Help with PA Conversion

[Jeru]

I'm converting a Muzak PA amp, basically just redoing the power supply and adding a dual triode 12ax7 preamp and vol/tone controls ahead of the 7199 -> 6V6PP power amp. It's a cool looking PA - pics are here.

I haven't been able to figure out the purpose of the 400ohm 20-wall resistor is between the PT's center tap and ground? If it's too small to read, The schematic says "Connect Jumper for 8 Watts" next to a dotted line in parallel with the resistor. I've searched a number of forums and can't figure out what it's doing there.

Schematic is below. Thanks.

[IMG][/IMG]

[Enzo]

It is reducing the B+ voltage available to the circuit. If I had to guess 8W was the higher power option so the amp could drive more speakers. With the resistor in the circuit, maybe it produces say 4 or 5 watts.

[Chuck H]

It looks to me like two voltages are specified on the HV rail. I'm not sure what the purpose of the resistor is but I can tell you what it does. A resistor in the CT like that drops voltage and adds sag. Just bypass it. You already have a rectifier tube that will sag some and 355V (the voltage when the resistor is bypassed) is a more typical guitar amp voltage.

It looks like you already have a 12ax7 in the amp that could be rewired in series ahead of the phase inverter for plenty of gain.

[Jeru]

Does bypassing it with a jumper as indicated on the schematic have
a different effect than simply removing it..? In the course of researching
and learning to complete several builds, I haven't noticed anyone placing
resistance between the PT-CT and ground this way..

Thanks.

[eschertron]

Does bypassing it with a jumper as indicated on the schematic have
a different effect than simply removing it..? In the course of researching
and learning to complete several builds, I haven't noticed anyone placing
resistance between the PT-CT and ground this way..

Thanks.

If by removing it, you mean replacing it with a straight conductor, then it works the same as the jumper, and the resistor has no effect. If you mean just cut it out of the circuit and leave the CT disconnected, then the difference will be that the amp won't work like that

edit: ...and yes, I've never seen a resistor used like that either. There are some other 'higher-tech' methods of power reduction I've seen on the forum, but not this.

[Jeru]

Of course -- I meant removing the resistor and securely grounding the PT's HV
center tap. I actually don't like where/how the builders grounded it anyway..
Thanks.

[Jeru]

edit: ...and yes, I've never seen a resistor used like that either. There are some other 'higher-tech' methods of power reduction I've seen on the forum, but not this.

HA - I've been looking into that too, to do with a different build/project. A topic for another thread, to be sure.

[eschertron]

Of course -- I meant removing the resistor and securely grounding the PT's HV
center tap. I actually don't like where/how the builders grounded it anyway..
Thanks.

The prevailing wisdom I've seen on this forum suggests the best place to land the CT is right on the negative leg of the reservoir cap (I think C13A on the drawing?) to minimize loop currents. Not sure if your layout allows that.

[Jeru]

Huh - will look into that too. I'd always thought that one of the PT bolts was the best
place for it. I do get a whole lot out of being wrong, though -- it's how I learn.
Thanks.

[eschertron]

Huh - will look into that too. I'd always thought that one of the PT bolts was the best
place for it. I do get a whole lot out of being wrong, though -- it's how I learn.
Thanks.

Well, certainly don't take my word for it. I'm just repeating what I read on the internet

But seriously - there are people here (with the engineering chops I lack) that have shared their ideas and the reasoning behind them. When the reasoning seems solid, I tend to accept the ideas presented. You'll discover who the reasoned thinkers are here, fairly quickly I wager.

[Dave H]

Huh - will look into that too. I'd always thought that one of the PT bolts was the best
place for it.

Don't connect the PT CT to chassis. It's better to connect it directly to the reservoir cap -ve as stated above. The rectifier current pulses travel between CT and cap negative and it's best not to have that current running through the chassis or other ground connections. There should be only one wire from PT CT to cap negative with no other connections to the CT.

[Jeru]

Thank you all for your time and assistance -- this is why I absolutely love places like this.

[Enzo]

Presumably the resistor is grounded where they wanted it to be grounded. Placing a jumper across the resistor wouldn't change where it was grounded. This is an amp intended for background music use, and not designed with high gain in mind. Make your conversion first, then worry about moving grounds around later.

[Dave H]

Make your conversion first, then worry about moving grounds around later.

Yes, it's not a new build so no need to fix it until you have broken it

[Jeru]

Yes, don't try and fix it until you have broken it

Not to worry -- I have every confidence I'll do both.