Demystifying the Cascode

Basic Cascode

This is what the schemo of the cascode looks like. R_K serves to establish the Q-Point bias for the lower triode, and R_G is its DC grid return. R_P is the passive plate load. The voltage divider connected to the grid of the upper triode establishes its Q-Point bias.

So why would you want to do this? What you have here is a cascade of a grounded cathode stage driving a grounded grid stage. The GC topology has the advantage of a Hi-Z input. However, its high frequency performance is impacted by a high C_Miller that only grows worse with increasing voltage gain.

The GG topology avoids C_Miller for excellent high frequency performance, but it suffers from a Lo-Z input. It's not very often that a Lo-Z input is desirable. However, you can combine the two in a manner that work together. The Lo-Z of the GG stage loads down the plate of the GC stage, reducing its gain greatly. The lion's share of voltage gain comes from the GG second stage. Reducing the gain reduces C_Miller to manageable levels while preserving the Hi-Z input. This gives the cascode a characteristic more like that of a small signal tetrode, with its reduced C_Miller, high voltage gain, less the screen grid "kinks", and partition noise. Unlike a tetrode, the V_GK of the upper triode remains negative, and so you also don't get the partition noise that tetrode screen grids produce. That's where the name comes from: a contraction of "cascade" and "tetrode".

It is for this reason that the cascode is frequently cited as a VHF small signal amplifier. As for what this means for audio amplification, the reduced C_Miller is helpful since a volume pot with a high resistance won't produce the roll-off that bothers designs that place the high gain triode stage up front. The cascode, having higher gain than a single triode gain stage also looked quite useful. As an LTP phase splitter, the cascode could give enough gain to eliminate a second gain stage.

As for sonic performance, there is very little information concerning this. Most of the information I could find related to the design of guitar amps where voltage gain was the emphasis, as distortion is much less of a consideration in such designs. Cascoded LTPs are used quite frequently in solid state designs. However, I could not come up with any examples of hollow state designs that used this topology. Was that just because it was "weird", or required another dual triode, or had this been tried and found to be sonically inferior?

This was another case of try it to find out. As for VT selections, the 6SN7 didn't provide enough gain, and the 12AT7 suffered a fast g_m roll-off with decreasing plate current. Digging into the RCA Receiving Tube Manual (RC-30), I came up with the 6BQ7A -- a dual triode designed specifically for cascoding and operation up to 300MHz. Its μ= 38 is considerably higher than that of the 6SN7, but is still reasonable for this particular design. The main problem with the 6BQ7 is that it isn't an audio tube, has no audio use specified in the spec sheet, and has a very peculiar plate characteristic (undocumented variable-μ feature?). Finding a good audio loadline is not so easy, and this type likes to see a V_PK that's higher than usual. Still, it looked doable, and since this is a balanced topology, it should greatly reduce harmonic distortion if most of that is h₂.

Cascoded LTP Design

There are a couple of bug-a-boos with this topology. The output voltage swing is rather small for the DC rail voltage, the PSRR is less than that of most triode-only gain stages, and the output impedance is very high. This last feature is desirable in an RF amp since that means less loading of LC tuners, and higher loaded Q's. For an audio amp that has to operate over a rather large range of frequencies, it's not such a good thing. A Hi-Z output will interact poorly with the input capacitance of a subsequent stage or other load. To prevent premature roll-off, it is necessary to operate into a Hi-Z, Lo-C load.

The design also uses an active (CCS) tail load. The CCS presents a very high impedance to the junction of the two cathodes. This greatly improves both the phase-to-phase AC balance, and also equalizes the harmonic distortion between phases. It is this distortion imbalance that gives a great many phase splitters inferior sonics. The CCS is a cascode of BJTs. This gives better performance than would the more traditional small signal pentode. The BJT, having much greater gain, gives a higher tail impedance, and a more nearly constant current, with a lower negative rail voltage. This is one area where solid state really is better.

O'scoping the output of the cascode directly resulted in poor square waves with severely tilted tops, indicating a loss of high frequencies. The -3db_v point was barely 20KHz. That's positively horrible! However these were artifacts of the C_i of the o'scope probe, cable, and vertical deflection amp. So this meant it required a "friendlier" load. For that, a cathode follower works nicely. A CF stage has the high input impedance, and the low input capacitance since there is no Miller Effect. Since the voltage at the cathode "follows" the grid, there is very little current through C_GK, so this capacitance all but disappears. When operated into a cathode follower, the true picture emerged: nice flat square waves to 10KHz, a -3db_v point of 117KHz, more than enough for an audio amp.

So how did it sound? In a word: excellent. There were no pentode-like artifacts at all, and the sonics were identical to what a good triode LTP would produce. It beats all the paraphase splitters in their various iterations. This definitely should see much wider use than it does.