My First Tube Amp: Check my work?

[Wigwam Jones]

Squid is just messin' with you! All Beam Power tubes or Power Pentodes have five elements (besides the heater), but the third "grid" is not always shown on all schematics. The third grid is called a Suppressor Grid in a Power Pentode, or a Beam Forming Element in a Beam Power tube, and is usually internally connected to the cathode, as is the case with this tube. Both elements act to repell electrons back to the plate, that if knocked off the plate in the process of bombardment, might otherwise be attracted to the screen grid rather than back to the plate if these elements did not exist. Without these elements, you have a Power Tetrode, which were known for much less power output capability (because of electron flow lost to the screen grid), and generally quirky screen grid behavior when the plate voltage was dropped low with the application of signal.

Any references you have seen to the 117P7GT being a Beam Power Tetrode is a factual error. There can be no such thing as a Beam Power Tetrode because any Beam Power tube has five elements in it as described above. The 117P7GT contains a Beam Power pentode section.

As for your amp, since you are using an isolation transformer, you will want to eliminate C1, by just shorting it out -- particularly since you are adding a new .01 cap in series with the hot signal from the input jack to the volume control. C1 is there precisely because as shown, it is a "hot chassis" design, and is included to prevent a direct connection to the ground side of the pickup, and the metal tone arm it was likely installed in back in the day. With an isolation transformer, the capacitor is simply no longer necessary, and will likely add some level of hum if included.

Good luck with your project!

Dave

Thanks! Great info, by the way. And Squid, why you messin' with the Wig, buddy? :scratch2:

 

[Squidward]

I got a good laugh out of this one. Gotta get my kicks where I can!

like Dave said, a lot of times a pentode will have the suppressor grid internally connected to the cathode, and not available as a discreet element via a pin. In the case where it is offered as a discreet element it is almost always tied to the cathode per design, but not always; I read a very interesting article that discussed using a 6AU6 pentode as a true triode by using the suppressor grid as the plate, and the plate as a shield! Neat concepts, and food for thought.

 

[Wigwam Jones]

I got a good laugh out of this one. Gotta get my kicks where I can!

like Dave said, a lot of times a pentode will have the suppressor grid internally connected to the cathode, and not available as a discreet element via a pin. In the case where it is offered as a discreet element it is almost always tied to the cathode per design, but not always; I read a very interesting article that discussed using a 6AU6 pentode as a true triode by using the suppressor grid as the plate, and the plate as a shield! Neat concepts, and food for thought.

Thanks, my friend. Always happy to provide entertainment.

FYI, I am spending my lunch break taking that volume pot out of the old R2R. Then I'll measure it. Turns out I was wrong, it doesn't have the on/off incorporated into it. I'll find one somehow on this pig.

And when I turned over the secondary inner chassis after disassembly, I found a Webcor EM87 magic eye tube. Ooh, now that looks like fun. Doubt I can use it in this circuit, though. I'll just tuck it away.

 

[Wigwam Jones]

I'd put an ohmmeter on it and turn the volume all the way up.

My multimeter says 220 Kilohms between pins 1-3, and 220 K between 1-2 with it all the way up and 220 K between 2-3 with it all the way down. So...I guess it's not a 1 Meg pot. But I don't know what this tells me. :scratch2:

 

[able.baker]

pot measurement

It tells you it is a 220K pot. You really only need to measure end to end (1-3).

 

[Squidward]

I'd give the 220K pot a try if you don't already have a 1M one.

Scavenging carcases is a great source for cheap experimentation parts. But then your basement starts to look like an electronics graveyard like mine, which is not a good thing.

 

[Wigwam Jones]

I'd give the 220K pot a try if you don't already have a 1M one.

Scavenging carcases is a great source for cheap experimentation parts. But then your basement starts to look like an electronics graveyard like mine, which is not a good thing.

OK, I will. Which reminds me, my basement already looks like yours. Differences are minor; mine is full of speaker guts and old cameras. I need to finish buttoning up my speakers so I can host a SMAC meet again.

 

[GordonW]

I'd give the 220K pot a try if you don't already have a 1M one.

Scavenging carcases is a great source for cheap experimentation parts. But then your basement starts to look like an electronics graveyard like mine, which is not a good thing.

Agreed. All that will really do, is change the input impedance from 1 megaohm, to 220K ohms. 220K is still a quite good value for input impedance- you should still be able to drive it from any decent source, tube or SS...

Regards,
Gordon.