I rebuilt my Mk III's and noticed a little hum on both. I just assumed it was the transformers, but the more I listen it seems to be coming from my GZ-34's. The amps were pretty much rebuilt stock except for an SDS Labs board in place of the quad cap. I hear no hum from the speakers. I was wondering if it was normal or if they might be getting too much juice from the 50uF section of the SDS board. Do tubes hum even the hum? It is fairly and hard to pin point. Really don't care about the hum just don't want to destroy the NOS tubes.
2 posters
GZ-34 Hum?
j beede- Posts : 473
Join date : 2011-02-07
Location : California
Have you tried powering up your amps with the rectifiers out to see if the hum goes away? Do you have silicon diodes ahead of the GZ-34 to "pre-rectify" the ac?
j beede- Posts : 473
Join date : 2011-02-07
Location : California
FYI: JJ, Telefunken, Philips, and GE all specify Cmax=60µF for GZ-34/5AR4. It is not unusual for experienced tube amp fans to limit capacitance on the rectifier side of the choke to much less than 60µF.
harpy- Posts : 94
Join date : 2012-02-04
Yes it's the tubes. Would the rectifier tube diode mode sticky apply for the pre-rectify?
harpy- Posts : 94
Join date : 2012-02-04
Also, J Beede are you running DQ-10's?
j beede- Posts : 473
Join date : 2011-02-07
Location : California
harpy wrote: Yes it's the tubes. Would the rectifier tube diode mode sticky apply for the pre-rectify?
Yes, I like to use diodes as 1/2 wave rectifiers ahead of the 5AR4. The "yellow sheet" mod in the sticky is worth doing. How much capacitance do you have on the side of the choke away from the GZ-34?
j beede- Posts : 473
Join date : 2011-02-07
Location : California
harpy wrote:Also, J Beede are you running DQ-10's?
No Dahlquists. Quad 2-ways and assorted Magneplanars. My avatar is one of my Quad ESLs from the "old" days.
...j
harpy- Posts : 94
Join date : 2012-02-04
I run 118.5 AC soft start into both. The voltages are good through out the amp. If I am on the right page the value would be 50uF (measures 53uf). Here's a shot the small yellow Teflon wire goes from the GZ-34 to the SDS Board.
harpy- Posts : 94
Join date : 2012-02-04
I tried switching out rectifier tubes and hum remains. I haven't done the yellow sticky mod yet because I would like to see if I can find the issue. The only thing I can think of is a bad ground on the SDS boards. The green wire is the grounding point and it grounds out where the 3 terminal board where the blue bias cap is. Any other thoughts?
harpy- Posts : 94
Join date : 2012-02-04
If you look at the picture you see the red transformer lines going to pins 4 and 6. If you look at my amp in it's present state you would see the red lines going to pins 4 and 5. Gremlins? 
No more hum.
No more hum.