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In Reply to: RE: 12bh7 substitution in pilot sa 260 posted by pennyford on February 26, 2012 at 18:57:32
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.
Follow Ups:
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.
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