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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 23rd Feb 2018, 8:09 am   #41
Argus25
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Default Re: Mullard 3 valve 3 watt amplifier circuits

One great advantage of the Class AB1 push pull audio stage, compared to class A, is that the relatively high power output that can be achieved without needing large iron cores, in the output or power transformer, or pushing the valve's dissipation too high.

Admittedly, the the power transformer still has to supply the output power & losses, however with music, seldom is the output power the full value as it is with a test sine wave, so for audio/music you can get away with a smaller power transformer with less iron. Also in the output transformer you do not need to worry about an air gap as there is no net magnetization of the core and you get away with a physically smaller transformer too.

It was interesting that when push pull was initially considered by RCA (compared to two valves wired in parallel for class A) was that they biased the valves into push pull class A. They were not initially aware of the power savings of increasing the bias and using the circuit in the class AB1 area.

Here are some remarks about push pull for those interested:

In RCA’s Radiotron handbook from the 1932, it was advised that two output triodes could be run in parallel class A to double the output power over one tube. Or one could boost the power to double in the push pull class A configuration. They stated that for the push pull connection, the power output was the same as running the tubes in parallel but the drive voltage requirement was double. This made the push pull class A connection seem unappealing to a designer reading the text, although there was a glib mention of the fact that more than double the output power was possible by increasing the bias.

In the RCA handbooks to follow and by 1934 in the Radiotron Designer’s Handbook, there was much more detail on the advantages of the push pull connection. They outlined how to bias the tubes into class AB1 and AB2 or class B for substantially more than double the output power from two tubes compared to what the same two tubes could safely generate in parallel class A. It appears that these concepts took a few years to gel in the minds of designers at the time.

In class AB1 for example, just as in class A, the grid voltage never exceeds the cathode voltage (no grid current is drawn, and hence the 1 suffix) but plate current flows for less than 360 degrees of the grid input voltage cycle at least at high signal drive levels. Each tube’s resting or “no signal plate current” is lower than in the class A condition due to increased bias voltage.
This increased bias allows a greater drive voltage before the grid to cathode voltage approaches zero, this allows more output from the tube without the tube’s overall dissipation being exceeded compared to class A. While one tube’s anode current is increasing, the other is decreasing hence the push pull naming.

Second harmonic distortion (typical of the single ended class A amplifier) as well as HT power supply ripple (hum) on the HT feed to the output stage tend to cancel in the push pull configuration and the increased efficiency is too hard to ignore.

Generally speaking, push pull output stags are affected by third harmonic distortion. At low drive signal levels in class AB1 each tube, with small excursions of plate current, behave as class A amplifiers but with maximum signal they shift toward the class AB where each tube is conducting for less than 360 degrees of the input grid voltage cycle.

In class AB2 the grid drive voltage is allowed to exceed the cathode potential and grid currents may occur at peak drive levels (indicated by the 2 suffix). This places a higher demand on the driver stage to deliver drive power. The same applies to class B in that the driver circuit now has to supply energy to the grid circuits of the output tubes.

In class B the plate current flows in each tube for 180 degrees of the input grid voltage cycle and the grids are generally drawing current during this cycle. Tubes designed for class B service (such as a type 49) are run at zero grid bias and the grids are driven positive with respect to the cathode (filament) to generate increased plate current and driven negative to cut off one tube while the other tube is conducting. This is why inter-stage transformers for class B use, to drive the output tube grids, are usually step down transformers (from the plate of the previous class A driver stage). On the other hand, inter-stage transformers for class A or AB1 use are generally step up transformers as the grid power demands are negligible and the extra gain (voltage magnification by the transformer) is helpful.

(It also pays to remember what it actually is that determines the maximum dissipation of any valve. It is the dissipation in any of the electrodes that takes the electrode temperature above what it was when the valve was manufactured and out-gassed. Above this temperature the metal structures release gas and poison the valve).

Since it is possible to attain a higher output power in the push pull AB1 configuration, compared say to two identical valves in class A, for any amount of valve dissipation, it is hard to argue that push pull is not the superior connection. However for relatively low power output scenarios, like mantel radios, class A certainly has a better economy of design.

Last edited by Argus25; 23rd Feb 2018 at 8:21 am.
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Old 23rd Feb 2018, 9:16 am   #42
kalee20
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Default Re: Mullard 3 valve 3 watt amplifier circuits

Quote:
Originally Posted by Synchrodyne View Post
Re the Mullard ECL80 push-pull design, would it be possible to apply NFB in shunt with the input, via a resistor from the appropriate side of the output winding? Then I think there would need to be a series resistor on the input side. Maybe this would not be of high enough value for say ceramic cartridges, but perhaps enough for modern sources? Power amplifiers that overall look like inverting feedback amplifiers seem to be rare.
Yes it would, and ultimately is the better connection.

Jim Williams of Linear Technology did some work on ultra-low distortion oscillators, finding that common-mode gain of even good op-amps is a limiting factor. The same would apply to the usual NFB injection point, ie an early stage cathode.

The twin-ECL80 amplifier does not allow for squirting NFB into a cathode, of course, due to the commonness of the cathodes.

Sensitivity would indeed be rubbish, but there is potential for a good improvement in distortion could be attained.
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