ST-120 Supplementary Cap Module - What Specific Caps These Days?

Hi all,

What are the specific caps (manf. part number) used in the SCM for ST-120 kits today?  After 12 yrs. I am going to retire the original Cornell Dubilier Electronics (CDE) 330uf, 400V, 85C caps.  They have served me well but there are probably newer caps available with higher temperature/ hours ratings.

With the CDEs mounted I do not see any identifiable part numbers. The amp is installed so mainly looking at physical dimensions (W x H) for something else that will fit in the space.  Probably will recap all electrolytics when the ST-120 is pulled apart.  

And what are the values of the two voltage balancing resistors across the SCM capacitors. They are not listed on the schematic supplied in 2012.  
Thanks in advance!

Download Cornell Dubilier Aluminum Electrolytic Capacitor Application Guide

There is a formula on p11 that will calculate the optimum balance resistance

330k 1w

Thank you! That is an interesting formula on page 11 and calculates the balancing resistors should equal to or less that 2.7M ohm. At V+ = 500V and 400V cap rating. So 330K ohms is definitely less and assures voltages across the caps never get close to exceeding their ratings. Each 330K resistor consumes about 1/6 watt on average in the ST-120.

There are lots of nice electrolytic caps available today with extremely high temperature ratings compared to the CDE model used. The power transformer is just right there baking those SCM caps. Capacitor height is a main consideration on making a change it seems. Pondering moving the choke to one screw mounting (like to the other side with TDR). Then flip the 5-post SCM terminal around where the SCM caps mount 180 degs and taller cap friendly.

The CDE caps at 85C rating have held up well but feel I am pushing my luck as they age. In reality they will probably outlive me!

The resistors are to bleed the caps after the power is disconnected. Nothing to do with voltage protection.
My RoT* is to use 25% of the calculated value. Anything lower just wastes power and generates heat.

Use the biggest C at a voltage 10% higher than the open circuit rectified DC, the voltage applied until the tubes conduct.

ESR increases as electrolytics age.

IMO, the TDR is not optimum. The delay is too short and applies full power into empty caps for a huge current spike. A soft start that ramps up the voltage is preferred.

* Rule of Thumb

Recapped the ST-120 electrolytics (12 yrs) and checking over the work before applying power.  Two of the original boards caps (upgrade versions) were right at their lower spec values (+10/-20%).  For instance one 100uf/400v cap measured 80.5uf and a 47uf/400V cap measured 40uf.  The quad Authenticap, bias, SCM capacitors all measured close to nominal. But everything was replaced with new.  

The quad Authenticap uses F&T brand caps inside and from experience with guitar amps F&Ts are very rugged so happy I found a direct replacement.  Used Nichicon UCY series for the board replacements.  Their endurance at respective ripple current is 105C for 10,000 hrs and recommended for power supplies/ ballasts. And finally the 330uf 400V SCM caps were replaced with Chemi-con GXAs with endurance of 125C for 3,000 hrs at 1.48A.  The GXAs are 35mm in diameter while the originals are 30mm (both 30mm in length).  But fit OK and was able to rework without removing the SCM terminal strip.  The Nichicons and Chemi-cons were purchased from Digikey.  

Did I need to replace the caps at this time?  No, but electrolytics don’t last forever and now have some peace of mind with hopefully upgraded spec parts.  Eager to get this amp back in full service.