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Old 22nd Feb 2024, 2:22 pm   #1
Electronpusher0
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Default The “Felpham” valve tester measurement circuit and pcb

This thread describes a circuit and pcb to provide the measurement functions of a valve tester or curve tracer.
The circuit allows all the voltages, currents and gm to be measured using a single (well +/- 15V) supply and every measurement referenced to 0V.

The key element is a high voltage differential amplifier constructed using closely matched resistors to translate the voltage across a 100 ohm resistor in the anode circuit (hence floating at HT) to a voltage referenced to 0V.
This is followed by two stages of bandpass filter to remove the 50 and 100Hz noise.
Additional circuits have been added to measure Anode and Screen volts.
Compensation has been added to null out errors due to loading by the measurement circuits themselves.
GM is measured by injecting a 100mV 1kHz Sine wave to the grid of the valve under test. I have included an oscillator for this is on the measurement board.
The ac Anode current is sensed across the same 100 ohm resistor and is amplified and filtered to eliminate 50Hz and precision rectified. The output is a dc voltage of 2V (or 200mV) for a reading of gm=20

I attach a pdf document that describes the circuit, set up and faultfinding.
Also attached is picture of the completed pcb and an errata sheet as there are 2 missing components on the pcb. If I order further boards I will correct this.

I have tested the circuit in both the Sussex and Mullard HSVT and notes on applying the board to these is added as an appendix.

Note on the name. Felpham is where I live, a small village in Sussex. Since one of the principle applications of this measurement pcb will be in the Sussex valve tester the name “Felpham” seemed appropriate.

Peter
Attached Files
File Type: pdf Valve Tester Measurement unit Schematic.pdf (119.5 KB, 184 views)
File Type: pdf Measurement Unit for Home Brew Valve Tester.pdf (285.0 KB, 134 views)
File Type: pdf Ver 1 pcb Errata.pdf (238.8 KB, 76 views)
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Old 22nd Feb 2024, 2:29 pm   #2
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Default Re: The “Felpham” valve tester measurement circuit and pcb

I forgot the pcb image.

Peter
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Old 22nd Feb 2024, 5:25 pm   #3
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Hi Peter,
I would be interested in how this board is applied to the "Sussex". I can't see a note on this.
-Jeremy
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Old 22nd Feb 2024, 8:57 pm   #4
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Default Re: The “Felpham” valve tester measurement circuit and pcb

I can't remember if the Sussex has a 'gas test' function, but could this PCB be used to measure the vacuum quality by looking for leakage? Likewise testing for cathode/heater insulation?
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Old 22nd Feb 2024, 10:10 pm   #5
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Evening Peter!

. . .One alternative method of effective stabilization of the Wien Bridge Oscillator output that gives potentially less distortion, is to replace R35 with a "B14" thermistor and replace R32 with a 3k3 resistor which gives automatic stabilisation of the gain from 3 to 3.3 times – as I wrote this, these thermistors are available in quantity from this seller :–

https://www.ebay.co.uk/itm/124302590...mis&media=COPY

. . . There's "10+ available" if anybody's interested in trying one – if this thermistor is used omit the diodes D1 and D2!

. . .Regarding "RichardGr's" question as to whether the PCB can read gas leakage current and heater/cathode leakage, I would say yes, but some quite involved switching would need to be added!

. . .The high–voltage dividing resistors would need to be short–circuited or taken out of circuit for a "gas–test" circuit – a "4 pole double–throw" switch can be used for this!

. . .For heater/cathode leakage tests, the heater/cathode insulation test voltage is applied across a VR37 10 M resistor between heater and earth, and then the "anode current measurement" inputs are connected across this resistor.

. . .I append two circuit suggestions below!

Chris Williams
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Old 22nd Feb 2024, 11:26 pm   #6
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Default Re: The “Felpham” valve tester measurement circuit and pcb

. . .Suggested circuits to try for using Peter's Valve Measurement PCB for
Gas Leakage Tests and Heather-Cathode Leakage tests!

