Voltage problem

[sith]

I have a transformer that after rectification gives 130-140V dc (on rails), but schematics says voltage should be 160V. Is it suitable for usage? What problems can it cause? I can't put extra turns on bobbin, it has no room.

Thanx...

 

[fdegrove]

Hi,

I have a transformer that after rectification gives 130-140V dc (on rails), but schematics says voltage should be 160V.

Can you tell us what the secondaries measure (AC) before recification?
What type of rectification did you use? Full wave? Full wave bridge?
Was the load attached when the measurements were taken?

Otherwise, if the xfromer can put out about twice as much current than the load attached draws, you may need to consider using a voltage doubler as your PS.

Cheers,

 

[sith]

AC voltage is 100V
solid state bridge
there was no load

It should be running preamp based on pair of 6sn7 or ECC's...

 

[fdegrove]

Hi,

AC voltage is 100V

That's really low for a tube based preamp, you could opt for the voltage doubler though if you know the VA rating of that xformer.

Cheers,

 

[Sch3mat1c]

Meh, you only got <5V signal output in a preamp, more than enough drive into the toughest loads if you have some good current flowing. My "All-out" preamp runs 125VAC for the supply...

Tim

 

[sith]

That's really low for a tube based preamp

Yeah, but that's AC voltage on secondaries, not rectified...

Tranny was scraped from device with 6 ECC88, so I thought VA rating isn't an issue.
I also think voltage doubler is not a good option 'cause I'll need to regulate voltage down to 160V.

 

[fdegrove]

Hi,

I also think voltage doubler is not a good option 'cause I'll need to regulate voltage down to 160V.

I still feel 160 V B+ for a preamp is way too low unless you're building a CF perhaps.

If you do the voltage doubler with a couple of single diode tube rectifiers + a PS choke you should arrive at about 250V B+ which constitutes a good starting point.
Otherwise you'll likely end up with something resembling a compressor instead of a preamp.

Cheers,

 

[sith]

OK, I think I'm stuck
Still, if I decide to make a voltage doubler how will that affect the working point ? Would it be any good?

 

[Sch3mat1c]

I still feel 160 V B+ for a preamp is way too low unless you're building a CF perhaps.

Something wrong with CFs?

If you do the voltage doubler with a couple of single diode tube rectifiers + a PS choke you should arrive at about 250V B+ which constitutes a good starting point.

Say, what is the transformer rated at anyways?

Otherwise you'll likely end up with something resembling a compressor instead of a preamp.

Now now Frank, don't scare the poor fellow. You know as well as I do that compression only happens when overdriving a cathode-biased stage (out of class A1 into A2 or AB).

(Which reminds me of an interesting tidbit, before I used Hept'AU7 and Frankenhouse for my system here, I used Revision 3 instead of one... at low volume, they would be equal; at high volume, Revision 3 - which is cathode, not fixed bias - was quieter. Near, but not over driven BTW. Since I won't tolerate imbalance I of course opted for the setup here...)

Tim

 

[fdegrove]

Hi,

Something wrong with CFs?

Like in if you want gain, that's what wrong with them...

Say, what is the transformer rated at anyways?

As it was used for feeding a sextet of ECC88s, it should be O.K. currentwise.

Now now Frank, don't scare the poor fellow. You know as well as I do that compression only happens when overdriving a cathode-biased stage (out of class A1 into A2 or AB).

I knew you'd trip over that one.
Should have said : "You'll have the impression of listening to a preamp that seems to compress the input rather than letting it flow" perhaps....

Still, if I decide to make a voltage doubler how will that affect the working point ? Would it be any good?

Dunno....What do you have in mind?

Cheers,

 

[Sch3mat1c]

Like in if you want gain, that's what wrong with them...

Pffbt. Two tubes can do 10-40dB gain and still have a low CF style Zo.

I knew you'd trip over that one.
Should have said : "You'll have the impression of listening to a preamp that seems to compress the input rather than letting it flow" perhaps....

Oh. One of those "audiophool" terms.

Tim