Re: Problem with a Fisher 400
- From: Jon Yaeger <jono_1@xxxxxxxxxxxxx>
- Date: Fri, 27 Jan 2006 19:36:54 -0500
in article 1138405715.834905.300240@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, fourkids
at dearly@xxxxxxxxxxxx wrote on 1/27/06 6:48 PM:
> All,
>
> I have searched and searched, and can't seem to find what I need.
>
> I have a Fisher 400 tube amp...family heirloom as it were...and I
> recently did a rebuild on it based on Charlie Kittleson (?) kit. I am
> an EE...so good and nerdy, fairly competent with a soldering iron, etc
> etc. I also have the original schematic my dad got when he bought the
> thing.
>
> So it works, but I have a couple of significant problems:
>
> 1) The tubes still run VERY hot...they ran hot before, and frankly, I
> think they are actually running hotter. I was a bit surprised. The
> rectifier has been replaced, which if I read things correctly, should
> almost make it run TOO cool. The mesh cover over the tubes is almost
> too hot to touch.
>
> I have read about checking the bias voltage, and I think that I know
> where I need to check it, but I am leary of having this thing open on
> the bench and on knowing the voltage levels in there. Besides, I am
> not sure how to get it turned over and on without damaging the
> tubes....
>
> 2) There seems to be a bit a signal leak at least at the headphone
> jack...with the volume all the way down, I get music through the right
> ear...
>
> The second one is pretty minor. I guess what I am looking for is some
> guidance on checking voltages, etc, to make sure I havn't screwed
> something up. The schematics show a number of voltage check points,
> which I can do. I have a goof Fluke MM, but no osciliscope, so I can't
> check signal anywhere.
>
> Anyone out there done a "rebuild" and have any pointer for me???
>
> Thanks in advance.
>
> Dave Early
> Colorado
Dave,
Fisher pushed the output tubes back "in the day" when the mains voltages
were about 117. Now that average voltages are higher, it pushes the tubes
even more.
You should raise the negative bias voltage on the 7868 tubes.
1. Install 10 ohm resistors between each 7868 cathode and ground. These
resistors also act as fuses and can protect the outputs in the event of a
short.
2. R34 is a 2.2K shunt resistor that follows R35, a 1K resistor. Raising
the value of the shunt resistor will raise (i.e. make more "negative") the
bias voltage.
3. I'd replace the 2.2K shunt resistor with a 1K resistor wired in series
with a suitable 5K pot. This will allow you to vary the bias.
4. Measure voltage across the 10 ohm cathode resistors. 0.25 volts
corresponds to 25 ma current. Set the bias between 15-30 ma (I usually pick
about 23 ma) per tube.
5. Since there are no individual bias pots you really need a matched quad
of 7868s. Failing that, you can install multiple pots to adjust pairs or
individual tubes.
6. You might have a leaky speaker switch or perhaps an imbalance between
the two outputs causing sound in the headphone jack. Note that the
impedance selector wires connect to each side of the earphone jack through
330 ohm resistors. Also check the value of these resistors.
- Jon
.
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