OpenBCM V1.06 (Linux)

Packet Radio Mailbox


[C.D.G. OpenBCM]

 Login: GUEST

G8MNY  > TECH     02.12.18 10:30l 160 Lines 8372 Bytes #3 (0) @ WW
BID : 47541_GB7CIP
Subj: Early AVO Valve Tester
Sent: 181202/0922Z @:GB7CIP.#32.GBR.EURO #:47541 [Caterham Surrey GBR]

By G8MNY                                (Corrrections May 17)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This early tester (1941) has no model number, but the 2 units that make it up
look something like this..

                      Holder  ÉÍÍÍÍ»    Tester
                  ÚÄÄÄÄÄÄÄÄÄÄÄп   ºÚÄÄÄÄÄÄÄÄÄÄÄÄÄ¿   The tester panel
The valve holder  ³()Ú¿()³  Úд ³~~~~~~~~~³ ³   has 6 rotary pot
panel has 14/15   ³()³______³()³  ÀÄ´ ³ `\      ³ ³   or switches for
types of valve    ³     o)     ³    ³  ~~~~~~~~~  ³   setting to the
base & a large    ³( )( )( )( )³    ³ db   ø   db ³   electrode volts,
9 way thumb       ³            ³    ³             ³   test modes, a
wheel switches.   ³( )( )( )( )³    ³ db   O   db ³   neon continuity
                  ³            ³    ³     (o)     ³   test lamp & a 
                  ³( )( )( )( )³    ³ db . o . db ³   test lever.

Valve Bases catered for are B7G,B9A,MO,IO,P,P5,B9,B7,B5/4,UX7,UX5,UX4, & 7.
(some added after 1941) I have even wired up the odd 4CX250B socket, but HTs
are too low for real tests!

As the switching is quite complex, a diagram it needed to do any faulting.
              º(________________  22V   ³                   ³              ³
              º __              ÀÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ260êÙo             ³
              º(__        o250          ³           SET  /³\ Off           ³
    Anode     º(__        o200          ³          ZERO  \³/               ³
  Transformer º(__        o150 ANODE    ³              ÚÄ1k5 o             ³
        ³    )º(__      ÚÄo100    o250  0.4ê          _³_  SELECT   ÚoCins ³
        ³ ___)º(__      ³ o80     o200  ÃÄÄÄÄÄÄo100mA /_\  ANODE    ³      ³
        ³³    º(__      ³ÚoRec   Âo150  À10êÄÄÂo mA/V  ³   D2oÄÄÄD2 ³      ³
        ³³    º(__      ³³oD     ÀoPen       47ê<ÄÄÄÄÄÄÁÄÄ>D1oÄÄÄD1 ³      ³
        ³³    º(__      ³³³       o100 SCREEN ³SETmA/V     A2oÄÄÄA2 ³      ³
        ³³    º(___560êÄ))Ù      Âo80 <Ä¿    38ê       NormaloÄÄÄA1 ³      ³
        ³³    º(____    ³³       ³o75   ³    +ÃÄÄÄÄ¿                ³      ³
        ³³    º(__  ÃÄÄÄ))Ä¿     ³o60  SCR  meter ===220u           ³      ³
        ³³    º(  ³ o   ³³ o mA/V³     yel    ÀÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄ>oÄÄÙ
  Ú220kÄ´³    ÃÄÄÄ´     ³³     ³                 50nF                  Cins³
 Neon   ³³    ³   ³     ³³     ³                                           ³
  o test³³    ³ÚÄÄ)ÄÄÄÄÄÙ³    G1                                           ³
  /prods³³    º(_Ú)Ä91êÄÄÙ                            Tester Socket        ³
  o     ³³    º(__³______________________GND gn             o A1           ³
  ³  on ³³  _ º                   ³      Cathode      G1 o     o A2        ³
LÄ)Ä/oÄÄÁ)Ä>ð)º(ð40V              o oCins           SCR o       o D1       ³
 100k    ³   )º(ð Heater          /³\                    o     o           ³
NÄÁÄÄÄÄÄÄÁÄÄÙ º(ð <ÄÄÄÄÄÄÄÄgn H+   ³                  GND  o o  D2         ³
       Heater º(ð Volts            ³                      H+  H-           ³
   Transformerº(ð2v        H-      ³                                       ³

The transformer HT - switch connections are too complex to draw here. The un-
loaded windings are not the same voltage as the switch would indicate, & the
screen voltages are also different to the anode voltages!

