Hiwatts..

[leadfootdriver]

What is going on here with V3B? It looks almost like a Cathode Follower, but not quite. I never seen this before.

[SoulFetish]

What is going on here with V3B? It looks almost like a Cathode Follower, but not quite. I never seen this before.

It is a catho.... wait a minute. I've never seen this before but I'm going to take a stab at it. What I CAN tell you is that it is kind of like a cathode follower, in that it is a grounded plate stage with a DC grid bias formed via a voltage divider (via the 1M/220K resistor from the B+ to ground). But, there is no AC signal at the grid. However there is an AC signal present at the cathode. Usually, when the grid is held at a steady voltage and signal is driven into the cathode, the grid is often grounded and it shields the cathode from the plate's capacitance charicteristics and extends the HF bandwidth (so on and so forth...). This type of stage is called a grounded grid. However the signal would be taken off of the output of the plate, so it would have some load impedance and not be AC grounded like it is in this stage. When looking at the cathode in this circuit, there are actually several AC signals present at this input. I only took a quick look but there are active filter networks via some negative feedback, so if I had to guess... This is a mixing circuit, with the balancing happening as each signal modulates cathode current?

*also, a grounded grid stage would have to be driving from a low impedance source, and these are NOT low impedance signals,


Edit: I read the schematic wrong here. See beliw.

[Malcolm Irving]

One way to look at it is that the CF triode is providing a ‘regulated’ DC voltage level to bias the LTP. But the 100k resistor to grid 1 of the LTP (compared to the 1M on the other grid) means that there is influence on the AC signal, as SoulFetish suggests. The low impedance at the cathode of the CF would make it harder to overdrive the LTP.

[Justin Thomas]

So, might this be a part of the "Loud & Clean" reputation they have?

Justin

[Zozobra]

http://music-electronics-forum.com/t39917/

I asked this question last year. Lots of good info up there and it was speculated that you could achieve the same thing using a potential divider from the B+ rail.

[eschertron]

http://music-electronics-forum.com/t39917/

I asked this question last year. Lots of good info up there and it was speculated that you could achieve the same thing using a potential divider from the B+ rail.

Isn't it a voltage source? Providing fixed bias to the PI?

[Mike Sulzer]

Notice that it has low current through it. While it is within its linear range it steals drive from the PI by loading the previous stage, as a result of its low output impedance. But as the drive increases it saturates in one direction, and turns off in the other. In either of those states it no longer has a low impedance; this effectively increases the gain of the stage driving the PI, since it is no longer loaded down. It might compensate for a non linear response in the output stage.

[SoulFetish]

I was thinking about it for a moment after I posted. And these guys are right. Normally you would take advantage of an opportunity to bootstrap and bias this type of phase inverter for a very high impedance. This way definitely loads down the previous stage

[leadfootdriver]

Thanks for the replies. So, it's a fixed bias LTP... Very clever. Fryette uses an AT7 forma PI Driver of sorts in a Deliverance 120, but I haven't seen a schematic. Fryette amps are referred to as Hiwatt like.

[Malcolm Irving]

... as the drive increases it saturates in one direction, and turns off in the other. In either of those states it no longer has a low impedance; this effectively increases the gain of the stage driving the PI, since it is no longer loaded down. ...

I like the idea of this, but I don't think the previous stage could provide enough output voltage swing to get the CF out of its linear range. The resistance to ground seen at the cathode of the CF is only about 620 ohms. Combining that in a potential divider with the 100k resistor (going to grid 1 of the LTP PI) means that an output swing of 162V from the driving stage would be needed to move the cathode voltage of the CF by just 1V.

[SoulFetish]

I like the idea of this, but I don't think the previous stage could provide enough output voltage swing to get the CF out of its linear range. The resistance to ground seen at the cathode of the CF is only about 620 ohms. Combining that in a potential divider with the 100k resistor (going to grid 1 of the LTP PI) means that an output swing of 162V from the driving stage would be needed to move the cathode voltage of the CF by just 1V.

Then i think we need to throw a pentode in between them and drive it to the rails.

[eschertron]

...means that an output swing of 162V from the driving stage would be needed to move the cathode voltage of the CF by just 1V.

So an argument in favor of voltage stability? Better than what you'd get with a conventional tail resistor?

[SoulFetish]

Oh, i realze now i totally saw connections that were not there. It's amazing to watch a circuit change completely in your mind after you move a few connection to where they need to be. Thats what I get for posting at 3 o'clock in the morning when I'm exhausted. I had time to take a second look, And I realize I read this wrong. (I friggin' always do that!) Anyway, the active tone control doesnt connect at that point on cathode with the grid leaks. This isnt a mixer at all. It does look like a bias circuit for the most part with active balancing component (I think)... That sucks though, what I thought was happening seem more interesting though.

[Malcolm Irving]

So an argument in favor of voltage stability? ...

The CF seems to supply a stable DC bias voltage to the grids of the PI. But why use a triode in place of a resistive divider with a capacitor to stabilise the voltage? The 100k grid leak on grid 1 of the PI (in series with the low resistance of the CF from cathode to ground) imposes a heavy load on the driving stage. Also, there is the signal through the presence pot in the mix.

Why have AC coupling into the PI, but a fixed bias scheme (instead of cathode bias) on the PI?

[Mike Sulzer]

I like the idea of this, but I don't think the previous stage could provide enough output voltage swing to get the CF out of its linear range. The resistance to ground seen at the cathode of the CF is only about 620 ohms. Combining that in a potential divider with the 100k resistor (going to grid 1 of the LTP PI) means that an output swing of 162V from the driving stage would be needed to move the cathode voltage of the CF by just 1V.

The CF holds its cathode voltage nearly constant due to its configuration derived feedback, yes, but it must sink/supply the current that allows this to happen. Since its bias current is low its runs out of current swing, going into shut off or saturation well before the previous stage runs out of voltage swing. When this happens, its output impedance is high, and so the loading on the previous stage is decreased and the gain into the PI increases.