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Old 25th June 2010, 11:25 PM   #541
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Quote:
Originally Posted by kevinkr View Post
Sorry, I seem to have muddied the waters a bit.. Yes, I am strongly in favor of absolutely conventional (fixed) grid bias with the 26 and all other DHTs, and use it extensively in my own designs. IMLE, yes grid bias is superior to filament based bias and I strongly encourage you to implement.. Easiest is a a pair of 9V batteries - one for each channel will last many years..
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Old 26th June 2010, 06:12 AM   #542
Richard is offline Richard  Australia
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Hi Kevin,

Thank you for your extensive reply. Your article on the 26 preamp still serves as my reference in addition to this thread. I値l save for the Lundahls since I wanted to use them for a 10Y pre. I have a pair of Hashimoto HL 20K that are just yearning to jump into the 26 pre. I値l get around to it one of these days.

Andy, I have to agree with Kevin and Coolzero in that battery grid bias is very good. I知 using a 133nF Russian SSG slver mica cap for the input. They are very transparent.

Coolzero, you like to try choke input on the filament supply. For me it gave a better sound and ripple is much lower with it. You may also like to put snubbing 220nF caps on either side of the choke, join them together and ground that connection. I still have a slight hum, but than may be due to the grounding in the preamp chassis itself.


Rich
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Old 26th June 2010, 06:14 AM   #543
barossi is offline barossi  Germany
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Originally Posted by revintage View Post
The only problem is that we canエt choose battery voltage so easy as the Lithiums are 3V.
Hi,

what about a mixture of ~2*3V Lithiums as Grid bias and 2 Ohms (2Ohm *1 Ampere=2W!!!) filament bias, to reach ~ -8V Ug?

KR Hauke
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Old 26th June 2010, 06:25 AM   #544
Richard is offline Richard  Australia
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Regarding grid bias with an input transformer, normally it’s applied it to one terminal of the secondary with the other terminal going to the tube’s grid, but I’m not so sure with a TVC. Can one just apply the grid voltage as per the diagram below? Or would the variation in the resistance of secondary windings cause a problem?

Rich
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File Type: jpg Grid bias with TVC.jpg (21.3 KB, 647 views)
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Old 26th June 2010, 07:39 AM   #545
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Barossi -

Great idea - combination bias. I'd forgotten about that. With 150v on the plate, this would give a little more flexibility with bias. Say 9v from a battery and 1 or 2v on the cathode. I'll have to try this!

Andy
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Old 26th June 2010, 07:44 AM   #546
barossi is offline barossi  Germany
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Originally Posted by andyjevans View Post
Barossi -

Great idea - combination bias. I'd forgotten about that. With 150v on the plate, this would give a little more flexibility with bias. Say 9v from a battery and 1 or 2v on the cathode. I'll have to try this!

Andy
Hi Andy,

please share your experiences then :-)

KR Hauke
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Old 26th June 2010, 08:20 AM   #547
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Quote:
Originally Posted by Richard View Post
Hi Kevin,

Thank you for your extensive reply. Your article on the 26 preamp still serves as my reference in addition to this thread. I値l save for the Lundahls since I wanted to use them for a 10Y pre. I have a pair of Hashimoto HL 20K that are just yearning to jump into the 26 pre. I値l get around to it one of these days.

Andy, I have to agree with Kevin and Coolzero in that battery grid bias is very good. I知 using a 133nF Russian SSG slver mica cap for the input. They are very transparent.

Coolzero, you like to try choke input on the filament supply. For me it gave a better sound and ripple is much lower with it. You may also like to put snubbing 220nF caps on either side of the choke, join them together and ground that connection. I still have a slight hum, but than may be due to the grounding in the preamp chassis itself.


Rich
Rich, what value that the chokes you used for filament supply. I'd like to try it if I could find the required components here. Presently with the current filament supply, there is no hum (no hum at all) and the sound is exceptionally good. It is a simple LM317 voltage & current regulator together.
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Old 26th June 2010, 05:11 PM   #548
Richard is offline Richard  Australia
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Coolzero,

They're 60mH 2A chokes I had custom wound, similar ones from 30mH upwards should be fine, but more inductance would be better. There are some similar spec'd Hammond chokes that are reasonably priced.

Rich
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Old 26th June 2010, 07:35 PM   #549
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Thank you Andy and all for your hard work. May I suggest that the difference between filament bias and battery bias on the grid is slight compared to the improvement of replacing the potentiometer (or transformer) volume control with optocouplers.

Another option in regard to bias supply is photovoltaic cells. ie... SOLAR CELL, 60MM X 60MM X2MM | AllElectronics.com

For linestage, I ended with the 210 tube with filament bias. Perhaps I should have spent more time with the 26? I am still playing with solar cells for bias in my phono amp, but I'd like to let you know how much I am enjoying this thread!
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Old 27th June 2010, 01:05 PM   #550
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Well, more interesting stuff. I've been using the Hammond 126C interstage which is really on the limits for a 26 - hasn't really got the inductance at 106H. I haven't got round to trying the 26 with a Lundahl LL1660, which I must do, but in the meantime I tried out the 10y with the 126C. Took me a few hours to get all the voltages right, since I'm still using a 150v glow tube rated 30mA and it's all pretty marginal, since I only have 15.5v on the filament supply voltage. After a lot of juggling I got some operating points that worked. I have 150v on the anode, 4.7v on the cathode and a 4.2 ohm cathode resistor. That should be about 12ma through the 10Y. That's about right since the 126c is rated for 15mA. Ideally I'd like some better operating points. What I have in mind is 210v on the anode, 10 ohm cathode resistor, 12mA, 12.5v on the cathode and 20v supply voltage. Not difficult at all - just have to change the filament transformer and use different glow tubes like 2 of 0C3.
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