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So now that we have info on output transformers, what about inter-stage coupling transformers? I assume similar theory holds true, but in my limited experience these seem to be far smaller cores with much less inductance. Likewise, how does one calculate the input impedance of a valve? I haven't seen it listed on datasheets , unlike the rated output impedance.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Sigurthr wrote ...
So now that we have info on output transformers, what about inter-stage coupling transformers? I assume similar theory holds true, but in my limited experience these seem to be far smaller cores with much less inductance. Likewise, how does one calculate the input impedance of a valve? I haven't seen it listed on datasheets , unlike the rated output impedance.
LF interstage transformers had a relatively short history and were seldom used after about 1950, when improved capacitor technology made cheaper RC coupling convenient.
Zin: (A) Impedance is a frequency dependent property, so you will find the interelectrode capacitances - g1 - k and g1 - a - given in data sheets, allowing the Miller capacitance to be calculated for any given stage gain. (b) Zin is also dependent on the value of grid bias resistor where employed. At LF, in general purpose circuits, the input impedance on g1 without a bias resistor can be considered 'infinite' for most practical purposes.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
LF interstage transformers had a relatively short history and were seldom used after about 1950, when improved capacitor technology made cheaper RC coupling convenient.
I'd like to point out something here, regarding pre-war and post-war amp-lifiers. After the war manufacturers started trying to economise on costs, etc, which lead to lower quality, and, eventually, miniaturization, with small, tinny sounding transistor radios.
A lot of the 'best quality' components were used in the pre-war years, and the use of these was slowly phased out in the drive to cut costs.
Ahh, interesting. I had assumed they were very high Zin by design (high Zout, low Zin is the opposite of an ideal amplifier) but didn't know if there was a certain figure aimed for when they designed the impedance matching and interstage transformers.
So I wonder what my earlier failures with interstage transformer coupling were due to. Until I had a real interstage transformer fall into my lap it seemed like I would need many more than two stages to reach a couple watts of output. I remade my earlier circuit exactly but used a real interstage transformer and two of the same triodes were more than enough gain. I guess it could come down to the step down ratio of the transformers used but I'm pretty sure I tried a 1:1 as well, though I think it was only 1900ohms, perhaps still too low to transfer power to an infinity load.
Is there a proper means of determining anode bias resistor values for use in capacitor coupling of valves for tubes whose datasheets do not list suggested anode resistor values? For example; I tried capacitor coupling of UX-226 triodes, and while I was able to get it to work poorly I was merely guessing at values. I know that for capacitively coupled valves the anode resistor forms the voltage divider that determines the operating range of the output. The lower value Ra is the larger the voltage swing seen by Ca between cutoff and full conduction, but Ra had to be at least the value of valve's output impedance from my experiments. In MOSFET capacitor coupling I've always set drain resistors to merely limit the maximum current through the MOSFET as saturation is basically a dead short, but these triodes have such a high output impedance that at their maximum B+ anode current or dissipation isn't a concern.
Thanks for answering all these questions, I know they must seem very basic, but it is hard to find info about valve implementation and design these days.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Sigurthr wrote ...
So I wonder what my earlier failures with interstage transformer coupling were due to. Until I had a real interstage transformer fall into my lap it seemed like I would need many more than two stages to reach a couple watts of output. I remade my earlier circuit exactly but used a real interstage transformer and two of the same triodes were more than enough gain. I guess it could come down to the step down ratio of the transformers used but I'm pretty sure I tried a 1:1 as well, though I think it was only 1900ohms, perhaps still too low to transfer power to an infinity load.
The triode-output pentode - a triode and and an output pentode contained in one envelope, such as ECL80/PCL80, ECL86/PCL86/UCL86, dominated domestic audio applications from the mid fifties until the end of the valve era. Here is a good quality design using ECL86: [/quote1386713960]
If more gain is needed at the front end than can be provided by a triode, a pentode would be used, but average quality radios and record players used the ECL8* series in their millions, while TVs used the functionally identical PCL8* series designed for series connected heaters in TV sets to cut out the need for an expensive heater transformer. Think of a triode having useful gain of up to 100, and a pentode up to a thousand, as a very broad outline. In a word, you should be able to make do with a triode and pentode as AF amplifiers for most domestic purposes!
Sigurthr wrote ...
Is there a proper means of determining anode bias resistor values for use in capacitor coupling of valves for tubes whose datasheets do not list suggested anode resistor values? For example; I tried capacitor coupling of UX-226 triodes, and while I was able to get it to work poorly I was merely guessing at values. I know that for capacitively coupled valves the anode resistor forms the voltage divider that determines the operating range of the output. The lower value Ra is the larger the voltage swing seen by Ca between cutoff and full conduction, but Ra had to be at least the value of valve's output impedance from my experiments. In MOSFET capacitor coupling I've always set drain resistors to merely limit the maximum current through the MOSFET as saturation is basically a dead short, but these triodes have such a high output impedance that at their maximum B+ anode current or dissipation isn't a concern.
I will try to answer the rest of your question when I have more time tomorrow,
The triode-output pentode - a triode and and an output pentode contained in one envelope, such as ECL80/PCL80, ECL86/PCL86/UCL86, dominated domestic audio applications from the mid fifties until the end of the valve era. Here is a good quality design using ECL86: [/quote1386713960]
If more gain is needed at the front end than can be provided by a triode, a pentode would be used, but average quality radios and record players used the ECL8* series in their millions, while TVs used the functionally identical PCL8* series designed for series connected heaters in TV sets to cut out the need for an expensive heater transformer. Think of a triode having useful gain of up to 100, and a pentode up to a thousand, as a very broad outline. In a word, you should be able to make do with a triode and pentode as AF amplifiers for most domestic purposes!
I will try to answer the rest of your question when I have more time tomorrow,
Thank you, many times over! Invaluable info here, even without the link to the ECL86. I had no idea the gain of a pentode was SO much higher than that of a triode. I'll have to read up on how to use a pentode, I'm clueless about the actual usage of the other two screens! I have two beam power pentodes left over from an old ham radio; one I've mucked around with by running it in triode mode, hehe. The gain of it in triode mode is even less than that of the UX-226 triode.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I enjoyed the article, nice little amplifier, the output transformer seems key to this project, almost as good as an LM380.
the rustling sound is me donning a flame-proof suit
P.S. I think it's good that knowledge of thermionics is kept alive and may have new uses also, I am amazed by the super-cheap class-d PAM8403 stereo amplifier performance powering speakers from a single cell (Lithium) or 3W/channel @5v e.g. under GBP1 including postage from China ......
sorry to diverge from the thread, waiting for output transformer discussion .......
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Sulaiman wrote ...
I enjoyed the article, nice little amplifier, the output transformer seems key to this project,
I think having an O/P transformer of the right spec will make all the difference between very mediocre performance and something really rather good considering the simplicity of the design.
The problem with old old stock - recycled - transformers, is that those made in the greatest number for use with ECL86 in cheap radios and TVs were made to a price. There was no FM to show up the weaknesses of poor bandwidth, and people's expectations of TV sound were low. Cabinets more often than not buzzed and boomed in sympathy with loud passages of music, and audio bandwidth had to be kept below 10 kHz in 405-line TV to keep out the 10,125 Hz line frequency, a continuous whine you could hear distinctly anyway, because of magnetostriction in the LOPT core.
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Output Transformer questions
