Showman tremolo on without footswitch

[WholeToneMusic]

The amp is a 6G14A model.
The tremolo is active even when the footswitch is not plugged in.
When a shorted rca plug is inserted the tremolo goes off. I checked the - bias supply both before and after the plug was inserted and it remains the same. From what I understand about the trem circuit two resistors go to the - bias supply. The negative voltage keeps the tremolo tube diconnected from the circuit. The footswitch grounds out the - voltage to activate the tremolo tube.
Anyone have any ideas to fix it?

[52 Bill]

The amp is a 6G14A model.
The tremolo is active even when the footswitch is not plugged in.
When a shorted rca plug is inserted the tremolo goes off. I checked the - bias supply both before and after the plug was inserted and it remains the same. From what I understand about the trem circuit two resistors go to the - bias supply. The negative voltage keeps the tremolo tube diconnected from the circuit. The footswitch grounds out the - voltage to activate the tremolo tube.
Anyone have any ideas to fix it?

What's wrong with the amp?

Looking at the schematic, the trem is supposed to work without the footswitch on this model. The negative supply voltage bias version was used in the later AB763 series.

[WholeToneMusic]

Thanks Bill, I knew that Fender used a few different designs for the tremolo, I thought all the switching functions for the tremolo functioned the same way.

Best Regards,
Brian

[Enzo]

Nope, that is why each one has its own schematic.

[WholeToneMusic]

I will compare the two schematics, the 6g14a with the ab763 to see the difference is between the two circuits.

[Dave Curtis, dB AudioTech]

Thanks Bill, I knew that Fender used a few different designs for the tremolo, I thought all the switching functions for the tremolo functioned the same way.

Best Regards,
Brian

From your website: "Our tech has 23 years of electronics experience." Is he not there anymore?

Also from your site:

"The sound system can be a whole other can of worms; if sound systems are not properly grounded they can have ground loops, which raises the floor level noise. The star grounding system is an effective way to minimize hum and crosstalk. The star grounding system is having all grounds lead to a common ground. Experienced amp builders and professional touring sound engineers know the importance of the star grounding system and use it faithfully, Sometimes venues do not implement star grounding and you can get a lot of floor level noise from various sources, because every piece of equipment that is grounded causes a signal path which in effect is a radio transmitter. (!?) The best way to achieve star grounding in your rig or a sound system is to have 1 piece of equipment grounded and lift the ground from every other piece of gear connected to your rig that is causing excessive noise, Lifting a ground should only be done to reduce noise that is excessive and can cause safety issues. Never lift a ground on equipment connected directly to a microphone; this can cause the singer to become the grounding point and cause serious injuries. Some amps and music equipment have a ground lift switch, if it doesn’t you can always buy the ground lift plugs at radio shack that you use to plug in your outdoor Christmas lights. I always keep a couple of them in my gig bag. Another technique to minimize grounding problems is to plug all the equipment into the same AC leg; this puts all the equipment at the same potential"


This is potentially very, VERY dangerous advise. NEVER defeat the safety (equipment) ground on any equipment. You've gotten it mixed up with lifting the SIGNAL ground, which is one accepted method of dealing with ground loop problems, and is what the "ground lift switch" does that you mention; it lifts the signal ground. Star grounding for equipment is *rarely* used at venues (if ever), and is really only beneficial for pieces of equipment that are quite some distance apart (works very well *inside* an amp). It is more often seen in studios.

See here:
http://www.epanorama.net/documents/g...m_solving.html

There's a lot of other good stuff there, too.

-Dave

[WholeToneMusic]

on the topic of lifting a ground it says-

Lifting a ground should only be done to reduce noise that is excessive and can cause safety issues. Never lift a ground on equipment connected directly to a microphone; this can cause the singer to become the grounding point and cause serious injuries.

Only a suggestion to reduce excessive noise. I do understand that earth ground is required by electrical codes, and can be hazardous when lifted, However it can cause ground loops when there is more than 1 path to ground. I have known a lot of musicians who have done this, and I only recommend it as a last alternative to playing in a club that has ground loops. Thanks for the link. I will take your first comment as a compliment because I do not claim to know everything. Yes I am still here, Learning something new every day. I have been studying and working on Electronic circuits for 24 years. You insinuate that I am not the brightest LED in the bin, but it's not for you to judge me, that is God's job. I enjoy coming to the forum to talk to techs and ask questions to techs who have more experience. I do appreciate all of your comments. This forum has been very helpful.

General Question From the Sound reinforcement handbook (pg. 332)- If you have 1 audio cable joining a console to a power amplifier can you have a ground loop? Yes! A ground connection through the AC cables and the chassis of the two units make the second connection. This, along with audio cable shield, constitutes a continuous ground loop in which noise currents can flow. One way to break this ground loop is to lift the AC ground on one piece of equipment, typically the power amplifier, with a two wire to three wire AC adapter. Leaving the loose green wire on the adapter unconnected breaks the ground loop, but also removes the AC safety ground. The system now relies upon the audio cable to provide the ground, a practice that can be hazardous.

Why would they suggest that the power amplifier ground be lifted if they state it can be hazardous? Because it is a means to eliminating a gound loop.

There are still alot of Vintage amplifier owners who use the two- prong ac cable. Is it dangerous? Yes, Do they still use it? Yes.
One more question Dave, If your trying to help me out then why did you hijack this posting and publically ridicule me on this forum? Couldn't you have emailed me?

[Dave Curtis, dB AudioTech]

Lift the signal ground, not the chassis ground. I don't care what handbook it came from; defeating the equipment ground can kill someone. So you leave the PA grounded and lift the chassis grounds on everything else. I'm playing guitar and reach up to adjust my mic (I sing, too). I get such a shock that both hands clamp up; one on my guitar neck, and the other on the mic. That makes the contact better so even more current flows from one hand to the other; right through my chest. Lifting the chassis ground is not an option in my book, because I can't hear how wonderfully hum-free the music is if I'm dead. There are much safer ways to eliminate ground loops.

"If your trying to help me out then why did you hijack this posting and publically ridicule me on this forum?"

Not ridicule, and I didn't intend for it to come across that way; I apologize.
You included the link to your website and I clicked on it (as others do, I'm sure). I saw what I consider to be bad advise. You might consider taking it down. I know I wouldn't want an accident like that on my consience; I am trying to help.

[WholeToneMusic]

Good morning Dave

I will revise the article and appreciate your concern. Please feel free to email me anytime you have a discrepency with anything that does not look right. I appreciate the free proof reading/editing counsel.