12AY7 Damaged??

[tom_kinki3]

Hi,

I've just completed my first 5e3 build. I'd made some stupid errors when first wiring it up, which caused one of my filter caps to blow, in quite spectacular style.

I now have the amp working, and the (bright?) channel it sounds great. It distorts pretty early, but this is kind of the tone i'm after, so hopefully not too much of a problem? There is pretty bad mains hum, but i'm going to put the 100R resistors of the 6.3V AC lines to ground to provide a reference, as i've heard this can improve the hum... I've also seen these wired to the cathodes of the power tubes - is this advisable?

BUT, i think its my normal channel that is very very quiet, and the volume control doens't work at all. One of the errors I made was wiring pins 1 and 7 of the 12AY7 the wrong way round.

Could this possibly have damaged the tube?? I guess it could have resulted in 250V being put on the grid right?

I'm going to double check all the wiring around this area again, but do you think it'd be worth just swapping the tube out as well?

Thanks is advance for your help!

Tom

[MWJB]

Just subbing V1 for any spare 12A#7 tube will help you identify whether the normal channel issue is down to the tube, or the wiring.

If you add the 100ohm virtual centre tap resistors, make sure that you disconnect the 6.3VAC centre tap wire, that you should have grounded from the PT at the moment. Use the 100ohm resistors, or the CT, not both. You should be getting up to 25vdc on the 6V6 cathodes...this may or may not be enough to negate your hum, it's worth a try. Any of these techniques will only help if your hum is coming from the heaters & not due to general grounding & layout of wires.

[tom_kinki3]

Unfortunately i haven't got any spare tubes - so I will double/triple check the wiring and if that fails, buy in another tube....

My PT doesn't have a centre tap for the heater windings - its a mojo export model. So the 100Ohm resistors is definitely a good bet for reducing hum...

Cheers,

Tom

[MWJB]

Get a couple of RCA male to 1/4" male adaptors & a stereo RCA interconnect cable. Run the cable from a CD/DVD player to your amp to provide a signal source, put in a CD & play it through your amp. Compare the AC signal voltage developed on V1 pin 1 (normal channel) & pin 6 (bright). Then check at the middle tabs of the volume pots, to see whether you are getting any AC on the normal channel at all & if you are, where does it stop?

[tom_kinki3]

should i be measuring the AC signal with a scope or DMM?

[MWJB]

A DMM will suffice, we're not looking for a specific figure, or a perfect sine wave, we are looking at this stage for any significant discrepancies between the Normal & Bright channels to identify why/where the signal is lost.

[tom_kinki3]

right. will do that tonight...

With regard to adding a ground reference to the heater circuit - are 0.6W resistors okay? If not, i have some 1W 120Ohm?

[MWJB]

Yes, 0.6W are fine, but the 120ohm 1W would work just as well. If you have your heaters wired as a twisted pair (one wire feeds pin 2 6V6, the other pin 7 6V6...rather than the single wire daisy chain Fender used in the 50's), but you have no 6.3VAC centre tap, nor any resistors wired as a virtual centre tap, then you will have significant hum.