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Re: TR-4 - Soft Finals?

 

Barry,
Something else to consider - Every TR-n that I've worked on has had the plate current meter resistors aged high.? The result is that plate current measurement is not correct which of course means that idle current is not set correctly.? Seems to me that idle current is the basic foundation for everything and if its not right than there may be some odd things happen.? There are two resistors.? One is a standard value and the other is determined by the Drake tech during final alignment before the radio was shipped.

Also, I don't recall if you mentioned checking it or not but the 68ohm and 15ohm resistors under the amp tubes are also prone to aging high.? I always check and usually replace those resistors and then measure the resistors for the meter and replace if they have drifted.? You can of course verify your idle current by measuring resistance and voltage drop across the three resistors under the amp tubes, calculating the current draw, and adding up the current draw for all three.

73,
Another Barry
KJ5GQM

On Sunday, May 18, 2025 at 05:23:44 PM CDT, n4buq <n4buq@...> wrote:


I tested all three tubes for gas and none were bad in that respect.? Interestingly, I decided to perform the "LIFE TEST" where the filament voltage is reduced (I think it goes down to about 5.8V or maybe 5.9V for a 6.3V filament).? While transconductance drops by an acceptable amount when that button is pressed, the fall in value looked a lot like what I see for plate current when the key is down.

With all the tubes back in place, I decided to monitor the filament voltage.? I've been running the rig between 110VAC and 115VAC and that was giving me a low-to-start-with filament voltage of just under 6VAC.? With that, key-down would cause that voltage to drop to around 5.7VAC.? I then bumped the autotransformer up to closer to 120VAC which gave me very close to 6.3VAC and, oddly enough, on key-down, that voltage would drop just a small amount - perhaps 0.1VAC to 0.2VAC.? That seems a bit odd that the sag was smaller but, perhaps, the transformer is more efficient at the input voltage closer to 120VAC but that's just a guess.

Also, with the input voltage closer to 120VAC, I think I'm seeing a good increase in plate current at resonance.? I suppose that's expected since the plate voltage also climbs with that input voltage.

Thanks,
Barry - N4BUQ

From: "Barry" <n4buq@...>
To: "DRAKE-RADIO" <[email protected]>
Sent: Sunday, May 18, 2025 1:03:51 PM
Subject: Re: [DRAKE-RADIO] TR-4 - Soft Finals?
Richard, Steve,

Thank you both for the responses.? I wasn't sure if "soft" was just a generic for "weak" and, in general, maybe that's true.

My three tubes do not exhibit some of the more obvious signs of excess gas (e.g. white coating or a "getter-like" spot but that's not a very good indicator of small gas amounts.? I'm going to pull them again and run the gas test as I didn't do that when I was testing them.? Maybe that will be revealing.

Thanks again,
Barry - N4BUQ

From: "Steve Wedge, W1ES/4 via groups.io" <w1es@...>
To: "DRAKE-RADIO" <[email protected]>
Sent: Sunday, May 18, 2025 12:37:22 PM
Subject: Re: [DRAKE-RADIO] TR-4 - Soft Finals?
Yep, that sounds like a textbook example. ?Power falls off first on the higherbands as the gain (transconductance) decreases. ?The electrons are generated at the heated cathode and eventually, the cathode material gets depleted.

I¡¯m starting to think that my TR-4Cw RIT is having similar issues, though strangely, the output only sags when on the antenna and not on the dummy load. ?The tubes in it are well-used.

Steve Wedge, W1ES

Time flies like an arrow. Fruit flies like a banana.




On Sat, May 17, 2025 at 10:46 PM, n4buq <n4buq@...> wrote:
Can someone please tell me what is meant by the term "soft finals"? I'm still having problems getting proper output from my TR-4 and, for all I know, that might be the problem. I've tested them for transconductance and all three are very close to the same value and, furthermore, that value is quite a bit above the minimum for a good tube.

Aside from low output (i.e. about 300 mA plate current maximum into a dummy load), I notice that if I hold down the key for a few seconds the plate current tends to drop just slightly and I think if I were to continue holding the key down, the current would continue to slowly decrease.

I measured the voltage across each cathode resistor and I notice the same phenomenon: the voltage climbs to about 1.6 VDC and then begins to drop a bit. Like the transconductance values, each cathode resistor is "matched" in value and behavior to the other two so I suppose that's one good thing.

Am I seeing the result of weak/soft tubes or is something else going on that would cause this?

Thanks,
Barry - N4BUQ






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