A2 Part 3 - Converting the 841 amp into a 572-160 A2 amplifier


The 841 amp was auditioned by several people in one of my lunchtime sessions. General consensus: very very quiet, very dynamic. Also quick with absolutely defined transients. Also confirmed my own observations that this amp LIKES to be played loud.

As noted in the previous installment, I had planned on being able to easily modify the amplifier to 572-160, and this article will describe that modification and the expected results.

I will note up front that this amplifier shares some of the sonic capabilities of its 841 progenitor, but has its own sonic character.


Maximum power at clip : 8 watts. (a bit more power than the 841)

Maximum sensitivity: 0.45 volts produces 8 watts. (considerably higher sensitivity)

Noise level: 800 microvolts. (80dB below 8 watts. This is slightly noisier, but still remarkably quiet. The added noise can be accounted for by the increased sensitivity).

Frequency Response: -2dB at 20Hz, and -0.9dB at 20kHz with no compensation. With modification to the input compensator described last time, response was within 0.1dB from 20Hz to 26kHz, which is similar to the 841 amp. The worse LF response is due to the higher plate resistance of the 572-160s.

Distortion. (see section below). In this amp, distortion is essentially all second order: the distortion spectrum is totally different than the 841 amp version.

Output impedance: 16 ohms on the 8 ohm tap. (higher Z out due to higher plate resistance).

The Modifications

There are some really easy modifications used, but otherwise, the "schematic" remains unchanged. These are:

  1. Unplug the 841s, replace with 572-160s.
  2. Change the 5R4 rectifier to a 5AR4. This provides about +490 volts.
  3. Change the 3k 15 watt resistors going to the 0A2 regulators to 3.75k. (Since I used 5k in parallel with 2 15k paralleled resistors, I simply removed one of the 15k resistors in each channel).
  4. Bias set to about +13 volts instead of +9 volts. (causes about 40mA idle current in the 572-160s).
  5. Filament chokes changed from 80mH to 4mH, 4A, 0.15 ohm and the capacitors changed to 44000uF on either side of the choke instead of 22000uF. I actually paralleled additional 22000uF capacitors. If I thought more about this up front, I would have initially gone with these values, since the "final" combination provides about 7.6 volts at 1.25 amps and about 5.9 volts at 4 amps, so would have accomodated either tube with no changes.
  6. If you use the "optional" frequency response compensation, changing the 18.2k resistor to 12.1k produces flat response from 20Hz to 26kHz.

There are no other required changes.


This amp produces a completely different distortion spectrum than the 841 amp. It consists primarily of second order and a little third order, with very little higher order components. This is essentially similar to A1 amplifiers.

For comparison, here was the 841 distortion characteristics:

Listening Tests / Discussion

This amp shares the 841 amps "like" to be played loud as well as it's dynamics, and almost invisible clip characteristics. In addition, you get a feeling of more "reserve" that isn't apparent from the less than 3dB increase in available power output. The higher output impedance may possibly be an issue with some speakers, but may actually aid other speakers. On my transmission line speakers, there is no evidence of bass muddiness.

This series is intended to present a useable form of A2 amplifier. There is some thoughts that A2 cannot be made to be as clean as A1. Noting the characteristics described above, this does no appear to be the case, if properly driven. (This may be one of those cases where a cathode follower does not harm the sound quality.).

This version of A2 amplifier also shows that in A2, at least, DC heated filament is no impediment to sound quality, and it remains a QUIET amplifier.