4th Oct 2013, 3:42 pm | #941 | |
Octode
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Re: The "Sussex" Homebrew Valve Tester.
Quote:
As I hope you can see, it is Martin who supplies the pcbs for the Sussex VT1. Drop him a PM. I hope this helps. Colin. |
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5th Oct 2013, 3:50 pm | #942 |
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Re: The "Sussex" Homebrew Valve Tester.
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5th Oct 2013, 6:53 pm | #943 |
Dekatron
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Re: The "Sussex" Homebrew Valve Tester.
Hi Andrew, you will find that you also have the option of e-mailing most forum members without being aware of their mail addresses, it is in the "contact" section.
Alezar, you may also find this option useful. Ed |
5th Oct 2013, 7:13 pm | #944 |
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Re: The "Sussex" Homebrew Valve Tester.
Ed
Thanks. Apparently you have to get your very first post past moderation before you are allowed to PM anyone. A sensible precaution against spammers etc. Anyway, PM now sent to you re. the transformers. Cheers Andrew |
6th Oct 2013, 9:39 am | #945 |
Dekatron
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Re: The "Sussex" Homebrew Valve Tester.
I'm part way through building the tester and wondered how hot the FET's get and whether a fan is vital. Reason for asking is to avoid cutting a large hole (difficult for me to do) if a fan isn't essential.
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6th Oct 2013, 2:55 pm | #946 |
Nonode
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Re: The "Sussex" Homebrew Valve Tester.
I don't have a fan fitted at all. The heatsink I used is a big one but it barely gets warm in my use.
Generally, those MosFETs are running at low power - dropping minimal volts at much less that their rated current. I usually find that the valves I test at high current need pretty high voltage, so the MosFETs are really only dissipating 3-5 watts (50v drop at 50mA is only 2.5W) so there isn't any appreciable heat to dissipate. The MosFETs are rated at 120W on a suitable heatsink..... Mind you, if you are going to soak test low-voltage power output valves at 75mA you might want the fan! Richard |
6th Oct 2013, 7:52 pm | #947 |
Dekatron
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Re: The "Sussex" Homebrew Valve Tester.
Thanks for the info. I have found a small fan, about 35mm and think I can cut a hole of about 26mm with an old Qmax cutter, so I think I might give that a go. I'll cut the hole and then wait until it's up and running before fitting the fan if it gets hot. Thanks again.
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6th Oct 2013, 8:43 pm | #948 | |
Octode
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Re: The "Sussex" Homebrew Valve Tester.
Quote:
Colin. |
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7th Oct 2013, 8:49 am | #949 |
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Re: The "Sussex" Homebrew Valve Tester.
I have built my Sussex in a reasonably large case with no fan but plenty of ventilation. I to have fitted a large heat-sink, 21 cm long and 8 cm x 8 cm and have found in use this hardly changes temperature wise .
Mike |
7th Oct 2013, 8:43 pm | #950 |
Dekatron
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Re: The "Sussex" Homebrew Valve Tester.
Hi Gents, yes, as Colin has said the transformer is not continuously rated for testing power valves, for that duty it would need to be much bigger and more costly. This was highlighted on one of the earlier pages of this thread.
Ed |
8th Oct 2013, 6:43 am | #951 | |
Nonode
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Re: The "Sussex" Homebrew Valve Tester.
Quote:
Richard |
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8th Oct 2013, 5:36 pm | #952 |
Tetrode
Join Date: Aug 2011
Location: Bristol, UK.
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Re: The "Sussex" Homebrew Valve Tester.
I am back troubleshooting my sussex today after a long gap; I know I know shame on me.
