Replacing rectifier tube with diode...

Externet

Joined Nov 29, 2005
1,960
Hi.
An old oscilloscope with a 1V2 rectifier tube on a 1170 VAC winding to yield 1600 VDC to a CRT grid is asking for a replacement. A diode on hand is DTV32F = 1500V 10A / TO220
Should I try ? What can be the worse it may happen ?

Joined Feb 20, 2016
4,251
Smoke!
You should go with a diode at least twice the voltage, and a bit more for safety I reckon.
And a 10Amp diode is a bit high.
And is the diode in a mains frequency circuit or a high frequency one?

Externet

Joined Nov 29, 2005
1,960
Smoke is fine It is 60Hz part of the power supply, and found that among my million parts cannot find a 560K and a 220K resistors needed too. Nor diodes for 2KV Tube equipment is hard to repair now... If I have to order a resistor they will want to squeeze $19.99 plus$19.99 shipping. Time to add a bunch of lower values

MisterBill2

Joined Jan 23, 2018
13,772
For this application I suggest three 1000 volt diodes in series, 1N4007's, rated 1 am and 1000 PRV.
And the worst that can happen is the diode fail shorted and the high voltage winding of the transformer would then fail open. Then you have a serious problem.

Externet

Joined Nov 29, 2005
1,960
1N4007 are 1KV. Will have to be in series. No 1N4008 after scanning 4 CRT TV cadavers in my junk... Just the mentioned one.

Joined Feb 20, 2016
4,251
You could try an EHT rectifier from an old TV or monitor. They may have a high forward voltage drop but could be worth a go.

DickCappels

Joined Aug 21, 2008
9,512
As dendad mentioned in post #2, the Peak Inverse Voltage (PIV) across a rectifier in a simple peak detector circuit is approximately 2X the peak input voltage.

This means that your rectifier must be able to survive a peak inverse voltage of 1,655 volts without damage.

Two 1N4007 diodes connected in series should work nicely provided the average current is well under 1 amp. (For a safety margin 750 ma should be the limit,, for higher currents diodes rated at higher currents or multiple diodes in parallel may be used.)

MisterBill2

Joined Jan 23, 2018
13,772
1N4007 are 1KV. Will have to be in series. No 1N4008 after scanning 4 CRT TV cadavers in my junk... Just the mentioned one.
You will not find any 1N4008 because they are not made. And post #4 is incorrect about the scheme for diode numbering. And what is an "EHT" rectifier? Not familiar with that acronym.

ericgibbs

Joined Jan 29, 2010
16,790
hi,
Extra High Tension.
E

MisterBill2

Joined Jan 23, 2018
13,772
hi,
Extra High Tension.
E
OK on "EHT", not a term I would use. That would be the second anode high voltage diode, often embedded into the horizontal output transformer. That would be a good alternative if it can be recovered. I have not tried to do that. Those devices seldom fail.

MrSalts

Joined Apr 2, 2020
2,617
And what is an "EHT" rectifier? Not familiar with that acronym.
EHT is extra high tension - (extra high voltage in modern terms).

Externet

Joined Nov 29, 2005
1,960
The ones that not exist :

Externet

Joined Nov 29, 2005
1,960

MisterBill2

Joined Jan 23, 2018
13,772
OK, I get the point. BUT for perhaps 40 years the 1N400n set only went up to 1N4007. So you got me with a new part.like
It is n that those will be among a stack of pieces left from projects completed a while ago.

Joined Jul 18, 2013
26,278
And what is an "EHT" rectifier? Not familiar with that acronym.
EHT was a familiar term for final CRT anode voltage used in the UK when I took my training!

DickCappels

Joined Aug 21, 2008
9,512
EHT is the thing in a CRT receiver or monitor that can cause one to do what one of my bosses used to refer to as "Do dah dance!"

If the subject diode needs to withstand 1655 volts. Whether one uses two 1N4008's or two 1N4007's in series the only effect would probably be cost and availability. Note that there are rectifiers, though not as cheap and the 1N400X series that are rated at 2 kV & up with faster reverse recovery times, not that you'd see any improvement at 50 or 60 Hz.

JWHassler

Joined Sep 25, 2013
299
The 1V2 has a voltage drop of 135 volts at 7mA. This may be an issue

Joined Jan 15, 2015
7,079
The 1V2 has a voltage drop of 135 volts at 7mA. This may be an issue
I gave that some thought also. When replacing some of the old vacuum tubes or valves (for those across the pond) the forward voltage drop might be a consideration but the more I thought about it I doubt it matters much in this application, with a vacuum tube having a 6.6 KV inverse plate voltage.

Ron

MisterBill2

Joined Jan 23, 2018
13,772
Consider that the device is "an older oscilloscope", it is likely that some additional high voltage for the CRT will be an improvement.