[sonata149]

After being impressed by the Bugle I had to try out the Cornet.  However I used the 6sn7 instead of the 12au7.  The result is so disappointing that I must have done something wrong.  I have measured the voltages around the valves and I got the following readings;

HT supply to 6sn7                                327V    327V
HT to V1-V2 (ecc83)                            288V    287V

Now this is where the problems start:

At anodes of ecc83 (V1, V2)                     3V        3V     (150V specified)
At anodes of 6SN7  (V3)                        322V    322V
At the grids of 6sn7                                  2V      3V
At cathodes of 6SN7                               56V     77V
At R219-C208 junction (on V3 cathode)      54V     76V     (R11-C6 on the Octal schematic)

I also checked the CCS circuit (same as for the ecc82)

Filament V+                                           6.33V
Base Q200 (MJE340)                               4.10V  (6.2V specified)
Emitter (Q200)                                       8V
Collector                                               50V     75V

I would very much appreciate your comments and suggestions as to the possible cause of such readings (and bad sound!)

Thanks in advance.

[hagtech]

Wow, where did you get a CORNET board?  I haven't sold one of these blank in about two years.  Did you get the pinout conversion on the octal right?  Is the transistor installed backwards (it has a reverse pinout)?  Wait a minute ... what did you build here?  This sounds like a mix between the CORNET and the CORNET2.  Q200 is not an MJE340.

jh

[sonata149]

Thanks Jim for your patience and support.  Maybe I could have explained myself better in the first message.

As you guessed, the circuit I built uses the first two (input) stages from the Coronet (12ax7) and the output (cathode follower) from the Octal version (using the 6sn).  I also included the CCS circuit published earlier on the Forum (with MJE340) replacing the CF 30K resistors. The CCS transistor has 510Rs on its base and emitter.

The section from Coronet 2 goes up to the grid of the 6sn CF, which uses the CCS circuit described above and the output components from the cathode (1uF and 300K) as per the Octal schematic. I did this as an experiment, seeing that the two circuits use the same topology and the 6sn7 seemed a stronger candidate in the output stage. Don't the two circuits mix?

Meanwhile I'll check my component layout (especially transistors) and wiring/soldering again.  Do you still have blank boards for the Coronet Octal? - I'm interested.

Thanks a lot,

Joe A

[hagtech]

Can you post a photo?  Makes it much easier to help debug.  I never made any octal CORNET boards.

jh

[sonata149]

Hello Jim,

Before taking the photo I rechecked everything and realised that what I had taken for granted were the two 0.1uF coupling capacitors on the first stage.  These were OS (old stock) PIO (paper-in-oil) caps. From dates shown on other similiar items these must be over 30 years old but I thought the PIOs would give me that special sound.  I decided to change them to ordinary polyprops and the result was spot-on.  The Coronet came to life with its renowned bass and sweet treble. Midrange is articulate and full-bodied.

Although I'm using a low powered triode-connected SE KT66, driving Yamaha NS1000 (claimed 90db/W sensitivity), the volume is still 'robust' in my 3 x 4 metre listening room (rather heavily furnished).  Signal input is from a Garrard 301/Rega/Nagaoka MP20 directly linked to the preamp section of a CJ DF1 cd-player. I feel there is good synergy between various system components.

Thanks to your input and suggestions I have been prodded to re-check and re-evaluate my work.  Needless to say,  Congratulations!!!  on a very well-designed and satisfying phono preamp. The preamp is still breaking in and I still have to check that the voltages at the various stages are correct.  But for the time being I'm captivated by the sound of the Coronet 2-6sn7!!

Best regards,
Joe A