need some help with my first build

[tonehound]

Hello everyone. I am new here. I discovered this forum while researching a hum problem after firing up my first build. It is a clone based on the A4 schematic. transformers are what I had on hand, Hammond 274BX and Marshall style 50 watt output. Hopefully, a set of Heyboers are down the road. After reading through the forum posts regarding hum problems. I have made the following changes to the grounding.

1. power filter caps B1,B2, phase inverter filter B3 grounds are grounded to the 1st filter cap ground lug. HV center tap is here as well as the bias supply ground. Had the heater c.t. here but removed it and put 2 100 ohm resistors from pins 1 and 8 on one of the EL 34's to ground at it's sockets mounting nut.

2. The 1st two preamp filter caps are tied to the ground buss at the same place as their cathode stages. The ground bus is grounded to a mounting bolt for the output transformer located opposite the pre amp circuitry. near the input jack.

3. The spkr output jacks( there are 3...4,8,and 16) have insulating washers and their jack ground lugs are tied together and grounded at the same place as the Presence control, at the ground bus where preamp filter cap B4 is grounded.

4. The input jack is grounded to the chassis via its mounting nut.

Here is the problem: When the amp has no guitar plugged in and the Vol is turned up the hum seems slightly louder than normal. Not too bad. When guitar (tried both Strat and Les Paul) humbucker is used, very loud hum that changes as I walk around room. When I use the in between settings on the Strat with a reverse wound middle pick up hum seems normal.

Any help would be greatly appreciated.

[UR12]

Do you have a bottom plate on your chassis?

[tonehound]

Yes. I have the bottom plate and tested the amp with it in place, however, to me there was no noticeable difference.

[tonehound]

As a follow up to my last post. what is troubling me at this point is why the Humbucking pickup is causing so much hum and the single coils on the strat are not. This may be a no brainer to more experienced amp builders here. Is there a simple explanation for this?

thanks in advance for any help.

[Darkbluemurder]

You did not mention which humbuckers you are using. If they are "Unbuckers", i.e. humbuckers that have coils with different turn counts then they will only partially cancel hum. The most popular "Unbuckers" are the Lindy Fralin Unbuckers, the PRS HFS and Vintage Bass as well as some DiMarzios (I believe PAF Pro, FRED and MegaDrive are all Unbuckers).
Of course if your humbucker has the option to split the coils it may be accidentally wired as single coil and hum but I guess you have ruled out that possibility.
You most likely have a strat with a middle pickup that is reverse wound and reverse polarity. If all pickups have the same number of turns then you will get a hum cancelling effect in the #2 (neck + middle) and #4 positions (bridge + middle).

Hope that helps. Good luck!

[tonehound]

The Les Paul has P.A.F. pickups with the patent pending stickers on the bottom of the pickups. The strat style guitar has a Allan Holdsworth pickup in the bridge position wired and two Duncan Hot stacks for the middle and neck position. I believe the middle position pickup is reverse wound but Im not 100% sure on that. It also has the Tom Anderson series parallel switching with the Blow switch. I also should mention that I did play both guitars through a different amp and they work as they should. I could be wrong, but at this point I believe the problem is with the amp. I'm just not sure where to go from here.

Thank you UR12 and Darkbluemurder for your help