The GUTZ

[Wavebourn]

Originally Posted by Halfmoon

Compressor

Nice!

Are 9-pin sockets current Chinese production?
Can you share design of PCBs for tube sockets?

[Halfmoon]

Originally Posted by Wavebourn

Nice!

Are 9-pin sockets current Chinese production?
Can you share design of PCBs for tube sockets?

I would if I could but someone else made it. Small quantity runs so I guess this would qualify as a boutique compressor.

[pan60]

bump

[A27Hull]

I recently did a head transfer/cabinet swap.

Here's a view of original-versus-reissue shots of the Fender Bassman.

The only "new" components in the classic 54-ish Bassman are two electrolytics. You can see the reissues' PCB and such.

Both the owner and I think the original Bassman sounds better than the reissue.

I put the original head unit into the reissues cabinet, because the old cab is literally falling apart, and had some major resonance/vibration problems. Even with the new cab and reissue Jensens, the old head still sounds better.

[A27Hull]

Up close:

[nosebleedaudio]

Originally Posted by A27Hull

Up close:

I would put a grounded AC cord on the old one to keep someone from getting the crap shocked out of them...

[A27Hull]

I'll pass that on, would it require a great deal of modification?

You know the owner, btw.

[nosebleedaudio]

No mods at all, the only problem for the guy with no or minimum tools maybe the problem of the new cable requiring a grommet and strain relief. I never reuse the old plastic grommet
Performed this upgrade this past weekend on an old Ampeg head.
The ground wire connects to the chassis, the black/hot connects to the switch/fuse leg, white/neutral connects to the power transformer.
If it contains the ground switch like the Ampeg did you disconnect it completely..
Leave the switch in the hole or cover it, your choice.

[A27Hull]

Thanks Mike. The ground switch is there, and the plug is 2 prong. The consequence to switching to 3 prong is the elimination of the ground lift switch then?

What would opening the neutral wire do in the 3 prong scheme? Put the 120 across that open? If the neutral wire is already connected to the chassis, would I need to find it and disconnect it also to implement the 3 prong scheme?

I ask because we've been fond of using the ground switch to eliminate/reduce hum with guitars.

I'm sure my ignorance is showing on this one, but I'm kinda playing on-site studio tech.

I've read that in three prong modern systems, the neutral wire is connected to earth back at the main breaker terminal, so I guess that a neutral short to chassis, the neutral being shorted to earth ground at the main breaker, would create a ground loop between the neutral/chassis and ground, and possibly in the operator touching the chassis and standing on earth ground.

[nosebleedaudio]

It is not a ground lift in the real use of the word.
Lifting the AC ground on a tube amp is a very bad idea, you lift the audio ground different ways.
I would always prefer the AC ground to be connected if possible...
But it is NOT allowed by NEC to use the neutral as a ground, yes it is connected at the main panel at one point and ONLY at one point, either at the meter panel or the main disconnect, but not at both and no where else.
This creates a parallel neutral current path, a bad idea.
NEC 250.

[A27Hull]

Thanks Mike,

It appears to be the amp circuit ground. I could be confusing the amp and audio ground though.

The Ground switch (left most) seems to open the neutral wire running from cable into the main xfrmer, but seeing as I'm looking at it through a picture, I'd need to verify it up close.

I'll check up on NEC 250. Could it be that those specs weren't adhered to in the 50's?

Perhaps the switch is wired parallel with the circuit, which would explain why switching it to off or open would cancel ground circulations between the switch and the circuit. The only problem is that I don't know where the currents would then go, providing they exist.

[zmix]

In principle, the "Ground " switch on a guitar amp switches a very small value capacitor ( connected at one side to the chassis ) between the "hot" and "neutral" leads of the A.C. line. There is usually a center-off position which disconnects the capacitor entirely.

When replacing a two-wire A.C. cable with a three wire, the green (earth) conductor is always connected to the chassis.

[A27Hull]

Originally Posted by zmix

The "Ground " switch on a guitar amp switches a very small value capacitor ( connected at one side to the chassis ) between the "hot" and "neutral" leads of the A.C. line. There is usually a center-off position which disconnects the capacitor entirely.

When replacing a two-wire A.C. cable with a three wire, the green (earth) conductor is always connected to the chassis.

