Electro Harmonix 6CA7 EH What should I set the bias?

Hey Guys, At what sould I set the bias on the 6CA7EH (new) quad of outpur tubes? I have the VTA-70 "low gain board" AND when rolling the driver tubes, do I need to re-bias the output tubes each time I swap a 12AU7 etc...?

Thanks, Ron

Bigron865 wrote:Hey Guys, At what sould I set the bias on the 6CA7EH (new) quad of outpur tubes? I have the VTA-70 "low gain board" AND when rolling the driver tubes, do I need to re-bias the output tubes each time I swap a 12AU7 etc...?

Thanks, Ron

6CA7 is very similar to el-34 , 40mA ( which is 0.4V in the testpoint(s))

No you don't have to reset bias when changing pre or driver tubes. But it never hurts
to check them anyway, especially with new 6CA7 tubes.

Thanks! Just what I needed. Does anyone have opinions on this amp/tubes using Advent Large speakers with the "fried egg" tweeter? Power hungry or a good match?

One of the tube websites says these tubes can handle .50 but I'd tend to trust you guys especially when it comes to eighty bucks for new tubes. Why, why, why? I know that gets tiring for you tube/dynaco experts but I appreciate your patience and willingness to teach/educate all of us freshmen. Thanks!

Hi Bigron865 -- "Advents with Fried Egg Tweeter"

I am currently driving the Advents with a ST70 in a 10 by 14 room with a 10 foot ceiling and they sound fine. Lots of punch, be it jazz or classical or something vocal.

Also driving another pair of the Fried Egg Advents with a Fisher 500C Receiver in amother similar sized room and it also sounds fine.

The amplifier sections in the ST70 and 500C are about 30 or so Watts per Channel.

I am not sure what it would sound like in a 20 by 20 sized room or something bigger.

Have fun,
Stew

Thanks Stew! Nice to make your acquaintance! I am in a small apartment but I'm currently using a pair of Klipsch RB61 speakers that are really efficient but I'm looking for more slam and wider soundstage. I just got a pair of tha Advents at a junk sale but haven't tried them with the St 70 yet. I'm chasing a sound that is alluding me. Perfect pitch and median income sucks!!!! But I'll give them a try. Thanks for responding. Ttyl!

Wow this is cool. I was just wanting to change out my tubes on my dynaclone to 6ca7's or kt77's and I did not know what to set the bias to. Heres the thing my amp from Triode has two ef86 (winged c) and an 12au7 (JJ) in the preamp section and curently JJ el34's for power and my instruction manual says to set the bias at 1.0 vdc. Thats a huge difference from the abovd posts. Any ideas?

AMCjeep wrote:my instruction manual says to set the bias at 1.0 vdc. Thats a huge difference from the abovd posts. Any ideas?

The Triode Electronics kit uses a common 10ohm cathode resistor for two output tubes. 1-volt across 10ohms gives you 100ma for 2 tubes so 50ma per tube. It's a little high. 40-45ma per 6CA7 won't change the sound but it will lengthen your tube's life.

EH 6CA7 are very nice sounding tubes. Love them.

So what would do you think is a better sounding tube KT77's or 6CA7's? I keep going back and forth on which to get, any opinions? Or is there even something better to consider like a better EL34 or different tube altogether for a ST70?

AMCjeep wrote:So what would do you think is a better sounding tube KT77's or 6CA7's? I keep going back and forth on which to get, any opinions? Or is there even something better to consider like a better EL34 or different tube altogether for a ST70?

What's great about tube amps is tube rolling. Meaning, you can try different types and brands. That doesn't mean you can put any tube, you have to be aware the amp capabilities. So, the only way you're going to know what tube you'd like, is to buy and try them out. ST70 can use KT66 and 6l6 too .

hawaii.ken wrote:

AMCjeep wrote:my instruction manual says to set the bias at 1.0 vdc. Thats a huge difference from the abovd posts. Any ideas?

The Triode Electronics kit uses a common 10ohm cathode resistor for two output tubes. 1-volt across 10ohms gives you 100ma for 2 tubes so 50ma per tube. It's a little high. 40-45ma per 6CA7 won't change the sound but it will lengthen your tube's life.

There are other factors in selecting a bias point, but the tube MUST be operating under its dissipation limits. Dissipation is measured in watts, and it relates to how much heat the internal elements can take. The formula is:

Dissipation (watts) = (Plate voltage - cathode voltage) x cathode current. Keep in mind that the cathode current is the plate and screen currents combined...

If you have a tube passing 50 ma (.05 amps) at idle and for purposes of illustration you have 450 volts across the tube, then the dissipation will be .05 x 450, or 22.5 watts. The 6CA7 EH has a plate dissipation limit of 25 watts, and a screen limit of 8 watts, there is no combined dissipation listed on the EH datasheet. So I like to use the plate dissipation as the maximum. 22.5 watts is about 90% of the plate limit, and that's about my limit for audio applications.

There is more to all this, but for tube replacement this should help. And as was posted elsewhere - BE SURE to dial the bias current down before installing the new tubes!! The bias setting for the old, tired tubes will likely be WAY off for the new tubes - and can cause damage. And remember - whenever you change output or rectifier tubes you must set the bias!!

Finally, I agree - 6CA7 EH tubes are outstanding - and a real bargain!!

AMCjeep wrote:So what would do you think is a better sounding tube KT77's or 6CA7's? I keep going back and forth on which to get, any opinions? Or is there even something better to consider like a better EL34 or different tube altogether for a ST70?

I believe the KT-77 Genalex (not JJ) is SLIGHTLY better than the 6CA7 EH - but it is very hard to say which is the "best" tube. Not only is it dependent on what it's used in - but at some point the decision of "better" is based on personal preferences, and those are different from person to person.

From the reports I get from a wide variety of customers, the KT-77 Genalex appears to be the tube of choice in a large majority of circumstances, with the 6CA7 EH a very close 2nd. FWIW, that's my personal opinion as well.