Tremoverb current draw numbers...

[lespaul6]

Hi guys, I'm new to the forum. Does anyone know the the correct bias for an EL-34 and 6L6 in a dual rec? I realize I'm not able to adjust the bias and I'm considering the bias mod. My question is, after the mod, how should I set the current draw per tube......30-35ma?
Thanks for the help.

 

[AdmiralB]

Around 40mA for 6L6GCs and around 33mA for EL34s. But those are baselines, you could go as high as around 52mA and 43mA, respectively. Your tubes' life is proportional to how hot you run them, though.

The lower numbers are 60% of rated maximum, and the higher set is 80%. I go for the lower end in amps that get most of their tone in the preamp, which is the case with 99% of Mesas. And if you want to go colder (lower numbers), certainly you won't hurt anything.

This, by the way, is assuming your plate voltage is around 460V. My TOV is.

 

[AdmiralB]

40 per side would be really cold, since that number is for two tubes. You'd want something like 70-80 per side on a DSL/TSL 100.

Mesa does indeed set theirs really really cold. It prolongs tube life, but it doesn't necessarily improve headroom - in fact, they set it so cold it probably reduces headroom, as you get crossover distortion.

IMO they do this so they can be fairly sloppy with respect to tube screening - set it sufficiently cold and you don't have to reject too many tubes for running too hot.

The three Mesas I have right now - a Heartbreaker, a 295 power amp, and a DR - all run under 20mA per, with Mesa-branded tubes. I run the Heartbreaker in 'tube rectifier mode' with a SS plug-in module, because the tube recto mode on those changes the bias. It runs a little over 30 this way.

On the 295, I got rid of the Mesa tubes in favor of Sylvania 6L6GCs and Siemens EL34s, picked for higher idle (about 40mA for the 6L6s, about 35 for the EL34s), but it is also substantially higher-voltage than the modern stuff.

 

[Boogiebabies]

Knowing Mesa's tube supplier, the tubes are already pre-screened for natural plate current and transconductance. Mesa then tests them again and color codes them. They know exactly what they are doing. Considering the warranty they give and the amount of tube newbies that buy the product I think it would be safe to say that a cold tube will suffice.
If you want to get the bias up, run a gray or white 6L6, but be careful with EL-34's. They react differently when you flip the bias switch. Personally, running a Mesa at 75% MPD is just too muddy for me. Interesting enough, a brand new SLO 100 comes biased at 17ma with Sovtek 6L6's.
I just really enjoy not having to pull the chassis and pop in some tubes.

 

[Core9]

Interesting enough, a brand new SLO 100 comes biased at 17ma with Sovtek 6L6's.
I just really enjoy not having to pull the chassis and pop in some tubes.

Yeah, but take those Sov 5881's to 36-38ma per tube in the SLO with 470-480 plate volts and it's a beautiful thing.

 

[AdmiralB]

Marshall uses 1-ohm resistors, so mV=mA. That's how most bias probe devices work too.

Factory setting is 35 per tube. On a 50W, that's 35 per side, on a 100 it's 70 per side.

 

[Core9]

When in doubt, look at the current draw of the tubes individually.

 

[lyman]

so is the colder bias approach true for the el84 Mesas as well? i've never had the draw measured on my amp (maverick) but i'm probably due for some new power tubes before too long and want to find some that are appropriately rated.

 

[AdmiralB]

I think the cathode-bias Mesas are set up for much more "mainstream" numbers. Cathode bias is a lot more forgiving, too.

The Mav has a 75-ohm cathode resistor; the typical AC30 uses 50 ohms (REALLY hot) and the early AC30s used 82 ohms, so I think the Mavs (and Blue Angels) are fine as-is.

 

[Boogiebabies]

Interesting enough, a brand new SLO 100 comes biased at 17ma with Sovtek 6L6's.
I just really enjoy not having to pull the chassis and pop in some tubes.

Yeah, but take those Sov 5881's to 36-38ma per tube in the SLO with 470-480 plate volts and it's a beautiful thing.

