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#26 pre amp
#26 pre amp
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Old 17th August 2010, 07:32 PM   #591
nicoch58 is offline nicoch58  Europe
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Nice Andy !

you know ,I will follow you in your way ,but och a new kg of iron
l-c-l .....thomas is great...


Hope Rod reg sound the same

Last edited by nicoch58; 17th August 2010 at 07:35 PM.
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Old 17th August 2010, 08:19 PM   #592
andyjevans is offline andyjevans  United Kingdom
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Try this - unfortunately when I upload it changes landscape to portrait:
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File Type: jpg 10Y Preamp-1002a.jpg (423.9 KB, 1205 views)
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Old 17th August 2010, 08:25 PM   #593
andyjevans is offline andyjevans  United Kingdom
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Layout
There are three boxes in the preamp:
1) Signal chassis
2) Power Supply chassis for HT
3) Filament supply chassis

You may wish to combine one or two of these.

Here's a first draft - please let me know if there are any obvious mistakes! I haven't actually tried this out - my version has 280mH chokes. I'm just assuming that 60mH chokes will work in this. I have some to try out but haven't done so yet. So the voltages in the filament supply are theoretical - untested in a working model, but should be pretty close. All the iron is Hammond and very good value. The 126C OPT is really good - bifilar wound and 10K. Don't underestimate it!! It likes the 10Y too.

Filament resistor is 10 ohms wirewound and as good quality as possible. I tried several types and they all sounded different. Avoid the alu clad and the thick film. Vitreous enamel are good, but best I tried were some obsolete surplus wirewounds. A 40W part here runs at 130 degrees C. And that's above the chassis in free ambient air. So I wouldn't use anything less than 40W, and more if possible (and definitely if going in the chassis)

andy
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Old 17th August 2010, 08:44 PM   #594
andyjevans is offline andyjevans  United Kingdom
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Just a note about using a 26 instead of a 10Y, since this is a 26 preamp thread. Mine just morphed into a 10Y along the way because it suited the Hammond 126C. You could do a 26 version with a Lundahl LL1660/5mA instead. My rough calculation is that you could run it at 150v on the plate with a single glow tube, and 4.5mA. This would require 13.5v filament supply after the last choke and a 12 ohm cathode resistor, which could be 30W. Bias would be 12v at top of the cathode resistor.

andy
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Old 17th August 2010, 09:02 PM   #595
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Not surprising that the last Choke makes a big difference, since without it you have LOW ac-impedance between the ends of the filament. This always sounds bad, since the flow of cathode-current (through the different parts of the cathode/filament) is changed by low-Z. It 'clamps' the charge movement within the filament.

In my regulator, the choke-equivalent is the Gyrator (for the first choke) and the CCS section is the equivalent of the last choke (high ac-impedance across filament ends).

With small changes, my PCB can drive filament bias circuits in an equivalent way - so it would be very interesting to compare.

Finally, please don't assume that a 60mH choke will have the same effect as a 280mH - the ac-impedance is about 380R at 1kHz for 60mH vs. 1.8K for 280mH.
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Old 17th August 2010, 09:11 PM   #596
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Here's a photo of the 26 (1.5V 1.05A) and 01A(5V 250mA) switchable filament driver. Just supply a raw dc of about 10V, and the board will heat 250mA filaments. Insert the PC-Motherboard Jumper link (in the centre), and the rated current rises to 1.05A for a 26. The extra large resistor soaks up the difference in voltages, instead of the transistor/heatsink - so these run cool in either mode.

In both modes, the 25-turn trimmer can accurately adjust filament current.

Special design for Cheung, these will be ready soon.
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Old 17th August 2010, 09:16 PM   #597
andyjevans is offline andyjevans  United Kingdom
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Hi Rod,

I should make it clear that when I chucked out the voltages regs they were simple LM1084s, not your circuit which I know from experience is much better! I haven't tried your circuit with filament bias.

Thanks for the warning about the smaller chokes. I'll try them out when I can and report back.

andy
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Old 17th August 2010, 09:43 PM   #598
kevinkr is offline kevinkr  United States
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#26 pre amp
Quote:
Originally Posted by Rod Coleman View Post
Here's a photo of the 26 (1.5V 1.05A) and 01A(5V 250mA) switchable filament driver. Just supply a raw dc of about 10V, and the board will heat 250mA filaments. Insert the PC-Motherboard Jumper link (in the centre), and the rated current rises to 1.05A for a 26. The extra large resistor soaks up the difference in voltages, instead of the transistor/heatsink - so these run cool in either mode.

In both modes, the 25-turn trimmer can accurately adjust filament current.

Special design for Cheung, these will be ready soon.
Rod, please let me know too as these are of special interest to me too! (26/12/01 usage)
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Old 18th August 2010, 03:06 AM   #599
greenvalve is offline greenvalve  United States
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Quote:
Originally Posted by andyjevans View Post
...
Here's a first draft - please let me know if there are any obvious mistakes! I haven't actually tried this out - my version has 280mH chokes. I'm just assuming that 60mH chokes will work in this. I have some to try out but haven't done so yet. So the voltages in the filament supply are theoretical - untested in a working model, but should be pretty close.... Filament resistor is 10 ohms wirewound and as good quality as possible. I tried several types and they all sounded different. Avoid the alu clad and the thick film. Vitreous enamel are good, but best I tried were some obsolete surplus wirewounds. A 40W part here runs at 130 degrees C. And that's above the chassis in free ambient air. So I wouldn't use anything less than 40W, and more if possible (and definitely if going in the chassis)

andy

Andy, if you have both left and right channel filaments fed from your 20V supply there will be anti-phase cancellation of stereo at the cathode. This widens soundstage but reduces image depth. Just thought I'd mention it
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File Type: jpg 10Y Tube Preamp Schematic.jpg (643.1 KB, 1164 views)
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Last edited by greenvalve; 18th August 2010 at 03:12 AM. Reason: Rotated Schematic
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Old 18th August 2010, 06:50 AM   #600
andyjevans is offline andyjevans  United Kingdom
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Hi - the schematic is for one channel only in terms of the signal and the filament supply. One filament supply for each tube. I used a 60VA 12+12 toroidal wired up for 24v for each channel - two toroidals, four chokes in all. The toroidals and the first chokes are in a separate box, and the last chokes are on the signal chassis. The filament box gets its power off the PSU box so there's only one cable to the mains. Umbilicals are 4 pin XLR for filaments and 4 pin Speakon for the HT. There's only one HT supply - I should have made that clearer. Thanks for that.

andy
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