OK, we all know about the post-PI cathode follower for increased drive current.....

[jrfrond]

BUT..... has anyone tried building a LTPI using TWO dual-triodes with both sections paralleled for increased current drive capabilities?

This idea has been on my mind.

[octal]

I'm not sure there's an advantage of "increased drive" in any sort of AC coupled PI circuit. You will not be able to achieve any sort of class AB-2 operation with an AC coupled PI. IMHO, all increased drive capability in the PI will accomplish is charging the coupling caps more when the power tubes reach grid clamping which will worsen any bias shift issues. But, you could certainly try it. I'm no Merlin / Enzo / RG grade authority!


Nathan

[Chuck H]

I agree. I don't think the lower impedance of the paralleled triodes will be a significant enough improvement to make a difference worth adding two tubes. It is possible that with the extra voltage gain you could use much lower value bias resistors at the power tube grids and that would certainly have the desired effect. But it's two extra tubes. A pair of cathode followers only needs one extra tube so I don't see the advantage unless you actually need the added voltage gain.

A current discussion on grid conduction concluded with the use of MOSFETs as cathode followers. I tend to be an all tube guy so I haven't tried it.

I did wire up a circuit once that used a cathodyne PI followed by a pair of triodes set up like virtual earth mixers. By changing the feedback resistor value for the virtual earth stages you can compromise between lower impedance and added voltage gain. It seemed to work very well for reducing grid conduction.

My circuit of choice though is the Paul Ruby buzz fix zener mod. It allows the coupling caps from the PI to discharge at what amounts to a 0 load while the power tubes are in cutoff. So no diode clipping is happening while the tubes are conducting but you cut the discharge time almost in half. It works brilliantly for eliminating grid conduction, and therefore crossover distortion in over drive conditions.

Chuck

[octal]

Chuck,

I think the OP is talking about adding one extra tube. Each half of the LTP is two tube sections in parallel, for a total of two dual triodes.

About ten years ago, I tried a DC coupled cathode follower circuit (using tubes) and if my memory serves, the clipping was so smooth and rounded that it didn't really give the "overdriven guitar amp" tone... it clipped so softly it just sounded more like compression!

When you refer to triodes set up as virtual earth mixers, you're talking about a triode gain stage with local negative feedback, right? Interesting. I thought about that myself. It sounds like you've concluded that the Ruby Zener mod offers the same results with less complexity though....

Nathan

[Chuck H]

I think the OP is talking about adding one extra tube. Each half of the LTP is two tube sections in parallel, for a total of two dual triodes

Aahh... Got it. In that case I'll retract my statements about voltage gain but side with you that any current capabilities are lost because it's coupled through a capacitor. The lower impedance would decrease the discharge time a bit but not enough to merit the trouble. This is just a guess and I always encourage trying something to see if you like it for any reason.

About ten years ago, I tried a DC coupled cathode follower circuit (using tubes) and if my memory serves, the clipping was so smooth and rounded that it didn't really give the "overdriven guitar amp" tone... it clipped so softly it just sounded more like compression!

That could be really useful in one of these modern gain machines where all the clipping is in the preamp. Did you use typical tube types?

When you refer to triodes set up as virtual earth mixers, you're talking about a triode gain stage with local negative feedback, right? Interesting. I thought about that myself. It sounds like you've concluded that the Ruby Zener mod offers the same results with less complexity though

The VE circuit is in many old tube circuit books. It IS a local feedback circuit with a series resistor to the triodes grid and a local feedback resistor of the same value taken from the downstream side of the coupling cap. The result is an output impedance rivaling a cathode follower and an input impedance that is so low that it inspired the name "virtual earth". As a mixing stage it's brilliant because you can use comparably low value mixing resistors compared to what you would see in most guitar amps and there is almost zero interaction and it has the added benefit of a low output impedance at unity gain. When I used this concept as followers for the cathodyne PI I raised the feedback resistor values to increase gain and still had more than enough drive and no grid loading. I tried it as a matter of tube economy. The LTP PI with a pair of cathode followers uses four triodes. My circuit uses three. I never tried both circuits in the same amp or even measured any parameters other than looking at my scope. I like using tube circuits but I do make concessions for non active SS devices like diodes. The economy of parts and difficulty along with the superb results have made me a big fan of the PR zener mod. Especially since it saved my a$$ two years ago when I was designing for 2009 winter NAMM and I needed a quick fix that didn't involve changing the layout. I was driving the piss out of a pair of el84's and with the PR mod this was possible without the typical consequences. I see no need to look back and my only regret is that I didn't come up with it first.

