Transformer to tube selection question

[jwhawk04]

Hello all first post
Just canned a transformer from an old piece of AV equip and was wondering if I could use it to power a two tube combo low watt amp. Took some volt readings With 120vac on the input sides I have 45vac, 5.2vac and 13vac on the output windings.
Any idea of tube combos I could use? Thanks..

Jay, Ac-130H Spectre Electrician

[Tom Phillips]

Hi Jay,

Ac-130H. Cool!

Based on your voltage readings, I'd say that your transformer came from a solid state piece of equipment. You don't have suitable windings to use it for a tube amp. Even a small one.

Regards,
Tom

[jwhawk04]

yep came from an old cd player, thanks for the info, was hoping there were some low voltage mini tubes. I tried looking through TDSL and was having no luck. Could I run this in paralell with another transfromer from the same AC source or should you have a dedicated power transformer for your power tube? I haven't seen this done on any diagram yet....

[Tom Phillips]

There are low voltage tubes but that's a whole different, non-conventional approach to a guitar amp.
I says it's not worth trying to incorporate your existing transformer at all. Best to start with a more suitable units and use currently available standard tubes such as a 12AX7 and a 6V6GT for a two tube amp. You will find other old tube equipment you can salvage from at garage sales, Ham fests and surplus equipment sales. Some old tube type test equipment is a good source. Also old PA systems, record players, tape recorders etc. if you don't want to buy new. Sometimes you end up with a suitable chassis and other parts of the circuit the same time.

[Rob Mercure]

Well, it depends on what you're looking for. If only a few watts -perhaps no more than around 8 at the extreme - you could use tubes that were designed for "line connected" sets with a voltage doubler/tripler for the B+. The "50" tubes and the "35" tubes as well as 60FX5, etc. (the variety is kinda endless with every variant from 16 volts to 117) could be used. These octal, e.g., 50L6 and miniature, 50C5, tubes are capable of putting out about 3-4 watts SE and about 8 in PP and they actually sound really nice. I've got several amps I constructed with "built in" isolation transformers.

So, let's say you get a 50C5 and run it on the 45 VAC heater winding (probably close enough - but there was a 42 volt output tube I seem to remember - any of these tubes will be a bit scarce but so cheap they might was well be given away - check old repair shops) and then shoot the 45 VAC source to a full wave voltage doubler giving you about 120 VDC ("sorta kinda") which will feed the 50C5 just fine. Then run a 12AX7 off the 13 VAC winding (OK, the 12AX7 is kinda "milk toast" but thre are lotsa curcuits to copy out there) giving you a low powered "champ." Or you you could use a PP arrangement with either two 50C5s or two 25C5s in series. Then there's the 7695 which will put out some power if you can find 'em. For these I'd use a voltage tripler and definitely go the PP route.

And there are scads of other great tubes with non=6.3 VAC multiple heater voltages you can use. The 20EZ7 - which really isn't applicable in this case - is a 12AX7 with a 20 volt heater and every one I've ever seen is a Mullard!

The only real hesitation I've got is sheilding on the PT as it was designed for mostly digital very low impedance circuits and may radiate some hum. Oh, check out the 26A7 - these guys really sound nice when they're cranked up.

Rob

[Arthur B.]

Hello all first post
Just canned a transformer from an old piece of AV equip and was wondering if I could use it to power a two tube combo low watt amp. Took some volt readings With 120vac on the input sides I have 45vac, 5.2vac and 13vac on the output windings.
Any idea of tube combos I could use? Thanks..

Jay, Ac-130H Spectre Electrician

The ECC86/6GM8 should work well in the preamp position, although I don't know anything that will work in te output stage with that low a voltage. IMO, the transformer looks to best used as a dedicated bias/heater tranformer.

[jwhawk04]

Thanks for all the help and info, looks like that little transformer won't go to waste. I like the idea of using it as a dedicated heater transformer.

[Rob Mercure]

Hawk,

It's been few years since I've had a chance to "rant and rave" about the boring and limited selection of tube types used by current manufactureres (some interesting NOS Russian stuff coming out though) and the resultant limitiations that designers are placing on themselves -probably subsconsciouscly to "conform" the same types that Fender and Marshall, etc., uses. Of course this is necessary if you're making hundreds or thousands of amplifiers but if you're making one of two - or even a tens - why not take advantage of the incredible "wealth" of tubes that are out there? You're not making volume purchases of 12AX7s from JJ so why not use the 6EU7 (rebased 6.3 V 12AX7) or the 20EZ7 (rebased 20 V 12AX7) or 12AD7 (premium, same base, 12AX7) or other NOS tubes. Now note, these are all still 12AX7s so I really haven't ventured far at all. But if you like these "6AV6" triodes (the 12AX7 is two 6AV6s - minus the diodes - with a center tapped heater) then there are some compactrons that have three of these triodes, or two and a 1/2 12AT7 or /AU7 or perhaps a nice signal or output pentodes in a nine-pin bottle. Check out 6AC10, 6C10, Q11, 6K11, ets,

Guess I'm spoiled but when I've needed a tube I just openend up on of the boxes and see what we've got.

Now I limited my earlier comments to fairly low powered amps - the 50C5/L6 types - cuz I didn't know what the current capacity of the tranny was. You could always use a line connected heater string if you maxed out your heater current - leaving no B+ current available - while retaining the tranny for B+ isolaton from the power line (while I've alway considered line connected circuits kinds "fishy" these types of curcuits were incredibly common during my childhood and on up into my 20s with no ongoing torrent of electrocutions or house fires so I'm pretty comfortable running heaters on line current,


Which brings us to another set of options. Other the DC audio is just about the least demanding tube application so lotsa tubes designed for other uses -Television hoirzontal and vertical output tubes - make great audio output tubes. Some famous HiFi designers - such as MacIntosh - used HO tubes in some of their designs.

So, back to our 50V, et al, tranny: If you've got sufficient current capacity you could use a H/V ouput tubes with a 50 volt heater - and the same voltage doubler - to produce a higher powered amp. There were lots of 5O VAC H/V ouput tubes and again you've got the option of running the heaters off the power line,. I can easily see "Vibrolux' style amp with a series heater string

Have fun.

Rob