Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bluesbear on October 09, 2011, 03:35:47 pm

Title: Need help determining a sag resistor value
Post by: bluesbear on October 09, 2011, 03:35:47 pm: Hi. I've been away for a while. My wife is ill and I've been spending most of my time helping her. However, I have to take time to finish the amp I started a few months ago.

One thing I really would like to do is dump the rectifier tube. The rectifier tube in my son's went south and took the filter caps and power transformer with it. Not good! So... my question is 2 part:

This amp is running BF Deluxe voltages with a 5Y3. I'd like to use diodes and a sag resistor to induce (simulate?) sag. What resistor would drop the voltage to emulate a 5Y3, 50 volts or so? What wattage resistor should I use? I've seen 10 watts mentioned but that actually seems a little light to me. Would an aluminum bolt down resistor with a heat shield be advisable?

Also, what's the consensis on the Weber Copper Caps? Any bad stories out there? I'd rather make my own (cheaper and I get to control the build and part quality).

Thanks!!
Dave
Title: Re: Need help determining a sag resistor value
Post by: FYL on October 09, 2011, 03:52:47 pm: Ohm's law is your friend.

Say the tubes draw an average of 120 mA. You want to drop 50 V, R = (50 / .12) = 416.67 ohms. P will be (50 * .12) = 6 watts, multiply it by 4 in order to get a good safety margin. You can use a 25W / 430R resistor (nearest std. values).

Re. Copper Caps I've never used or tested them but I've read a lot of negative reports. IMO you'd be better off building your own version.
Title: Re: Need help determining a sag resistor value
Post by: tubeswell on October 09, 2011, 04:52:30 pm: I use the diode protection between the HT winding and the rectifier tube's anodes when using tube rectifiers. (SS diode's cathode pointing towards the rectifier tube's anode)

For the resistor, try somewhere between 10R and 220R. There is no 'magic number'. Use a high power resistor.
Title: Re: Need help determining a sag resistor value
Post by: jjasilli on October 10, 2011, 10:24:09 am: Ditto to FYL & tubeswell. If the rectifier tube fails by going open, it turns off the B+, but no physical harm is done. If it fails by going short internally, then physical harm may result. The SS diodes solve that issue.

Ohm's Law is the solution. The Duncan PS calc may also help. Use of an inductive power resistor will yield reactance -- it's induction instead of impedance -- but it may help simulate the behavior of a tube rectifier.
Title: Re: Need help determining a sag resistor value
Post by: Willabe on October 10, 2011, 02:10:19 pm: Quote from: jjasilli on October 10, 2011, 10:24:09 am
Use of an inductive power resistor will yield reactance -- it's induction instead of impedance -- but it may help simulate the behavior of a tube rectifier.

Who makes an inductive power resistor and where can you buy them? All the wire wound power R's I see at Mouser and such all claim to be low induction.

Maybe I missed seeing them?

Brad :dontknow:
Title: Re: Need help determining a sag resistor value
Post by: Tone Junkie on October 10, 2011, 02:15:30 pm: Is this what you guys are talking about in adding diodes to the tube.
Thanks Bill
I thought a picture might be helpful
Title: Re: Need help determining a sag resistor value
Post by: tubeswell on October 11, 2011, 04:30:59 am: Quote from: Tone Junkie on October 10, 2011, 02:15:30 pm
Is this what you guys are talking about in adding diodes to the tube.
Thanks Bill
I thought a picture might be helpful

Yep, that's one way of mounting them
Title: Re: Need help determining a sag resistor value
Post by: spacelabstudio on October 11, 2011, 08:00:51 am: Quote from: bluesbear on October 09, 2011, 03:35:47 pm
Hi. I've been away for a while. My wife is ill and I've been spending most of my time helping her. However, I have to take time to finish the amp I started a few months ago.

One thing I really would like to do is dump the rectifier tube. The rectifier tube in my son's went south and took the filter caps and power transformer with it. Not good! So... my question is 2 part:

This amp is running BF Deluxe voltages with a 5Y3. I'd like to use diodes and a sag resistor to induce (simulate?) sag. What resistor would drop the voltage to emulate a 5Y3, 50 volts or so? What wattage resistor should I use? I've seen 10 watts mentioned but that actually seems a little light to me. Would an aluminum bolt down resistor with a heat shield be advisable?

Also, what's the consensis on the Weber Copper Caps? Any bad stories out there? I'd rather make my own (cheaper and I get to control the build and part quality).

Thanks!!
Dave

If you know the current in your amp and the voltage drop you want, then it's fairly easy to apply Ohm's law and get your answer as others have suggested. Merlin Blencowe's power supply book also has a chart showing average internal resistances of common tube rectifiers. 5Y3GT is 300 ohms, so a 300R resistor might/should get you in the ballpark.

Chris
Title: Re: Need help determining a sag resistor value
Post by: jjasilli on October 11, 2011, 09:01:39 am: Tube data sheet shows 50Ω or 135Ω, depending on AC supply voltage: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/5/5Y3GT.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/5/5Y3GT.pdf)
p. 4 top. Confused :dontknow:
Title: Re: Need help determining a sag resistor value
Post by: bluesbear on October 12, 2011, 03:58:41 pm: On the whole, I think I'll just use a 5Y3 with the diodes. I KNOW what that will sound like and I repositioned the PT for a bit more room to make it all fit.
Thanks for all the info. Between everyone, this place is like a super-computer.
Dave
Title: Re: Need help determining a sag resistor value
Post by: Jennings on October 13, 2011, 03:19:51 am: How do you determine what value diodes to use...or is this a fixed value...or have I (as usual!) missed something?!
Title: Re: Need help determining a sag resistor value
Post by: John on October 13, 2011, 06:18:50 am: I think it's the 1N5408. 3A/1000V. And only 50 cents! :smiley:
Title: Re: Need help determining a sag resistor value
Post by: tubeswell on October 13, 2011, 01:04:07 pm: Quote from: Jennings on October 13, 2011, 03:19:51 am
How do you determine what value diodes to use...or is this a fixed value...or have I (as usual!) missed something?!

Any SS diode that is rated to deal with the current and PIV will work - a 1N4007 for instance.