• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Stuck on Standby

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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
 
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
 
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?
 
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.
 
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
 
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.
 
Charlie Slee said:
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
 

Attachments

  • soft start.jpg
    soft start.jpg
    49.1 KB · Views: 233
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!
 
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
 

Attachments

  • schematic1.jpg
    schematic1.jpg
    86.9 KB · Views: 239
Geez, how did you pick that design? (1) It's not a purposely designed microphone preamp (EDIT: oh, is it a microphone preamp you're building, I seem to remember that from another thread?), it looks like 1 channel of a hifi preamp, (2) it's obviously very dated, as some caps are much smaller than one would use for hifi at today's prices.
 
Charlie,

When the B+ rail voltage is below 400 V., cathode stripping is not much of an issue. That's especially true for comparatively fast warming preamp types.

IMO, the suggestion to use a negative temperature coefficient (NTC) inrush current limiting thermistor is the way to go. The "fix" entails very little fuss and muss, while doing good things for the tubes, rectifying diodes, and power trafo. Check this data sheet out. You should be able to source those GE/Thermometrics parts or something similar in the UK. The CL90 looks like a candidate for this job. Wrap the part in wool or acrylic yarn to minimize the operating state resistance. Place the thermistor between the rectifying diodes and PSU filter. Doing so allows a few seconds head start for the tube heaters, while taming the turn on surge.
 
Like I said previously, I wish I found this website when looking for a preamp design. The design im using is very dated, I think its a 1950s design, unfortunatly it is far too late in the day to change it now, which really is a shame.

Richy - to be honest im not sure if its missing a resistor to ground, would you recommend doing that?

Eli - I will certainly try that, you have been very helpful, thanks.

Richy - I am studying an Audio technology BSC and have only ever been taught about basic transistor amps. Everything I know about valve amps is all self taught. I am not making excuses I am mearly asking for advice. Im sure everyone on this forum was a 'newbie' at some point, and when they were 'newbies' im sure they all wanted positive comments and advice. That said I thank you for your schematic and will look into adding it to my circuit.

Thanks for your comments


Charlie
 
Cathode-stripping.....

Grossly overrated subject....

There have been literally millions of valve-radios and TV sets where ther full HT is applied directly on turn-on. The valves in these especially the signal valve often lasted the life of the set, say 8 years of daily use/cycling. Its only the main O/P bottles that generally suffered, and these were mostly 'Killed' by another component going faulty, say, a Flyback transformer or boost-cap etc....

I have built a bread-board OTL using the in-famous 6C33, a valve allegedly prone to being damaged by HT on before fully hot. I used this in testing/refinement for a long time with no issue relating to the performance of the valves. It had Full HT at turn-on from cold--never a problem.

I have a 1930 Pilot radio, with allegedly original valves. Was used daily for many years. It still works, no sign of issues, and has a directly-heated (read, fast warm-up, rectifier, type 80)....

Only ever found 2 valves that had gone 'low-emission'--possibly through stripping--maybe through just old-age, Both these valves were cheapo eastern manufacture things....and I was a TV engineer for at least 10 years over the time Transistor sets started coming in..

For a Pre-Amp, Nah--You'll have no problems, Go for it!!:D
 
Charlie, keep up the good work. I've enclosed a small pic of another sim Baxandall design with the 1M5 grid leak fitted on that 2nd tube section. The tone schematic is shown bit different" but the principle is the same. Otherwise the grid will be bouncing producing distortion.

richy
 

Attachments

  • baxand.jpg
    baxand.jpg
    82.6 KB · Views: 102
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.