Home » Audio » Thermionic Emissions » Fisher 500C-how hot is too hot?
Fisher 500C-how hot is too hot? [message #9175] Sun, 04 September 2005 07:22 Go to next message
hurdy_gurdyman is currently offline  hurdy_gurdyman
Messages: 416
Registered: May 2009
Illuminati (1st Degree)
Can anyone give me an idea of just how hot the output transformers should be on a Fisher 500C after running all day? Mine are hot enough that I can only leave my hand on them for about 8 seconds or so. I've heard that these receivers run hot, but want to make sure that these aren't too hot.

I can't just check bias voltage and such by comparing to schematic. The receiver was long ago converted from 7591 to EL34. Bias was adjusted by lowering screen voltage. The tubes don't have any sign of strain even after running all day, no orange glow at all on the plates.

There is about 440 VDC at the plates, about 320 VDC (was 375) at the screens, and -20 VDC at the grids.

Dave



Re: Fisher 500C-how hot is too hot? [message #9176 is a reply to message #9175] Sun, 04 September 2005 17:59 Go to previous messageGo to next message
Damir is currently offline  Damir
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Registered: May 2009
Illuminati (2nd Degree)
With OP - Ua=440V, Ug2=320V, Ug1=-20V, you can expect more then 100mA Ia+g2 current through each EL34. Much more then Pa max for EL34. For "resonable" bias of about Ia~50mA (and Ig2~5mA) you should change the "bias voltage" Ug1 to about -26V. If you don`t have adjustable "bias" pot, you can change the values of voltage divider resistors in "bias" supply to "enlarge" Ug1.

Re: Fisher 500C-how hot is too hot? [message #9177 is a reply to message #9176] Sun, 04 September 2005 21:52 Go to previous messageGo to next message
hurdy_gurdyman is currently offline  hurdy_gurdyman
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Thanks for the help. I've never been any good at math formulas.

I changed the resistor on the bias bridge rectifier-to-ground from 15 ohms to 2 ohms and got the voltage to -24 VDC. This also, unfortunately, raised the fillament voltage on four 12AX7's higher than what is good for them, so I added a 16 ohm resistor in series whith them and have them right where they should be.

I'm a bit worried about reducing the bias resistor anymore. Not sure if the transformer tap and rectifiers are up to a lower resistance than this or not (not sure if it's up to the valule I have on it now.) Is -24 V close enough, or should I try an even lower resistor?

Dave

Re: Fisher 500C-how hot is too hot? [message #9179 is a reply to message #9177] Mon, 05 September 2005 05:43 Go to previous messageGo to next message
Damir is currently offline  Damir
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Registered: May 2009
Illuminati (2nd Degree)
I found the schematic, actually the voltage across two series connected 12AX7 heaters wired for 12,6V operation must be 25,2V.
I`d leave 15 Ohms resistor in place, but I`d change the series connected 5k6 resistor to about 2k2-2k7. This resistor with 15k resistor forms voltage divider, and "set" the bias voltage.
I think that -24V is still on the "hot" side, you can measure the current through EL34s by installing 1 Ohm (or 10 Ohms) "sense" resistor between the cathode and ground on each EL34 (pin 8). By measuring the voltage drop you can calculate the current by Ohms Law (say, 60mV through 1-Ohm resistor is Ia+Ig2=60mA, and that`s about max. you can use with respect for max. Pa data).

that's power iron hot... [message #9180 is a reply to message #9175] Mon, 05 September 2005 08:47 Go to previous messageGo to next message
Thrint is currently offline  Thrint
Messages: 43
Registered: May 2009
Baron
a 7591 running at +400 is going to pull less than 70 mA before developing plate glow. So, what is creating the heat? I-squared-R in the primary winding? What is primary DCR? Do the calculation, and measure idle current.

Is there any other source of heat? you mentioned the rectifier a bit ago.

Have you actually measured temp? 8 seconds is a long time when holding your paws to heat. There is a diff between hot and inducing damage, and hot and able to burn fingers. 85-C is not going to allow 8 seconds of good finger contact, I can assure you...

Still, none of my OPTx's run hotter than I can stand, so I would urge you to discover the heat source.

