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My new amplifier came with 4 SED Winged C EL34, biased at 30mA by the seller, to extend tube life, 40mA is recommended by the manufacturer...Sunday I changed bias to 40mA, when I switched off the amplifier, the speakers made a loud noise, same noise Monday...
So today I changed back to 30mA, switched off, no more strange noises... I saw people recommending 50mA, saying it sounds better...
What could be the problem, tubes or amplifier...
Antonio Melo Ribeiro
Follow Ups:
Sunday I changed bias to 40mA, when I switched off the amplifier, the speakers made a loud noise, same noise Monday...---------------------------------------------------------------------
when switched off the amplifier, As my experience: If EL34 Plate & Screen Grid voltage decreasing slowly, when reaching at low B+ region (beyond 100V ? )Screen grid current will rise sharply and causing a short period oscillation till exhausting B+ current then stop loud noise.
I use a B+ bleeder resistor to eliminate this effects.
May be biasing will affect it!
Thanks,I can see now that it is more a feature than a problem, it doesnt happen always and it varies from one channel to the other, or nothing at all, so I will just turn volume down before I switch-off the amplifier...
Antonio Melo Ribeiro
Less plate current means higher negative bias to grid, it can be have more ability to suppress the unstable status,
also if supply voltage Vsg of screen grid too high will causung this problems, El34 can withstand higher B+ than KT88 also get high power output from this smaller tubes, I use only +280V regulated Vsg for EL34 and 30 ma of plate current for pushpull 55 watts output to 8 ohm speaker.
The built-in meter shows mA, so I have no idea of the supply voltage...In the manual it is recommended 40 to 45 mA, the seller set it to 30mA, I tried 40mA, but in a quick test diddnt hear any difference, so it is back to 30mA, less switch-off noise, maybe later I will try again higher bias...
Antonio Melo Ribeiro
Sound quality from setting 30mA to 45mA Ip are nearly no difference,even using distortion meter to check it when working at pushpull mode, but maximun power output will dropping may be 20% at 30mA status.
But if at single end mode, higher plate current means lower negative bias and also lower distortion lower life time of tubes.
It is pushpull...Well, my speakers are 96db, so power is not an issue, actually at the same pre volume it is louder than the more powerfull KT88 amplifier I had before...
If I have a party I can always bump the bias... ;)
Antonio
Catch 22 has a point and I was thinking that same thing in that you may want to swtich your preamp off first and then your amp and see what happens..
Im wondering if a mod was done on your amp and maybe the bleeder resistors were taken out or maybe they need bleeders installed.All this seems like somthing could have been altered in the amp before you got it..Did you buy the amp used?
The amp is new, and no mods, as far as I know, I tried to get the seller to hard wire one of the inputs directly, no relays and no volume on signal path, but he said the inputs go direct into a 3 layer PCB, making it difficult to do, but relays have silver contacts, shouldnt be too bad...He also said it is normal for this amp to do switch-off noises, maybe it is so purist that it doesnt have those bleeder resistors, it also doesnt have remote...
I use it as power only, volume max, and I did try switching off pre first...
Funny thing is that now that I pay more attention, sometimes it does only right speaker, others left, sometimes none, or both...
Antonio Melo Ribeiro
In what order are you turning on and turning off your components? Try turning your amp on last, after all the other gear is turned on. Turn the amp OFF first, before the other gear.Sorry if this seems too elementary, but I've had amps that would thump the heck out of my speakers if I didn't do this. I suppose you could also have a tube that is starting to short, but this would more likely effect only one channel.
I tried both ways...
Antonio
It depends on the amp but you may have a bad el34..Is happening in both channels..If not switch the tubes around.
Thanks Michael,The noise is from both speakers, it looks very strange, to me, no noise whatsoever with bias 30mA !!!
Antonio
Can you check the B+ voltage at the el34s? Just take out one tube if you dont want to take the bottom off and measure at pin 3 and tell me what you get.
Michael,I replaced a Cayin A-88T with this Classic 16.0, only because I can adjust bias (change tubes), without even moving the amplifier, I dont have any multimeter, other than the one built-in...
Last night it made the switch-off noisa again, even with 30mA... :(
Antonio Melo Ribeiro
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