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Old 18th Oct 2020, 1:53 pm   #1
Dick Glennon
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Default Metal Rectifier in Pam TR 26

Hi again, I want a bit of advice on replacing the metal rectifier in a Pam TR 26. As can be seen from my sketch I was going to replace it with a IN 4007 but I would have to insert a resistor in the circuit to compensate for the loss that was the rectifier. If I am right in what I am going to do what ohms and wattage would that resistor need to be?

Thanks again, Dick.
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Old 18th Oct 2020, 2:06 pm   #2
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Default Re: Metal Rectifier in Pam TR 26

Hi.
R14 is the surge limiter resistor. When substituting a silicon rectifier this resistor will have to be increased in value as you know. You can leave R14 in circuit and add a further resistor in series. The issue is of course we don't know the current drawn from the HT once the set is fully warm so an exact value is hard to work out. What I do especially as if the old rectifier is tired and giving low output.
What I do is to add a 100 ohm resistor in series with the surge limiter and then measure the HT on a fully warmed up set and see if the HT is high or low and trim the value as necessary. Once worked out you can add the value to the value of R14 and fit a single resistor of the combined value instead of the existing R14 to make a neat job. Also add a capacitor in parallel across the 1N4007 of 1nf and rated at least 250v ac to remove switching noise from the Diode.
Hope this helps.
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Old 18th Oct 2020, 2:38 pm   #3
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Default Re: Metal Rectifier in Pam TR 26

Sorry I forgot to say.
The wattage of the resistor to be on the safe side would be a 7 or 10 watt. Normally in a radio I use a 7 watt as that is what I have. Once you have the HT at the correct voltage just measure the current or the voltage drop on the resistor and work out the wattage. A 2 watt resistor is OK purely for the select on test value.
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Old 18th Oct 2020, 6:12 pm   #4
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Default Re: Metal Rectifier in Pam TR 26

"Also add a capacitor in parallel across the 1N4007 of 1nf and rated at least 250v ac to remove switching noise from the Diode."

I think 250VAC will not be enough as the maximum voltage will be higher than this as it's the DC HT plus the maximum negative peak of the AC waveform. Maybe 450VAC or 1000VDC would be better.
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Old 18th Oct 2020, 9:06 pm   #5
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Default Re: Metal Rectifier in Pam TR 26

..or why not leave the metal rectifier in circuit and put the 1N4007 in series with it. That leaves the same series resistance, but protects the metal rectifier from nasty reverse breakdown. It also keeps the set looking more original (you can sleeve the 1N4007 to hide it).
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Old 18th Oct 2020, 9:19 pm   #6
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Default Re: Metal Rectifier in Pam TR 26

The OP hasn't said why he wants to replace the rectifier,maybe its giving poor output? Les
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Old 18th Oct 2020, 9:46 pm   #7
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Default Re: Metal Rectifier in Pam TR 26

Ah yes, if it’s gone very high forward resistance that would not allow my solution. Good point. I have used the series diode solution before where I wanted to protect an otheise fairly good rectifier.
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Old 18th Oct 2020, 9:52 pm   #8
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Default Re: Metal Rectifier in Pam TR 26

Hi.
In fact you can place the snubber cap between the anode of the 1N4007 and chassis, a 0.01uf can also be connected to cathode and chassis (across the reservoir in most circuits) and the result is exactly the same.
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Old 21st Oct 2020, 2:55 pm   #9
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Default Re: Metal Rectifier in Pam TR 26

If you're concerned about the PIV rating of the diode, just fit two in series.

I've never needed to fit caps across rectifier diodes in valve sets. There may or may not be interference generated, but if it isn't audible it hardly matters.

The value of the new surge limiter resistor isn't critical, and neither is the HT voltage. I use 200R simply because I've got a big bag of suitable 200R resistors. You can fine tune the value if you like, but there isn't much point.
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Old 21st Oct 2020, 10:21 pm   #10
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Default Re: Metal Rectifier in Pam TR 26

Posts discussing ratings of rectifier and capacitor moved here

https://www.vintage-radio.net/forum/...d.php?t=172318

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Old 21st Oct 2020, 10:37 pm   #11
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Default Re: Metal Rectifier in Pam TR 26

Many thanks for splitting the thread Mike
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Old 22nd Oct 2020, 8:01 pm   #12
Dick Glennon
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Default Re: Metal Rectifier in Pam TR 26

Hi again,I have looked at all your comments and discussions but I must confess most of them are out of my league here. I have only the forum to ask as there are no one interested in valve stuff that I know of. One other point as well, the set only has three valves ,UBC 81 (14volts) UBF 89 (19volts) and UCL 83 (40volts). R16 has gone open circuit. What wattage would I need to replace it and what voltage should I have where R 16 meets V1.Also I would be very grateful if any of your forum members could do a little sketch of the new layout of the rectifier. The original rectifier was held on with one screw and one wire. There was a BY 127 diode and 300 ohm 5watt resistor in the cabinet. Thanks again, Dick.
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Old 22nd Oct 2020, 8:44 pm   #13
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Default Re: Metal Rectifier in Pam TR 26

I don't know what the value of R16 is as I don't have the service details but the power that will be dissipated by R16 will be 0.01 multiplied by the resistors value and whatever that comes to double the value for a suitable wattage rating for the resistor.

The voltage at the junction of R16 and V1 heater should be approx 76 volts AC with respect to ground, 76 volts being the sum of the valve heater voltages when working.

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Old 22nd Oct 2020, 9:52 pm   #14
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Default Re: Metal Rectifier in Pam TR 26

R16 = 1320 Ohms.
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Old 22nd Oct 2020, 10:26 pm   #15
Dick Glennon
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Default Re: Metal Rectifier in Pam TR 26

Hi again, Would I be right in saying that the the value of R16 is 1320 ohms and therefore 1320x0.01 equals 13 x2 equals 26 watts, Regards, Dick.
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Old 22nd Oct 2020, 11:07 pm   #16
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Default Re: Metal Rectifier in Pam TR 26

P=IxIxR
0.1x0.1x1320=13.2w

Valve heater are 100ma or 0.1a
A 20watt resistor should be ok.
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Old 23rd Oct 2020, 12:22 pm   #17
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Default Re: Metal Rectifier in Pam TR 26

Hi Dick.
I have one of these nice radios in my collection and was thinking of replacing the Metal Rectifier.
Cheers
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Old 23rd Oct 2020, 2:21 pm   #18
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Default Re: Metal Rectifier in Pam TR 26

Quote:
Originally Posted by paulsherwin View Post
I've never needed to fit caps across rectifier diodes in valve sets. There may or may not be interference generated, but if it isn't audible it hardly matters.
I've also never found the need to fit any. This is one of those cases where the 'theory' says they're needed, but in reality they never are. There may be the odd unusual case and there can be a first time for anything. So don't worry about the diode unless there's an obvious problem.
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Old 23rd Oct 2020, 2:30 pm   #19
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Default Re: Metal Rectifier in Pam TR 26

A capacitor across a half wave rectifier in valve receivers was often fitted originally in some models.

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Old 24th Oct 2020, 9:08 am   #20
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Default Re: Metal Rectifier in Pam TR 26

Hi.
I've found that adding the cap across the rectifier will cure stubborn cases of modulation hum.
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