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Old 20th Dec 2009, 2:16 pm   #41
oldticktock
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Default Re: Newb Stumped on a Derwent

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Originally Posted by vinrads View Post
is the 220k ohm red red yellow, resistor connected to pin4 (grid1) and the other end to chassis make sure that thick wire goes to chassis ,i take it the 220k is ok it looks new,what is the reading from pin 5 (cathode) to chassis in ohms with set off.

220k resistor is wired as follows, one pin from speaker to chassis loom has a wire rail attached, then the 220k is next on the rail, from that the 220k is connected to a 0.1 uf yellow cap which links into a wire that connects to pin4 in one direction, and chassis in the other.

500ohms is the answer for pin 5

Last edited by oldticktock; 20th Dec 2009 at 2:24 pm.
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Old 20th Dec 2009, 2:24 pm   #42
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Default Re: Newb Stumped on a Derwent

Given 30V DC on the cathode and a 500 ohm cathode resistor that gives a combined anode and screen current of 60mA. I would expect to see about 40mA.

However more bias should give less current, so something doesn't add up. Can you locate the cathode resistor and check it's colour coding?
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Old 20th Dec 2009, 2:28 pm   #43
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Default Re: Newb Stumped on a Derwent

Yes quite right the thick wire could be HT are we then saying that the 220k is anode load to the 75 ,
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Old 20th Dec 2009, 2:30 pm   #44
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Default Re: Newb Stumped on a Derwent

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We'd better establish whether the thick wire is earth or HT. With the set on what voltage do you get between chassis and the thick wire?
350v and I think the cathode resistor is the cruddy one in the better angle view chassis pic. It's hard to read as it old and flaking I did not change it because i'm not sure of it. looks like green yellow brown making it 540ohms the other one i did change in that circuit i did replace with a new one and its 510k it hooks in with one end of a 50v electrolytic with neg end joined with the cruddy( cathode resistor?) and the new 510k.

gahhhhh! my heads turning into spaghetti wiring
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Old 20th Dec 2009, 2:31 pm   #45
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Default Re: Newb Stumped on a Derwent

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from that the 220k is connected to a 0.1 uf yellow cap which links into a wire that connects to pin4 in one direction, and chassis in the other.
So the grid is connected to chassis via the wire then?
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Old 20th Dec 2009, 2:31 pm   #46
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Default Re: Newb Stumped on a Derwent

Measure it with the set off. Let's see what it is.

Cheers,

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Old 20th Dec 2009, 2:41 pm   #47
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Default Re: Newb Stumped on a Derwent

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So the grid is connected to chassis via the wire then?
yes
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Old 20th Dec 2009, 2:44 pm   #48
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Default Re: Newb Stumped on a Derwent

So we have HT rail----220K resistor-----------0.1uF capacitor-----------chassis.

Is there anything conected to the junction of the resistor and the capacitor?
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Old 20th Dec 2009, 2:53 pm   #49
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Default Re: Newb Stumped on a Derwent

yes it spits down to the 75 looks like the first small pin after heater

just to be clear, the chassis end point appears to be a tag which is on the base of valve 75's holder looks like the chassis rivet point with a leg on it, the neg end of the smoothers are on this as well.

one pin of (4 pin loom)...... (ht rail).....(220K).....split here off to 75.(0.1uf).... hits centre of new wire one side to pin 4 grid 1 other to chassis (tag on rivet of 75 holder
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Old 20th Dec 2009, 2:56 pm   #50
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Default Re: Newb Stumped on a Derwent

Yes, but is there anything connected to the junction of the resistor and capacitor? If not I cannot see what useful purpose these components serve.
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Old 20th Dec 2009, 3:03 pm   #51
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Default Re: Newb Stumped on a Derwent

this might be easier than my numpty ramblings,sorry if i'm frustrating you guys you've all be more than helpful.
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Old 20th Dec 2009, 3:16 pm   #52
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Default Re: Newb Stumped on a Derwent

That looke OK apart from the link between the two valves - Pin 4 on the 42 to the unmarked pin on the 75. That won't work - check again here. If it's right, disconnect this link from the 75. Also, the wires to that capacitor to the HT and the valve. Should be joined surely.

The cathode volts are high - check that resistor - what value is it? Higher than it's marked at a guess.

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Old 20th Dec 2009, 3:18 pm   #53
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Default Re: Newb Stumped on a Derwent

more detail

the link from 42 to 75 is a chassis tag on the 75 valve holder
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Old 20th Dec 2009, 3:22 pm   #54
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Default Re: Newb Stumped on a Derwent

The capacitor goes to pin 4 of the 42 valve. Take the other link from this pin off. Then try it.

Cheers,

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Old 20th Dec 2009, 3:22 pm   #55
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Default Re: Newb Stumped on a Derwent

where is the grid leak for the 42, the grid seems to be connected directly to chassis
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Old 20th Dec 2009, 3:39 pm   #56
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Default Re: Newb Stumped on a Derwent

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The capacitor goes to pin 4 of the 42 valve. Take the other link from this pin off. Then try it.

Cheers,

Steve P.
did that an no change
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Old 20th Dec 2009, 3:48 pm   #57
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Default Re: Newb Stumped on a Derwent

Cut the wire which goes to pin 4 of the 42 and connect a high value resistor between pin 4 and the wire.

Anyone got the pin out for the 75?
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Old 20th Dec 2009, 3:54 pm   #58
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Default Re: Newb Stumped on a Derwent

yes, the 510k you have going to pin 5 should go to pin 4, the link from pin 4 to chassis should be cut, also the 50 uf seems to be connected the wrong way round, the + should go to pin 5, - to chassis
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Old 20th Dec 2009, 3:55 pm   #59
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Default Re: Newb Stumped on a Derwent

75 tube pinout:-

http://tdsl.duncanamps.com/show.php?des=75

Regards, Mick.
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Old 20th Dec 2009, 4:02 pm   #60
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Default Re: Newb Stumped on a Derwent

The cathode bias resisitor is a 1% type and seems to be green-brown-black-brown which is 5K. Which is just as well as I*I*R says it should be a 2W part and I would guess this is a 0.6W.

The wire wrapped around the 330 resistor looks like a dry joint.
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