May 17, 2017

VT25, VT25A Push Pull rejuvenation Part 1

This amp have been my music companion for more than two decades and the only one I kept among the few I built until I came across the Philips MC1/60 triode.
It had all the qualities expected from the 10 family of triodes and I especially enjoyed the VT25 Visseaux in this configuration for its rich texture and refined sound.

Both amps remained for a while in my system but listening after listening it became evident that the big triode SE was better in terms of speed and impactness. The VT25 amp finally went to the attic as I have no room for two amps.

I never totally gave up the idea of a better VT25 amplifier but was not in the necessity to ask myself "what makes the difference between these two amps ?" Moreover reflection time being always beneficial to find solutions to a given problem, even unconsciously, I let things slowly growing.

During the past year the few electronic devices I built made me realize the essential contribution of the PSU in the final result, and naturally came to me this question : Is my VT25 amp lack of punch a supply issue ?
So, I put it back on the bench for major modifications including revisited supply, better implementation, different bias point....and new driver.

In my very first prototype the bias point was quite in the middle of the load line and the amp worked almost A1, current flowing all the time at 22mA. This is not the best for triodes push pull operation, neither for power efficiency (even if I am not really concerned with 104dB speakers) nor distortion and I decided to move AB1 with a setting current in the 8/10mA range. The biasing resistor was just increased to a 3,5 Kohm value. On the new amp the resistors are 3K, 1% sorted, for a 10/12mA flowing current. For some years now I use the vintage Sprague Koolohm non inductive resistors or Kiwame carbon in tubes cathodes with good results.

I also totally reconsidered my work which was not the most pertinent in term of good implementation: long path from the decoupling caps to the active devices, potentiometer to set input level (excellent for unwanted hum and noise), multiple wiring points (the best for ground loops), poor filament filtering and bad supply time constant. Despite these negative points the amp was performing very decently and I can expect some clear improvements with the right modifications.

Prior to change the power supply and the amp being completely stripped, I modified the driver on one amp. I wanted to take advantage of the very good results of the SRPP stage in the MC1/60 amp and performed some blind test to see (...hear) which driver was the best. The 5687 transformer loaded SRPP is the winner, no discussion. Excellent image and tempo, deeper sound stage, extended low end (no current through IT primary means wider bandwidth). Better linearity and lower distortion are this setup assets when coupled to the Tango NC14. Furthermore the good PSRR of this circuit will help to simplify HV supply. A good point and first step to an improved amplifier.

5687 µ & ρ

5687 SRPP resistor, gain and current calculation

5687 DC load line. Tube will work in a more linear region of characteristics than the previous stage

One minor drawback is a more demanding stage. I need to provide a 2Vrms input voltage when 0,9 where sufficient with the former one to swing the amp full power. However I can switch to the E182CC / 7044 for higher gain if necessary.

Good 5687 and E182CC chosen from my stock for this test

Amplifier revisited.

A 5µF coupling cap will insure wide frequency response with a -6dB cut off @ 6Hz for a 5 Kohm AC load but like for a parafeed load the coupling cap is dependent of the transformer primary inductance and is calculated large enough to avoid resonance at an audible frequency. In this case, considering a 50H primary inductance for the NC14, the resonance frequency f0 = 1 / 2π √LC is about 10Hz. Lowering this value can give some bass boost to the circuit. Interesting for those who feel the VT25 family of tubes lacks of solid bass extension.

To be continued

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