Federal AM 864/U -- 60Hz Issue help

[cannikin]

HI Guys,

I'm wondering if you could help me with my 60hz issue in my federal limiter Clone.

First: I redesigned the Power supply and moved it farther away from the rest of the circuit.
Second: i replaced all wires with shielded cable except for the Heaters (but they are tightly wound).
Third: moved the AC inlet, power transformer, and 5Y3 farther away from the rest of the circuit. ( that was laborious :? )
Fourth: I swapped 6SK7 with different set (that helped a tiny bit but definitely not the source of the problem).

After all that -- not much change.

So I proceeded to try some other things:

After Reading this-- http://www.aikenamps.com/InputRes.htm -- a few times.

Grid Stoppers the 6SK7s (input tubes) nothing there. Next a 1Meg Grid (to ground) Stopper on 6SQ7 (compression tube)

:thumb: BINGO reduced the Hum by at least 50%

So my HUM is somewhere in the Power Supply to 6SQ7, which makes sense when you look @ the schematic 5 or six times a day.....

I proceeded to try one more thing: I added a Second Choke from the 5Y3 (PIN 8) to the first Cap (C4) of the Power Supply, I have been searching and reading about Power Supply designs on the Net and found a few designs utilizing 2 Chokes -- Figured what the hell, give it a try.

:thumb: BINGO reduced the Hum greatly BUT I don't think this was a 100% because of better filtration -- it was because of it created a power sag in the B+ My B+ is supposed to be 200V (which it is with one Choke), with the Second Choke the B+ Sags to 160V.

This created some interesting artifacts: the Compression/Limit seems to act a faster in the attack and recovery @ 160V... Is this Possible -- or am I once again -- delusional? The other thing which I like is -- the Output signal isn't as hot, therefore no need for a PAD This also Cures the Oscillation/Motor Boating in V4 when you turn R10 (Threshold pot) past 3 o'clock.

Ideally, I would like to get this working in the way it was originally designed - Correct B+ (200V), No 60Hz Hum, and and some kind of PAD.

I need to order some other 6SQ7 tubes in case my tube is bunk -- But any other tips, thoughts, and suggestions you may have -- I'm all Ears.
(unfortunately I don't have a scope).

As always thank everyone

 

[Rob Flinn]

David

I have been where you are myself & know how frustrating these problems can be. I have built something with really neat wiring, which has had mains hum. I have then stripped it down, moved things around, rewired it (never as neatly as the 1st time I did it) & still have hum.

I can't supply you with an answer to your particular problem, but in almost every case for me it has turned out to be something relatively simple, which didn't require all the radical changes/rebuilding I did. I know from what your saying that you feel you have explored every avenue, but have you dismissed anything simple, like so simple you think it couldn't possibly have an effect ?

Have you scoped your HT lines to see if they have ripple on them ?? If they do then you may find it better add a bigger cap rather than an extra choke because the rail won't sag, but it will have the same effect.

Did you make the body of the the grid stopper resistors as close to the valve base as possible, no long leads ?

 

[gang of elk]

I had a similar problem with an old tube mic power supply, rebuilt it to no avail. New everything, still hum. Then reread an old post with a suggestion by dale116dot7 to raise the capacitance of the heater filter. Bingo. In my case I quadrupled the value from 1000uf to 4700. Maybe worth a try?

 

[cannikin]

thanks Rob -- I have a feeling the issue is right under my nose --

GofE -- interesting ... really?? I'll do some searching -- thanx

[quote author="dale116dot7"]1N4004 diodes will be fine but you may need to adjust R11 to get 6.3 volts at the heater of the tube. The Coke Bottle microphones make a lot of hiss if you underheat the filament. Also, the silicon rectifiers may make more 'hash' so if you get buzzing in the filament, raise the input capacitances a lot. I used 4700 uF x 2 for mine, that made the buzz disappear.[/quote]

I already increased my Filter caps from 12uf to 22uf --- but up it even More like 450uf or something

 

[cannikin]

going to give this a try:

Balancing the Filament Circuit w.r.t Chassis

To minimize filament AC feedthrough to the Cathode of the tubes both legs of the filament circuit may need symmetry adjustment with respect to the chassis potential (0-volt). As well, a pair of dedicated wires must run to the filament pins on the tubes, ie. the chassis should not be used on one side of the filament circuit as a means of avoiding the creation of a shared impedance scenario (also known as a Ground Loop scenario). A balancing "Hum-Adjust" network should then be installed as a means of tuning out imbalances between both legs of the filament winding on the power tranny. A 1watt 100-ohm wire wound pot is typically used for this.

If you find that the lowest hum position happens to be when this pot is turned completely to one side then a simple resistor can be used instead and the pot saved for another amp though I usually leave it in just in case things change with different tube sets.

 

[emrr]

I bet hum was a fact of life with these, and having the voice grade transformers of most originals you'd never hear most of it. I wonder if switching one of the grounded filament sides of one compression tube would help, since you have all four 6.3 filaments in parallel to ground. Do your filaments strictly follow the #'s on the schematic? Don't know if it'd matter or not. OR; are you using 6.3AC with no ground reference at all? That would be the problem, if so.

Related: I have yet to meet a Gates SA-39 that didn't improve greatly with the later factory DC filament mod for the 1612 compression tubes. Loud hum hard to change without it. They made those for 12 years before the DC front end shows up, so they must have had better tubes than I've ever found. Maybe they bought factory matched from Gates. Should be easy to find the SA-39A online.

I can't stress enough how important it is to buy extra compression tubes so you have a big pile to match from. I can go through ten 1612's to find a best SA-39 pair. It is the most important part of your comp, so spend the freakin' money. There's almost no way you can buy just a pair and get lucky; you certainly can't compare.

 

[Rob Flinn]

going to give this a try:

Balancing the Filament Circuit w.r.t Chassis

If your heater is not connected to ground at all then you can expect hum. Even if you just connect 1 side it makes a vast difference

 

[Gus]

60 Hz, heaters AC powered?

If so try the old school trick of rasing the heaters above ground.

link to the schematic and any variations to the circuit you built?

 

[Gus]

gang of elk

Yes the DC heater section of a tube mic power supply need attention to detail.

 

[cannikin]

Thanks Gus

Here is the Link to the Schematic:

http://waltzingbear.com/Schematics/Misc/AM-864.htm

Question for you guys: why would putting a 1Meg Resistor to Ground on V4 Grid (Pin 2) of 6SQ7 compression tube lower the 60Hz hum?

 

[cannikin]

As another idea that I'm contemplating if I can't find a solution:

The Heaters are AC.

what if I changed them to DC?

with something like this:

http://www.tentlabs.com/Products/Tubeamp/Tubeheater/index.html

 

[Rob Flinn]

[quote author="cannikin"]As another idea that I'm contemplating if I can't find a solution:

The Heaters are AC.

what if I changed them to DC?

with something like this:

http://www.tentlabs.com/Products/Tubeamp/Tubeheater/index.html[/quote]

probably uneeded & you will require more than 1A anyway.

Have you got your heater circuit connected to ground yet ???? Either via a pair of resistors or a 100R 1W pot.

 

[cannikin]

Nope not yet..

thats the first thing I'm going to try.

 

[cannikin]

it worked! :grin: :grin: :grin:

I used a 100 Ohm 2W pot on the heaters, wiper to ground I was able to dial out almost the all of the 60Hz

There's still a little bit BUT at least now they are useable for recording.

Now I have a little hiss that I need to get out of the system and I'm ready to rock -- I'm assuming it might be the 6SK7 input tubes because if I adjust the input pot the hiss increases.