Question about "Grid Stoppers"

[R.G.]

One thing that is confusing me is why did they use a TCR5305 (constant current diode) off of the cathodes on V3?

It's a biasing element for the PI - which is actually a differential amplifier.

A diffamp works best with a true current source as an emitter biasing element, and a constant current diode is an easy way to do that. I'd have to delve through the circuit to get an idea about how well this one is likely to work. But it's probably OK if it's not dead yet.

[davidjon_99]

It's a biasing element for the PI - which is actually a differential amplifier.

A diffamp works best with a true current source as an emitter biasing element, and a constant current diode is an easy way to do that. I'd have to delve through the circuit to get an idea about how well this one is likely to work. But it's probably OK if it's not dead yet.

Thanks RG. Is there a way I can tell whether it is actually working or not? I've read that an "official" Mesa modification is to replace it with a 1k5 resistor.

[R.G.]

Is there a way I can tell whether it is actually working or not? I've read that an "official" Mesa modification is to replace it with a 1k5 resistor.

The TCR5305 is a 2ma diode that tolerates up to 85V across it. It's a TO-18 package and is obsolete. That's probably why Mesa says use a resistor.

The easy way to find out if it's working is to measure the voltage across it and the current through it, then compute. Voltage across it means it's doing something, and if the current is 2ma, it's working.

I would measure the voltage across it with the amp idling, then measure the voltages across both the plate resistors, compute to get both plate currents and add them. If the result is about 2ma, it's good.

A CCD gives better performance with a diffamp. The resistor will give a cathode impedance of 1.5K, the CCD 375K min. The higher the impedance, the better for a diffamp. You can make up a CCD composite device from other stuff to do the same but still get high impedances if you need to. Not as simple as a resistor, but better performance.

[davidjon_99]

Excellent info. Thanks RG!

One more question. Can the TCR5305 be replaced by this?
http://www.mouser.com/ProductDetail/Sem ... 6x3ETZ4%3d

[R.G.]

One more question. Can the TCR5305 be replaced by this?

I'd sure try it. It looks to be as good at what I ...think... the purpose is as the original. Even rated for more voltage.

It would be really good to know what voltage the original was supposed to operate at, which is a factor of the circuit, not the diode.

[Smitty]

The 68K/68K resistor networks at the input jacks of most amps are usually there for making a high-gain/lower-gain input from two jacks.

I thought it was a resistive mixer so the accordion player could plug in as well.

"Fred, I'll plug my guitar into 1 and you plug the electric accordion into 2. We'll use the channel with reverb for the mics. Hope were not too loud."

[sluckey]

I thought it was a resistive mixer so the accordion player could plug in as well.

Yes, that too.

The clever two input jack wiring as seen on most Fenders allowed a Hi or Lo input level when only one jack is used. But as soon as you plug in two instruments the 68K resistors simply become mixing resistors and both jacks will have the same input level.

Back in the early/mid '60s it was still common to see more than one instrument plugged into an amp.

[R.G.]

Back in the early/mid '60s it was still common to see more than one instrument plugged into an amp.

The Thomas Organ Beatle amps were perhaps the ultimate in this idea. They had two inputs each to three preamps, a brilliant, normal and bass, each of which had different effects available. I think it was designed as an amp for a one-amp band.

But it was LOUD.

[SilverFox]

What is the benefit in using the current limiting diode on the cathode? I would think that would lead to a sterile tone but obviously it does not.

silverfox.

[davidjon_99]

I'd sure try it. It looks to be as good at what I ...think... the purpose is as the original. Even rated for more voltage.

It would be really good to know what voltage the original was supposed to operate at, which is a factor of the circuit, not the diode.

I was looking through several pictures I had taken of the inside of the amp. The attached show that the leg of the TCR508 (which either Boogie or someone else put in place of the TCR5305 mentioned on the schematic) that is supposed to be connected to ground doesn't even exist. Seems someone, maybe Boogie, added a presence circuit much like the bassman 5F6 to this amp. The blue wire going to the .001uF cap comes off the wiper of the Master Volume and the white wire comes from a 68k resistor connected to the 8 ohm speaker jack. The bare wire comes from one of the lugs off the 5K ohm presence pot.

Not sure why they didn't just remove the thing from the socket.

EDIT: Added quick hand drawn schematic of presence circuit. Hopefully I got it right.

[R.G.]

What is the benefit in using the current limiting diode on the cathode? I would think that would lead to a sterile tone but obviously it does not.

Can you tell me why you think that would lead to a sterile tone?

I can't come up with any good reason why that should be so. Always looking to learn.

[SilverFox]

What I'm thinking is, constant current would lead to a stable amplification signal. A stable amplification signal would not have much harmonic content.

But I don't know so that's why I asked.

silverfox.

[teemuk]

If it's obsolete, its not trivial to engineer a replacement.

[img:328:318]http://sub.allaboutcircuits.com/images/05269.png[/img]

[Leo_Gnardo]

Our friends over at Audio Asylum have been discussing constant current devices for some time. You don't need to be a member to search & read. Hours of fun for those so inclined.

[R.G.]

What I'm thinking is, constant current would lead to a stable amplification signal. A stable amplification signal would not have much harmonic content.

But I don't know so that's why I asked.

OK. Now I understand your question.

That's probably true if the PI diffamp is the only, or the major amplification of the signal. I think that in most tube amps, every tube in it is contributing harmonic content, so you're not missing much.

A second version of the same train of thought is that in most amps there is overall feedback around the entire power amp. The effect of this feedback is to "hide" any nonlinearities of the inside of the feedback loop. Changing the effective gain of the PI in such a setup changes the degree of hiding a little, not a whole lot.