And I'm ready to put forth another conclusion:

If I measure xxx volts on one end of a resistor.....and ZERO volts on the other end

Guess what


 

It's time to go ISO

 

What it does mean is:

The circuit is open.

 

Today I learned:

Just because a resistor has no voltage drop across it and measures the same voltage at each end with respect to ground

Doesn't mean the resistor is shorted

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Quote from: joecool85 on April 11, 2024, 03:53:39 PMI recommend making sure that the input jack is clean and connected well to both line and ground.  If not, it could be only letting some signal through causing these issues.

I will, thanks.

Hi. Did you manage to make much of an improvement to the amp? I've just picked one up for £20 on marketplace. I'm looking to tinker a little too!

Quote from: saturated on April 09, 2024, 05:20:43 AMThank you sir I am glad I did not have to touch any power tubes...in the back of my mind I had heard about tube biasing and was dreading having to do something like that.  But I looked it up and evidently preamp tubes are just plug and play...?

The two 12ax7 I switched positions were in there really tight.  To avoid broken glass and blood I put on a leather work glove.

I really appreciate everyone's help with this idk what we did but it's sounding really crazy nice it still has a little hum at low volume but I ain't touching it right now.

A little hum at low volume may well be normal for this amp.  It certainly is for others.

I recommend making sure that the input jack is clean and connected well to both line and ground.  If not, it could be only letting some signal through causing these issues.

Quote from: g1 on April 11, 2024, 12:24:42 PM

Quote from: RookieRecurve on April 11, 2024, 10:09:19 AMMy main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues? 

It can cause issues with ground loops or worse.  Some amps require their output (-) terminal to be isolated from ground for things like current feedback schemes.
If you want it to be safe as possible, use a Cliff S2 style jack, which will be insulated from chassis, and also will allow switching of both the positive and negative wires.

Thank you!  This is what I had suspected, but your answer confirms this.  Much appreciated.

Quote from: RookieRecurve on April 11, 2024, 10:09:19 AMMy main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues? 

It can cause issues with ground loops or worse.  Some amps require their output (-) terminal to be isolated from ground for things like current feedback schemes.
If you want it to be safe as possible, use a Cliff S2 style jack, which will be insulated from chassis, and also will allow switching of both the positive and negative wires.

Quote from: Tassieviking on April 10, 2024, 03:33:55 PMThe safes way to do this is to add a socket behind the speaker that the speaker goes to directly.
Also a speaker out socket on the back of the amp chassis, then you need a short speaker jumper cable with a plug on both sides to attach the combo amp to the combo speaker.
Edit: I have done this to some of my combo amps as it turns them into a separate head and separate speaker cab.
Sometimes I want to play the Marshall through the Fender speaker or the Sunn speaker, I just un-plug and plug into another combo for the sound I want.

Thanks for the reply!  Yes, I have considered doing what you have done, and adding two jacks with a short jumper.  To me, this makes things more simple.  The reason I am considering an alternative approach is that it is even more idiot-proof if it works in the same way that a headphone jack does, where plugging in disconnects one, and connects another speaker.  My main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues?  I am comfortable that I would not have both heads on at the same time, but if I ever sell this, or someone else is using this amp, that it is safe for them to use it as well.  I am pretty sure the 13A has a common ground terminal.  I have seen other jacks referred to as 'cliff' jacks that I am unsure about if the ground is common or not?  I have also seen 12A 'tip shunt' style jacks.  I might just order up a few different style of jacks, and see if any of them fit my intent of switching the hot and the ground when a plug is inserted.  My other question still remains though of whether or not a ground can be shared between amps.