# 240vac xformer and tube amp

M

#### Michael

Jan 1, 1970
0
I've been in electronics a long time but it's been years since I've messed
with tube stuff but I thought I "may" build a small guitar (practice) amp.

First question is...

Has anyone explored the possibility of using a simple (and generally
inexpensive) 120 to 240vac step up transformer with a solid state bridge
rectifier to get high voltage for a tube?

Is there a design/schematic someone can refer me to on a particular tube
that could operate on the voltage from this arrangement....I would imagine
you could get close to plus 280-290 volts or so under load?

D

#### Doggone

Jan 1, 1970
0
You need Duncan's very fine tools.

Go to:
http://www.duncanamps.com/software.html

And download PSU Designer. While you're there, you'll want to get the other
tools too. TDSL is my fav. You'd be hard pressed finding better value on the
net.

J

#### John Larkin

Jan 1, 1970
0
I've been in electronics a long time but it's been years since I've messed
with tube stuff but I thought I "may" build a small guitar (practice) amp.

First question is...

Has anyone explored the possibility of using a simple (and generally
inexpensive) 120 to 240vac step up transformer with a solid state bridge
rectifier to get high voltage for a tube?

If it's an autotransformer, and most are, it won't be isolated and
will be a shock hazard.

John

M

#### Michael

Jan 1, 1970
0
I see where Hammond makes a few 230vct transformers, one that will supply
..35a for about 20 bucks....not an autotransformer.

Wonder if anyone has seen a design with these for a small practice amp with
just a few watts?

J

#### John Popelish

Jan 1, 1970
0
Michael said:
I see where Hammond makes a few 230vct transformers, one that will supply
.35a for about 20 bucks....not an autotransformer.

Wonder if anyone has seen a design with these for a small practice amp with
just a few watts?

That's an 80 VA transformer. Use a bridge rectifier on it and get 325
VDC out. You could get more than a few watts out of that with a class
B output stage. Use the center tap as a 162 volt DC output for the
earlier stages.

B

#### Bob

Jan 1, 1970
0
I've been in electronics a long time but it's been years since I've messed
with tube stuff but I thought I "may" build a small guitar (practice) amp.

First question is...

Has anyone explored the possibility of using a simple (and generally
inexpensive) 120 to 240vac step up transformer with a solid state bridge
rectifier to get high voltage for a tube?

Is there a design/schematic someone can refer me to on a particular tube
that could operate on the voltage from this arrangement....I would imagine
you could get close to plus 280-290 volts or so under load?

That's almost all I use! (Sorry, my schemo is a proprietary device!)

The peak voltage will be around 330 but will drop under full load back down to
250 or so - perfect for a 6BQ5 power tube. If you run a class A SE amp the
voltage will be stable... don't go over 300...

Preamp tubes like the 12AX7A will run on anything over 150 volts with no
problem...

Don't forget you need a 6 volt filament supply at a few amps...

Oh, I hope you have experience with wiring a high Z high gain wide bandwidth
device - or it won't work... it'll either hum or oscillate or both, probably...

Then there's the matter of Tone...

Bob
Amp builder

B

#### Bob

Jan 1, 1970
0
That's an 80 VA transformer. Use a bridge rectifier on it and get 325
VDC out.

Not at load... I get 275 volts after a 150 ohm filter... about 10 less than the
bridge output, for a SE EL84 5 watt amp. Higher loading would reduce it more...
You could get more than a few watts out of that with a class
B output stage. Use the center tap as a 162 volt DC output for the
earlier stages.

You can't use the center tap if you use the full secondary with a bridge...

Bob
Amp builder

J

#### John Popelish

Jan 1, 1970
0
Bob said:
Not at load... I get 275 volts after a 150 ohm filter... about 10 less than the
bridge output, for a SE EL84 5 watt amp. Higher loading would reduce it more...

You can't use the center tap if you use the full secondary with a bridge...

Why not?

D

#### Doggone

Jan 1, 1970
0
Not at load... I get 275 volts after a 150 ohm filter... about 10 less
than the bridge output, for a SE EL84 5 watt amp. Higher loading would
reduce it more...

You can't use the center tap if you use the full secondary with a
bridge...

Bob
Amp builder

Hi Bob.
You can use the center tap. It will be at exactly 1/2 the B+.

B

#### Bob

Jan 1, 1970
0

Sorry - my bad - you can... I must have been pre-occupied!

B

#### Bob

Jan 1, 1970
0
Hi Bob.
You can use the center tap. It will be at exactly 1/2 the B+.

You're right - I must have been thinking of getting the bias from there, which
is negative...

P

#### Phil S

Jan 1, 1970
0
Bob said:
That's almost all I use! (Sorry, my schemo is a proprietary device!)

The peak voltage will be around 330 but will drop under full load back
down to
250 or so - perfect for a 6BQ5 power tube. If you run a class A SE amp the
voltage will be stable... don't go over 300...