The unit tests valves with AC power. So prolonged soak testing of large power
valves will overheat the unit, as the HT transformer will saturated with DC.
The half wave valve load on the HT transformer may explain the nead for over
voltages so the valve warms up properly!
N.B. Two of the anode voltages come from the heater transformer!

Care must be taken when selecting the 9 valve pin thumb wheels to correct 10
option valve electrode connection. Plug in S G A electrode wander lead
connections available for top cap & side screw types.

This must be done with the POWER OFF as any mistake may destroy the valve or
damage the tester!

       THUMB WHEEL                                   SOCKET       VALVE
       0 = Earth                                        5           5
       1 = Cathode              ALWAYS DOUBLE        6     4     4     6
       2 = Heaters -           CHECK ELECTRODE      7       3   3       7
       3 = Heaters +             VALVE BASE          8     2     2     8
 (G)   4 = Grid   (G1)          CONNECTIONS!          9   1       1   9
 (S)   5 = Screen (G2)                             PIN NUMBERING CONVENTION
 (A)   6 = Anode 1        Normal \                      2           2
       7 = Anode 2         A2     | Select          4       3   3       4
       8 = Diode Anode 1   D1     | Anode               5           5
       9 = Diode Anode 2   D2    /  Switch
                                                        1           1
                                                  [Not Side Contacts types!]

Multiple section valves are catered for, with up to 2 Anodes & 2 separate Diode
Anodes. Grids are normally paralleled up with other sections, & for Mixer
valves you use A2 as G2/G3 etc.

Insert valve in correct socket! Then power up, watching out for unusual signs
of over bright heaters, no heaters, glowing anodes or screens. Switch off
immediately & check settings & valve type.

CINS test uses 3 changeovers to rewire much if the anode meter bridged test
circuit to measure the leakage cathode to heater when 80V (105V!) AC is applied
though the 50nF cap. (put Screen to 60V) [250kê is a GOOD insulation!]
N.B. The test uses Anode to Cathode rectification to drive the meter so the
valve must be warmed up, with the Anode circuit connected!

The heater transformer has an internal mains taping adjustment if needed.
Valves can be tested after about 1 min warm up for indirectly heated types & a
few seconds for directly heated ones. Over doing the heaters e.g. 7.5V on a 6V
valve may well quicken the warm up time, but lower initial voltage is best at
1st for heaters. Never over run heaters above the normal cathode temperature
(dull red) as this WILL vaporise a cathode coating & stop emission!
For dual/tapped heaters types use parallel connection.

As the whole valve warms up there may be differences (improvements) in
measurements is the valve is soaked for 30 mins or so.

On the valve holder panel there is a heater 1/7 transformer to enable small
directly heated valves to be lit at 1.4V when it is switched in & heater volts
set to 10V, so never through this switch back to normal on 1.4V valves!

This applies 14V via the 560ê & you measure the current on the 10mA scale with
mA/V pot at to highest end.

This applies 32V via a 91ê & you measure the current on the 100mA scale.
e.g. mA/V pot set to 100.

Select Anode & Screen voltages as needed (from AVO ref book!). Screen voltage
must not be used without anode connected, as a Screen only current can damage
some valves.

Valve Gain "gm" is measure by Zeroing the meter with DC offset from the SET
ZERO" control, then throwing the "mA/V" key. With the "SET mA/V" set to the
"mA/V" the "gm" position, the "gm" can be directly read off the meter scale.

The mA/V test switch applies either 1.2V or 1.1V AC to the G1 & the difference
in anode current can be measured, or if the GM value is dialled into the SET
mA/V pot, & Anode current bridge balanced SET ZERO, a GOOD/FAIR/BAD gain
indication results.

The mains neon has 2 switched probe sockets (S), if plugged in these probes
allow full mains voltage insulation testing on the leads for testing the
unplugged valve. As there are 2 high value Rs on this circuit so it is quite
safe really!

As there is no fuse in old appliances like this, I use a 1A mains plug fuse!

See my Tech buls on "Valves (Tubes)" & "Old Valve Radios"

Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

Read previous mail | Read next mail

 09.12.2018 20:55:00lGo Back Go up