There is *NO* magic involved, so examine it thoroughly.
cheers,
Douglas

Re: that's power iron hot... [message #9181 is a reply to message #9180] Mon, 05 September 2005 09:38 Go to previous messageGo to next message
hurdy_gurdyman is currently offline  hurdy_gurdyman
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Illuminati (1st Degree)
Aser the conversation with Damir below, I'm zeroing in on the problem. It all started because I converted it to EL34 several years ago when there were none being made that would fit it. Hasn't been used in a few years, now I'm getting it going again and working out the bugs.

Dave

Re: Fisher 500C-how hot is too hot? [message #9182 is a reply to message #9179] Mon, 05 September 2005 09:45 Go to previous messageGo to next message
hurdy_gurdyman is currently offline  hurdy_gurdyman
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Registered: May 2009
Illuminati (1st Degree)
The series resistor for the bias voltage has been completely by-passed a long time ago. There just isn't enough voltage there to reach 26 volts without changing the 15 ohm resistor to ground. The way it is currently hooked up with the 2 ohm resistor replacing the 15 ohm, I am getting:
Plate= 440 VDC
Screen= 300 VDC
Grid= -24 VDC

Also, some time ago I replaced the 1.2 ohm resistor between first and second stage in the high voltage supply with a 7.2 ohm. This is what lowered the screen voltage.

It gets complicated, but it does seem to play well and sounds wonderful. The things I'll do to save money. EL34's are still cheaper than 7591's. Besides, I love the sound of EL34's.

Dave

Re: Fisher 500C-how hot is too hot? [message #9183 is a reply to message #9182] Mon, 05 September 2005 11:32 Go to previous messageGo to next message
Damir is currently offline  Damir
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Registered: May 2009
Illuminati (2nd Degree)
With lower Ug2=300V and Ug1= -24V, current through the tube(s) is now in the "safe area", lower then Pa+g2 max. Enjoy...:-)

Re: Fisher 500C-how hot is too hot? [message #9184 is a reply to message #9176] Mon, 05 September 2005 13:07 Go to previous messageGo to next message
metasonix is currently offline  metasonix
Messages: 103
Registered: May 2009
Viscount
I have repaired and upgraded SCORES of Fisher receivers. Please let me offer some advice.


Yes, your output tubes are running too hot, plate current must be decreased. -26v is not negative enough.

Also, the heaters of the EL34s draw a LOT more current than 7591 heaters. This is dangerous. The original power transformer is barely adequate to run the original load. If I had to put EL34s in a 500C, I'd either use an external power transformer or remove/bypass some preamp tubes (or maybe even the tuner) to decrease the load.

>) you should change the "bias voltage" Ug1 to about -26V.
>If you don`t have adjustable "bias" pot, you can change
>the values of voltage divider resistors in "bias" supply
>to "enlarge" Ug1.

That's good advice. If this were my receiver, I would assume even worse, and modify the output stage to insert a cathode standoff resistor on the cathode of all 4 tubes, to further decrease plate current. I suggest a big 100 ohm wirewound resistor bypassed with a 1000 uF, 25v capacitor.

Another advantage to this scheme: the cathode resistor helps limit inrush current at turn-on.

Another thing I always do to old tube receivers is to put a large resistor in series with the AC mains input. 5-10 ohms at 10 watts also limits inrush, to help prevent power transformer death. Remember, AC line voltages have crept up over the last 40 years.

If modern EL34s conformed to the old data sheets, -26 would be adequate, but modern EL34s NEVER conform. They always seem to need more negative grid bias.

Re: Fisher 500C-how hot is too hot? [message #9185 is a reply to message #9184] Mon, 05 September 2005 15:32 Go to previous messageGo to previous message
hurdy_gurdyman is currently offline  hurdy_gurdyman
Messages: 416
Registered: May 2009
Illuminati (1st Degree)
When I first did the EL34 conversion I mounted a couple of Radio shack transformers on the back of the chassis to handle voltage for the power tube heaters, so thats not a problem.

Will it hurt the power transformer to change the bridge rectifier-to-ground resistor to an even lower value than the 2 ohms I'm presently using (origional value was 15 ohms)? I've looked at schematics of amps from other companies and see that many times the bridge rectifier is hooked directly to ground without any resistors.

I was wondering earlier about using cathode resistors but didn't know how to adjust the values. I think I'll try this soon.

Dave

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