Thanks Zmix,

I agree with you and Mike about the green going to chassis. Thanks for the info on the cap. So the cap should switch between hot and neutral to ground.

I'm just trying to learn a little about vintage equipment, and how to correct their ancient power scheme.

[A27Hull]

Well I found a better schematic; it may not be the exact model, but its close.

You are exactly right Zmix, it has a cap switch across the hot and neutral.

Do the grounds shown refer to a chassis ground or just a reference to the circuit ground?

[zmix]

Originally Posted by A27Hull

Well I found a better schematic; it may not be the exact model, but its close.

You are exactly right Zmix, it has a cap switch across the hot and neutral.

Do the grounds shown refer to a chassis ground or just a reference to the circuit ground?

Both, the circuit ground will be tied to the chassis, usually through the input jacks.

The A.C. leakage through the cap on the ground switch is what gives you a nasty shock when the ground switch is in the 'wrong' position and you touch another amp or a microphone...

[A27Hull]

Thanks Zmix.

I guess everybody working with this amp has been lucky to this point.

Granted its still a 2 prong device.

Is it, electronically speaking, that using the switch with the small value cap in the modern situation (3 prong) would create a short of 120 to the chassis, via a bypass of the intended circuit, at a different impedance of that that would normally exist between circuit ground and chassis?

[nosebleedaudio]

Originally Posted by A27Hull

Thanks Zmix.

I guess everybody working with this amp has been lucky to this point.

Granted its still a 2 prong device.

Is it, electronically speaking, that using the switch with the small value cap in the modern situation (3 prong) would create a short of 120 to the chassis, via a bypass of the intended circuit, at a different impedance of that that would normally exist between circuit ground and chassis?

Yes you are correct with respect to the 60's grounding; they did not exist back then.
I did not mention the cap being switched because I would NEVER use them with a properly grounded cable/chassis.
One more note about the cap being switch; it can be a very dangerous thing, have seen those caps shorted and then you have a full 120V on the chassis.
The caps can and do leak current into the chassis, and if not bleed off via a real ground you can have 120V on the chassis.

Today with NEW unbalanced 2 prong gear you can simply reverse the AC plug and can change the ground noise, if that is a problem.
It changes the AC phase going to the power transformer which sometimes helps with hum/noise.
The internal grounding is usually floating, since it has no REAL AC ground point, and when the unbalanced signal is plug in to something that IS grounded is uses THAT for its ground also.
Hope this makes sense

[zmix]

Originally Posted by nosebleedaudio

Yes you are correct with respect to the 60's grounding; they did not exist back then.
I did not mention the cap being switched because I would NEVER use them with a properly grounded cable/chassis.
One more note about the cap being switch; it can be a very dangerous thing, have seen those caps shorted and then you have a full 120V on the chassis.
The caps can and do leak current into the chassis, and if not bleed off via a real ground you can have 120V on the chassis.

Today with NEW unbalanced 2 prong gear you can simply reverse the AC plug and can change the ground noise, if that is a problem.
It changes the AC phase going to the power transformer which sometimes helps with hum/noise.
The internal grounding is usually floating, since it has no REAL AC ground point, and when the unbalanced signal is plug in to something that IS grounded is uses THAT for its ground also.
Hope this makes sense

Let's deduce what we are actually dealing with.

a 0.05µf cap has a capacitive reactance ( i.e. impedance) of 53K at 60 cps..

This cap will flow 2.2milliamps of current. Not enough to hurt you but will give you that instant reminder that the 'ground' switch is in the wrong position!

You can still use this when employing a 3 wire plug, as it *willl* reduce RF and other A.C. induced noises....

[nosebleedaudio]

Originally Posted by zmix

Let's deduce what we are actually dealing with.

a 0.05µf cap has a capacitive reactance ( i.e. impedance) of 53K at 60 cps..

This cap will flow 2.2milliamps of current. Not enough to hurt you but will give you that instant reminder that the 'ground' switch is in the wrong position!

You can still use this when employing a 3 wire plug, as it *willl* reduce RF and other A.C. induced noises....

Will have very little effect on RF on the primary.
I would still not use it, but thats just me...

[A27Hull]

Thanks to you both Zmix and Mike,

Its beginning to make sense. I'm a beginning electronics tech with a nasty habit of asking "why" all the time. I guess thats why I'm doing engineering as well. I have a lifetimes worth of knowledge to learn still.

