need help~ express hum when speaker changing

[reroe]

after tons of trouble finally finally i did my express build
but one more problem I have...

first i don't have any speaker cabinet
so i decided to use one in my combo amp (cut original wiring and attach 1/4 jack to speaker)

it works with the speaker in my tiny, cheap amp(maybe 4inch?, don't know impedance)
there's no hum, no squeal (thanks god)
but when i change the bigger one (in old line6 spider amp, 8inch, 16ohm)
it starts to squeal and pitch(tone) of hum changes when turning volume or eq knob (every knob, even they're "0")

please help me~

[Structo]

Strange it doesn't do it on the other speaker.

Check to be sure you have the phase of the output transformer wires correct on the power tubes.

Test this but disconnecting the Negative Feedback wire to the presence control.
If the squealing stops, switch the primaries on the power tubes.

Or if it is easier, switch the two wires on the plates of the phase inverter.

[reroe]

thanks structo~

actually i don't understand what you mean, "phase"
(i'm not good at making amps, this is my 1st build...)

primaries of my output trans consists of 3 wires, 6.6k * 2 + 1 B+
each 6.6k wire is connected to pin3 of two EL34s

do you mean that...
1) disconnect NFB
2) switch 6.6k wires connected to out tubes each other
3) or switch lines connected to pin1, 6 of the PI tube ?

eagerly hope your reply...

[M Fowler]

He means to switch the primary leads for the output transformer (OT). Just reverse the leads from the OT going to your power tubes.

Also if you have prescense pot connected to the 4 or 8 ohm tap of the impedence selector that can cause troubles.

[Structo]

Yes if when you disconnect the NFB, the squealing stops, then that means you are getting positive feedback, the squealing.

So if it stops, swap the primary wires on the primary of the OT.
If you have cut the wires too short to swap them, you can switch the plate wires of the phase inverter, same difference.
You are changing the phasing by 180 degrees when you do this, so that you are getting negative feedback to the phase inverter as it should be.

It might not even be that but that is a common probably people run into on a new build.

Hope this helps.

[Cliff Schecht]

It may not be from the negative feedback. My first Express clone has an issue with certain speakers in that it will squeal with some speakers but not others. This squeal completely goes away with an attenuator but I find that without one, if I turn the presence past about 70% an audible whine does appear (not with EVERY speaker though). Your amp isn't fully stable, perhaps it's a wiring issue or a bad wire run somewhere. The Express is notoriously unstable so my bet is that the amplifier doesn't like the speaker/cable/connector combination you are using.

[Cliff Schecht]

Another thought is that with the 4" speaker, the frequency response isn't low enough to actually produce the hum you describe but the 8" speaker does. Try the amp with another speaker and see if the problem persists.

[reroe]

thanks everyone
i'll swap wires and try another speaker..