tube preamp question

[David Deen]

Hello all, I wanted to build a tube preamp that would use a 12AX7 or eq. to drive an EL84 into saturation so my question deals with how the output should be built. I've never used a pentode as a 'preamp' tube but I figure it's no different than a triode except for some more versatility in biasing.

On a single ended tube power amp the usual setup is an output tranformer that provides the tube with proper power. Then the OP tranny serves also as a means to match output impedence to a load and eliminate any high DC voltage at the output. So if I were to build this tube circuit simply with the intent to get an EL84 to distort would it be adviseable to go with an output transformer for the output OR could I do the same thing as many common tube preamps do and use a DC blocking cap from the plate then run it through some variable resistor as a output volume control? The question is mainly, for low power applications, what would be the difference between the two configurations: output transformer or DC blocking capacitor leading to the output?

Thanks for any comments or suggestions.
-David

[csj]

Well for starters...
Other than giving the tube a place to dump it's load the OT doesn't supply the tube with anything let alone power. The power is determined by the combination of the plate supply voltage (which comes from the PT not the OT) and the operating point of the tube (which is the voltage interplay between the cathode and the signal grid). The standing DC plate voltage gets shaken up by the cathode current flowing through the tube which is set by the grid bias point and the signal as it rises and falls. This current passes through the OT primary and finds it's way back to the power supply thereby completing the circuit.
I'm not sure I understand what your question is...

You're trying to get away from using an OT so you want to dump, say 50ma of EL84 current into a coupling cap which is hooked up to a pot?
What, for the sake of the argument, would you hook the pot to? Assuming somehow you did get as far as I think you're saying - you'd have a loose tail of voltage gyrating around nothing at the back half without something loading the plate and serving to close the loop back through the power supply. Maybe I've misunderstood what you're asking.

[puretube]

for mere pentode overdrive, you could use EF86 or eq.
small signal/low power tubes;
will sound different than a "loaded" EL84, though...

[David Deen]

To csj,
Thank you for the quick response. Alas, you caught me typing faster than I was thinking. I do understand that the OT does not supply the tube with 'power'. I meant to say in a single ended power amp setup the OT couples the output tube with the power supply thereby setting the plate bias of that output tube. Then the OT serves as an impedence matching device for a loaded amp plus it prevents any high DC through to that load.

As for my question, on many tube preamps say the Real McTube, or the Matchless Hotbox, or many tube preamps the 'end' or output of the preamp comes off of the last tube gain stage's plate through some DC blocking cap and possibly into a pot as a volume control; so I was wondering, IF I were to build a preamp using an EL84 could I use the aforementioned setup (that being a DC blocking cap into a pot w/ the wiper of that pot going to a 1/4" jack) or will I need to couple the output tube to a 1/4" jack using an output transformer? So the question is what is the difference between the two configurations?

I'm sorry for the poor explaination, I hope this is better.
-David

[David Deen]

Hmm, EF86? I've never used one of those. I know it's a subjective question but how do they sound overdriven? I might have to try this. I was going for the EL84 only b/c I've built an amp that will overdrive EL84s and I really like how they break up.

-David

[csj]

Hi David,
Honestly, I've never built a tube based pedal. I don't think I've ever even looked at a schematic of one. I haven't heard one yet that I've liked so I've never paid that much attention (sorry guys).  The closest I've come to something like it is cloning the Fender 6G15 reverb unit. It's run very similar I suppose...taking the wet signal off the plate through a tone pot and then a mixer pot for the wet/dry signals. The plate has the typical 100k PS tie in resistor.

I was just trying to follow your explanation and I couldn't see any way to account for plate loading and I was left scratching my head. Thanks for straightening me out.

I've heard the EF86 has a so called warmer tone. Maybe, like puretube says, it might be better suited for the economy of the circuit. I wish you good luck on the build. I'll help if I can... puretube's your main man here though.
Clay

[jazzyfingers04]

Well, about this tube... It was originally used in the early vox ac30s. It had a beautiful treble sound -round and sorta hollow sounding, no icepick there!

Pentode preamps are not used much in guitar amps. Early Fenders, Matchless Clubman, Vox AC-4 and the Dr Z Route66 are the only ones I'm familar with. The EF86 has a very high gain, typically around 150 - it can be easily driven into distortion by most guitar pickups. Pentodes have an interesting characteristic in that the screen grid current increases as the tube is driven harder. The screen grid can be used as a gain control - lowering the screen voltage reduces the gain of the tube. A typical EF86 preamp has a 1m screen resistor to B+ and a cap of .1u or so filtering the screen voltage. When the EF86 preamp is overdriven it exhibits "screen sag" - the screen voltage drops because of the increase in average screen current and the large screen resistor. As the screen sags the gain drops - its a compressor. Pentode screen compression is the subject of a patent by Randall Smith of Randall Amps, although his circuit applies to power stages. The compression effect is quite noticable in this amp, more so with higher output pickups. When the EF86 distorts it has a different sound than a triode preamp - its more like power amp (power pentode) distortion. The net result is the amp sustains really well and can produce nice distortion tones at low volume.
        (from :http://www.storm.ca/~rheslip/richsamp.htm)

The only trouble is that the tube is notoriously vibration sensitive. It is highly microphonic. This could be a slight problem for pedals unless you house the preamp in some sort of heavy wooden enclosure. Maybe a small head-like device with a channel select switch like most modern amps would be a solution to this issue.

[Chill]

on many tube preamps say the Real McTube, or the Matchless Hotbox, or many tube preamps the 'end' or output of the preamp comes off of the last tube gain stage's plate through some DC blocking cap and possibly into a pot as a volume control; so I was wondering, IF I were to build a preamp using an EL84 could I use the aforementioned setup (that being a DC blocking cap into a pot w/ the wiper of that pot going to a 1/4" jack) or will I need to couple the output tube to a 1/4" jack using an output transformer? So the question is what is the difference between the two configurations?

The Real McTube and Hotbox are both 12ax7 only IIRC.  They are voltage amplifiers.  The EL84 is a power pentode and will usually act as a power amplifier (i.e. it will give you a voltage gain around 10-20 and a large current gain compared to the preamp stages).  If you use a resistor for loading a power amp, you'll need a high wattage R and it may still get hot...but it has been done.  One of O'Connor's TUT books mentions Traynor using a resistive load on an EL84 for driving a reverb tank.

If I were doing what you describe, I'd use an OT, a power resistor on the secondary (say 4, 8, or 16 ohms as appropriate, and probably 10W to be safe), and a volume out pot much larger than the resistor (say 1K or even 100K) to limit the current going through the pot to allow you to use standard 1/2W pots.  With the power resistor on a switching speaker out jack, you can also use the thing as an amp.

Check out the ax84 P1 project, which is basically the amp you describe.  A bunch of people have built it and there's good documentation for the project.  

If you want to do it on the cheap and save yourself some time and hassle, find a used Electar Tube10 or silverface Champ and mod it.  If you go that route, maybe get a single yellowjacket so you can use EL84's.

HTH!

[Lonestarjohnny]

EF 86. Think DR. Z, Buddy Whittington with Jon Mayall use's a Z amp on the last Bluesbreaker CD, Killer tone he's got !
JD

[David Deen]

Thanks for the replies guys, this is a wealth of information. That EF86 reputation has now got me salivating. I think I'll check out that AX84 project as well.

The OT makes sense, though my first intention was to stay away from building a single ended power amp and build a preamp strictly for aquiring the sound of an overdriven pentode.

Thanks again guys.

-David