Ultra Gain Tube Preamp

[Roly]

I've been having a close look at this circuit and there are more than a few curious features that deserve comment.

I have redrafted it so that it has component designators for identification.

Starting at the input, a minor point; it is more conventional to place the grid leak (R1) after the grid RF stopper (R2) than before it, as here.  Unlike in an output stage where the function of a grid stopper is to prevent self-oscillation, in preamp stages the grid stopper is to prevent external Radio Frequencies entering the amplifier, and is therefore normally placed as close as possible to the input socket.

What initially grabbed my attention when glancing at this circuit is the value of R3, the first stage cathode resistor, in conjunction with the value of the anode resistor R4.  The Mullard databook give typical operating conditions for valves such as the 12AX7, and for around 300 volts HT with a 220k anode load (R4) the value for the cathode resistor R3 should be 3k9.  The operating current of this stage is around 0.6mA and the effect of such a low value for R3 is to reduce headroom and increase stage distortion.

The cathode bypass, C1, has been set to a fairly low value meaning that the stage has about a 4dB rise from 100Hz to 1kHz, clearly aimed at tenor guitar.

There are a number of components that seem to have no useful function, and R5 is the first we encounter.  Since there is already a DC path from C2 to ground via R6 and gain control U5, which are also of much lower value, there seems to be no point including R5.

The gain control circuit, R6, C3, U5, seems to be at odds with itself.  The effect of R6 & C3 is to provide some fixed treble boost, however this is minor and doesn't come into effect until about the fourth or fifth octave above Middle-C.  It would be more normal to omit R6 altogether and take C3 to the wiper of the gain pot, then becoming a more conventional "top coupling" cap.

Again we seem to have a component, R7, which has no function and can be omitted.

Given that some top boost has been applied via C3 the inclusion of R8 more than throws it all away again.  The object or function of R8 is obscure, but its effect is to act as an ill-defined top-cut filter in conjunction with the Miller capacitance of the following valve, U2.

An important aspect of circuit design is to try and make the circuit as insensitive as possible to the aging of components, and to replacements during repair.  The main effect of R8 is to make this stage more sensitive to changes in the Miller capacitance of U2, and to different Miller capacitance of any replacement valves inserted at that position.  Similarly R8 is large enough to add bias due to grid leakage and thus make the actual DC operating point of U2 less predictable.  It is also worth mentioning that because the grid leakage current can flow through the wiper of the Gain control it will tend to make this scratchy in operation, through no fault of its own, and which no amount of cleaning will cure.

The value of cathode resistor R9 is spot-on for its anode load (R10) of 100k.  Again the low value of the cathode bypass (C4) shapes the low frequency response of this stage towards tenor guitar.

Having obtained around x60 gain from U1 and x55 from U2 (~x3300) we now encounter R11 and 12 which form a fixed attenuator of x0.6, reducing the effective gain at this point to 3300*0.6=1980 or about x2000.  The reason for doing this is not at all clear.

Again we encounter a grid series resistor, R14, which has a much lesser effect than R8, similarly seems to only make this stage less predictable and more sensitive to device variation in U3.

More predictable is the effect of C6 across the anode load of U3 (R15).  This provides a heavy roll off above about 1kHz, negating the effect of C3.

I would personally reserve the title "Ultra gain" for a preamp where all the stage anode loads were constant current sources, thus making them Maximum Available Gain or MAG stages, but in stages U3 and 4 we see that the cathode bypass resistors aren't bypassed, meaning the application of local negative feedback and quite a bit less than MAG from each stage.

Similarly R17 at 470k provides less stage gain than by using a 1Meg.

Again we have a large value grid series resistor in R18 which has a similar effect to R8 discussed above.  By the time we get to the entry to the tonestack at C8 we have about 120dB of available gain, but we also have a pretty savage overall roll off above about 1kHz.

And again in C8 and R21 we seem to have two components that serve no useful function.  Both could be omitted (as they are in many similar amplifiers) and the anode of U4 taken straight into the tonestack since the DC path is already blocked by C9, 10 and 11.

Phil has already touched on having the unbuffered output of a High-Z tonestack subject to following loads, and with this particular Marshall configuration nothing less than about 1 megohm load will produce acceptable performance.  The use of loads as low as 47k, particularly at high setting of the volume control U9 will result in savage tilting of the response downwards into the bass from about 1kHz.

By re-positioning the tonestack between U3 and U4 it could be fed either conventionally from the anode of U3, with R16 bypassed to provide high gain (or perhaps from U3 as a cathode follower), and U4 could certainly be used as a cathode follower output buffer which would make the tonestack totally indifferent to output loading.

Taken overall this is a preamp that isn't sure of what it really wants, with contradictory and ill-defined response shaping, gain mixed with attenuation, and the use of stages (particularly U3 and 4) which aren't employed to best effect.