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 Post subject: Re: Brown Tubes?????
PostPosted: September 6th, 2015, 4:23 pm 
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Joined: February 28th, 2013, 3:31 pm
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Switch to transistors. When they get brown inside you can't see it :crazy:

Tom


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 Post subject: Re: Brown Tubes?????
PostPosted: September 6th, 2015, 5:02 pm 
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Joined: March 2nd, 2013, 2:43 pm
Posts: 216
Location: Potomac, MD
Hi All,

This thread could be turned into a listening get together to listen to various P-P pentode-type amps to see how preferences go as to how hot to run the tubes. Yes, a hot bias keeps the amp more in class A, but on the other hand it makes it more difficult to keep the idle current balanced in the output transformer. I have always preferred lower bias current because of this as it seems to improve transparency, even though there is more crossover distortion. I just completed an EL34 P-P amp and I would like to get some ears on it and it would be fun to compare. My operating parameters for this amp are unusual in that my plate-to-plate loading is 18 k ohms if an eight-ohm speaker is used and 9 k ohms if a four-ohm speaker is used. The plate voltage is 730 V and the screen voltage is 275 V. Idle current is 11 mA per tube. In my case I am not using a transformer.

Any interest on such a listing session focused on this topic?

David


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 Post subject: Re: Brown Tubes?????
PostPosted: September 6th, 2015, 8:17 pm 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
Sorry Randy -- no one has said to lower the grid voltage -- what was said was to essentially dial in a cathode current for the best sound and with that -- a lower cathode current -- you extend the life of the tube(s). A lower grid voltage = higher cathode current -- a higher grid voltage = lower cathode current. No grid voltage = a run-a-way tube. Cathode bias is another matter entirely. The voltage drop across a cathode resistor creates a condition where the cathode goes positive with reference to the grid. The grid input is then negative with respect to the cathode. Sorry for the instruction but it could be confusing to some. It certainly has been confusing to me in the past.

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 Post subject: Re: Brown Tubes?????
PostPosted: September 6th, 2015, 10:53 pm 
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Joined: December 14th, 2013, 2:19 pm
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Wow, I just spent a half hour writing a detailed and helpful reply. Nearing the end, POOF!, it disappeared!

Call me if you get the chance, Chris.

Stuart (410) 919-8750. Tomorrow is fine.


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 Post subject: Re: Brown Tubes?????
PostPosted: September 7th, 2015, 7:57 am 
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Joined: January 15th, 2015, 7:19 am
Posts: 1697
Location: Baltimore MD
Walt (SoundMods) has been helping me rebuild my ASL Hurricanes. The project has overwhelmed me and Walt's generous and patient help is getting me to the end of the project. I would have given up long ago if he was not there to lean on.


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 Post subject: Re: Brown Tubes?????
PostPosted: September 7th, 2015, 11:22 am 
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Randy -- with all due respect -- I have been talking about cathode current. Power tubes in my experience have a "sweet spot" with regards to the cathode current and I have found that it is not the maximum current the tube can tolerate. That said -- with fixed bias you apply a negative voltage to the grid. The higher the voltage the lower the current through the tube. The lower the voltage the higher the current. OK? With cathode bias (preferred by the late Sid Smith) the voltage drop across a cathode resistor (say about 300-ohms as a possible example) creates a positive voltage with respects to the grid potential. The cool thing about cathode bias is that it "tracks" the music flow. Many arguments out there discuss the benefits of both methods. The French company Jadis has used an old G.E. trick using both methods together. A negative voltage on the grid as a starting point together with cathode bias.

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 Post subject: Re: Brown Tubes?????
PostPosted: September 7th, 2015, 11:28 am 
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Location: Parkville, Maryland
As far as Paul Elliott's pair of Hurricanes are concerned -- Paul has undertaken the kind of project akin to restoring a vintage airplane or car. It is horribly tedious -- the devil is in the details -- and any die-hard DIY hobbyist would have a problem with the project. Scratch building is tough enough, but undoing two previous owner's follies and incompetence is much worst. So -- I am chairman of Paul's cheer leading squad. :clap:

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 Post subject: Re: Brown Tubes?????
PostPosted: September 7th, 2015, 12:13 pm 
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Joined: April 20th, 2014, 5:21 pm
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This brown stain has nothing to do with bias. Some import tubes get that way from average tube heat. No need to run out to the shed and start changing cathode resistors.

Doesn't seem to effect the operation of the tube in the immediate situation, but I do note that NOS USA tubes that don't get brown marks last a heck of a lot longer than the import tubes that do!


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 Post subject: Re: Brown Tubes?????
PostPosted: September 8th, 2015, 12:24 pm 
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Joined: March 12th, 2013, 1:49 pm
Posts: 214
SoundMods wrote:
Randy -- with all due respect -- I have been talking about cathode current. Power tubes in my experience have a "sweet spot" with regards to the cathode current and I have found that it is not the maximum current the tube can tolerate. That said -- with fixed bias you apply a negative voltage to the grid. The higher the voltage the lower the current through the tube. The lower the voltage the higher the current. OK? With cathode bias (preferred by the late Sid Smith) the voltage drop across a cathode resistor (say about 300-ohms as a possible example) creates a positive voltage with respects to the grid potential. The cool thing about cathode bias is that it "tracks" the music flow. Many arguments out there discuss the benefits of both methods. The French company Jadis has used an old G.E. trick using both methods together. A negative voltage on the grid as a starting point together with cathode bias.


Correct me if I am not right, but the less negative the grid to the cathode, the more of the electron cloud can pass thru the grid to the plate, thus increasing the current flow to the plate as current is a measure of the number of electrons flowing in a circuit, and thus the more power dissipated in the plate. The cathode current is the sum of the current flow in the plate and any current flow in the screen grid, there should be NO current flow in grid 1, the current flow in grid 2 a good bit smaller than the plate current. Assuming that you are operating on the linear portion of the load line for the grid voltage you are using, you will get a linear result of the amplification from the tube, operating in a non-linear portion will add non-linearity to the amplified signal. As far as cathode bias tracking the music flow, I believe that is the whole point of amplification. Whether you let the cathode biasing track the music or grid 1 track the music, the resulting signal should be the same if linear amplification is desired.


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 Post subject: Re: Brown Tubes?????
PostPosted: September 8th, 2015, 1:59 pm 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
That's what I said. ?????

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