Remember These?

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Cressy Snr
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#271 Re: Remember These?

Post by Cressy Snr »

Ray P wrote: Sun Dec 30, 2018 12:30 pm

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 suitable 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.
Maybe the guy who tested them did that, but nevertheless, just to be on the safe side....
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Ray P
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#272 Re: Remember These?

Post by Ray P »

Thanks Steve. I'm waiting for a transformer to power the heaters from Toroidy in Poland so I'll do that when it arrives.
Sorry, I couldn't resist!
Wolfgang
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#273 Re: Remember These?

Post by Wolfgang »

It’s really about protecting the output tubes in OTLs from extreme stress every time the amp is turned on. It also depends on the amp type.

In case of PP OTLs a soft start is recommended to protect some of the tubes from conducting initially a lot more current than for what they have been set up in the circuit. It will also protect the speakers from high DC levels during the warming up phase of the tubes. Any serious book about designing OTLs will mention this.

In the case of SE OTLs – like the 300B SE - with parallel output tubes a soft start helps to protect tubes from stress through higher current and that some tubes are worn out faster than the rest (the strongest tube always gets the hit). The CCS does not 100% control this situation despite the expectation or belief that it should or would.

In order to get final clarity I recommend to actually measure what’s going on in the tube/psu/amp comparing different ways of switching the amp on. The results might slightly change with the output tubes/amps.I did this with my OTLs (PP,SE 300B, SE 6C33). Here is what happens with the 6C33 OTL.

Without soft start: It takes about 20s to heat up the cathode before the tube starts to conduct. During that time the current through the tube ramps up slowly with B+ voltage at full peak level (unloaded B+ psu). As the current rises the voltage drops until the tube reaches its normal operating point (130V). At around 140V one can hear a “plop” which signals that the speakers are connected .
With a soft start the voltage rises slowly and stops at the tube’s operating point (the tube has already started to conduct because it takes about 5 sec for the voltage to reach 130V) while the current also ramps up continuously. As soon as the current is strong enough and the tube begins to conduct more current the voltage starts to drop again to about half its value until the relay bypasses the resistors. The “plop” is softer and smoother and happens at around 60V B+ instead of 140V.
Delayed B+ wouldn’t do the same but the fully open tube will be hit with the full peak B+ voltage every time B+ is switched on. At least the caps should be protected with a thermistor which also should be bypassed for better SQ.

As always: in the end it’s a personal choice depending on our preferences, good, bad, or otherwise.
Cressy Snr
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#274 Re: Remember These?

Post by Cressy Snr »

Interesting.
This to me, given the amount of current these tubes are capable of sourcing, reinforces the view I've had for a while, that OTL tube amp power transformers should be complete stupid overkill, especially the ones supplying the HT.
The difference between the unloaded condition of my 1KVA toroid and when drawing the full 950mA (475mA per secondary) of tube current is precisely 5V! That is a great figure by any standard and makes sure that excessive voltage can never be applied to the plates of the power tubes. God knows what lab equipment these toroids were used in, but the quality is outstanding. Those of us who bought them off Phil (pre65) got bargains, that's for sure.

The caps in my amp are protected with a CL60 NTC thermistor, on the primary side of the main power transformer.

I appreciate that there are better, cheaper solutions in the commercial world, such as active regulation, enabling smaller, less expensive power transformers to be used, but us DIY'ers don't have those constraints.

The trafos in my amp get barely warm and the power supply chokes are stone cold, even after hours of continuous playing.
By contrast the trafos on the Mini-Beast got too hot to touch after an hour. But then Bruce is running a business not a charity.
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Cressy Snr
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#275 Re: Remember These?

Post by Cressy Snr »

Wolfgang wrote: Sun Dec 30, 2018 3:16 pm
In order to get final clarity I recommend to actually measure what’s going on in the tube/psu/amp comparing different ways of switching the amp on.
I had a go today at measuring what is going on with the HT switch.

Switching on with the HT switch in the 'on' position causes an instant voltage (168V) to appear at the plates. As the tube warms up, it starts to conduct after about 25 seconds, whereupon the voltage drops to 163V and settles there.

With the HT switch in the "off" position, I let the heaters warm for 1 minute. Upon throwing the switch, there is a very soft, low frequency thud from the speakers, then the voltage rises over the space of about one second from zero to 163V as the power supply caps charge. There is no overshoot; the voltages rise smartly to their working value and stay there

Seems that with the delayed HT, the 13E1s are being treated as gently as is humanly possible, with the resources I have to hand.
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Nick
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#276 Re: Remember These?

