Re: [Amateur-repairs] Looking for Hi pot to test some tubes - mostly 3-500's

Hi Mark et al,

Here is what I did when testing 8122 tubes. I have a more complete writeup that includes building an adapter to use a TV-7 to get a relative indication of tube gm.

Bob,  K7DYB

On 12/13/2025 11:37 AM, Mark via groups.io wrote:

Merry Christmas,

Does anyone have a reasonably priced hi-pot for sale? 

73

Mark

W0NCL

_

1.           Hi-Pot Testing

Warning: Disconnect any leads to the tube tester before proceeding.

Out of concern that a tube might short or flashover when full voltage is applied, it is prudent to perform a Hi-Pot test between the Anode and Screen Grid (G2) using a current limited voltage source that can deliver at least 500v or higher. Hi-Pot testing is performed on a cold tube with no filament voltage applied.

I purchased a "CX-20 High Voltage DC Power Supply 12-24V Input Adjustable Module 1-10KV Output" from ebay for approximately $30. This supply uses a 12 vdc input (from an external wall wart) and provides an adjustable output from 1 to 10 kilovolts.

The power supply is mounted in a small plastic enclosure that provides 12vdc in and HV out connections, High Voltage metering, an on/off switch with LED indicator and a voltage adjust control. The original 10k ohm voltage adjust potentiometer was removed from the circuit board and replaced with a panel mounted control. The value of the metering resistor will depend on the meter used. I used a 100x probe for my VTVM to find the value needed for a full-scale reading of 10 kilovolts (in my case the resistor was 150 megohms).

Figure 13. HV Supply Schematic

During HiPot testing, the output of the supply is routed through an external 50ua meter in series with a 10 megohm current-limiting resistor. This provides a degree of protection for both the supply and meter in the event of a short or flashover.

While leakage current is commonly associated with gas, I would not rule out using a tube with slightly more leakage current than was typically observed. The following were typical values observed between the Anode (+) and G2 (-):

Voltage between Anode (+) and G2 (-)	Observed Typical Leakage Current
2 kv	< 1ua
3 kv	<3 ua)
4 kv	6 ua
5 kv	<10 ua
6 kv	<10 ua