That's to protect the 12AT7 cathode follower from excessive heater to cathode voltage differential.
The rated maximum heater to cathode voltage of most tubes is +/-100V.
Exceeding that maximum will degrade the cathode of the tube over time. You will eventually hear loud crackles from that tube.
With the heater supply grounded to 0V, and with the 12AT7's cathode at 151V, the 12AT7 cathode will be 51V over the maximum rating of +/-100V.
Lifting the heater supply up to 70V reduces the heater-cathode voltage to 151 - 70 = 81V, so the 12AT7 cathode will be +81V above the heater supply, while the 12AX7 tubes' cathodes will be about -70V below the heater supply. That way both tubes' heater-cathode voltage will be less than 100V.
The voltages you posted don't quite compute.
Can you post the voltages at the plates of the 12AX7 triodes? That would be at pins 1 and 6, referenced to ground.
Last edited:
First step, check voltages of both channels, at each tube socket pin.
Voltages should match channel to channel within about 10% or so.
If the silent channel is much different from the working channel then there must be a wiring mistake somewhere.
I'm glad to hear you have sound from both channels now. However, the voltages don't look right.
If the B+ to the second stage is 283V, and the B+ to the first stage is 256V, at the other end of the 36k resistor, that's 27V / 36k ohms = 0.75mA
That means the first stage 12AX7 is drawing 0.75mA plate current.
Since the plate resistor of the first stage 12AX7 is 150k,
150k ohms * 0.00075A = 112.5V dropped across that 150k resistor
256V - 112.5V = 143.5V
The voltage at the plate of the first stage 12AX7 should be 143.5V, not 150V.
As JC Morrison once said to me, "It's Ohm's LAW, not Ohm's suggestion."
If the B+ to the second stage is 283V, and the B+ to the first stage is 256V, at the other end of the 36k resistor, that's 27V / 36k ohms = 0.75mA
That means the first stage 12AX7 is drawing 0.75mA plate current.
Since the plate resistor of the first stage 12AX7 is 150k,
150k ohms * 0.00075A = 112.5V dropped across that 150k resistor
256V - 112.5V = 143.5V
The voltage at the plate of the first stage 12AX7 should be 143.5V, not 150V.
As JC Morrison once said to me, "It's Ohm's LAW, not Ohm's suggestion."
You're right. Sorry.
Value of resistors form B+ to B++ not as showed because my phono include preamp. There are total 6 tubes: 12AX7 x 3 + 12AT7 x 1 + 12AU7 x 2
You help me calculating again resistor.
Thank you.
Hello, is this possible to replace the above 1/2 12AT7 cathode follower with a source follower ? The reason is limited chassis surface (two noval sockets available). I'm considering usage of STU9HN65M2 (N-channel 600V MOSFET also applied in Baby Huey as output tube driver). Any hints for such an approach? Is the CCS also recommended to be applied, or the resistor between source and ground is fine? The load of the above phono stage will be a volume potentiometer of 100 kOhm (preceded by input switch). Thanks ahead for any advice.
That is OK voltage ... 143.5V and 150V not a big difference. Who knows if this tubes are 100% good, etc. You have simulation voltages which you cannot 100% get in real life.
Now it would be good me measure RIAA curve of this built phono.
I'm not comparing the measured voltages against the simulation voltages. I'm trying to square the measured voltage readings with Ohm's Law. It could be that the resistor values in the built circuit are different from what is shown on the schematic.
I would definitely be interested in measurement of the RIAA response.
I would definitely be interested in measurement of the RIAA response.
0.7mA to 0.75mA is nothing. ;-) maybe resistors, but maybe even tubes. I would not be concerned about that little difference.
RIAA curves would show if everything is by the book.
Regards
RIAA curves would show if everything is by the book.
Regards
Yes, of course.
You will want to choose a MOSFET rated for high enough drain-source voltage (500V or more would be good), and with low input capacitance (Crss) because the 12AX7 has low plate current of only 0.75mA which cannot drive a high capacitance load without possible slew limiting.
STU9HN65M2 looks like a good choice, with Crss = 0.85pF (very low!) and Ciss = 325pF.
A CCS is not necessary. The MOSFET source follower with resistor load is already cleaner than a triode cathode follower.
A 100k ohm load is a fairly light one. A 12AT7 CF should be able to drive that well.
No, I'm not worried about that.
I want to see the voltages match up with the resistor values to make sure the circuit was built as designed.
For instance, if the plate resistor on the first stage 12AX7 is different from 150k, that can affect the source resistance presented to the RIAA filter.
Thank you for your replies. Highly appreciate. The source follower transistor has been selected by author of Baby Huey PP amplifier (EL34 based), so I wanted to hear your opinion about its applicability for the output stage of RIAA preamp. As I said before, the MOSFET source follower will be loaded with a 100kOhm (volume potentiometer), this value seems to be somehow common for a currently built tube amplifiers intended to be driven also by CD/DVD/PC. Thanks!
In a follower, Crss isn't usually a problem even with normal MOSFETs because it's still much smaller than the Miller capacitance a12AX7 usually has to deal with. Even your grandfather's IRF820 would do.
Well, that's good to know! Somehow I'd gotten the impression that the old IRF840 or similar old MOSFETs would at least theoretically cause some likelihood of distortion rising with frequency, getting to be a (admittedly minor) problem as you up to 15kHz or so. But if that's not a concern even with a 12AX7, then there you go. Use any old MOSFET with a high enough Vds and there you go. Good thing, too. I've collected a bunch of IRF820s and IRF840s from surplus sales. I was using them for power supplies, but I guess they're good for source followers too.