biasing 6v6 with bias rite question

[rpavich]

Hi,
I just recently purchased a Drz Z28.

It's got 2 6V6's from groove tubes and they're rated a 5 each.

I also purchased a bias rite so I could do my own biasing.

I checked the amp (which sounds great) before turning any pots and here is the result:

Tube A
Plate dissipation: 386v
Current draw: 40.8ma.

Tube B:
Plate dissipation: 386v.
Current draw: 38.5ma.

Dr. Z's web site says that the numbers should be

375v and 22ma to 28ma.

I was a little alarmed at the high current numbers so I adjusted it and got this:

Tube A
Plate dissipation: 394v
Current draw: 26.8ma.

Tube B:
Plate dissipation: 395v.
Current draw: 25.4ma.

It seems to be a balancing act between the numbers; PD goes up...draw goes down....is that right?

I have a 5AR4 rect tube in there also..

Have I missed something? Is there a way to adjust plate dissipation without changing current draw?

I'm sorry I'm such a dunce for asking such simple questions....

I'm not going to turn the amp back on until I get a reply for fear of roasting something...

bob

 

[calieng]

Typically you need to drop down to a 5Y3 rectifier if the voltage is too high with a 5AR4.

But Dr Z designed it for 6V6 with 5AR4 so maybe try setting it around 25mA and see how it works. The EH6V6s are pretty solid tubes.

 

[rpavich]

Calieng,
Hey, I'm from SD originally...lived in Kearny Mesa....

Just to let you know....the 6V6's are stock for this amp...

bob

 

[calieng]

yep...

Maybe the house voltage is just a little high when you were testing?

Anyway the rectifier swap is sometimes an option when you go from 6L6s to 6V6s. It would drop the voltage in this case as well. By the way I assumed the GTs were EH relabelled...is that what you got?

You can certainly drop the voltage with a soldering iron and an assortment of resistors too if you want to bust open the chassis!

Good luck!

P.S. Where did you move to?

 

[calieng]

...duh!

I just checked the EH web page and the tubes are rated up to 475 volts....

http://www.ehx.com/ehx2/Default.asp?q=f&f=/Catalog/29_Tubes/6V6EH

sorry, it's still early here on the west coast and I haven't had my coffee yet.

If the GTs are something else like JJs you are still fine.

Just set around 25mV and enjoy!

Please let me know how you like the amp... I was seriously thinking about getting one...did you buy it here? There was a nice 2-10" for sale last week.

 

[deeval]

You are Running way too hot,the 386v and 40ma is 15wattage dissapation should be lower,You might have to change a risistor.

 

[rpavich]

Calieng,

I moved to West Virginia...a job related move...

I don't know about the tubes; just that they're GT's.

Dr. Z just got back to me on this and he said go to 36ma at about 380v so that's what I did...

And do I like this amp? Very much....it's "somewhat" fenderlike but the Dr. has his own style....

The tone controls are amazing as they are on all of the Dr Z amps...

I'm scouting for a Ghia right now as a matter of fact...

I noticed you're "calieng"....short for california engineer?

Where do you work?

bob

 

[calieng]

Originally posted by deeval
You are Running way too hot,the 386v and 40ma is 15wattage dissapation should be lower,You might have to change a risistor.

25.4mA X 395V = 10 watts per tube. With two tubes gets you 20 watts for an amp rated at 22 watts.

He does need to set the bias down around 25mA.

The EH tubes are spec'd to 475V and JJ's are good to 450V or more so the voltage is ok.

You are right it was too hot at the 40mA setting.

 

[calieng]

Originally posted by rpavich
Calieng,

I moved to West Virginia...a job related move...

I don't know about the tubes; just that they're GT's.

Dr. Z just got back to me on this and he said go to 36ma at about 380v so that's what I did...

And do I like this amp? Very much....it's "somewhat" fenderlike but the Dr. has his own style....

The tone controls are amazing as they are on all of the Dr Z amps...

I'm scouting for a Ghia right now as a matter of fact...

I noticed you're "calieng"....short for california engineer?

