Help on Paramount Soft Start Kit installation

[stevesong]

I spent many hours trying to install Soft Start Kit on my Paramount 300B amps with MagneQuest iron.
I installed one amp and resistance check indicated to me it is well within spec.
Only thing with my attention is that T9 and T19 hasve value of 298, where manual said 124K with "*" mark.
So, possibly and hopefully my meter is interfered by capacitance.

When I did the voltage check everything looked good except the following
(I set 5670 plate voltage successfully set it to 175VDC prior these measurements):

T10  233V -> mine has 0
T11  164 ->  224
T12  0    ->  155
T13  94  ->    0
T15  26  ->    18

All LEDs light up and tubes glow and physically they look good with no smoke, etc.
I went through installation procedure several and could not fine what I have done wrong.

All other terminals measure with spec and I don't want to start my other amp until this is well and figured out.

Please help and thanks in advance for advise from you "Experts".
     

 

[Paul Birkeland]

T10 should indeed be zero, it looks like that's a typo.

The other terminals have a string of zener diodes connecting the regulated voltage go ground.  If OA and OB measure perfectly, I wouldn't be too concerned with those readings.

 

[Paul Joppa]

Yeah, the Zeners are not functioning once the circuit is running - they exist only to protect things until the regulator kicks in, around 20-30 seconds after startup. So the actual Zener voltage depend on the miniscule leakage currents of the individual diodes, e.g. on how much fingerprint was left on the diode.

The only time it is important is the initial check, page 36 of the upgrade manual. Here the initial voltage of 345v is set by the Zeners, and the eventual voltage of 300v indicates the shunt regulator has taken over. After that, if the regulated voltage is right and you can adjust the driver plate voltage to the correct value, then you can ignore all the readings on that terminal strip.

 

[stevesong]

Hi,
Yes I did check the B6 voltage to be about 294V and adjusted the OB driver plate voltage to 175V.
Whew, I am so glad to know I did  not make any serious mistake.

Really appreciate both of your extremely fast HELP!

Time for another long hours of second soldering

 

[stevesong]

Just inserted the completed mono amp in my system to see if it works.
It works great and sounds wonderful.
Thanks again,
Steve

 

[saildoctor]

I just looked at the checklist I had when I changed my Paramounts to use the soft start (I was using 300b's then).  My values are similar to yours.  FWIW my T11 measured ~230 as well.  Rock on!

 

[stevesong]

Thanks everyone for help. Can't believe I didn't mess it up.

I have been soldering the second amp tonight.
One of the 220 ohm carbon resistor measured about 217 ohm on my meter and I installed one end to B5 and the other end to
18U.  The one soldered to B5 (one of the tube pins) is supposed to go very close to the pin.
After soldering it measured about 251 ohm.  Is it possible for the heat to increase the value of carbon resistor?

I found a 235 ohm carbon resistor from the old board.
Would it be ok to use this resistor when it's off by about 7%?

 

 

[Paul Birkeland]

The value of these resistors functionally can be 200 Ohms to 1000 Ohms, and they are a carbon resistor, so the tolerance is inherently not all that great.  (In other words, don't worry too much about them)

 

[stevesong]

Thanks Paul always for your quick help.
I will keep soldering today...

 

[stevesong]

Finally finished up installation of both amps.
Some stupid mistakes along the way but caught them on double (or triple?) checking before firing them up, so not harm done.
Measurement results are almost identical on both amps, so hopefully I didn't make the same mistake on both.

I just hooked them up in the system and they sound very sweet as Doc described it.
Sound may change as the amps break in, but so far very very positive with WE396A input tubes.

It's quieter, more delicate, very musical.

Many thanks,
Steve