parallel SE, Class A build: hum

a fine line between stupid and clever

parallel SE, Class A build: hum

Postby centervolume » Sat Feb 13, 2016 8:31 am

marnin, yawl (that's mississippi for hello)

finishing up my first custom circuit build (cathode bias, class A with 5881 and toggle to activate 2nd 5881 in case of emergency, has full assortment of treble bass mid controls [5f8-a tone stack] all in a tweed vibrolux box w/ alnico 10, 20 watt OT). She's passing a nice full signal but with what sounds like a 60 Hz hum in the power tube section.

I grounded all of the following to a single point:

1. filter caps
2. red/yellow PT center tap
3. power tube cathode bias assembly (this is the one I currently suspect as needing to be separated given its proximity to signal... does this sound
like something I should ground to another location?)

I double checked my heater pairs, they hit the same pin #s on both 5881s and the same ones on the 2 ax/ay7s (I don't recall if they are supposed to alternate for example pins 4/5 on v1 go to pin 9 on v2 or if pin 4/5 on v1 should go to pin 4/5 on v2 etc)

I don't have a choke installed.

The heater has a center tap (green yellow) that I soldered to the inside wall of the chassis with my 80 watt solder canon.

Got to nail down that blasted hum!!
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Re: parallel SE, Class A build: hum

Postby cdmawolf » Sat Feb 13, 2016 12:37 pm

How do you have the input wired? Well are you using shielded wire and where is it routed.
Are the heater wires routed against the chassis?
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Joined: Thu Nov 29, 2012 3:07 am

Re: parallel SE, Class A build: hum

Postby centervolume » Sat Feb 13, 2016 4:40 pm

Hi cdmawolf

I pulled the 2 preamp tubes and the hum persists until I pull the power tubes, so I am interpreting that to mean the hum is not in the preamp stage. Overall the amp is dressed as per the 50's fenders, with which I am more familiar. The input jacks are wired in the style of the 50's fender chassis, one of which I have on hand to verify my interpretation of the schematic follows into application. They are self shorting, and #1 of 3 has a 1M resistor between tip and ground. The amp is dressed with cloth covered solid core wiring, in the mode of the 50s as well.

The twisted pairs are not down in the corner of the chassis as per the 50's Fender factory examples, rather lifted up above the sockets as per the 1960s fender tolex amps. I may try to lay them down to see if that helps.

Right now I am wondering about the heater center tap from the power trans (green-yellow) that I have soldered directly to the chassis. Perhaps I should isolate that and use the "artifcial center tap" approach (2 x 100 ohm resistors hanging from pilot light assembly)
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Joined: Tue May 12, 2015 7:46 pm

Re: parallel SE, Class A build: hum

Postby Gingertube » Sun Feb 14, 2016 9:15 pm

Do you have power and output trannies mounted with their cores at 90 degrees to ech other? That stops magnetic coupling of hum from power to output tranny.
Do you have garanteed good (and sufficiently high value) B+ filter cap(s) for the output tube B+?
Do you have garanteed good (and sufficiently high value) screen supply to the output tube - the screen supply is MORE important than the anode supply in determining tube current, so it is MORE important that this supply is squeeky clean.

And finally:
Excess current draw by the output tube from parasitic (ultrasonic) oscillation can cause excessive hum. Make sure you have grid stop of at least 1K5 and screen stop of at least 150 Ohms on each output tube, resistor bodies right against the tube socket pin.

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Re: parallel SE, Class A build: hum

Postby centervolume » Mon Feb 15, 2016 9:12 pm

thank you for your thorough response. It is most helpful to keep my perspective in order.

Here is my PT (

All of my caps are rated at or above 450VDC, my first cap is rated at 500, my b+ is currently 415.

I may have to remount the output transformer, it is currently at 45 degrees relative to PT.

However, I have started to see spikes in power tube current draw from 50ma (cathode bias)
all the way up to .2; as you can imagine there is some hum and this is a much more pressing
need. I'll double check the cathode bias cap for shorting. But I must say, your suggested
values for screen and grid resistors are most helpful.

Is there a general rule of thumb regarding screen voltage as either equal to or some percentage
of plate voltage? some schematics I have consulted show them as identical and others show
the screen about 10-15% under the plate.

best regards
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Joined: Tue May 12, 2015 7:46 pm

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