[octal]

Chuck,

Re: DC coupled cathode followers... I went though several types of output tubes in that amp, but I think I was using EL84s in fixed bias when I tried the CFs. I loosely copied a Heathkit power amp (I think it was the W6-M) which used 12BH7s for the followers. I think I subbed 12AU7s. I think an all tube DC coupled CF is a risky proposition though- if the CF fails, the output tubes are going to fry due to lack of bias. Also, EL84s aren't a good choice for AB-2. You really want a beam power tube, not a true pentode. The screens looked VERY unhappy when I was load testing the amp.


Nathan

[Amp Kat]

Whoever said you get increased drive by using a Cathode Follower in the 1st place ? I can think of know way a circuit that has less than unity gain will get increased drive. IMO driving from the plates is a much better sound and at least has some drive.

[mooreamps]

My circuit of choice though is the Paul Ruby buzz fix zener mod. It allows the coupling caps from the PI to discharge at what amounts to a 0 load while the power tubes are in cutoff. So no diode clipping is happening while the tubes are conducting but you cut the discharge time almost in half. It works brilliantly for eliminating grid conduction, and therefore crossover distortion in over drive conditions.

Chuck

My circuit of choice is using combination bias for a push-pull power amp. When the grids on the power tubes are driven into conduction, the front and back resistors tend to keep the grid current in check.. It also allows me to toggle between class A and class A/B very nicely without having to any lifting of the power tube cathode grounds ; or open circuit on the bias when toggling from one to the other..

-g

[Chuck H]

Well I know better than to ask for specifics since I'm sure you consider the technology proprietary (but hard to protect as are most electronic circuits), but if I understand you correctly your circuit has the amp in AB1 until the grids begin to draw too much current for the time constant of the coupling circuit. At that point your circuit begins reducing signal drive while simultaneously morphing the bias into class A. I'm curious. Especially since class A has a lower peak output than AB1 so the end result would be an amp that has a very similar percieved volume both clean and overdriven. Very handy for the guys who like to use their guitars volume knob as a distortion control.

[redelephant]

Whoever said you get increased drive by using a Cathode Follower in the 1st place ? I can think of know way a circuit that has less than unity gain will get increased drive. IMO driving from the plates is a much better sound and at least has some drive.

They are talking about increased current drive, not voltage gain...

[defaced]

A current discussion on grid conduction concluded with the use of MOSFETs as cathode followers. I tend to be an all tube guy so I haven't tried it.

I am using this setup in an amp with a SLO type preamp and much past the fourth gain stage, you can't hear the addition of the MOSFETs. Before that, you can tell and it's not pleasant - at least not with out re-biasing and really working the circuit. I currently use some big guys like the IRF820 for the post PI source followers and the LND150 for the cathodyne and preceding gain stage. It would be interesting to hear some feedback about how the setup sounds with more clean preamp/dirty power amp setups.

That said, I have not convinced myself that the source followers are necessary. I need a scope to do that. They require some interesting hoops to jump through to get to work. The bi-polar power supply being the most annoying.

[Amp Kat]

They are talking about increased current drive, not voltage gain...

There is no current drive or very little. Tubes are voltage source devices not current devices. There is no current drive and there is no gain. The voltage that was present at the plate gets transformed to the Cathode and the impedance changes from high to low but current stays the same what very little ua's there is.

[redelephant]

There is no current drive or very little. Tubes are voltage source devices not current devices. There is no current drive and there is no gain, in fact it is less than unity gain.

Whatever dude.

Followers/buffers are basic building blocks. Go buy electronics 101.

[Amp Kat]

Basically a Cascode follower aka a White Cascode and there are some other variations like boot strapping to get a tad more drive out of it but it's still a follower with best unity gain.

[redelephant]

Whatever dude.

Followers/buffers are basic building blocks. Go buy electronics 101.

Sorry man. That last line was uncalled for. My immature self not being able to resist.