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Old 10th October 2010, 11:30 AM   #11
Ian444 is offline Ian444  Australia
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Thank you, I can work with that. The B+ from the power tranny I have may be a bit on the high side, I think I can get it down to a reasonable value. Will start construction this week. And no problem trying lower G2 voltages too...
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Old 10th October 2010, 01:24 PM   #12
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Actually I have a question about audio transformer. Digging in a carton box I've found an all black transformer, I believe it was manufactured by Philips between 1959 and 1966. The secondary, depending on straps, is good for 14 or 7 ohm speakers. The code number is PK 51099.
Anybody can tell me about its specifications and if it is worth to use it for a PP amp of EL34 ?
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Old 11th October 2010, 07:59 AM   #13
Ian444 is offline Ian444  Australia
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Quote:
Originally Posted by 45 View Post
The 1K load line is fine for class AB1. The max power curve is meant for the quiescent conditions. When the device is modulating power the average anode dissipation will be not that much.... otherwise you wouldn't get anything at output!
You can go up to 400V 60 mA (i.e. 24W) safely.
So where is the safe operating area for Class AB1, anything less than say 48W? (Provided the tube operates in that area for only half the time?) The 24W power dissipation curve is really only for Class A1 and quiescent conditions then? That would make sense to me...

45, can you answer this question please? When a loadline passes way below the knee of the 0V grid curve, like in the loadline I posted, and the tube grid1 is driven to 0V, what actually happens to the tube or amp in this situation? I know its a bad idea, but I don't understand what happens, does it just square wave the output of the tube?

300b-luc, you can measure the primary impedance of your transformer, this will let you know what tubes it would be suitable for. There are a few threads around that describe how to do this. If you can't find one, let us know.
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Old 11th October 2010, 09:03 AM   #14
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Ian444, even if I measure the primary impedence, due to my poor instrumentation I cannot get enough information to decide that the transformer is worth to be used for a PP amp. I hoped that someone had the Philips data sheet.
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Old 11th October 2010, 11:05 AM   #15
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Quote:
So where is the safe operating area for Class AB1, anything less than say 48W? (Provided the tube operates in that area for only half the time?) The 24W power dissipation curve is really only for Class A1 and quiescent conditions then? That would make sense to me...
A valve does not suffer from second breakdown so the concept of safe operating area is meaningless (the SOAR plot is rectangular), providing the operating frequency is high enough that mechanical fatigue from cyclic thermal load on the plate is not an issue then you can drive the valve as hard as you like as long the average plate dissipation is within ratings and the grids are not overloaded. The screen grid has much less thermal mass than the plate, with a constant voltage screen supply and a hard driven control grid AB2 or C the plate voltage should fall below the screen grid voltage causing it to capture more electrons and possibly melt or distort if overloaded.
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Old 12th October 2010, 08:46 AM   #16
Ian444 is offline Ian444  Australia
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Metalsculptor, thanks very much for the info. I have found very little info on PP loadlines regarding how hard you can go before something blows up, I guess *average* power dissipation is taken for granted by those who know the subject well. Now I understand...

I have begun drilling the top plate, work is in progress...

Edit: 300b-luc, put 10 or 20V AC into the primary and measure the V AC at the secondary with no load. From that the primary impedance can be easily calculated. All you need is an AC supply and a multimeter.

Last edited by Ian444; 12th October 2010 at 08:52 AM.
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