Chris Williams
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Old 23rd Feb 2024, 10:01 am   #7
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Hi Peter.

Do you have a bill of materials and suggested source of components i.e. Farnell, RS etc?

Regards, Malcolm.
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Old 24th Feb 2024, 2:33 am   #8
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Hi Folks,note that I can supply the Sussex transformer with a +/- 15v meter winding in place of the current 3 off 7.5V windings.

Ed
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Old 24th Feb 2024, 6:35 pm   #9
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Hello all, sorry for the delay in replying, I am having to deal with a family matter at the moment.

Ed, that's great thanks.

Malcolm, sorry, I do not have a bill of materials yet, I designed this for my own use and have probably published it before I have crossed all the t's as regards things like BOM but to be honest if I waited I probably would never have got round to it.
I will try and find time to produce one.

Chris, re the Wein bridge, I looked at the various methods of stabilising it and decided that the diode method was the simplest and the purity of the sine wave was not that big an issue. Of course others are free to experiment.

Peter
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Old 24th Feb 2024, 6:38 pm   #10
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Default Re: The “Felpham” valve tester measurement circuit and pcb

I have received requests for all the remaining pcbs and will be sending payment details shortly.
I also have one member on the waiting list for a possible second batch.
If anyone else would like one please let me know, I really need a minimum of 5 to make it worth while.
Peter
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Old 24th Feb 2024, 11:03 pm   #11
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Hi Folks, one of the reasons for using a high purity oscillator is that it is also possible to use a PC sound card (or digital scope) to measure the distortion introduced by the valve under test.

The distortion level introduced by the oscillator will have little effect on the static test results. The phase shift ? oscillator in the Avo tester is not that pure

Ed
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Old 25th Feb 2024, 7:02 pm   #12
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Hi I would be interested in buying a PCB for this project. Ross
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Old 26th Feb 2024, 10:17 am   #13
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Default Re: The “Felpham” valve tester measurement circuit and pcb

OK, that's 2 requests for a possible 2nd batch. I will make the mods to add the 2 missing components and see how many requests I get in say a week and then order another batch.

Peter
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Old 27th Feb 2024, 10:12 am   #14
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Default Re: The “Felpham” valve tester measurement circuit and pcb

I am happy to take 5, have them posted to a UK address, collect them when I am there in April, and then be able to offer some to other members based in the EU. It has become very expensive to receive parcels from the UL nowadays. I pay a £7.50 admin charge just for handling the VAT service of the post office here, whereas parcels anywhere else on the continent have no additional charges.
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Old 28th Feb 2024, 11:54 am   #15
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Looks like the second batch is a goer then.
Peter
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Old 29th Feb 2024, 6:57 pm   #16
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Default Re: The “Felpham” valve tester measurement circuit and pcb

I have added the missing components and up issued to version 1.2
15 boards are on order and 6 of these are spoken for.
They will take about 2-3 weeks to arrive.
The good news is that the price is the same - £6 including postage to mainland uk.

Peter
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Old 2nd Mar 2024, 3:59 pm   #17
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Thanks Peter, PCB received, I've drawn up a provisional BOM . There may be mistakes.

Questions, I couldn't find R31. Are all the RV's 10T presets? Lastly why all the paralleled 47n caps on the power supply filtering instead of big electrolytics?

Andy.
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File Type: pdf Felpham Valve test PCB - BOM - Provisional..pdf (17.0 KB, 36 views)
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Old 2nd Mar 2024, 5:23 pm   #18
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Suppression capacitors on the +/- supply lines to the 7 chips
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Old 2nd Mar 2024, 11:32 pm   #19
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Supply line suppression with 47nF caps as close to chips as possible is for RF suppression as E caps ESR rises at higher frequencies, so acts as RF bypass. Standard technique in industry

Ed
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Old 3rd Mar 2024, 11:00 am   #20
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Default Re: The “Felpham” valve tester measurement circuit and pcb

Ta, missed that. Looking at the PCB each IC has a cap across the rails.

Andy.
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