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Old 10th December 2012, 08:48 PM   #641
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buzz. i'm gonna have 0.6A bias pr fet. and wondering if the best way is to use external sinks or main sinks for the diodes. the main sinks will run pretty hot i guess. 125+W pr side on 2x 10.080" profiles, 6" long.
on the other hand. with external sinks. how will the transaction be, when you WANT the diodes to conduct. i'm way out of the 1A ranch. maybe connect the diode sink with the main sinks with 2 L profiles?
i can not crank up the bias with rail voltage of +/-50V
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Old 11th December 2012, 08:24 AM   #642
Variac is offline Variac  United States
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I sent the payment on the 7th of Dec, the day I posted here that I finally ordered. It's in the board fab and Fed- Ex's hands now. I promise to get them in the store ASAP when they arrive.

Last edited by Variac; 11th December 2012 at 09:00 AM.
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Old 11th December 2012, 09:11 AM   #643
Marra is offline Marra  United Kingdom
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What would be a suitable alternative for the 2SA1837 and 2SC4793.They don't seem to be available from Farnell or RS and the only supplier I've found are asking close to a fiver each.
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Old 11th December 2012, 09:14 AM   #644
AndrewT is offline AndrewT  Scotland
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does rapid or dalbani stock them?
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Old 11th December 2012, 10:13 AM   #645
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Marra,
I'll send you some with your diodes.
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Old 11th December 2012, 03:33 PM   #646
Marra is offline Marra  United Kingdom
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Andrew I did'nt check those two.I tend to deal mainly with Farnell.

Buzz you are a gent thank you.
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Old 11th December 2012, 03:57 PM   #647
BigE is offline BigE  Canada
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Sanity check time for me....

V3, 4 pairs per channel, biased to 3,0 amps per polarity, 50 volt rails. That gives 0,75 watts/FET, or 0,35 Volts. Total bias current is 6 amps.

Now total dissipation is 8 * 50 * 0.75 = 300 watts. That can easily be spread over 4 regular sized F5 heatsinks at 75 watts per heat sink -- each heatsink carries one pair of FETS.

Efficiency is around 33% at full class A power. Does that sound about right?


As for Transformer VA, clearly 300 VA is needed just to run the bias. Now, add the load requirements. P=150 rms = 300 peak.

So, P=IV , V = 50 says I = 6 amps peak. That is another 300 VA to power class A operation. (That checks with doubling the bias current to come out of class A).

So far, the transformer has to be 600 VA *bare minimum*.

What do you spec for Class A/B overhead? Do you limit the VA to provide 50% of the steady state operating current of the output transistors?

Also, how do the diodes impact the heatsink size?

Last edited by BigE; 11th December 2012 at 04:01 PM.
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Old 11th December 2012, 03:58 PM   #648
6L6 is online now 6L6  United States
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Gentlemen,

I am helping a forum member with his F5T -- I will save you the details and ask;

Would there be any problem building a v3 with ~ 42v rails and only 2 pair of output transistors?

Thank you,

-Jim
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Old 11th December 2012, 04:05 PM   #649
Zen Mod is offline Zen Mod  Serbia
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there is no point making current hungry load oriented solution , without full current capability

catch is - what's limiting factor , responsible to just two pairs choice ?
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Old 11th December 2012, 04:11 PM   #650
6L6 is online now 6L6  United States
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The reason for the high voltage rails is due to a mistake when ordering the transformer. So am trying to see if there is a good solution to using the parts on hand.
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