Power pentodes as preamp tubes?

[Austin]

I am thinking of building my amp with all power tubes from the input. Is there any reason I shoudn't? I have all these octal slots available and alot of power tubes at my disposal along with huge supply transformers.. Like 6g6g 6f6g etc, I could triode strap them still have plenty of gain I think. Like using the 6g6 as a pentode as input voltage amplifiers and using a 6f6 as a cathodyne phase inverter then using two more 6f6 as dc coupled cathode followers to drive two 6L6 as power stage. Makes sense to me but why doesn't everybody do it? I saw a data sheet listing the curves for ab2 operation when looking at the 6L6 data sheet last night.. I guess I will try it and see how well it works. Any thoughts anybody?

[Steve Conner]

Well, the classic tube amps were being manufactured and sold at a profit, they were subject to economics. You wanted the most gain per dollar in the preamp, and preamp tubes were designed to deliver that.

Now tube amps are an artform and you can do anything you want.

My Ninja Corvette essentially uses an EL84 as a preamp stage in high power mode.

[Austin]

Well, the classic tube amps were being manufactured and sold at a profit, they were subject to economics. You wanted the most gain per dollar in the preamp, and preamp tubes were designed to deliver that.

Now tube amps are an artform and you can do anything you want.

My Ninja Corvette essentially uses an EL84 as a preamp stage in high power mode.

Hi Steve, thanks for the reply. What do you mean exactly by "high power mode"? Pentode mode? I have this cool choke that was configured on the cathode of a 12ax7 with a bias tap. I was considering using it as a plate load for the 6g6 and using the bias tap as the screen supply, but what do you guys think?

[Gregg]

I've seen Hi Fi amps using EL84 and other power pentodes as preamp tubes so it's been done already. Whether they are suitable for guitar amps is another story.

[Austin]

I've seen Hi Fi amps using EL84 and other power pentodes as preamp tubes so it's been done already. Whether they are suitable for guitar amps is another story.

This is intended to be a clean P.A. system I can play mp3s and sing through with my gal.. Nobody has a tube pa system and I imagine tubes might sound sound nice with the vocals but we will see... I already have a guitar amp that I like.

[Steve Conner]

It's worse than pentode mode. The EL84 drives the bases of a couple of hefty power transistors through a Valve Jr. OT, which is used as an interstage transformer in this case. It makes about 40W and sounds remarkably good for what it is. I got the idea from RG's writings on the Thomas Vox amps. You can google "ninja corvette" for more details.

One problem I had is that the transistor output stage also amplifies the microphonics of the EL84. Preamp tubes are designed for low microphonics, power tubes aren't, as they are usually not operated with gain between them and the speaker.

I tried a few EL84s to select a quiet one, and also ended up with a silicone gasket from a coffee maker wrapped round the tube to damp it.

[Alan0354]

One question. What about the noise performance? I was just reading about the grid current( from RDH4 where there is current even at -ve bias), that would be a source of noise

In^2=2q(Ig)(BW) where In is the noise current, Ig is grid current and BW is the bandwidth.

I don't have the noise model of vacuum tubes, since the electrodes are all metal, I assume there is no significant resistance inside the tube to generate thermal noise. So my assumption is the grid current is the main source of noise refer to the input. Sounds like the power tubes has more grid current and has more shot noise. Question is would that be a problem. This is particular important if there is a large grid stopping resistor at the front end as was talked about in a post not too long ago in radio interference and how to stop it.

Also, the electrode capacitance and the inter electrode capacitance must be a lot higher for power tubes that might affect the performance.

[loudthud]

This is intended to be a clean P.A. system I can play mp3s and sing through with my gal.. Nobody has a tube pa system and I imagine tubes might sound sound nice with the vocals but we will see... I already have a guitar amp that I like.

Vocal harmonies through a tube PA sound awesome. I read that Elvis used five Dynaco Mark III's for a while in his PA.

[Austin]

One question. What about the noise performance? I was just reading about the grid current( from RDH4 where there is current even at -ve bias), that would be a source of noise

In^2=2q(Ig)(BW) where In is the noise current, Ig is grid current and BW is the bandwidth.

I don't have the noise model of vacuum tubes, since the electrodes are all metal, I assume there is no significant resistance inside the tube to generate thermal noise. So my assumption is the grid current is the main source of noise refer to the input. Sounds like the power tubes has more grid current and has more shot noise. Question is would that be a problem. This is particular important if there is a large grid stopping resistor at the front end as was talked about in a post not too long ago in radio interference and how to stop it.

Also, the electrode capacitance and the inter electrode capacitance must be a lot higher for power tubes that might affect the performance.

I was wondering about the capacitance too, I guess I just have to try it and see how it sounds. Shoudn't I get lots of voltage gain like 300 or so using a pentode? I could use one pentode as a ccs plate load for another pentode, and dc coupled cathode folowers using power tubes might sound pretty good.

[Alan0354]

I was wondering about the capacitance too, I guess I just have to try it and see how it sounds. Shoudn't I get lots of voltage gain like 300 or so using a pentode? I could use one pentode as a ccs plate load for another pentode, and dc coupled cathode folowers using power tubes might sound pretty good.

