Hey guys,

I'm looking to build my own preamp based on pentodes, but I want it to be OTL. Are there any good basic designs out there that I can easily tailor to whatever tube I choose, and would it be as simple and using the specified suggested parts (grid resistors, plate capacitance, and all that jazz) from the datasheets?

:scratch2:

I guess what I'm asking for is a bare-bones 'input-here-output-there' design with tubes in the middle and no heavy iron to deal with.

:smoke:

Jeez, it sounds like you're talking Bottlehead stuff. Lots of folks have done lots of cool things with them. Seems like a good place to start. Have you checked into the DIY forum here?

Cheers,

David

I'm looking to build my own preamp based on pentodes

Why pentodes?


I guess what I'm asking for is a bare-bones 'input-here-output-there' design with tubes in the middle and no heavy iron to deal with.

Sounds like a simple triode cathode follower would do.

Jeff

Why pentodes?

Thought it might be fun :)

Sounds like a simple triode cathode follower would do.

Jeff

Would I be able to triode-strap the pentodes in a cathode follower preamp design? :scratch2:

Pentodes take a little more work to get right, but the rewards can definitely be worth the trouble. The 6AN8 is a good one, or the low noise EF86 is too. Those are popular in the hi-fi world, but there are lots of good pentodes under the radar too. I'd start by taking a look at some popular schematics of integrateds that used pentodes and tweak from there. Heathkit comes to mind right off, but there are many others.

The Roberts tape recorder amps that are so popular for Hi-Fi used the EF86 up front. That might be a good one to study too.

Would I be able to triode-strap the pentodes in a cathode follower preamp design? :scratch2:

Possibly, just don't ask me how.:D

Jeff

Would it be a trick to use low-voltage sub-mini pentodes in a cathode follower design, just tweak the passives to get the correct currents and voltages?

:scratch2:

The Roberts tape recorder amps that are so popular for Hi-Fi used the EF86 up front.

And the Mullard 5-20. The Quad II used them as drivers.

Jeff

Would it be a trick to use low-voltage sub-mini pentodes in a cathode follower design, just tweak the passives to get the correct currents and voltages?

What about output Z? Maybe easier to use a triode in this config.:headscrat

What about mixing pentodes and triodes in SRPP?

Jeff

I understand that pentodes wired as pentodes actually can make very good CFs but I am having trouble understanding the use of a pentode as a voltage amplifier in a line amp. Quite a bit of gain to be thrown away no?

mike

I would recommend the Akidio design. You can make it work with a whole host of tubes.

My personal preference is one using 6DJ8/6922/7308 with 5687 driver tubes. The Akidio can also use 6CG7 in place of 6DJ8 as a front end tube. The 5687 is one of the best driver tubes out there.

Okay, I'll come clean. It's the case of 'what can I do with this tube.'

I wanted to try my hand a modifying a simple tube circuit to work with a bunch of 6612 tubes (datasheet here (http://www.mif.pg.gda.pl/homepages/frank/sheets/138/6/6612.pdf)) I bought on a whim, then make the unit a portable headphone amp.

I figured what the hell. :)

If I read the data sheet right then the plate resistance of this puppy is 180K (.18Meg). That would mean even in a cathode follower (if I'm right) it would have a pretty high output impedance. Plus cathode followers don't have much in the way of gain, in triodes they actually have a gain of less then 1. I would think the way to go on this for a preamp would be use this as a gain stage with output off the plate (very conventional way to go) then follow it with a cathode follower off tube with much lower impedance. The 6DJ8 might be an ok choice for this.

If I read the data sheet right then the plate resistance of this puppy is 180K (.18Meg). That would mean even in a cathode follower (if I'm right) it would have a pretty high output impedance. Plus cathode followers don't have much in the way of gain, in triodes they actually have a gain of less then 1. I would think the way to go on this for a preamp would be use this as a gain stage with output off the plate (very conventional way to go) then follow it with a cathode follower off tube with much lower impedance. The 6DJ8 might be an ok choice for this.

What properties are desirable about the the 6DJ8 tube to use it in this application?

*Edit* If in a cathode follower topography, would the high output impedance match up with a high input impedance for an OPAMP?

Try triode wiring the pentode for the VAS. As I understand it gm has more to do with CF Zout than Rp. If you are going to be driving an opamp I imagine that the opamp will dominate the sonic character of the whole.

mike

Try triode wiring the pentode for the VAS. As I understand it gm has more to do with CF Zout than Rp. If you are going to be driving an opamp I imagine that the opamp will dominate the sonic character of the whole.

mike

Thanks Mike, I suppose you're right... :scratch2: I've been reading up on the Tube CAD Journal site (http://www.tubecad.com/2007/04/blog0104.htm) and it looks like there a good number of configurations available to me.

Okay, so the Zout for the tube CF'd is going to be high, would I be able to calculate a theoretical value for it? Wouldn't I be able to parallel a lower-value resistor to bring the Zout down to something usable?

