• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

The Red Light District - another PP EL84 amp

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#161
Complication, complication. Is that bandwidth worth it? Are there other problems lurking? Can I afford to miss a mortgage payment?

My own opinion is that the accomodation of toroid quirks to get more bandwidth is probably not worth it, but there is certainly a school of thought that an amp flat to 100kHz is better than one flat to 40kHz.
 
#163
Reading and lurking.

I was thinking "C" cores as well for the gap. One wonders if Plitron isn't already heading that way. I don't think a toriod is worthwhile here.

I don't even like them for power transformers unless there is no height for anything else.

-Chris
 
#164
There again, it's not as if you can just run down to the corner and get some c-cores. but they do offer geometry that would favor high BW.

I'm still working on winding technique for E-I's that might be novel... have to bring that up with the family one of these days.

:xeye:
 
#165
anatech said:
I was thinking "C" cores as well for the gap. One wonders if Plitron isn't already heading that way. I don't think a toriod is worthwhile here.
Click to expand...

Somehow i doubt we'll see a C-core from Plitron. I had the wonderful experience of sitting in on an hour or 2 of OPY discussion between 2 of the world's most knowledgable transformer guys -- Menno van der Veen (Plitron head of R&D and the toroid man) & Bill Perkins (big C-Core fan and capable of making some pretty special OPTs). Both had (what seemed like) very valid reasons for their approaches (the detail of the technical discussion was quite beyond me) and neither swayrd the other, but both went away with tidbits to help them improve their product.

dave
 
#167
SY said:


Quite the contrary, that's a particularly linear spot to operate it. At the full 11V peak output needed to drive the output stage, THD of the voltage amp is less than 0.03%.

Regarding LED reliability, you might check out what Bob Pease had to say in his book "Troubleshooting Analog Circuits."
Click to expand...


Bob Pease examining a SPICE printout:
 

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#169
Hi Stuart,

Can you please help me with the maths for working out the dynamic impeadance of the leds, I have read the artical several times but can't work it out. I have set up the test jig as per the artical and have a scope and sig genny. My maths are not too good. The leds I bought drop 2.1v @ 10ma.
Thanks. 😕
 
#170
Hi, Kopite, I had some difficulties responding to your email (bounces). In any case, here's the text of what I sent you:

If you get 2.1V drop across the LED, it's not a "normal" red LED- those should have about 1.7V drop. So the biasing won't be right. I hope they weren't expensive! What you want to avoid is any LED that's "high efficiency" or "high output." You want surplus and cheap- the Jim-Pak LEDs that I bought at Fry's (the local computer/electronics emporium) were pretty good; the best ones I used were from a surplus place and ran a nickel each. I bought a few of each one they had, tested them, then went back and bought a couple thousand of the best ones.

If you want to be double-sure that you're getting normal LEDs, take a voltmeter, a 9V battery and a 1k resistor along with you. Put the battery and resistor in series to feed the LED (you need the resistor to limit current), and when it lights up, measure the DC voltage across it. 1.6-1.8V is normal.

For the test circuit, it's best to adjust the input level so that you have 4.7V peak (or peak-to-peak, if you'd rather) across the 4.7k resistor. Then when you read the ac voltage across the LED (it will be in millivolts), that will correspond to the impedance. For example, if you adjust the plate ac voltage to 4.7V, then read 6 mV across the LED, the dynamic impedance is 6 ohms. If you read 10mV, the impedance is 10 ohms. And so forth.
Click to expand...
 
#172
Kopite
I got a pack of 100 cheap red leds from local Aztronics store who are Altronics agents.
The Altronics catalogue shows Kiwi agents as Global PC with stores in Christchurch which I recall was a real nice place (except full of b... tourists) or in Riccarton and I no idea where that is or even which island.

Other options are the Jacar Agents as follows:
Electric Blue in Masterton
Plug N Play Electronics in New Plymouth
Booneens Electronics in Tauranga
Thames Television Systems in Thames
Booneens Electronics in Whakatne

Hopefully one of those is not too far away.

Cheers from across the ditch in Oz.
Ian
 
#173
I'm desperately short of my best ones (I've built two RLDs and am in the middle of building four more, plus a higher powered version), but I still have some good usable ones with the right drop. If Ian's sources don't work for you, let me know and I'll send you some. At what they cost, I'm not worried about getting the five bucks from you.😀
 
#175
Hi Stuart, I tested the leds as per your guide and they came out at 6.5 ohms. Just for interest I had some HP leds left over from another project HLMP-6000 and these measured 1.5 ohms. I only have a few of these but in the Farnell catalogue they go for 82 cents NZ each. That is about $50 per hundred US I think.
 
#176
That's a pretty typical number. My "best" ones measured out at about 4.5 ohms, but I had to sort through half a dozen bins before getting anything that low. And really, I don't think the extra ohm or two will make any difference. Assuming any reasonably low impedance, the voltage drop is still the most important parameter in this application.
 
#177
Just a reply for folks in this part of the world searching for suitable LEDS, the cheapest and best I have found so far is from Farnell, cat # 102-1267 @ 28 cents each NZ per 100. These are a 3mm Fairchild led MV50640 max Ia is 30mA and the dynamic ac impeadance is 3.5 to 3.1 ohms from 10mA to 20mA and 4 ohms @ 5mA. They have a voltage drop of 1.77v @10mA.
These are cheapest and best value I have found and beat Jaycar and Trade Tech for price and specs.
Now to start on my RLD amp.
 
#179
That resistor, as far as I know, forms a zobel together with the 20muF capacitor. The application note for the 317 also includes such network, but with slightly different values. I remember that somewhere in this thread SY mentioned that this 4r7 and 20muF gave the best results for this application.

Erik
 
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