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What makes a tube good or not good for A2

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Posted on March 25, 2015 at 15:49:07
RayP
Audiophile

Posts: 726
Location: Maryland
Joined: June 30, 2005
I've been wondering about this for a while and I suspect there is more to this than meets the eye.

Could you give examples of both yeah and nay with the reasons why.

ray

 

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RE: What makes a tube good or not good for A2, posted on March 25, 2015 at 16:19:41
I have only played with A2 with thoriated types such as 3C24 826 and also the larger HK54 . I used a James 6123 output transformer on the 5k tap which didn't perform particularly well . No matter what valve I used , HF was a tad bright , although rolled off at 20kHz . Feedback is definitely required for decent bass . It's all down to the driver and the type of feedback used , it can make your speakers sound very different compared to an A1 amp . I used cathode feedback , but I suspect driving the cathode and taking feedback from the anode is the way to go . If you are thinking of driving positive into the peaks of an AB1 or A1 amp then this is totally different and not worth the hassle IMO

Al

 

RE: What makes a tube good or not good for A2, posted on March 25, 2015 at 17:56:53
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
The control grid has to be able to dissipate some power. Quite a few types will fail in a positive grid current regime.

Among multi-grid types, the 6L6 family and the KT88 are specifically rated for positive grid current. Some popular DHTs are OK too.

Don't forget that the driver has to be low O/P impedance and capable of delivering power, not just a waveform. Historically, cathode followers DC coupled to g1 or step down interstage trafos have been used. Tubelab's PowerDrive, which uses MOSFETs, is the "modern" way and enjoys an excellent reputation.


Eli D.

 

Powerdrive, posted on March 25, 2015 at 19:46:14
RayP
Audiophile

Posts: 726
Location: Maryland
Joined: June 30, 2005
I've used Powerdrive a couple of times and it does work well.

ray

 

There are tubes designed for A2, posted on March 26, 2015 at 05:32:35
Chip647
Audiophile

Posts: 2633
Location: The South
Joined: December 24, 2012
Higher Mu transmitter tubes. Eimac "TH" tubes, things like 809s. As Eli says they need to have a grid designed for substantial current. They are really hard to drive due to the current, but the Powerdrive works on these tubes as well, provided that you use bigger mosfets and heatsinks.

 

Which A2?, posted on March 26, 2015 at 08:25:21
Triode_Kingdom
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Posts: 10011
Location: Central Texas
Joined: September 24, 2006
I don't think that question can be answered unless the mode of A2 is stipulated. An amplifier that only enters A2 on peaks is very different from one that draws constant grid current. Many tubes that are suitable for the former may not have sufficient grid dissipation to be used in a circuit biased to draw constant grid current.

 

RE: Which A2?, posted on March 26, 2015 at 09:45:03
RayP
Audiophile

Posts: 726
Location: Maryland
Joined: June 30, 2005
Interesting.

Which leads to another question. Some data sheets such as the Svetlana 811 and 812 show DC grid current. A typical 845 or GM-70 datasheet does not show this information.

Is there a way to calculate grid current for an 845 or GM-70?

And then, how much grid current would be too much for constant operation and how much for peak?

ray

 

RE: Which A2?, posted on March 26, 2015 at 10:42:00
Triode_Kingdom
Audiophile

Posts: 10011
Location: Central Texas
Joined: September 24, 2006
"Is there a way to calculate grid current for an 845 or GM-70?"

Not that I know of. All else being equal, grid current is a function of the physical construct of the grid. Unless you have access to the specific design criteria for the tube, grid current must be measured under actual operating conditions. Personally, I wouldn't operate an expensive tube in A2 unless A) the manufacturer provided data for same, or B) other designers before me had successfully done so.


 

RE: What makes a tube good or not good for A2, posted on March 27, 2015 at 15:33:22
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
Look at the curves for a given tube, if the negative grid bias lines are all scrunched up and at obnoxiously low currents, then running some grid current will be necessary to move into the more linear portion of the curves.

This will also generally be accompanied by a G1 dissipation rating, which is a good indicator that grid current is acceptable.

The plate voltage on an 845 needs to be obscenely low (maybe 200V) to bias the tube in A2, which is probably a decent part of the reason that no grid current measurements are given.

 

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