Pardon my ignorance, but what does VAS stand for? :dunno:


Thanks,
Philip.




*edit*

VAS = Voltage Amplifier Section? Like this?

http://www.triodeel.com/7199f.gif

Found it in an RCA application (http://www.triodeel.com/7199.htm) note thru Google.



Sorry for all the questions, glass interests me and I want to get as comfortable with tubes as I am with solid state devices.

Pentodes take a little more work to get right, but the rewards can definitely be worth the trouble. The 6AN8 is a good one, or the low noise EF86 is too. Those are popular in the hi-fi world, but there are lots of good pentodes under the radar too. I'd start by taking a look at some popular schematics of integrateds that used pentodes and tweak from there. Heathkit comes to mind right off, but there are many others.

I was digging thorugh my stash and found a set of brand old stock mullard EF86's. Hmm......... and idea is brewing.

Time to go back and read the rest of the thread - see if someone posted schematic.


Edit - nope not yet. Time to look for one of my own.

I was digging thorugh my stash and found a set of brand old stock mullard EF86's. Hmm......... and idea is brewing.

Time to go back and read the rest of the thread - see if someone posted schematic.


Edit - nope not yet. Time to look for one of my own.

You don't want to use those nasty old pentodes. Send them to me and I'll toss them out for you.:smoke:

You don't want to use those nasty old pentodes. Send them to me and I'll toss them out for you.:smoke:

Jay, thanks for the offer, I always know who to turn to when I need help!:yes:

I don't recommend using these kinds of pentodes for a conventional line stage, although they'd be good if you are building a high-gain phono stage and using something else for output. I also don't recommend wiring pentodes as triodes, since there are plenty of fine cheap triodes available new, and good pentodes are only available NOS today. Instead, consider a high-transconductance pentode where you can use a small plate load, which will translate to a low output impedance without using a cathode follower. Like a ~1k load on a 12BY7-family tube running at ~15mA. The pentode will also require an exceptional power supply - almost all the noise will be on the output, so many filter stages are needed. Overall, the design is non-trivial compared to a standard triode line stage.

Otherwise, I'd recommend an Aikido kit ... with a 6DJ8 output you'll have ~100 ohm output impedance. A 6CG7/6DJ8 combo should be excellent. Or 6CG7/6N6P.

Yes, that is a VAS. To triode strap connect R3 to the plate instead of the power supply. Zout of CF is approximately 1/gm give or take but as a practical matter you also want to idle the tube (bias) at several mA if possible to drive the capacitance of the next stage. Paralleling a resistor across the output just makes the load more difficult to drive. In other words it lowers the effective Zin of the load rather than the Zout of the source. If these are rf pentodes then a grid stopper will be nearly mandatory me thinks.

mike

Just went back and had a look at that datasheet. Interesting little tube. If you got them why not give it a go? Worst case is you don't come up with anything satisfactory.

You could try a triode strap as mentioned but watch out for that screen dissipation. I have read of instances of using the screen as a triode plate alone (not using the real plate) but I suspect this tube couldn't take it.

Another possibility is to set it up as a pentode direct coupled to a CF and then wrap a bit of negative feedback around the pair. There is a fancy name for that configuration that I can't recall but it just might do the trick for your application.

Another thought since you are thinking low B+ headphone amp is to pair this up with some space charge "power" tubes in push pull for a low voltage all tube headphone amp.

mike

The EF 86 is very highly thought of in the audio circles. Run it in pentode like it was designed. IMHO the trick to this tube (and other pentodes used as voltage amps) is getting the operating parameters perfect. Use a pot (variable resistor) for the cathode and plate voltage resistors so you can really dial things in. Then you can remove the pot and measure the resistance you ended up with. Solder in a resistor of that value and you're done. Pentodes can sound very nasty, or very good, you just have to get them set up just right.:yes:

The EF 86 is very highly thought of in the audio circles. Run it in pentode like it was designed. IMHO the trick to this tube (and other pentodes used as voltage amps) is getting the operating parameters perfect. Use a pot (variable resistor) for the cathode and plate voltage resistors so you can really dial things in. Then you can remove the pot and measure the resistance you ended up with. Solder in a resistor of that value and you're done. Pentodes can sound very nasty, or very good, you just have to get them set up just right.:yes:

I know I'm talking about this oddball 6612 tube I came across... :dunno:

The plot thickens with the 6612 tubes-- I bought 8 x 6205 tubes (http://frank.pocnet.net/sheets/137/6/6205.pdf) (which is a premium version of the 5840, and the 5840 is like a 6205) and comparing the datasheets (6612 v. 6205), they look strikingly similar (with my limited experience) except that the 6612 requires ~1/10th the power. The 6025 is used for stomp-box applications in guitar circles.

Maybe they're not totally useless? They're in the mail and I should have them sometime in the next week or two.