You are asking the wrong person here!!! I am new to the tube stuffs. I am going to try working on this a little. I look at 6L6GC, the gm is 4700umho vs 1600umho for 12AX7. So the gain is definitely higher. The plate resistance rp of 6L6GC is about 30K vs 62K of the 12AX7.

Gain of the stage u'=gm(rp//RL)

Where RL is the load resistance. So

Gain =u'= gm (rpXRL)/(rp+RL)

Using RL=100K

For 12AX7, (rpXRL)/(rp+RL)=37.5K

For 6L6GC, (rpXRL)/(rp+RL)=23K

So gain for 12AX7 is gm(rpXRL)/(rp+RL)=.0016X37500=60

Gain for 6L6GC is 0.0047X23000=108.1

From my calculation, the 6L6GC is about double the gain of 12AX7. Please few free to check and challenge my calculation, I don't know any of these 3 days ago!!! Just learnt it, fresh out of the oven!!!!

[Alan0354]

Still back to my original, how about the grid current. I have no data on this, this cause noise. Common sense tells me that the grid of a big pentodes is going to be a lot bigger, if there is any electrons from cathode or plate or others hitting the grid, you get more surface area and therefore more grid current. That will raise the noise. If you have a high gain design, that might give you a lot of hiss.

[Austin]

Still back to my original, how about the grid current. I have no data on this, this cause noise. Common sense tells me that the grid of a big pentodes is going to be a lot bigger, if there is any electrons from cathode or plate or others hitting the grid, you get more surface area and therefore more grid current. That will raise the noise. If you have a high gain design, that might give you a lot of hiss.

Why would grid current cause noise?

[Alan0354]

Why would grid current cause noise?

Because if you look at current at any given time, it is not smooth. Current is a flow of electrons, say you have average of 100 electrons per second passing through a junction A, it is not as if there is evenly one electron pass through A per 1/100 of a second. It is totally random. You can have two or three electrons bunch together and pass through at the same time, then there might be a dead spot for say 1/10 of a second, then 20 electrons passing in the next 1/10 of a second...............It is totally random. The only thing you can say is on "average" you have 100 electrons pass through junction A per second.

So by itself, it is not smooth, it is bumpy. So there is a noise involve in it as long as there is a current flow through. This is called shot noise ( vs thermal noise that only from resistance). and it is represented by a noise current In where:

In^2=2q(Ib)(BW)

Where Ib is the current flow ( grid current in this case), q is charge per electron which is 1.6X10EE-19 coulomb per electron. BW is the bandwidth of interest. In your case for guitar amp, BW=5KHz.

When you calculate noise, it is not at all obvious what is the cause of the noise problem. It all depend on the input impedance, the input current etc. That's the reason I kept challenging in the post about the grid stop resistor. Because until you have all the data, it is not obvious. You cannot lump everything into thermal noise generated by resistors.

In your case here, you still need to find the voltage noise of the tube. Just because I don't see where the resistance can be from a tube don't mean there is none. What do I know about tubes? That's the reason I asked more than once whether anyone has the noise model of the tubes.

Also, the calculation I gave on the gain was based on the gm given in the operating condition of the 6L6GC at the given plate current. At the low plate current that you are trying to run, is the gm still 4700? If it drops off, then you loss the advantage.

[Austin]

Well I didn't decide how much current to let the tubes run at but isn't it true that it will be more linear working into a higher resistance? I have no shortage of tube slots or tubes or power supply so gain is not an issue at all. Right now it has 1 6v6 1 6g6g 3 6f6g and two 6l6gc in there per channel and I was going to use the 6v6 as a plate load ccs and use the 6g6 as the preamp and use one 6f6 as cathodyne phase inverter and two 6f6 as driver tubes into tthe 6L6 power stage. Everything triode strapped for now just to get it working until I make a regulated screen supply. I will try and post some pics tomorrow.

[J M Fahey]

Not to rain on your parade, but 6L6 (or any other power pentode) transconductance at, say, 1mA plate current (you are feeding it through 100K after all) is WAY lower than at usual power tube current levels.
You want a very rough, no graphs voltage gain estimation for a 6L6 ? (EL34/84 can be calculated in a similar way):
Just consider this:
Idle: plate voltage=V+ , say, 430V ; grid voltage (bias)= -52V
Saturating: Plate, say, 60V ; grid: 0V .
(Delta means "variation")
Plate variation (delta voltage)= 430-60=370V
Grid variation (delta voltage) = 52-0=52V
Effective gain: 370/52=7 ..... incredible, isn't it?

So, what about that huge transconductance, is it a lie? Not at all, but is is the name to describe a *type* of Physical phenomenon: that when voltage changes, current changes.
In this case , it does not mean huge voltage gain but huge current swing for a relatively small (grid) voltage swing.

So, in a nutshell, use small triodes (or small pentodes if you wish) for gain and big pentodes for power.
Can't go wrong.

Hasn't been wrong for the last 90 years, go figure