| resident |
Hum problems again! :(
I finished my amp today (third time!) and I have a strange (for me) problem.
I build it like it is in the schematic I have attached. I only have changed the stoppers at the output tubes to 1K, the UL resistors to 1K2 and the cathode resistor at the phase splitter to 15K. I don’t think these changes affect my problem but…just to know.
And, I haven’t applied FDB. I’d like to see my amp’s behavior without FDB before I apply any.
Since, I don’t like my input stage as it is, I’d like to use 6SL7 at this stage, I build it like this because I don’t have receive it, yet. ;) I also like to see the differences. So, I was thinking to try it bypassed and un-bypassed. The strange thing is that my amp hums when the first stage is un-bypassed! When it is bypassed is dead quiet. I can’t understand why. So, any help? :confused: |
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| EC8010 |
| Well, yes, it will if you haven't paid sufficient attention to your heater wiring. Is the 6.3V centre-tapped to the chassis? (Either by a transformer CT or a pair of 47R resistors.) Another possibility is that the input valve has leaky heater/cathode insulation. |
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| resident |
| quote: | | Well, yes, it will if you haven't paid sufficient attention to your heater wiring. Is the 6.3V centre-tapped to the chassis? (Either by a transformer CT or a pair of 47R resistors.) |
Yes it's center tapped to the chassis by a transformer CT.When it's not, my amp hums louder. :mad: | quote: | | Another possibility is that the input valve has leaky heater/cathode insulation. |
It's a russian 6SN7 (6H8C).Do you believe it's over here the problem? Not very expensive tubes and I saw that are microphonic,too.
Any other possibility before I buy new tubes? :( |
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| EC8010 |
| Hmmmm. That sounds as though it might be heater/cathode insulation.. You could always buy a valve tester for about £800 to confirm it, but I suspect you'd rather pop a different 6SN7 in and see if that works... |
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| resident |
:cheerful:
Yes you have right!!
I pop a different one and no hum.
I change it with the phase splitter tube and hum went away.
Now it's deaeaeaead quiet!
Thanks for help!
I'll change those russians,mabe with EHX or Sovtek.I bought three russians for my project hoping to be good.But they're all microphonic.The other two not like this one I had at the input stage.But they're a little bit,too. :(
Very cheap,though.
My amp works very nice.
Frequency response 10Hz-20Khz (flat).
I'm wondering how much is at -3db.I didn't have enough time to check it.
Power undistorded 19,5W. 6L6GC EHX work at 320V / 77mA.
And without feedback,yet! It's on the workbench so I haven't heard it.Hope it sounds good,too.
I'd like to try it with more current at 6L6s,say at 80-82mA.
What do you say to change the first 6SN7 with 6SL7? |
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| EC8010 |
| Glad to hear your hum is sorted. You can't just pop a 6SL7 in place of a 6SN7 and expect it to work - it would need a complete redesign. Why not listen to what you have? |
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| resident |
A complete redesign?I was thinking to change only the first stage.6SL7 has more gain than 6SN7. I'm afraid that with 6SN7 I'll not have to much gain to drive the output stage when I'll use feedback.I have measured the grid to grid voltage at the output of the phase splitter and I have around 60Vp-p before clipping.That's not much to drive the 6L6s easilly.
Of course I'll change some resistors to bias the 6SL7 correctly.And I'm expecting better results.
I'm a newbie though.So,I need help.
Do you believe that if I'll leave it as it is it'll be ok?
I have ordered one 6SL7.Not give it a try? |
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| resident |
Hmm maybe I'll not change the 6SN7.My amp sounds very good and there's no any problem with large amounts of FDB.
Only at 15db and more the input sensitivity is too high.But now I'm using a passive preamp.
In two or three days,if everything will go fine,my new 12B4A preamp,will be ready.
I can't choose yet how much feedback I like.I have put a pot to change it from 1dB to 25dB.I think at around 10dB is ok.But I need some more listening tests to decide.
A problem is that my power transformer is getting hot :hot: Temperature stays constant.
I can touch it though but is this normal?
The output stage is working almost in class A.So maybe is from this,right? |
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| SY |
Resident, let me expand a bit on EC's comments, though it may be moot after your last post. Changing over from SN to SL involves changing the idle current and static voltages via changes in the plate and cathode resistors. This will also change the amp's gain and the feedback, so you'd have to go back and redo the compensation and, most likely, resize the feedback resistor.
It's doable, but not trivial. If you're happy with what you have, leave well enough alone. But, in theory, you should be able to boost the amp's test bench performance by such a redesign- no comment on sonic impact of those changes. |
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| resident |
Hmmm…
I don’t have any problem to change the feedback resistor since I haven’t put it, yet.
I’m using a pot right now to change it easily and when I decide the amount of feedback I like I’ll put the right fixed resistor.
But I forgot that if I change the 6SN7 with 6SL7 this will change the anode voltage of 6SN7, too. :bawling:
So, I must redesign the phase splitter. This is a problem!
I’m happy with what I have right, now. When I’ll finish some other projects that I like to build, maybe I’ll do an experiment just for curiosity. |
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| effindi |
REsident,
Your project looks simple and interesting. What is the total output power? And what output transformer are you using.
Perhaps would like to give this a try.
Thanks, |
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| resident |
Effindi,
Yes it’s a simple project and those “purple hazes” at 6L6GCs are too cool.
Output power is around 20W undistorted (I measure 19W exactly before I saw any clipping at waveform).
If you’d like to know frequency response at -3dB let me know.
I haven’t measure it, yet, because my amp is at my listening room right now.
I do some listening checks to see how much feedback I like.
Power and output transformers are from GKlab. There’s no any site from this company but if you’d like to contact with the winder I can post his e-mail.
The other parts are Audyn MKP caps for coupling and electolytics bypass,Nippon Chemicon electolytics, Electoharmonix output tubes and Russian 6SN7 (that I’ll change with EHX or sovtek).
If you’d like more details just let me know. |
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| effindi |
Is that ~20W per channel?
Looks like quite a decent power there.
I think I might be able to handle to output trans so long I got the specifications.
The next step will be to give me a link to the full shematics.
Thanks. |
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| resident |
The schem is posted in this thread. Some small changes that I have done are posted, too.
Over here you will find the PSU attached.
One more change is that I have bypassed the first stage with 470uF capacitor and after 1K//470uF I have put a 100 ohm resistor to ground for feedback. I haven’t decided how much feedback I like, yet. Now my amp is with 12dB of feedback but I’d like to do some more listening checks.
I don't know if I had notice before but I have bypassed all the electrolytics with Audyn MKP caps following the ratio 1/100.
For example the 470uF with 4u7, the 150uF with 1u5,e.t.c.
The output transformer has 12K5 anode to anode primary impedance and 8 ohm secondary.
I have biased the 6L6GC with 220 ohm resistors. So current at each 6L6 is about 78mA (class A). When I’ll decide about feedback I’ll change this, too. I’d like to bias it at class AB. Then I’ll play with feedback again. Then I’ll keep what I like better.
I’m thinking to try triode configuration, too. Maybe after all these I’ll do it! |
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