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Old 24th May 2021, 10:56 am   #41
PaulR
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Default Re: Fisher 400 output valves current draw

Ah thanks. It doesn't look like 7w and I didn't check. I have ordered a 10w one.

I did replace the bridge with a very similar looking 600v 2A one, but I am wondering about going the whole way and replacing the mains caps which, looking at the layout in the Audiokarma thread, will mean using the space formerly occupied by C60. In that case one that fastens to the chassis would be more stable.
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Old 24th May 2021, 12:30 pm   #42
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Default Re: Fisher 400 output valves current draw

Could you measure the AC volts from the transformer winding that feeds CR1?
I'm guessing, but suspect it should be 16 volts. That gets you about -22.5 volts DC at the smoothing cap C60B. If everything is normal.

A little more research (google) gives a figure for the static cathode current for the 7876 in the 400. Recommended is 32 - 35mA per valve. That seams sensible with an +HT of 400 volts. And gives a dissipation of 14 Watts per valve, fine for a 19 watt valve.

One strong recommendation is to replace the grid leak resistors R117, R118, R119 and R120. They are nominally 330kΩ, but that is a little outside the manufactures spec and 220kΩ is advised. Especially if you go with the Electro Harmonix replacements. Check them anyway, if they go high there is a chance of thermal runaway once you get the DC voltages correct.

Once you get the replacement R41 in we need a new set of voltage readings.
Personally again, I would replace the Mains resistor and use a bucking transformer for further testing. Once everything is in order you can have the original rewound or what ever suits...

PS. Looking back at your pictures, the resistor in the white circle does not look original. It it part of the bias circuit? Connected to C60A and or B?
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Old 24th May 2021, 4:13 pm   #43
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Default Re: Fisher 400 output valves current draw

Al -

I am getting 22v on the AC side of the rectifier and 26v DC out.

Noted regarding the grid leak resistors.

I am a bit reluctant so spend money on a bucking transformer then to have the mains transformer rewound. I will wait to see how things are when the new resistor arrives and what my son decides about the power supply

That white transistor replaces R138. I increased the value from 15R to 22R in an attempt to make the bias more negative, but there is now apparently more wrong than that. I suppose that if it ends up more negative than per the circuit diagram that is no bad thing.
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Old 24th May 2021, 4:58 pm   #44
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Default Re: Fisher 400 output valves current draw

Quote:
Originally Posted by PaulR View Post
... I am getting 22v on the AC side of the rectifier and 26v DC out.
That white resistor replaces R138. I increased the value from 15R to 22R in an attempt to make the bias more negative, but there is now apparently more wrong than that. I suppose that if it ends up more negative than per the circuit diagram that is no bad thing.

Ah ha, increasing R138 will reduce the bias voltage. The bias voltage supplies the heaters for V8, 9, 10 and 11 as well, so adding more resistance, in series, lowers the voltage at the heaters and that is where the bias is taken from via R35.
The schematic is a little odd, have a look at it as I have re-drawn it. It might be a bit easier to understand. Sorry it's scruffy...
Put the 15Ω resistor back in.
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Old 24th May 2021, 5:00 pm   #45
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Default Re: Fisher 400 output valves current draw

Ah right I will put it back.

I have replaced the grid leak resistors for 220K ones. Fisher must have used good quality ones as they were spot on 330K.
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Old 25th May 2021, 11:25 am   #46
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Default Re: Fisher 400 output valves current draw

Hi Paul.
I think that you are going down the wrong path here.
You have proved that the 4 o/p valves have different degrees of low emmission, and that the mains supply has been bodged for the set to work on 230v.
So, before you go any further, you should fit 4 new o/p valves, replace the original value resistors, remove that resistor from the mains supply, and then feed the tx with 120v, and then make tests to determine what fault, or faults exist.
Reading through the posts, it seems to me that you are trying to alter component values to counteract faults. As the valves continue to loose emmission you will be back to square one.
This really is not the way to repair a vintage amp.
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Old 25th May 2021, 12:01 pm   #47
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Default Re: Fisher 400 output valves current draw

Thank you John.

This all started with the valves drawing far less current than they should and low bias voltage. The valves are what they are and my son, who owns the amp, is inclined to buy the NOS ones from Watford valves. The bias is probably low because the feed resistor has gone way out of spec. I am waiting for a new one as a start.

We have determined that for some reason the output transformer has a 220V primary and I think that the best way forward is to fit the new bias resistor and see the effect of that then get the transformer rewound to give it a 240V primary. If it was a 120V USA one I would get a matching transformer but as it is an odd voltage I feel that the most satisfactory approach is to get it rewound to take it to UK voltage. I have had a quote for doing this.

Amazingly is sounds quite good with the wildly mismatched, low emission valves but I agree that they need to be replaced. The only alterations remaining are tacked in 10R cathode resistors which give something across which to determine the current draw and reduced value grid leak resistors which I fitted following Al's advice.

Let's see what the new bias resistor achieves then I can ask for more advice. I am not looking forward to desoldering the mains transformer but I feel it is the best approach.

Thank you
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Old 25th May 2021, 1:20 pm   #48
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Default Re: Fisher 400 output valves current draw

I still think you would be far better off leaving the mains transformer alone and adding a bucking transformer. Most far east countries use 220V and if your son ever decides to sell that is where it is most likely to end up!

R41 is causing the screen voltage to be low and a low screen voltage will reduce the anode current. If we are lucky we will end up with a good pair so only one replacement pair needed.

