2204 style amp hum and buzz

[paul525715]

I have recently built a 2204 style amp with a few minor changes, the main ones being a relay controlled boost, a ppimv, a vox style cut control and two separate bias circuits. The relay is floating to minimise hum and is powered by a 5v tap on the PT. The bias controls are directly below the output jacks (as can be seen in the video below). I had originally used the larry grounding scheme but for whatever reason (perhaps the shared can cap cathode) the hum and buzz was terrible. So, I rewired the grounding in the same way that a typical 2204 is wired, with a bus bar.

The bus bar is as follows (from one end to the other)...

V1 cathode
Input jack ground
Preamp gain pot ground
V2 cathode
Relay boost ground (a simple voltage divider in the circuit that is switched in and out)
Preamp can cap ground
Mid pot ground
Fixed presence circuit ground
Bias ground
Star ground near PT

The star ground near the PT also has the main and screen can cap cathodes and pt centre taps.
The output jack ground and pin 1/8 of the output valves are grounded together near the output valves.

The amp is still horribly full of hum / buzz.

The voltages are as follows...

V1 pin 1 202v, pin 2 0v, pin 3 2.2v, pin 4 & 5 -, pin 6 236v, pin 7 0v, pin 8 2.7v, pin 9 -, B+ 295v
V2 pin 1 159v, pin 2 0v, pin 3 1.2v, pin 4 & 5 -, pin 6 303v, pin 7 158v, pin 8 176v, pin 9 -, B+ 304v
V3 pin 1 224v, pin 2 12.7v, pin 3 40v, pin 4 & 5 -, pin 6 240v, pin 7 14v, pin 8 40v, pin 9 -
V4 & V5, 496v, 492v and -46v on anode, screen and grid respectively.




[youtube]http://www.youtube.com/watch?v=iBHCWAX_x9A[/youtube]

Can anyone help me out?

Thanks.

[kek]

Tell us more about the relay driver circuit. Is it driven directly off of un-referenced 5VAC? Try ripping it out. Also try shortening the leads to the master volume, it doesn't look appealing as is. Unbolt it and connect it using short leads and see where that leads you.

Try isolating one of the problems, start with the 50Hz buzz. Locate the outlet with the best earth connection in your house and see if it makes a difference. Try elevating the heaters, and try DC heaters. Use a multimeter or a scope to find the source. Work methodically both ways up and down the signal path, hillbilly style shorting inputs, pulling tubes, or like an engineer using proper equipment.

If the hum is a 100Hz one, you will find its source in the power supply.

[paul525715]

Tell us more about the relay driver circuit. Is it driven directly off of un-referenced 5VAC?

The relay circuit is driven from rectified 5v. The 5v ac supply goes through a bridge rectifier and a filter cap. The dc that is made is not ground referenced. It is floating.

Also try shortening the leads to the master volume, it doesn't look appealing as is.

The master volume leads are as short as they can be without removing the pot and making it float above the chassis.

Locate the outlet with the best earth connection in your house and see if it makes a difference. Try elevating the heaters, and try DC heaters.

I will try these as soon as I can.

Use a multimeter or a scope to find the source.

I can get a signal generator and a single channel oscilloscope. How do I use these to find the source of the buzz / hum?

Thanks for your help.

[paul525715]

It was the relay power supply. I've removed it... the amp is silent. Lovely!