Tube Circuit Basics ?

Tom Gorr · Post by **Tom Gorr** » 26 Feb 2005 8:27 pm

I need to finesse one of my tube preamp's voicing - but don't know much about the electronics of this.

1) what are the names of the different parts of the tube (eg. base, emitter, etc? ) and what do each of them do?

2) What do the resistor and capacitor in series between the circuit ground and the "bottom" of the tube do? I've heard its 'cathode bypass', but this doesn't mean too much to me...Does it affect tone?

3) How do I recognize the sections in a tube preamp circuit responsible for voicing? I don't mean the tone stack - which in the specific case comes at the end of 4 tube gain stages. I'm referring to the 'hardwired' voicing.

4) How do I calculate the voicing frequencies, bandwidth, and cut/boost depth?

5) Any tube preamp gurus on this forum that are willing to step me through a high-level explanation of what the circuit is doing? I'll pay for the phone call anywhere in North America.

Thanks,
Tom Gorr

John Daugherty · Post by **John Daugherty** » 27 Feb 2005 5:03 am

Tom, I think you will find a lot of info and circuits if you do a "google search" for "tube circuits guitar amp preamp" or something similar.
BTW, the tube elements from bottom up,in a typical picture are: filament,cathode,control grid,screen grid,supressor grid and plate. A triode is the same,but the only grid is the control grid.

Ken Fox · Post by **Ken Fox** » 27 Feb 2005 6:35 am

Good page to visit:
http://www.duncanamps.com/

1) What are the names of the different parts of the tube (eg. base, emitter, etc? ) and what do each of them do?

Parts are differents in some tubes, a basic 12AX7 has a plate (B+ here), control grid and a cathode (ground, "0" volts). It is a dual triode, 2 tubes in one package.

2) What do the resistor and capacitor in series between the circuit ground and the "bottom" of the tube do? I've heard its 'cathode bypass', but this doesn't mean too much to me...Does it affect tone?

The capacitor bypasses the ac signal around the typical 1500 ohm resistor in the cathode circuit of the preamp tube. When this cap goes open, gain in that stage is tremendously reduced. The 1500 ohm resistor is part of this unique self-biasing, class "A" circuit.

3) How do I recognize the sections in a tube preamp circuit responsible for voicing? I don't mean the tone stack - which in the specific case comes at the end of 4 tube gain stages. I'm referring to the 'hardwired' voicing.

The different preamp tubes in the 12A*7 series have different gain and plate load impedances. That will alter tone and gain both. The tone stack in a Fender Blackface or Silverface amp comes right after the first gain stage, not at the end! Also coupling caps between stage, value and type can alter tonal characteristics.

Ken Fox · Post by **Ken Fox** » 27 Feb 2005 6:37 am

http://www.pacificrecone.com/JackDarrBook.html

That is a link to a complete book by Jack Darr on tube amp repair, the absolute best book I ever got and it is free!

Donny Hinson · Post by **Donny Hinson** » 27 Feb 2005 7:57 am

I have the Darr book, and as Ken says, it's a good one. Here are some other good books I'd recommend...

"The Ultimate Tone" (Modifying and Custom Building Guitar Amps) by Kevin O'Connor

"Great Tube Amp and Guitar Mods" by Mike Thompson

"How To Service Your Own Tube Amp" by Tom Mitchell

"Vacuum Tube Guitar and Bass Amp Theory" by Tino Zottola

"Inside Tube Amps" The Book on Tube Amps by Dan Torres

I'd also suggest any version of the "RCA Receiving Tube Manual" which gives pin diagrams and specs for all tubes, plus some theory and simple projects.

Will Holtz · Post by **Will Holtz** » 27 Feb 2005 12:06 pm

Be careful if you aren't experience working with tube amplifiers or other high voltage systems. Voltages in tube circuits often get up to 200V, and even after the unit is turned off for a long time much of that voltage may be retained in a capacitor.

Jim Jasutis · Post by **Jim Jasutis** » 27 Feb 2005 6:23 pm

I still remember taking electronics in high school. We had this large metal rack with a radio reciever on it, divided into sections, so that we could work on them one at a time. We were working one day trouble shooting. My partner was behind the rack and touched the bleeder resistor on the power supply. He must have jumped 3 feet. I asked him what happened. He said "all I did was touch this" and he proceeded to touch the same resistor again, getting the exact same result. :<)