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Low Voltage 6N6p-OTL by H. Ahammer - Page 3

post #31 of 89
Thread Starter 
#2 Thanks, I planned to.

#1 Yeah, I was (and still am) confused as all hell about those numbers. Sticking to your numbers, and a wide safety margin, I'll just go with this a 390R Ohmite 10W resistor from Mouser.

Will order it soon and get it over here.
post #32 of 89
Quote:
Originally Posted by nikongod View Post
Since nobody has come up with a better number, Im sticking to my previous one that the output tubes idle at 25mA/section, 50mA/tube. Both channels together for 100mA.
.

Yea, it wouldn't make sense for the 6n6p to be biased at 50mA per section, thats 25% of max dissapation.
post #33 of 89
Thread Starter 
I think I'll drop that voltage dropping resistor from 390R to 300R to get about 150V unloaded. The transformer I'm using is an Antek unit so it's 200V measured unloaded unlike traditional transformers which measure it loaded. I'm getting 188V unloaded before the resistor and a 300R will drop 39V (assuming all tubes are pulling 130mA combined).

On Doug's recommendation, I'm not going to use the 160V BlackGate output caps and will go with something higher rated.

It also means I'll have a pair of BGs to use for yet another project...

Those 4K 1/2W resistors I got are also getting replaced with 3.9K 2W to be safe.
post #34 of 89
Quote:
Since nobody has come up with a better number, Im sticking to my previous one that the output tubes idle at 25mA/section, 50mA/tube. Both channels together for 100mA.

My reverse engineering of the gain tubes puts them at 6mA/section, 12mA/bottle. Both together for 24mA.

All together for 124mA. At this current the single 390 ohm resistor dissipates about 5W, which requires a little planning to make work but is not impossible. Id buy some safety factor, but wire wound resistors are very easy to find in 10W or even larger power ratings.
Since I am the originator of the first schematic posted by FallenAngel I might as well throw in some of my thoughts... as stated the schematic is based on the Ahammer one and uses less expensive/ easier to find tubes.
The original article on Headwize goes through some of the calculations, but as Nikongod researched correctly I planned on running the tubes some hotter... especially the output 6N6P's. His numbers are quite correct actually...;-)

It should be easy to find aluminum cased WW's in 25W at your favourite sources... I got most of my stuff at RS Components in Germany.

I haven't built the schematic myself since I got "distracted" by a 2 month business trip to Tokyo and am planning a 6N6P-Pushpull amp using toroids as output transformers for my 300ohm cans...

Quote:
Those 4K 1/2W resistors I got are also getting replaced with 3.9K 2W to be safe.
The 4K resistors are specced as 1W which should be okay but 3.9K 2W is perfectly acceptable.

Good luck with the project!
post #35 of 89
Thread Starter 
Got this baby build and only scared the crap out of myself once - when I shorted the big power cap to the case.

Impressions:
1) It works and plays music
2) It has some serious transformer hum
3) It sounds distorted at the top and bottom end

Photos coming in a bit
post #36 of 89
Quote:
Originally Posted by nikongod View Post
160Volts is given at the plates of the gain tube
I just re-read this, and it dosnt make sense. 160V is given at the plate of the output tube... Sorry about that.


Good news it is running sort of

Post pics and someone will probably be able to guess where the hum comes from.
post #37 of 89
Thread Starter 
The layout is "interesting" to say the least! I have wired the "amplifier" part on the top of the chassis but the "power supply" is attached to the bottom. There is a high-voltage wire going from the bottom of the chassis to the top and the heaters are are twisted together and also running to each tube from the a pair of terminal blocks on the bottom.

Photos would be kind of hard to do, but I'll try. As for hum, I'm not positive where it comes from but I'd guess the layout, and the distortion, I'm clueless about.
post #38 of 89
I feel an "uggh" moment coming on. but bring it!

My guess so far is to say:
Wire everything on 1 side. you are making life HARD on yourself this way.

I would look at how the heaters are arranged as another thing.

Is it star grounded?
post #39 of 89

Impressions:
1) It works and plays music

 

That's good!


2) It has some serious transformer hum

 

Maybe coupling from the transformer into the signal when you have things on top of each other. Otherwise it is most likely a grounding issue (know from personal experience )


3) It sounds distorted at the top and bottom end

 

Have you checked ALL voltages? Like B+ but more important the voltages across the respective cathode resistors? Nikongod did some pretty good reverse engineering ... verify those voltages to make sure everything is biased up properly or you will be running your valves biased in areas they are not supposed to make music.

 

post #40 of 89
Thread Starter 

It's all packed up to be moved back to Canada with me.  I'm flying out in just 2 days!  It's going FedEx across (yes, I know, there will be hefty customs and brokerage fees but it's traceable and USPS won't take boxes this big anyway).  I should be able to look at it again in about 10 days.

 

B+ is 160V and heaters are a steady 6.2VAC.  I haven't checked bias voltages but it will be first to look at.  Ground looks surprisingly close to the schematic as the "grounds" on the "amp side" are actually all connected to a terminal strip and that terminal strip has a wire going to the star ground.

post #41 of 89
Thread Starter 

 

This project is back on the table! :) Troubleshooting discussion very likely to come shortly, as well as photos.

post #42 of 89

Thought I'd try this with 6N6Ps throughout, since I have 4 doing nothing. Gain of only 2.

 

 

 

Get a bit more gain with some tweaking...

 

 

Works so much better without the LTP, but of course you get an inversion...

 

 

 

w

post #43 of 89
Thread Starter 

Well, as I've had this on the workbench for some time and could not resolve the buzzing issue, I've completely disassembled the amp and am going to rebuild it.

 

The PSU will stay as is as I see nothing immediately wrong with it (aside from the choke and 5V transformer having ground wires attached close by instead of at the star ground (the tower looking thing in the middle, just before the blue/green AC heater wires).

 

 

I'll post build photos soon, and any suggestions for how to lay out this build would be much appreciated. I'm currently thinking of building the amp part P2P on the top side again, but as "clean" as possible. A single star ground on the top and a single star ground on the bottom, connected via 1 wire.

 

I am a little concerned about running the AC heaters so close to signal wires, but not sure how to avoid that at the moment.

 

Also I was reading that I should have connected pin 9 of the 6N6p tube to ground as it might create buzzing, so that's also planned.

post #44 of 89
Thread Starter 

Looking over the schematic, I'm not completely understanding the purpose of the 100uF / 400V caps on pin 7 of the E88CC to ground between B+ and Ground.

Do these need to be as close to the tubes as possible to make sure there's enough current available at all times or can these be a couple of inches away, closer to the star ground?

post #45 of 89
Thread Starter 

I've been busy today :)

 

 

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