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In Reply to: RE: Tube Rectifiers - Need Expert Help posted by Lord Soth on September 24, 2012 at 05:39:58
5U4 draws 50% more heater current than the others, so don't try it unless you're sure the transformer can take it. Add 5V4 to your list - same as GZ32, really. Expect to rebias since different tubes will affect B+, have different voltage drop.
Follow Ups:
I replaced the 5Y3GT Tung Sol with another Tung Sol 5U4G (Coke Bottle).
Nothing's blown up yet! :)
I did have to rebias the amp though and set the bias lower from around 0.9 mV to 0.7 mV as per the manufacturer's recommendation.
My 5U4G is a NOS specimen so I'm letting it burn in for about 20 hours before I proceed to carry out a more indepth comparison.
You say you rebiased the power tubes after the 5Y3 was replaced with the 5U4. Did you measure the new B+ and calculate the new bias current or did you just set it to the manufacturers 5Y3 current number?
With the 5U4, I would expect your B+ to be considerably higher. You would then be operating the EL34s at a greater power setting, current being unchanged. There may be no harm in doing so, but to really compare rectifiers you should do your comparisons at the same power settings.
My Tube amp has an easy fixed (adjustable) bias feature.
I don't really have to measure the B+. There is a built in resistor in the bias circuitry which requires me to simply re-set it to 0.7mV as long as EL34 tubes are used.
This is because after I subbed in the new rectifiers, from 5Y3 to 5U4G, the bias naturally was changed.
I just had to re-adjust it according to the manufacturer's specs.
Not that it matters, I am currently using the RCA 5R4GY rectifier and it blows away the 5Y3 and 5U4G in terms of sonics. I got back my (previously missing) low end bass along with a warmish mid-range and sonic clarity from top to bottom.
The top-end also sizzles in a pleasant way.
I'm done with rectifier tube rolling for the time being. ;)
Like Tom Bavis says, don't put in a rectifier with a larger current draw. This is designed for 2 amps filaments. So check the data sheets and don't be tempted to use any old octal rectifier. And as stated, your HT will vary - could go up or down, most likely up with the GZ32 and 34.
Don't be too hard on the 5Y3, it's the octal equivalent of the 80 which is a super rectifier and as good as most except exotic mesh plate ones (AZ1 etc. but they're different again, and expensive). If you can trust yourself with a multimeter you can measure the HT, but you'd need to understand the circuit before going inside for that to be of much use, and if you don't I wouldn't risk your fingers in a live chassis.
So look at the data sheets (you need to understand what rectifiers are capable of providing), tube roll with caution from amongst the 2 amp choices, and don't do more than short listening tests. If you hear something you like then get somebody to help adjust the circuit.
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