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Old 2nd May 2012, 11:58 AM   #1
loopy is offline loopy  Australia
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Default All tubes red plating

Hi. I've spent countless hours reading and experimenting with red plating, and my last option was to ask here, which seems like the most active forum in regards to tubes.

Firstly, this amp is different to what is generally discussed here. It's a 4-tube RF amp, not an audio amp, but I'm guessing or hoping that the solution may be something that applies to tube amps in general.

In case the answer isn't a simple one, I can go into more detail, else I'll try and keep it brief. At first there was always least one tube red plating, even with low input driving the tubes nowhere near what they're capable of. I've tried several tubes, all with similar results. There was originally one resistor to control the bias for all 4 tubes, but I now split the tracks for all 4 bias voltages, and can control each one separately. Currently it seems it's all quite equal, as
they're red plating equally lol. I've tried -8volts (seems fine for the output) and -50volts (which drops the output a bit too much) to no avail.


The tubes in question are 6P45S, which I've been told apparently require some sort of auto-bias system.

What I wish to know is, where to from here, any suggestions? Is there a general rule to prevent all tubes from red plating at the same time?



Thanks.
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Old 2nd May 2012, 02:26 PM   #2
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Is this amp a linear power RF amp?. If this is the case, it must be biased for AB1 with only a small idle current. And if it is a class C amplifier, it always be hot red if not RF input AND in the frequency / band for that is is tuned.
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Old 2nd May 2012, 02:49 PM   #3
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Quote:
I've tried -8volts (seems fine for the output) and -50volts (which drops the output a bit too much) to no avail.
The tubes in question are 6P45S, which I've been told apparently require some sort of auto-bias system.
What is your quiescent current and how have you measured it ?
-8 V bias voltage seems very low for 6P45S.
What is your Ug2 and Ua ?
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Old 2nd May 2012, 02:52 PM   #4
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Quote:
Originally Posted by Osvaldo de Banfield View Post
.... if it is a class C amplifier, it always be hot red if not RF input ....
In class C and with no input the tubes should be cut off and run "cold".
Or did I miss something in your message ?
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Old 2nd May 2012, 03:05 PM   #5
loopy is offline loopy  Australia
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Quote:
Originally Posted by artosalo View Post
What is your quiescent current and how have you measured it ?
-8 V bias voltage seems very low for 6P45S.
What is your Ug2 and Ua ?
Sorry, not sure. I haven't measured any current, and Im unfamiliar with the terms Ug2 and Ua. I wasn't really keen on having to learn the ins and outs of tubes, but seeing as I've bumped into this issue, it's become a bit of a project to work out, but I haven't given up just yet.

To explain the bias voltage - originally the tubes were PL519's, which require 40 volts for the heater. This voltage is also used for the bias, via a diode and cap to convert it to DC.

When I converted to the 6P45S's, which require 6.3 volts for the heater, rather than try to drop the 40 volts AC down with a resistor (which I believe would've been too unstable), I added another transformer with 6.3 volts AC output. That output is also used for the bias. However, I also have the option of using the now unused 40 volt line, which dramatically reduces the output, but doesn't eliminate the red plating of the tubes.
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Old 2nd May 2012, 03:10 PM   #6
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Quote:
Originally Posted by artosalo View Post
In class C and with no input the tubes should be cut off and run "cold".
Or did I miss something in your message ?
Most class C RF tube based amplifiers, get the cutoff voltage from grid rectification from the input signal, drawing some amount of power from preceding stage, particularly if that previous stage is in the same equipment. In case of abnormally low signal input voltage, the tubes get only low bias and then gets red plates. The same can occur if the tuned circuits at the plate of the tube arenīt tuned to the incoming signal, or a integer multiple of such frequency, in that case the stage is a frequency doubler, tripler, etc., although signal level is right. But if it is independent set, may be biased externally by means of a -C power supply. I donīt know exactly the case of the guy is making the question.
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Old 2nd May 2012, 04:01 PM   #7
DF96 is offline DF96  England
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Using an RF power amp with apparently so little knowledge of what is going on could be dangerous. Could the OP reassure us that RF PA biasing is just a minor gap in his knowledge and he is otherwise competent and licensed to use this equipment?
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Old 2nd May 2012, 04:04 PM   #8
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A schematic would certainly help.
It sounds like this is some kind of linear amplifier. If it's a linear, it's not class C.
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Old 2nd May 2012, 04:13 PM   #9
loopy is offline loopy  Australia
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Originally Posted by Frank Berry View Post
A schematic would certainly help.
It sounds like this is some kind of linear amplifier. If it's a linear, it's not class C.
Yeah sure thing: http://www.rmitaly.com/download/manuals/KLV1000-manual_rel_210.pdf
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Old 2nd May 2012, 04:25 PM   #10
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The link does not work. Access is forbidden.
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