Direct coupled screen drive amp...

[Stuart Polansky]

Ray, take a look at those resistor wattage! That 3.4k is 25 watts! Some if the others are 5 and 10 watts. Physical size and heat dissipation will preclude PCB, perfect or turret board mounting.

Stuart

[SoundMods]

Ray, take a look at those resistor wattage! That 3.4k is 25 watts! Some if the others are 5 and 10 watts. Physical size and heat dissipation will preclude PCB, perfect or turret board mounting.

Stuart

Also -- oversized resistors will minimize hysteresis issues -- important for good sound reproduction.

[ratbagp]

Ray, take a look at those resistor wattage! That 3.4k is 25 watts! Some if the others are 5 and 10 watts. Physical size and heat dissipation will preclude PCB, perfect or turret board mounting.

Stuart

I am not planning to use a PCB. I just like to use PCBExpress to plan a layout since I tend to lose layouts drawn on paper.

Yes, the 25 watt resistor will be a problem. I have a couple of 25 watt 5K resistors and I will initially put them in parallel with a series combo of a 10 watt 10K and some other power resistors to make the required 3K4. The combo needs to be 10K7. If the amp works and is a keeper, then I can order the single resistor from Mouser in my next order. Or replace the 3.4K resistor by an LM17 in series with a resistor as shown on Alex Kitic's site.

http://rh-amps.blogspot.com/2018/10/rh6080he-headphones-edition-its-been.html

And yes, I will need to think about the heat issues. By the way, each tray in the trolley has a perforated bottom so ventilation will not be a problem.

ray

[Roscoe Primrose]

Ray, take a look at those resistor wattage! That 3.4k is 25 watts! Some if the others are 5 and 10 watts. Physical size and heat dissipation will preclude PCB, perfect or turret board mounting.

Stuart

I am not planning to use a PCB. I just like to use PCBExpress to plan a layout since I tend to lose layouts drawn on paper.

Yes, the 25 watt resistor will be a problem. I have a couple of 25 watt 5K resistors and I will initially put them in parallel with a series combo of a 10 watt 10K and some other power resistors to make the required 3K4. The combo needs to be 10K7. If the amp works and is a keeper, then I can order the single resistor from Mouser in my next order. Or replace the 3.4K resistor by an LM17 in series with a resistor as shown on Alex Kitic's site.

http://rh-amps.blogspot.com/2018/10/rh6080he-headphones-edition-its-been.html

And yes, I will need to think about the heat issues. By the way, each tray in the trolley has a perforated bottom so ventilation will not be a problem.

ray

FYI, the 3.4k resistor in mine measures at more like 3.7k. It's actually 5 680ohm 10W resistors in series. The best part for the job is the one you already have, right? I've got stuff coming tomorrow to finish the second one and play a little with optimizing the values. Stay tuned...

Roscoe

[Roscoe Primrose]

OK, played a little bit with some of the values....

The amp may be a little easier to get biased properly if you either drop the B+ to 400V, or increase the output tube cathode bias resistor to 220-250ohms. My B+ is actually about 420, and the current is a little high (90mA) with 220ohms.... Of course, some of that is due to a change I made in the driver plate resistor, which is now 3k3....

Changed both cathode bypass caps to 820uF/35V.

Changed the filter cap on the driver screen grid to a 4.7uF/450V film cap.

As far as grounding the cathode and using a variable negative control grid voltage to set the bias current, I think that's a bad idea in this circuit. With no cathode resistor, there's nothing to stabilize the current in the output stage if the driver stage drifts a bit. That approach might be OK in a cap coupled or IT coupled amp where the output tube is living in a vacuum from a DC perspective, but not viable in a DC amp... And no, I don't have any interest in some kind of servo setup to control the negative voltage on the control grid. Gross violation of the KISS principle.

Roscoe

[ratbagp]

Thanks for the tips and the update. I still plan to build it.

It will be January 8 before my stuff arrives. There was an outbreak of the virus in Sydney and the border between NSW and Queensland was closed. Fortunately, my daughter was able to fly up from Sydney and we have just opened our Xmas presents.

And best wishes to all of you. Hopefully, next year will be a lot better.

ray

[SoundMods]

Fortunately, my daughter was able to fly up from Sydney and we have just opened our Xmas presents.

And best wishes to all of you. Hopefully, next year will be a lot better.

ray

Australia doesn't have a boxing day like the Uk or Canada?

[ratbagp]

Yes, we have Boxing Day and in reality, most businesses are closed from Xmas to the New Year or longer. Imagine Xmas day, July 4 and summer all rolled into one gigantic holiday period. The second cricket test match with India starts tomorrow in Melbourne and there is a good chance it will last the full five days.

We will start our Xmas dinner with shrimp and scallops taken from the bay right outside our window. Then we will have what we call a lamb blob followed by pavlova topped with local tropical fruit.

This was breakfast. Decadence in Excelsis Deo!



ray

[ratbagp]

Roscoe, could you check this out? I can play with R4 as you suggest.



Playing with PSUD I think I can get pretty close to 400 vdc.

ray

[SoundMods]

Yes, we have Boxing Day and in reality, most businesses are closed from Xmas to the New Year or longer. Imagine Xmas day, July 4 and summer all rolled into one gigantic holiday period. The second cricket test match with India starts tomorrow in Melbourne and there is a good chance it will last the full five days.

We will start our Xmas dinner with shrimp and scallops taken from the bay right outside our window. Then we will have what we call a lamb blob followed by pavlova topped with local tropical fruit.

This was breakfast. Decadence in Excelsis Deo!



ray

I'm jealous!!!!