........ except that for the price I haven't ruled out the Genz-Benz Black Pearl. Not so much the Cat 5s because I prefer single channel amps.
 
Re: A bit of EL84 tube talk

Looking at the tube types and the wattage ratings, you can generally tell. A quad of EL84's in an AC30 is roughly 30 watts (more like 22 clean) which is cathode biased. Same quad of EL84's in my Carvin Nomad I used to own was nearer to 50 watts, fixed bias. Most amps with a single output tube go cathode biased, often because it's simpler. Pairs/quads of 6L6's and EL34's are going to be fixed most often. There are exceptions of course, but most commercially available amps are going to fixed bias with those sorts of tubes.
 
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Re: A bit of EL84 tube talk

trevorus said:
Looking at the tube types and the wattage ratings, you can generally tell. A quad of EL84's in an AC30 is roughly 30 watts (more like 22 clean) which is cathode biased. Same quad of EL84's in my Carvin Nomad I used to own was nearer to 50 watts, fixed bias. Most amps with a single output tube go cathode biased, often because it's simpler. Pairs/quads of 6L6's and EL34's are going to be fixed most often. There are exceptions of course, but most commercially available amps are going to fixed bias with those sorts of tubes.
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What about 6V6s?

Not knowing any better I would think that four EL84s at 30W would outlast four at 50W. Obviously there's more to it than just the biasing?
 
Re: A bit of EL84 tube talk

In general, the amp manufacturer will tell you if the amp is fixed-bias or cathode biased. If I remember right, you were looking at the Remedy from Dr. Z. Based on my very unscientific microscopic inspection of this gut shot, I would guess that they're cathode biased. I don't see a bias pot anywhere, so I don't think it's fixed bias.
 
Re: A bit of EL84 tube talk

Lazarus1140 said:
What about 6V6s?

Not knowing any better I would think that four EL84s at 30W would outlast four at 50W. Obviously there's more to it than just the biasing?
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6V6's are in the same "family" as 6L6's kind of... Just depends on the amp. The Fender champ is a single 6V6, cathode biased, whereas a Deluxe Reverb is 2 6V6's fixed bias. Like I said, depends on the amp. The other thing is, the Class in which the amp operates plays a big role as well, not Just the type of bias system.
 
Re: A bit of EL84 tube talk

Lazarus1140 said:
I am primarily looking at Dr. Z, Matchless, and Category 5 amps ........ except that for the price I haven't ruled out the Genz-Benz Black Pearl. Not so much the Cat 5s because I prefer single channel amps.
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Any one of those amps is really nice! I've had a Carmen Ghia for about 5-6 years. I tweaked it a bit to get a little more of what I wanted as a base tone. I love that amp! Loud for 18 watts, very nice response (feel) in the amp, great tone ........ I don't play crazy loud or every day, but I've only had three sets of EL-84's in that time and all of those changes were preventative/fear/extra cash swaps, not because it ate tubes. I could probably be on the second set still. The Ghia is very efficient that way. Also, Dr. Z is probably the least expensive of the 'boutiques' if that is a concern.
As for changes, I changed the V30 to an Eminence Red Fang Alnico and I swapped the V1 tube for a NOS Mullard 12AT7. Fantastic cleans, nice natural O.D. cranked, and with a good pedal, smoke and fire!
 
Re: A bit of EL84 tube talk

Very interesting. I've been playing most of my life, and am just now in the last five years really trying to learn more about amps/tubes/tone, etc.
I picked up my first EL84 amp a little over a year ago...a Mesa TA/15. Within three months I has an EL84 go bad. They were good about sending me a new matched set under warranty. I now run a small fan behind the head whenever it's on for extended periods. I don't know if this will have any benefits or not. Myles?
I also have a little six watt ValveTrain that's very basic...volume; tone; reverb. I run it wide open always, no problems at all so far. It runs a single 6V6.
 
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