DIY CAT SL1 preamp

[HVfanatic]

Hi,

I got the point with the lower cathode resistor. Could you link the datasheet you used?

What I don't understand properly: I thougt, the drive resistor is determined by the wanted drive to the upper valve. Im my opinion, the 10K brings only 25% of drive, so modulation of the upper valve is poor. Or does the higher current improve the amplification factor of the valve, so that only 10 is needed??? Or does the voltage drop becomes unacceptable now with 47K?

I always thought, the SRPP hast to low drive to the upper valve, because the cathode resistor is only determined by bias requirements, and upper valve modulation is a happy sideffect...

I always thought, u-follower is more perfect push-pull then SRPP... And betafollower would be constant current anode load alike.... MHMMMM...

[DF96]

The data I used was on a sheet of paper in a book, but I'm sure google can find a 12AX7/ECC83 datasheet for you or an RCA receiving valve handbook.

Raising the 'drive resistor' reduces the output impedance from the upper cathode. It raises the effective anode impedance of the active load - roughly mu times the resistor. As the ECC83 is happier with higher anode voltage we want to drop as little as possible. 10k gives us a 1M effective load for the lower valve. You could try going a little higher but there is a balance to be struck. The upper valve already has the same drive as the lower valve just from having the same cathode resistor.

Mu-follower is not push-pull hardly at all. It is really a hybrid of active load and bootstrapped cathode follower. Only the SRPP is balanced, and only that when properly designed. There is a reasonable write-up on Merlin Blencowe's site: ValveWizard. The upper valve modulation in the SRPP is not 'a happy side effect' but the whole point of the circuit.

[merlin el mago]

ECC83 datasheet

[jonusgrumby]

This is the circuit I built on the eBay board that I purchased a few years ago. The 6DT8 is basically a 12AT7 without a center tapped filament. I did test this circuit as I built it and I feel the front end is biased in a more linear operating point than the original circuit that used a 12AU7 (in the directions and schematic that came with the board). I used a lower closed loop gain as well. I'm not using this preamp currently but it functioned fine for me.

[HJWeedon]

Hi there.

I took a look at the characteristics of the 12AX7 as published by GE in December of 1961. These characteristics curves show that at 1mA plate current the mu of the 12AX7 is just about 100, essentially independent of operating conditions. When a device behaves that linearly, essentially no significant change in mu over a range of anode currents, the tube has to be the "cat's meow" as far as amplifier linearity is concerned.

In other words, if you operate the 12AX7 as a resistance coupled voltage amplifier with a 210V power supply rail the near optimal operating point should be with a 70k resistor in the plate-lead, (essentially equal to 68k) and the bias of the 12Ax7 should be adjusted to 140V across the tube and 70V across the plate resistor, a little more than 1mA in the plate-circuit.

Vacuum tubes are, in general, so well behaved and so linear, that it is nearly impossible to cobble together a circuit using a 12AX7, that has poor harmonic performance.

Greetings: Hans J Weedon

[HVfanatic]

Originally Posted by DF96

The upper valve already has the same drive as the lower valve just from having the same cathode resistor.

But this sets the idle current, the additional resistor modulates the current to a wished symmetry upper/lower valve... So my unerstanding of SRPP is such:

Lower valve cathode resistor is "local feedback", upper valve cathode resistor derives a drive signal.

u-Follower:The additional resisor ca be used to "replace" the drive from the upper cathode resistor with a correct sized drive, so that upper an lower valve are driven in a symmetric manner. I the current set is couple with 1M, so drive is only done with the additional resistor of the u-Follower...

(My explanation is no load condition-what is the case in the CAT1, because a cathodefollower added...

So, getting to the point Hans showd: Looking at the u-curves of the added datasheet, it seems that 3mA could be a stable u Area.

Hans said, 1mA gives a stable u... I can't see this. Please explain...

[HVfanatic]

Hi jonusgrumby, but:You are using the 1K instead of the 1M in the u-Follower. What is also strange: The cathode follower work resistor is very low valued. I think, this is bad for distortion figures...

[DF96]

Originally Posted by HJWeedon

I took a look at the characteristics of the 12AX7 as published by GE in December of 1961. These characteristics curves show that at 1mA plate current the mu of the 12AX7 is just about 100, essentially independent of operating conditions. When a device behaves that linearly, essentially no significant change in mu over a range of anode currents, the tube has to be the "cat's meow" as far as amplifier linearity is concerned.

In other words, if you operate the 12AX7 as a resistance coupled voltage amplifier with a 210V power supply rail the near optimal operating point should be with a 70k resistor in the plate-lead, (essentially equal to 68k) and the bias of the 12Ax7 should be adjusted to 140V across the tube and 70V across the plate resistor, a little more than 1mA in the plate-circuit.

To get the benefit of constant mu the valve has to see a high (ideally, infinite) anode load. This is because the transconductance (and hence anode impedance) are strong functions of current. 70k is far too small a load for an ECC83. You need at least 10 times the anode impedance, if possible. For that you need either a very high supply rail voltage and a high value resistor, or an active load.

Originally Posted by HVfanatic

u-Follower:The additional resisor ca be used to "replace" the drive from the upper cathode resistor with a correct sized drive, so that upper an lower valve are driven in a symmetric manner.

In a mu-follower the extra resistor adds to the drive from the upper cathode resistor, not replace it. In most cases this means the balance is lost, so we now rely on the linearity of the valve. That is why SRPP is best for non-linear valves, but mu-follower is best for linear valves.

Running an ECC83 at 3mA with low anode voltage means that the grid voltage is low and inside the grid current region. This adds distortion by loading the previous stage. ECC83 is a wonderful valve when used in the right circuit, but behaves badly in the wrong circuit. This may be why it is so unpopular with some people.

[HJWeedon]

Hi guys.

I may have been wrong in my understanding about the operation of the 12AX7 and optimal plate-current. The enclosed GE<PDF> data-sheet shows that the recommended load resistor in the recommended operating region is between 100k and 510k.

But I do agree that 3mA is not good that is in the forward biased region of Grid#1. Looking at the curves for mu, though on page 4 I may have been mislead.

Hans J Weedon.

[HVfanatic]

... and looking in the attached data, around 3k for the cathode resistor seems quite ok.

another point of trouble I may see here: A much higher voltage coule improve a lot: around 600V for the u-follower could be very nice...