Valve amp transformer bodge

  • Thread starter Gareth Magennis
  • Start date
G

Gareth Magennis

Guest
Hi,

I have a WEM Dominator Bass Mk1 with a blown mains transformer (shorted
primary).
http://www.vintagehofner.co.uk/britamps/watkins/schematics/dommk3.html

As a possible alternative to a rewind, which I haven't investigated yet
......

if I got 2 x 240v : 6v transformers (I am in the UK) and connected both
secondaries together to get 240v back out for the HT, could I use the 6v as
a heater supply, or is it best to use a separate transformer for this (or a
better way to get the 6.3v)

I realise that the rectified 240v may be a little high for this amp, but
maybe loading the 6v tap with heaters might be just the thing to bring it
down somewhat, or I could use another method.

(I also note the 6.3v winding is centre tapped and grounded on this circuit)



Cheers,


Gareth.
 
Gareth Magennis wrote:
Hi,

I have a WEM Dominator Bass Mk1 with a blown mains transformer (shorted
primary).
http://www.vintagehofner.co.uk/britamps/watkins/schematics/dommk3.html

As a possible alternative to a rewind, which I haven't investigated yet
......

if I got 2 x 240v : 6v transformers (I am in the UK) and connected both
secondaries together to get 240v back out for the HT, could I use the 6v as
a heater supply, or is it best to use a separate transformer for this (or a
better way to get the 6.3v)

I realise that the rectified 240v may be a little high for this amp, but
maybe loading the 6v tap with heaters might be just the thing to bring it
down somewhat, or I could use another method.

(I also note the 6.3v winding is centre tapped and grounded on this circuit)



Cheers,


Gareth.
Click to expand...
Hi Gareth,

Have you no isolation transformers on your side of the pond?

Remember that the maximum current you can draw is the ratio of the 6.3V
to its amperage compared to 240V. So if the 6.3VAC was rated at 10A,
then the other side of your proposed pair would be 240V @ 262.5ma
(assuming 100% efficiency). I suspect you will be having trouble with
your idea if the 6.3V output current is even less!

John :-#)#

--
(Please post followups or tech enquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
 
On Nov 7, 7:54 am, "Gareth Magennis" <sound.serv...@btconnect.com>
wrote:
Hi,

I have a WEM Dominator Bass Mk1 with a blown mains transformer (shorted
primary).http://www.vintagehofner.co.uk/britamps/watkins/schematics/dommk3.html

As a possible alternative to a rewind, which I haven't investigated yet
.....

if I got 2 x 240v : 6v transformers (I am in the UK) and connected both
secondaries together to get 240v back out for the HT, could I use the 6v as
a heater supply, or is it best to use a separate transformer for this (or a
better way to get the 6.3v)

I realise that the rectified 240v may be a little high for this amp, but
maybe loading the 6v tap with heaters might be just the thing to bring it
down somewhat, or I could use another method.

(I also note the 6.3v winding is centre tapped and grounded on this circuit)

Cheers,

Gareth.
Click to expand...
By bodge, do you mean what us west of the pond folks call kluge?

Anyway, the voltage rating of the capacitors on rectified output will
determine if the rectified 240V is excessive or not. The tubes can
take it. Also, you will have double losses going from your 240 input
to 6V and back to 240V, so you will probably only have 230V on the
output or maybe less. Tapping the 6 V for filaments is reasonable, I
would connect a couple of resistors in series across the 6V and
connect the midpont to ground to approximate their grounding scheme,
it probably is to reduce hum from the input amplifier tubes. Make the
resistors 10K or so to not overload the first 240 - 6V transformer.
After things are running, check both transformers that you are not
overheating anything<g>.
 
"John Robertson" <spam@flippers.com> wrote in message
news:nt2dnb61KqY2GAfNnZ2dnUVZ5v6dnZ2d@giganews.com...
Gareth Magennis wrote:
Hi,

I have a WEM Dominator Bass Mk1 with a blown mains transformer (shorted
primary).
http://www.vintagehofner.co.uk/britamps/watkins/schematics/dommk3.html

As a possible alternative to a rewind, which I haven't investigated yet
......

if I got 2 x 240v : 6v transformers (I am in the UK) and connected both
secondaries together to get 240v back out for the HT, could I use the 6v
as a heater supply, or is it best to use a separate transformer for this
(or a better way to get the 6.3v)

I realise that the rectified 240v may be a little high for this amp, but
maybe loading the 6v tap with heaters might be just the thing to bring it
down somewhat, or I could use another method.

(I also note the 6.3v winding is centre tapped and grounded on this
circuit)



Cheers,


Gareth.





Hi Gareth,

Have you no isolation transformers on your side of the pond?

Remember that the maximum current you can draw is the ratio of the 6.3V to
its amperage compared to 240V. So if the 6.3VAC was rated at 10A, then the
other side of your proposed pair would be 240V @ 262.5ma (assuming 100%
efficiency). I suspect you will be having trouble with your idea if the
6.3V output current is even less!
Click to expand...


Hmm, I haven't really thought this through, I didn't consider an isolation
transformer!

I guess an isolation transformer and a heater transformer would be a lot
less iron overall.




Thanks,


Gareth.
 
