[Boatanchors] Globe (WRL) V-10 VFO - Problem with voltage regulation

[Garey Barrell]

Jerry -

Are you measuring with a high impedance meter??  VTVM or GOOD DMM??  The Harbor Freight DMM for instance is a pretty low 
input impedance and will load this kind of circuit down.

You're dealing with fairly low currents here.  The power supply will approach peak voltage with very light loads.  With 
the VR lit, it's drawing at least 5 mA.  The 2700 ohm resistor is dropping 15V so it is carrying ~ 5.5 mA.  This is 
about what I would expect with the VFO 'unkeyed'.  But if you 'key' the VFO, the voltage would drop another 24 V across 
the 2700.

The 220 ohm resistor is not needed.  Is the 176 V before or after the 220 ohm?  At these currents, the forward drop 
across a selenium rectifier is about one (1) volt / plate.

If I was designing this circuit, with 185 VDC as the specified HV, I'd specify about 11 mA through the VR tube, and with 
the 6BH8 drawing the 9 mA it does 'key down', the resistor would have to be 1.8k.  'Unkeyed', the VR tube would have to 
sink 9 more mA, which is well within it's ratings.  I don't see any way the VR tube could stay 'lit' key down with the 
specified 4.7k resistor, as with just the 9 mA for the oscillator the voltage on the VR would drop to only 143 V.

I would start by removing the 220 ohm resistor.  Then set the AC line voltage to a more typical 120 VAC and measure the 
B+ across the second filter cap with the VR tube out.   Take that voltage, subtract 150 V, and divide the remainder by 
0.02. This will be the value resistor needed.  Multiply that remainder by itself (squared) and divide by the resistor 
value just calculated.  Multiply that by 2 and that is the size resistor needed.

Example:  185 VDC - 150 = 35 VDC / 0.02 = 1750 ohms.  35 x 35 = 1225 / 1800 ohms standard value = 0.68 x 2 = 1.36 so 2W 
resistor.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>

Jerry wrote:
> Hi Gary
>
> I tried  the R4 = 2700 ohm but only get 164vdc B+ with the plate on the 0A2
> showing 149vdc.    This is all with a variac at 115vac .    I measured
> voltages after pulling the 0A2 and  B+ was 176vdc.  I have fresh lytics and
> diode.  I am dropping about 6vdc because I added a 220 ohm resistor in
> series to the diode when I replaced the selenium with silicon 1N5408.  Any
> idea why with the 0A2 striking my B+ is only 164vdc?
>
> 73 Jerry
> K1JOS
>
> -----Original Message-----
> From: Garey Barrell [mailto:k4oah at mindspring.com]
> Sent: Friday, November 08, 2013 22:12
> To: Jerry; Boatanchors
> Subject: Re: [Boatanchors] Globe (WRL) V-10 VFO - Problem with voltage
> regulation
>
> The parts list shows R4 as a 4.7k @ 1W.  The B+ is shown as 185 VDC on Pins
> 5 and 6 of the 6CB6.  So 40V drop to the 0A2 is about 9 mA.  The 6BH8 is
> drawing about 9 mA when keyed, so doesn't leave anything for the 0A2.  I'm
> not surprised it didn't fire!  A 2700 ohm @ 2W would be a better choice.
> With the higher line voltages today you'd have enough to regulate without
> overloading the 0A2 when the VFO is unkeyed.
>
> Dropping 40V across the 1.5k would result in 26 mA keyup and 17 keydown,
> well within it's limits.  BUT, you would leave as much as 1W in that 1.5k
> resistor.  Really ought to be a 2W.
>
> 73, Garey - K4OAH
> Glen Allen, VA
>
> Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
> and TR-4/C Service Supplement CDs
> <www.k4oah.com>
>
> Jerry wrote:
>> Hi Lee
>>
>> Many thanks.  Will get to it tonight.  Did you also add a low value
> resistor with the silicon diode to compensate for the extra 8-10v drop from
> the old selenium?  My V-10 had two large selenium stacks in series so it
> could have been as much as a 20vdc drop across them bringing the B+ from
> 175vdc to 155vdc.  I though this may have been part of the problem so I only
> put in a 220 ohm resistor to drop it by about 7 volts.
>> 73 Jerry
>> K1JOS
>>
>> -----Original Message-----
>> From: L L bahr [mailto:pulsarxp at embarqmail.com]
>> Sent: Friday, November 08, 2013 17:56
>> To: D C _Mac_ Macdonald
>> Cc: boatanchors at mailman.qth.net; Jerry
>> Subject: Re: [Boatanchors] Globe (WRL) V-10 VFO - Problem with voltage
>> regulation
>>
>> Jerry,
>>
>> This is your LUCKY DAY!!!!  I know exactly what the problem is as I had
> the same problem with my V-10.  I bought it off of the E-pay place and of
> curse it did not work.  I too replaced the selenium rectifier with a diode
> and changed out the filter caps.  Then I turned it on and my 0A2 would not
> strike.  The problem is people have diddled with these VFOs and tried to
> rebuild them.  There big mistake was the circuit diagram is wrong!!!!!  That
> dropping resistor they show as 47k cannot work.  If you use ohms law, the
> 47k defies science.  There is no way the 0A2 can fire with a 47k resistor in
> there. WRL might have meant a 4.7k.  However,  I put in a 1.5k 1 watt and it
> fires off nicely and the resistor stays cool and everything is now happy!!!
>> Lee, w0vt
>>
>> ----- Original Message -----
>> From: "D C _Mac_ Macdonald" <k2gkk at hotmail.com>
>> To: "Jerry" <jsternmd at att.net>
>> Cc: boatanchors at mailman.qth.net
>> Sent: Friday, November 8, 2013 4:10:22 PM
>> Subject: Re: [Boatanchors] Globe (WRL) V-10 VFO - Problem with voltage
> regulation
>> Try the regulator to give you 105V regulated with the specified dropping
> resistor.  I'd bet the VFO would run just fine with the lowered B+ voltage.
> IIRC that's the 0B2.
>> 73 - Mac, K2GKK in OKC
>>
>>
>> Sent from my iPhone
>>
>> On Nov 8, 2013, at 15:51, "Jerry" <jsternmd at att.net> wrote
>>
>> I found a V-10 in great shape but someone had cut the line cord close
>> to the chassis which made me worry that there were some potentially
>> serious issues with the VFO.  The inside looks new, however before
>> applying power, I checked to make sure the power transformer was not
>> shorted.  The xfrmr works fine supplying filament 6.2vac and separate
>> 175vac to a half wave selenium
>> (65ma) that I replaced with 1n5408 plus a 200ohm dropping resistor.  I
> also replaced the two 30uF lytics and left the large (~10mH) choke between
> the two filter caps as it was not shorted either.  Thes B+ supplies the
> plates to 6BH8 and 6CB6 using a 0A2 voltage regulator.  I cannot get the 0A2
> to strike at 175vdc from the filtered supply.  I replaced the 0A2 twice with
>> known tubes and still not striking.   The dropping resistor between the B+
>> and 0A2 is 47K, 1W per the schematic but in the VFO I found a 560 ohm
>> 1 watt instead.  I cannot tell if this was done by the previous owners
>> to get the
>> 0A2 to strike earlier or if this was a factory mod not documented on my
>> schematic.   Can anyone offer advice?
>>
>> 73 Jerry
>> K1JOS
>>
>