Hi, I'm starting to build a 12AX7-6K6 SE amp with EZ81 rectifier and my question is about the heaters.
I have two 230V-11.6V 14A toroids which I'm planning to use back to back and get the heater voltages from the 11.6V of the first transformer and B+ from the 230V of the second transformer. The issue is that the 6K6 has 6.3V 0.4A heaters and the EZ81 has 6.3V 1A heaters so I just can't connect them in series to get the heater voltages. Should I just use dropping resistors?
Resistor calc:R=V/I
6K6 11.6V to 6.3V at 0.4A = ~12R 5W-10W
EZ81 11.6V to 6.3V at 1A = ~4.7R 10W or more
OR should I add there 6V fixed voltage regulators? Or do you know any other Full Wave Rectifier tube with separate heaters at 0.4A?
-Pete
I have two 230V-11.6V 14A toroids which I'm planning to use back to back and get the heater voltages from the 11.6V of the first transformer and B+ from the 230V of the second transformer. The issue is that the 6K6 has 6.3V 0.4A heaters and the EZ81 has 6.3V 1A heaters so I just can't connect them in series to get the heater voltages. Should I just use dropping resistors?
Resistor calc:R=V/I
6K6 11.6V to 6.3V at 0.4A = ~12R 5W-10W
EZ81 11.6V to 6.3V at 1A = ~4.7R 10W or more
OR should I add there 6V fixed voltage regulators? Or do you know any other Full Wave Rectifier tube with separate heaters at 0.4A?
-Pete
6K7 = .4A
ECC83/12AX7 = 0.3A
place those two in parralel = 0.7A
EZ81 = 1A.
Place the EZ81 in series with the other two in parralel. Place a 27R across the EZ81 heaters. Place that across the 11.6v supply.
ECC83/12AX7 = 0.3A
place those two in parralel = 0.7A
EZ81 = 1A.
Place the EZ81 in series with the other two in parralel. Place a 27R across the EZ81 heaters. Place that across the 11.6v supply.
Hi, thanks! I didn't think that I could use the 12AX7 @ 6.3V there also!
I also have the pilot lamp. I could put it in parallel with the 6K6 and 12AX7 too to get total near to 1A.
( \--------\-------\
) 6K6 12AX7 LAMP
( /_____/____/
) |
( | \
) R EZ81
( | /
Is this what you meant?
I also have the pilot lamp. I could put it in parallel with the 6K6 and 12AX7 too to get total near to 1A.
( \--------\-------\
) 6K6 12AX7 LAMP
( /_____/____/
) |
( | \
) R EZ81
( | /
Is this what you meant?
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This doesn't add up. When placing the EZ81 (which needs 1A at 6.3 V) in series with the paralleled 6K6 and 12AX7 (which need 0.7 A at 6.3 V as a duo) than where is the extra 0.3 A for the EZ81 coming from? The 27R resistor in parallel with the EZ81 makes things even worse since it will lower the total resistance of the duo 'EZ81/27R' so it will need more than 1A to stay at 6.3 V.
Question for TS: What are the voltage and wattage of the pilot lamp?
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Thanks for the input! The lamp is 6.3V 190mA. So it would add the total 0.9A (12AX7+6K6+lamp) which is maybe close enough. No need for the resistor then?
No resistor parallel to the EZ81 and a resistor of 56 Ohm in parallel to the 6K6/12AX7/dial lamp.
But your transformers are rated for 14A so you could connect all tubes and the dial lamp in parallel to ensure they all get a full 6.3 V. All of them would take 1.89 A. The needed voltage drop is 11.6 - 6.3 = 5.3 V, so the needed dropper resistor would have to be R = V / I = 5.3 / 1.89 = 2.8 Ohm. The dissiaption in that resistor would be P = I x V = 1.89 x 5.3 = 10.02 Watt so it has to be rated 15 W (or a bit more to be safe). Ofcourse this would cost more power/generate more heat.