I made a block diagram to simplify the situation. It would seem that in a three prong system, shorting either neutral or hot to the chassis by any means (whether circuit ground or fault) would create a current loop between the earth ground and that neutral or hot wire. Anything touching the chassis would become a impedance mismatch to earth, causing a voltage drop across that object and a current flow of some amount. In a sense the object creates an additional parallel path to earth.

If this is true, why connect the circuit ground to chassis/earth at all?

[nosebleedaudio]

Originally Posted by A27Hull

Thanks to you both Zmix and Mike,

Its beginning to make sense. I'm a beginning electronics tech with a nasty habit of asking "why" all the time. I guess thats why I'm doing engineering as well. I have a lifetimes worth of knowledge to learn still.

I made a block diagram to simplify the situation. It would seem that in a three prong system, shorting either neutral or hot to the chassis by any means (whether circuit ground or fault) would create a current loop between the earth ground and that neutral or hot wire. Anything touching the chassis would become a impedance mismatch to earth, causing a voltage drop across that object and a current flow of some amount. In a sense the object creates an additional parallel path to earth.

If this is true, why connect the circuit ground to chassis/earth at all?

To eliminate a shock hazard, for one thing...
Do this: with an amp that is NOT grounded and with a VOM measure on AC scale between the chassis of the amp an a KNOWN AC ground point, an amp that is grounded for example and see what you get...with the ungrounded amp ON the other makes no difference.
I have measured over 150 volts on an amp chassis before.
Also the noise, stray currents ect. want and need to go to the source ground point to be dissipated or lowered to a very low point.
Noticed I said the SOURCE ground point, a separate and unconnected ground rod will NOT be the same.
Most gear I have seen that was properly designed was cleaner and quieter when grounded.
A growing problem in clubs ect is harmonic distortion on the AC circuits that the band uses for their amps ect, and if it's bad enough can truly destroy audio gear.
A GOOD, dedicated home run circuit is the best, and IM refering to all 3 wires...
The problem is most clubs will NOT provide this.

[A27Hull]

I went back and checked the amp/room. The Ground switch on the amp has only two positions.

Room circuit, Big slot=neutral, Small Slot=hot, Grnd=Grnd.

In the room circuit, from hot to ground = 120, from hot to neutral = 120, and from neutral to ground = 0.

With the amp in Standby Mode, with Ground (what I consider to be the "off"state) switch off: ground to chassis = approx. 0.

With the amp in Standby Mode, with Ground switch "on": ground to chassis = 120.

With the amp in "Run" Mode, with Ground switch "off": ground to chassis = 46-47 VAC

With the amp in "Run" Mode, with Ground switch "on": ground to chassis = 120 VAC

In short, what you initially told me was correct. Their does exist a shock potential if something is touching both chassis and something grounded. Good thing to know.

Thanks to you both,

Andrew.

[Wavebourn]

My new baby.

One part of my Festival gear: a powered phased array. There are 2 of such chests, plus a horn loaded woofer.
Hybrid amp, 120W RMS nominal, about 1 KW peak, from 300 Hz to the sky. 130 dB max SPL

[nosebleedaudio]

Originally Posted by Wavebourn

My new baby.

One part of my Festival gear: a powered phased array. There are 2 of such chests, plus a horn loaded woofer.
Hybrid amp, 120W RMS nominal, about 1 KW peak, from 300 Hz to the sky. 130 dB max SPL

I think you need MORE speakers... Glad it's not my job to move them from gig to gig..

[Wavebourn]

Originally Posted by nosebleedaudio

I think you need MORE speakers... Glad it's not my job to move them from gig to gig..

I can easily fit them in my Nissan Armada (2 columns and a bass horn + console +cables + other gear). Ordinary speakers from Guitar Center capable to give similar sound pressure (but never sound quality) will need a big truck to carry them.
That speakers you see on the picture are 4.5 inch only.

Edit: Oops... :(
They did not fit: when I designed them I did not count length of legs/wheels...

[tINY]

A recent swap I had to do on a new product I was borrowing/reviewing.

Quite a study in compact amplifier design...




-tINY

[tINY]

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[tINY]

-tINY

[tINY]

This thing is still screwy...



-tINY

[tINY]

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