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Old 8th Nov 2018, 1:34 pm   #1
G3PIJpeter
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Default FT200 mains power supply

I have recently revived a car boot sale Sommerkamp FT250, which is the same as the Yaesu FT200. Considering the heterodyne oscillator crystals that I have replaced, the previous owner seems to have been a 27 MHz CB devotee and MARS (6.5-7.0 MHz) contributor. One of the 'sweep toob' 6JS6s also had a shattered envelope and a large hole burned through its anode. Substituting 6146Ws with 600 V on the anodes and 160 V on the screens, I have an output of 50 watts carrier / 70 watts peak SSB on 80 and 40 metres.

The accompanying FP200 mains PSU is advertised as putting out 600, 150, 300 and -100 volts. All of these are correct, except the 300 V line which measures at 370 V (on-load). Various sources on the internet speak of this HT line being either 300 V or 350 V. Nothing runs super-hot, even the class A audio output stage, which takes the full 370 V on its anode.

My questions are:
1. What should the HT line voltage be - 300 V or 350 V?
2. Is my HT running at 370 V likely to inflict lasting damage, despite the apparent absence of over-heating?
3. Just what should the screen voltage of AB1 6146s be? (Data books say 180 but many designs use 250 V or more).

Many thanks

- Peter G3PIJ
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Old 8th Nov 2018, 3:37 pm   #2
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Default Re: FT200 mains power supply

That is within 10% and most components in the valve era were 20%.
I would sign if off as fine.
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Old 8th Nov 2018, 5:46 pm   #3
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Arrow Re: FT200 mains power supply

370 v.d.c. when you're expecting 300 v.d.c. If available, might be worthwhile studying the cct. diagram. There might be an adjustment for that HT voltage - or, OTOH, there might be a fault condition.

Al.
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Old 8th Nov 2018, 5:48 pm   #4
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Default Re: FT200 mains power supply

I got the impression that most units have a rated value of 350V.
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Old 9th Nov 2018, 1:09 am   #5
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Default Re: FT200 mains power supply

The 2 x 135V AC (RMS) voltage would give about 380V DC rectified filtered voltage with no load.
So that 370V DC sounds about right.
The question remains why would they call that 300V DC?

Regards, Peter
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Old 9th Nov 2018, 1:01 pm   #6
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Default Re: FT200 mains power supply

Quote:
Originally Posted by orbanp1 View Post
The 2 x 135V AC (RMS) voltage would give about 380V DC rectified filtered voltage with no load. So that 370V DC sounds about right. The question remains why would they call that 300V DC?
That's exactly what I was thinking! But I still cannot understand how the AF output ECL82 / 6BM8 survives. It has the full HT (370V) on its anode and the screen grid is fed from the +150V line through a 1k resistor (R106). The data sheet at http://www.r-type.org/pdfs/ecl82.pdf suggests 272V for the anode and screen and a cathode resistor of 650 ohms - FT200 circuit Rk = 390 ohms (R101).

I have set the primary taps on the mains transformer for the maximum of 2 x 117V = 234V and feed this in via a variac, so I'm not over-running. However, it all seems to work and nothing is cooking - but I remain puzzled.

- Peter
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Old 10th Nov 2018, 12:49 am   #7
orbanp1
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Default Re: FT200 mains power supply

As you indicated, the voltages you list are the suggested operating voltages.
The maximum voltage ratings are higher, the values depending on which datasheet you look at.
See the RCA 6BM8/ECL82 datasheet:
http://frank.pocnet.net/sheets/049/6/6BM8.pdf
According to that datasheet the 6BM8 should survive in the FT-200 application (max voltages).
On the other hand, a quick simulation of the circuit shows that the pentode plate dissipation exceeds the max listed value, but that could be because of an "imperfect model" of the tube.
The model I used was from the Duncan web-site.

Regards, Peter

Last edited by orbanp1; 10th Nov 2018 at 1:06 am.
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Old 10th Nov 2018, 1:20 am   #8
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Default Re: FT200 mains power supply

On my set I wired in a 5k6 1K in series from the HT and the top of the 6BM8 output
transformer/screen grid, which is now decoupled by a 10uF/450V cap to ground.
Cooler valve, less hum.

Note the 6BZ6 can be replaced by 6CB6.
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Old 10th Nov 2018, 1:45 pm   #9
orbanp1
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Default Re: FT200 mains power supply

Quote:
Originally Posted by orbanp1 View Post
...
On the other hand, a quick simulation of the circuit shows that the pentode plate dissipation exceeds the max listed value, but that could be because of an "imperfect model" of the tube.
The model I used was from the Duncan web-site.
...
I did find another set of SPICE models for the ECL82, this time by Ayumi Nakabayashi.
The plate dissipation is still excessive, about 14.8W, while with the Duncan models the dissipation is 15.7W.
So it looks like the tube is indeed stressed in that application.
The RCA datasheet quotes the max plate dissipation as 5W, if the plate voltage is more than 250V, and 7W if the plate voltage is below 250V.
In the simulation, with 370V supply voltage, the plate-cathode voltage is about 330V.

Regards, Peter
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Old 10th Nov 2018, 2:47 pm   #10
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Default Re: FT200 mains power supply

Quote:
Originally Posted by orbanp1 View Post
... the voltages you list are the suggested operating voltages. The maximum voltage ratings are higher, the values depending on which datasheet you look at. See the RCA 6BM8/ECL82 datasheet at http://frank.pocnet.net/sheets/049/6/6BM8.pdf ...
That's a very interesting addition to the data sheets I have accessed so far. The general consensus seems to be: output power = 3.5 W with an anode dissipation of 5 W, which gives a total input power of 8.5 W. At 370 V, this power represents an anode current of 23 mA.

The suggestion of Restoration73 to wire a 5.6k resistor in the HT feed represents a voltage drop of 56V for every 10mA of anode current - a useful option. The alternative is to add some droppers to the PSU (as suggested by G3RZP elsewhere] to bring the HT down to somewhere between 300 and 350V for the whole set. Time to dig out some large WW resistors.

Many thanks to all

- Peter G3PIJ
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