[ColoKurt]

Hello all, I just finished building a Cornet 2.   The word "I" should be in quotes since a friend of mine soldered the board.  Anyhow, when I power it up the LED goes from red to green just fine, but some of the voltages inside seem high.  For example, the 150V test point at R212L measures at over 170V instead of 150.  The value at R212R was more reasonable at 159V.  I was going to check the transformer voltages and the pins on the rectifier tube but I don't see anything to compare them to.  Is there a schematic available other than what's on the Cornet 2 website?  I've looked at all those and I don't see those reference voltages listed anywhere.  It's been a while since I looked at a schematic though so maybe I'm just missing it.  Any help will be appreciated.

[hagtech]

What does you AC mains measure?  Are you sure you have the right tubes in the right spots?

jh

[ColoKurt]

I checked the mains in the lab where we did the final assembly and I did the initial measurements and it was ~123V.   I'm sure that the 12AU7 tube was in the right spot.  I  guess I can double check that the tube I bought really is a 12AU7.   The rectifier tube seated itself very nicely in the socket but the three smaller tubes really did not.  I was actually worried that they were going to pop out but I pushed them in as hard as I dared to.

[ColoKurt]

All the tubes seem to be in the right places.  I'll measure the mains at home and some more test locations.  Should I measure the pins of the rectifier tube and post the voltages?   I assume that all the voltages past the rectifier tube are measured DC to ground.  I'm thinking at least some of the rectifier pins should be AC voltages.  Right?  Any further suggestions?

[poty]

Did you check if the device produces some sound?
Many of the voltages are marked on the board. The increase can be from the rectifier tube - what type do you use? Even with all parts as marked there could be some deviations in the voltages.
Other voltages can be counted from the schematic. For example, first stage draws 1ma (300V[on top of R202]-150V[on bottom of R202]/150k[value of R202]). There is 300V/330k=0.9mA through R206. Second stage draws the same - 1mA. The current in the third stage is marked on the schematic - 6mA. Total is around 9mA. Then at the plus wire of the С103 there should be 330V[on top of R216]+9mA*3.6k[R221+R220] ~ 362V.
You should also check the voltages across the C209=C210=C211 (heating voltage). It should be around 6.2V.

[ColoKurt]

I have the SovTek 5Y3GT.  Thanks much for the suggestions.  I'll have time tomorrow to play with it.

[ColoKurt]

My Cornet 2 does produce music and with no hum, so that has to be a good thing!  I haven't done any really critical listening yet but there's nothing obviously wrong with the sound.  The noise floor is about the same as using the internal MM stage on my Mac C220 preamp.  I noticed in pictures that some folks twist the short leads to the RCA pins.  I didn't do this.  The wires are only about an inch long but I can do this if it's important.

I tried to check out the square wave response on a scope but for some reason I couldn't get any signal out.  At first I thought it was the inverse RIAA filter I was using, but that seemed to work fine when I was playing music through it using my phone.  I guess I can try this again later.

As far as the internal voltages go, the only ones I see that are somewhat out of spec are 177V (should be ~155) at R219L and 346V (should be ~365) at R107.  Also, the voltage at R212L is a little high at 170V but perhaps more worrisome is that it seems to drop over time, maybe 1V every 30 min.  Is this to be expected with tubes?  I stopped measuring after about a half an hour after powering up but it did seem to be dropping steadily.

My heater voltages are only at 5.6V.  The game here then is to tweak the value of R223 in order to get this as close as possible to 6.2V?

[poty]

My Cornet 2 does produce music and with no hum, so that has to be a good thing!

Well if both channels are working - at this point there should be something not very important.

...177V (should be ~155) at R219L ... at R212L is a little high at 170V

The voltages are closely related (the voltage at R212 defines the voltage at R219 together with the bias setting cathode circuit). So - the problem is in the 12AX7s part.

... the voltage at R212L ... seems to drop over time, maybe 1V every 30 min.

You always mention L - left channel. Is it the same in the right channel too?
I guess something wrong (cold solder joint?) with the R206. When you measure the voltage you add another leackage path through the DVM, which adds some voltage drop across the R211. There are C201 which support the voltage for some time then started to gradually discharge - that is why the voltage is changing over time.

My heater voltages are only at 5.6V.  The game here then is to tweak the value of R223 in order to get this as close as possible to 6.2V?

Really strange. Maybe the value of the R223 is wrong?