The best choice will depend on the actual voltage you will get under load. I would not be surprised if the actual voltage will be a bit higher than the 11.6 V since your amplifier will only use a fraction of 14 A. In that case the first solution seems the best.
But your transformers are rated for 14A so you could connect all tubes and the dial lamp in parallel to ensure they all get a full 6.3 V. All of them would take 1.89 A. The needed voltage drop is 11.6 - 6.3 = 5.3 V, so the needed dropper resistor would have to be R = V / I = 5.3 / 1.89 = 2.8 Ohm. The dissiaption in that resistor would be P = I x V = 1.89 x 5.3 = 10.02 Watt so it has to be rated 15 W (or a bit more to be safe). Ofcourse this would cost more power/generate more heat.
The best choice will depend on the actual voltage you will get under load. I would not be surprised if the actual voltage will be a bit higher than the 11.6 V since your amplifier will only use a fraction of 14 A. In that case the first solution seems the best.
I'm building something similar (Fender Champ style SE) and also using EZ81 Rectifier instead of the 5Y3. I am trying to use an old power transformer that has got the center tap of the HT (250-0-250) connected internally to the center tap of the filaments (3.15-0-3.15) and all filaments are powered from this supply as there are no other filament windings. as allt he valves are indirectly heated, including the rectifier i'm hoping it should work OK. Can anyone see a problem with this set up. The transformer seems to have enough current to power the heaters, i tested it using half the heater in a 6c33 (3.5 amps) and still measured a little over 6 volts.
You can safely hook up the heater of an EZ81 and the heaters of the other tubes to the single grounded heater winding. There were millions of tube radio's and tube amplifiers in which this was done this way. The maximum heater to cathode voltage of the EZ81 is 500 Vdc. With your HT winding of 250-0-250 you will stay far below this maximum.
Using two transformers back-to-back rarely works out well. The second transformer tends to put a very heavy load on the first transformer, making it hot and cranky. No-load current of little transformers can be surprisingly high, particularly when used backwards.
I've read a number of threads on the Aussie Guitar Gearheads forum in which someone tried to use two transformers back to back. Most of the time, it didn't work out.
On the other hand, you have a rather high-current pair of transformers, so it might work for you.
If not, the Triad N-68X ( https://www.digikey.ca/en/products/detail/triad-magnetics/N-68X/1887210 ) is the go-to cheap power transformer for DIY tube guitar amplifiers. Use it backwards, plugged into 115V AC, and you get a nominal 230 volts AC RMS from the secondary. Rectified with silicon diodes (or a silicon bridge rectifier), that will get you around 320 - 340 volts DC.
That works on paper, but has a dicey failure mode. If any of the three parallel elements fails open, the other two end up with too much voltage across them / too much current through them.PFL200 said:
In other words, failure of the dial lamp could cause cascading failure of the 12AX7 and 6K6 heaters, too.
With the current shortage of tubes, and no better prospects in sight for the foreseeable future, it's probably a good idea for all of us to be hyper-protective of whatever few tubes we still have. 🙂
-Gnobuddy
Hi, thanks for the info but I've used these same transformers back to back many times earlier. I've even made a dual power supply with three of these same transformers. Never had a problem with them. This one is the first amp I'm using AC for heaters, not regulated as earlier. I got the amp working and the heater voltages are a bit off but not too much. Didn't use the 56r resistor. Still have many of these transformers for future projects..
Excellent. That's because of the high 14-amp secondary current rating of your transformers. That's much bigger than typical transformers used in valve amps.
If you have an AC wattmeter, Kill-A-Watt (http://www.p3international.com/products/p4400.html), or something similar, it would be interesting to find out just how much no-load AC power the pair of back-to-back transformers are drawing.
What is the heater voltage (of the 12AX7/6K6) if you remove the dial lamp, which simulates what will happen when the lamp burns out?
-Gnobuddy