B+ Delay - Is this a problem?

[Rob11966]

Hi Folks,

I am building an 845 amp. The circuit has a soft start to limit inrush current but I am contemplating a delay on the B+ to help prevent cathode stripping.

The question is this - is it a problem to delay the B+ by 10 seconds or so but not delay the bias voltage. In other words, the bias voltage will be applied on amp power up but the B+will be delayed by about 10sec.

I have attached a link to the schematic for reference.

Thanks for the advice,

Rob

http://www.audiodesignguide.com/New845/New845v2.html

[Tweeker]

How are you switching the 900VDC B+ for the delay?

Turing on the bias first is fine.

[DigitalJunkie]

Originally posted by Tweeker
How are you switching the 900VDC B+ for the delay?

Turing on the bias first is fine.

Turning on the bias first is even preferable,that way the tubes can not be in an 'uncontrolled' state where there is plate voltage,and no bias.

[chrish]

Good luck with the build Rob! I am sure from looking at your other work that it will be beautiful! I hope it sounds better than it looks in the photos you linked to, there is a lot of money crammed in to one butt ugly box!

Cheers,

Chris

[Rob11966]

Thanks for the replies.

Hi Tweeker. As you can see from the schematic, the 900v is a combination of two lower voltage transformer windings (about 380 from memory, I can't access the schematic from work). I was going to place a double pole relay (with 440v ac rated contacts and delay circuit) in either -

a) The secondary of both 380v windings before rectification or

b) The same arrangement after rectification but before they are joined to become 900v

I am inclined to go for option (b) as the soft start on the primary side of the transformer will protect the caps from the inrush current, however, with this option, the caps will be charged without the B+ circuit being closed for 10 seconds or so. With option (a) there will still be an inrush current into the caps.

I would welcome any thoughts on this; advatages, disadvantages?

Hi DigitalJunkie, without the delayed start, the bias would start at the same time as the B+, I would not be applying B+ without bias so it should be OK but I am pleased to hear that delaying the B+ with bias on is better.

Hi Chris, great to hear from you. Yes a few people have commented on the original box. You will be relieved to hear that I have modified this part of it greatly. I hope when finished that it will be a work of beauty and I will post pictures when complete.

Cheers all, will look forward to some more advice.

Rob

[Tweeker]

(with 440v ac rated contacts and delay circuit)

You may have to switch before rectification to avoid violating your switch/relays ratings, most are seriously derated or not rated for DC as you dont cross through zero preventing arcs from being extinguished etc.

[Tweeker]

For example, a 440VAC 10A switch might typically be rated for 380VDC .2amp resistive/ .1amp inductive load.

[brady]

why not use separate switch?

[Rob11966]

Originally posted by Tweeker


You may have to switch before rectification to avoid violating your switch/relays ratings, most are seriously derated or not rated for DC as you dont cross through zero preventing arcs from being extinguished etc.

Thanks Tweeker, I think that this resolves the issue of placement.

As an aside, the inrush current issue is probably not as bad as I thought because the transformers with the B+ also have heater windings on them.

Hi Brady, Do you mean a seprate manual switch for the B+? I was trying to avoid this as I want a 'one switch start' (so that anyone can start the amp simply) and also, I would like to avoid accessible switches with the whole B+ running through.


Rob

[ray_moth]

A popular arrangement, which I think offers the best solution, is:

1) Inrush current protection by using a thermistor in the primary of the power tranny, giving a few seconds delay, at most, to all circuits (heaters, B+ and bias); if you're using more than one tranny, drive them all through the thermistor.

2) SS rectified bias, causing the bias voltage to rise quickly its proper level once the thermistor has reached its operating temperature.

3) Thermionic rectified B+, using either a center-tapped HV secondary and a double-diode such as GZ34 or a pair of damper diodes; or a single winding with a rectifier bridge which can be hybrid SS/dampers.

Thermionic rectification gives 20-30 seconds of B+ delay. You definitely want the bias to come up well before B+. Some designers also build in a safeety switching-circuit, to kill B+ if the bias ever fails for any reason. Obviously the switching needs to be after the B+ filter circuit, in order that B+ will die immediately if necessary.