Marcel's scratch build AC15 based clone

[DrNomis_44]

It could be as you said Doc.

In this amp build the heaters for the 12AX7's are from the grounded centre tap 6.3VAC winding, so if the centre tap isn't exactly dead centre then there will be a imbalance that could cause hum. As this has a solid state rectifier diodes the EL84 heaters are on the otherwise unused winding that would have powered the rectifier tube with two 1W 100 ohm resistors to ground to balance up that sub-circuit. However from initial operational diagnosis and knowing the circuit as well as I do I'm more inclined to think it is a 22uF filter cap beginning to fail and starting its climb up the ESR ladder. When I open it up it will one of the first things I check.

I also have a 2.9H 120mA Fender champ power supply filter coil that I want to incorporate, so that may also go some way to eliminating all hum.

As for the Tweed amp Doc... I've lost count on how many times I've had to stop myself from getting a Tweed kit.. or even a small kit from one of those places Simon recomended... I gotta get my spending under better control before my wife gets fed up and takes over control by hiding my credit card...lol

Have you thought about converting the 6.3V AC heater supply from AC to DC?, that may reduce the hum level a bit, I think some well-known guitar amp manufacturers use a 6.3V DC heater supply in some of their high gain amps that they manufacture.


Cheers for the links to the two pdfs, I downloaded the Mullard pdf and I'm in the process of downloading the other one, the Mullard pdf has some really interesting projects in it, most of the theory in the Mullard pdf is geared towards HiFi applications, but it still applies to guitar amplification applications as well, definitely worth downloading a copy of each of the pdfs.


Note that the RCA pdf has lots of good valve theory in it, good reading material if anyone is interested in learning about how valves operate.