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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: actually, it's rather easy posted by Sidewinder on June 17, 2019 at 13:15:03:
1) CCS plate loads for the input and driver stages. K&K Audio sells inexpensive kits that can be used for this purpose. The operating point in the Sun for the input stage 6SN7 was pathetic, 80V@2-3mA if memory serves. Changed this to something like 160V@6-8mA biased with 2 cheap red LEDs in series (-3.4V).
2) Had to ditch direct coupling with driver and RC coupled (not sufficient B+ for 6BX7) After allowing enough voltage headroom for the CCS (~30V) and enough voltage swing to drive the 2A3 to full output (say 70V total for both) you are left with about 210-220V on the plate of the BX7 which is kind of low but seems to work @19mA bias current. I will have to open amp to check the value of the cathode bias resistor that I used. The BX7 has a higher filament current requirement and the extra B+ current demands were already resulting in the power transformer running somewhat hot so a separate filament transformer for the BX7 is needed.
3)The driver stage CCS now connects to the raw B+. For the input stage use RC filter for ~220V.
4) With the active plate loads I did experience the occasional misbehavior on start-up with the B+ coming on before the signal tubes were ready to conduct. So I added a 30 second delay to the rectifier tube's filament supply.
Probably not that important: I created a center ground for the input tube filament (Sun left it floating) with two 100R resistors. The new filament transformer for the BX7 has a CT and both were biased positive 30V and AC grounded with a capacitor.
Not relavent: My line stage has floating transformer output. This is important for my DIY balanced mono-blocks. Both sets of amps are connected to the line stage all the time so the Sun got a pair of Lundahl LL1690AM input transformers. Got rid of the input "pots" and replaced with separate volume and balance stepped attenuators.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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Follow Ups
- RE: actually, it's rather easy - JKT 06/19/1923:28:26 06/19/19 (0)
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