# Tube MC Phono Stage - RIAA Calculator / Formula Help?

#### rhthatcher

Paid Member
I'm troubleshooting a friend's MC Phono stage. I think I found the problem with a ground wire connection that was intermittent.

In going through the unit I sketched out the schematic and was looking to learn a bit more about the design. The RIAA EQ components don't look like the "standard" passive or split circuits on RIAA calculator tools. I'm pretty sure this is a low-production volume commercial unit. I stripped out the measured voltages and component values to protect the IP of the manufacturer.

Does anyone know a similar design I can study? Is there a calculator, spreadsheet, or website with more info?

The unit uses 4 12AX7 tubes in total - 3 triodes per channel, one unused. Signal goes from MC transformer into 1/2 of a 12AX7, then EQ, then parallel both triodes in the next 12AX7. The 2nd triode in stage 1 is grounded out.

Also the filaments are run at about 9VDC. I found a little when searching for "starved filaments", but not much. Any theories about this design choice?

Paid Member

#### 6A3sUMMER

9V on the 12V filaments?
AC or DC?
Are they using a 9V Alkaline battery to reduce hum?

What is the B+ voltage at the top of the 12AX7 plate load resistors?

The calculation of the passive RIAA network needs to account for the parallel resistance of the input triodes plate impedance, rp, and its plate load resistor.
The plate impedance, rp, is increased by the mu (u = 100) of the 12AX7 times the un-bypassed cathode resistor.
Example: If the cathode resistor is 500 Ohms, then the rp is increased by 100 x 500 = 50,000 Ohms.
With a normal rp of 58,000 (only normal if there is a bypass cap across the cathode self bias resistor),
the new rp = 58,000 + 50,000 = 108,000 Ohms.

The output of that stage goes to the RC coupling network. That first resistor in the network is also the output tubes grid return resistor, Rg.
That coupling cap and Rg, form a high pass filter.
Perhaps those R and C values attenuate the typical 3Hz that can result from a warp in the vinyl.

Attention to details and careful analysis, proves a design to be good, bad, or mediocre.
Just my opinions.

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#### rayma

Filaments operated at 9V instead of 12.6V would not work well in a phono preamp.
Are you certain there is no problem in the filament power supply?

#### rhthatcher

Paid Member
6A3Summer

Filaments are DC. They have a bridge to CRCRC power supply. The R's a 25W parallel with 10W resistors. My notes show 8.7VDC at the filament power supply when loaded.

I had 4 hours in the car last night and was thinking about this circuit for some of the drive. A thought occurred to me and I need to take another look. I measured the filament supply, but not at the tube socket. I need a closer look at and measurement of the actual wiring at the socket. From memory: + is to pin 4 or 5. The other is to - (to star ground?). And pin 9 is wired to chassis (ground). So it could be that one triode has the filament lit up, and the other doesn't. This got me wondering if the tube is starved (8.7VDC for both triodes) or hot (8.7VDC for a single triode). I'll follow up on that point.

B+ on top of Stage 1 plate resistor is 242V. Stage 2 B+ is 230V
Stage 1 is running ~2.8mA, Stage 2 is ~3.1mA

#### rayma

Either way would be bad.

The series Rs may have increased in value, and so could be dropping more voltage than they should.
Audio tube filaments are very seldom operated at less than 6.0VDC (5% under spec) per section.

#### rhthatcher

Paid Member
Update on Fils.

One triode per tube lights up. One is dark.

Picture is worth 1000 words, right? The dots in the middle are the break between external PSU and the main phono stage PSU.

I confirmed 8V7 on Pin 4, 0V/GND on Pins 5&9.
After the power resistors two wires split off to feed each channel. There is a 270u at the 2nd of 2 tubes in each channel.
Each channel draws about 400mA.

#### rayma

Looks like you'd be close to spec if you rewire the power resistors in series rather than in parallel,
in both locations. The capacitors are woefully undersized, though. Each should be 3300uF or more instead.

See if the waveform on the first filter capacitor looks reasonable, without excessive ripple.
If there's more than 1V p-p ripple, which there likely will be, increase the first capacitor to at least 3300uF.

But this will increase the DC voltage, and require all the resistors to be in series, and possibly will need
even a bit more resistance, to keep the voltage down to 6V.

Looks like the owner had a bunch of 470uF caps, and decided to go with those instead of a more reasonable value.

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Have to point out the contrarian view that perhaps there is nothing wrong with the phono stage design at all. Sure, using a starved filament design may be unconventional, but do you have any evidence that the designer didn't purposely do that for reduced noise and didn't properly design the RIAA eq for the reduced emission? Just fix the intermittent wire and if you want to dig deeper at least MEASURE if something is amiss with the eq.

#### Depanatoru

12AX7 has series 12,6V filament between 4 and 5 , pin 9 is the central tap , unused in this case , what is there is completely wrong
When you want parallel 6,3V filament pins 4 and 5 are shorted together and use pin 9 as the other terminal
Like it is now one triode filament is shorted and the other one is receiving 8,7V instead of 6,3V

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1 user

Did you measure 8.7V with the tubes pulled?

#### rhthatcher

Paid Member
8V7 with tubes installed. One half glows. One half does not. I'm going to try to confirm with the designer... It's easy enough to adjust the CRCRC chain with parts from my box to land at 6.3V. I did a similar thing on a Juicy Music Blueberry recently.

If only half glows then your schematic is wrong.

#### Depanatoru

8V7 with tubes installed. One half glows. One half does not. I'm going to try to confirm with the designer... It's easy enough to adjust the CRCRC chain with parts from my box to land at 6.3V. I did a similar thing on a Juicy Music Blueberry recently.
But you don't need both triodes in one tube ? How is working without filament ?

#### Tom Bavis

Yes 9.8V seems low for the first cap - its value may have gone down or ESR has gone up. A larger cap will be better, with less ripple and lower ESR.

10-11V has traditionally been used for phono preamp heaters - the lower noise outweighs the lower transconductance and emission.

What's with those 10W and 25W resistors? 1/2W would do fine.

If heaters are really wired as shown, half of each is unused and voltage is smokin' high, not low! Pin 9 should be unconnected.

#### rayma

Have to point out the contrarian view that perhaps there is nothing wrong with the phono stage design at all. Sure, using a starved filament design may be unconventional, but do you have any evidence that the designer didn't purposely do that for reduced noise and didn't properly design the RIAA eq for the reduced emission?

Glutinous (40% high), not starved.

#### Depanatoru

Probably the circuit was made for tubes with no central tap , only 4 and 5
Then usually there is a shield at pin 9 connected to ground too .
And with all the filaments running ( and the correct tubes ) the voltage would be lower about 6,3V

#### krca45

There is possibility that only one triode is wired for heaters and other is dormant / non-working ( saved for later use ) - some guys used that so when the wired triode should be changed the tubes just change locations from one channel to the other and the active triode then is the one which was not used previosly.