Single Ended Hum

[leadfootdriver]

Messing around with my little Combo monstrosity. I've used it to try different things over the year.

Right now the Preamp is based around a JCM 800 into 1-EL84. But it seems to Hum, a lot!
It Hums close to a Bb note on the guitar. I don't think it's Ripple. I have 100uf after the Rectifier and 2-22uf's for the Preamp.
I tried doing a fake Center Tap on the Heater with 2-100R's to ground, but it made no change.

What do you guys think? I just don't remember the Hum being this loud..

[DrGonz78]

AC ripple will measure 120hz hum in an amp. Whereas a ground hum will emit a 60hz frequency. Of course this is 100hz and 50hz where line voltage is 220-240v. The B flat frequency is 116.5hz so perhaps you are hearing an AC ripple hum.

[leadfootdriver]

You are right! I threw another 100uf in there and the Hum got a bit quieter.

[g1]

Single ended amps require better filtering to get the ripple (on output) down to what you are used to with push-pull type amp.

[leadfootdriver]

Scrambles to find more Caps..


Surprised I overlooked this.. Poking at a Hiwatt DR103, he has around 760uf of filtering after the Rectifier..
What is the goto value then??

https://el34world.com/charts/Schemat...100w_dr103.pdf

[Helmholtz]

60Hz also sounds close to Bb, just one octave lower. Also there are harmonics i.e. multiples of the fundamental frequency.

A scope pic could help.

Do you get the hum with the input shorted?

Please post a drawing of the power stage + power supply.

[mozz]

Put a R in between 2 C's, rinse repeat, it will get quiet.

[Helmholtz]

Scrambles to find more Caps..

Surprised I overlooked this.. Poking at a Hiwatt DR103, he has around 760uf of filtering after the Rectifier..
What is the goto value then??

Where do you see 760µ? No sense just adding up capacitance of all filter caps.
Effective smoothing/reservoir capacitance is 110µ. But it's a 100W PP amp.
In your case (low wattage SE amp) 100µ at the rectifier should be sufficient, that is if the cap is good.

[loudthud]

Does adjusting any of the pots affect the hum in any way ?

How is the input jack grounded ?

[Chuck H]

This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I don't doubt that the problem is related to the single ended nature of the amp, BUT...

You do report that the hum is worse than you remember. If the hum was acceptable at some point then there's no reason that can't be restored with repair. The first thing to do is isolate the problem. Preamp tubes CAN become noisy or otherwise misbehaved. And ground connections at non soldered contacts or cold solder joints can cause problems. Hum can likely be improved from the original design by idealizing filtering and practice as it applies to single ended amps. I think you should first rule out any potential circuit failure for the best result. In other words...

If the hum level was acceptable once upon a time then reducing hum in other areas where inherent hum already existed may give satisfactory results, but you still may not have fixed this new problem. I think it would be best to fix the problem first and THEN do any general improvements for an even greater result. That means isolating the hum as it exists right now. If you start making power amp and filter arrangement changes arbitrarily it can only confuse remedies for the problem at hand. Isolate the problem first before doing anything.

JM2C

[leadfootdriver]

Does adjusting any of the pots affect the hum in any way ?

How is the input jack grounded ?

No.
1M to ground, Cliff Jack.

[Chuck H]

No.
1M to ground, Cliff Jack.

I think he was asking about the phone plug sleeve connection. "1M to ground" would seem to be the jack to first triode grid circuit.

[leadfootdriver]

This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I don't doubt that the problem is related to the single ended nature of the amp, BUT...

You do report that the hum is worse than you remember. If the hum was acceptable at some point then there's no reason that can't be restored with repair. The first thing to do is isolate the problem. Preamp tubes CAN become noisy or otherwise misbehaved. And ground connections at non soldered contacts or cold solder joints can cause problems. Hum can likely be improved from the original design by idealizing filtering and practice as it applies to single ended amps. I think you should first rule out any potential circuit failure for the best result. In other words...

If the hum level was acceptable once upon a time then reducing hum in other areas where inherent hum already existed may give satisfactory results, but you still may not have fixed this new problem. I think it would be best to fix the problem first and THEN do any general improvements for an even greater result. That means isolating the hum as it exists right now. If you start making power amp and filter arrangement changes arbitrarily it can only confuse remedies for the problem at hand. Isolate the problem first before doing anything.

JM2C

I've had this thing for a long time. I'm no designer, but I can burn things with a soldering iron!
I just go for it, trying to pick up things here and there. I don't do it everyday.
I don't know that it was ever really quiet after I gave it life..
It used to be a Crave V58. It had a SS for the 1st gain stage. I gutted it and used the Tranny's.
I added a 2nd 12AX7. I've tried different things over the years, copying and trying other peoples designs.
Right now I'm trying to do 3 gain stages and then a cathode follower into a Marshall tone stack out to 1-EL84. The 3rd state is giving me problems as the sound is blatty..
Like I said, it's my monstrosity..

V58.pdf

[Chuck H]

Ok. I thought it was inferred that the hum was worse than you remembered. But if this is a sort of closet shelf project then it's hard to go by memory. I know this as well as anyone.

FWIW the schematic seems to show that the filament secondary has a center tap. Did you lift this when you implemented the 100R resistors for a "false" center tap? Because you should NOT have both.

You also have your grounds "daisy chained". As in, you've wired a single buss across all board ground points for the whole circuit. Ideally you should remove that and wire each ground point with an individual lead to location near the input jack. This is called "star grounding" and it works pretty well for simple circuits. I don't know if that ground bus lead is finalized to ground at the input jack end of the chassis or the power amp end. If it's grounded at the power amp end that could be one source of hum with everything daisy chained like that. You may have more than one source of hum.

[leadfootdriver]

Looking at the Schematic, I for sure did not do the Diode and cap network for the Heaters.. I bet that would do it!
I haven't looked at that thing in a decade.. I just gutted it and started..
'Need to get on Mouser!