True, but I much prefer the GT KT-66HP biased as a 25 watt tube. At 30 watts it starts to glow orange. On the flip side, I was biasing the SLO one hot day and a hair from my are hit full voltage. It was like Batman. Pow. Bam, OOOMMMMPPP !!!! A big blue flash and my fillings were stinging.
Even after biasing amps for years strange things can happen. For newbs, I really recommend leaving the bias circuit alone. If they must, take it to a tech for a retube. Don't forget, a cold yellow or red 6L6 may draw 17ma, but in the EL-34 bias setting it may draw 35-40.

 

[Core9]

Interesting enough, a brand new SLO 100 comes biased at 17ma with Sovtek 6L6's.
I just really enjoy not having to pull the chassis and pop in some tubes.

Yeah, but take those Sov 5881's to 36-38ma per tube in the SLO with 470-480 plate volts and it's a beautiful thing.

True, but I much prefer the GT KT-66HP biased as a 25 watt tube. At 30 watts it starts to glow orange. On the flip side, I was biasing the SLO one hot day and a hair from my are hit full voltage. It was like Batman. Pow. Bam, OOOMMMMPPP !!!! A big blue flash and my fillings were stinging.
Even after biasing amps for years strange things can happen. For newbs, I really recommend leaving the bias circuit alone. If they must, take it to a tech for a retube. Don't forget, a cold yellow or red 6L6 may draw 17ma, but in the EL-34 bias setting it may draw 35-40.

OUCH!!! Yeah, you gotz to be carefull.

I've got a set of matched KT88's I've been thinking about swapping into my SLO.

 

[lyman]

I think the cathode-bias Mesas are set up for much more "mainstream" numbers. Cathode bias is a lot more forgiving, too.

The Mav has a 75-ohm cathode resistor; the typical AC30 uses 50 ohms (REALLY hot) and the early AC30s used 82 ohms, so I think the Mavs (and Blue Angels) are fine as-is.

thank you for the info.

 

[lespaul6]

Thanks for the replies guys, very helpful.

I didnt realize you could run a cold set of 6l6'd in the EL34 mode to bring up the current draw!!!! :idea:

 

[siggy14]

Most people that use mesa's leave it fixed bias. As you crank the amp the bias gets better, not to mention you can order tubes from Eurotubes, or the tube store or almost any place that sells tubes, tell them what amp you have and ask for a tube that will run hotter in it.

ok so do most of the professional Mesa amp users leave it in fixed bias or do the mod to make it adjustable (cathode biased is what that's called I think)??

In terms of flexibility in the power section, this is where Diezel destroys the competition...

 

[siggy14]

Mesa has been shipping there amps latley with a hotter bias then they use too. I guess they got to many complaints or started reading forums more.

Mesa does indeed set theirs really really cold. It prolongs tube life, but it doesn't necessarily improve headroom - in fact, they set it so cold it probably reduces headroom, as you get crossover distortion.

IMO they do this so they can be fairly sloppy with respect to tube screening - set it sufficiently cold and you don't have to reject too many tubes for running too hot.

The three Mesas I have right now - a Heartbreaker, a 295 power amp, and a DR - all run under 20mA per, with Mesa-branded tubes. I run the Heartbreaker in 'tube rectifier mode' with a SS plug-in module, because the tube recto mode on those changes the bias. It runs a little over 30 this way.

On the 295, I got rid of the Mesa tubes in favor of Sylvania 6L6GCs and Siemens EL34s, picked for higher idle (about 40mA for the 6L6s, about 35 for the EL34s), but it is also substantially higher-voltage than the modern stuff.

 

[siggy14]

No, for some reason mesa went the opposite way with the EL84's, they are at the right bias or damn close to it.

so is the colder bias approach true for the el84 Mesas as well? i've never had the draw measured on my amp (maverick) but i'm probably due for some new power tubes before too long and want to find some that are appropriately rated.