[Bill Epstein]

Clarinets been super for a couple of months, rolled in some JAN Mullards tonight and 2 hours later I heard a noise like the needle zipping across the record and 1 tube was glowing red.

I tested both CV4004s and theyre dead, one tube glows red.

Put the unit on the bench and, sure enough, the bias supply test point is getting the full B+.

No smells, no scorches to be seen and all the resistors in series with the tubes ohm out.

I had an issue with the board at C103 and direct connected the film cap I used to R107.

Haven't pulled any of the caps yet thinking maybe one of the Qs died? Would the LED shorting cause this?Those leads are mighty close together.

[tubesforever]

Bill I am running a 9k ohm and 12k ohm step down at R 304 and R 311 to achieve 280v at R306. 

Did you check these resistors to see if you were dropping down enough voltage? 

I am running a Bendix 6106 rectifier tube and had to alter a few resistors to hit the appropriate voltages on the test points. 

Sorry to hear about the Mullards.  These are pretty fine tubes. 

Bill we are talking about your Clarinet line stage aren't we?  Isn't the CV4004 a 12AX7 tube?  It would draw a bunch more current than the 12AU7 circuit is designed to handle--at least 10% more from my memory.   That might cause the tubes and the circuit a major head ache to be sure.

Drop in some 12AU7 tubes and let us know where the R 306 and R311 voltage runs.

Happy New Year Bill, you have been an inspiration for me.  You deserve some fine tunes and broad smiles! 

[Bill Epstein]

Upon further review, the tubes are CV 4003, 12AU7.

[smbrown]

Having a bit more than 280v wouldn't be a big problem since it is split across both halves of the 12AU7. First off, are both channels out? If so, most likely a power supply problem (since it's common to both channels). I'd pull the tubes and check your basic voltages, expect the 280 to be high since there isn't any load on the supply with the tube pulled. Check your heater bias point, where R107 and 108 meet. I'm guessing you should have about 6 or 7 volts there? LEDs correctly lit?

[david62]

Bill,

  I hope that you get your Clarinet sorted out without too much trouble.Most of us just need another project.Happy new year.
Dave

[Bill Epstein]

Still looking for a clue, have full B+ (389vDC without driver tubes) at R314L and R.

R304 and R314 both measure the ciorrect 10k.

Does that make C300 the culprit? But shouldn't R304 and 314 drop the voltage before it gets there?

I guess I'll use the old roady trick of replacing parts until it works

[GRD]

R304 and 314 will drop voltage when current flows through them (Ohms law - current times R = voltage drop).  With the AU7 tubes pulled, no current flows so you'll see the entire B+ at R314.

One guess (I'm no expert) is V301 failed and that gave you the full B+ on R311, and probably took out V300.  If that's the case, then the question would be why it failed.

Also, C103 should tie to ground, not R107. 

[tubesforever]

Hey Bill, have you plugged in some other 12AU7 tubes to check the voltages?

I really want to know where your voltages are with some tubes plugged in and running.

[Bill Epstein]

Getting  370 vDC at both R314 and R311 with 2 known good AU7s, gonna check that again as soon as I have time tonight

[tubesforever]

Bill that's 370V AC right?

I suspect the drop down resistors are fried or you would have the 280 V you need going into the circuit at the B+ drop down rail.  The problem is if the drop downs are cooked then why is there voltage hitting the next point in the circuit? 

What resistors are you using? 

[Bill Epstein]

Problem solved and may thanks for everyones concern and advice.

Epsteins Law: it's always, always something incredibly stoopid

The uber high voltages were on the AC      side of my new and, in my defense, unfamiliar DVMs.

When I switched to DC voltages   all readings are normal.

Apparently, one of the Mullards failed, took the other with it and I got to the shut-off before any damage was done.

We return you now to the discussion of the Cornet 3...

[smbrown]

Bill, that's a relief, I was about to say just pack up all the stuff that isn't working and send it to me and I'll sort it out. Now you're back up and going fine. Good to hear. You like the sound?

[david62]

Bill,

  Is it common for a Mullard tube to fail?Were they vintage tubes?
David

[Bill Epstein]

Not common on true NOS from AES.

When I bargain hunt on E-Bay I expect to lose a third, those that become or arrive noisy. The highest failure rate in my experience is with 9 pin miniatures, 12Axx, etc. 6550s would be next but maybe because a lot of designs tend to flog them. You can back your truck over 6SN7s and they still work. Most amazing is that of the 8 UX-45s, all more than 70 years old, only one has failed.

You want to have rugged, cheap tubes to sacrifice if necessary for your testing after the build is complete. Then you might try NOS from tubes and more or Andy at Vintage Tube who has the most sophisticated testing equipment around. He's slow and sometimes needs to be poked with a stick. His prices may seem high but it's because he only has the best.
http://vintagetubeservices.com/

SAme is true for Kevin Deal.

[david62]

Bill,

   Glad to hear that your Clarinet is working well again.Happy new year.
Dave