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Old 26th May 2021, 9:48 pm   #61
PJL
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Default Re: Fisher 400 output valves current draw

That looks promising. Try swapping the wayward valve with another just to confirm it is the valve that is at fault. Assuming it is the valve, you could try measuring the grid voltage. Is the gettering on this valve different from the others? You did replace the 0.047 coupling capacitors?
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Old 27th May 2021, 9:24 am   #62
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Default Re: Fisher 400 output valves current draw

Hi Paul.
You must sort out the tx problem, as the current increases through the o/p valves the voltage will increase across the series resistor in the mains feed reducing the voltage across the tx primary causing you more problems, i.e low heater voltage, etc..
Those o/p valves do get very, very hot, providing the anodes do not glow red, and the screen grid wires inside the anodes are not glowing red, the valves should be ok.
I agree with you in that the power supply needs sorting before you go any further.
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Old 27th May 2021, 9:42 am   #63
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Default Re: Fisher 400 output valves current draw

The increased current draw does follow the valve and the bias becomes less negative on that pair as well. The getters on all the valves are thin but it seems thinner on that one. I did replace the coupling caps.

I thought that the valves were getting hot before I replaced the screen feed resistor but they are now getting very hot. I was going to try to get a case or have one made but I wonder whether it will be better left without one. It could do with some forced ventilation.

I will arrange for a transformer rewind as that is clearly the next step so there will be a hiatus until that is done.

Thanks to everyone for the suggestions and help so far. I feel that we are progressing well.
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Old 27th May 2021, 10:28 am   #64
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Default Re: Fisher 400 output valves current draw

It's drawing grid current and either has some air in it or some internal leakage between the valve pins. If it's air and still has some getter it might recover but needs to be run for a while in a setup that does not allow grid current to impact the bias point. If it's leakage between pins it might be possible to blast it away using a gas hob spark generator.

I still think having a working high power mains transformer rewound is a risk not worth taking.
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Old 27th May 2021, 11:01 am   #65
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Default Re: Fisher 400 output valves current draw

I wonder if there is anyone in your area Paul who has a Variac they could lend you for a couple of days?
That way you could ascertain the real 'mains voltage' suitable for the transformer you have. Once you get that, the bias voltage, HT voltage and heater voltage will fall into place and you can make a proper assessment of the smoothing caps, valves etc. etc.

You will also have more exact AC voltage data if you do decide to have the transformer rewound.
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Old 27th May 2021, 12:04 pm   #66
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Default Re: Fisher 400 output valves current draw

The 7868 datasheet quotes max dissipation as 19W anode and 3.3W screen and the 55mA cathode current is around 20W total dissipation so it is unlikely to be thermal runaway.

Fixed bias that is not adjustable is a weakness here along with the design that requires the DC supplied heaters to reach thermal equilibrium before the bias voltage settles and the bias voltage falling if the overall current demand on the mains transformer increases. Reworking the bias arrangements looks like a very good idea.

I agree that the poor regulation caused by the resistor in the mains input does not help us understand what is going on. Do you have any low voltage transformers that you could hook up as a bucking transformer?

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Old 27th May 2021, 12:32 pm   #67
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Default Re: Fisher 400 output valves current draw

Ok. I cannot ignore the advice of someone who knows far more than I do about this so I have ordered the toroid transformer and will use this permanently. My inclination would have been to go for the conventional one but I think the toroid might be easier to mount in a box. I might use a 5A round pin plug as the connector. That would mean it couldn't be plugged into the mains without at least a bit of thought. Alternatively there is something called a 3 pin Agro plug and socket on ebay that might make an interesting alternative

I don't know anyone who has a Variac. I would buy one as it would be useful for other things but ones that have a 240V primary seem quite expensive on ebay.

I would be very interested in suggestions for alterations to the bias supply.
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Old 27th May 2021, 12:59 pm   #68
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Default Re: Fisher 400 output valves current draw

Regarding the Variac, I wondered if anyone reading this thread might offer or if you put a 'Wanted to borrow' in the Wants Section? It might still be worth an ask.

The bucking transformer will get you a fixed mains voltage at your transformer.
You can make it 'fool' proof I'm sure...

The bias voltage is set on this amplifier by the voltage from the tail of the valve heaters V8 and V10. That then feeds to R35 (1k). R35 and R34 (2.2k) form a potential divider so that the voltage at the end of V8/10 is dropped by approximately 1/3rd. So if you had 24 volts DC at the heaters, you should see about 18 volts on the grids. Give or take.

You can adjust the bias two ways. First change the ratio of R34 to R35 and that changes all the output valve bias voltages equally. With a good reasonably matched set of valves that might be adequate.
More sophisticated is to have a separate adjustment for each output valve. That is covered in the links I posted way back in the thread as ''Output Tube Balance and Bias Circuit''.

