ST-120 - Noise in R Ch After Recapping Electrolytics

The ST-120 recapping went well but now have crackling noise in the R ch.  Progressively gets worse and right channel sound is low and distorted.  I see no error in rework recently performed to replace the electrolytics.  Basic troubleshooting reveals 410V on the plates of the right channel phase inverter. Left channel looks normal around 225V.

Seems the high plate voltage indicates the 12AU7 phase inverter is not drawing current.  Probing points to the LM334 constant current device in the phase inverter cathode circuit.  Have some LM334s on order.  

The noise sure sounds like a faulty capacitor popping or flaky tube so spent time exploring what I did recently to the circuit.  But high plate voltage tells me the phase inverter 12AU7 is drawing little to no current.  I now see why the board electrolytics are rated 400V.  If the tubes draw no current then no voltage drop across the B+ resistor feeds to the tube circuits.  The new Nichicon UCY board caps are rated 420V and are holding up well.

LM334 constant current regulator replaced in the right channel and amp performance restored. The VTA ST-120 plays way above the 250W/ch transistor amp substitute. Better bass thanks to KT-120s, silky highs, huge staging, and black background. Love the dynamics/ bloom and perfect match for my 8 ohm medium wattage, bass shy speakers. Does not get much better than this right now.

What's the heater current capability?

KT-120s can draw close to 2A and if the heater supply can't supply it, low heater current can shorten tube life.

Measure the heaters and make sure they are 6.3±0.5.

FWIW, I limit heater voltage <±2%, preferably ±1%

Used KT120s in the ST-120 for close to 9 yrs and no problem. Besides being rugged/ reliable KT120s were chosen for sound character in my system.  Also use solid state rectification which saves the power transformer from supplying an additional 15-20W heating a rectifier tube.  

I'm sure someone may be using KT120s, tube rectification, and three 6SN7s which should be the greatest load on the power transformer heater
circuits.

As Bob Latino said many moons ago. “This tube (KT120s) should work fine in any Dynaco Mark III or VTA ST-120. It could also be used with any Dynaco ST-70 or Mark IV as long as you have an upgraded power transformer.”

https://dynacotubeaudio.forumotion.com/t325-new-tungsol-kt120-output-tube-for-dynaco-amps-first-impressions-on-4-16-10?highlight=Kt120

It's all about tolerance level.

Nowhere in the referenced article is the KT120 heater voltage mentioned as measured.

Tubes are typically spec'd for ≈±10% on heater voltage. If an amp can drive KT120 @ 6.3v then it's going to be way over on 6550 et al. If it drives 6550 @ 6.3v it's going to be way under on a KT120.

Gm falls faster over time the higher the heater current. Below about 6v, other issue can occur. However, some makers run 5v and the tubes last forever. Sonics unknown.

My ST-120 right channel heater voltage measures 5.6Vac at the input tube pins.  This is 10% lower than spec but I'm using 115V AC input voltage to keep B+ below 500V with solid state rectification. Functioning normally for years. These older style amps with unregulated power supplies never operate with nominal conditions.  Yet they work for years.

Yes, they operate, but not optimally.

Underheating a tube is more damaging than overheating.
Ions rather than just electrons leave the cathode and deposit metal on the other elements and the glass.