Where do you work?

bob

Bob,

Here's the specs for JJ 6V6 just as an example:

http://www.cedist.com/new/scripts/s...evclients/cedist/ELEVATOR.FXP?item=T-6V6-S-JJ

So 36mA at 380V gets you just about 14 watts which is at the top end of the specs for pentode operation...you may want to crank down the bias a bit to save tube life but I'm sure the good Dr. knows best.

I work for the local office of a national engineering consulting firm. I mostly work on highway, rail, and aiport projects (civil engineering) so electronics and amps is something that I am just learning as a hobbie. I paid my way through college in a band and unfortunately only have time for weekend jams with friends now.

I hope you enjoy your new location...I hope at least the housing prices are better over there!

I'll give a Z a try soon...I had a Maz 18 briefly once but haven't tried any other models. Right now I have a Brown Sugar kit on the way from Allen Amps. The Encore I just finished was so amazing I just had to build another one of his amps.

Cheers.

 

[rpavich]

Guys,
Thanks for your thoughtfull replies...

What do you think of the good Dr's suggestion of 36ma at 380v?

bob

 

[calieng]

Originally posted by rpavich
Guys,
Thanks for your thoughtfull replies...

What do you think of the good Dr's suggestion of 36ma at 380v?

bob

Per the message above...maybe a little hot but he's the expert!

I usually run 6V6s at 25mA and 6L6s at 33mA as a rule of thumb. They usually sound good at that setting and are not too hot to kill tube life.

 

[rpavich]

Calieng,
thanks again for the info...very interesting....

Housing prices WV vs CA? lol...

My house here was 65,000. When was the last time you saw that in SD?

then again....you'll make 65K in the first 6 months of owning any house in south CA.


bob

 

[calieng]

...not for much longer...the bubble is about to pop...or at least the heat is starting to come off the market...should be fairly flat for a while on average. Of course Coronado Island and La Jolla still are favorite locations so they still may appreciate.

A lot of people I know who bought real estate on spec are starting to get a little bit concerned. If you own a place back here it may be time to cash out...but who can tell what the future will bring...all I know is that only 7% of the population can afford a medium price home here now and that won't work for much longer...

http://www.nbcsandiego.com/news/4700178/detail.html

The news article say 9% can afford a median priced home here but I was just at a SANDAG meeting where 7% was the new number. You can buy a whole lot of amps with the difference in a $65,000 house cost versus a $550,000 house cost...and that get's you a knock down in Lemon Grove with drive by shootings!

 

[Trevor Bond]

rpavich, in your first post:

Plate dissipation: 386v

386V is really plate voltage, not dissipation. Dissipation is power. Static dissipation is what you are setting when you adjust the bias current. As the bias voltage goes more negative, this causes the tube to cut off. Don't worry about the plate voltage magnitude... it is what it is... you do need this value though to correctly calculate the static dissipation and thus the bias current

for current production 6V6 tubes, about 60% max tube dissipation is a good place to be. you can move around this pivot point, a bit more and somewhat less. More current and a warmer muddier tone and shortened tube life... less current and colder punchier tone and extended tube life...

assume a 6V6 is 15W max dissipation

so:

15W * .6 = 9W

9W / 386Vdc = 23ma

i.e. Plate Voltage * Bias Current ~= Tube Static Dissipation

 

[calieng]

Originally posted by rpavich
Dr. Z just got back to me on this and he said go to 36ma at about 380v so that's what I did...

...hmmm...

Dr. Z must have been drinking again...oh dear....watch out for another bad batch of Maz Jr's....

just kidding.

 

[rpavich]

I re-biased my Z28 last nite.

I couldn't stop thinking about if 36ma was too hot so I dropped it back to:
25ma at 375v and didn't see any difference in sound..so i'm feeling better...

Maybe longer tube life with this setting?

bob

 

[Trevor Bond]

Originally posted by rpavich
... so I dropped it back to:
25ma at 375v and didn't see any difference in sound..so i'm feeling better...

bob, 25ma is a more sane setting for the 6V6...

.025 * 375 = 9.4W

trev