Fisher 400 output valves current draw

[PaulR]

After several years in the loft I resurrected my Fisher 400 (strictly it is my son's).

I have replaced the coupling caps, the bias rectifier and smoothing caps and have put 1/4 watt 10 ohm resistors between the cathodes and ground. A previous owner has put a wire wound resistor in the mains line which reduces the voltage at the mains transformer to 200v. I am not sure why this has been done as the other voltages are low.

Anyway, I measured the voltage drops across the new cathode resistors and got, from left to right as seen from below

60mV, 114mV, 66mV, 148mV

The voltages remain similar at each socket if I swap the valves about.

The bias at each grid is -13v and the HT on each is 368V. These are lower that shown on the circuit diagram but this could be because of the mysterious dropper resistor. The set is using about 100W.

Push pull amps are rather outside my experience but I assume that each valve should draw about the same current and that if there are any differences, it would indicate a problem with the particular valve. In this case the ball park figures remain at the same valve holder.

Can anyone help with this, please?

Thank you

[ms660]

According to the valve data, 6, 6.6, 11.4 and 14.8mA sounds like low emission for whatever reason:

https://frank.pocnet.net/sheets/168/7/7868.pdf

Lawrence.

[snowman_al]

Some things there do not make any sense?

First Yes, the current draw for each half of the output pair should be near equal. And yes that will be dependant on the valve.

But the figures quoted are all way too low for a 7868 output valve. Even the 148mV figure says the current through that valve is 14.8mA, you would be expecting 40 - 50mA at least.
Also and possibly related is the 100 watt consumption figure. The heaters alone consume 40 watts.

Questions,
Is this a 240 volt version, most are 120 volt USA mains? What value is the wirewound resistor?
Are the output valves marked as 7868? Perhaps they have been replaced by another type.
Has it ever worked / does it work now at all?
Have you put the 'resistors' in the correct location?

Which model is it and do you have the service sheet / schematic? HiFi Engine have them. Easier to reference using their numbers than 'left to right' as we do not know the front or back...
Alan

[theredhouseinn]

Hi Paul.
Do you have a circuit diagram?
Are there screen resistors and decouplers?
Why have you put 10ohm resistors in the cathodes?
Measure the resistance of each half the primary winding on each o/p transformer, they should be identical for respective halves. The innner winding half may be slightly lower than the outer winding half.
Feed in a 1kHz signal and measure the level at each anode. Does the sine wave look distorted.
Measure the distortion level at each output into an 8ohm load.
John

[theredhouseinn]

Hi Paul.
Just found the maual on hi-fi engine site.
I repaired one of these about two years ago.
The mains tx was missing as were most of the ht supply caps and resistors, and most valves.
I assumed that it had been plugged into 240v mains as this model is for use on 120v American supply.
Take out the resistor in the mains supply and run it from an auto tx, or, I use one of those yellow site tx's used for power tools for this type of amp. supply.
When I finally got mine working the quality of reproduction was awsome.
john.

[ronbryan]

It could be that the fixed bias on the control grids of the 7868s is incorrect, leading to the valves being nearly cut off. Perhaps you could check that the control grids are at -17V as shown in the service manual.

Ron

[PJL]

From above, the bias is -13V which if anything sounds too high (should be more -ve). I think we need all of the valve voltages (g1, g2, anode, and heater AC) and confirmation they are 7868 types before we can determine if the valves are OK.

[PaulR]

Thanks for all the replies. I will try to answer the easy ones first then I will look at it again..

I added the cathode resistors to allow me to calculate the current draw of the output valves. I used low wattage ones so that they would burn out if the valves drew too much current. One of the output transformers had been replaced by a non-Fisher one and I didn't want to risk them again. My son bought the correct transformer several years ago so they are now both correct. The resistors as on pin 3 of each valve which was previously connected to ground.

The added resistor is dropping 40 odd volts to bring the voltage on the mains transformer down to just over 200v. The set is marked 110v but I assume that the transformer is a 240v one as something nasty would have happened if it was run at 200v.

