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Low Voltage 6N6p-OTL by H. Ahammer

post #1 of 89
Thread Starter 
Hey guys,

So I started building one of these and so far have most of the PSU constructed.

There is a couple of concept here that I don't seem to grasp:

1) With the way the tube rectifier is wired, I'd be getting quite a bit of AC on the output - (I measured 76V)! How is this a good idea?

2) I looked at 5U4G application notes and they show using a 2x 2.5V transformer wired differently from the 5V transformer in the amp schematic. The scheme from the notes look kind of cool considering I'm actually taking the "middle points" of the 2 transformers - wondering if there's any benefit to this versus a single 5V winding.

Thanks
LL
post #2 of 89
Quote:
Originally Posted by FallenAngel View Post
1) With the way the tube rectifier is wired, I'd be getting quite a bit of AC on the output - (I measured 76V)! How is this a good idea?
Something's not right -- you should have ~200V DC depending on current draw, and ~2.5mV of ripple.

Quote:
Originally Posted by FallenAngel View Post
2) I looked at 5U4G application notes ... wondering if there's any benefit to this versus a single 5V winding.
In practical terms, not much. If you have a center tapped 5V winding, there is no harm, but don't go out of your way to find one.
LL
post #3 of 89
Thread Starter 
PSU isn't "done" yet, just measured output of the tube rectifier with no load. I have 188VDC output (Antek toroid at 2x200V so it's rated 200V unloaded) and 76V AC on the output. I'm sure ripple will go down after the caps and choke, just wandering how THAT much went through.
post #4 of 89
I wouldn't worry about it. It's a meaningless measurement. Do the same with a 24VAC transformer and some solid state diodes and see what it looks like.
post #5 of 89
I'm proberbly going to be shot down in flames but that circuit looks very different to the one on headwize

HeadWize - Project: A Low-Voltage Class-A Tube Headphone Amplifier by Helmut Ahammer

don't know if you've seen it

cheers
FRED
post #6 of 89
Thread Starter 
Doug - Thanks, I'll keep building the PSU, should have the rest of the parts needed tomorrow (and of course I forgot to order a 220uF cap. Have a couple of Rubycon 100uF/400V for the rest of the amp, but no 220uF).

Fred - Saw that one, the version I posted is the "newer" version using cheaper and easier findable tubes and an ever so slightly higher B+
post #7 of 89
I would bias the heaters above ground. It can help a lot with noise & hum.

The second one here:
post #8 of 89
Quote:
Originally Posted by nikongod View Post
I would bias the heaters above ground. It can help a lot with noise & hum.
Looks like the max k-/h+ voltage on the e88cc is 60 volts, and the max k/h on the 6n6p is 100 volts.

Based on experience, how large a bias would you use?

I guess if cost isn't an issue, the other approach would be to provide the 6n6p it's own heater supply.
post #9 of 89
I *assume* the cathodes of the output tubes are about 55-65V above ground. I tried to reverse engineer the exact operating points, but couldn't make it work. I probably forgot to take something into account.

Anyways, I would probably bias about 40V above ground but the exact value isnt too important. Anything from 20 to 60V should work nicely. 40V gives you a a lot of screw-up room on either side which is nice This keeps the heater voltages much higher than the cathodes of the gain tube, and does not come anywhere near testing the Vh-k limits of any of the tubes. Im not quite as concerned with noise pickup in the output stages: IME cathode followers run quietly with the heaters way below cathode voltage.
post #10 of 89
Thread Starter 
How would I do that with this amp design? The way the 6N6p heaters are wired, it's taking output below them. Yes, I'm very confused - solid state guy making his first foray into tubes.
post #11 of 89
The bias supply is usually made up with a resistor divider with a cap across the lower resistor. Click the photo above, it links to a page all about tube heaters.

I would put the bias supply right after the choke. It dosnt need super-clean voltage to work right.

The schematic you posted dosnt have bleeder resistors shown. The bias supply can be used to bleed the power supply caps.
post #12 of 89
Thread Starter 
Sounds good.

I'll add a 100R/5W bleeder after the 200uF cap in the HT supply.

For the heater bias supply, that's a low voltage cap right? Also, what values should the resistors be?
post #13 of 89
Quote:
Originally Posted by FallenAngel View Post
I'll add a 100R/5W bleeder after the 200uF cap in the HT supply.
You only want the bleeder to pull a few mA.
Quote:
For the heater bias supply, that's a low voltage cap right? Also, what values should the resistors be?
100V has a nice voltage overhead. Fairly low capacitance is adequate: 10-33uf.

Voltage divider calculator
post #14 of 89
Thread Starter 
100K bleeder should pass 1.6mA off 160V supply. Sounds reasonable.

For the bias supply, really not sure of resistor values - only hint in there is not to go above 100K for R2. The VD calculator doesn't help me much as I'm not sure what to look for.
post #15 of 89
Quote:
Originally Posted by FallenAngel View Post
100K bleeder should pass 1.6mA off 160V supply. Sounds reasonable.

For the bias supply, really not sure of resistor values - only hint in there is not to go above 100K for R2. The VD calculator doesn't help me much as I'm not sure what to look for.
It's just a voltage divider that sets the bias voltage.

Probably the best introduction to this topic that I've seen, is here:

The Valve Wizard
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