HX/DA 50 Watt Bias Adjustments

[WeFixFlats]

Using these tubes(unmatched), after a good 15 minutes of warmup:

https://www.tubedepot.com/products/mullard-el34-new-production-power-vacuum-tube

 

[László]

Sweet! How do they sound?

 

[Deleted member 5962]

Yeah, how do they sound? I've considered a pair of those to try in my Custom 50

 

[WeFixFlats]

Since I'm on a budget these are the only make and model of tubes I've put in the amp since I've had it. So I can't compare how they sound to other brands, boutique or otherwise. I can only say that the amp sounds like it's in 'excellent health': no noise and loud as hell when cranked thru the Big Mouth.

Note: These were bought as 'non-matching' even though it was zero(0) cost. And no '24 hour burn-in' either at 1.50 a tube, to save 3 bucks. It's just dumb luck that both averaged within ~1mV of each other. How these wear over power-up/playing time over the next weeks and months should be the real world test.

 

[vchizzle]

I've always liked the new production Mullard El-34's. It's been awhile since I've compared to others but I ran them in my Bad Cat amps for awhile.

 

[Kred]

I have the new production Mullard EL34 (also bought from Tube Depot) in my homemade JTM45++ modded for 2 years. They are still going strong. I have them biased at 35ma, which is about 65% of maximum dissipation with my 456 volt plates.

 

[C&M Music - Lafayette]

Question??? How does one trim pot bias adjust two different tubes? Secondly, I was always told that a bias of 30 was on the cool side, a safe bet, and done by manufacturing for ease of warranty issues more so than sound preferences?. Biasing to 45 or even 55 is supposed to give you breakup a little earlier and of course ends your tube life a little earlier. In rig run downs for Stevie Ray and Eddie Van Halen they were biased at 55 because they liked the breakup point better. I am not saying I know what I am talking about, because I do not. I am asking questions because this is what I consistently heard. Lastly, In my HXDA, can I swap out the EL34s for 6L6's without changing the sockets they go in? Do tell? Thank you

 

[C&M Music - Lafayette]

Bump. Hoping to get an answer for my post above.

 

[Dusty Chalk]

It can't really bias two tubes differently -- but it will change the bias of both of them, as it's just a voltage.

It should say right on the back of the amp which tubes you can put in it. If not, then I wouldn't swap them without checking with customer service.

I wouldn't run it anywhere near 45 or 55 -- there are other things at play, mostly the transformer, which you don't want to overheat -- it could warp and then change its value. I wouldn't go much higher than 35, and if you do, get one of those infrared thermometers and monitor the temperature of the iron. Maybe if you're in a particularly cool, well ventilated environment, it can handle it for short periods of time, but if you break it, I'm pretty sure that's out of warranty.

 

[C&M Music - Lafayette]

Thank you Dusty.

 

[WeFixFlats]

Looking thru the PRS amp catalog, the only amps that have the option of either 6L6 OR EL34 Power Tubes are the Archon 100 or 50 that I could find.

Putting 6L6 tubes in a HX/DA would defeat the entire concept, design and character of the amp: two 'english' amps in one box.

 

[Boogie]

Question??? How does one trim pot bias adjust two different tubes?

To expand on Dusty's explanation, it's a balancing act. You're looking for the adjustment that brings the reading for both tubes as close together as possible. You should be able to get two matched tubes within a few tenths of a millivolt. The target value recommended by PRS (and that's the sound I bought the amp for) is 30mV.

 

[C&M Music - Lafayette]

Thank you guys. I appreciate the answers. Yes, I bought the amp for the same sound each of you purchased it for. I needed answers to get my head around the opinions I heard elsewhere. The bias trim pot was the big one though. Did not understand that. Now I do. Thank you!

 

[Deleted member 5962]

having not played with this yet, I think the reason guys ask about it is because it's widely accepted that biasing a little higher would give warmer, more power tube breakup, tone at the expense of some tube life. This done in moderation could be an upgrade in tone, which makes it a viable tradeoff. If there is not tonal improvement, then stay at 30. Without trying this though, and since it appears nobody here (that I have read) has tried it much, I'm not sure yet if there IS a tonal improvement. Be interesting to find out.

Also note, if you never crank the amps up loud loud, biasing it a little warmer will give you better tone at low to mid volumes, at maybe no expense at all in tube life. With many amps, it's commonly accepted that you can bias for "cranking" or bias for "home." Now, home doesn't mean "bedroom volumes." Home in this instance is generally half or less on the volume knob. Given that I never turn my Custom 50 up to half (for long anyway), and that this would be loud enough for any gig anyway, many would suggest me to bias warmer anyway for better tone a the volumes the amps is used it.

This is all amp dependent, and general info. May not be "universal" to all amps. YMMV. Consult your physician for high volume playing that lasts over 4 hours.

 

[Boogie]

... it's widely accepted that biasing a little higher would give warmer, more power tube breakup, tone at the expense of some tube life...I never turn my Custom 50 up to half (for long anyway), and that this would be loud enough for any gig anyway... May not be "universal" to all amps.

I was going to comment that not all amps are created equal. You covered that in your legal disclaimer. I run my SuperD's master at 2:00 for 90% of my applications. If I tried that with my MkIII, I'd be banned from playing in this state. Those are two different designs that leverage their power stage in two completely different ways. I'm not saying that you couldn't experiment with biasing, just that the results will vary with the amp, the application, and the tubes.

 

[HANGAR18]

Question? can I swap out the EL34s for 6L6's without changing the sockets they go in?

Yes, 6L6's and EL34's use the same sockets.

 

[Kred]

You need to know the plate voltage to calculate what 30 or 35 means. Here is an example:

35 ma x 450 volts = 15.75 watts. The EL34 tube is rated for 25 watts maximum dissipation, so 15.75/25 is 63%.

But if the plate voltage were 350 volts, then 35 ma x 350 volts = 12.25 watts. This would be only 49%.

Most tube manufacturers recommend 60-80% for long tube life. Amp manufacturers try different bias to find what sounds best to them, then recommend a bias setting. Whether the output transformer can take a hotter bias depends on how much design margin the amp manufacturer specified.

Amps designed with different negative feedback may recommend different bias settings. More negative feedback is a cleaner sound, and needs a hotter bias for the same amount of breakup. I can select the amount of negative feedback on my homemade EL34 amp, it makes a big difference.

 

[Kred]

Related info...

"Presence" controls change the amount of "high frequency" negative feedback.
"Resonance" controls change the amount of "low frequency" negative feedback.

They are essentially tone controls in the negative feedback loop of the power amp.