Any advice for tube pre power supply? Old-Fashioned or New-Fangled?

[slor]

I own a few Grommes G5M tube mixers which I love, and for years I've wanted to make a stereo version of one, with four inputs switchable to either or both outputs.

The design itself should be fairly easy to replicate, with a switching network of 4-pole 3-position switches. My question concerns the power supply.

I could easily hack the supply from an existing unit; I never use the 12AU7-driven output stage, so removing that tube and adding the requisite 12AX7 for the second output / EQ stage would give me a total of 8 x tubes, so a current draw somewhat less than 10 mA (the original transformer is rated at 320-0-320, 20 mA.

That said, I'd rather not cut up a perfectly good mixer. My question is:

-Do I try and replicate the original power supply, which means finding a high-voltage, low-current transformer, which seem oddly difficult to come by?

-Do I take advantage of modern components and build a solid-state, even a regulated supply?

Even a brief scan of the forums--and I am an avid reader--shows no consensus on tube power supplies. Many people feel that unregulated is fine, even AC on the heaters is okay. The original unit uses a 25V secondary for DC heaters, which may be one of the reasons it's so quiet.

I'd love any expert opinions. My gut is to keep it as simple as possible, but that doesn't mean forgoing modern technology if it's called for.

Many thanks! 

 

[ruffrecords]

The original supply uses cascaded RC circuits for the HT smoothing and dc heaters. I see no reason to change either. Semiconductor rectifiers can handle much higher peak currents than rectifier tubes so you can significantly increase all the smoothing caps with a consequent reduction in ripple. A 12V or 25V dc supply can easily be made using an LM317 on a suitable heatsink.

There are plenty of transformers available that can fit these needs.

Cheers

Ian

 

[PRR]

> a high-voltage, low-current transformer, which seem oddly difficult to come by?

As Ian says.... get the 24V DC some other way (24V packs are very common).

Then find a Fender Champ Replacement Power Transformer. Typically 650V AC CT (325V each side; same as your 320V each side) and safe for up to 80+mA. Which is far more than you need, but Fender Champ/DeLuxe PT is a super-popular part, and usually your best-buy for tube-boxes from 10mA to 100mA.

http://www.tubesandmore.com/products/P-TF22772
http://www.hammondmfg.com/guitarLinePWR.htm
Watch for Epi Valve Jr parts on eBay. The Jr is a cheap Champ, and someone either strips the PTs for upgrades or imports barrels of Jr PTs, I dunno which. Note: the Jr uses a 4-diode Bridge rectifier instead of the classic 2-diode form.

 

[slor]

Gents: Thanks much for the replies and tips. My major concern was that by drawing far less current than the transformer supplies, my voltage would soar through the roof.

Oh and by the way, I checked the parts bin and found that I already have an Epi jr power transformer! I'm going to have to do a bit of head-scratching to integrate that into the project, but thank you so much for the lead!

 

[PRR]

> soar through the roof.

PT regulation is 10%-20% (30% on VERY small junk).

Plate windings *tend* to be better regulated. (The "optimum" wire size is annoyingly small, so they often use bigger wire than "needed".)

325V will tend toward 460V DC. If you keep a hollow-state rectifier it will be well below that when all tubes are warm. You have a C-R-C-R-C filter with two resistors to adjust to get desired final voltages. Since 325V==320V for practical purposes, and tube amps are not fussy, no great change should be needed.

The Jr PT needs a different rectifier, most conveniently Silicon, and IIRC the Jr is over-volted, so up-size the filter resistors considerably for smoke-test then work your way to desired voltages.

 

[slor]

The Epi Valve Jr transformer supplies 260v, 12v, and 6.3v (only the 12v winding is center-tapped). I am going to read a bit more on the subject and see how this is going to play out. Again, many thanks for the guidance!