Preamp tubes like the 12AX7A will run on anything over 150 volts with no
problem...

Don't forget you need a 6 volt filament supply at a few amps...

Oh, I hope you have experience with wiring a high Z high gain wide
bandwidth
device - or it won't work... it'll either hum or oscillate or both,
probably...

Then there's the matter of Tone...

Bob
Amp builder

Hey Bob!
You're the one who told me to tie two secondary (82v + 125v) windings of
that old Heathkit TV power tranny in series and it would give more than
enough B+ for a Champ like thing. I haven't seen you around in a while.
Thought I'd mention that it worked out just like you said it would. As I
recall, B+ was in the neigborhood of 250VDC. I built it with a diode
bridge, 12AT7, and 6AQ5. Works great! Amp building is addictive. I built
two more since then (5E3-almost, and JTM45).
Phil

B

#### Bob

Jan 1, 1970
0
Hey Bob!
You're the one who told me to tie two secondary (82v + 125v) windings of
that old Heathkit TV power tranny in series and it would give more than
enough B+ for a Champ like thing. I haven't seen you around in a while.
Thought I'd mention that it worked out just like you said it would. As I
recall, B+ was in the neigborhood of 250VDC. I built it with a diode
bridge, 12AT7, and 6AQ5. Works great! Amp building is addictive. I built
two more since then (5E3-almost, and JTM45).
Phil

Hi Phil - I remember you!

I don't come here too much because of the off topic political bullshit - some
people can't understand this is a guitar amp group...

As for addictive amp building - I'm now working on number twenty something...
I'm trying toroids this time!

P

#### Phil S

Jan 1, 1970
0
Bob said:
Hi Phil - I remember you!

I don't come here too much because of the off topic political bullshit -
some
people can't understand this is a guitar amp group...

As for addictive amp building - I'm now working on number twenty
something...
I'm trying toroids this time!

Well you dropped in at a decent time. Between the filters and whatnot,
things are down below a dull roar at the moment. I don't think it will stay
that way. Anyhow, it was nice to see you drop by. Toroids, huh? What's
the deal with them?
Phil

D

#### Doggone

Jan 1, 1970
0
Hi Phil - I remember you!

I don't come here too much because of the off topic political bullshit -
some people can't understand this is a guitar amp group...

As for addictive amp building - I'm now working on number twenty
something... I'm trying toroids this time!

Amp junkies can be itentified by the solder burns on their hands.

Do you know of a source for toroidal transformers with both a high voltage
and filament windings or do you wind your own? I found Hammond have decent
toroidals but don't have mixed windings. They also tend to be quite pricey. I
find it had to justify the extra cost when recycled iron can be had so cheap.

Is there such a thing as a toroidal output transformer?

<thinking>......... R.K. style......................................

Would the core end up being unrealistically large? What do you think?

B

#### Bob

Jan 1, 1970
0
-snip-
Well you dropped in at a decent time. Between the filters and whatnot,
things are down below a dull roar at the moment. I don't think it will stay
that way. Anyhow, it was nice to see you drop by. Toroids, huh? What's
the deal with them?
Phil

A toroid has the advantage of a more compact magnetic field, and I thought that
would give an advantage to players who like to sit on their amps, and get hum
broadcast into their single coils. I'm putting the toroids on the bottom of the
amp chassis, and there should be no radiated field outside the amp.

I will test it with my Strat, a sort of $1000 magnetic field finder! Toroids aren't all peaches and cream though, their wide frequency response can bring more line noise into the amp... I may put a ferrite choke on the power wires... not to mention their price, about double a standard unit. I need 2, for filament and plate supply, cost me about$100.

B

#### Bob

Jan 1, 1970
0
-snip-
Amp junkies can be itentified by the solder burns on their hands.

Do you know of a source for toroidal transformers with both a high voltage
and filament windings or do you wind your own? I found Hammond have decent
toroidals but don't have mixed windings. They also tend to be quite pricey. I
find it had to justify the extra cost when recycled iron can be had so cheap.

I'm trying a pair of Hammond's right now! I will do some tests between identical
amps with and without torus transformers. I've never seen a tube type power
toroid... you'd have to order it made.

I might also try a standard filament tranny with a toroid plate tranny... I
bought a box of E-I filament trannys cheap - about \$7 each, with slight damage
cosmetically speaking... 12.6VCT @ 2A ... was a good deal so I want to use them.
Is there such a thing as a toroidal output transformer?

I haven't seen one - don't know if there is an advantage to one...
<thinking>......... R.K. style......................................

Would the core end up being unrealistically large? What do you think?

I'm not a tranny expert by any means, although I have wound a lot of them! I
couldn't tell you how a torus would do as an output... but for fun I may try a
toroid filament tranny as an output... it would match an EL84 to 4 ohms! But it
may saturate with the DC...

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