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UK Vintage Radio Repair and Restoration Discussion Forum (https://www.vintage-radio.net/forum/index.php)
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-   -   EHT warm-up delay in pre-war sets (https://www.vintage-radio.net/forum/showthread.php?t=139808)

Panrock 14th Sep 2017 4:10 pm
EHT warm-up delay in pre-war sets
 
1 Attachment(s)
Quote:
Originally Posted by Argus25 (Post 975666)
... I came up with a very simple way of delaying the EHT in electrostatic sets until the heaters had warmed up. In the Andrea KTE-5 for example, the EHT is derived from a separate transformer & 2X2, it came up much faster than the scan stages, as did the CRT heater. So the beam could burn the crt phosphor in that interval before the HT and scan got established.

One day I notice that a typical audio output tube, like a 6v6 (and many others) have about a 270R to 330R cathode resistor and it drops roughly around 12V for the bias. Suddenly the penny dropped and I realized the cathode resistor could simply be replaced with a 12V, 270R to 300R relay coil from a common garden 12V relay.

So if the relay coil is used or incorporated into the audio output tube's cathode, the audio stage functions normally...but, it takes time for the tube to warm up and get enough current to close the relay and the HT has to be established to do that and by then all other stages are running. A perfect delay, the relay contacts were simply put in series with the mains supply to the EHT transformer primary, to delay the EHT. So its a quick & easy way to create a delay after turning a set on, if you need to, so as to benefit some part of a circuit in the set.
Is this what is known as lateral thinking? Whatever, it's quite brilliant!

Trouble is, it won't help in the case of the HMV 904 that's currently being restored on my bench. This is because the EHT transformer also supplies the heater feeds to the scan sections, among other things. We want the scans to warm up before applying EHT.

Originally, the onset of EHT in this set was delayed by the warm-up of the heater of the U17 EHT rectifier valve. In this case though, the valve was dud and I didn't have another to hand. So it's been physically left in place (with new black EHT cable to anode) for appearance, but its work is actually now being done by an EHT stick below decks.

Any bright ideas how I can incorporate EHT delay in this case, in a similar 'organic' way?

Steve

Boater Sam 14th Sep 2017 4:19 pm
Re: EHT warm-up delay in pre-war sets
 
Could you put another eht rectifier valve in the lead to the tube with its under-run heater fed from a current shunt off the eht?

Boater Sam 14th Sep 2017 4:21 pm
Re: EHT warm-up delay in pre-war sets
 
No, perhaps not.

Guest 14th Sep 2017 4:30 pm
Re: EHT warm-up delay in pre-war sets
 
Some reed relays are rated at a very high voltage. I have two inserts (the reed no coil) here (gifts from a relay manufacturer) rated 7kV, they are different, one at least will be 7kV. Yours for the asking Panrock. (drop a PM)

This time I have found the thing before offering it!

Panrock 14th Sep 2017 4:33 pm
Re: EHT warm-up delay in pre-war sets
 
I should mention the cathode bias on the KT41 sound output valve, (heater fed from the other transformer) is a little low for this purpose, at 4v. Just read merlinmaxwell's post above. This is a get-out-of-jail-free card. To say I am grateful is an understatement! Anyway, let's see what other bright ideas come up in this thread first.

Guest 14th Sep 2017 4:51 pm
Re: EHT warm-up delay in pre-war sets
 
Quote:
is a little low for this purpose, at 4v
You will have to wind your own coil anyway!

Panrock 14th Sep 2017 5:08 pm
Re: EHT warm-up delay in pre-war sets
 
Yes, and it would have to be about 100 ohms. That's a lot of fine wire I may not have. I'd probably look into operating your relay some other way - or find a pre-wound coil.

turretslug 14th Sep 2017 5:17 pm
Re: EHT warm-up delay in pre-war sets
 
I'm sure that there will be 5V relays around with mains-rated contacts that might swing in with 4V on them, especially if a little driver saturation voltage lee-way had been allowed for.... There's usually documentation somewhere out there with minimum operating voltage quoted.

Panrock 14th Sep 2017 5:24 pm
Re: EHT warm-up delay in pre-war sets
 
The attraction of merlinmaxwell's reed relay is that it can actually switch at EHT voltages. Unfortunately mains switching of the EHT transformer input isn't an option here because a feed from this energises the timebase heaters.

turretslug 14th Sep 2017 5:39 pm
Re: EHT warm-up delay in pre-war sets
 
Oh, yes- overlooked that, I get your drift now! It rings a bell now (from the days when supplier catalogues were within elbow-distance) that some offered curious long, thin encapsulated relays with exceptional voltage ratings- presumably they had these reed switches in them.

