Peavey 450 Tour

[lowell]

I don't get it. Yes P19 has the zobel network and triac b/t itself and ground. But how does the power transistor signal get transferred to the zobel when it's shorted to ground?

[nickb]

I don't get it. Yes P19 has the zobel network and triac b/t itself and ground. But how does the power transistor signal get transferred to the zobel when it's shorted to ground?

Peavey used to do this quite a bit. The current flows through the load via the caps C30 and C31. Thus the collectors are really the outputs. The advantage of this is that the output stage has a little gain. It's odd until you get your head around it. There a name for it but I've forgotten.

[52 Bill]

There a name for it but I've forgotten.

Fender used to call it a Flying Rail design.

[Enzo]

I call that "flying rails" which I think I got from Crown when I serviced a lot of Crown amps with the same approach. I think grounded emitter might be another term.


The output of the amp does not come from the power transistor emitters, it comes from the CT of the power supply. Note the CT power supply is NOT grounded.

[lowell]

I was thinking that may be the case. So why then doesn't the AC signal on the collectors get shorted to ground thru the filter caps? In a tube amp there is a resistor (plate resistor) that separates the ac output from that tubes filter cap.

[g1]

The filter caps are not grounded, they go to the PT center tap (which is not grounded).
I only know this because I made a fool of myself discussing the same issue here :http://music-electronics-forum.com/t26258/

[nickb]

I was thinking that may be the case. So why then doesn't the AC signal on the collectors get shorted to ground thru the filter caps? In a tube amp there is a resistor (plate resistor) that separates the ac output from that tubes filter cap.

Here's a couple of drawings that hopefully make it clearer. All that's really happened as far as the output is concerned is the ground reference has moved.

[g1]

A couple older circuit examples using this type of design. My question would be whether the filter caps wear out faster than conventional type designs.

[Enzo]

No, they wear out the same, they are doing the same job. The signal doesn't come through the caps, it comes through the transformer winding. The +V and -V rails bounce up and down with the signal, and so then must the hot terminals of the caps. But so does the other end. If you ground a scope and watch those rails, you will see your signal. Now "ground" the scope to the CT of the transformer and probe the hot ends of the caps and see the same steady DC you;d see on a conventional amp.


Peavey used this arrangement on MANY MANY amps over the years, as do any number of other amp makers. This is really quite common.

The nickb drawing really says it all. The circuit is a loop, and it really doesn;t matter where you decide to connect it to ground.

[lowell]

Wild concept. The whole PS is wigglin up and down. Well since the triac is burning hot I assume there is DC on the output. I measured at speaker jack but nothing so I'm also guessing that the protection relay has switched off the output. I've yet to measure at the output inductor and will do so.

[g1]

If the triac is getting hot because it is doing it's job (shorting DC on output to ground), then I think you will need to remove the triac to see that DC on the output.

[lowell]

Good point.. If the triad is doing its job should it be burning with smoke? This makes me think something ain't right.

[Jazz P Bass]

The triac does nothing when the gate is low. (if it is in proper working order)
If the gate goes high, (easy enough to check) because of a perceived fault (Vdc on the output or a gate circuit fault) it turns on the triac.
If there is not any Vdc on the output and the amp is idling, the triac will do nothing. (even with a high gate)
It is when the triac is on and there is Vdc or a signal on the output that the triac lends a path to ground for the output.
That is when it will get hot.
In a severe enough output section failure the triac will give itself up & most probably go short.

[J M Fahey]

Not necessarily *smoke* buy you already said it was very hot.
And DC must be measured before the Relay (doing its job), disconnects the speaker jacks.

[flyingdutchman]

In a severe enough output section failure the triac will give itself up & most probably go short.

Hopefully blowing the rail fuse(s) or the mains fuse in the process.

Many older Peavey amps have such a triac "crowbar" circuit on the output. It is meant to protect the speakers from large DC voltages. Most other amps use a relay contact in series instead of a crowbar circuit, but the relay is, by some, perceived as less reliable as it may cause dropouts because of dirty or worn contacts.