Andrea,
Just to let you know... I tried two different 6FQ7 tubes in my Aikido today and the results made me wonder if maybe you aren't suffering the same fate. One tube was perfectly dead quiet and the other tube sounded like it was a hiss amplifier. I switched the tube back and the hiss was gone. Is there a possibility that your noise is caused by substandard tubes?
Just a thought...
Just to let you know... I tried two different 6FQ7 tubes in my Aikido today and the results made me wonder if maybe you aren't suffering the same fate. One tube was perfectly dead quiet and the other tube sounded like it was a hiss amplifier. I switched the tube back and the hiss was gone. Is there a possibility that your noise is caused by substandard tubes?
Just a thought...
Quote:
Hi,
I would not call Mullard E88CCs and CV2492 substandard tubes (I tried more than just a couple of them).
My point was even with NOS tubes one could get a noisy tube. This unfortunately can even happen with a Mullard or CV.
Just for grins I tried a pair of 6BK7's that have a MU of 40 and the result was no noise when I put my ear next to my 100+ db efficient speakers. By the way the 6BK7's might be worth a try if you happen to have a pair
Hi,
I would not call Mullard E88CCs and CV2492 substandard tubes (I tried more than just a couple of them).
My point was even with NOS tubes one could get a noisy tube. This unfortunately can even happen with a Mullard or CV.
Just for grins I tried a pair of 6BK7's that have a MU of 40 and the result was no noise when I put my ear next to my 100+ db efficient speakers. By the way the 6BK7's might be worth a try if you happen to have a pair
Good news...
I rewired the Aikido using stropboards and now the noise is almost gone, surely acceptable.
I also founf that the mechanical structure has to be pretty robust to minimize microphonics.
Probably the use of a lower mu triode for the first position can help.
I have some E182CC laying around... might use these but I have 2 doubts:
-Does it make sense to use a higher current tube in the first position?
-Will it fit my chassis? (have to check but I'm afraid it won't..)
Cheers
Andrea
I rewired the Aikido using stropboards and now the noise is almost gone, surely acceptable.
I also founf that the mechanical structure has to be pretty robust to minimize microphonics.
Probably the use of a lower mu triode for the first position can help.
I have some E182CC laying around... might use these but I have 2 doubts:
-Does it make sense to use a higher current tube in the first position?
-Will it fit my chassis? (have to check but I'm afraid it won't..)
Cheers
Andrea
Hi all,
Just finished mine this morning using 12SN7 tubes and really enjoy what I'm hearing so far except for this.. there is a faint hum on the right channel woofer(but acceptable), no hiss from the tweeters. however, on the left channel, there is also hum and a sort of constant hiss or static from the tweeters.
I have already swapped tubes, replaced tubes but to no avail. I have checked my star grounding and the chassis is grounded via the input jacks. I have measured voltages and have what i expected, input tubes run at 4-5mA while output tubes are at 8mA. Wire dressing (used shielded wires for signal) and layout was done with care.
I power the heaters with AC from a 6.3-0-6.3 and referenced the "0" to an elevated +80V supply. My ht is a CRCRCRC and measures +298V. Plan is to use an ss voltage follower but due to excitement it hasn't been implemented.
I don't think noise is from psu since one channel only has hum while the other channel has hum+hiss/static noise. Could it be the heaters due to the AC supply instead of DC?
Any thoughts, recommendations would be greatly appreciated.
Many thanks!
JojoD
Just finished mine this morning using 12SN7 tubes and really enjoy what I'm hearing so far except for this.. there is a faint hum on the right channel woofer(but acceptable), no hiss from the tweeters. however, on the left channel, there is also hum and a sort of constant hiss or static from the tweeters.
I have already swapped tubes, replaced tubes but to no avail. I have checked my star grounding and the chassis is grounded via the input jacks. I have measured voltages and have what i expected, input tubes run at 4-5mA while output tubes are at 8mA. Wire dressing (used shielded wires for signal) and layout was done with care.
I power the heaters with AC from a 6.3-0-6.3 and referenced the "0" to an elevated +80V supply. My ht is a CRCRCRC and measures +298V. Plan is to use an ss voltage follower but due to excitement it hasn't been implemented.
I don't think noise is from psu since one channel only has hum while the other channel has hum+hiss/static noise. Could it be the heaters due to the AC supply instead of DC?
Any thoughts, recommendations would be greatly appreciated.
Many thanks!
JojoD
I'm with you on this problem also. I too have been having some problems with hum and hiss on my pre amp.
In my case, I was using 5687s and trying to use AC for my heaters. I don't think they like AC heating and I didn't feel like building a DC circuit so I just rewired the socket and tried some 5963 tubes I had lying around. The amount of hum went down and now isn't too bad. Not great but better. I can now hear some hiss that was behind the hum which I am trying to deal with.
I'm not sure if playing with the B+ or current flow through the tubes would help or not. I'm not smart enough to figure this out. Right now I have about 250VDC B+ and about 7ma flowing through each section. Seems fine to me but what do I know.
I'm also having problems with too much gain because I can only turn the volume pot less then a quarter to get full output. I might try a pad at the input to see if that helps in this area.
Don't give up yet. I've been playing with this amp for about two months now. It started out looking great but now is a mess because of all the things I've tried. Hummm, maybe thats why there is so much noise in my circuit. Time to rebuild and start over??? 