Brigham 14th Sep 2017 5:45 pm
Re: EHT warm-up delay in pre-war sets
 
Finding the right valve might be a better bet.
Otherwise adapting the circuit to use a more readily-available one might be possible.

Panrock 14th Sep 2017 5:57 pm
Re: EHT warm-up delay in pre-war sets
 
I'd be twitchy at trusting in the delay from the "correct valve" on its own, since its cathode is directly heated (the filament).

Even though the original designer did!

Boater Sam 14th Sep 2017 6:13 pm
Re: EHT warm-up delay in pre-war sets
 
How about a simple timer on the CRT cathode to cut the beam off?

Panrock 14th Sep 2017 6:40 pm
Re: EHT warm-up delay in pre-war sets
 
Yes Sam, a possibility. The contacts though, I suspect, would still need to be EHT rated. A concern here is that the tube is cathode fed with video. Any additional capacitance here would be undesirable.

Barry Lloyd 14th Sep 2017 7:18 pm
Re: EHT warm-up delay in pre-war sets
 
Quote:
A concern here is that the tube is cathode fed with video. Any additional capacitance here would be undesirable.
Alternatively the tube could be cut off by pulling the grid down to 0 V.

FERNSEH 14th Sep 2017 9:56 pm
Re: EHT warm-up delay in pre-war sets
 
Employ an ST&C DLS10 delay switch to hold the EHT off until all the timebase valves are fully warmed up.
Some time ago I modified an HMV 801 radiogram to employ a pair of triode connected KT66 tetrodes to replace the hard to find PX25 triodes. A DLS10 was used to delay the HT supply until the indirectly heated output valves had warmed up.
The Marconi-Osram EHT rectifiers have directly heated cathodes. The Mazda U21 and U22 EHT rectifiers have slow warm up indirectly heated cathodes.

http://www.radiomuseum.org/tubes/tube_u17.html

DFWB.

Argus25 14th Sep 2017 10:32 pm
Re: EHT warm-up delay in pre-war sets
 
Quote:
Originally Posted by Panrock (Post 975684)
Any bright ideas how I can incorporate EHT delay in this case, in a similar 'organic' way?

Steve
Steve,

Yes the 904 issue is a different proposition to the KTE-5.

I found two significant threats to the 904's CRT phosphor, especially in light of the fact that non aluminized crt's are very easy to burn.

In the case of the 904, using a 5FP4 crt at least (I have never owned the 3/1) the problem occurred at turn off, or when the TV was running and then switching it to radio which causes scan collapse. It was less elegant leaving the scan stages running on switching to radio, but it protects the crt.

The fixes I used are here:

http://worldphaco.com/uploads/CONVER...A_5FP4_CRT.pdf

(One circuit on pg 4 creates a negative pulse created by two exponential waves, to blank the CRT beam at turn off).

Back to your issue with the stick rectifier instead of U17: I think you simply need to fit a U17 and you will get the normal EHT warm up delay, it is not worth risking damage to the CRT phosphor. Are U17's getting hard to get ? Then have a look out for those "turn off" or "change to radio" bright spot issues.

That is a great looking chassis you have restored.

Freya 15th Sep 2017 7:40 am
Re: EHT warm-up delay in pre-war sets
 
Could you use a slow heating 6D3 in the CRT Grid, keeping the grid at chassis potential until the heaters have reached their correct temperatures.

Panrock 15th Sep 2017 8:02 am
Re: EHT warm-up delay in pre-war sets
 
Thanks for all these ideas, which I am now studying! In the meantime beery has directed my attention to a good U17 for sale online, and I have now ordered this.

Steve

Argus25 15th Sep 2017 10:33 am
Re: EHT warm-up delay in pre-war sets
 
1 Attachment(s)
Steve,

The HMV 904 certainly has some nice valves in it, including the U17. The valve that I think is the most interesting is the X41C, it is such a beautiful valve. While X41's appear easy to get, the "C" or ceramic base version they used is rarer. When I went looking for them I could only find two from UK suppliers, I have attached a photo. They are electrically, very much like an ECH35, except for the heater.
Hugo


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