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In Reply to: RE: My homemade EL34 amp sounds better with 5881s posted by PeterI on April 02, 2014 at 13:38:39
Of course it sounds better..It's a 6L6..No bias on my part..LOLAnyway,I have found the same thing with the stock Dyna st70 where the 5881s sounded better IMHO.The same holds true with the VTA version.
Here is the thing Pete.EL34s typically load from 4.2k to 5k at about 420vdc on their plates. The 6L6 family, typically loads at 6.6k.
The 5881 if you look at the tube data,would have you closer to optimal operating points if you drop the B+ to about 380vdc at the plates.
Are you running an ultra linear or pentode circuit?5881 Application Data
Class Va Vg2 Vg1 Ia Ig2 Ra S Zout Pout
A S/E 350 250 -18.0 53.0 2.5 48,000 5.2 4,200 11.3
A S/E 250 250 -14.0 75.0 4.3 30,000 6.1 2,500 6.7
AB1 P/P 360 270 -22.5 88.0 5.0 3,800 18.0
AB1 P/P 360 270 -22.5 88.0 5.0 6,600 26.5
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/02/14Follow Ups:
Thanks Mike. I knew I was pushing it at over 400V on plates. I have 40% UL tap on Edcor OPTs. And there's 412V on those screens thru 150 ohm R. I had looked at the datasheets for 5881 and 6L6WGB but there are a lot of varying parameters.Hey, while I have your ear, Mike, is there any advantage to putting an approx 150ohm R on the screen of the 7189s in a Sherwood S5000. I'm ordering parts for mine.
Also, can't you lower all voltages in the S5000 PS by increasing the resistance of that first 33ohm 7W resistor off the rectifier? say to 50-75ohms, or more? Or does that cut current too?
Thanks
Pete
Edits: 04/02/14
You are really pushing it at 400v if you had the 6.6k load or higher.Since you have UL,you stay right where your at and as long the driver circuit can 84vac of signal,you are fine where you are at.
I got email on that Sherwood and I thought I answered it but don't change that 33 ohm resistor...If you want to 39 ohms,that's fine but just get rid of that sand block on there and put in a Mills or Vishay wirewound.
You also need to put in a 35uf poly off the GZ34.Mount the cap in place of where the filter cap front sits for DC fil and negative bias..
Honest amplification is better than excessive 2nd order distortion anytime.
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