Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on July 01, 2023, 12:04:58 am
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Alright here's the big one, power supply and power amp I stole from a dual rectifier. I just have a few questions before I draw up a BOM.
-If I use a 2nd power transformer (on account of needing 1.5x the VA rated 6.3v line of a standard dual rectifier for DC heaters) for the heaters is there anything I should keep in mind as far as orientation? It will be fine just right next to the main transformer? Does anyone have any suggestions for a power transformer that can handle that much current for the heaters (350v for B+, 50v for bias, and 6.3v 44VA for heaters)?
-If I add the switch to be able to use EL34s or 6L6s is it really just as simple as a different voltage bias?
-The Wendigo preamp this is based around was originally running something like 50v cooler on the plates than this design, is this a major design concern?
I really would appreciate any and all design critiques for this, I am learning so much designing my own gear and I love it.
Original mesa boogie schematic attached.
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The signal at the power tubes is like 500 times bigger than the signal at the preamp tubes, so 500 times less sensitive to heater hum. There's really NO point feeding big tubes DC. And it is intensely difficult to filter AC to big DC, especially at low impedance (high current).
Your baby needs well-filtered water. Your elephant-wash does not.
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OK I see your point haha. Considering I wont have an FX loop on this design I think I wouldn't be drawing too much current off the heater winding if I just feed the 12ax7s DC.
This does make things significantly easier as far as choosing a power transformer goes.
Was for sure thinking too hard on that one. I will just use a standard dual rectifier power transformer.
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Updated power supply:
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Is this a center tap or a bias winding at the HT powertransformer winding? Can't be both me thinks. And to have AC and DC heaters you are giving up heater AC ground reference …
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50v tap off the HT for bias. Good catch I did forget to add in the ground reference for the heaters. Looks like MB uses x2 100r making a virtual ground, I'll just do that.
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Ah again good eye Will. there is a dedicated CT on the dual rec power transformer that I did not add in the schematic.
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That 470k plate/ 820ohm cathode caught my attention. I've never seen that before that I can remember?
I typically see things more in the 100k/1.5k, 120k/1/8k, 150k/2.2k, 180k/2.7k, 220k/3.3k ........ etc .........
Maybe build it like your schematic shows but then consider a 220k/3.3k if you're not pleased with the tone of the 470k/820ohm.
You could also consider a 5879 pentode as V1?
With respect, Tubenit
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The preamp is basically an exact copy of ceriatone chupacabra, and when i built that into a standalone preamp (using the output of soldano x88r preamp) it absolutely floored everyone who played it.
That being said this is its incarnation as a full amp head in my design. I will definitely be revisiting your suggestion if i get it built and its not up to my standards.