Preamp Deal of the Century

So, I tried the Mullard GZ33 tonight, and I like it a lot! I bought a pair, and both had good getter flashing, full lettering, and tested 100%+ on my tube tester (set to 5V4, 5U4 and 5AR4 settings). Compared to the GE 5AR4, the background is cleaner, the soundstage is more focused (I can tell more easily where the instruments are) and the bass is a bit tighter and deeper. Cymbals are cleaner and have more focus. I swapped the two tubes back and forth twice each (I didn't want to stress the preamp with too many rapid on/off cycles). It just sounded more powerful and focused with the GZ33 than the GE 5AR4 - I'm keeping it in.

Hello,

Good for you! I hope you are posting this as a result of your "own" experiment only and not for the others who have no time to understand what is going on in the circuit to try.

With all due respect, are you saying that the preamp PSU (5V filament supply) rated for 5Y3/5AR4 current draw (2.0 amps) according to the circuit design is good for a GZ33 with current draw of 3.0 amps? Very dangerous situation for those who are not aware of this limitation I would say. Did you check the filament transformer rating of the rectifier supply or noticed any change in heat from the filament transformer? If not the proper rating and noticed an increase in heat from the PT, turning ON/OFF the unit as frequent to avoid stress to the PSU is the last thing I would be concerned of.

Another thing, the 5AR4 drops 17 V at 225 ma, the 5V4 is 25 Volts at 175 mA, the GZ33 being similar to the 5U4 drops 44 V at 225 mA for G/GA and 50 V at 275 mA for the GB. Not really the same (21/27 volts change!) and I bet that is why the change in sound because you are changing the OP of the tubes. A Mullard GZ33 cost a lot. It seems that a 25W aluminum ohmite dropping resistor on the B+ line will work also no?

Do you have a measurements of the B+ from GZ34 to GZ33 and see the difference? Since you are being adventurous, what is the reason why you did not try a 5U4?

I am only reporting back what has worked for me, YMMV. Remember, my unit is not exactly stock anymore, as I stated above. The Mullard GZ33 is actually less expensive than the Mullard GZ34 - I bought two for the price of one GZ34. I am very aware of the larger voltage drop of the GZ33, and was not expecting better sound from it, yet that's exactly what I got.
Mick recommends the GZ37 as an alternate for the 5AR4 in the manual for the Chenin, and it also has 3 amp current draw, so that does not seem to be a problem...

Hello Ait,

Oh, do not get me wrong, I am just curious following your experiment. Yes, I am aware that your unit is not stock anymore. The same reason why I asked if you know the rating of the rectifier filament supply.

Yes, you are right, on the manual the GZ37 is listed as "ok" replacement of a 5AR4. He also listed a GZ32 but not a GZ33 or a 5U4. That is my point. I thought the GZ37 has a current draw of 2.8 amps max, 200 mA amps difference from a GZ33 (with 3 amps).

regards,

Abe

I have to say, I just went back and listened again to the difference between the 5AR4 and the GZ33 in my preamp. It's very clear to me that the Mullard GZ33 is superior to the GE 5AR4. The sound is clearer, cleaner, more focused, and overall just has more guts to it. Not what I expected from lowering the B+, but it's what I got.

Is the OP of the tubes changed? Most certainly! Is that a problem? No, isn't that the whole point of rolling rectifier tubes? It's not like the signal goes through them, you know...

No argument here as far as rolling the rectifier. Yes, no signal goes to it except the dc voltage it provides is the whole signal that is amplified. Besides, your unit is different now compared to stock. Let's say your preamp is a new design using the psu of a Supratek preamp and any experiment you have may or may not be applicable to stock Chenin's.

Oh well, as long as u have fun doing what you are doing.

Regards,