The fixed bias voltage MAY need fiddling to work with new valves but there are other issue to resolve such as the low LT before we get to that point. At the moment the bias voltage is -14V(-17V) but there is a 330K resistor in the path so we would need to know the inpu impedance of the meter.
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Old 25th May 2021, 1:39 pm   #49
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Default Re: Fisher 400 output valves current draw

I cannot ignore your advice, PJL. Could you point me in the direction of a suitable transformer and I will leave the final decision to my son?

The meter is a Tenma 72-14610 and the handbook quotes the input impedance as 10M. I am actually bidding on a valve voltmeter as I have always wanted one!
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Old 25th May 2021, 2:26 pm   #50
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Default Re: Fisher 400 output valves current draw

There are plenty on ebay. I would suggest two 0-12V secondaries and 50VA should more than do it. The two secondaries will give you an extra adjustment 220, 232, 244.
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Old 25th May 2021, 4:57 pm   #51
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Default Re: Fisher 400 output valves current draw

What should I search for on ebay?
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Old 25th May 2021, 6:13 pm   #52
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Default Re: Fisher 400 output valves current draw

There is a toroid here: https://www.ebay.co.uk/itm/121206467447
Or conventional here: https://www.ebay.co.uk/itm/383694537154

Does anyone else want to chip in to confirm the rating is OK?
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Old 25th May 2021, 7:28 pm   #53
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Default Re: Fisher 400 output valves current draw

It's a pity there isn't anywhere in the cabinet to fit a transformer. It would have to go in a separate box outside it. I have to say that I am still in two minds as to the best solution.

The bucking transformer would mean there was no need to remove the existing one and would be much simpler but it would need a separate case. Having the existing one rewound by Ed would mean a fiddly job of disconnecting and then reconnecting it. I don't think that my son would want to sell the amp so any reduction in value would not be a problem and the amp could be plugged straight into the mains.

I mentioned "elegant solution" in an earlier post regarding having the transformer rewound but I can see that reducing the mains voltage using a bucking transformer could be regarded as more elegant.

I think I will leave the final decision to him.

I assume that I wouldn't need to attempt to replicate the existing phasing of the various secondaries if I removed the existing transformer for a rewind?

Thank you
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Old 26th May 2021, 12:17 pm   #54
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Default Re: Fisher 400 output valves current draw

I think the 50VA is probably best as that gives 500W at 240V and the amplifier includes some USA external sockets for accessories that should not be used but I guess someone might have a go.
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Old 26th May 2021, 1:08 pm   #55
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Default Re: Fisher 400 output valves current draw

According to the owners manual (page 12) the receiver consumes a maximum of 200 watts a full power and 160 watts normally.
So, assuming the mains transformer is actually a 220volt unit, the bucking transformer secondary needs to be at least 1 Amp, either of the ones PJL references is fine, both have secondaries able to deliver approximately 2 Amps and that gives a good margin and will run cool.

Thinking about the bucking transformer, why not have it in a box with a short lead with a 13 amp plug as input. For the output use a Euro 220 volt type socket and fit the appropriate Euro plug to the amplifier lead? Just a thought.

If you do get the original transformer rewound, you do not need to worry about the 'phasing' of the secondaries.
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Old 26th May 2021, 1:22 pm   #56
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Default Re: Fisher 400 output valves current draw

If there is a possibility that your son may sell the amplifier, do not underestimate the stupidity of some potential purchasers. Some will ignore instructions regarding special precautions and special requirements, blow things up, and then turn litigious. A clean solution reduces these risks (nothing eliminates them)

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Old 26th May 2021, 1:28 pm   #57
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Default Re: Fisher 400 output valves current draw

Thank you for the suggestions. I discussed it with my son last night and he would like to go down the path of having the transformer rewound. I can see the advantages of both solutions so I thought I would leave it up to him. He said he never envisages selling it, particularly if he buys a set of the NOS ones from Watford

For the moment I am going to get the voltages as near as possible to being correct with the current set up taking into account the low voltage on the transformer then sort out the power supply. I have also sent off for a class Y cap to replace C64.
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Old 26th May 2021, 1:51 pm   #58
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Default Re: Fisher 400 output valves current draw

Personally I would remove C64 and R44.
C64 is commonly known as 'the death cap'...
If you do replace it you Must ensure it is connected between the Neutral and Earth. In which case R44 is redundant.
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Old 26th May 2021, 1:56 pm   #59
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Default Re: Fisher 400 output valves current draw

Well, I did consider simply removing them as I would do that with an "ordinary" radio. I will do that and just keep the Class y one for future use
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Old 26th May 2021, 9:37 pm   #60
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Default Re: Fisher 400 output valves current draw

The lower rated 1.2k resistors arrived today so I thought I would tack one in to see the effect.

The output valves are now getting very hot - I can smell the heat.

The heater voltage is 5.5 volts so the voltages will be a factor of 6.3/5.5 low

The Screen voltage is now 295V which equates to 338 (345 per the circuit diagram)

The valves are drawing
296mV
466mV - This was increasing as it warmed up. I switched it off when it got to 550mV and rising
302mV
390mV

The Bias is now only -11.5V though. In view of the hot smell and the increasing current draw of that valve I didn't leave it on for long though.

EDIT With no output valves the bias is -13.85 which taking into account the low voltage equated to 15.86V

I need to sort out the power supply don't I?
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Last edited by PaulR; 26th May 2021 at 9:45 pm.
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