The HT is 368v on each valve. I did check the voltage on the screens and it was correspondingly lower than that shown on the circuit diagram but I cannot remember exactly what it was. I haven't rebuilt the main power supply as everything seemed ok

They are the original Fisher 7868s. The voltages remain similar at each valve holder when the valves are moved about i.e. it doesn't follow the valves

It did work fine when it was put away but I haven't actually put a signal through it. I have no speakers connected but there is a speaker off switch which is off. I have headphones connected. The nearest to a signal I have had is the white noise from the FM tuner which sounds ok through the headphones.

I can certainly put a sine wave through it an analyse that through an oscilloscope,

It is a Fisher 400.

Does that information help?

[PaulR]

PS. I have just played a CD through it and it sounds very good through headphones.

[theredhouseinn]

Oh no! Paul. a CD digital junk. At the very least play a cassette tape through it.
Connect an aerial to it and listen to Radio3 or classic fm. or vinyl l.p. into phono sockets.
Analogue will never die.
John.

[PaulR]

Well, I could have set up a turntable and played an LP but there isn't space and a CD is more convenient.

[PJL]

Paul, based on the voltages across the 10 ohm resistors it is far from operating correctly as the cathode currents are all too low and very mismatched. It could be the output valves all need replacing but we can't tell without the voltage measurements, anode, cathode, grid, screen and AC heater.

Are you sure your added resistors are 10 ohm?

[PaulR]

I had the same thought about the resistors and measured them. They are all 10 ohm. I even tried a different multimeter just in case!

The voltages, except for the cathodes, are the same for them all.

A 373, Sc 244, Heater 5.95 AC Grid -13v

I cannot find a component plan to show which valve is which but these are the cathode voltages. These are similar even if I swap the valves about - they do not follow the valves themselves.

LH channel V13 V14 - 121mV, 63mV

RH channel V15 v16 - 116mV, 58 mV

EDIT

Apologies, the voltages do follow the valves. I have to keep turning it over to get the valves out and it weighs a ton. New set of valves

[PJL]

There is something very wrong then as swapping V13 and V14 should also swap the cathode current measurements! Are the 10 ohms definitely connected between the cathode (pin 3) and chassis? I wonder if it's a problem with your meter as there will be an AC component.

The screen voltage looks low, anode 373(395), screen 244(345). As you are 20V down on the anode I would expect a slightly smaller drop on the screen so it should read >325V.

[PaulR]

Sorry, I misled you. The particular voltages do follow the valves. I must have done a "double swap" earlier and put the ones with the lower cathode voltages in the equivalent sockets. If I swap the "high cathode voltage" ones for lower ones then it is clear that the voltages are particular to the valves.

[PJL]

It looks like you need new valves but don't buy the Russian 6P41S as they are not compatible. Tracking down the low screen voltage fault will improve the anode current a little. Measure the voltages along the chain R105, R104, R103 and R95 then compare to the service sheet.

[PaulR]

Thank you, I will do that. I see that new 7868s are made but it is really up to my son as it is his set. It sounds amazingly well in view of the badly mismatched valves.

I will report back tomorrow.

[Diabolical Artificer]

Watch out using low wattage sense R's on the cathodes,a lot of resistors don't fail OC, they go high, biasing the valve at high current. An easy fail safe is to use the bias to power a relay, the relay contacts across the HT or heater supply, if the bias fails relay drops out cutting HT.

For now pair the 116mV & 121mV valves and the 58mV & 63mV . With the OP valves out try giving the bias adjust pot a twiddle and if you can a squirt of cleaner. Whilst your at it measure the DCR of the OPT from CT to anodes. Put the valves in as suggested them measure the voltage from CT to each anode,then uses ohms law to find Ia. If there's a screen grid resistor you could do the same there.

It sounds like something is off, even if the valves are tired they should pull more current than that. A clean of the valve bases might help too.

Andy.

[theredhouseinn]

Hi Paul.
You can get new National 7868 gold line valves from Watford Valves @ £55.00 each + VAT
John.

[snowman_al]

@ Andy (DA), unfortunately the bias is not adjustable, it is derived off a transformer tap and bridge, what you get is what you get. It also powers the heaters for the pre-amp stages.

@ Paul, there are several versions of the 400, there are subtle differences, but component identification changes from one schematic to another. You need the serial number to match the schematic. We can follow you then. Can you find the serial?

And as PJL says do not sub the often quoted replacement 6P41S.