Zenith SET amp capacitor sub, change in sound quality here?

[tom3]

Little Zenith 4l21 amp sounding really good but had a pretty noticeable hum that varied with the volume control. Saw this old paper cap and checked it first. Got "unknown/defective part" so I started looking for a sub. Have a 33 uf and the 47 uf. Put in the 47 and it fixed the hum problem but the SQ was some different and not for the better to my ears. Lower treble/upper mid range is some "shrill" sounding that wasn't there before that I could hear. Would the 33 uf cap be better in this spot? Parallel a couple 22 s? Does the cap in this position have an effect on the SQ - or is it just me? (sure some difference in size in the 60 years)

 

[Tom Bavis]

Without it, there will be out-of-phase crossover between channels, and loss of bass. Don't know it would affect hum, except by unbalancing the hum cancellation in the output transformers (the reason for the tap). As for "shrill" quality, may be other issues too.

 

[Mr Boochie]

Without it, there will be out-of-phase crossover between channels, and loss of bass. Don't know it would affect hum, except by unbalancing the hum cancellation in the output transformers (the reason for the tap). As for "shrill" quality, may be other issues too.

The bypass cap has a direct effect on the frequency response of the output section. If the response is shifted too high, try a larger bypass cap. Maybe, 100mfd/100VDC, and see what happens.

You need matched EL84 output tubes, as there is no balance adjustment between the two cathodes --- to get best hum-free performance.

 

[zackthedog]

Little Zenith 4l21 amp sounding really good but had a pretty noticeable hum that varied with the volume control. Saw this old paper cap and checked it first. Got "unknown/defective part" so I started looking for a sub. Have a 33 uf and the 47 uf. Put in the 47 and it fixed the hum problem but the SQ was some different and not for the better to my ears. Lower treble/upper mid range is some "shrill" sounding that wasn't there before that I could hear. Would the 33 uf cap be better in this spot? Parallel a couple 22 s? Does the cap in this position have an effect on the SQ - or is it just me? (sure some difference in size in the 60 years)

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In an SE amp that bypass cap is going to be heard pretty clearly. I would suggest Elna Cerafine, AudioNote or Nichicon Fine Gold. Some will scoff and say it can't make a difference, but you yourself can hear it does. Stick with 47uF and anything from 25 to 50vdc rating.

 

[Mr Boochie]

In an SE amp that bypass cap is going to be heard pretty clearly. I would suggest Elna Cerafine, AudioNote or Nichicon Fine Gold. Some will scoff and say it can't make a difference, but you yourself can hear it does. Stick with 47uF and anything from 25 to 50vdc rating.

I like using 100VDC rated caps, not that they'll see anything close to that voltage (hopefully), but they are better in heated conditions which they may be placed (near the output tubes and cathode resistors). Even new-issued 100VDC rated ones are small compared to those old school waxed 25VDC puppies.

 

[tom3]

Thanks for the info, I sure came to the right place for help. All the tubes are Zenith originals and test over 90% with my old basic tester. I'll experiment some with caps at that spot and see what works best. This amp also has 4 or 5 bumble bee caps, some say they are awesome, some say trash, especially at this age. Replace-- or test and leave alone if on spec??? Pretty surprising how good this really simple amp sounds - or sounded. I'm playing it through the original console speakers with an added piezo tweeter each.

 

[gadget73]

have never found a bumblebee cap that didn't have excessive leakage problems. Re-install them in the trash and put in caps that work correctly.

 

[Buck Rogers]

Separate the two output cathodes.

Install a 300 ohm resistor and a 47uf cap on each output tube cathode.

Check the voltage across it divide by 300 and you will have the current value.

Much better setup.

 

[Mr Boochie]

have never found a bumblebee cap that didn't have excessive leakage problems. Re-install them in the trash and put in caps that work correctly.

I replace any old bumblebee or black beauty caps. Any paper or wax covered get replaced, too. Nice modern poly film caps will last forever.

 

[gadget73]

Separate the two output cathodes.

Install a 300 ohm resistor and a 47uf cap on each output tube cathode.

Check the voltage across it divide by 300 and you will have the current value.

Much better setup.

the one and only reason I didn't do this in my SE amp is I didn't have two appropriate resistors on hand, so it retains the original shared resistor. There really isn't any reason to combine the cathodes in this kind of amp other than it reduces parts count for the mfg.

 

[Mdpaudio]

I second the splitting the bias resistors into two 300 ohm 5 watt units. Bypass them with a 47uf cap on each. It’s less than $10 all in and is a much better way to go as you don’t need matched tubes but they would be nice. The loss of bass might be elsewhere, check the coupling cap to see if it’s leaking.

 

[Mr Boochie]

I second the splitting the bias resistors into two 300 ohm 5 watt units. Bypass them with a 47uf cap on each. It’s less than $10 all in and is a much better way to go as you don’t need matched tubes but they would be nice. The loss of bass might be elsewhere, check the coupling cap to see if it’s leaking.

Matched tubes are always nicer. Unless you are looking for some different harmonics, at volume.