Low Voltage 6N6p-OTL by H. Ahammer

[FallenAngel]

You were asking about my transformer earlier, it's this Antek (totally cheap) - http://www.antekinc.com/as-1t200-100va-200v-transformer/
So we're expecting about 190VAC output at full load, rectified to around 270VDC, with the tube giving a 50V drop, that's an optimistic 220V before filtering.
 
I was not clear about your PSU description, what do you mean by "33R (at each diode plate)"
 
In terms of the full PSU, a diagram would really be appreciated, it's not super clear in words (and I'm a visual person, just don't tell anybody since people seem to enjoy my writing).
 
I'll try to upload a modified Aikido-like schematic soon with proposed values.

 

[00940]

Ok, here's a drawing. The 33r resistors are only needed if the secondaries' DCR are too low. If the 200V secondaries each measure above 33r (just a simple dmm check), the 33R resistors aren't necessary. See: http://www.freewebs.com/valvewizard/fullwave.html 
 

 

[FallenAngel]

Resistance between the secondaries is around 32R.
 
I already have a 100uF/400V cap for each channel as per amp schematic, should I replace it with a 220uF?
 
Come to think of it, could I just leave the PSU mostly intact except for the 33uF cap instead of the 3.3uF (probably will just add a 33uF cap in parallel, easier, and I have no other use for a 3.3uF huge film cap like that), and changing the 100uF cap in the PSU to a 220uF?
 
What's the purpose of the 4x15K 0.6W (60K / 3W) resistors?
 
The 100uF/100V cap is a complete mystery as well...

 

[00940]

  Resistance between the secondaries is around 32R.

 
If it is 32R from CT to each end, you can reduce the two 33r to 10R or 22R. If it is 32R from the extremities, keep 33R.
 

 I already have a 100uF/400V cap for each channel as per amp schematic, should I replace it with a 220uF?  
Come to think of it, could I just leave the PSU mostly intact except for the 33uF cap instead of the 3.3uF (probably will just add a 33uF cap in parallel, easier, and I have no other use for a 3.3uF huge film cap like that), and changing the 100uF cap in the PSU to a 220uF?

 
The caps values after the choke aren't extremely critical. With unregulated supplies filtered by LC or RC cells, you have a trade-off. It is good to have a final RC filter per channel to reduce crosstalk. But if you put too high a resistor and too low a cap, you actually increase the ripple (because of the varying current draw) compared to the ripple after the choke (it's an easy sim in ltspice if you wanna see it). So put a decent value cap after the choke (100uF is fine) and the biggest cap you can after the 220R resistors going to each amp.
 

 What's the purpose of the 4x15K 0.6W (60K / 3W) resistors?  
The 100uF/100V cap is a complete mystery as well...

 
With an aikido, the top tubes' cathodes are sitting at B+/2. To avoid stressing the tubes, it's best to bias the heaters somewhere around B+/4, so that the cathode to heaters voltages aren't too big. So the 15K resistors are a voltage divider (so you cannot use a single resistor but you could use 47K-15K) tied to the heaters CT (or fake CT with resistors). The 100uF cap in // filters that reference point. Its value isn't critical (voltage rating is).
 
It was already explained at length by Nikongod at the beginning of the thread.
 
Edit1: wrt the transformer: I see two 6.3VAC windings, no 5VAC for the rectifier ? Do you have resistors to drop the excess 1.3V ?
Edit2: I'm going to be a bit nasty: you could keep the tube rectifier for the nice look  but disconnect it and replace it below the hood by a pair of uf4007. Voila, instant 50V gain.

 

[FallenAngel]

It's 32R between the secondary to CT (well, since it's a dual winding transformer, it's for each secondary). I should have some 10R / 5W resistors around.
 
Looks like I'll keep PSU to 200uF (bypassed by 1uF) after the choke. I'll add a 100uF/100V cap between the heater supply resistors and ground.
 
Would it be fine to just put 2x 100uF/400V caps per channel after the resistors? Just so I can skip placing an order and use parts on hand.
 
Edit1: I have another 5V transformer for the rectifier. The 6.3V windings are AC heaters for the 4 signal tubes.
Edit2: Uhm... very evil

 

[00940]

Originally Posted by FallenAngel /img/forum/go_quote.gif
 
I'll add a 100uF/100V cap between the heater supply resistors and ground.

 
I just re-read the first posts. If you already had a 10uf to 33uf capacitor there, as suggested by Nikongod, it's enough, it's not very critical. The virtual CT of the heaters must be held at 40 to 60Vdc though, that's the critical point.
 

 Would it be fine to just put 2x 100uF/400V caps per channel after the resistors? Just so I can skip placing an order and use parts on hand.

 
Sure.
 
Quote:

 Edit1: I have another 5V transformer for the rectifier. The 6.3V windings are AC heaters for the 4 signal tubes.

 
Oh, ok, didn't catch that.

 

[FallenAngel]

How does this look?
 

 
 
 
Aikido schematic

 
EDIT: Updated Schematic after 00940 post

 

[00940]

Mostly fine (if a bit weirdly drawn  ). Two little things: the 100R resistor doesn't need to be 5W and the two 1meg resistors at the output should go before the output cap, not after.

 

[FallenAngel]

I just figured 100R/5W since it said it had to be rated for B+, if it's fine, I'll just use a 1/4W as the rest.
 
Updated the schematic. Not sure of the last bleeding resistor is necessary or even should be 1M, I see anywhere from 10K-1M here.

 

[00940]

No, it is said that the caps with * should be rated for b+. For resistors, the * means they have to be of "good quality". I'd rather use 0.6w resistors rather than 1/4w though, as suggested by Broskie.

 

[FallenAngel]

I usually use Vishay Dale RN60, they technically 1/4W, but I'm pretty sure they're actually 1/2W.
 
Just to confirm, what rating should the 100R resistors for the heaters be?
 
I'm probably going to a local electronics store tomorrow to pick up the parts I'm missing.

 

[FallenAngel]

The new amp is on the work bench and I'm building it this weekend.
 
Just a little question; can I make the assumption that anything connected via 1M can be safely connected to something going to that destination (Anode to B+ connects through 1M resistor to signal, I'm planning to just connect the 1M between the pins and run a wire to B+).

 

[FallenAngel]

Just built rebuilt the PSU by adding a 33uF cap before the choke in parallel with the 3.3uF, added the 2x 10R/5W resistors on the output of the transformer and wired up the 100uF/100V cap with 15K/45K resistor divider.
 
Testing just the PSU, unloaded, I got 274V DC before the choke and 273V DC after... which looks quite excessive.
 
The amp is ready to be plugged in and tested, just wanted to verify that this isn't a little crazy first.

 

[wakibaki]

This is typical of CLC filtered PSUs. A supply with capacitor input filter CL will have an output voltage of ~1.41*RMS voltage input for all capacitors over a certain size. A supply with choke input will have an output voltage of ~0.9*RMS voltage input. If a parallel cap is put in front of the choke, then as the capacitor size increases, so the output voltage increases asymptotically to 1.41*RMS input. This is sometimes used to control the output voltage.
 
These figures above are approximations, say nothing about the ripple, choke resistance, load.
 
w