Stuck on Standby

[CSlee]

Hello all,

I am currently building a valve preamp for my dissertation and seemed to have got stuck on the rectifier.

I planned to use a solid state rectifier to save money, but have come across a problem. Having a basic bridge rectifier and reservoir capacitor, when i turn the power on i will cause a surge and therefore cause cathode stripping.

Because I am running out of time to finish my preamp, I was wondering what peoples views are on (just for the purpose of testing) leaving out a standby relay circuit? Would that cause too much damage to the cathodes?

Or do you think I would be better off building a standby circuit, or even switching straight to a valve rectifier? Is it common practice to use valve rectifiers in valve amps - am I just being too cheap?

Hope that makes sense

Cheers


Charlie

[Majestic]

You should deference my reply to one of the more knowledgable members of the forum, but if I could temporarily offer a quick-and-dirty suggestion: How about inserting a thermistor to mimic a slow turn-on?

[ErikdeBest]

Hi Charlie

Interesting project for a dissertation... what are you exactly doing: developing the whole thing or just parts of it? Can you show a schematic?

I have no hard evidence, but from reading here and there I have the impression that cathode stripping is a somewhat overrated thing: sure it can happen, but mostly with larger (power) tubes, such as the regulator devices (6AS7, 6528, etc). As you are building a preamp I suppose you are using a small tube (ops, valve for the British) which are less prone to cathode stripping.

I do not know if researching other circuits was part of your assignment, but you would have seen that there is a large amount of preamps (and even poweramps) that just use SS diodes in the PS without any delay or whatever...

Erik

[boywonder]

Search the tubes forum for "cathode stripping" and "start up delay", there are loads of posts about the subject.

Many folks here do not think it's a problem for B+ less than a kilovolt or so, although Morgan Jones seems to worry about it a bit in his book.

Another practical issue with SS rectification (depending on your B+ voltage) is exceeding the voltage rating of your electrolytic caps during start-up.

What is your B+ voltage and how much voltage overshoot are you seeing during start up?

What tubes are you using?

[Jeb-D.]

As Majestic implied,

Use an inrush current limiter (ICL) if you are worried about it and want to keep SS rectification.

It's a negative temperature coefficient thermistor. It's resistance is large when room temp and then the resistance decreases as it's temperature increases. Mouser carries a large variety of them. Check the datasheet to pick the one most suitable for your application.

[CSlee]

Hello,

It was my call to build a preamp, and have only just found this site, which although is an amazing resource, i wish i found it earlier for such designs. But it was always going to be a valve amp from the start.

One of the books i have been reading is Morgan Jones' Valve Amplifiers, and from what you said, he is very concerned on the matter of cathode stripping even on preamp rectifiers.

One thought i didnt say earlier was - will my reservoir cap charging up give me that few extra seconds so there isnt such a massive surge?

Im afraid that im not used to all the terms used on this site, and from Erik have only just realised that OPS means valve. But ill show you the schematic... sorry for the poor quality image

Im using just the microphone parts of the circuit, an EF86 pentode and a 12AX7 triode. I was planning on changing the output to a cathode follower to lower the output impedance...

Your thoughts would me massively helpful


Cheers


Charlie

[Jeb-D.]

will my reservoir cap charging up give me that few extra seconds so there isnt such a massive surge?

The cap is what causes a surge (the cold filaments contribute as well). It acts like a short circuit when power is initially applied. By placing a ICL in between the rectifier and cap, it will limit the turn on current which in turn will also bring up the B+ voltage more slowly allowing the tubes to heat more before the full plate voltage is applied.

Or, you could place the ICL on the primary of the transformer, which will also limit the extra current drawn from the cold filament. This configuration will protect more against turn on surge, but less for cathode stripping when compared to the configuration above. Not that I really think cathode stripping would be a problem in this case.

[richwalters]

Originally posted by Charlie Slee
Hello all,

I am currently building a valve preamp for my dissertation and seemed to have got stuck on the rectifier.

Cheers

Charlie

As an ex teacher that's no excuse to be stuck by a rectifier. As you mentioned solid state, and since you are doing a dissertation the question of mosfet polarity at this stage should be well understood. A little digression in the enclosed pic should bring clear. The relay was used in a fixed bias amp as if the bias failed the contacts would turn mosfet off; ie, an HT/B+ interlock.

The voltage rating of the mosfet be chosen with good margin.
& low Rds reduces Power loss.
Ferrite beads in each drain and gate avoid RFI.
Ingenuity unlimited.


richy

[ErikdeBest]

Hi Charlie

Im afraid that im not used to all the terms used on this site, and from Erik have only just realised that OPS means valve. But ill show you the schematic... sorry for the poor quality image

I just don't want to confuse you more, so I have to say that the word 'ops' doesn't mean anything related to audio. It is actually a word used a lot in Portuguese, and means something like: hold on!

[CSlee]

Clearly the schematic didnt upload properly, so ill do it again.

Thanks for your comments Jeb-D, I shall certainly look into using an ICL. Adding to that, I think you are all right in saying that it might not be a problem. From the people I have spoken too it seems that using the valves I am wouldnt make that much difference in the end.

Boywonder - I am afraid I havnt even turned it on yet, still in the building stage. Time is running out!! Like I said im not hugely cut out with the lingo at the moment, what do you mean by B+? Hopefully it will be on the schematic anyway.

You have all been most helpful, and any comments you have on my schematic, and any thoughts on having a cathode follower at the output, instead of driving through the anode would be very helpful indeed.

Cheers

Charlie x