Backstar 5 watt combo to tweed Princeton. Issues

[phsyconoodler]

I'm gutting a blackstar 5 watt combo and converting to a tweed Princeton circuit but the heater supply is driving me batty. It has a +15 -15v supply but not a dedicated heater supply.. 
  In the schematic it shows the heaters connected to the +15v side and has a resistor and all the heaters wired together in series. I've not encountered that before.
 Weird to me. Normal for them and their insanely complicated amp design. I'm not going to wore all the heaters in series and then to ground or is there something I'm not getting here?
  Any ideas? I can rip out the PT and replace it easily enough

[kagliostro]

Can you show the schematic ?

Which are the AC voltages on the PT ?

Franco

[phsyconoodler]

DC heaters. Duh. Just gotta figure out how much of this circuit I need to get 12v on the heaters

[phsyconoodler]

It has 3 ac taps. 2 blue and one green. The blue read 98vac and 18vac and the gree reads 7.8vac.
Now if i have a brain(jurys out on ttat yet) the hreen is for the power tube heaters and the blues are for the ÷ - 15v supply. It rectifys the +15v and uses a couple resistors to get the preamp tube heaters online. Right or wrong?

[John]

It has 3 ac taps. 2 blue and one green. The blue read 98vac and 18vac and the gree reads 7.8vac.
Now if i have a brain(jurys out on ttat yet) the hreen is for the power tube heaters and the blues are for the ÷ - 15v supply. It rectifys the +15v and uses a couple resistors to get the preamp tube heaters online. Right or wrong?

Makes sense to me. Do you see some diodes & caps in the heater string anywhere?

[tubenit]

phsyconoodler,

Great to see you on the forum! Hope life is going well for you.  Missed seeing you around. 

I don't know the answer to your specific question, but I might be inclined to simply purchase a small 6.3v transformer rather then a new PT to run the heaters on the preamp tubes.

With respect, Tubenit

[phsyconoodler]

Well it had two tubes in it. One el84 and one 12ax7.
  Just confusing figuring out how the heaters are wired. It used a + -15v supply to run its Miriad of transistors and other ss nonsense. Thee must be a way to wire it up. It shows the heaters in series run off the 15v +

[jjasilli]

Ditto to kagliotro:  Can you show the schematic ?


Meanwhile, I'm thinking this.  Heaters can be run in a series string; then their voltages add-up.  So a 6V preamp tube + 6V power tube = 12V.  BUT, these tube's filaments have different current draw, which skews the simple analysis.  The different current draw is cased by less resistance in the power tube filament, allowing more current to flow. dropping the voltage to Presumably this can be fixed with a series resistor which equalizes the tube's current draw, while simultaneously dropping the heater voltage from 15 > 12.

But this is just a guess.

[sluckey]

You can't put tube filaments in series if they don't have the same or very, very close current requirements. Well you can if you get clever with some parallel and series resistors. You'ld probably get a headache trying to figure it out.

I suspect the 'heaters in series' comment from phsyco refers to the two filaments inside the 12AX7 bottle. Then a resistor would be placed in series with that to limit the current to the required 120mA for 12V operation.

[phsyconoodler]

I'm just going to change the transformer. I have a good one here. Saves a lot of hassle on my end.
 Thus original circuit is the most reficilous thing I've ever seen inside a 5 watt amp ever. 

[kagliostro]

Just to know, is this the +15v -15v PS to which you refer ?



--

The Blackstar HT5 schematic I've, shows only one tube, a 12ax7 tube, no Power Tube section

so the current available on that PS is less than that you need for a 12ax7 plus a power tube, good to purcase a dedicated heater transformer

But, if the amp has SS power section, consider that on the original PT there isn't enough current for B+ to supply also the Power Tube ......

Ciao

Franco

[PRR]

> is this the +15v -15v PS to which you refer ? .... no Power Tube section...

But notice the voltage-doubler at bottom, making a "BIAS" point. That suggests a power tube in there somewhere?

Then "V1c" would be a 0.150A 12.75V (12.6) heater. Or "83" is a 12V 0.050A lamp (makes 12.6V).

[DummyLoad]

> is this the +15v -15v PS to which you refer ? .... no Power Tube section...

But notice the voltage-doubler at bottom, making a "BIAS" point. That suggests a power tube in there somewhere?

Then "V1c" would be a 0.150A 12.75V (12.6) heater. Or "83" is a 12V 0.050A lamp (makes 12.6V).

i believe it's a 12BH7 in P-P - see attached schema: 2nd page, header to socket on right of page.


--pete

[sluckey]

I don't think that's the correct schematic. Phsyconoodler  said "Well it had two tubes in it. One el84 and one 12ax7." And does it even matter? He's gutting it and building a tweed Princeton and he's gonna use another PT.

[kagliostro]

On the schematic I see the bias connected to ..... a pair of IRF830 ?? ( ... NO there are capacitors on the path .... ??)



My SS knowledge is so poor .......

--

Only Phsyconoodler can say if we are looking to the correct schematic (I didn't see any 12BH7 on the schematic ... ??)

when he was talking about a pair of tubes may be I misunderstand, but I was thinking to the Princeton, not to the Blackstar

Franco

p.s.: Why they draw schematics that way ? It's a pain to follow the signal path !

-----

EDIT: I've find this other schematic (see the attached .pdf) that has 12BH7 Power Tubes, DummyLoad is right !

[sluckey]

I didn't see any 12BH7 on the schematic ... ??

Look at the very top of that last pic you posted. What do you see written just above CON1? (Hint... It's written vertically so you may have to turn your monitor on it's side. 

when he was talking about a pair of tubes may be I misunderstand, but I was thinking to the Princeton, not to the Blackstar

Maybe. But he specifically mentioned 12AX7 and EL84. Phsyco is a big Fender guy. He knows there ain't a EL84 in a tweed Princeton. However, if he just took a quick look at the amp he may have confused a 12BH7 for a EL84.

a pair of IRF830 ?? ( ... NO there are capacitors on the path .... ??)

Those are the LTP PI.

[kagliostro]

Many thanks Steve

as I told those schematics confuses me (more than I'm allways    )

Thanks Again

Franco