Jun 4, 2017

VT25, VT25A Push Pull rejuvenation part 2

On the bench for surgery.

This major changes just leave transformers, choke and the 4 pin sockets on the chassis.
I also kept the hum and push pull balance potentiometers but removed the electrolytics (replaced by Philips 021 low ESR) and made a new ground line from solid 1mm2 (17 AWG) silvered OFC wire.

Amp being almost completely stripped first thing to do was to change the 5687's socket.
Originally I used a phenolic Chinch one but after several tube tests the contacts became loose and the way I built my amp gave me no possibility for an easy replacement.
It is important to select good sockets to prevent any unwanted noise. When troubleshooting an old tube equipment, this is the first thing to check.

Today I use excellent Russian military ceramic ones with heavily silvered and super tight contacts. So tight that I can lift the amp when removing the tube! Not to be confused with cheap Russian's manufacture with thin clad metal frequently found on the web. These are the best I have on hand with old Schurter silver or gold plated ones (West German made in the 60's).

Supply choice

I had to make a decision, C or L input filter ?
Setting the amp for AB1 operation will favor choke input filter as there is some current variation depending on how deep I modulate the amplifier. In this way the choke acts like a constant current device, reason why it was widely used in class B amplifiers.

On the other hand I have noticed better dynamic with SRPP when using a CLC filter.
I finally opted for the choke input filter mainly because its inductance helps the current to change very little during the AC cycle thus providing an almost perfect DC to the whole circuit (E130 amplifier). Moreover this kind of filter is less stressing for the transformer and the rectifier and permits the use of a quite large capacitor behind choke without sacrificing the network time constant. In that way I can calculate a well filtered supply with just two cells (push pull configuration have a very good PSRR). Minimalism and efficiency.

It reminds the great electronics of the past with just two small capacitors in the main supply and no hum at all. These guys knew their job.

One important point when using a choke at input is a «starting current» through circuit. Below this minimum current the choke acts like a resistor only.
The minimum amount of Henries depends upon the total resistance in series with rectifier Rs and the internal resistance of the circuit Ri, which is the voltage to current ratio behind choke (Ohm's law).

                       Lmin ≥ ( Rs + Ri ) / 6πf                 where    f = supply frequency

Usually Rs is small compared to Ri and can be neglected so we can use a simplified formula.

                        Lmin ≥ Ri / 940 for 50Hz    and     Lmin ≥ Ri / 1130 for 60Hz

In this case, with an estimated current of 30mA (VT25A 2x10mA + 5687 10mA) @ 440V, the minimum would be ~14666/940 or 15,6 Henries. My choke is 10H / 95 ohm, to work properly I need to pump 15mA more and the simplest way is a bleeder resistor.
The resistor value will be 440V/15mA or 29,3K, closest standard value 30K and 6,7W dissipation. Need to use a 30W one and it will be hot, thus have to be cleverly located away of any heat sensitive part like electrolytic capacitor.

Two filtering cells command a quite large smoothing capacitor. For example a 200µF C1 capacitor will provide a very low impedance path to the 100/120Hz AC while keeping a time constant below 20 milliseconds for fast recovery and good transients. The ripple on C1 will be 2,64 10-1 Vpp or 9,3 10-2 Vrms (6J5 line preamp for calculation). A second cell with only 8/12µF will floor the ripple to negligible value to properly feed the SRPP.

30W bleeder resistor

Push pull balancing and hum potentiometers. Shunt resistor straight from choke to the point where all grounds will return.

PSU schematic

more to come