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Re: Fisher 500C-how hot is too hot? [message #9188 is a reply to message #9185]

Tue, 06 September 2005 21:03

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

>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.

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oops [message #9189 is a reply to message #9188]

Tue, 06 September 2005 22:01

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

I wrote the wrong cathode and grid voltages above. My tired eyes was reading the scale wrong. It should have read:

Plate= 450 V
Screen= 350 V
Grid= -24 V
Cathode= 3 V

I'm not using my old faithful Eico VTVM I've used for many years because the 1 meg precision resistor in the probe has finally drifted to much. I'm using an old archer with different values for each scale. I have it straightened out now.

I also experimented with a resistor in the AC line. All I had on hand was a pair of 50 ohm, 10 watt, so I wired them together for 25 ohms. Here's what I got.

Plate= 420 V
Screen = 330 V
Grid = -22 V
Cathode = 2.6 V

Looks like I'm just not going to get enough negative bias.

I'm almost ready to rewire this back to origional and just buy a set of 7591's. Darn. I prefer EL34's, and dang if this thing doesn't sound sweet.

Dave

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Re: oops [message #9190 is a reply to message #9189]

Wed, 07 September 2005 11:23

Russellc
Messages: 397
Registered: May 2009

Illuminati (1st Degree)

Not to worry, the 7591 also sounds excellent as well!
Good luck with the 500C,
Russellc

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New day, more tinkering, new results [message #9191 is a reply to message #9184]

Wed, 07 September 2005 12:47

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

Not tired anymore (much) and started playing with cathode resistors. I have a single 5000 ohm wirewound resistor hooking all the cathodes together, bypassed by a 1000 uF cap, and get these results:

Plate= 440
Screen= 340
Grid= -25.5
Cathode= 15

What voltage should I be shooting for on the cathode resistor seeing as I've reached the maximum the bias power supply can put out?

Dave

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Re: New day, more tinkering, new results [message #9192 is a reply to message #9191]

Wed, 07 September 2005 13:23

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

With too large common cathode resistor (5k), you`ve "biased" your EL34s to about I=15/5000=3mA total, or just 0,75mA per tube!
The original thought was one 100-Ohms resistor between each cathode and ground - that way (with "combination" bias) you`d get probably less then 40mA per tube, OK. If you want one common resistor for all four tubes, then you can use 25 Ohms. But, better to "separate" two channels and use 50 Ohms for L (2xEL34), and 50 Ohms for R ch (2xEL34 again). Two capacitors, of course.

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Re: New day, more tinkering, new results [message #9193 is a reply to message #9192]

Wed, 07 September 2005 14:42

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

OK, somehow I got things backward in my head. I have an old artical on converting 7591 to 5881, and it calls for a 390 ohm resistor on each cathode. Oh well, I corrected it.

All I have on hand is a 20 ohm resistor, so I tried it. here's the results:

Plate = 400
Screen = 270
Grid = -23
Cathode = 1.1

I can get a pair of 50 ohms the first of the week. I'm leaving for a music festival tomorrow and won't be back till Sunday evening.

Thanks for the patience.

Dave

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Hmm... [message #9194 is a reply to message #9193]

Wed, 07 September 2005 23:46

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

Are you sure that your resistors values are correct, and your measurements, too?
In this last case, (20 Ohms for 4 tubes), you have large drop of Ug2 and Ua, and your Ia+g2=1,1/20=55mA, or 13,75mA per tube- something isn`t quite right. I suppose that you doing it right - disconect all the cathodes (pin 8), solder them together and ground it through 20 Ohms resistor. Pin 1 (g3) of each EL34 is connected to pin 8, right?

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Re: Hmm... [message #9195 is a reply to message #9194]

Thu, 08 September 2005 07:20

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

Looking at the pin wiring, pin 1 is grounded on each tube. Pin 8 was grounded as well, but I lifted this and hooked all four pin 8's together and grounded them through the 20 ohm resistor. Should I un-ground pin 1 and wire it to pin 8?

Dave

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