5C5 Hummingbird

[4 Strings]

Hi!
A few months back I finished an amp build based on the 5C5 circuit which proudly bears the name “the Junkmaster” .

The amp is up and running and sounds good too, but there is an awful lot of hum in it too, at least compared to my 5E3’s and 5F1’s. I don’t intend to use it for studio recordings and in live situations the hum shouldn’t be too much of a problem, but anyway…

Which are your impressions with the 5C5, is the hum inherent in the (rather obsolete) 5C5 circuit or should I expect it to be reasonably quiet?

Now except for using correct component values and a Weber fibre board nothing much about this amp is “stock” really; I have used 9 pin preamp tubes, the PT is a custom wound piece without the 5v filament winding, hence SS rectification and 188 ohms of sag resistors. The PT also lacks end bells, which may cause some of the hum. The OT is a 1990’s Vibrolux Reverb replacement tranny from Magic Parts, the chassis is a hefty piece of 2,5 mm stainless steel and so on.

The amp actually sounds good, especially through a 10A125 and a 10A125-O, but as I said it also has quite a bit of hum. To battle this, I first installed a ground buss to enhance the grounding, given the low conductivity of the stainless steel chassis, but to no apparent avail. Next, I put shielded wire on all the grids, but still no real improvement.

Any more thoughts on how to approach this?

Thanks!

A.

[sjhusting]

- does the hum change volume with the volume control
- is it 60 (or 120) Hz hum?
- did you lift the heaters with 100-ohm resistors, or use a heater center tap?
- where did you ground the CT (or the virtual CT)?
- did you use the original 5C5 layout with one leg of the heaters over the chassis?
- how's your lead dress?


steven

[4 Strings]

Thank you Steven!

- does the hum change volume with the volume control

With both volumes turned down and nothing plugged in there is a bit of static hum. Not too annoying, but more than in my other tweed builds (5F1's and 5E3's).
Turning Vol 1 up increases the hum.
With Vol 1 back down to zero (back to that a-bit-of-static-hum level), turning Vol 2 up kills some of the hum, though not to a very dramatic effect.
With Vol 1 up to 12 (yielding the loudest hum), the effect of turning Vol 2 up is a bit more noticeable. Going from 10-11 to 12 on Vol 2, the static hum decreases some to reveal the usual hiss from a maxed out amp.

- is it 60 (or 120) Hz hum?

I dunno...

- did you lift the heaters with 100-ohm resistors, or use a heater center tap?
- where did you ground the CT (or the virtual CT)?

Yup, I made an artificial center tap using two 100 ohm resistors grounded to the chassis on one of the PT bolts. I have not yet tried to lift the CT from ground, perhaps this is what I should do next? To which power tube pin do I solder the CT to do this?

- did you use the original 5C5 layout with one leg of the heaters over the chassis?

No, I used the modern style twisted heaters.

- how's your lead dress?

Actually, this is the one place where this amp really shines. I paid attention to achieve a neat and tidy lead dress.
I have changed the front end grid wires to shielded wire and the wires to the power tube grids are twisted.

Except for the static hum described above - which is enough to annoy me, but perhaps it could be fixed by lifting the CT from ground - I believe that much of the noise comes from the un-shielded PT and from the fact that I play a one mic single coil Esquire/Partssquire picking up whatever dirt there is. I plugged my Esquire into a 5E3 with the 5C5 being turned on, and as soon as I got anywhere near the 5C5 the Esquire mic picked up its hum so it could be heard through the 5E3.

[sjhusting]

Originally Posted by 4 Strings

Turning Vol 1 up increases the hum.

Make sure you're using a good cable.

Turn off any flourescent lights. Get away from any other possible sources.

Test it with a humbucker pu guitar.

Try a different tube in V1.

Chopstick the leads around V1.

tie the artificial CT (where the 2 100R resistors are soldered together) to the high side of one of the 6V6 cathodes - that is, directly to pin 8 instead of the transformer bolt.

Read the Hoffman info on grounding. His grounding scheme has always produced dead quiet amps for me.

You can also disconnect the heaters from the 6.3v winding and connect them to a 6V lantern battery. If the hum disappears, you know it is related to the heaters. If it doesn't, you know it's not the heaters.

steven

[TNO]

More powerful amp=slightly more noticeable hum at low levels.