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Re: need more headroom

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3/18/2002 8:56 PM

Bruce /Mission Amps

Re: need more headroom

quote:
"I just put a Sovtek 5Y3 in per your suggestion and am now seeing 342 v DC on the plates.... much better. Am also seeing 21.79 v DC voltage drop across the final bias resistor giving 87 mil bias. Your suggeston of a 270 ohm bias resistor would give 80 mils."

Yes, that's because you are using a choke instead of the 5K resistor in the power supply.
Your screen voltage is almost the same as the plate voltage.
Higher screen voltage will make the tube bias MUCH hotter.
You could try sticking a 2K-5K resistor from your choke lead to the screen lugs.
That will lower the screen voltage a little bit and cool your idle current a few ma.
You still will have more screen voltage the the rest of us!
Other then shorter power tube life, I don't see anything wrong with what you are doing if the power tube's plates are not glowing red after playing hard for a few mins.

Bruce

3/19/2002 5:19 AM

Bruce: I was under the impression that the choke was an IMPROVEMENT. Am I wrong ? I heard it 'smoothed out the sound'. I realize a lot of this is subjective... but.. perhaps I shouldn't be 'reinventing the wheel'.
Let's get spacific on the dropping resistor for the screens .. You're suggesting I run 2k to 5k in series with the 470 ohm resistors that I already have in the screen lines ?
/ frankenstein

3/19/2002 6:09 AM

Bruce /Mission Amps

If you are happy with the way it sounds, then leave it that way.
Improvement? Well, the large value dropping resistor is there to isolate the plate and screen/preamp filter caps and lower the voltage to the screens and lower the plate voltages of the preamp tubes.
Your choke does make for a better filtered supply but IMHO, it's overkill in this little amp and it could make the amp sounds and play differently.
It's obvious that the power tubes idle hotter because of the extra screen voltage and the preamp tubes are probably running higher voltages too.
None of that is bad, just different.
If you take the lead that feeds your 470 ohm screen resistors with B+, (from the second filter cap and the B+ return from the choke), and insert a 2W, 2K to 5K resistor there, you'll drop the screen voltage a little more and the tubes will run cooler.
Will it sound different? Maybe, maybe not much.
Your 470 ohm screen resistors are mildly needed with your circuit (choke) but not with the single 5k resistor in the stock 5E3.

Bruce

3/19/2002 3:07 PM
frankenstein	Bruce: Understood. Why not just put a dropping resistor in series with the return from the choke ? Wouldn't that get you better regulation AND lower voltages to the screens of the finals and the plates of the preamp tubes ? Best of both worlds ? If so what value ? 5.1k (2 watt) minus resistance of choke ? / frankenstein

3/19/2002 3:54 PM

Bruce /Mission Amps

quote:
"Why not just put a dropping resistor in series with the return from the choke ? Wouldn't that get you better regulation AND lower voltages to the screens of the finals and the plates of the preamp tubes ?"

Yeah, that's what I ment to say for you to try.
Try a 4k7 or some other standard value close to there.
Again, you might like what you have better so keep an open mind.
Bruce

3/19/2002 8:46 PM

Bruce: OK. Excellent. I'll give that a go and report back to youse guys ! I'm building another 5E3 next week (when I get the chassis) and will make it a 'test bed' with a DPDT switch - choke in one position, resistor in the other - and will report in on what, if any, differences I can articulate. I'll also try the 270 ohm bias resistor (if needed) and then go from there.
Thanks VERY much for your helpful suggestions Bruce. This board is great !
/ frankenstein

3/19/2002 9:01 PM

Bruce,

Re: "sound different, maybe, not much." I hate to disagree (because the amount of valuable info you have shared in this forum makes you a revered mentor to me) but in my experience the diff between a choke and a 5K resistor is pretty dramatic and very audible. I have a graph from when I tried it a month or so ago when I was putting together my 5e3 clone. Its a totally stock 5E3 circuit with a Fender output transformer at 1kHz:

http://members.aol.com/wsjcrane/choke.gif

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