HA-2 Wheatfield headphone amp query

[Brit01]

Have a quick question regarding this design.

Does the cathode resistor of the 6AS7 CF output stage need to be bypassed?

I assume yes as it shows 2 links to ground off the cathode resistor of 2K 20W.


http://www.pmillett.com/Wheatfield/manual.pdf

 

[DougL]

No,

Bypassing that resistor would effectively ground the output for AC.
The grounds shown are probably unused pins on something.

HTH

Doug

 

[leadbelly]

That resistor is labelled as a TO-220 device, so I guess there are probably 4 pads on the PCB for it, and the middle 2 should be grounded.

 

[Brit01]

Thks Doug. That makes sense.

 

[whitelabrat]

no bypassing on that 20w 2k resistor. i used a 25w, thermal grease, and an el grande heat sink. that sink gets hot too, but not painful.

i'm having a real bear of a time hunting out some bad hum on mine. i'd suggest avoiding the 200 series hammond transformers like the plague. mine has some serious mechanical noise.

 

[Brit01]

i used a 25w, thermal grease, and an el grande heat sink. that sink gets hot too, but not painful.

Thks for the heads up. I've got a couple of 2.2K 20Watts I was going to try. Maybe attach them to a heatsink I've got hanging around.

 

[Brit01]

i'm having a real bear of a time hunting out some bad hum on mine. i'd suggest avoiding the 200 series hammond transformers like the plague. mine has some serious mechanical noise.

What current are you running your tubes on? I was hoping I could use a little less B+.
I plan to use 6080's.

I see he's using about 95 mA.

 

[Brit01]

I could probably get about 240/250 B+ out of my transformer with max 200mA.


psu (275V secondary, 52ohms): (CRCRC)

100uf
225 ohms
470uf
225ohms
470uf

 

[leadbelly]

100 uF is too big for a tube rectifier.

 

[Brit01]

Sorry leadbelly forgot to mention I'm going to use a SS power supply.

And yes you are very right. I use 0.47uF in my tube rectifiers.

 

[whitelabrat]

I get nearly 300v on the B+, but I had to use a 5ar4 to get there. My 6H 120 ohm choke has more resistance than the Hammond 193H that I'm guessing is used in the real thing so I needed a bit of help.

I've observed that keeping the voltage up is important to sound quality. As B+ goes down, distortion goes up according to my ears. 250v doesn't seem to be enough.

I don't think I've taken any measurements other than B+

You may be able to parallel the 20W with a 22k 5W wire wound?

n_maher's "The Menace" is a sort of perfection of the HA-2 in my opinion.

 

[whitelabrat]

Using the solid state rectifier is a good move to get the B+ up, but you may have to tolerate some switching noise. In that case 100uF would be fine as the first cap. I use 33uF with the 5AR4.

 

[Brit01]

You may be able to parallel the 20W with a 22k 5W wire wound?

Thought about that. Think the closest I've got is 15k AT 5w

Not so easy to get parts down here in Uruguay. Have to wait weeks for anything to get delivered from Europe.

As B+ goes down, distortion goes up according to my ears. 250v doesn't seem to be enough.

That's a pity. Don't fancy forking out for a new piece of iron at the moment.


What would be the lowest resistors in the psu I could use at 5 Watts? I could parallel some I guess.
Let me try the psu2 again.

update: 100 ohm resistors in the psu get me up to 290 V B+

 

[whitelabrat]

Be aware that your transformer should be able to put out at least 180ma at 300v...

 

[Brit01]

Be aware that your transformer should be able to put out at least 180ma at 300v...

yep..I'll be pushing my transformer to its limits with this one.
I have a custom built rated at 200mA.

Trying to get some quotes for new ones at 400-500mA.

 

[Brit01]

You may be able to parallel the 20W with a 22k 5W wire wound?

Hit a spot of luck. My boss is visiting from Dallas in 2 weeks.

So she'll be bringing a package for me with lots of goodies including 2 x 25w aluminum resistors.