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Operating points for 6h30s

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Tjj226

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Joined 2012
#1
So I am in the middle of building a 6h30/300b amp and I have a couple questions about choosing operating points for the 6h30.

I am basically trying to make a toned down and simplified version of the Murkur V1 amp here

http://www.4tubes.com/SCHEMATICS/LUNDAHL-TRANSFORMERS/!LEVEN-Germany/300B-SE/MERKUR-V1_complete.pdf

The power supply will be a hammond 300BX transformer into a GZ37 into a 47uf input capacitor with a 10henry choke followed by a 100uf cap. Super simple design. Assuming I have a 200ma load on the PSU, it should give me a B+ of around 435v.

Now from what I understand from other threads, the 6h30 likes to be at or above 20ma of current. So for the first half of the 6h30, I am looking at doing another RC filter stage to bring down the supply voltage to 180v and then us a 4.5K plate resistor. That seems to give me a nice linear load line between 0-4v.

Next up is the second half of the 6h30 which needs to swing the 110+ volts for the 300b.

This is where I get confused and doubt myself. The merkur amp above supplies 440v with a 12.2K plate load resistor which gives a nice linear line, but it lists the bias voltage at 48.5v? I feel like I am missing something.

Any feedback would be appreciated.
 
#3
Every download page ends with a 404 error : (

The irony is that the picture of the load lines on the page you linked was one of the potential load line options I already came up with.

350v B+, 10K plate load, and bias the tube at the 8v grid line.
 
#4
#5
I can get 100v p-p out of 6922 driving 20k load with 140v B+. my amp needs 40v p-p to reach 75 watts rms output.
in order to maximize your output you need to have 55% B+ across the tube.
6H30 gives way more current then nescessary for this application
 
#7
The thing you may be missing is the first and second stages are direct coupled and the bias is determined from the difference in voltage between cathode and grid. The schematic shows -7.5V difference (41V - 48.5V)

I graphed a quick load line for that second stage. I think there's a typo on labeling the cathode voltage. I think it should be 43.5V, not 48.5V. Sure looks like to me there should be -2.5V difference between cathode and grid for the load line to pass through the quiescent point.
 
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#8
Tjj226 said:
Now from what I understand from other threads, the 6h30 likes to be at or above 20ma of current.
Click to expand...
Who told you that? The 6H30 can work nicely in a wide range of its curves, including at low voltage low current and it will still be quite linear.
In fact the measurements of THD of that amplifier tell me that the input+driver deliver very low THD and the overall THD is mostly coming from the 300B with the typical extremely dominant 2H. I would have used different driver to cancel some 2H, or added some feedback or used different transformer and working condition (will less power output) but that's personal choice...

Tjj226 said:
This is where I get confused and doubt myself. The merkur amp above supplies 440v with a 12.2K plate load resistor which gives a nice linear line, but it lists the bias voltage at 48.5v? I feel like I am missing something.

Any feedback would be appreciated.
Click to expand...

The second stage is DC coupled so the cathode voltage of the 2nd tube is the plate voltage of the first + the bias voltage.
 
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#9
I had read a few different posts on this forum saying that the tube should probably have a higher current for bias such as this one here. Favorite 6H30 Operating Points

The operating points for the 300b will be different. I am looking at a few options for transformers for it, but I am probably going to go with the good old sowter SA08s. If push comes to shove, I have some james transformers that will work, and then I could go between the 3.5 and 5K primary taps to see which one sound better.

And lastly, thank you and Kward. I usually avoid 3 stage amps like this, so I never have had to bother with DC coupling. This build is for a friend and he wants to use 6h30s for some reason despite the face that I have some perfectly good 5842s that we could use 😛
 
#10
Tjj226 said:
I had read a few different posts on this forum saying that the tube should probably have a higher current for bias such as this one here. Favorite 6H30 Operating Points
Click to expand...
The final answer in your link says "it depends"! What is the task of the input tube? What is the task of the second tube?
The second tube looks like it is working with about 20 mA current. Does the input tube need to do the same? The answer is no, it only needs to deliver a couple of volts.

Tjj226 said:
And lastly, thank you and Kward. I usually avoid 3 stage amps like this, so I never have had to bother with DC coupling. This build is for a friend and he wants to use 6h30s for some reason despite the face that I have some perfectly good 5842s that we could use 😛
Click to expand...

If you make your amp similar you won't need a preamp. If you make a 2-stage amp with the 6H30 you will need a preamp so it's always 3 stages at least....
A single stage with higher gain tube might be enough. A 5842 has a mu of 43. If you need to swing 90-100V peak the with a max gain of 43 you will need 2-2.5 Vrms input. However it's a less linear tube. Likely not linear enough for this task with a simple anode resistor unless you want to burn a lot more supply power which might even be impossible to do for practical reasons. You need to think about CCS, gyrator etc....
I would not change the 6h30 and the design of the input stage.

The 300B seems to be loaded with 3K. This gives the typical second harmonic largely dominant THD. It's not bad at all but it could be less at low level. Say 0.2-0.3% @1W, for example. In this case I would use some feedback if I wanted all that power.
 
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#11
Awesome, thanks so much.

I am gonna bread board the amp before putting it in its case. If the distortion is too much of an issue, I can fix that latter. Not too concerned about it for the moment.

I just want to get the thing up and running so that I can see what the thing sounds like and make adjustments from there.
 
#12
By the way, with a 180V supply on the first stage, you'd need a 16K plate resistor. That will set that stage just right to bias the second stage correctly at 41V on its grid.

The bias point on the second stage will allow a voltage swing of about 140v peak to peak to drive the 300B.

Given the raill voltage you quoted for the second stage, the plate and cathode resistors on the second stage need a bit of tweaking. I did the math last night and don't quite remember the values right now though.

It looks like it will work with those tweaks given your rail voltages quoted.

The Sovtek 6h30 is a decent sounding tube, nice and quiet.
 
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#13
I see a 10W resistor with metalcasing used as anoderesistor. This resistor should better be replaced with a wirewound resistor more suitable for this application, like a decent built enammelled one with brazed contacts, not crimped ones.
 
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