power for 6aq5

[Andy Le Blanc]

I found I had some 6aq5 in the bench spares, and thinking maybe....
They seem to generically want a 10k-ish load, in the 6v6 family,
but a lower max plate voltage (small envelope) and a couple less watts max dissipation.
I found a deluxe opt , 8.5 K, thats in the ballpark, And at the published
design center plate voltage of 275V max dissipation two will want to draw
only 100ma., so the power requirement shouldn't be too bad.

So I was thinking of noodling around with a couple 120/12v 3A pt back to back.
The resulting V out of a doubler is in the ball park too.
I can get the heater consumption down by running them series, and with other heaters from the pre, the consumption could be .5A to .75A.

Will 3A be enough? This back to back PS works great for pre-amps and effects.
I've seen schematics for single ended amps using the notion, but haven't
tried it out for a power side yet.

[Alexo]

So we figure 1A for heaters, leaving 2A.

2A @ 12V = 200ma @ 120V. Double the voltage, halve the current leaves 100ma to play with.

Welp, not ideal, but odds are you can get away with it. FWIW, I have a push-pull 6AQ5 amp running at 180 volts wit a 65ma PT, pulling 67 ma at idle. That PT gets hot as heck but it can run for hours on end, but it's a Hammond.

If you run fixed bias, you'll need less idle current... I say go for it. After all, isn't the ever-present danger part of what makes tubes so much fun?

[Andy Le Blanc]

It looks like it'll work, I have a back up PT in case the notion really is impractical.
With the small envelope, I'm reluctant to run a fender PT.
The benefit of the scheme is that the whole power side becomes 3 transformers
all the size of a deluxe OPT, only a few holes in the chassis, or mount them inside.
Pretty slick to see a tube unit with no visible tranny. Very compact, if its practical.
for a small push-pull power side.

[Cliff Schecht]

Andy, I will gladly join you on the back-to-back transformer adventure. I have a build planned where all of the iron I use is not meant for the purpose I'm using it. I haven't quite decided on the topology yet but I'm aiming for about 2 Watts audio power which a 6AQ5 will do with room to spare. I'm thinking perhaps another 6BM8 build first. The back to back trick is cool because you can match a 12.6V transformer to a 6.3V guy to really step up or step down the voltage as you want. I may play around with a HV/LV switch that chooses between either a 6V or 12V tap on the second transformer (first transformer is 6V).

The output transformer will be a 10W line-to-speaker transformer. If you square the voltage rating (70.1V) and divide by the power (choose 1W) then you end up with about a 4900 Ohm SE primary and an 8 Ohm secondary. Just keep the audio power down so you don't saturate the core, especially in a SE build!

[Andy Le Blanc]

Here a scheme from heaven, anybody ever try out a rick tone?
I first saw the B2B thing in a glassaudio/audioxpress articles some years back.
I think it was Joseph Norwood Still, and he'd get pretty creative with it.
You can use a walwart, and step up the AC before rectification, and it allows
you to take advantage of PT w/o heater or bias taps.

[Alexo]

If you wire those 12V transformers out of phase and put them close enough together, will it buck that pesky magnetic hum?

[Cliff Schecht]

I keep looking at the dual transformer trick as an option but it just is not something I would want to do in my production amps. Neat trick, but not any more economical than just using a proper transformer. Edcor and others have usable low plate voltage transformers for $20-$30. Allied has one I'm interested in as well. And if you don't mind toroidal, AntekInc makes a solid PT.

[Andy Le Blanc]

It does work well for tube effects and pre, were the current requirement is low.
The cost and small size becomes very attractive. Its also a way for DIY to
use whats laying around affordabley. I'm very curious to see if holds up for
a power side. In general it seems to be quiet.

[davent]

This Audioxpress article is on the creative use of wall warts but towards the end of it has a good section on sizing b2b transformers for powering a tube circuit. http://www.audioxpress.com/magsdirx/ax/ ... er2874.pdf

dave

[Andy Le Blanc]

Thats fun, was that off the web articles? Hadn't seen that one yet.

So...shoot for a 250 v supply max dissipation for 2 6aq5 will be .112A
double it, say .2... it'll be on a 120/12.6 pt..... .2 x 9.5 that nearly 2A
plus .225 plus .3 for heaters thats around 2.5A.... its close

Anybody measure the max signal plate current.... wait, should be able to eyeball for 6v6 at 250 plate, AB.
so the base assumption will really only be around 1 A , on the first pt, 1.5A plus the heaters, that looks better.

reality will be fun. Have to scab up a PS and little power side to measure.

[zambo]

http://www.youtube.com/watch?v=H1Cfejdd7Ew

http://www.youtube.com/watch?v=pAc2FTzLwbk

Hi! I am a noob here so forgive any ignorance. Reading your posts I thought this might be of interest to you as both of these amps tun on a single 9 volt walwart into a super small smps putting out 200v at around 37ma as far as I can tell. smps is a 1363 from taylor edge electronics purchased for 13 bucks with shipping included. Not bad, but they whine a lot if not loaded fully it seems. I dont have "real" schematics for these as I just kind of guess and build as I go. Dangerous but fun. Good to see Cliff on here! Any improvements on either of these is more than welcome.

[Cliff Schecht]

I will eventually finish my little tube SMPS but right now that is not the direction I want to go with the amps I'm building. I played around with that route quite a bit and IMO the sound and look of a tube amplifier with a real HV transformer/rectifier setup is more attractive.

Andy, while I think the dual filament transformer trick is cool, there are other affordable options. I just got a quote from Marvel Magnetics for about $12 a piece for a power transformer and output transformer that I bet would work great for what you are trying to do.

After a bit of reading I found the dual filament transformer trick to not be as economical as I originally thought, at least for Champ sized and smaller amplifiers where the iron isn't ridiculously expensive. For higher power stuff, a high current filament transformer for the filaments and a smaller transformer for the HV is fairly economical, but not something I would do in production amplifiers.

[Andy Le Blanc]

Thats www.marvelelectric.com ? same corp as magnetic components
They are very reasonable.

That PS has its place, for effects its great and it allows you to use what you have
on hand or have to scrape up as DIY on a buget. I was surprised that might
power a amp over just a few watts.

[Cliff Schecht]

Thats www.marvelelectric.com ? same corp as magnetic components
They are very reasonable.

That PS has its place, for effects its great and it allows you to use what you have
on hand or have to scrape up as DIY on a buget. I was surprised that might
power a amp over just a few watts.

It really is a heck of a deal. Same guys that do Mag Components as well. They are sending me free samples for the two transformers I ordered which I'm pretty excited about. The transformers are the last piece of the big puzzle for the production unit prototype I'm working on so hopefully I can really push towards getting some production models out the door soon!