no HV center tap

[stingray_65]

I've recently aquired a few old power transformers

i've got two i would hope to be able to use for tube amps, problem is no HV ct

is there a safe way around this?

one has 330vac at 200ma wich would be pefect for a small 6v6 PP
the other has 297 vac at 190 ma would love to do a 18w marshall lite with that.

it just seems when I use the Duncan PSU simulator the ripple is WAY huge if i set it up 1/2 wave and a 5Y3gt recto

on that same note if i put in a choke I loose alot of B+

any suggestions?

[R ski]

the only option would be full wave solid state rectifier bridge, via the HV taps. the only draw back is tapping negative bias via half wave isn't possible. then using fixed bias on the output tubes is the way to go.

[stingray_65]

yah I got to thinking about that after I posted.
I have a nephew who keeps bugging me to make him an amp.
He plays a lot of metal type rock, so I'm kind of thinking the marshall 18 watt

build it like hoffmans stout 18, no temolo and he uses the unused 1/2 of the 12ax7 as a second gain stage just with a ss recto.

it ought to suit him for the style he plays.

I just won't know what to expect tone wise

[tubeswell]

Hybrid options

There are some hybrid options for PTs with no HT CT if you want to use a tube rectifier. (Pictures courtesy of Mr Valve Wizard)

[stingray_65]

Tubeswell!

You're a crafty Kiwi no doubt!

That 3rd example is exactly the solution I'm looking for I believe!

I was working off memory earlier and I transcribed a few details between the transformers.

I'm going to use an old PAECO

115 primary
289 VAC @200mA secondary
207 VAC @175mAsecondary
and 3
6.3 VAC @ 4.5 A
and 1
6.3 VAC @ 1.4A

YOU NOW BRING UP 2 QUESTIONS (math)

First: the two limiting resitors. what am I looking to calculate here? are they current limiting? (my guess) and how do I calculate thier values

Second: (more ohms law) (I told you I get confused with it )

with no 5VAC supply for the 5Y3GT I want to add a voltage dropping resistor on one of the extra 6,3 VAC filiment supplys

so to figure the resistance of the dropping resistor:

OHMS ( what i'm looking for ) = voltage / amperage
is the voltage the drop i'm anticipating (1.3)/ the amps of the 5Y3gt heater (2.0)

.65ohms=1.3v drop/2.0 A

or is it some combination of supplied voltage and current?

oh yeah and 6.3V x 2A = 12.6w of power to dissapate


HELP!

[unparalleled]

Tubeswell!

You're a crafty Kiwi no doubt!

That 3rd example is exactly the solution I'm looking for I believe!

I was working off memory earlier and I transcribed a few details between the transformers.

I'm going to use an old PAECO

115 primary
289 VAC @200mA secondary
207 VAC @175mAsecondary
and 3
6.3 VAC @ 4.5 A
and 1
6.3 VAC @ 1.4A

YOU NOW BRING UP 2 QUESTIONS (math)

First: the two limiting resitors. what am I looking to calculate here? are they current limiting? (my guess) and how do I calculate thier values

Second: (more ohms law) (I told you I get confused with it )

with no 5VAC supply for the 5Y3GT I want to add a voltage dropping resistor on one of the extra 6,3 VAC filiment supplys

so to figure the resistance of the dropping resistor:

OHMS ( what i'm looking for ) = voltage / amperage
is the voltage the drop i'm anticipating (1.3)/ the amps of the 5Y3gt heater (2.0)

.65ohms=1.3v drop/2.0 A

or is it some combination of supplied voltage and current?

oh yeah and 6.3V x 2A = 12.6w of power to dissapate


HELP!

You could use 6X4 or 6X5GT if you want octal rectifier, these will give you 70mA.
for more HT two in parallel 140 mA. Both are 6 volt heaters.

[stingray_65]

I like the Idea, but like all my builds i'm on a serious budget. 5Y3's I got in spades.

unless I find some in a donor chassis I'll likely not get any unless they are CHEAP

as it is i'll probably have to horse trade to get some el34's

any one got a need for 4 7868 and their novar sockets fresh pulls from a Conn organ? hehehe

[Don Moose]

So is a (6.3 - 5)/2== 0.65-ohm, 2*2*.65== 2.6W (use a 5W) resistor easier/cheaper to get, or a 6X4/6X5GT?

** some retentive types would have you use a separate 0.325 resistor on each side of the heater.

Hope this helps!

[tubeswell]

Or there's the EZ81 (probably more reliable than a 6X5) still in production. The limiting resistors are surge protection for when you first flip the on-switch. I don't think they're that critical, but there is an article of the valve wizard site for calculating them. (Same place I got those pics from)