What to do with 90v/8v transformer? Leakage tester?

[Steve Blackdog]

I picked up a 1950s battery eliminator for a couple of quid and disassembled it to pass the time.

I thought of restoring it, but I have no need for a battery eliminator designed for a portable valve radio receiver.

The baby transformer steps 240v down to 90v and 8.2V (with a 2x4.1v centre tap). I have been wondering about using this little tranny as a power supply to make a capacitor leakage meter to use with high voltage caps.

Does that sound worthwhile? Any better ideas.

Cheers

steve

[tedmich]

With a FWB this might get ~ 120VDC for a starved 12ax7 preamp design like the Real McTube,*
http://www.dogstar.dantimax.dk/tubestuf/mctube.htm
if the 8v tap has enough current for the heater (regulated down to 6.3VDC)

*the designer, the late Fred Nachbaur, was a great guy and answered all my dumb newb questions back in the day (and his family maintains the web site after he passed)

A leakage tester would be cool too but my credo is if it doesn't make sound I don't make it

[Mick Bailey]

A leakage tester really needs to operate a the test capacitor's rated voltage to give best results, or at least the voltage you'd find in-circuit in a tube amp. You could use a half-wave voltage multiplier to get the secondary up to a decent test voltage.

I too would build it into an audio project - maybe a tube preamp pedal. Make sure the transformer is actually good before deciding on anything. I've sometimes had great plans for a transformer only to find that the gear it came from was probably scrapped off because the transformer was bad.....

[Steve Blackdog]

With a FWB this might get ~ 120VDC for a starved 12ax7 preamp design like the Real McTube,*
http://www.dogstar.dantimax.dk/tubestuf/mctube.htm
if the 8v tap has enough current for the heater (regulated down to 6.3VDC)

*the designer, the late Fred Nachbaur, was a great guy and answered all my dumb newb questions back in the day (and his family maintains the web site after he passed)

Brilliant idea! That’s a perfect project. Roughly what size enclosure does it need?

[tedmich]

I'd have better luck guessing your shoe size!
What is the dimensions of the transformer?
The McTube is not terribly high gain but as with most tube preamp projects room and good layout practices can make all the difference. They also recommend an enclosure on the site I think?

[Steve Blackdog]

I'd have better luck guessing your shoe size!
. They also recommend an enclosure on the site I think?

Yes indeed! I have found the details on the website.

so now I am wondering about using the mctube circuit as a pre-amp and, rather than use it as an effects pedal, adding on a power amp circuit using a
TDA7294 100w audio amp DMOS chip (I have one lying around).

[tedmich]

Yes indeed! I have found the details on the website.

so now I am wondering about using the mctube circuit as a pre-amp and, rather than use it as an effects pedal, adding on a power amp circuit using a
TDA7294 100w audio amp DMOS chip (I have one lying around).

Those chip amps require a bipolar supply (up to +/- 40VDC) a bit higher than the LM3886 which I've used a ton. Do you have another center tapped PT for this or would you use a pair of SMPS? For real small SMPS lately I've been using tiny Vicor units off Ebay, A 300VDC front end (current version FARM) combined with two 300V input DCDC converter can be had dirt cheap sometimes. I have two box fans and multiple amps running with these for years.

[Steve Blackdog]

Those chip amps require a bipolar supply (up to +/- 40VDC) a bit higher than the LM3886 which I've used a ton. Do you have another center tapped PT for this or would you use a pair of SMPS? For real small SMPS lately I've been using tiny Vicor units off Ebay, A 300VDC front end (current version FARM) combined with two 300V input DCDC converter can be had dirt cheap sometimes. I have two box fans and multiple amps running with these for years.

I haven’t given this much thought. I do have a 30-0-30 tapped PT. I am a bit of a novice, and I am not familiar with using SMPS. I will investigate!

[tedmich]

I haven’t given this much thought. I do have a 30-0-30 tapped PT. I am a bit of a novice, and I am not familiar with using SMPS. I will investigate!

The 30VAC center tapped could work for an 8 ohm load, you'd want <27VAC for 4 ohms with a vanilla PS, maybe try out Duncan Amps PS software to see.

Several cheap TDA7294 PCBs on the ebay have everything for the PS except the trannie, but some lack schematics and component values...good luck!

[J M Fahey]

Just adding: with your 30-30VAC transformer, a bridge and two 2200 or 4700 uF x 50V caps you will get around +/-42V rails, perfect for a TDA7294 driving an >>>8 ohm<<< load and getting very loud 70W RMS ; not reliable enough driving 4 ohm loads.
Unless you drastically lower voltges, but you already have the PT, so ...

In my book TDA7294 is an unreliable 100W amp, and a very good 70W one, go figure.

Notice it is exact same size as 3886 and that dictates power dissipation, I couldnīt care less about voltage or current handling if it can not dissipate all that heat.

[Steve Blackdog]

Just adding: with your 30-30VAC transformer, a bridge and two 2200 or 4700 uF x 50V caps you will get around +/-42V rails, perfect for a TDA7294 driving an >>>8 ohm<<< load and getting very loud 70W RMS ; not reliable enough driving 4 ohm loads.
Unless you drastically lower voltges, but you already have the PT, so ...

In my book TDA7294 is an unreliable 100W amp, and a very good 70W one, go figure.

Notice it is exact same size as 3886 and that dictates power dissipation, I couldnīt care less about voltage or current handling if it can not dissipate all that heat.

70W is more than enough. I am already deaf in one ear - I need to look after the other one!

[Helmholtz]

Don't forget to make sure that your 2x30V PT can supply the current necessary for 70W output.

[Steve Blackdog]

Don't forget to make sure that your 2x30V PT can supply the current necessary for 70W output.

Thanks for the advice. It actually came from an old 80w amp, so should be ok.

[Steve Blackdog]

Just adding: with your 30-30VAC transformer, a bridge and two 2200 or 4700 uF x 50V caps you will get around +/-42V rails,
.

Please can you explain the calculation that you have done above. How do work out that capacitors should be 2200 or 4700 uf? I realise this is a hole in my learning!

thanks in advance.

steve

[Jazz P Bass]

" How do work out that capacitors should be 2200 or 4700 uf"

Technically, it is not simple.
Realistically, there is a small available range of capacitance values.
Too small of a value can result in excess ripple under severe load.
Too high of a value can cause an unnecessary inrush current.

The true capacitance value depends on voltage, current draw, resistance of the transformer and the diodes........

Here is a good read:
http://www.skillbank.co.uk/psu/smoothing.htm