6V6 line preamp

[grovergardner]

Oooh, that looks nice!

 

[grovergardner]

Congrats for the build. What about the SQ of the Fostex output coupling caps?

Oh, what a wonderful cap! Hard to describe--the transparency and speed of Teflon with the warmth and richness of oil. Totally balanced, crisp top end but never bright or etched, nothing jumps out, rich deep soundstage. Pity they stopped making them. Madisound still has 3.3uF in stock. Only drawback is 250VDC rating, but for a preamp that's usually not a problem. Run don't walk. ;-)

 

[Salas]

Cout 250V voltage rating should be good enough when the bias current is nominal. Thus voltage drop across the plate resistor has fully developed. Same for rail elements like RC filters. Since its on the plate node it will see about 165V DC + AC output swing peak.
I put 400V Cout schematic note rating to be >B+ safe in case of current source heating which is soft start and takes significantly longer time to develop full emission.

 

[grovergardner]

Yes, voltage at the plate is 150 or thereabouts. There's a bit of surge at turnon but the Maida board quickly drops this to 330 volts and that then drops down further as the 6V6s warm up. I figure a few seconds won't hurt the cap. I might also add a slow-start board to the HV.

 

[Salas]

Exactly how much plate DC after tube bias has settled depends on 6V6 brand and vintage tolerances. But there...150-165V in the G3X when with 330V B+ rail.

 

[grovergardner]

I have been listening all evening to this. I hear nothing wrong and a lot right! Tubes are nothing special, some Groove Tube Russian-made guitar amp tubes. HIP Baroque bands are tangy but never "screechy," piano is full and rich, classic jazz recordings have great "PRAT" and deep, clean bass. Gain is perfect for my system. Everything I put on sounds natural and "right." I'm waiting for that one recording that comes out "wrong" but haven't found it yet. ;-) I have some Bendix 5992's on the way. Eager to hear them.

 

[Salas]

The Bendix has unrivaled aviation spec construction. Possibly most expensive one to get among NOS "6V6".
Has an electrical difference which is 45sec slow warm up 0.6A filament structure.

 

[grovergardner]

Thanks for reminding me about the filament current, I'll need to adjust the non-regulated supply a bit.

 

[Bas Horneman]

Transformer mounted.

Bell end is standard. Drilled holes and glued stainless mesh. Initially for a friend in South Africa because of the heat. Just for looks in this amp though.

 

[Bas Horneman]

Wood on end of copper glued so the copper plate can handle the downward pressure of inserting tube without having to use more bolts in the top
plate...to keep a cleanish look.

 

[merlin el mago]

Hybrid Graetz rectifier? What tube?

 

[Bas Horneman]

Yellow sheet mod. Just some help for the tube 5AR4/GZ34.

https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-yellow-sheet-diode-mod

 

[Bas Horneman]

Inside

 

[Salas]

The torn "wood" top remains sabotage the inside looks. Other than that great. Useful transformer taps values by the way.

 

[Bas Horneman]

I agree wholeheartedly. Ugly as sin.

 

[merlin el mago]

My new power tx arrived, it's a gift of my friend JOIMON

I didn't wanted 300VAC to not pass the max. 400V surge voltage of BG....

 

[Bas Horneman]

Good to have friends!

PS. Cathode bypass caps. What do you guys use? I have panasonic fc and nichicon fine gold in my parts bin with values suitable for this project.

Another favourite pastime: prepping (spent hours trying to find four 15k resistors)

 

[grovergardner]

For bypass caps I have some old Black Gate 470uF/160V that are pretty smooth. Otherwise I usually use Elna Cerafines. Mills makes a non-inductive 7.5K 12 watt resistor I have on order, plus a pair of Mills 390 ohm 5 watts for the cathodes.

 

[Bas Horneman]

I'll wait to do the boutique bypass cap later....after listening for a long while. That way I would find out if I can hear the difference. Anyway..this sub-assembly came built up. It is from a fellow diyaudio member v4lve lover. 6,3V AC in 6,3V DC out.

 

[grovergardner]

Oh, where do you get that?

ETA: I found it.