PTP Vibrochamp Hum

[LegendaryCheddar]

Hi Amp Garage legends,

Long time reader first time poster, from the UK.

Currently finishing up a first PTP AA764 Vibrochamp, and it sounds lovely-but has a heater hum (50hz for me in UK), and I cannot fathom it. I’ll attach some pics, and if anyone can see what the dickens I’ve missed I would really appreciate it.

I have narrowed it down to the 6v6 tube-detached the cap on pin 5 from the last stage before 6v6 grid, moved the grounds.

For your interest my ground scheme is a buss wire that is only attached to chassis near the input, and the order is as follows:

EARTH (chassis)
Buss:

HT CT, heater CT, Reservoir Cap first GND 1 group

Then Screen, Speaker ground, V2B cathode GND 2

Then Preamp cathodes and grounds GND 3-then chassis.

All jacks are cliff style, and the general hiss and hum excluding the heater one is dead quiet.

I’ve tried to implement Merlin Blencowe’s guidelines.

Anyway, please help!

T

[lonote]

I am sure that extra sets of eyes will notice things to check.


It look awfully busy around the preamp tube, my guess is some lead-dress/proximity issues in that area, but it will be interesting to see.


You might try elevating the heaters to see if that helps. Just move the ground point of your heater artificial center tap to the high-side of the powertube cathode resistor/cap combo.

[xtian]

Quickest cure for hum in SE amps is an additional filter stage, called Pi filter. Add another 20u/450v cap and 100-470ohm/1w resistor just prior to the existing first filter cap.

[Stevem]

Other then the noisy way Fender did the filaments this works for a good layout.

[angelodp]

Many have added a choke to quiet up this SE circuit.

[Stevem]

While you’re still in there you should seperate your output tube cathode resistor and bypass cap by at least 1 inch, otherwise that cap will have a short life.

Adding a choke to a Champ changes the whole great feel of the amp for the worst!

[R.G.]

Wiring for low hum info:
https://www.dropbox.com/scl/fi/vpijj3al ... y7v7f&dl=0

[LegendaryCheddar]

Thanks everyone, all really useful stuff. First champ Ive built and didnt realise that the lack of filtering was such a big deal on it..

Id like to implement the Pi filter-looks like that’ll probably do it.

So do I pop another 20uf/450v next to my reservoir cap-straight after the 470r resistor? Never done one before..

Amazed that anyone replied and with such great info, so kind of you all.

[Jered]

Wiring for low hum info:
https://www.dropbox.com/scl/fi/vpijj3al ... y7v7f&dl=0

Wow! Thank you RG ! A ton of great info

[xtian]

So do I pop another 20uf/450v next to my reservoir cap-straight after the 470r resistor?

Each filter stage is a cap from B+ to ground, and a decoupling resistor in series with B+. Use a 100 to 150ohm resistor (you can think of this resistor serving the same purpose as a choke). So you're going to add a new 20uF cap immediately after your rectifier, and a 100R resistor to connect to the existing first filter cap.

[LegendaryCheddar]

Thanks everyone-all sounds amazing now…

ill post some pics of it in the cab Im building at some point.