[gurevise]

Hi guys,
I have a question:
I have had original Cornet for a while. It worked great. Recently, I noticed that the lower registers sound a bit thin / restricted compared to other phono stages.
So, I have done some Cornet measurements using HagTech iRIAA.
SigGen -> iRIAA -> CORNET -> Scope

Frequency Level relative to 1kHz
20            -2.1 dB   
30            -1.6 dB
40            -0.6 dB
50            -0.5 dB
100            0 dB
1k              0 dB

Also, here are 100 Hz, 1kHz and 10kHz signal images (square wave input signal):
100 Hz signal


1kHz signal


10kHz signal


So, does the frequency response numbers look correct ? I'd like to tweak low end UP a bit. Which resistor/cap to tweak?

Also, I'm not sure why 1k signal has this strange roll off at the leading edge.
What do you think?

Thanks,
Sergy
Dayton OH
USA

[poty]

I have had original Cornet for a while. It worked great. Recently, I noticed that the lower registers sound a bit thin / restricted compared to other phono stages.
So, I have done some Cornet measurements using HagTech iRIAA.
SigGen -> iRIAA -> CORNET -> Scope

Frequency Level relative to 1kHz
20            -2.1 dB   
30            -1.6 dB
40            -0.6 dB
50            -0.5 dB
100            0 dB
1k              0 dB

Taking into consideration that at first it worked fine, I could deduce that some parts have been aged. Most probably - tubes. The EQ in the build is dependent of the tubes and their operating conditions. Then you should try to measure all voltages (to eliminate power supply problems) first, replace 12AX tubes - second.

Also, here are 100 Hz, 1kHz and 10kHz signal images (square wave input signal):
100 Hz signal
So, does the frequency response numbers look correct?
...
Also, I'm not sure why 1k signal has this strange roll off at the leading edge.
What do you think?

The 100Hz square looks like the lower frequencies are too low. The "roll off" at 1kHz means that there is a fall in the higher frequencies.

I'd like to tweak low end UP a bit. Which resistor/cap to tweak?

It may be not so easy, but I'd better learn what has happened with original build.

[WGH]

Like poty writes, maybe you have a weak tube. I have found different rectifier tubes can have a dramatic affect on the amount of bass. A RCA 5Y3GT wasn't my cup of tea so I switched to a 5V4GA which works very well with my speakers in my room.

The heater voltage (H+) also effects how much bass you have, get it close to 6.3 volts (check your manual, the original Cornet may be different) for optimal bass.

Wayne

[gurevise]

I measured plate voltage on AX7 tubes.
They are at around 150 VDC (it varies by less than 5 volts) depending on variety of AX7 tube used.
I think it's normal.

Filament voltage is also at 6.3 to 6.6 depending tubes used. I think, Cornet should work well, as long as filament is at least 6.3 VDC.

I also noticed that supply voltage on AU7 plate varies from 330 to 360 depending on tube selection. I'm not sure if this is a problem.

All in all, I do not see any DC issues.

I'm more concern with how 1 kHz square signal looks like. I simulated combined iRIAA + Cornet schematics in pSpice. According to simulation square signal shall look pretty good, much better that what I measure. I'm scratching my head here. Am I doing measurements wrong or what?

I'm bit uneasy about getting into endless tube swapping for "perfect" sound game
I already have MIT EQ and Auri output capacitors installed.

I'm trying to figure out what is the reasonable measurable target signal.

Sergy
Dayton OH
USA

[hagtech]

Your best bet is to try different 12AX7 tubes.  They make a difference on EQ.

jh

[poty]

I measured plate voltage on AX7 tubes.
They are at around 150 VDC (it varies by less than 5 volts) depending on variety of AX7 tube used.

It's OK.

Filament voltage is also at 6.3 to 6.6 depending tubes used.

More than 6.3 could be slightly a problem.

I also noticed that supply voltage on AU7 plate varies from 330 to 360 depending on tube selection. I'm not sure if this is a problem.

Do you have CCS in the cathode circuit of AU7? I think it is definitely some kind of problem. There is an inconsistence with voltages on AX7s too, because the voltages should be vulnerable to the voltage on the AU7, but you have not had the dependence.

I'm bit uneasy about getting into endless tube swapping for "perfect" sound game

It's not tube rolling game, it's about replacing an "aged" tube with "normal".

I'm more concern with how 1 kHz square signal looks like. I simulated combined iRIAA + Cornet schematics in pSpice. According to simulation square signal shall look pretty good, much better that what I measure. I'm scratching my head here. Am I doing measurements wrong or what?

To be sure all is normal you should look at the signal from your source ("SigGen"), check the iRIAA (it could be the source of bad signal too). Then - check all the caps - maybe some of them are out of the capacitance. But it seems that you have bad tubes though...