In which class works the Hiwatt 400W ?

[kagliostro]

I would like to have explanation about the use of the ecc83 tube (cathode connected) on the Hiwatt 400W between the PI and the Power Tubes and about the operation class of such a Power Section

Many yeard ago PRR explained something to me but my brain don't want to collaborate

Many Thanks

Franco

[pdf64]

I think it’ll just be operating in regular AB1, same as 99.99% of other push pull valve guitar amps.
It can’t operate in AB2 because the 22k grid stoppers (and inability of a ECC83CF to deliver much current) will cause distortion when the signal level + bias tries to pull the control grid voltage above the cathode voltage.
ie the control grids can’t be driven positive of their cathode, which is the prerequisite for AB2.   

The CF is used to provide high undistorted signal level across the 100k grid leaks (which also serve as the CF cathode resistor).

[PRR]

> to provide high undistorted signal level

And not go into grid-blocking.

[kagliostro]

Many Thanks Friends

A question

If instead of a 12ax7 triode the CF use a 12au7 something will change (if necessary the components around CF will be modified) ?

Franco

[cfortner]

Most DR405 use 12AT7/ECC81. In my clone it works fine, the amp delivers little more than 300W.
Due to the grid stopper of 22k grid current isn‘t possible, with 2,2k little more power, but much more danger to the KT88.

[cfortner]

Update:

Meanwhile I built a DR405-Clone with 2,2k and Va 850V, it delivers stable 400W in AB2-operation.

Elder DR405 had 790V on the plates, that may be the reason for 400W.

Edit: With 2 ECC81

[pdf64]

Update:

Meanwhile I built a DR405-Clone with 2,2k and Va 850V, it delivers stable 400W in AB2-operation.

Elder DR405 had 790V on the plates, that may be the reason for 400W.

Edit: With 2 ECC81

Have you verified the AB2 operation, eg scoped that the driver is pulling the control grid voltage above 0V on signal upper peaks?

[PRR]

Have you verified the AB2 operation,....

Do we even care?

1) The driver and coupling network (in the plan in the top post) are able to go several mA into grid current, although the 22k prevents big grid current

2) At 400 Watts, who can hear slight distortion if any?

[pdf64]

Have you verified the AB2 operation,....

Do we even care? … At 400 Watts, who can hear slight distortion if any?

The increased power output, and the proposed mechanism by which it was achieved, was seen to be worth posting, reading and responding to, so I suppose we do seem to care, even though the dB change makes it something only the golden eared might notice

1) The driver and coupling network (in the plan in the top post) are able to go several mA into grid current, although the 22k prevents big grid current

So the signal wave at the grid would be kinda clipped off at the top, hence maintaining linearity into grid current operation seems unlikely?

[HotBluePlates]

1) The driver and coupling network (in the plan in the top post) are able to go several mA into grid current, although the 22k prevents big grid current

So the signal wave at the grid would be kinda clipped off at the top, hence maintaining linearity into grid current operation seems unlikely?

I will forecast that PRR will remark that "linearity into grid current" isn't the goal as much as "graceful recovery from brief overdrive."

Mainly because IMO any "Class AB2 amp" that isn't using a power tube & a driver transformer isn't actually serious about driving power into a positive-going grid.

[cfortner]

I measured the voltage over the 2,2k (not 22k!) grid resistor while increasing the audio level.

Above 300W the 2,2k voltage increased from 0 to little more than 20V, that means 10mA at 800V for each bottle. I think, thats AB2.