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#26 pre amp
#26 pre amp
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Old 29th October 2011, 01:51 PM   #1321
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Quote:
Originally Posted by merlin el mago View Post
Thks Rod, wich B+ are you using for battery bias schematic?

Felipe, If you use 9V battery, you can operate at about 145V (135 to 155V maybe). The anode current will be about 6 to 8mA. Salas HV shunt will need some extra current, so you can set the current-source to 20mA or more (please check the instructions for the SSHV).

.
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Old 29th October 2011, 01:55 PM   #1322
merlin el mago is offline merlin el mago
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Thank you Rod
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Old 29th October 2011, 01:55 PM   #1323
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
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Originally Posted by johnsciolino View Post
Rod,
All I can say is Holy $hit!! The DHT board has made my amp using a 26 to drive my 2A3 DEAD QUIET. And preserved the sound of an AC heated triode. I think its time to tear apart my friends Welborne amps and rip out the poorly designed DC circuit on the 300B. It sounds lifeless so I suspect your board will perform like my #26.
The build was a piece of cake using that heat sink you recommended. I had to drill some holes in the sink since the holes were too large and use self tapping machine screws. be sure to clean and sand smooth the hole since it could short the transistor to sink.
Thanks John, I'm glad to hear you like them. Unlike most dc regulators, there's no noisy reference IC anywhere, so no noise is added at all, except the transistor junctions & resistors, which are minimal.
And, so far as the cathode-current is concerned, there's no feedback at all. Meanwhile, all the mains-noise is screened out by the pass-transistors, which even AC-heat can't do!
This allows the regulators to work even with RIAA stage DHTs.
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Old 29th October 2011, 01:57 PM   #1324
regal is offline regal  United States
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Rod could you elucidate the use of your filament to give cathode bias without a cathode bipas cap, seems the best of both worlds, no need for an input cap to bias the grid and no cathode bypass cap. There has to be a catch?
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Old 29th October 2011, 02:00 PM   #1325
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Andy runs the 26 with the filament current (1.05A) running through a 9-10 ohm cathode resistor to provide the bias. It's this low value that allows the omission of the capacitor without degrading the gain.

With a normal cathode bias setup (~1K resistor) the value is too high, and must be bypassed.

The price is the need for a big trafo to power 16V at 1.05A......
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Old 29th October 2011, 02:40 PM   #1326
andyjevans is offline andyjevans  United Kingdom
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It's worth reminding 26 builders that filament bias is the brainchild of Thomas Mayer, and a very worthwhile way to bias a tube in my opinion. I use it, but the idea comes from Thomas! He likes choke input supplies and so do I for filament bias.

Andy
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Old 30th October 2011, 09:14 AM   #1327
massimo is offline massimo  Italy
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Quote:
Originally Posted by Rod Coleman View Post
Andy runs the 26 with the filament current (1.05A) running through a 9-10 ohm cathode resistor to provide the bias. It's this low value that allows the omission of the capacitor without degrading the gain.

With a normal cathode bias setup (~1K resistor) the value is too high, and must be bypassed.

The price is the need for a big trafo to power 16V at 1.05A......
What do you suggest Rod? A std 5A/12V (60VA) trafo should be fine IMO
8A (or 16A) bridge (four discrete TO220s) followed by a couple of 10-15mF/25V caps and a suitable low ohm resistor in between.
What about the mod to your #26 std kit to get 11V out?

Last edited by massimo; 30th October 2011 at 09:17 AM.
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Old 30th October 2011, 09:44 AM   #1328
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Hi Massimo, yes you can try filament bias, when you have the Filament Regulators supplied by me:

2x 10000uF/35v; 1N5822 rectifiers. resistors adjusted to give 16,0V at the regulator input terminals.

The Regulators are modded:

C1= 47uF/35V Panasonic AM (From Farnell. this has been tested.... Nichicon VR should be OK too, from Mouser)
R8 change to 470 (if you have 220 there now); or change to 1,8K (if you have 1K there now,)

The down side to filament bias is a big increase in rectifier pulse current. The trafo/rectifier/caps must be wired with VERY short leads to prevent magnetic coupling from these pulses into your amp. Put them in a shielded box, away from the tubes!

Alternatively, find a big choke 10-22mH or more (at 2A) and use PSUD2 to work out a choke-input supply. This also should be kept away from the tubes, but potentially offers the ultimate in performance.
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Old 30th October 2011, 10:08 AM   #1329
massimo is offline massimo  Italy
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Wow, faster than light, Rod!
R8 is now 220 ohms, thus will become 470 1/4W, C1 from 100/16 to 47/35 pana FM (this should be the right band, not AM I guess ) I have some 47/35 pana FC, but they are 6.3 mm dia. instead of 5.... they might not fit between trim and BJT.
What I have in mind is a two chassis build, thus all trafos and choke will stay far from the tubes, while your boards just under them.
Is 16V at the reg input the very minimum? With 12V AC and dropping something in the Pi filter it will be difficut to get more than 15 V. OTOH a 15V AC trafo means drop nearly 5V with a big power resistor in the Pi filter and a lot of heat.
Or I can slightly reduce the Rk value changing the #26 operating point accordingly....

Last edited by massimo; 30th October 2011 at 10:11 AM.
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Old 30th October 2011, 10:22 AM   #1330
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
yes, minimum dc voltage at the regulator input terminals must be 4,0V higher than the output. It will work better (more HF blocking) at 5V above output.

I agree, best to adjust Rk to meet the dc-input voltage from the transformer you have. Please let us know how it sounds!
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