Tube Amps / Music Electronics
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|7/4/2003 9:05 AM|
||What are the voltages for a 5F1?|
I've looked at every schematic and website I can find and have yet to locate the voltages for a 5F1 Champ.
Can someone tell me what they should be?
Thanks a lot!
|7/4/2003 6:07 PM|
|7/5/2003 2:50 AM|
Dan, I'm not an expert. I have built a 5F2A. I think as long as your plate voltage is 330-380 and your plate is dissipating 13-14 watts, you should be okay. Your screen will be 10-25 volts less than the plate, with about 3ma of screen current. About 2ma total to the 12AX7. If someone disagrees, PLEASE don't hesitate to correct me.
I never have figured out how to do a search here. If you go to the AX84 site and do a search on "Champ voltage" or "6V6 voltage" you'll probably find out more than you want to know.
|7/5/2003 4:36 AM|
I have, for the 6V6GT: +340VDC B+, +295 screen, +18 volts off the cathode to the cathode resistor (470 ohms)
The 12AX7 has +150VDC onto pin 1 and onto pin 6. 1500 ohm cathode resistor onto each half of the duo-triode.
This is using the 5Y3GT rectifier.
|7/5/2003 6:09 PM|
Newt, I think you're in the ballpark on your estimates. I do have a Fender published schematic that shows voltages on the B+ rail to be 340vdc on the plate, 295vdc screen, 250vdc to the preamp. As with most of the Fender published specs of this era, these tend to be a bit low. I would consider the 5F1 B+ design target to be about 350vdc at the primary voltages of the day. I don't make notes on these things, but IIRC, the 5F1 champs I've worked on tend to run about 365-385vdc with 120vac on the primary.
Having said that, my recommendation when building these would be to shoot for about 350vdc. In my experience, to my taste anyway, I think the key to best performance in class A for guitar amps is in lower voltages/higher current.
With the 470ohm cathode resistor and the B+ at 350vdc, you will have the cathode voltage at about 20vdc, and a typical 6V6 dissipating about 14-15watts. Not the best point for tube life, but I think it's acceptable for class A with the more rugged tubes, and when you start lowering the current, the tone seems to chill pretty fast.
I haven't experimented much with the B+ lower than 350vdc in these, may deserve some merit, but I have turned the variac down a bit and found no discernible tonal difference, outside of slightly earlier breakup as you approach 330vdc.
Anyway, that's my .02. Some like the performance of the higher voltages with the current pulled back. You may gain some headroom that way.
As with everything else in this biz, beauty is in the ear of the beholder.
|7/6/2003 5:11 PM|
||How do you calculate "plate dissipation"?|
How you I go about calculating plate dissipation on my Champ?
|7/6/2003 6:21 PM|
||Re: How do you calculate "plate dissipation"?|
ok, cathode bias yes? of course.
1)measure cathode current (voltage drop across Rk divided by value Rk)
2)subtract 5ma from that figure to represent screen current (or you can measure this across the screen resistor if you want)
3)measure plate voltage
4)subtract cathode voltage from plate voltage
5)plate diss=plate current * plate voltage
hope that helps
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