Amplifiers on the bench

After purchasing a (Tonghui) dB-Millivolt meter, I am able to more easily (and more accurately) undertake frequency response measurements of my Preamp and Power amp.

This enables me to directly read relative dB signal voltage output from my amplifier (or preamp) without laborious 20Log Vout/Vin calculations. (Thanks to advice from my pal Clarry.)

I will check out my recently built WOT Preamp and my usual 2A3 Push-pull amplifier.


First, the 6SN7 WOT Preamplifier :


Frequency 1/3 octave spot measurements were taken at 12 o’clock volume setting (-24dB attenuation), with output load 100kΩ and 0.106V sine wave input.

Preamp Gain at 1kHz at maximum output = 13.65dB (4.8x).  This is around 50% lower than my previous 76-6SN7 cathode follower linestage, due mainly to the 4 : 1 OPT now used.

I’m listening at around 11 o’clock volume setting – which is -27dB attenuation, still plenty of power headroom in this preamp.

Frequency response:


20 – 20kHz, within 0.55dB at ’12 o’clock’ volume – very close to Tango specification.

I’m happy that this (ISO) Tango iron lives up to reputation – decently wide bandwidth.

However, preamp residual noise could be improved, -60dB (ref maximum output), but same at all volumes.

I need to try:

  • 600Ω OPT secondary loading (presently 1.2kΩ)
  • 76 tube instead of 6SN7


Next, 2A3 Push-pull power amplifier :


This amplifier, with NOS 2A3 tubes, produces 7.6 Watts into 8Ω load, before clipping at 1kHz (with no Negative Feedback).   Input sensitivity is high, 340mVrms. Gain is approximately 27dB.

Approximately the same power as 300BSE but triple the 2.5Wpc power available from self-biased 2A3SE.

I am also using 6k6 Output Transformers (not 5k), which will reduce power output a little.

However, plenty of power for my 100+dB/W horn loudspeakers.  Only 0.025W will generate over 85dB SPL at the listening seat at 4m distance.

(The original Japanese designer (Valves’ World) achieves ~12 Watts (50% more) using higher dissipation Sovtek 2A3s.)

Sep 18 1
Frequency response:


20 – 20kHz. at 1W output, 20 Hz -0.4dB, 20kHz -1.6dB

Bass is surprising good.  The OPTs are vintage Partridge 6k6 : 8Ω, 25W.

High frequency rolls off a little quicker, but not bad.


My next plan is try a variation of this design, with IT (interstage transformer) coupling and phase-spitting.

This may reduce the (small) amount of ‘crossover distortion’ due to the valve/tube phase-splitter (see below).

Input Sensitivity will be reduced unfortunately, but we’ll see……

10k Sep 18

10kHz square wave output at 1W power.


2 thoughts on “Amplifiers on the bench

  1. Hi Ianmo – the Tonghui was purchased s/hand from an auction site. I think any good quality dB-millivolt meter will do the trick. This one happened to be inexpensive & so far, adequate for these occasional tasks. – Cheers, Owen

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