Remember These?

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Wolfgang
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#256 Re: Remember These?

Post by Wolfgang »

I noticed that the resistor for setting the current with the TL783 is not at the output where it should be. Just a typo?

The 0.1uFcap between the 2 triodes of the input stage was part of the phase problem with the feedback loop in the original 300B OTL DC which caused the motorboating/bass driver problem and was changed to 1uF. Just saying, doesn't need to be an issue here.

I don't want to be a nudge but these amps/tubes are precious and should get a soft start. OTLs/output tubes without soft start will fail over time or wear down a lot faster. A thermistor doesn't protect the tubes, only handles the inrush current and protects the caps in the psu. I use for the 6C33/B+ psu a simple resistor based soft start without solid state parts. Heaters, input stage psu caps have to be protected with a thermistor. Everything is turned on at the same time but the current/voltage through the output tube will ramp up slowly .


B+ soft start.png
Cressy Snr
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#257 Re: Remember These?

Post by Cressy Snr »

Wolfgang wrote: Sat Dec 29, 2018 8:19 pm I noticed that the resistor for setting the current with the TL783 is not at the output where it should be. Just a typo?

The 0.1uFcap between the 2 triodes of the input stage was part of the phase problem with the feedback loop in the original 300B OTL DC which caused the motorboating/bass driver problem and was changed to 1uF. Just saying, doesn't need to be an issue here.
Damn! Yes its a typo. Welcome to my world. I've cocked up so many schematics in the past, I wonder why I bother putting them up anymore. Of course I'm going to have to delete it. As you say OTLs are hairy as is without me putting something up that someone could copy and get more than they bargained for ie a big bang! :roll:

My amp is stable as is, so I'll leave that bit alone for the moment. I was unaware that the 300B original had the 0.1uF coupling cap as mine does.

I appreciate your concern about soft starting these big tubes, but this is a breadboard. The finished job will have a soft start arrangement. Ed has kindly sent me his redundant HT delay board and this will be incorporated in the final boxed up version. :)
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#258 Re: Remember These?

Post by Cressy Snr »

Just been reading up a bit on the 13E1 and it seems to be a real kick-ass monster of a tube.

This from The National Valve Museum http://www.r-type.org/addtext/add020.htm

....The 13E1 has been used as both a high power pulse generator (it will deliver 7 kW pulses at modest duty factor) and as a long wave) broadcasting transmitter. Both types remained in the lists until the 1980s and the last known professional use of the 13E1 was in 1997.

I think that given the above, it has to be the best candidate for single tube OTL operation in existence. Should last for years on modest audio output duty.
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#259 Re: Remember These?

Post by jack »

They are built like tanks. The ones I have have by far the thickest glass of any tube I've seen.

They're beasts...
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Ray P
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#260 Re: Remember These?

Post by Ray P »

Can I just clarify what people mean when they talk about soft-start? is it;

(a) - apply power(filaments and B+) together but ramp up the B+ slowly

or

(b) - pre-heat the filaments before turning on B+ after some period of time

or

(c) - pre-heat the filaments before slowly ramping up the B+ voltage after some period of time

or

(d) - something else

In my build I plan to preheat the filaments of both the driver (30sec) and output (60sec) valves before turning on the B+ (achieved by switching relays for the appropriate transformer secondary windings). As it happens, the Neurochrome regulator I will be using for the driver stage B+ ramps up to its operating voltage.
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Nick
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#261 Re: Remember These?

Post by Nick »

Any literature that indicates an advantage using soft start (of any sort) for indirectly heated valves? I have been trying to work out what it could be about not using it that would reduce lifespan.
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#262 Re: Remember These?

Post by Cressy Snr »

The 6336A data sheet does specify a 30 second cathode warm-up time before HT is applied, which seems sensible, given the size of the cathode surface area, in these large regulator valves. Interestingly the less powerful regs such as the S11E12, 12E14, 6080, 6AS7, don't have any delay requirements, presumably because their cathodes are physically smaller.

