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Messages - LJ King


Two ohms is the correct minimum load for this amp. You might also want to remember for when you need it that the extension speaker jack puts the extension speaker impedance in series with the internal speaker impedance.

The real output impedance from a solid state amp is usually very low. The minimum load impedance is usually a "stated figure" to prevent the amp from destroying itself.
Amplifier Discussion / Re: Fender champ distortion issue.
February 06, 2008, 09:32:51 AM

New tubes,
New electrolytics (caps),
New speaker.
Amplifier Discussion / Re: marshall G50 R CD repair..?
January 18, 2008, 07:00:05 PM
Quote from: joecool85 on January 18, 2008, 06:07:40 PM
I knew it wasn't anything kinky, but I know google would think it is.  Thanks for the explaination though, I'd never heard of that.

You don't really need to wet the finger either. Try it on your input cable. (duh)
Amplifier Discussion / Re: Broken Knob! Serious issue!
November 26, 2007, 03:49:10 PM
Quote from: teemuk on November 26, 2007, 12:20:04 PM
Never mind the pictures.

There is an old saying: "one picture is worth a thousand words".

Amplifier Discussion / Re: Broken Knob! Serious issue!
November 26, 2007, 10:10:29 AM
You don't have permission to access /fender/f1.jpg on this server."

Same here.
Amplifier Discussion / Re: Peavey Musician 400 (1970's)
November 20, 2007, 04:45:39 PM

If there is no feedback with the reverb control at zero then it can't be feeding back through the reverb springs - there is no other path through to the preamp output but the reverb control.

There were (at least) two models of 400 Musicians during the 1970s. Does your amp have a master volume and reverb control... along with both series and parallel input jacks?

The Newcomer's Forum / Re: A general power supply?
November 02, 2007, 04:55:28 PM

Why would anyone use a switching power supply for a guitar amp?

With a single power supply there is really no choice but to have a common ground. Think of ground as the return path for currents back to the power supply (current always needs a complete circuit path, otherwise it don't flow).
Preamps and Effects / Re: Motor Boating?
October 30, 2007, 11:32:31 AM
Motorboating is almost always caused by a low frequency feedback path through the power supply.

Increase the value of the power supply decoupling caps, or replacing old ones that have lost some capacitance usually corrects the problem.


The simple cause of those symtoms is not having a ground connection at the 1A lug of the mid pot. Validate that it is actually grounded.
#0 - The first rule is that there are no rules.

#1 - Typically a power amp wants to see a line level signal to be driven to full output. This is usually a one volt level. What you are calling resistance is actually impedance, even though both are measured in ohms. A lower output impedance from the preamp with a higher input impedance for the power amp insures the transfer of maximum voltage.

#2 - refer to #0. There are examples of both. Generally, when cost is a primary issue, both will be powered from the same power supply.

#3 - Don't have a clue.

#(4) - Yes, lower voltages can be derived from a higher voltage supply. A stage of RC filtering, or zener, or a voltage regulator can reduce the higher voltage to a lower supply voltage.
Schematics and Layouts / Re: Baldwin Professional
October 22, 2007, 05:48:46 PM

Yes, I have seen the light bulb in series with the speaker in later models. Doesn't make much sense to me as the back is enclosed making a short highly unlikely... But we can always assume the action of a cosmic ray from a galaxy a zillion light years away as being the cause of a possible short!!

Interesting about the phantom supply, as the voltage available would seem to be quite high. Maybe the Baldwin/Burns guitar that had a preamp capable of using this voltage would also have some means of further filtering... and voltage reduction.

At any rate, hope this helps!
Schematics and Layouts / Re: Baldwin Professional
October 20, 2007, 07:21:57 PM

I must have skimmed past the tremolo modulation stage.

The 6.8K feedback resistor was changed to 3.3K as part of the gain modification.

The 100K and 470K resistors are actually capacitors - values are 100pf and 470pf. The emitter-collector cap is correct at 470pf. The oscillator and modulator driver stages are correct.

Backing up to the channel 1 tone controls, the 82K and 150nf should be 8.2K and .015 micro farad.

I remember the NPNs are silicon. I only replaced a few, and I'm thinking I used 2N3391 or 2N3391A - it's been a while.

The "massive heatsink" is simply sheet metal, aluminum IIRC approximately 1/8 inch thick - I'm guessing as I never measured it, maybe slightly thinner. The early versions it was flat and in later versions it got a right angle bend at the bottom of it, pointing towards the back of the amp and the TO-3's were mounted on the bottom horizontally with the transistor case facing downward.

Power supply is conventional - center tapped transformer, full wave rectification, dual +/- 43 volts at the first electrolytics. No regulation, just RC filtering to provide the other voltages.

There is one strange aspect though. The positive side has a separate leg of filtering that isn't directly used in the amplifier, separate from the filtering that provides the +32 volts (390 ohm/500 microfarad). After a 150 ohm 5 watt to a 500 microfarad electrolytic paralleled with a 3.3K 1 watt... it's split into TWO resistive voltage divider sections, looks to be a 700 ohm 5 watt (print is bad) that is grounded with a 2.2K 1 watt.

Here is the strange part - the voltage at the junction of the voltage dividers are simply designated "Y" and "Z". These sources are connected to what would be a "ring" connection on the "high" input jack of each channel with a .01 microfarad cap to ground. ("Y" to channel 2, "Z" to channel 1).

It could be a phantom supply, but I've never heard it advertised as such. I'm thinking it's purpose is to load down or reduce the +43 supply voltage (and the +32 which is derived) when the high gain inputs are in use... since it would be grounded when a normal T/S input plug is used.

That seems kind of contradictory... as that would seem to me to make the high gain distort earlier, but that's what the schematic shows.

Here is the Supersound portion of the schematic. The 68 micro input to the bass "format" is really a .68 micro - I missed that before I think.


What solder paste are you talking about?


Flux is essentially a cleaning agent. I've never thought about it being electrically conductive. Can't answer that.

With rosin core solder I've never had to use any extra flux for any electrical connection.
Quote from: joecool85 on October 20, 2007, 07:54:14 AM
Sounds like you put the caps in backwards.


And very lucky they didn't explode all over you.