Post by Nick »

Still being a curmudgeon, but I think there is is a degree of anthropomorphising going on here, As long as the valves are within their design limits, why would it matter? And if the circuit design means (like the example Steve gave of the regulator) that during startup the valves are potentially taken outside that limit, then (like the regulator doc suggested) the circuit design should be modified to avoid this if at all possible, My reason for saying this is that the same problem will likely happen if a valve fails and cause havoc around it. Protecting the output from DC is valid, but a speaker protection relay will do that, and again may be useful when a valve does fail if its driven by checking the output for DC instead of just a timer.

And of course power problems like brief mains drop outs can cause problems if the amp expects to be nursed into life each time.

Just saying...
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Mike H
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#277 Re: Remember These?

Post by Mike H »

+1 for the speaker relay idea, this is frequently used in commercial designs; many years ago I had a 100W per channel Pioneer amp which had such a relay, not just to stop on/off plops (which it could), but in case one of the output trannies went t!ts up.

There are purpose designed speaker protection chips around, think I've got a couple somewhere.

There was a caveat re the Pioneer speaker relay, occasionally I had to clean the contacts.

HTH
 
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Ray P
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#278 Re: Remember These?

Post by Ray P »

Sorry, I couldn't resist!
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#279 Re: Remember These?

Post by Ant »

My Samson stereo 260 has a speaker protection relay that causes 99 percent of the breakdowns with them! However it did stop melted voice coils when an output device went bang on it, so I like speaker protection relays. Shame Samson saw fit to use crap relays for the original version of them, they seem to fail on their own quite often. Sorted with a better relay as recommended by them when I spoke to them about fixing the fried output device
From a sound quality point of view, would speaker fuses cause much of an issue? It's not something I've considered much to be honest
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Cressy Snr
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#280 Re: Remember These?

Post by Cressy Snr »

I think I have a reasonably safe amplifier, and I really don't want to have to festoon the thing with electronic protection circuitry about which I understand neither how it works, or how to implement it..

I feel this has got a bit out of hand and moved too far away from away from keeping things dead simple, which was the main attraction of this power cathode follower.

So I'm seriously moving towards doing a Bruce Rozenblit and just putting fuses in each HT feed, delaying the HT and having done with it. Seems to work fine for his designs.

Time to wrap up the development and just get building I think?
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Mike H
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#281 Re: Remember These?

Post by Mike H »

Ant wrote: Mon Dec 31, 2018 5:26 pm From a sound quality point of view, would speaker fuses cause much of an issue? It's not something I've considered much to be honest
Did for me! I thought pretty hopeless really. So came out after just the first listening sesh. Have also had similar problem with them being in the supply line. But OK if followed by an electrolytic. This is how my germanium o/p amp is configured, each o/p stage has a fuse BUT followed by a 'lytic, fairly large value. 1,000 uF in that case.
 
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Cressy Snr
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#282 Re: Remember These?

Post by Cressy Snr »

So much for stopping the development! :lol:

Anyway, I did 3 mods to the setup today:
Image

First one was a fuse in each HT line after the last power supply cap. I have, at the moment, 500mA slow-blow fuses in the fuse holders. The 13E1s idle at around 450mA, so If anything steps out of line, the problem will be terminated straight away. If I get nuisance blowing, with nothing wrong I'll just up them to 750mA. There were no ill effects on the sound that I could hear, so HT line fuses will do perfectly well for me on the final build.

Second mod was to do something I had forgotten to do. Nick's advice to Mick, regarding bypass caps on his thread, prompted me to fit a 1uF LCR 630V polyprop cap to ground, right at the plate connection on each 13E1 tube socket. This gave a surprisingly good improvement to the top end smoothness and detail. It was a bigger improvement than I remember the NVA clone giving, when I bypassed its caps, but then the 10,000uF PSU caps in that amp were of far better quality in the first place, than the 2200uF Panasonics I dug out for this OTL bitza.

Last mod was to replace the nonedescript 1000uF cathode bypass caps on the driver stage, with some nice 100uF, 50V Cerafines I scrounged out of the bottom of a box full of light blue LCR Maplin motor run caps. These cleared a slight fog at the bass end, so again a decent afternoon's work. I now think I've done as much as humanly possible, to neutralise the effects of any catastrophic fault condition that might crop up, whilst not compromising the excellent sound quality too much.
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Ray P
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#283 Re: Remember These?

Post by Ray P »

I see the amp has acquired a small label, screwed down to the front edge of the breadboard?

Keep squeezing Steve!
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Cressy Snr
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#284 Re: Remember These?

Post by Cressy Snr »

I'll make another attempt at a schematic sometime this week. You never know, I might get it 100% right this time. :)
Last edited by Cressy Snr on Tue Jan 01, 2019 8:03 pm, edited 1 time in total.
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#285 Re: Remember These?

Post by IslandPink »

It'll be nice to see that. Looks like a great project.
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