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Hi everyone.
I was wondering if i could substitute this tube safely in the 12au7 position in the pilot amp.
I tried it briefly and the difference was not subtle.
I understand that i have to watch the heater current but i dont know what or how to check.
any help would be appreciated.
the schematic can be found here
http://www.ainamoi.com/pilot.htm
thanks
Follow Ups:
Thanks for all the replies.
really appreciated.
A look at the schematic shows the 'U7 (1 section/channel) being used in a common cathode gain block, outside the NFB loop. The 'U7 is NOT linear. Switching to the pin compatible and linear 12BH7 rated to (and did) offer a sonic improvement.
As the posts below indicate, you are probably safe, as long as you don't use the preamp power socket.
If you decide to install 5R4s, be certain to use the tough "potato masher" variety. The B+ rail voltage can be made "perfect" by tweaking the value of R48 downwards.
The bias rectifier (M1) rates to be selenium, which makes it a ticking toxic time bomb. I suggest you replace M1 with a pair of 1 A./100 PIV Schottky diodes. Doing so may require the value of R49 to increase.
FWIW, a nice, linear, medium μ twin triode that draws only 300 mA. of heater current is available, the 7AF7. To use the 7AF7, sheet metal "surgery" replacing the Noval socket used by the 'U7 with a Loctal socket is necessary. If you look at the data sheet linked below, you will see that the 7AF7 is similar to the 7N7. The 7N7 is electrically equivalent to the 6SN7 and the 6CG7/6FQ7. ;> )
Eli D.
The plate resistance is a little lower but nothing untoward will happen.
For what it's worth, with minor rewiring you can use a 6CG7 instead of a 12AU7.
Almost any 6CG7 will stomp a 12AU7 for sonics.
The Mind has No Firewall~ U.S. Army War College.
Thanks for the reply
i have heard about the wonders of the 6cg7.
i am reluctant to change anything in the amp though.
may be i will in the future
cheers
What Eli stated{and you are already aware of} has merit about the filament current being inadequate on the 6 volt winding. Your schemo says 6.9 amps for 6.3 volt winding, that's CLOSE to the edge being your 6CA7's {x4} are drawing down 6 amps right there, leaving .9 amps to power the 12AU7 and the two 12AX7's.
If you are truly impressed by the sound, a 'safe' {albeit inelegant} way to achieve your goal without hacking or modifying the circuit would be by using two 5R4 rectifier tubes instead of the two 5U4's. Your power transformer will run cooler than you will ever recall because the 5 volt winding is now only supplying 4 amps instead of 6.
For what it is worth, the 5R4 is one of the most rugged rectifiers I have ever used and can supply the same amount of current the 5U4 does.
because you are now running the whole circuit with approximately 17 to 23 volts less {forward voltage drop of 5U4 is 44-50 volts and forward voltage drop of 5R4 rectifier is 67 volts} the sound WILL{?} change
You could insert the two 5R4 rectifiers and listen for the change and decide from there.
The Mind has No Firewall~ U.S. Army War College.
Wow thanks for that.
I will try the 5ar4's and compare the sound see if it still has the qualities i like.
It is really amazing how that one tube transformed the sound of an already really good amp.
Cheers
One tube can make or break an amp. The first tube I go after are 12AU7s & I am sure to some I sound like a broken record.
As for transformer heating the percent of current increase is important. With 6 amps already needed for the power tubes, the percent increase is minimal. 3. amp x 6.3 volt increase is a tiny 1.89 watts. That is so small that your transformer is better with that load increase vs a 100 degree ambient environment. And, the transformer will survive both situations at the same time.
On the other side a 6.3 volt @ .6 amp winding maxed out and adding .3 amps or 50% more load is a large percent increase and may overheat the winding to distruction.
5AR4 has a lower voltage drop than 5U4, so voltage will increase. 5R4 will decrease voltage - a safer approach.
There is a sonic difference with each type of rectifier. With 5U4s, drawing that 6A of 5V filament does help to cause that power tranny to run hot after a few hours. Changing to 5V4s or 5AR4/GZ34s or 5R4s will only draw 4A off the 5V fil winding; definitely cooler running.
With 5U4s as the "standard" sound, changing to 5AR4s or even 5V4s, will undoubtedly yield more bass oomph and possibly even more bass detail, but at the expense of midrange sweetness. 5R4s will undoubtedly yield sweeter mids(the sweetest of these three types) and less bass than these other choices. The voltage drop is significant enough to change sound character and the tube type is also contributing to this change in sound character. These sonic results are typical...
Also, something else to consider: Two different brands or two differently testing tubes of the same type will not necessarily have the exact same voltage output ! I can take two good testing 5V4s and their voltages might be differing on the same circuit. With parallel rectifiers that are not split internally, this might cause problems. It's a rare two rectifier tube amp that splits the tube internally in circuit. Matching tubes might be best.
John Atwood originally split the halves of dual rectifiers and then switched to paralleled plates in each leg. JA does know what he's doing.
Eli D.
Both the 12BH7 and 6CG7/6FQ7 present the same issue. Does the filament winding of the power trafo have enough extra current? The substitutes draw 2X the heater current, when compared to the 'U7.
Eli D.
The schematic shows a "aux power socket". I assume this is to power the preamp.
The difference between a 12au7 and a 6cg7 is .3 amps.
I would think a preamp would draw at least .3 amps of heater current.
The schematic for the non-powered preamp (that I assume goes with this power amp) shows the heater line (pins 1 and 3) being rectified to feed 4 12ax7 heaters.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The powered by the amp Pilot preamp, model 210, requires a filament AC Voltage of 24V, IIRC; not simply more current drawn off the 6.3V AC fil. winding of the amp. The Pilot 232 and 260 amps' power trans have the 24V winding in addition to the 6.3V winding. The power trannies from Pilot were not cool running; even without the preamp being powered. However, like Tre, I don't think .3A of 6.3V will blow the power tranny. It might not even make any difference to the power tranny. If the 6.3V changes to 6.1V, at the same "in from the wall" voltage, then I would be concerned. Otherwise, I would keep using that 12BH7 or 6CG7...
I'm really getting sloppy.
I didn't see that the X Y winding was dedicated to the preamp.
Sorry for any confusion I have caused.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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