(https://audiokarma.org/forums/index....er-400.689121/)

There are several options ideas for this on other threads as well.
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Old 27th May 2021, 1:43 pm   #69
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Default Re: Fisher 400 output valves current draw

Must look at the circuit properly! As you say, looks easy enough.
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Old 27th May 2021, 4:24 pm   #70
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Default Re: Fisher 400 output valves current draw

If the resistors are as shown in the circuit and the mains voltage is correct then it will operate as intended by the manufacturer. There’s no need to modify it.
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Old 27th May 2021, 6:59 pm   #71
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Default Re: Fisher 400 output valves current draw

Quote:
Originally Posted by vidjoman View Post
If the resistors are as shown in the circuit and the mains voltage is correct then it will operate as intended by the manufacturer. There’s no need to modify it.
In a perfect world, but these valves are now rare and originals are expensive and risky. There is a current production 'replacement' but they are not perfect running at higher anode currents. Being able to change the bias would also mean it can be optimised to make best use of the existing used valves.
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Old 28th May 2021, 1:16 pm   #72
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Default Re: Fisher 400 output valves current draw

When the transformer arrives how should I wire it up to get the required bucking effect?
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Old 28th May 2021, 1:28 pm   #73
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Default Re: Fisher 400 output valves current draw

Look at the info posted at #30 https://sound-au.com/articles/buck-xfmr.htm
It explains 'bucking' and 'auto transformer' operation. Figure 4 is my preferred method.
A quick check with your meter will get the phasing sorted if in doubt.
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Old 28th May 2021, 1:40 pm   #74
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Default Re: Fisher 400 output valves current draw

Ah - I see. That is a very useful diagram. The transformer I have bought has two 12v secondaries. Do I connect these in series to get the required reduction in voltage?
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Old 28th May 2021, 2:13 pm   #75
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Default Re: Fisher 400 output valves current draw

Yes wire the 2 secondaries in series. That will subtract about 24 volts from your mains voltage depending on the load etc.
(You may have 2 primaries as well, wire them in series too!)

Once you have it working as a normal transformer, next step is to join the secondary to the primary. So Live to the secondary point X, then secondary point Y to the primary A. Neutral goes to primary point B.
Points A and B are now the 220 volt take offs.
Test the voltage between point A and B using a 40 or 60 watt incandescent light bulb.
Now swap the secondary round so Live goes to point Y and the point X goes to A. Measure the voltage again. Which ever test gives the lowest voltage between A and B is the correct phasing for an auto transformer.

Easier to do than write...
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Old 28th May 2021, 4:43 pm   #76
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Default Re: Fisher 400 output valves current draw

Don't connect the amplifier up whilst you are doing this! It might be easier to measure the AC volts between A and B with your DMM, if it is higher than mains then swap X and Y over.
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Old 28th May 2021, 6:59 pm   #77
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Default Re: Fisher 400 output valves current draw

With a bucking transformer, you have to get all the windings in the correct phase. If you have some the wrong way round, it will INCREASE the mains voltage rather than decrease it.

Be sure to do a test without the amplifier connected.

Toroidal transformers usually identify their winding connections with different colour sleeving on flying leads. There should be info with it on which colours tobridge and which to power to make a 240v primary (get this wrong and hopefully having a small mains fuse will save the transformer from damage) They will likely tell you the colours to bridge and use to add the secondary voltages.

But using it as a buck (or boost) transformer is unusual and they might not tell you the relative phasing between primarie and secondaries, so you have to go by trial and error. Your first attempt is as likely to give you a boost in voltage as it is to give you a buck.

No problem if you're expecting it. You just reverse the secondary pair to change boost to buck. Power it from a plug with a 1 amp fuse, just in case you have the primaries wrong.

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Old 28th May 2021, 7:02 pm   #78
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Default Re: Fisher 400 output valves current draw

Great, thank you. I am looking forward to its arrival. I think I will simply wire the box containing the transformer directly to the amp.

I will do lots of testing before I connect it up. I was thinking of testing it using low voltage AC first.

Edit - It looks like a Vigortronix transformer in which case the colour codes are given here

https://www.vigortronix.com/uploads/...c6623fe796.pdf
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Old 29th May 2021, 7:36 am   #79
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Default Re: Fisher 400 output valves current draw

If that is the transformer you get try this first.
It keeps the Blue = Neutral and Brown = Live output convention.
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Old 29th May 2021, 9:01 am   #80
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Default Re: Fisher 400 output valves current draw

I have a little trmr Paul you can have if you want to up your bias voltage, it's 0, 110, 200, 220, 240v pri & screen and 45v 50mA & 12v 120mA secondarys and measure 2" x 2" sq - 2 3/4" high. Using the 45v sec would give you about - 60v. Also happy to knockup a little board to rectify and smooth and adjust. Footprint should be about 2" x 2 1/2" combined.

Andy.
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