Two Amps in Parallel Produces Squealing

[bclick]

I am trying to run two amps in parallel but get a loud squealing from the 18watt when I turn up very loud. One amp is an 18 watt Ceriatone/Marshall TMB-EF86 and the other is a 1971 Fender Super Six (basically a Twin).

My desired end result is: wireless output into pedal board, 2 outputs from Boss DD-5 into an Ernie Ball stereo volume pedal. One vol pedal output to the Fender, the other vol pedal output to a StarTouch A/B+Y pedal that feeds both channels on the 18watt head.

For troubleshooting, I have used ONLY the volume pedal into the Fender & 18watt. Also tried the A/B+Y pedal alone. Same squealing. Sounds like going into oscillation. Happens on both channels of the 18watt.

I vaguely remember reading that you could not daisy-chain the channels on a Fender, but not sure that would be applicable here.

Any ideas appreciated!

[David Root]

Oscillation is caused by uncontrolled positive feedback. Something is making the Ceriatone 18 watt unhappy but the Fender is not affected.

You could try a 100pF 500V mica or ceramic cap across the phase inverter plates, pins 1 and 6. Somehow I feel the problem is deeper than that.

Also, just a thought but are you powering both amps off the same wall outlet? That can cause ground loops in some equipment, especially those with old two pin plugs. Not that ground loops are your problem, but that could be a sign of other instability.

[bclick]

Thanks for the ideas David. Tonight I will try a ground lift on one of the amp power cords to eliminate a ground loop. I also have another amp (David Hunter Two Stroke) to try in place of the Fender.

[NickC]

I am always careful to have the amp and pedal power supply (regulated and isolated DC feeds to each pedal) on the same AC outlet. I never lift a ground on anything. Ever.

However, I have had problems at venues with dirty AC, lots of neon signs and florescent lighting injecting hum and noise.

I've also had ground loop trouble caused by different gear (FOH vs guitar rig with line-out feed going to board) being on different circuits (different ground potentials). I have had good success using Ebtech Hum Eliminators to break ground loops in those situations.

0.02 HTH

[bclick]

Thanks Nick. These are on the same A/C outlet, with no connection to mixer or anything else.

I was under the impression that pedalboards using a 9VDC power supply were isolated through the transormer in the power supply.

I will look into the Ebtech product.

[vibratoking]

I am always careful to have the amp and pedal power supply (regulated and isolated DC feeds to each pedal) on the same AC outlet. I never lift a ground on anything. Ever.

I respectfully disagree. There are many instances where using the same AC outlet will still result in ground loops and induced hum. I have experienced many situations where lifting ground is the ONLY practical solution. One could modify each unit to install chassis ground lifts or disconnect shielding at key points, but that is not practical or desirable in many situations. This is especially true with AC powered multiple effect setups and lot's of shielded cable. The fact that you are connecting to the same ground source does not preclude ground loops in any way whatsoever.

BTW, I have never seen a multi amp setup that caused a proper functioning amp to oscillate. I would first be looking elsewhere for the problem.

[NickC]

................ I was under the impression that pedalboards using a 9VDC power supply were isolated through the transormer in the power supply.....

Those typical pedal power supply daisy-chains can wreak havoc (not always). When I migrated to the Voodoo Lab Pedal Power (I have no affiliation with them, just a satisfied customer) those problems went away.

Good luck finding, and eliminating, the hum gremlins. Seems I'm constantly chasing them in the studio, but the live rig is hum-free now (knock on wood).

[crbowman]

Have you tried the 18 watt by itself just guitar into amp and cranked it?
I would try that, then amp + vol pedal, then both amps + vol pedal and see at what point the oscillation occurs.
Just wondering if the problem is simply in the amp itself and not a result of other variables.

[bclick]

Yes, the 18 runs fine by itself with full pedals, etc.

More testing this evening.

[NickC]

I am always careful to have the amp and pedal power supply (regulated and isolated DC feeds to each pedal) on the same AC outlet. I never lift a ground on anything. Ever.

I respectfully disagree. There are many instances where using the same AC outlet will still result in ground loops and induced hum. I have experienced many situations where lifting ground is the ONLY practical solution. One could modify each unit to install chassis ground lifts or disconnect shielding at key points, but that is not practical or desirable in many situations. This is especially true with AC powered multiple effect setups and lot's of shielded cable. The fact that you are connecting to the same ground source does not preclude ground loops in any way whatsoever.

BTW, I have never seen a multi amp setup that caused a proper functioning amp to oscillate. I would first be looking elsewhere for the problem.

Yes, all that is true enough. Still, I try reducing the number of variables when trying to solve a complex problem. Using a single AC outlet to power everything is better, from a problem solving standpoint, than using multiple AC outlets. As you say, that isn't a guarantee of eliminating it. My ensemble of gear works without needing to lift grounds, and is hum-free. The hum didn't go away by accident. It was a long painful process.

[bclick]

I vaguely remember reading that you could not daisy-chain the channels on a Fender, but not sure that would be applicable here.

After re-reading this I think you cannot daisy-chain the 2 channels on the Fender because they are out of phase. I have only tried the "vibrato" channel on the Fender - not the "normal" channel. Not sure if that would make a difference or not, if they are not completely isolated.

[Firestorm]

I vaguely remember reading that you could not daisy-chain the channels on a Fender, but not sure that would be applicable here.

After re-reading this I think you cannot daisy-chain the 2 channels on the Fender because they are out of phase. I have only tried the "vibrato" channel on the Fender - not the "normal" channel. Not sure if that would make a difference or not, if they are not completely isolated.

Right about that. Just for jollies, try using the normal input and see what happens. Amps used together sometimes have phasing issues that either cause frequency cancellation or reinforcement.

[bclick]

More test results: the 18 Watt does not play well with others. It squeals with any combination of the Fender or the Two Stroke.

The Fender and the Two Stroke are fine together.

[vibratoking]

Perhaps a diagram of your setup will help. Take things step by step and introduce the next component in the chain only after you verified there is not a problem with the existing components. When you use only the 18W with the ABY does the 18W sqeal either in A mode, B mode, or Y mode? If not, then add the volume pedal only. Tell us when the squealing starts and what component seems to be the cause.

[bclick]

AB/Y into 18 watt only is OK. I have used this for a year to select/combine channels.

AB/Y pedal only, into 18 and another amp = squeal. Either Fender or Two Stroke for the other amp.

AB/Y pedal only, into Fender and Two Stroke = ok.

Stereo volume pedal by itself yields similar results as AB/Y pedal.