[ColoKurt]

That's great feedback poty!  Thank you very much.  I do have some other 12AX7's I can swap in so I'll try that.   Maybe I'll just swap the tubes left and right first and see if anything switches sides.  The voltages that I measured on the right side are all within +/- 10V of the specified value; it's only on the left that there was deviation.  I may not have a chance to do anything tonight since I have to run to the airport, but I should be able to do some more poking around tomorrow after work.

[hagtech]

Yes, try and bring up the heater voltage a bit.  5.6V is too low.

jh

[poty]

I do have some other 12AX7's I can swap in so I'll try that.   Maybe I'll just swap the tubes left and right first and see if anything switches sides.  The voltages that I measured on the right side are all within +/- 10V of the specified value; it's only on the left that there was deviation.

Sorry, I should have been more precise. When I said "12AX7s part" I mean part of the circuit, stages, not exactly the 12AX7 tubes. By the way, the first and the second 12AX7s work for both channels, not "one tube - one channel", so switching the tubes won't switch sides.
If the problem is only in one channel then my guess about R206L is becoming more solid. Reflow the resistor connections and try to measure once more.
The suggestion about the heating voltage is also good.

[ColoKurt]

Son of a gun.  You learn something new every day.  I always thought it was one tube per channel.     I guess that's why I never was very good at circuits.

[ColoKurt]

I finally got back to my Cornet 2 after some diversions with good weather.   Anyhow,  the progress was good.  I swapped out R206L and now all the voltages on the left side look to be within 10V of spec.  Thanks poty!   I also replaced R223 with a parallel combination of three 4 ohm, 3W resistors for a net resistance of 1.33ohms.  This was all I could find at the local electronics shop.  This brought the heater voltage to a nice 6.15V.   (One side note here is a reminder to use a good voltmeter.  The cheap one I was using was reading a little low so I had to bring one home from work.)

Once I hooked it up and started to listen there was an obvious improvement in the sound quality and, unfortunately, some hum.  I currently have the arm grounded to my SUT.  There was no improvement when I grounded the arm instead to the Cornet.  However, the hum did completely disappear when I added a ground wire from the SUT to one of the nuts securing the transformer to the case.  I'm guessing this has to do with my choice of the anodized aluminum case.  Since the anodized layer is non-conductive does this mean I have to scrap it off at all the points where the sides and top/btm come together?   This can be tricky if you don't want to affect the appearance.  Or should I just run an interior strap from the transformer nut to the grounding post?

[poty]

I swapped out R206L and now all the voltages on the left side look to be within 10V of spec.  ...  I also replaced R223 with a parallel combination of three 4 ohm, 3W resistors for a net resistance of 1.33ohms.  ... This brought the heater voltage to a nice 6.15V.

Two news and both good! Congratulations!

Once I hooked it up and started to listen there was an obvious improvement in the sound quality and, unfortunately, some hum.  I currently have the arm grounded to my SUT.  There was no improvement when I grounded the arm instead to the Cornet.  However, the hum did completely disappear when I added a ground wire from the SUT to one of the nuts securing the transformer to the case.  I'm guessing this has to do with my choice of the anodized aluminum case.  Since the anodized layer is non-conductive does this mean I have to scrap it off at all the points where the sides and top/btm come together?   This can be tricky if you don't want to affect the appearance.  Or should I just run an interior strap from the transformer nut to the grounding post?

Unfortunately I don't understand what cured the problem. I do not have the Cornet 2 and know about its layout just by manual. Maybe someone can help you better in this regard.

[ColoKurt]

Yeah.   I went with the anodized case for looks but I don't see what good it does to attach the grounding post to the case when there's a non-conductive coating on it.  I did scratch the coating off inside where I attached the post but since each side of the case is coated this seems like a limited benefit.  If I had to do it again, I'd just get the non-coated aluminum case since I didn't foresee this complication.

[ColoKurt]

Since the entire case isn't grounded because of the non-conductive anodized layer, I added an internal wire grounding the transformer mount.  That solved the hum problem, so long as I don't put my SUT too close to the power transformer on the Cornet 2, that is.   Thanks for all the help Jim and poty.  I'm a happy camper.  The sound is very nice and much better than it was with a heater voltage of 5.6.

[mucool]

Hey ColoKurt,

Just wondering whether bumping the Heater voltages, dropped the other test point voltages? I have similar problem with my C2. After my initial test voltages were high, I changed the R223 to bump up the Heater voltage. But even after bumping it up it still is at 5.7. But now going further lowers all other voltages below the specs.
Did you observe the same thing? Thanks!