Greetings -
The build project I'm developing uses a PT with no bias tap. Simple enough - rectify the bias supply from the B+ windings with inverted diodes. My question is whether I want to put those bias rectifiers before or after the standby switch.
It seems to me that having the bias at full value before applying B+ would be a Good Thing(tm), but is there a reason to have it come up with B+ instead?
Bias and standby
Moderators: pompeiisneaks, Colossal
Re: Bias and standby
Take the bias from before the standby switch. That's the safest, easiest, most reliable way.
Re: Bias and standby
I'd do the same. However, this wouldn't work in a situation when you have the Stand-by switch on the CT of HT winding (what I prefer).
Cheers,
-Tomek Drabas
___________
‎"One should not pursue goals that are easily achieved. One must develop an instinct for what one can just barely achieve through one's greatest efforts."
—Albert Einstein
-Tomek Drabas
___________
‎"One should not pursue goals that are easily achieved. One must develop an instinct for what one can just barely achieve through one's greatest efforts."
—Albert Einstein
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JazzGuitarGimp
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- Joined: Mon Jul 23, 2012 4:54 pm
- Location: Northern CA
Re: Bias and standby
Keep in mind that if you place the bias tap after the standby switch, you'll need to locate the standby switch between the transformer secondary and the input to the main B+ rectifier, rather than the more-common location of in the B+ circuit. If the design uses a center-tapped, two-diode full wave bridge for the main B+, you will need a double-pole standby switch - one pole for each of the secondary high-voltage AC windings.
Lou Rossi Designs
Printed Circuit Design & Layout,
and Schematic Capture
Printed Circuit Design & Layout,
and Schematic Capture
Re: Bias and standby
How about not bothering with a standby?
If that's not acceptable, how about switching the screen grid supply, or switching the power tube cathode ground return?
Pete
If that's not acceptable, how about switching the screen grid supply, or switching the power tube cathode ground return?
Pete
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JazzGuitarGimp
- Posts: 2357
- Joined: Mon Jul 23, 2012 4:54 pm
- Location: Northern CA
Re: Bias and standby
According to Merlin Blencow in "Designing Power SUpplies For Valve Amplifiers", There is no need for a standby switch in guitar amps. He says there are transmitting tubes that will suffer cathode depletion if B+ is applied before the filament is hot, but this is not a concern with receiving tubes.pdf64 wrote:How about not bothering with a standby?
If that's not acceptable, how about switching the screen grid supply, or switching the power tube cathode ground return?
Pete
Lou Rossi Designs
Printed Circuit Design & Layout,
and Schematic Capture
Printed Circuit Design & Layout,
and Schematic Capture
Re: Bias and standby
I don't see any sense in using a standby switch either, the trouble is now everyone expects each amp to have one.JazzGuitarGimp wrote:According to Merlin Blencow in "Designing Power SUpplies For Valve Amplifiers", There is no need for a standby switch in guitar amps. He says there are transmitting tubes that will suffer cathode depletion if B+ is applied before the filament is hot, but this is not a concern with receiving tubes.pdf64 wrote:How about not bothering with a standby?
If that's not acceptable, how about switching the screen grid supply, or switching the power tube cathode ground return?
Pete
Mark
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Cliff Schecht
- Posts: 2629
- Joined: Wed Dec 30, 2009 7:32 am
- Location: Austin
- Contact:
Re: Bias and standby
The bias supply should come up when the heaters are fired on. If this is not the case, the tubes will have no bias voltage (i.e. set to draw max plate current!) for a short instance and while this usually doesn't kill them instantly, it stresses the power tubes every time you fire on from standby (especially with a quick SS rectifier). I don't break the ground for the HV CT for exactly this reason.
On my personal amps I usually leave off the standby completely. I don't have a use for them. But when it's expected, I usually put the switch in the traditional spot that breaks the B+. Yeah it's not the best spot but in practice I've never had a heavy duty SB switch fail, even when feeding a C-L-C network before the plates ala 5E8A. In this case (with no SB) the heaters are cold when the bias supply and B+ are coming up and so no current will flow until the cathode has developed an electron cloud (via the heaters).
On amps that use a bridge rectifier, the raw AC voltage has to be capacitively coupled to the bias circuit. I've had this detail nip me in the ass before!
On my personal amps I usually leave off the standby completely. I don't have a use for them. But when it's expected, I usually put the switch in the traditional spot that breaks the B+. Yeah it's not the best spot but in practice I've never had a heavy duty SB switch fail, even when feeding a C-L-C network before the plates ala 5E8A. In this case (with no SB) the heaters are cold when the bias supply and B+ are coming up and so no current will flow until the cathode has developed an electron cloud (via the heaters).
On amps that use a bridge rectifier, the raw AC voltage has to be capacitively coupled to the bias circuit. I've had this detail nip me in the ass before!
Cliff Schecht - Circuit P.I.