Fisher 500C voltages
Regats
I recently acquired an old, long unused Fisher 500C receiver which
I brought it up to normal AC voltage slowly, over two hours, using a Variac.
I haven't put any in it tubes yet.
The power amplifier part of this receiver uses one 12AX7 phase-splitter and
two 7591A output tubes for each stereo channel.
I found a schematic for this receiver:
This is what the schematic calls for (in parentheses) and what I'm reading
on a Fluke 27 F/M meter:
12AX7 pin 1: (310) 461
12AX7 pin 2 & 6: (130) 441
12AX7 pins 3: (131) L:126.4 R: 128.1
12AX7 pins 4 & 5: 3.34 VAC to ground
12AX7 pin 8: 1.1
7591A pin 2: .4 VAC to ground
7591A pin 3: (430) 489
7591A pins 4 & 8: (375) 474
7591A pin 6: (-17) 0
7591A pins 7: 3.43 VAC to ground
House AC current: 122 V AC
Both sides measure the same.
I don't want to risk damaging some new EH tubes if these voltages are as
grossly out of spec as they appear - or is it normal that some voltages
measure much
higher than spec when there is no resistance from the tubes to bring them
down?
Many thanks for your time and expertise.
____________ Marc Stager
NYC
Jon Yaeger
9/14/08 4:40 PM:
1. You should never apply full line voltage to an amplifier with no output
tubes. As you have correctly surmised, the voltages will be much higher
without the current load that the tubes present in circuit;
2. You might damage the filter capacitors under the conditions previously
described;
3. It is not unusual for the selenium rectifiers in Fisher gear to have
failed. A reading of 0 volts on pin #6 of the 7591s indicates such a
failure. Plugging in a good tube without sufficient negative bias will soon
destroy it and maybe even the output transformers;
4. The original Fisher circuit ran the output tubes very hard. Once the
selenium rectifier is replaced with silicon, a dropping resistor should be
added and the shunt resistor changed so that you get about -21 volts of
bias. The original Westinghouse or other US tube could take the punishment,
but it might be too much for the EH unless you change the circuit as noted.
5. I always insert 10 ohm 1/4 watt resistors between the cathodes of the
output tubes and ground to act as a fuse and to provide a convenient way to
measure bias voltage and current.
Jon
Lanzo Garbanzo De Ceased
After months went by, I finally got back to working on this old Fisher
receiver.
It is in pretty beaten up shape so I decided to just use it as a power
amp, so I cut the wires from the output of the volume controls and
connected them to two unused RCA jacks.
It turned out that R142, the 15 Ohm resistor at C97 was not connected.
After I wired it back in, I put in a nice set of matched 7591A EH tubes
and a pair of EH 12AX7's.
I got a bias voltage of -24V, but the other voltages were still higher
than spec. But nothing exploded so I thought I'd give it a listen.
Pleasant surprise! A sweet, beautiful sounding 500C amplifier! (Just
using it above 300Hz as the upper half of a biamped setup).
So now I want to gild this lily, so to speak.
There was no noise or hum so it appears the PS caps are OK, and the
rectifier is working fine.
I replaced the CR5 and 7 diodes with 10 am HexFreds
I also ordered Kawame resistors for R121 - 129 and the 47K at the
inputs. (Are those really necessary?)
The coupling caps are (West) German EroFol .047 uf caps.
Maybe these could be improved with FT-2 0.047uF 600V Teflon caps
(eBay # 380108073840) or Paper-in-oil K40Y-9 0.1uF 630V (eBay #
380109913556)? I've read some great things about those
Russian caps, and they seem to be very promising.
Will the slightly larger .1uf caps be an improvement or should I
just stay with the stock value of .047 uf? ...
Or should I just leave the existing caps as they are?
Since I'm not using the phono section, that leaves a nice DC filament
supply unused. A simple matter to run them to the phase splitters - but
the supply is meant for four tubes (V14-17) so running to two (V12 and
13) might not work well? (The four phono pre tubes are in a
sereis-parallel array. Perhaps a 34 ohm resistor (which is the DC
resistance between pins 4 and 5) across pins 4 and 5 of each
phase-splitter tube might compensate?) Incidentally, I'm still getting
24 V of bias at pin 6 of each of the 7591A's. Maybe DC at those
filaments would bring that down a bit, since there is nothing damping
the DC voltage from the rectifier. (Is it OK to just leave it at -24 or
will the sound suffer?
Thanks again, Jon, for you expertise in these matter.
Best,
Marc
Lanzo Garbanzo De Ceased
After months went by, I finally got back to working on this old Fisher
receiver.
It is in pretty beaten up shape so I decided to just use it as a power
amp, so I cut the wires from the output of the volume controls and
connected them to two unused RCA jacks.
It turned out that R142, the 15 Ohm resistor at C97 was not connected.
After I wired it back in, I put in a nice set of matched 7591A EH tubes
and a pair of EH 12AX7's. I got a bias voltage of -24V, but the other
voltages were still higher than spec. But nothing exploded so I thought
I'd give it a listen.
Pleasant surprise! A sweet, beautiful sounding 500C amplifier! (
Just using it above 300Hz as the upper half of a biamped setup).
So now I want to gild this lily, so to speak.
There was no noise or hum so it appears the PS caps are OK, and the
rectifier is working fine.
I replaced the CR5 and CR7 diodes with 10 amp HexFreds
I also ordered Kiwame resistors for R121 - 129 and the 47K at the
inputs. (Are those really necessary?)
The coupling caps are (West) German EroFol .047 uf caps.
Maybe these could be improved with FT-2 0.047uF 600V Teflon caps
(eBay # 380108073840) or Paper-in-oil K40Y-9 0.1uF 630V (eBay #
380109913556)? I've read some great things about those
Russian caps, and they seem to be very promising.
Will the slightly larger .1uf caps be an improvement or should I
just stay with the stock value of .047 uf? ...
Or should I just leave the existing caps as they are?
Since I'm not using the phono section, that leaves a nice DC filament
supply unused. It's a simple matter to run them to the phase splitters -