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Old 21st Sep 2021, 5:27 pm   #21
bobhowe
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Default Re: Roberts P4D

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Originally Posted by FERNSEH View Post
Hi Bob,
At 78V the anode voltage of the DAC32 is too high. A figure near 30 volts should be expected. Unless there is a negative voltage on the grid and the valve is cut-off it would be safe to say the triode has gone low emission.

DFWB.
Hi making good progress changed these capacitors and there is no distortion now & the anode voltage is dropped to 56V still too high as what Davis says the valve has gone low emission . Spoke to David this afternoon and he says a 1H5GT valve is the American equivalent to the DAC32 kind regards Bob
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Old 21st Sep 2021, 6:11 pm   #22
ms660
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Default Re: Roberts P4D

If the anode current figure given in the service sheet (R18) is to be believed then the anode voltage of the DAC32 should be approx. 50/60 Volts when measured if the HT is 90 Volts and your meters input resistance is 10meg.

Lawrence.

Last edited by ms660; 21st Sep 2021 at 6:37 pm. Reason: extra info
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Old 21st Sep 2021, 6:52 pm   #23
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Default Re: Roberts P4D

According to the information from the British Radio and Television service sheet issue R18 July 1951 the voltages were measured using an AVO meter model 7.
The anode current of the DAC32 is given as 0.024mA.

DFWB.
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Old 21st Sep 2021, 6:56 pm   #24
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Default Re: Roberts P4D

The input resistance of the Avo 7 on the 1,000 Volt range is 500k.

Lawrence.
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Old 21st Sep 2021, 7:56 pm   #25
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Default Re: Roberts P4D

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Originally Posted by ms660 View Post
If the anode current figure given in the service sheet (R18) is to be believed then the anode voltage of the DAC32 should be approx. 50/60 Volts when measured if the HT is 90 Volts and your meters input resistance is 10meg.

Lawrence.
Hi Lawrence i am using Davids diagram & the 828 Trader service sheet my meter is not far out as when i checked the anode voltage on the DL35 Valve the service sheet says 78V and my Fluke DVM reading is 81V as in post 18 picture 3 kind regards Bob
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Old 21st Sep 2021, 8:01 pm   #26
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Default Re: Roberts P4D

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Originally Posted by FERNSEH View Post
According to the information from the British Radio and Television service sheet issue R18 July 1951 the voltages were measured using an AVO meter model 7.
The anode current of the DAC32 is given as 0.024mA.

DFWB.
Hi David just bought a NOS Mullard DAC32 for £ 5.95 + postage & the DAC32 in my set looks the odd valve out as it is the only valve with a metal Base kind regards Bob
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Old 21st Sep 2021, 8:12 pm   #27
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Default Re: Roberts P4D

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Hi Lawrence i am using Davids diagram & the 828 Trader service sheet my meter is not far out as when i checked the anode voltage on the DL35 Valve the service sheet says 78V and my Fluke DVM reading is 81V as in post 18 picture 3 kind regards Bob
The anode voltage measurement on the DAC32 (measured with your Fluke) looks about right.

The low voltage as measured in the service sheet is due to the loading effect of the Avo 7 meter, basically a voltage divider consisting of the anode load resistance and the meters input resistance which is 500k on the 1,000 Volt range in parallel with the valves DC resistance (which is considerably higher) If the anode current was 24uA as given in the service sheet then the actual voltage on the anode would have been 66 Volts with an HT of 90 Volts.

Lawrence.
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Old 21st Sep 2021, 8:40 pm   #28
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Default Re: Roberts P4D

Thanks Lawrence kind regards Bob
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Old 22nd Sep 2021, 9:41 am   #29
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Default Re: Roberts P4D

Correction to my earlier posts, that should be 82 Volts HT and 58 Volts on the anode, sorry about that.

Lawrence.
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Old 22nd Sep 2021, 4:09 pm   #30
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Default Re: Roberts P4D

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Correction to my earlier posts, that should be 82 Volts HT and 58 Volts on the anode, sorry about that.

Lawrence.
THnks kind regards Bob
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Old 24th Sep 2021, 4:45 pm   #31
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Default Re: Roberts P4D

Hi the DAC32 valve arrived today it as a genuine NOS Mullard British made for £ 5.95 + postage ( 8k1 date code 1951 ? ) and the price of the valve then was 11/6 + purchase tax . tried it and there is further improvement on sound anode voltage is now 59.6v . Going to change ( C14) V4 G.B bypass capacitor to the correct voltage rating as the one i have temporary fitted is 100V kind regards Bob
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