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Hello,

Still happy with my new amp but I was wondering if there is suggestions to improve hum and hiss. the amp is playable, that's not the problem.
1) AC hum : I know the way we wire the ground circuit is important to avoid this. I have 3 ground points : one for chassis ground (just next to the AC plug), one for power ground (next to the power transformer) and finally one near the input jack and linked to the bus ground : it's the basics I suppose.
I first power on the amp. Everything's fine, but when I turn off the stand by switch, there is this hum from AC. It's not awfull but it's there. I suppose it comes from the power supply, since this hum is always at the same level, no matter how the volumes and master are set. The trannies come from Ingo Georges. I'm sure about my circuit, I've checked it several times before testing it. Cap can come from Ceri@tone, the other caps in the power supply chain are F&T caps.

2)Hiss :
OK I know this is a usual "issue". Master past 3 or 4 and it begins to hiss quite a lot mostly when the bright channel is set past 4.
I know this can't be cured perfectly. This is an old style circuit and we have to accept this. But I can test some tricks or make some change to see if I can improve this. Maybe by putting metal film resistors instead of carbon comp at some locations in the circuit? Where would be those resistors to swap?

Last question : I've put the common of the output transformer to the power ground. I don't know what is better for this. We sometimes see common to ground and sometimes not (as on plexi brown from Stephen).

thanks for any kind of help.

Ed.

OK, for the AC hum, it's definitely 50Hz (Europe), so from what I've read it's probably du to electromagnetical prb, between heaters and DC. Some leak trhough the signal. I've made a test : this 50Hz hum is present even if the three 12Ax7 are out the chassis, I may not suspect them. Does it mean that the prb could come from the wiring near the power tubes?
The 6,3V (green) for heating power and preamp tubes is center taped.

I don't know if i correctly wired the heaters. here are some pics.

thanks to let me know if you see something like :

PS: when I see the mess in my '73 princeton rev with no hum at all, i'm shocked.

Make sure the heaters wires go to the same pins on all the tubes. i.e. keep the polarity the same.
You can connect the output jack common directly to the OT. The Jack can be isoalted from the chassi or not.
See if the output tubes are not badly mismatched.
The twisted heater wires look Ok but it seems the issue is at the output stage
Tale a look at one of our layouts in the resource section and compare the grounding and output layout.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Hello Stephen,

thanks.
I've compared the layouts. I've got the same wiring (i took the plexi from Unimind thread to wire the amp). Pin 4 of V4 is linked to Pin 4 of V5. This is the same as on your layout.
The only wire I'm not sure about its use is the "screen wire" from the main transformer. I've connected it to the ground.
Really don't know why this happens.

I'm not far from giving up...
I've redone the wiring of the power section. Here are two pics, showing before and after the changes.
The amp still amplifies 50 cycles (with or without all 3 preamp tubes)...
in the second pic (the actual wiring), the blue and red twisted wires are B+. B+ and grounds lie on the chassis. on the left, the brown one (from OT) and the red one (from board) go to the cap can. The blue one on the right is lifted and is the hot wire from AC outlet.
I've also disconnected ground to common on OT.

I've read some installs a hum pot for this kind of issue, as Fender did during Silverface period. Don't know if it could change anything here.

if you have others suggestions...

thanks,

Yes a hum pot can,ale a difference. You can also lift the heater ground off and use two 100R from each to ground in case the 0V is not perfect. You could measure that as well.
Finally, you can 'lift' the heater voltages to a DC level by connecting the heater centre. tap to the power tube cathode point. See the 18W build by kurtlives

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks Stephen.
I'll try this (2 Resistors, then the pot, then the one in Kurtives thread (have to read it) if it still doesn't make a difference).

To measure the 0V, I put a 1 Ohm resistor between ground and CT and measure intensity? Or is it the difference between (ground and heater 1) and (ground and heater 2)?

What would cause the 0V not to be at 0V?

thanks again,

Ed

To measure the heater AC voltages, measure between (ground and heater 1) and (ground and heater 2)

Slightly Incorrect winding of the transformer would cause the voltages to be unequal?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Hello Stephen,

just an update.
I've changed power tubes (valveart) for JJs. Powered the amp on and standby switch : hum was gone! The amp is really quiet.
The pot I ordered (with the new tubes) is useless but maybe i'll need it in another project.

Conclusion : be carefull with chinese tubes.

thanks,

rgds,