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PostPosted: 07 Aug 2010, 03:33 
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kkkittehz wrote:
If you can get even 500 hours listening time out of them then the $29 per tube isnt too expensive, especially if they sounds pretty good. I know Bruce Heran's guide to tube amp building recommended that tubes not be pushed further than 80% of their max plate dissipation BUT if it works for you then so be it!

This is all ok as long as when they die they don't ruin the rest of your amp i.e. short out and fry everything

Firstly: Big announcement: I may have a compact valve tester in the next few weeks. I can then test the tubes - running at 14W, regularly and ensure their emissions are good. Also I will know whether I can get 6 or 12 months out of a tube.

Only having to replace one tube is not too bad. As kkkittehz said above $29 X 2 a year is no big deal if the sound is worth it.

Definitely in the Pinot "Shin", the next SET to be built, I will limit the current a little.

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PostPosted: 07 Aug 2010, 12:53 
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mwhouston wrote:
I can then test the tubes - running at 14W, regularly and ensure their emissions are good. Also I will know whether I can get 6 or 12 months out of a tube.

Will be interesting to see how long the 6AS7 lasts with a plate dissipation of 14W (6AS7 is rated at 13W). I know you don't like the look of the 6080 and while it has the same plate dissipation rating as the 6AS7 it may be able to take more abuse (6080 = ruggedized version of 6AS7G).

FWIW - here are Steve Bench's notes on Inverted Triodes - Investigating the 6AS7/6080 Family.
Cheers

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PostPosted: 07 Aug 2010, 18:14 
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If I wasn't so lazy I'd drop a couple of resistors at the plate of the rec tube and be done with it.

I have purchased 10 X 6AS7 so no more tubes for me. I should just fix the one I have.

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PostPosted: 09 Aug 2010, 08:42 
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On placing some new SED Winged "C" tubes in the Pinot "KiSharn" I found the tube to be dissipating about 14W. This will dramatically shorten tube life.

I have been feeling guilty about running the new 6AS7 SED Winged "C" at 14W. From some rough calcs I figured I needed to drop the HT to the 6AS7 by 10 to 20V. I wanted to maintain a higher HT to the 12AX7 driver tube. My calcs.claim I need a 150ohm resistor in series with the HT+

First I tried a 330ohm R from the HT+ to the B+ of the OPTs. This dropped the voltage too much. So now I tried my guesstimated value of 150ohms. This proved close to perfect. I still wanted to drive the tube hard just not cripple it. With the 150ohm 5W resistor in place Plate to Cathode V was 176 with a HT+ of 270V. Bias V = 75V

Idle current was 67mA. Tube dissipation is now 11.8W. Just nicely under the 13W max rating. This makes it 90% of max. I know Bruce would rather see an 80to 85% but he is producing a high performance commercial product and purchases are right to expect both good performance and good life from their tubes. For me I'm happy to replace the tubes once or twice a year!!

Based on some predictions Bruce has made I should get better than 1000hours of good life from the tube. I hope to have a tube tester of my own soon so I can keep an eye on tube degradation.

So no more Tube Cruel, I'm just working them hard.

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PostPosted: 09 Aug 2010, 10:02 
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Well since youve changed your philosophy you need a new name : Mark 'The Benevolent' Houston :angel:

:D

Any change in the sound for the amp?

Lewis

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PostPosted: 09 Aug 2010, 17:29 
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I will get a chance today to have an extended listen. I'll post some notes. I'm not expecting any difference because really it has been only a small drop on power dissipation. BUT now I do have an additional 150omhs in the HT leg. Not sure if this will effect anything. It shouldn't when you think the primary of the OPT is 1.7K so 150ohms is an order of magnitude smaller.

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PostPosted: 09 Aug 2010, 20:44 
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mwhouston wrote:
I have been feeling guilty about running the new 6AS7 SED Winged "C" at 14W. From some rough calcs I figured I needed to drop the HT to the 6AS7 by 10 to 20V. I wanted to maintain a higher HT to the 12AX7 driver tube. My calcs.claim I need a 150ohm resistor in series with the HT+

Draw the power to the 12AX7 before the 150 ohm resistor to keep the voltage up on the driver tube.
Cheers

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PostPosted: 09 Aug 2010, 21:30 
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Gio wrote:
mwhouston wrote:
I have been feeling guilty about running the new 6AS7 SED Winged "C" at 14W. From some rough calcs I figured I needed to drop the HT to the 6AS7 by 10 to 20V. I wanted to maintain a higher HT to the 12AX7 driver tube. My calcs.claim I need a 150ohm resistor in series with the HT+

Draw the power to the 12AX7 before the 150 ohm resistor to keep the voltage up on the driver tube.

Gio

Yes I did this. The driver gets full HT+ where the 6AS7 gets the reduced HT.

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PostPosted: 13 Aug 2010, 07:39 
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I sadi I would post the final values of the 6AS7 SET amp and here they are. They are still a work in progress so expect some more minor changes:
Attachment:
retro-thermionic-Pinot-KiSharn-6AS7-SET-Schematic.png
Attachment:
retro-thermionic-Pinot-Power-Supply-Schematic.png


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PostPosted: 13 Aug 2010, 17:50 
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You will notice in the amp schematic above the 150ohm resistor feeding the OPT. This is to drop the HT+ just a little going into the 6AS7. Reason is that the 6AS7 was dissipating over 14W. Not good for a tube rated 13W. With the 150ohm R the tube dissipation falls to under 12W. Still high but these days I like to run the tubes hard.

Gio made the comment that I should have a filter cap where the 150ohm R joins the OPT. And he's right. This would add just a little more filtering to an altready quiet PS and a little more reserve on big transients.

I was thinking something small like 10 to 47uf. A small fast electro snubbed by 0.1uf poly. I thought about it when adding the 150ohm but it is not going to be an easy job. I may still do it next time I have the bottom plate off.

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