So I seem to get pretty reasonable test results with twin triodes (ECC81,82 and 83) including the reference valve that mole42uk passed my way I also got book values on an EL84 and 6L6 which makes me think that fundamentally my sussex is working ok. However I do get strange results on several EL34: Ia = 85mA Gm = 2.0 - should be 11 EL34 pin measurements (dc, then ac) G3 = 2.7mV HTR = 42v 6.31 AC A = 222v 4.3 G2 = 254v 0 G1 = 13.6 0.7 (scope says 280mV p-p though) NC = 200mV HTR = 42 0 C = 2.7mV 0 Some observations and questions
I am bit unsure where to go next in my troubleshooting so looking for some suggestions and inspirations of what to check next. |
8th Oct 2013, 7:09 pm | #953 |
Nonode
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Re: The "Sussex" Homebrew Valve Tester.
That's pretty much what I get when I try to check EL34s. Everything else is okay, just like yours, but not EL34s.
Richard |
8th Oct 2013, 8:57 pm | #954 |
Dekatron
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Re: The "Sussex" Homebrew Valve Tester.
Hi Gents, the mains transformers on these sets are wound with several taps to enable a close match the actual mains voltage at your place, and at the time you are testing, as some areas do vary a lot.
One suggested mod would be to fit a meter to one of the windings with a green band showing mains tolerance and switch primary taps with a suitable switch to stay within that band. Note that transformer regulation will also reduce the voltage on all windings slightly when load is drawn, so off load readings will be higher. Ed |
9th Oct 2013, 5:33 pm | #955 |
Tetrode
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Location: Bristol, UK.
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Re: The "Sussex" Homebrew Valve Tester.
@mole42uk Thanks for confirming similar Experience Richard, it makes me feel less paranoid. Do you think this could be current limiting?
@ed_dinning thanks and understood. I don't know what mains variation is typically I will keep an eye on that. Next steps for me are to compare the pin voltages on the other tubes to see if gives an insight into where the issue lies e.g wiring, oscillations etc Keen to hear from anyone with success on EL34 testing. |
9th Oct 2013, 7:49 pm | #956 |
Heptode
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Re: The "Sussex" Homebrew Valve Tester.
Hi Mike
I have just quickly tested a new JJ Tesla EL34 on my Sussex. I have took a photo of the results, hope it is clear enough. Frank |
9th Oct 2013, 10:12 pm | #957 |
Tetrode
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Re: The "Sussex" Homebrew Valve Tester.
Hi Frank,
That's interesting so yours is working much better result than I get although the Avo states Gm should be 11 for EL34 so isn't yours is a bit low. What does stand out as a clue to me is that your Va is exactly where it should be where as mine is dropping by 30v under load. As a comparison, when I test a 6L6 and I do get a sensible value, my Va has stayed up high. Another test was to adjust the grid bias to get a 75mA Ia which was about -16v when I do this the Gm makes an effort and comes up to about 4. I think this tells me that it is working but inaccurate - does that sound correct? PS: I like your sussex layout nice and clear. |
10th Oct 2013, 8:18 pm | #958 |
Dekatron
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Re: The "Sussex" Homebrew Valve Tester.
Hi Gents, perhaps one of the knowledgeable members may care to comment on the likely spreads to be expected on this type of valve and which parameter should be the "standard" setting to get a particular reading and which one is of secondary importance.
Ed |
10th Oct 2013, 9:12 pm | #959 |
Heptode
Join Date: Feb 2006
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Re: The "Sussex" Homebrew Valve Tester.
For what its worth increasing the grid bias to -15.5V or reducing the screen voltage to 230V will get the GM up to 11 on the EL34. See the photos below.
By the way my Anode volts drops back very little under load, while testing the EL34 at the correct settings it just drops back by 2V while under load. Frank |
10th Oct 2013, 9:29 pm | #960 |
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Re: The "Sussex" Homebrew Valve Tester.
I think you have to know whether the valve data being used had the Gm value recorded at a specified grid bias, or at a specified anode current.
I believe the AVO data books worked along the lines of "Apply -xx grid volts, and here are the anode current and Gm you should expect." Which is why the later VCMs had elaborate grid voltage scales. Also the EL34 is quite a difficult valve to test from the point of keeping it stable with wiring harness connections cross-coupling. When the natives are restless, placate them with beads - ferrite ones. David
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