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Old 2nd January 2005, 10:45 PM   #1
sith is offline sith  Croatia
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Default Voltage problem

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.

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Old 2nd January 2005, 10:51 PM   #2
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Hi,

Quote:
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.

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Old 3rd January 2005, 08:59 AM   #3
sith is offline sith  Croatia
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AC voltage is 100V
solid state bridge
there was no load

It should be running preamp based on pair of 6sn7 or ECC's...
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Old 3rd January 2005, 09:04 AM   #4
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Hi,

Quote:
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.

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Old 3rd January 2005, 09:30 AM   #5
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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
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Old 5th January 2005, 06:08 PM   #6
sith is offline sith  Croatia
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Quote:
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.
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Old 6th January 2005, 03:15 AM   #7
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Hi,

Quote:
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.

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Old 6th January 2005, 06:26 PM   #8
sith is offline sith  Croatia
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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?
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Old 7th January 2005, 01:36 AM   #9
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Quote:
Originally posted by fdegrove
I still feel 160 V B+ for a preamp is way too low unless you're building a CF perhaps.
Something wrong with CFs?

Quote:
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?

Quote:

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
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Old 7th January 2005, 02:18 AM   #10
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Hi,

Quote:
Something wrong with CFs?
Like in if you want gain, that's what wrong with them...

Quote:
Say, what is the transformer rated at anyways?
As it was used for feeding a sextet of ECC88s, it should be O.K. currentwise.

Quote:
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....

Quote:
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?

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