Greetings from FixitLand!
OKeh, now it's a whole new ballgame. I see you've added the 100-ohm screen resistors on the EL84s. I think your buzz issue is down to the fact that you're not on a metal chassis, and that there's no ground plane to speak of.
Glad you've got plenty of volume now. The schematic shows no feedback loop of any kind; adding one would help things quite a bit. I can walk you through it. But first, I still need to know what your signal source is -- what audio are you playing through this amp?
Take care,
--
J. E. Knox "The Victor Freak"
OKeh, now it's a whole new ballgame. I see you've added the 100-ohm screen resistors on the EL84s. I think your buzz issue is down to the fact that you're not on a metal chassis, and that there's no ground plane to speak of.
Glad you've got plenty of volume now. The schematic shows no feedback loop of any kind; adding one would help things quite a bit. I can walk you through it. But first, I still need to know what your signal source is -- what audio are you playing through this amp?
Take care,
--
J. E. Knox "The Victor Freak"
Greetings from FixitLand!
Sorry...our posts seem to be "mis-timed". I see you're using a MacBook Pro as an audio source. (Good choice...I'm typing this on one.) The MacBook Pro puts out a line-level signal that *might* overload an amp with no negative feedback. I'm headed to a meeting right now, but I'll get back to you as soon as I can.
Take care,
--
J. E. Knox "The Victor Freak"
Sorry...our posts seem to be "mis-timed". I see you're using a MacBook Pro as an audio source. (Good choice...I'm typing this on one.) The MacBook Pro puts out a line-level signal that *might* overload an amp with no negative feedback. I'm headed to a meeting right now, but I'll get back to you as soon as I can.
Take care,
--
J. E. Knox "The Victor Freak"
I use my phone and my MacBook Pro. Sadly i don't have any high-end CD player, or FM receiver, or anything. :/ I want to buy a high-end sound card, but that's the future. 🙂
Disconnected the capacitor, nothing changed. I measured the AC voltage on the 470kOhm resistors, and the results are 1Volts. I also measured the PSU's AC voltage, it's 1.1Volts.
YES, YES, i solved the problem. Put a 100uF capacitor to the ECC82s pin6, and the buzz disappeared. 😀
Greetings from FixitLand!
Pin 6? That's the plate (anode) of the *unused* unit, isn't it? Very interesting that it would clear up the buzz like that.
One thing you could also do would be to connect the two triodes together -- pin 1 to pin 6, pin 2 to pin 7 and pin 3 to pin 8. Or, build a second amp for stereo and use the second half of the ECC82 for *its* phase splitter.
Congrats on fixing the buzz issue! Enjoy your tube amp!
Take care,
--
J. E. Knox "The Victor Freak"
Pin 6? That's the plate (anode) of the *unused* unit, isn't it? Very interesting that it would clear up the buzz like that.
One thing you could also do would be to connect the two triodes together -- pin 1 to pin 6, pin 2 to pin 7 and pin 3 to pin 8. Or, build a second amp for stereo and use the second half of the ECC82 for *its* phase splitter.
Congrats on fixing the buzz issue! Enjoy your tube amp!
Take care,
--
J. E. Knox "The Victor Freak"
No, i used the pin 6-7-8. 🙂 But i have a new problem. I just grounded the ECC82's anode with the capacitor, i think. It cleared up the buzz, but i think it's also reducing the output signal. You know, capacitive reactance...
Greetings from FixitLand!
Will have to be a very quick reply...yet another meeting in five minutes. (Whew!)
Whoops. Yes, you did indeed AC-ground the anode, effectively reducing the drive to the output. But if the buzz is gone when doing so, I'm thinking you may still have an oscillation problem. For laughs: Do you have some small-value capacitors such as, say, 330 pF (preferably 500 V micas). Try tacking one from each EL84's pin 2 to ground. If this gives no improvement, and they are in fact 500 V mica caps, connect them from each EL84's pin 7 (anode) to ground. I have an amp circuit where that's done, and have never noticed oscillation with it. Good luck!
Take care,
--
J. E. Knox "The Victor Freak"
Will have to be a very quick reply...yet another meeting in five minutes. (Whew!)
Whoops. Yes, you did indeed AC-ground the anode, effectively reducing the drive to the output. But if the buzz is gone when doing so, I'm thinking you may still have an oscillation problem. For laughs: Do you have some small-value capacitors such as, say, 330 pF (preferably 500 V micas). Try tacking one from each EL84's pin 2 to ground. If this gives no improvement, and they are in fact 500 V mica caps, connect them from each EL84's pin 7 (anode) to ground. I have an amp circuit where that's done, and have never noticed oscillation with it. Good luck!
