New Dynaco ST-35 bias control upgrade kit from Dynakitparts, Mine expirence.....

I am lucky, I have a new ST35 & a rebuilt SCA35 (as power amp only). Both I have put in the ST35-BCU bias upgrade kit to & have done 3 additional critical changes. The Bias kit was constructed per instructions & I added the optional ground wire from the board for both amps. C1 was removed from both amps.

ST35. I also added 22uF cap & a 1uF auricap after 50 ohm resistor. Replaced C4 & C5 with .47uF caps (5x original value, biggest I could fit in).

SCA35. The power supply on this was already way over done, all sections are bypassed with 1uf auricaps & it is split into channel A & B right after the diodes. I replaced C4 & C5 with 1uF auricaps.

Biasing, followed all setup instructions. I then turned on the amp & let set for 10 minutes. Set bias per instructions.
Both amps were dead stable still a week later.
ST35 32ma
SCA35 34ma

Sound after the above work done EXCELLENT! This is a term I seldom use. The amps were ok, but I have been around long enough & heard so many systems to know what was lacking. The high treble is now perfect in its delicacy. The deepest bass exists now with all the air. I found the sound staging much more focused & wider. The sound was all around clearer more enjoyable. What’s so interesting is just a few changes made a dramatic improvement in sound. I would highly suggest these mods to anyone owning ether amp.

Go away, have fun!
Tazsmonn
05.31.09

I will be writing a bit shortly on installing it in a non Dynaco amp, I have been rebuilding this winter. So look for it on the this Forum.

Hello.. I am brand new to this forum so hello to everyone. I just ordered an ST-35 bias kit from Dynakit and also some Dynapads.

My ST-35 was rebuilt some 5 years ago and it was intermittently used. Now I have it as my daily amp with a PAS-3 (with the VTV upgrade kit a NOS Mallory FP, Si Diodes) and mated to some Focal 806V and fed by a Marantz BD-7004. Sounds amazing !

My question is about the bias kit. I had never seen a Cathode Bypass cap in series with a 1K resistor. Does this resistor serve a purpose or is it needed because of the clever design on the variable Cathode Bias bias circuit?

I always wanted to modify my ST-35 with four individual cathode resistors and bypass caps, this kit has been the answer to my prayers.

Thanks,
Rich

the cathode bypass cap is NOT in series with a 1K resistor.
There is a 1K resistor in series with a trimpot and in series with the 10 ohm bias measure resistor, and a bypass cap in PARALLEL with all of that. That is a normal bias setup
for the DIYtube ST35 amp on which the Bias Kit for the Dyna ST35 was based on.

Excellent! This is what I had concluded after scratching my head and trying to understand the schematic sent to me. I ordered the Bias Board and counting the days... Here's the diagram that confused me. I guess it will be corrected by Dynakit soon.

I came back from a 1 week business trip and finally (anxiously) was able to install the ST-35 bias upgrade kit into my vintage ST-35 amplifier. Some 2 hours after I began the amp was ready. One nice thing I noticed was one weak EL-84 tube out of the four I had, so I swapped them out for a fresh new quad. Biased up at 0.35V (35ma) on each tube. Absolutely wonderful sound. Highly recommended upgrade. The kit also includes a filament resistor mod as well to provide a balanced resistive center tap to ground. The quality of the PC board is excellent. All of the parts are premium quality, the instruction manual is first class. The ST-35 now provides more inner detail to the music, and the overall sonics have noticeably improved, especially the Bass.

As mentioned I wrote of putting The bias kit in a non dynakit amp. This posting can be found elsewhere on this forum.
Tazsmonn
04.27.11