"John Robertson" <spam@flippers.com> wrote in message
news:nt2dnb61KqY2GAfNnZ2dnUVZ5v6dnZ2d@giganews.com...
Gareth Magennis wrote:
Hi,

I have a WEM Dominator Bass Mk1 with a blown mains transformer (shorted
primary).
http://www.vintagehofner.co.uk/britamps/watkins/schematics/dommk3.html

As a possible alternative to a rewind, which I haven't investigated yet
......

if I got 2 x 240v : 6v transformers (I am in the UK) and connected both
secondaries together to get 240v back out for the HT, could I use the 6v
as a heater supply, or is it best to use a separate transformer for this
(or a better way to get the 6.3v)

I realise that the rectified 240v may be a little high for this amp, but
maybe loading the 6v tap with heaters might be just the thing to bring it
down somewhat, or I could use another method.

(I also note the 6.3v winding is centre tapped and grounded on this
circuit)



Cheers,


Gareth.





Hi Gareth,

Have you no isolation transformers on your side of the pond?

Remember that the maximum current you can draw is the ratio of the 6.3V to
its amperage compared to 240V. So if the 6.3VAC was rated at 10A, then the
other side of your proposed pair would be 240V @ 262.5ma (assuming 100%
efficiency). I suspect you will be having trouble with your idea if the
6.3V output current is even less!

John :-#)#
Click to expand...


OK, so this amp, according to my calculations, requires a minimum of just
over 15 Watts of 6.3v heater power, having 3 x ECC83 @ 300mA and 2 x EL84 @
760mA.

In the real world, how many ballpark Watts of power supply transformer
should you be thinking about for the rest of the amplifier?





Cheers,



Gareth.
 
"hr(bob)

Also, you will have double losses going from your 240 input
to 6V and back to 240V, so you will probably only have 230V on the
output or maybe less.

** With no load, the output voltage will be close to 240V as the combined
turns ratio is exactly 1:1. If two 50VA trannys are used, each will have a
regulation loss under load of about 10% - so the actual output voltage will
be under 200V.

A far worse problem is the primary magnetising current which the first
transformer must supply to the second. The typical amount for a 50VA
conventional tranny is 60mA on a 240V primary, but if operated in reverse it
increases according to the turns ratio.

It becomes 40 times 0.06 or 2.4 amps - eating up almost 30% of the VA rating
of the first tranny.

Things get better if two toroidal trannys are used, as the magnetising
current is far lower.


..... Phil
 
"Gareth Magennis"

Hmm, I haven't really thought this through, I didn't consider an isolation
transformer!
Click to expand...
** Go for a 240V / 220V isolation step down of say 50VA rating.

Plus a 6.3V, 3 amp tranny.


..... Phil
 
Gareth Magennis wrote:
"John Robertson" <spam@flippers.com> wrote in message
news:nt2dnb61KqY2GAfNnZ2dnUVZ5v6dnZ2d@giganews.com...
Gareth Magennis wrote:
Hi,

I have a WEM Dominator Bass Mk1 with a blown mains transformer
(shorted primary).
http://www.vintagehofner.co.uk/britamps/watkins/schematics/dommk3.html

As a possible alternative to a rewind, which I haven't investigated
yet ......

if I got 2 x 240v : 6v transformers (I am in the UK) and connected
both secondaries together to get 240v back out for the HT, could I
use the 6v as a heater supply, or is it best to use a separate
transformer for this (or a better way to get the 6.3v)

I realise that the rectified 240v may be a little high for this amp,
but maybe loading the 6v tap with heaters might be just the thing to
bring it down somewhat, or I could use another method.

(I also note the 6.3v winding is centre tapped and grounded on this
circuit)



Cheers,


Gareth.





Hi Gareth,

Have you no isolation transformers on your side of the pond?

Remember that the maximum current you can draw is the ratio of the
6.3V to its amperage compared to 240V. So if the 6.3VAC was rated at
10A, then the other side of your proposed pair would be 240V @ 262.5ma
(assuming 100% efficiency). I suspect you will be having trouble with
your idea if the 6.3V output current is even less!

John :-#)#




OK, so this amp, according to my calculations, requires a minimum of
just over 15 Watts of 6.3v heater power, having 3 x ECC83 @ 300mA and 2
x EL84 @ 760mA.

In the real world, how many ballpark Watts of power supply transformer
should you be thinking about for the rest of the amplifier?





Cheers,



Gareth.
Click to expand...
If you have the original transformer you can get some sort of an idea of
the current requirements by checking the wire gauge for each winding.

That's about all I can think of other than figuring out what style of
output you have and then calculating (based on) the maximum plate
dissipation (usually in watts) and convert that to current as you
roughly know the DC voltage.

Beyond that, pull out some tube amp design books and see what they
recommend for transformers for your style outputs...

John :-#)#
--
(Please post followups or tech enquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
 
"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:ag0m8dFsgtfU1@mid.individual.net...
"Gareth Magennis"


Hmm, I haven't really thought this through, I didn't consider an
isolation transformer!

** Go for a 240V / 220V isolation step down of say 50VA rating.

Plus a 6.3V, 3 amp tranny.


.... Phil
Click to expand...

That sounds like the plan.


Thanks,


Gareth.
 
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