Harmony H303 Hum and Static

[scstill]

Restored this guy several years ago but never fully tamed the hum and static. For me its tone is great.
Dusted it off a few days ago trying to reduce Hum and Static
Thinking the hum might be reduced if I created Artificial ground for the heaters.
I have done Artificial Ground on 6.3v before but wondered if there would be any difference with this 50v
The schematics below.

Other considerations:
Hum - is likely 60cycle hum that does not change with volume, maybe it is a ground loop or poor filament circuit
Tried to combine the PS and 50L6 grounds, but did not help.
Thinking that maybe separating the Preamp grounds from the PS grounds might help
And maybe lifting them as separate stars with buss to single chassis ground
But first will up the C6 20uf to 40uf . However when you consider C6 and C5 are in parallel already the 35z6 is already seeing 60uf from the factory. When adding another 20uf in parallel to C6, no impact to the hum

Static - sounds like interference from a poor antenna, it is intermittent,
Relacing C1 and C2 had no impact.
BTW - C1 in schematic is .005 but in circuit was .003 replaced with .0047; C2 is .05 in schematic but in circuit is .01 replaced with .047. These larger values should provide a lower bass floor.
Also tried adding 470pf cap from grid to ground to possibly filter it out,  but created an awful buzzing, thinking not the solution
Any thoughts?

[scstill]

backside

[tubeswell]

Thinking the hum might be reduced if I created Artificial ground for the heaters...
Any thoughts?

12SQ7 = 12.6VAC for 150mA
35Z5 = 35VAC @ 150mA
50L6 = 50VAC @ 150mA


What are you going to 'balance'?

[sluckey]

I would not mess with the heaters. That ground from the PT is shared with the B+ winding too. Probably mess that up.

[scstill]

Am I stuck just living with the hum?

[shooter]

probably
unless you want to scrap the field coil speaker, the PS, then use SS rectifier, more filtering, then maybe, probably not though.

[scstill]

Yeah probably not those mods. The hum is not too bad.
What about the intermittent "radio" interference/static, any thoughts?

[shooter]

is it there with nothing plugged in?
actual radio signal getting in?


check the ground on the input jack, loose, cracked, flaky solder connections. 
leaky plate R's

[scstill]

Static was present without anything plugged in.
I replaced the orange wire from vol wiper to 50L6 grid with coax which seemed to reduce the interference
(was quite subtle and very intermittent to start just describing like a radio type of static, but not a true radio signal).
Might do same dress on the coupling cap from 12SQ7 to Vol

Still have the annoying hum.
To me it seems that the heater design is causing much of this. But if that is not a fix option is there anything else?
When I increase the reservoir cap from 40uf to 100uf the hum is noticeably lower
The 35Z5 is rated for 40uf. Is increasing the cap a good risk?

[shooter]

the PS is 1/2wave, 1 diode, so hum is "normal", given a lo-power SE....


I measure "normal" on the old amps by setting the Amp on 7, guitar 0
If the hum no longer bothers me by the time the guitar is 1-2, it passes the hum test.


Not a tube rec guy so i'd probably use the 100uF til the rec fails, then replace with silicon 

[sluckey]

When I increase the reservoir cap from 40uf to 100uf the hum is noticeably lower
The 35Z5 is rated for 40uf. Is increasing the cap a good risk?

There's an easy experiment you can try... Switch to ss rectifier. Move the PT lead that's connected to pin 5 of the 35Z5 over to pin 4. Now connect a 1N4007 to the 35Z5 socket, anode to pin 4, cathode to pin 8. Now you'll be running SS so pile on the filter caps. Keep in mind that the B+ will be a bit higher but you can also put a dropping resistor between the recto socket pin 8 and C6. And add a 47µF cap between pin 8 and ground. This will greatly decrease the ripple on the B+.

Keep in mind this is a half wave rectifier so the B+ is much harder to smooth out than a full wave rectifier. But the simple mod I suggested will be very effective at reducing hum due to the B+. Won't do anything for filament hum though.

[scstill]

In Antique Radio forum thread they discuss radio factory design using a small resistor before the reservoir cap.
https://antiqueradios.com/forums/viewtopic.php?f=6&t=237258
In the Silvertone radio case below they use a 80uf Cap preceded by a 15ohm resistor for protection. It feeds a similar small 35L6 output tube but also the Radio tubes (12sk7, sa, sk, sq)
https://antiqueradios.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=156767&g2_serialNumber=1

I have tried this with 10ohm and two parallel 47uf, seems better,

I agree with the guitar noise test. Don't hear when playing, only when I'm in a quiet room not playing.

[shooter]

don't do that, move to another room or power the amp off. 


I got a Gibson PSE G8 to that point, the owner thought I modified his amp, I spent 15 minutes calming him down, even showed the before/after pics!


 

[scstill]

Are you referring to tone sap? or hazard operation?
The tone doesn't seem to change with the added cap.
To me it sounds good with or without.

[thetragichero]

add a cap and a small choke before the output transformer

[PRR]

Fix the factory's bad solder joints.

Get the power switch wiring off the volume pot! Keep the 117V AC AWAY from the audio.

As said, the 52V is common with the 115V AC so you can't have a floating or even un-grounded heater circuit. I sure would not go inside the 50 year old PT to try to break the fine wire.

You MAYBE can bodge a DC heater supply. Here is a sketch but I think I have a hot flaw in it.  {EDIT: redacted, see below.}

[PRR]

OK, I did get the rectifier wrong. Done right, the two half-wave rectifiers 'balance' as 10 to 11, making the core flux balanced (for practical purpose). The heater DC will be very clean and can be routed somewhat carelessly. The link from PT's "0V" tap to both sets of main filter caps must be SHORT and direct or you get 60Hz buzz everywhere.

[sluckey]

Don't forget what this amp is...

[PRR]

Don't forget what this amp is...

A high-class KENT. $10 more, some isolation, 2 more inches of cone, mighty 50L6.

Yeah, I'm turd polishing. The H303 was not supposed to last until 1964. Something went wrong and the dang thing asks $100-$500 on the auction sites. Another $50 and some labor to make it QUIET might thrill somebody.