Last resort is to try and find a scope. Its easier to troubleshoot with one. Or try using your voltmeter and set to AC millivolts and probe around. Hopefully you'll find something. Good luck.
The last-last resort is to buy Bas' PCB of this Aikido pre-amp. If my last efforts don't pan out then I'll be getting a pair of those real soon.
In my case, I was using 5687s and trying to use AC for my heaters. I don't think they like AC heating and I didn't feel like building a DC circuit so I just rewired the socket and tried some 5963 tubes I had lying around. The amount of hum went down and now isn't too bad. Not great but better. I can now hear some hiss that was behind the hum which I am trying to deal with.
I'm not sure if playing with the B+ or current flow through the tubes would help or not. I'm not smart enough to figure this out. Right now I have about 250VDC B+ and about 7ma flowing through each section. Seems fine to me but what do I know.
I'm also having problems with too much gain because I can only turn the volume pot less then a quarter to get full output. I might try a pad at the input to see if that helps in this area.
Don't give up yet. I've been playing with this amp for about two months now. It started out looking great but now is a mess because of all the things I've tried. Hummm, maybe thats why there is so much noise in my circuit. Time to rebuild and start over???
Last resort is to try and find a scope. Its easier to troubleshoot with one. Or try using your voltmeter and set to AC millivolts and probe around. Hopefully you'll find something. Good luck.
The last-last resort is to buy Bas' PCB of this Aikido pre-amp. If my last efforts don't pan out then I'll be getting a pair of those real soon.
JojoD818 said:
I have built a nice sounding Aikido 6SN7
, BUT, microphony is killing the poor thing.Strangely, not at first , but looks like it gets worse over time. Tapping the case now sets a 'oscilation' into motion of about 4-60 Hz that won't go away until after some time. Only the left channel is prone to this. I measured it with a frequency meter and saw that it was around 56 Hz, now, and other low frequencies at other moments. It is like a drumming sound. After some time is fades away, the frequency drops. When it is active, the natural hiss I see DISAPPEARS. Thats strange. The oscilation is quite 'clean'. I had a 2nF across the feeding cap.
It started all at once.
I never heard that the Aikido is an amplifier to 'turn on once only'. I put the scope on the first anode and saw that the fualt lies in the first stage.
I have changed tubes ('rolled' tubes as I understand that this means) and found e.g. my Kenrads very microphonic; now have Sylvania but the amplifier is still prone to oscilate. RCA got into a metalic noise that stopped when I tapped the tube. Etc.
Hypothesis:
1) to me this might come from the elevated heater. This might identify why it deteriorates over time... I have it at 90V. And 90V should be OK. The GE datasheet says max positive 100V. And maximum negative (the top triode) is 200V. So I should reduce it to 50V and reduce the Vb to below 250 volt maybe?
2) The first stage runs at 3,5 mA (at 1k5 cathode resistor. now installed a lower value of 820 ohm) but it remains. (output is 434 ohms and 8 mA)
3) I used to have 6.3 V AC but now I have a LM350K regulator at 6,3 V (which has a nice max current of 3A which softens the start-up) Still that did not take out the hum.
4) I have a LCR feed with a choke up front and 1.200 muF, dead silent, really. At 2MV no single movement seen. No hum from there. Each channel has 40 muF dedicated, taken with a 870 ohm resistor from a common 40 muF. This really should
5) the DC V+ might be too high, at is at 323 Volt. I added a resitor to reduce it to 285 volt. Still no improvement.
This leaves me with the spagetti.
6) At a mu of 20, I really wouldnot expect 'singing around' to be the source of the trouble. I have gridstoppers on all tubes. 100 ohm (right channel) and 47 ohm (left channel, the one at fault). Broskie has 300 ohms. I change the ones from the first stage to the second to 330 ohms . yes --- some improvement!
7) or the power supply. Maybe, because the two top triodes in mathematical terms are a single tube (this is what once was derived in the magazine L'Audiophile when describing a 211 schema) MAYBE there should indeed be a single feeding wire.
8) I took out the PS-feedback that John Broskie so nicely added. No resolution.
I'm sure I'm not the only one seeing these problems. Who has an idea? I've burnedfingers quite often, doing it for ages, but still -- strange.
9)
Well, believing...
Still, 1) the elevated heater looks like the most possible cause; next is the PS with separte feeding lines which introduces an induction (who said something about 'lead' in the amplifier?).
Solder getting 'lazy', connections being messed up...
Glad I'm not alone here!
albert
triode_al said:
I have built a nice sounding Aikido 6SN7
--> Solder getting 'lazy', connections being messed up...
After a lot of changes, I have now fixed the problem of 'microphony'.
It was due to reused octal chassis parts. The pins had lost contact and in one of the sockets, solder had gone between the ceramic and the pin-squeezer so this could not move freely. This resulted in bad contacts, maybe with some diode action
(the sockets came from an OTL 6AS7 power supply). At a certain moment I could see this on the scope as well (two stable states). As well I rechecked the soldering with a magnifying glass, and found solder was not always applied really evenly on a part or a wire.
Instead of installing new sockets I cleaned them, bent them and the sound is now OK, both just after power up (the expanding of the metal parts created the movements) and when tapping the tubes (this just changed the contact a bit and that sound is what I heard).
I also restored the AC 6.3 v heater (which I had thought a possible source of trouble) and this gives no hum (very little, it can be decreased depending on the actual tubes, with a hum-balancing pot).
Now I'm happy again with my Aikido.
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