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In Reply to: RE: That's correct! posted by FenderLover on March 27, 2016 at 18:10:06
Hi, FL , Tre suggested that there might be a short between the cathode and heater pins on the tube socket. If that is not the case, you are saying that the CT on the 6.3v line should be left ungrounded? regards, Dak
Follow Ups:
6x5gt
Hi FL, thanks for the link to the radio site. While, the one guy at the site was very negative, it seems that my use would be very easy on the 6x5 since the amount of current draw from the 2 6sn7 will be small and nowhere near its limits. Besides, it is a cool looking rectifier. cheers, Dak
So, what is the verdict? I thought this might be the answer to rectifying my preamp. I am using the 6.3vac filament circuit for the rectifier, and then taking the 5v filaments rectifying that to get 6vdc and using that for the heaters of the 6sn7.
Interesting discussion, I wasn't aware there were any common issues with these tubes. My little 6V6 SE amplifier uses a 6X5, and it's been running fine for years. However, I just went back to review the schematic, and discovered something interesting. The power transformer is from an old Japanese Akai tape deck, and it was intended to power a number of 6 volt tubes in addition to a 6X4 rectifier. In the Japanese design, the rectifier filament is powered from a separate 6.3V winding, and it's not grounded. All the other tubes are on a different winding. This makes me wonder if Akai was aware of the problem even back in the '60s. After reading the discussion at the link posted in this thread, I don't think I would use the 6X5 in the way you have it wired. Maybe you can find a small 6.3V transformer and use it just for the rectifier. If the circuit can handle the extra voltage, Schottky rectifiers might also be a viable alternative.
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Buy Chinese. Bury freedom.
Hi TK, my power trans 6.3v winding is rated at 2A. It will only be used for the 6X5 tube. Seems like that would be a fairly light work load. cheers, Dak
I didn't realize you had a separate winding just for this. Yes, better to let it float. Incidentally, I'm a fan of the 6X4/6X5 for preamp and low power SE use. Last time I looked, there was still a good supply of NOS around, and not too expensive. Good luck with the project!
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Buy Chinese. Bury freedom.
"Over a period of time, the 6X5 would first develop a heater to cathode short. As one side of the heater circuit is grounded, that means you will eventually have a B+ short to ground."
Interesting.
Why would one want to ground the heater at all?
With the heater winding floating it won't matter if the tube "develops a heater to cathode short".
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The 6X5 and other 6V IDH rectifiers were designed to operate off the same filament supply as other 6V tubes in the circuit. Therefore it possessed designed-in h-k insulation capable of withstanding a typical B+ of roughly 300V. In most circuit designs of the era the 6X5 came from, the cathodes of the other tubes would be grounded either directly or thru a relatively low value resistor. The filament circuit of these tubes was also usually referenced to ground thru a center tap or by grounding one leg. If the 6X5 develops an h-k leak, B+ will be shunted to ground theu the heater circuit. If the filament supply is floating, a 6X5 h-k leak will raise the filament circuit to B+ level thus far exceeding the h-k voltage rating of the other tubes. Either case leads to circuit failure, sometimes catastrophic. Bottom line is that a 6X5 is defective if there is an h-k leak and shouldn't be used.
"The 6X5 and other 6V IDH rectifiers were designed to operate off the same filament supply as other 6V tubes in the circuit."
I did not know that.
That sounds like a dangerous practice to me.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
...and it saves a PT winding. Also eliminates need for a rectifier heater winding in 6V car radios where 6X5 was originally employed. Just connect all filaments directly to the battery.
Another type of rectifier where superior h-k insulation is employed is the TV damper. These usually come with a 900V h-k rating, avg current ratings in the multi hundred mA range and an unusually long warmup resulting from the additional h-k insulation mass.
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