Powerful tube amplifier. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Powerful tube amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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This is a development somewhere in the late 80s. During this time, it proved to be worthy and versatile: it is suitable both for lovers of high-quality sound (composed for myself), and for musicians who need power :).

(click to enlarge)

scheme

Short lyrical introduction. At one time, the amplifier published in the Radio magazine in 72 was very popular. I also repeated this pattern. Its shortcomings are known to many who repeated it: low linearity, poor stability at IF, insufficient RF stability (from which a corrective air conditioner was introduced into the circuit), a narrow frequency range, and something else that I don’t remember now. And most importantly - the sound left much to be desired.

I could not stand this at home: my ears are not state-owned :) The first thing I started the modernization with was the replacement of the output trance. The changes made to the output trance suggested themselves - to tighten the connection of the feedback windings (ultralinear) with the rest of the windings, than to reduce Kg at higher frequencies, and to improve the frequency and phase characteristics of the output stage. In the version that I used in the new design, it was possible to expand the frequency range, increase the stability at high frequencies, and lower the output impedance. The sound has noticeably improved, but now all the circuitry (a clone of the so-called "Williamson circuit") began to seem far-fetched in Hi-Fi - it was somehow "head-on", the weak link was weak stability with OOS at infra-low frequencies, increased non-linear and frequency distortions (especially at high frequencies).

Further improvement resulted in the complete rejection of this scheme. Many different circuit solutions have been tried. Attempts to find the best option led to the solution that I propose. At the input, I used a cascode UN with high linearity, then a phase-inverted cascade with a shared load, which has the highest linearity. At the same time, I connected them directly in order to reduce phase shifts along the signal path. At the output, however, the familiar ultra-linear output stage remained with minor changes (for the purpose of ease of setup and increased stability), and, as already mentioned, with an improved output trance. In the diagram, I conditionally divided the preliminary stages, a bunch of triodes in which, in fact, are my know-how;), and the output stage, instead of which you can connect any suitable one. With a properly manufactured and adjusted amplifier, the maximum amplitudes on the control grids of the output lamps must be at least 80V in a 47k load. And this made it possible to completely swing the 6P45S. And what is important, for all its merits, the scheme turned out to be even simpler than the one that had to be abandoned.

The result is an amplifier with a sound that (with proper measures) may well claim to be hi-end;) The amplifier is absolutely stable, so it can be used both with deep OOS and without it at all - the linearity of all stages ensures low distortion open loop OOS.

From two 6P3S, I managed to get> 150 watts, from two 6P45S -> 220 ;), and in the version with grid currents (especially for musicians) - 400 watts of peak power! But that scheme is already noticeably different from the one shown.

I can’t give detailed parameters of the amplifier now - I haven’t measured it for a long time. For those who need sound and not parameters, I have given enough information for repetition, and if it is very necessary, I can (albeit very scrap) remeasure. For a magazine, I would probably measure it. And then it will do :o)

As for the setup, it is simple:

assemble a standard scheme for measuring parameters;
turn off the OS;
turn on the force and warm up the cathodes;
resistors R10 and R11 set the quiescent currents out. lamps 30 ... 60mA (0,06 ... 0,12V on cathodes), but always the same;
without applying a signal to the input, set the regulator R2 on the cathode of the phase inverter 105V;
apply a signal to the input until a voltage is obtained at a load of 15 volts (for a 6-ohm version);
resistor R9 sets the minimum of the 2nd harmonic at the output;
restore the OS (optional).

Point 7 can be skipped if you replace R8 and R9 with one, with a resistance of 12k (this may not even affect the quality in any way, especially with OOS).

To power the amplifier, additional voltages were needed: 410V (10mA / channel) and stabilized 68V (b / t). The diagram shows one of the options for obtaining them from the available ones. Here you can do it in different ways. I, for example, have a source stub. + 220V to power the preamp, so I got +68 as a divider.

At one time, the scheme was shrouded in trade secrets :). Now please - let anyone who wants to try. I repeat that the UN-FI link is universal, and can be used to drive various output PP stages (triode, pentode, class A, AB). For each specific case, you may have to recalculate some elements, which is done very easily. This is where I can help those in need.

PS: "Surf" amplifiers lend themselves well to such alteration - the quality improves noticeably.

Author: Vladimir Vasilyev, vivNOZPAM@linknet.dn.ua; Publication: cxem.net

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