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Old 4th Feb 2023, 2:52 pm   #1
_Garak_
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Default Hacker Mayflower ll RV20

I recently purchased this radio which was working but I noticed that both of the ECL86 output valves were running very hot.

I was always planning to replace out-of-spec components which I have now done (including re-stuffing the two capacitor cans) and installing silicon diodes in the rectifier along with a series resistor to drop the output voltage a tad.

This has all been completed and I have checked voltages around the two ECL86's against those listed in the schematic and these voltages are OK however the valves still run very hot.

I have been informed that these valves in this particular set do run hot but I don't know anyone who has one to ask for advice, so can anyone here who owns one of these tell me if these valves run hot please.

BTW, I also have a 1st generation Mayflower RV14 which uses the same ECL86 valves and these run much cooler.

Mayflower ll
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Old 4th Feb 2023, 3:00 pm   #2
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Default Re: Hacker Mayflower ll RV20

If you're worried about cooking the ECL86s you could always try increasing the values of the cathode-resistors to bias them back a bit and reduce the standing current.

Yes you will lose a bit of volume/maybe increase the distortion at full-volume, but it will reduce the heat and prolong the lives of the valves .

[I often fit a 500-Ohm cathode resistor in place of the standard 270-Ohm one to radios using 6V6 output-valves].
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Old 4th Feb 2023, 5:38 pm   #3
Goldieoldie
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Default Re: Hacker Mayflower ll RV20

Have you tried swopping the valves over with the good Mayflower .
Iíve had this problem and it was the valves themselves
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Old 4th Feb 2023, 6:49 pm   #4
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Default Re: Hacker Mayflower ll RV20

What was wrong with the wire wound resistors?
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Old 4th Feb 2023, 7:06 pm   #5
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Default Re: Hacker Mayflower ll RV20

A cathode resistor that has changed its resistance over time, or a leaky cathode bypass capacitor can alter the effective grid bias and then cause excessive current in the valves.

Do you know the actual resistance of the cathode resistors?

If you can measure the voltage across the valves and the voltage across the cathode resistors you can calculate the power dissipated by the valve.
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Old 5th Feb 2023, 11:47 am   #6
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Default Re: Hacker Mayflower ll RV20

Quote:
Originally Posted by Silicon View Post
If you can measure the voltage across the valves and the voltage across the cathode resistors you can calculate the power dissipated by the valve.
Looking at the picture in post #1 it looks like most components have been changed, which I guess means the cathode resistor and bypass capacitor are new.
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Old 5th Feb 2023, 7:42 pm   #7
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Default Re: Hacker Mayflower ll RV20

Calculate the anode dissipation of the pentode section and see whether it is less than 9W. Leave your valve some "wiggle room". In a nutshell:

(Anode voltage minus cathode voltage)*anode current = anode dissipation
Anode current + screen current=cathode current.
You can also get the anode current from the voltage drop across the output transformer

Read here:
https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Also monitor the cathode voltage for 15 minutes. I don't know if the ecl86 tends suffers from thermal runaway but I've had some gassy el84s in the past

Why did you change so many resistors? 20% tolerance is generally acceptable in most locations
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Old 6th Feb 2023, 7:14 pm   #8
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Default Re: Hacker Mayflower ll RV20

I have a Blaupunkt "Sultan" that has a single-ended ECL86 as the output valve. When I first got it, it would sound good for about 5 minutes then start distorting which got worse. Somebody suggested 'grid emission' and to check the anode current by measuring the voltage across the cathode resistor. It started off from cold at a reasonable value - can't remember it now - but after a few minutes the cathode voltage started to rise and the distortion set in. It took three other ECL86's before I found one that didn't have the effect.
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Old 8th Feb 2023, 2:16 pm   #9
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Default Re: Hacker Mayflower ll RV20

Manufacturers often pushed the valves beyond their design limit so they sounded 'better' than the competition, not so good now replacement valves are not available off-the-shelf.
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Old 8th Feb 2023, 3:30 pm   #10
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Default Re: Hacker Mayflower ll RV20

1) ECL86s tend to run hot.
2) ECL86s are often 'troublesome' valves. Their design attempts to get a quart out of a pint pot.

If, after all that you've done, the set seems to work fine, then I'd just accept the valves running hot and now enjoy it.

You may be interested in the tone correction mod: https://www.vintage-radio.net/forum/...d.php?t=110877
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Old 9th Feb 2023, 9:28 am   #11
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Default Re: Hacker Mayflower ll RV20

The TV equivalent the PCL86 proved to be very reliable more so than the PCL82.
The few PCL86's I have had to change in TV's were due to the valves going microphonic or just going low and distorted. Sometimes we would find the cathode bias components needed changing and a few times the coupling capacitor.
Reading all the previous posts I have seen no mention of checks or changing of "That Capacitor" or the coupling capacitor.
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Old 9th Feb 2023, 10:08 am   #12
Leon Crampin
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Default Re: Hacker Mayflower ll RV20

The PCL and ECL86 are the same, heaters aside. The reliability of the PCL in TVs is almost certainly due to lower HT voltages and hence lower dissipation for a given anode current. I would agree, that at their published ratings, these valves are overrun for long life. The same applies to the EL84.

So, it's tempting to use the cheaper PCL as an ECL substitute and re-arrange heater supplies. The problem I found with this was that the PCL heater voltage is ill-defined. The actual specified voltage varies a bit between suppliers, but when I measured 3 samples from different makers when supplied with a constant 300mA, the measured voltage differed by about 1.5V. Optimum parallel heater operation therefore may be difficult in terms of achieving the right cathode temperature at nominal mains voltage.

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Old 9th Feb 2023, 10:39 am   #13
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Default Re: Hacker Mayflower ll RV20

Quote:
Originally Posted by stevehertz View Post
1) ECL86s tend to run hot.
2) ECL86s are often 'troublesome' valves. Their design attempts to get a quart out of a pint pot.

If, after all that you've done, the set seems to work fine, then I'd just accept the valves running hot and now enjoy it.
I like that attitude, but before sitting down with a cup of tea, I would compare voltages, and current through the OP transformers between the two sets.

It would be sad if faulty valves burned out the OP and/or mains transformer.
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Old 10th Feb 2023, 7:40 pm   #14
_Garak_
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Default Re: Hacker Mayflower ll RV20

Quote:
Originally Posted by G6Tanuki View Post
If you're worried about cooking the ECL86s you could always try increasing the values of the cathode-resistors to bias them back a bit and reduce the standing current.

Yes you will lose a bit of volume/maybe increase the distortion at full-volume, but it will reduce the heat and prolong the lives of the valves .

[I often fit a 500-Ohm cathode resistor in place of the standard 270-Ohm one to radios using 6V6 output-valves].
I replaced the 220Ω cathode resistor with a 470Ω for each output valve and the valves are running cooler, I'm going to keep my eye on the situation but after a long soak test everything seems to be fine.
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Old 11th Feb 2023, 3:34 pm   #15
PJL
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Default Re: Hacker Mayflower ll RV20

You should check the HT voltage now you have lowered the current.
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