OC3 VR Tubes

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simon
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#1 OC3 VR Tubes

Post by simon »

I've just switched on the HT series reg for the phono for the first time. I can get 250V tweaking the 100k pot and nothing has gone bang yet. Before I leave it on for a while and start measuring voltages etc. I just thought I'd ask a no doubt simple question, but I haven't used 0C3s before so now seems an appropriate time to ask. :)

First, they have a beautiful purple glow, much more so than 0A2s - I certainly won't need an LED to tell me it's switched on. But it makes a very low buzzy hum - nothing like when I've had 0A2s oscillating, but more than I expected. Is this normal, or do I need grid stoppers on the ECL82 or something? Circuit is pretty much the first one on Steve Bench 3 but with a paralleled 6080 rather than EL509.
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Nick
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#2

Post by Nick »

That doesnt sound right, have you checked the current thats passing through them?
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andrew Ivimey
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#3

Post by andrew Ivimey »

I agree, its probably the current. I had a brief time of buggering about OD and OC3s and really love the glow (very worthwhile), but I never got them to hum/buzz.

They only do 30ma, I believe.
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#4

Post by simon »

I did try to measure the current through the the tube but the meter didn't give a reading. There could be (at least) three reasons for this, in order of likelihood:

1 I'm taking the reading incorrectly. I've put the probes to anode and cathode of the 0C3 - is this right?

2 I've not got the resistor values in the circuit quite right and it's not passing any current.

3 The meter isn't working properly. I did try measuring the current with my brand spanking new autoranging all singing and dancing meter too but as soon as I did the VR tube extinguished. When I removed the meter it lit again. See point 1.

The datasheet says min 5mA max 40mA. 0mA isn't between those unfortunately. :cry:
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#5

Post by simon »

Oh, after checking the voltages looked reasonable I couldn't help myself but plug it in to see if it worked. It does, but there's a fair bit of hum, more than the big L and C passive. I checked the output HT with the AC setting on my meter. Whereas the filament reg gives me 0.001V variance, it can't provide a reading at all with the HT reg so I'm guessing the reg isn't working properly. Bum.
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Nick
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#6

Post by Nick »

Regulators can be tricky. Biggest problem is I think you need to use a scope to see whats going on in them. Though the voltages at various point can help.

To measure the current through the valve, the meter probes will need to be in series with the current, in current mode you will just short the VR out, or more likely blow a fuse in the meter.

Simple and easy way, put a 100R resistor in series with the VR tude, and measure the voltage across the resistor.
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#7

Post by Dave the bass »

Nick wrote:
To measure the current through the valve, the meter probes will need to be in series with the current, in current mode you will just short the VR out, or more likely blow a fuse in the meter.
Beat me to it Nick, I was just typing up a response to help Simon, I was trying to find out if he knew about meters in series when measuring current without sounding condescending :oops:

If he had the meter set to current and put the probes across the VR toob he's probably supplied a low current path around the valve and hence it 'going out'.

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#8

Post by simon »

Condascend away chaps :). I didn't think I was doing it correctly, but reading the manuals and books I have didn't exactly explain how reading current should be done... When the VR tube went out I figured I'd provided an easier path for the current, so I quickly unplugged and went to work.

When I was checking voltages to see what was going on it struck me straight away that this would be an excellent job for a scope. So, next I need to work out how to use a scope properly. Anyone fancy doing a teach-in at Witham?
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#9

Post by Nick »

If not at Witham, you always know where I live.
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#10

Post by simon »

Cheers Nick, see how we go.
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#11

Post by shaun »

hi simon

sorry for not posting sooner. work work work

can i just ask if you have used choke input. if so what value?

have you used a cap on the reg O/P i found that i had a sort of low level buzz when i built my first one and this sorted it so you could try one just to iliminate it from your enquiries. just clip one in and see :D

i know you've most probably got the above covered but i thought i'd ask.

just a stab in the dark :)

keep us posted

take care

shaun
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#12

Post by simon »

Okay, there's 23mA through a 100R resistor on the anode.
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#13

Post by simon »

Hi Shaun,

Choke input, 10H into 100uF. I'll draw it up and post in a bit.
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#14

Post by simon »

Here it is, fire away.

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Nick
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#15

Post by Nick »

That all looks fine (assuming the feedback signal is going to g1 not g2 (I assume a typo), 26ma make perfect sense given the 6k between the HT out and the 0c3.

Maybe its a faulty 0c3.

What I would so, is use a ap with adiquate voltage rating, connect one end to the output voltage, the other to your scope probe, and see how stable the voltage is.

Eventually you will want to add a zobel across the oc3, but thats not a big deal at the moment.

One thing I would do is add a 100R resistor (1w or so) in each of the 6080's cathodes, before they become commoned. This will help balance the current through both triodes.
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