Hello All,
My knowledge of tubes is very very limited, so I would appreciate some help with a preamp problem.
I built an Assemblage L-1 preamp from a kit back in 1999. It has performed beautifully since I first powered it up. I leave it powered up 24-7. This is a line-level only preamp with a single tube that is shared for both channels. In 2011, I did the first tube replacement (Sovtek 6922). A few months ago I noticed some microphonics in the right channel. I did some measurements with a QA401 and found the right channel gain was about 0.3dB lower than the left channel.
So, I replaced the tube with a Genalex Gold Lion E88CC/6922. The gain is now identical between the two channels but has much more variation across frequency. The first figure is before the tube change while the second figure is after the tube change. I also measured THD across frequency, and, on average it is about the same for both channels (0.01% to 0.02%), but does show a little more variation after the tube change . My question - is the variation with frequency simply a function of the tube such that a different brand might perform differently, and hopefully with less variation.
Thanks in advance for any help.
ceulrich
My knowledge of tubes is very very limited, so I would appreciate some help with a preamp problem.
I built an Assemblage L-1 preamp from a kit back in 1999. It has performed beautifully since I first powered it up. I leave it powered up 24-7. This is a line-level only preamp with a single tube that is shared for both channels. In 2011, I did the first tube replacement (Sovtek 6922). A few months ago I noticed some microphonics in the right channel. I did some measurements with a QA401 and found the right channel gain was about 0.3dB lower than the left channel.
So, I replaced the tube with a Genalex Gold Lion E88CC/6922. The gain is now identical between the two channels but has much more variation across frequency. The first figure is before the tube change while the second figure is after the tube change. I also measured THD across frequency, and, on average it is about the same for both channels (0.01% to 0.02%), but does show a little more variation after the tube change . My question - is the variation with frequency simply a function of the tube such that a different brand might perform differently, and hopefully with less variation.
Thanks in advance for any help.
ceulrich
Attachments
That second picture is not normal at all.
Assuming the measurements were done correctly, something went really wrong.
Assuming the measurements were done correctly, something went really wrong.
That is not normal, perhaps due to noise or instability. Is it repeatable?
Try a different tube. Can you post the schematic and some photos?
Try a different tube. Can you post the schematic and some photos?
Put the old tube back in and check your measurements again. The second image you posted looks like something went very wrong with the measurement process.
should be flat, my 6922's are flat to 0.05db 25hz - 40khz 30 volts p-p.
the low rolloff is due to the .33 uf coupling cap to the volume pot.
the low rolloff is due to the .33 uf coupling cap to the volume pot.
I suspect there's some sort of intermittency, which shows up simply because the sweep takes time. Check the socket is tight and free of corrosion.
Basically a wide-band amplifier cannot show rapid changes in response with frequency, there's no physical mechanism to allow it as nothing should be resonant anywhere in the band. Therefore this is a time-domain issue.
Basically a wide-band amplifier cannot show rapid changes in response with frequency, there's no physical mechanism to allow it as nothing should be resonant anywhere in the band. Therefore this is a time-domain issue.
Thanks to everyone that responded.
You have confirmed my suspicions that I probably have a measurement problem not a tube problem.
It was repeatable over a four-hour period on 05-08-20, as I tried different settings for the QA401 software. I have used the QA401 for a couple of years now, on many different projects, and have not had this problem before.
I trashed the old tube a few weeks ago, and I only bought one new tube, so if I do not get this solved in a day or two, I will order more tubes.
Assemblage is no longer in business, but they were part of The Parts Connection which is still in business, so, it is probably illegal to publish the schematic. Perhaps a moderator could confirm this.
In my unit the input coupling cap is 1µF.
When I get this solved, I will report back.
Thanks again all.
You have confirmed my suspicions that I probably have a measurement problem not a tube problem.
It was repeatable over a four-hour period on 05-08-20, as I tried different settings for the QA401 software. I have used the QA401 for a couple of years now, on many different projects, and have not had this problem before.
I trashed the old tube a few weeks ago, and I only bought one new tube, so if I do not get this solved in a day or two, I will order more tubes.
Assemblage is no longer in business, but they were part of The Parts Connection which is still in business, so, it is probably illegal to publish the schematic. Perhaps a moderator could confirm this.
In my unit the input coupling cap is 1µF.
When I get this solved, I will report back.
Thanks again all.
Yes, there is some gray area regarding posting schematics. Since Assemblage is long out of business and the product is long ago not for sale, I think you're probably OK posting the schematic. If anything, the moderators can just take it down if anybody is unhappy about it.
Hello All,
I did have a measurement problem and not a tube problem. I ran the QA401 in loop-back mode and got the same strange results. The QA support staff sent me straight to the source of the problem, I had accidently misadjusted one of the settings. My preamp now measures ruler flat, and sounds great again.
Cheers,
ceulrich
I did have a measurement problem and not a tube problem. I ran the QA401 in loop-back mode and got the same strange results. The QA support staff sent me straight to the source of the problem, I had accidently misadjusted one of the settings. My preamp now measures ruler flat, and sounds great again.
Cheers,
ceulrich