The gain required was only ≈10dB (≈3.2x) and the output impedance needed to be lower than 2kΩ ~ The distortion spec. was 0.1% at 8dBu (≈2V^_rms) output ~ Frequency response from 2Hz to 40kHz (at –3dB points) ~ Output load was not specified other than "long cables"

There are many ways to meet the above spec. and many were discounted due to overall complexity increasing the size of the finished layout which had to fit in place of the QUAD22 EF86 valve bases ~ Use of a Pentode/Triode such the ECF80 looked like an option but the end user of the amplifier had many 12AT7 12AU7 and 12AX7 based products and had these and many ECC81–83 as spares so using one of these valve types became the first choice

Topologies like the µ–Follower and the now popular Shunt Regulated Push Pull (SRPP) were avoided as they both require a higher HT supply voltage than that available inside the QUAD 22 ~ In practice both these arrangements also suffer from gain variation due to valve type and condition unless feedback is somehow applied

As the "boys own book of valve amplifiers" states ~ When using a triode valve the anode impedance r_a restricts the gain and the maximum gain or µ is only achieved with a constant current anode load ~ The cathode must also be held at signal ground with either a large value bypass capacitor across any cathode resistor or by using a "fixed" bias scheme (designs that require batteries or multiple odd value supplies should be avoided)

Having achieved the maximum gain it should not be reduced by loading so a cathode follower should be used to buffer the gain stage from the output load ~ However in practice a specific gain is often required and this is achieved along with a reduction in distortion by use of overall negative feedback which may introduce instability and increases the complexity and size of the "design"

A simple way (not from the boys own book) to get a reasonably high and predictable gain with low distortion and low output impedance and without having to worry about overall d.c. and a.c. feedback calculations etc. is shown below ~ This circuit will provide 20dB or more wideband flat gain with a low output impedance

The gain of a valve amplifier can be calculated if two of the values µ or g_m or r_a are known or can be determined from published data for the valve used ~ There are several ways this can be done ~ The traditional method of measuring from published 'curves' is very awkward when operating at low anode current or voltage as this amplifier does

The ECC81 as used above will be operating with an anode voltage of about 70V ~ The cathode is at 30V so V_ak is only about 40V ~ Extrapolating from 12AT7 or ECC81 tables for V_ak=100V gives g_m≈2.6mA/V and µ>52 ~ For a lower V_ak  µ will tend to be higher for the same I_a and the r_a curve tends to "flatten out"

The ECC81 internal anode impedance r_a for V_ak ≈40V is increased by R_k =27kΩ to r_a' and will be about 20k + (52 x 27k) + 27k   or   r_a' ≈1.3MΩ .  .  .  see calculations from characteristic curves ~ The values of µ g_m and r_a may be lower than expected when operating at low V_ak or a high I_a due to grid current ~ see ECC83 comments

The anode load R_L' used for gain calculation is R23 100kΩ in parallel with r_a' 1.3MΩ or R_L'≈93kΩ ~ The reduced gain due to R_k feedback is ≈R_L'/R_k or about 3.45x ~ In practice the gain after the cathode follower is about 3.2x or 10dB ~ You may think you have seen this circuit already but here V1a is not auto biased because R_k is far too high

In a normal auto bias arrangement R_k would be a lower value and in many circuits would be bypassed by a large electrolytic capacitor but in this circuit the grid of V1a is raised +ve by R22 to prevent the reduction in I_a due to the large value of R_k ~ Resistors R22 or R29 or both can be adjusted to set the optimum voltage at the anode of V1a

A way to view the gain of V1a is that the input voltage at the control grid appears across the cathode resistor due to 'follower' action so the mutual conductance g_m of the valve with R_k is a lower value g_mk ≈ 1/R_k and the gain A_v = g_mkxR_L' = R_L'/R_k ~ For higher gains by using lower value R_k the total R_L' is also reduced by r_a' not being increased so much but it will still be much higher than the valves native r_a

For higher gains >15dB the ratio R_L/R_k should be adjusted to be higher than calculated ~ The amplifier signal path requires no high value or electrolytic capacitors apart from the external PSU decoupling and possibly across C15 for an extended Bass response ~ The input impedance is set by R34 R29 and R22 and could be made higher as the parallel combination of R22 and R29 can be the manufacturers maximum g1 resistance for auto bias

Like my Cascode driver for the QUAD II this amplifier topology was developed during 1980s but not made into a PCB at that time ~ For this project I decided to go straight to PCB layout without breadboarding as I knew the design would meet the spec. ~ The only drawback ~ if there is one ~ is that very good HT smoothing and regulation may be required but then good amplifiers should have this anyhow

Like my QUAD II Cascode driver and the QUAD 22 RIAA PCB these PCBs are available to purchase either as plain PCBs or as a kit of parts including valve bases and mounting hardware but not the resistors and capacitors which will be determined by your design and the HT and gain expected for the valves used