Take care,
--
J. E. Knox "The Victor Freak"
I have ceramic capacitors; 101K, 221K. I think this means 100pF and 220pF. Can this capacitors handle 200Volts?
Greetings from FixitLand!
Hard to tell. I don't think these will be suitable for connecting to the EL84 anodes, but they'll be OKeh to try on the grids. This is just a troubleshooting exercise, so don't sweat it too much. Try the 221K (yes, that's 220 pF) caps on the EL84 grids. If the buzz noise subsides, the stage was definitely oscillating. If it's still there, it would be worthwhile to get hold of some 33-pF 500-V capacitors and bypass the EL84 anodes with them.
Is there no voltage rating specified on those caps?
Wish I could be there in person...would like to hear the buzz noise. An oscilloscope would also be handy too, but... Anyway, you said you had a multimeter, correct? Digital or analog?
Take care,
--
J. E. Knox "The Victor Freak"
Hard to tell. I don't think these will be suitable for connecting to the EL84 anodes, but they'll be OKeh to try on the grids. This is just a troubleshooting exercise, so don't sweat it too much. Try the 221K (yes, that's 220 pF) caps on the EL84 grids. If the buzz noise subsides, the stage was definitely oscillating. If it's still there, it would be worthwhile to get hold of some 33-pF 500-V capacitors and bypass the EL84 anodes with them.
Is there no voltage rating specified on those caps?
Wish I could be there in person...would like to hear the buzz noise. An oscilloscope would also be handy too, but... Anyway, you said you had a multimeter, correct? Digital or analog?
Take care,
--
J. E. Knox "The Victor Freak"
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Greetings from FixitLand!
I looked up the meaning of the 'K' code following the '100' and '221' -- 'K' just means a 10% tolerance (which is good). It doesn't mean a 1-kV rating. Somewhere there should be a voltage rating. If they're physically not very big, or especially if the leads are of thin, slender wire, they have a low voltage rating and won't work bypassing the anodes -- but would be OKeh bypassing the grids.
Take care,
--
J. E. Knox "The Victor Freak"
I looked up the meaning of the 'K' code following the '100' and '221' -- 'K' just means a 10% tolerance (which is good). It doesn't mean a 1-kV rating. Somewhere there should be a voltage rating. If they're physically not very big, or especially if the leads are of thin, slender wire, they have a low voltage rating and won't work bypassing the anodes -- but would be OKeh bypassing the grids.
Take care,
--
J. E. Knox "The Victor Freak"
Digital multimeter. 🙂 I measured the AC voltage at the EL84s anode, it's 10.5-11Volts. I think this is the hum that goes thru the output transformer. The capacitors are very very small, the diameter is around 3-4millimeters.
I did measurements, and i have some news. The hum that goes to the speaker, is 1.6Volts. I decided to check the psu again, and realized: there isn't enough bypass capacitors. I disassembed the PSU, made a Graetz rectifier on it, and added more capacitors. So the set is now: 470uF 200V capacitors in series, then a 120Ohm 5W resistor, then 330uF 200V capacitor in series, plus another 470uF 200V in series, plus another 100uF 400V... 🙂) So, i have 685uF now. Turned it on, measured the hum; it's 0.4Volts. I decided to buy new, good quality bypass capacitors. Solen and Kendeil. What do you think?
You now have an awful lot of smoothing. I suspect that the normal P-P ripple cancellation is not working too well, perhaps because your output valves are not well balanced. In addition, many amps will use negative feedback to further reduce the hum.
So, i need to use NFB? How can i do that? (I know how to do it with an op amp, but hey, this is a tube amp)
No difference !
If you remove the bypass cap in the cathode of the EF86 you create an "inverting input".
No matter if its impedance/sensitivity is not the same that the "non inverting" one.
Then, you can pick a fraction of the output voltage from the OPT secondary and feed it back to the inverting input.
Of course the way you tie the OPT will swap the input role: the inverting becomes non inverting and vise et versa !!
Yves.
If you remove the bypass cap in the cathode of the EF86 you create an "inverting input".
No matter if its impedance/sensitivity is not the same that the "non inverting" one.
Then, you can pick a fraction of the output voltage from the OPT secondary and feed it back to the inverting input.
Of course the way you tie the OPT will swap the input role: the inverting becomes non inverting and vise et versa !!
Yves.
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