I can see why it might be desirable to delay the HT with these giant cathode 13E1 and 6C33C but I can't see there being any problem with my manual 'warm up the cathodes, then switch the HT on' routine apart from the lack of automation circuitry to do the job for me. Automation is for me a desirable operational feature rather than a make or break one.

After warming the cathodes, the HT is still not strictly instant when the switch is thrown. If the switch is on the AC side of the rectifier, which it should be, then in my particular situation, 4400uF of caps per channel, have to charge before full voltage is available to the power tubes. I observed about a seconds worth of charge build up, from zero to 163V DC, at the last cap, when I threw my HT switch.

Another thought I had is, how does an IDH rectifier survive turn on?
GZ37/5AR4 etc, last for years and they are always presented with full, high voltage AC on the plates and no heat when the equipment is first powered up. Granted, they do arc over eventually, but that's often decades later with the old ones.

I personally like the idea of a "missile style" red cover for my HT switch, when I do my Steampunk version.
https://www.switchelectronics.co.uk/swi ... yle-covers. Combined with the five second countdown display of Ed's delay board, it should make quite an impression.

But perhaps I'm being unduly flippant.
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Nick
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#263 Re: Remember These?

Post by Nick »

The 6336A data sheet does specify a 30 second cathode warm-up time before HT is applied, which seems sensible
Yep, but is that for reasons of performance or lifespan?
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#264 Re: Remember These?

Post by Cressy Snr »

Nick wrote: Sun Dec 30, 2018 10:27 am
Yep, but is that for reasons of performance or lifespan?
No idea.
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pre65
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#265 Re: Remember These?

Post by pre65 »

Cressy Snr wrote: Sun Dec 30, 2018 11:09 am
Nick wrote: Sun Dec 30, 2018 10:27 am
Yep, but is that for reasons of performance or lifespan?
No idea.
Could be both ?
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#266 Re: Remember These?

Post by Cressy Snr »

OK
I've done a bit of digging and here is the relevant extract from the 6336A application notes:
Image

Looks like it applies to one particular instance, see below:
Image

The circuit above does not appear to apply to what we are doing with a power cathode follower, which is effectively what a single tube OTL is.

Given the above, I would have thought that, warm the cathode for one minute then turn the HT on either auto or manual is perfectly acceptable.
Last edited by Cressy Snr on Sun Dec 30, 2018 11:43 am, edited 1 time in total.
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Nick
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#267 Re: Remember These?

Post by Nick »

Good spot. So at least in that case not inherently about lifespan.
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#268 Re: Remember These?

Post by Cressy Snr »

Nick wrote: Sun Dec 30, 2018 11:43 am Good spot. So at least in that case not inherently about lifespan.
Another question:
If the cathodes are already warmed up, won't the CCS stop any possible residual 'shocks' to the system when the HT is applied?
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Ray P
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#269 Re: Remember These?

Post by Ray P »

Cressy Snr wrote: Sun Dec 30, 2018 11:36 am Given the above, I would have thought that, warm the cathode for one minute then turn the HT on either auto or manual is perfectly acceptable.
That's what I'll stick with...

Given the soft-start of the Neurochrome regulator I think I'll dispense with the delayed turn-on of the B+ for the 6SN7 driver stage too.
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#270 Re: Remember These?

Post by Cressy Snr »

Ray P wrote: Sun Dec 30, 2018 12:30 pm
Cressy Snr wrote: Sun Dec 30, 2018 11:36 am Given the above, I would have thought that, warm the cathode for one minute then turn the HT on either auto or manual is perfectly acceptable.
That's what I'll stick with...

Given the soft-start of the Neurochrome regulator I think I'll dispense with the delayed turn-on of the B+ for the 6SN7 driver stage too.
Just a thought Ray. With your 13E1s, you don't know how long they've been idle, so I would warm them through off a transformer for an hour or so before using them in anger for the first time. It'll give their getters a chance to mop up any residual gas that may have leaked in past the pins over the years.
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