E88CC dissipation in Aikido

[Andypairo]

Hi all,
while rebuilding my Aikido (E88CC/E88CC) I noticed that the tubes that were used in the output position had some (small) black spots on the glass.. not actually real "spots" but rather a small localized darkening of the glass.

It looks like a result of tube overheating... I used to run the input tubes at about 5-6 mA while the output tubes are at 10mA.
B+ is 220V.

A quick calculation and.. well it seems that I exceeded the 2W total package dissipation limit.
However I picked up values from Broskie's table... and he even lists a 10mA with 250v B+.

I never noticed this fact but now I'm afraid I shortened my tubes' life.... and since they are Mullards this makes me

How hot are you running your Aikido?

Cheers

Andrea

[SY]

Dropping the B+ to 200V won't hurt your linearity very much, especially at these small swings. And that will take your dissipation down to recommended limits.

edit: Distortion measurements here:

http://www.diyaudio.com/forums/showt...210#post859210

These were done at 12.5VRMS, at 2.5VRMS, which will clip nearly any power amp, the distortion would be expected to be about 14dB better.

[Andypairo]

Of course from now on I'll be taking in account plate dissipation , this post was meant as a warning for all Aikido builders not to trust all that is written without using your own brain.

Since these values for the ECC88 resulted in a excessive plate dissipation it might be true for other tubes also....

Cheers

Andrea

[Sheldon]

Originally posted by Andypairo
It looks like a result of tube overheating... I used to run the input tubes at about 5-6 mA while the output tubes are at 10mA.
B+ is 220V.

A quick calculation and.. well it seems that I exceeded the 2W total package dissipation limit.
However I picked up values from Broskie's table... and he even lists a 10mA with 250v B+.

Andrea

You sure? Are you talking about the B+ for the amplifier. If so, the B+ per section has to be divided by 2, as the tube sections are stacked. I think your total dissipation at 220V B+ and 10mA would be more like 1.1W per tube or 0.55W per section - well within the limit.

Sheldon

[Andypairo]

In the Aikido the tubes have each half B+ but both pass the same current, so each plate dissipates B+/2*10mA = 1.1W and total dissipation is 2.2W.

Cheers

Andrea

[Sheldon]

Originally posted by Andypairo
In the Aikido the tubes have each half B+ but both pass the same current, so each plate dissipates B+/2*10mA = 1.1W and total dissipation is 2.2W.

Cheers

Andrea

Yup, you are correct. Guess I shouldn't be trying simple math before bedtime. So, you can do as SY suggested and lower B+, or you can increase the value of the cathode resistors on the follower. Larger resistors will increase your output impedance a bit, but shouldn't be a problem unless you have a very low impedance to drive.

Two other thoughts (and given the quality of my earlier answer, you will want to confirm for yourself): Some of the E88CC data on the TDSL indicate 10,000 hours at 2W. You are just over the rating, so your tubes my yet not have lost too much of their zip. Makes sense, of course, to get below the rated max. Other thought is that you might want to use a somewhat less precious tube in the follower position. Save the exotic NOS for the first slot, where individual tube characteristics should be somewhat more apparent.

Sheldon

[Sheldon]

I have a phono pre with an Aikido buffer on the output. I checked the current and found I was a little high on the dissipation. So I increased my cathode resistors. Then, I also checked a few 6DJ8, 6922, 7308. Though they spec. the same, the current variation is as much as 2 fold at the extremes. The 7308 were higher than the others by about 25%. But one of the 6922's was half of the highest 7308. So, check the calculation but, if you are running near the limit, check the individual tube bias too.

Sheldon