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Old 20th August 2006, 01:49 PM   #1
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Default How can i change bias 6h30pi tube

First i'm a beginner in the tube diy.
I bought a diy preamp ls-32 board with 2* 6h30pi tube's.
I'm verry happy with it but it can always better.
All over the forum i read you can change the bias for better results.
Can sombody help me how to change the bias.

Click the image to open in full size.

Thanks for helping in advance
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Old 20th August 2006, 02:34 PM   #2
SY is offline SY  United States
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Quote:
All over the forum i read you can change the bias for better results.
"All over the forum" you can read all kinds of silly stuff from self-proclaimed experts.

You can change the bias voltage (and hence idle current) by changing the cathode resistors R12, R13. The chances that this will improve anything are close to zero- presumably, the guy who engineered this picked a good operating point, and random component substitutions are unlikely to improve matters.

If there's anything that one could do to make an improvement without a total rebuild (and even this is not certain to), it would be to get the magic eye circuitry out of there by clipping out R9 and R10. It's possible that the nonlinear base impedance of the bipolar could cause a bit of distortion. But maybe not.
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Old 20th August 2006, 04:39 PM   #3
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Thanx for the answer.

took out the eye but there was no change there is however a minor buz.
Also the eye is already halfway with no signal there must be qait some signal to make it move do you now how to improve this?
eye looks need.
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Old 21st August 2006, 02:04 AM   #4
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Actually, the bias point looks a little on the hot side. Will be very linear and powerful. But I think the tube is gonna cook. This is outside of it's specification in terms of power dissipation. You will need really good cooling. You're on the edge here. I tried the same thing in my Cymbal but ran into thermal runaway and grid current problems.

Agree that removing the magic eye could be an improvement. Or at least add another R10 to the other channel, so that they are summed. This would make added distortion the same for both. Try also making R10 a larger value. As high as you can get away with.

You can also improve the sound by removing C5. I think it will be quite a noticable change. Gain will be cut in half, so you might need to double the resistance of R4R5 for the same output level. Also, make these 5W resistors.

If you want a milder, warmer sound, I change R12 to 470 or even 680 ohms. Such will also solve the thermal issue.

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Old 21st August 2006, 02:07 AM   #5
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Oh yeah, make C3 about 5x to 10x larger. You can also try a 0.1uF to 1uF across the heater lines. Take the spare 0.15uF and put it across C1.

Oops - I misread the size of R4R5. 10W size is fine. I thought it said 2W.

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Old 21st August 2006, 08:28 AM   #6
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Originally posted by hagtech
Oh yeah, make C3 about 5x to 10x larger. You can also try a 0.1uF to 1uF across the heater lines. Take the spare 0.15uF and put it across C1.

Oops - I misread the size of R4R5. 10W size is fine. I thought it said 2W.

jh

Thanx, tonight I'll try your sugestions and let you now, the output signal is very high so maybe I solve that problem to by removing the C.
For the eye, do you now if there is a way to use an optocoupler to send the eye?
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Old 21st August 2006, 01:37 PM   #7
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To pick up on what SY and Hagtech said about the magic eye: If you really must have the eye (they ARE retro cool), what you need is a buffer that doesn't put a non-linear load on the output of your preamp. An optocoupler probably would be a troublesome load. Try replacing Q1 and its associated parts with something like a 12AX7 stage, since you’ve already gone tubular. Find a 12AX7 of undistinguished heritage to use here; no since wasting something expensive on an eye tube buffer. Increase R10 to 100K or even higher; the higher the better so long as the eye still does its thing. Connect another “R10” to the other channel to sum both channels together. Connect the 12AX7 grid directly to the two R10s without C10. The two channels’ 470K output resistors will serve as grid resistors. I’ll leave the 12AX7’s plate and cathode resistors up to you, but you may need to bypass the cathode resistor to get enough gain to drive the eye. If you are truly adventurous, you could use both halves of the 12AX7, each as a buffer for its own channel, with output summing resistors connected to the 12AX7 plates, the summing junction connected to C7.
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Old 21st August 2006, 02:44 PM   #8
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Originally posted by Brian Beck
To pick up on what SY and Hagtech said about the magic eye: If you really must have the eye (they ARE retro cool), what you need is a buffer that doesn't put a non-linear load on the output of your preamp. An optocoupler probably would be a troublesome load. Try replacing Q1 and its associated parts with something like a 12AX7 stage, since you’ve already gone tubular. Find a 12AX7 of undistinguished heritage to use here; no since wasting something expensive on an eye tube buffer. Increase R10 to 100K or even higher; the higher the better so long as the eye still does its thing. Connect another “R10” to the other channel to sum both channels together. Connect the 12AX7 grid directly to the two R10s without C10. The two channels’ 470K output resistors will serve as grid resistors. I’ll leave the 12AX7’s plate and cathode resistors up to you, but you may need to bypass the cathode resistor to get enough gain to drive the eye. If you are truly adventurous, you could use both halves of the 12AX7, each as a buffer for its own channel, with output summing resistors connected to the 12AX7 plates, the summing junction connected to C7.
A lot of info but very welcome, as a beginner I can't have anough input. I don't now when I'm going to experiment but I will sertenly try.
Thanx
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Old 23rd August 2006, 08:15 PM   #9
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Originally posted by wrangler



Thanx, tonight I'll try your sugestions and let you now, the output signal is very high so maybe I solve that problem to by removing the C.
For the eye, do you now if there is a way to use an optocoupler to send the eye?

Oke, changed r12 en r13 en seems better also took out c6 and c5 took sound better but i wonder what purpus do they have? Output level is much better not to high as it was. And what happens when i change the c3 and c4?
Put in a riaa correction preamp for the PU but it is pikking up a nasty buz, still working on that (based on a tl 072 for now)
Anyone any sugestions.

Thanx
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