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# Low Voltage 6N6p-OTL by H. Ahammer - Page 2

Well, as I'll likely have a single 100uF/400V cap left over after I use 3 in this amp, I'll stick that into the bias supply.

As for resistors to set bias voltage, what should the bias voltage be?

luvdunhill - I read that article a couple of times (it was actually posted by nikongod above ), still not sure what it should be.

AHA! 20V-90V. As I'm expecting should have ~160V after the choke, R1 of 100K and R2 of 30K should give me 37V bias. Sounds reasonable?

Quote:
 Originally Posted by FallenAngel AHA! 20V-90V. As I'm expecting should have ~160V after the choke, R1 of 100K and R2 of 30K should give me 37V bias. Sounds reasonable?
... okay, now compare this to the Vhk (max heater to cathode voltage)of the tube in question.
I guessed at where I thought the cathodes of the output tubes were in my above post. I spent some more time with the calculator reverse engineering the schematic. I misread the value of the cathode resistors as 30ohms earlier, they are in fact 38ohms. Everything makes much more sense now.

I got the following operating points.YMMV, etc.
The idle current of EACH section in the output stage is about 25mA, BOTH sections in parallel at 50mA.

The Cathodes of the output tubes are about 52V above ground at idle.

PS:
How do you make a "not equal to" symbol on these forums? I wanted to write 30 (not equal to) 38 in that above line for dramatic effect, but could not.
!= ORR <>
OK, I'm royally confused. Teach me oh mighty tube gods!
Something to look out for with tubes is the Heater-cathode voltage limit. It is basically the maximum voltage the tube can safely operate with between the heater and cathode. Depending on the tube it may be different positive and negative, so that should be noted. If the voltages are exceed you run the risk of the insulation (the heater is electrically insulated from the cathode) breaking down and the tube failing.

The issue is particularly problematic in DC coupled amps because the cathodes of the tubes may be at such significantly different voltages. The 2 common ways of solving the problem are avoidance and using several heater supplies.

It is not as difficult of a problem to solve in most cap coupled amps although they are not immune to this issue at all, so it is always worth checking that the voltages don't exceed the limits.

There is always a bit more to it... i would get a copy of "Valve Amplifiers" by Morgan Jones at the very least. You'l pick it up really quick.
Some people will claim Vhk numbers are merely guidelines and can be exceeded. You find this very often in commercial gear, by vendors of good report (RSA for example) and those of not-so-go report (Singlepower) I suspect in both cases (unfortunately), these people don't really understand the basics of what you're trying to understand, so kudos there.
Ok, next part of PSU is done - choke installed along with the 3.3uF and 100uF cap. After the those two, I get 188V DC and my auto-randing multimeter goes haywire trying to find AC. When I select range in 100V range, it shows about 3VAC, the small cheap DMM shows 605VAC
some cheap multimeters will struggle to read the ac on the high DC.
Thanks, that's probably it, it's a DVM850BL, bought off some surplus store for under \$20.

Did a little casework - the 4 small 9-pin tube sockets are installed and I've planned out most of the layout of components. Still need to drill a hole for the 5U4G, and it's larger than any stepper bit I've got, will likely have to take a wood bit to it. Either that or make the largest hole I can, then take some time with a hand knife. Still surprised me how steel cuts aluminum.
For large holes Greenlee punch are unbeatable.
Ok, so I have most of the parts - a couple of things i'm not clear on in the PSU portion.

1) The 390R resistor between the 100uF and 200uF (250uF in my case) capacitors. What rating should this resistor have? If I'm expecting roughly 180mA (based on 15mA Ia per 6922 tube and and 30mA Ia per 6N6P - I'm assuming Ia is measured by section, not for whole tube) going through it; following V=I*R to get voltage drop, that's almost 70V I'm dropping across that resistor! First of all, that's absolutely nuts as I only have 188V before it and would probably want the B+ to be closer to 150V (I have 160V BlackGate NH output caps I want to use). Second of all, I can't imagine a 70V voltage drop without burning a resistor.

2) The 4K 1W resistor following the 200uF cap. With my layout, it's actually easier to have a pair of these, one for each channel. Is there anything wrong with doing this and as the current through each should be halved, can I stick with a 1/2 watt resistor?

Quote:
 Originally Posted by FallenAngel Ok, so I have most of the parts - a couple of things i'm not clear on in the PSU portion. 1) The 390R resistor between the 100uF and 200uF (250uF in my case) capacitors. What rating should this resistor have? If I'm expecting roughly 180mA (based on 15mA Ia per 6922 tube and and 30mA Ia per 6N6P - I'm assuming Ia is measured by section, not for whole tube) going through it; following V=I*R to get voltage drop, that's almost 70V I'm dropping across that resistor! First of all, that's absolutely nuts as I only have 188V before it and would probably want the B+ to be closer to 150V (I have 160V BlackGate NH output caps I want to use). Second of all, I can't imagine a 70V voltage drop without burning a resistor. Thanks in advance.

I'm guessing he is using a separate PS for each channel, this would give you the roughly 35V drop. Also suspect this by judging the size of the transformer. Its common to do with unregulated tube power supplies to keep crosstalk out of the equation. Download PSUD from Duncan Amps it really helps when setting up a tube power supply.
I downloaded PSUD2, put the circuit in and got 70mA across R1. At about 190V that's should be a 13.3 Watt resistor Guess I'm going with a huge metal Ni Power resistor rated for 50W when bolted to the chassis.

As my PSU is shared, it's probably a bad idea to keep the 160R (100mA) choke as it would be running twice that current and replace it with a 90R (200mA) choke.

Judging by transformer is tough - 200mA 2x so it easily pulls 180mA total.
First off, work with the best numbers you have. If you have 2 or 3 solid numbers check them against each other to see if they make sense.

I think your estimate of current is too high. With such high currents the voltages around the PSU dont add up. 160Volts is given at the plates of the gain tube and the voltage at the first cap can be gotten from the data sheet for the rectifier. 160V+70V(voltage across the resistor)+28V(voltage across the choke@180mA) = 258V>>190V (5u4gb datasheet says 190V with 200vrms to each diode, cap input, 180ma.)

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.

160V+(390ohm*.124A)+(160ohm*.124A)=227V (5u4gb datasheet says 210V under conditions above BUT @124ma) Its not exactly what the 5u4gb data sheet says, but within a much smaller margin of error.

Regarding Question 2:
I would MAKE SURE to use 2 separate 4Kohm resistors each with their own cap on the gain side regardless of what you do for the power stage.
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