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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 4th May 2021, 12:01 pm   #21
Radio Wrangler
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

The Labgear modulator for a 813 flat out in their LG300 used a pair of KT88

David
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Old 4th May 2021, 2:44 pm   #22
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

The Circlotron design looks scary - it's the sort of thing I might have come up with if the rewuirement was to circumvent an existing patent!

Transformer-wise, I've got a big 1950s Woden one which is about an 8-inch cube: modulation transformers are always big-and-heavy because the secondary winding carries the standing DC current to the RF amplifier and so there's steady magnetisation of the core.

For HT I've got a 550-0-550V@250mA one with plenty of LT windings: a bunch of 1Kv-rated diodes and some nice big electrolytics [the type used on the mains side of computer SMPS are cheap...] should suffice.

Phase-splitter/driver stages have always fascinated me - one of my favourites is the 'starvation mode pentode' approach - it gives plenty of gain in an intriguing way. Driven by something like a little 6BR7 such a splitter should provide enough 'oomph' to happily work the pair of push-pull 5B/254M I have sitting here.
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Old 4th May 2021, 2:49 pm   #23
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Quote:
Originally Posted by Radio Wrangler View Post
The Labgear modulator for a 813 flat out in their LG300 used a pair of KT88

David
The LG300 is indeed part of my inspiration here; G5RV's "Elizabethan" is helping to get the design-juices flowing too. Though given the price of KT88s for modulation I'll be using STC's little 807-alikes which are more-sensibly priced [i.e. free as I've already got them!]

and maybe a 4CX350 instead of that 813, if only because of the 813's unconventional heater-needs.
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Old 4th May 2021, 11:31 pm   #24
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Er hmm just a question, If you want big modulation why don't you just run a couple of 811A's or 813's in push pull? That will most likely modulate the whole street's power supply .

Joe
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Old 5th May 2021, 10:54 pm   #25
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by Radio Wrangler View Post
Quote:
Originally Posted by Synchrodyne View Post
The first “hi-fi” use was in the Acoustical QA12 amplifier of 1947:

Attachment 233215
Quad claimed that its circuit allowed the use of different anode and screen voltages without the complication of separate transformer windings. That claim was made in the third page of the attached article.
That's a wonderful diagram, Synchrodyne, and really gives the game away on Walker's sectional transformer winding scheme.
It's the sort of diagram that makes my eyes go funny! Great for understanding the transformer construction, but not so obvious how the circuit works!

I suspect that it was done that way to highlight the nature of the transformer as a key feature. It’s the only QA12 schematic that I have on hand, but there may well have been more conventional versions. The same format was used for the original Quad amplifier schematic in the operating instructions, both when it was current as the Q.U.A.D., and post-facto when it was renamed as the QUAD I.

Returning to the main theme and in particular the anode and screen voltage relativity, I think it can be said that the need in some cases for differential voltages was addressed from the start, and in various ways. And as Robert has noted, suitable transformers for the separate windings approach remain available, which in turn suggests that there is still a demand for them. How much effort was ever put into studying and developing the other methods of obtaining different anode and screen voltages is not readily apparent.

A broader look at the main theme suggests that attempts to improve the UL amplifier by making fundamental changes rather than by refinements and attention to detail might have been few and far between.

One was the “Lowther Linear” circuit of c.1956, which added feedback via the output valve (EL34) suppressor grids:

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I have not seen any analysis of this in say a journal article or an institutional paper, so how it worked and how well it worked remain unknown.

The basic UL circuit was subjected to detailed analysis by Langford-Smith and Chesterton, in AWV Radiotronics for 1955 May, June and July. This was reprised in a WW 1956 January article “Tetrodes with Screen Feedback”, whose title indicates that the UL operating mechanism was seen as involving feedback as well as distributed loading. The same issue also included an article “UL Output Transformers” by Leakey and Gilson.


Cheers,
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Old 6th May 2021, 12:10 am   #26
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

Ooooh Narsty!

Lowther put the entire cathode current of both EL34s to the slider of a pot!

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Old 6th May 2021, 12:46 am   #27
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

The Goldenears amp also has a cathode current balancing pot. I am using 50 watt wirewounds with a large carbon brush similar to a Variac. I am hoping it wont go bad as it has FOUR cathodes hanging off it.

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Old 6th May 2021, 7:22 am   #28
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Default Re: Improving the 'Ultra-Linear' push-pull amplifier.

It might need to get moved occasionally for the contact to wipe and self-clean. High power pots tend to be wound with wire metallurgy dedicated to high temperature use rather than lowest tarnish.

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