We are pleased to offer you our ULTIMATE Dynaco ST-70 upgrade kit with the AB-Q-ST70/120 Auto Bias module, the Dyna-70AB.
We have developed this kit to bring your tired old ST-70 power amp up to modern tube amp standards with many improvements over the original Dynaco driver circuit and using the AB-Q-ST70/120 Auto Bias module for the output tubes!
This is indeed a RADICAL upgrade to your ST-70, really changing how this amp operates and sounds.
We assume that all the transformers in your existing ST-70 are in working order.
Speaking of transformers, do take note of this VERY IMPORTANT point with regards to the PA 060 power transformer.
Dynaco had two types of PA 060 power transformers they used. The PA 060 transformer used in the very early kits had a very low 'stack' about 1"/25.4mm high. Then Dynaco came out with a transformer with a notably higher stack, about 1 3/4"/44.5mm high. This is the size transformer we are using in our test ST-70 and we've had NO issues with using this size. Yes, it gets warm, but this is quite normal!
Do also note, that since with our kit the 5AR4 tube rectifier tube is no longer used, the 5VAC secondary winding is also no longer used which will lessen the overall load on the PA 060 keeping its running temperature lower!
We cannot guarantee that the very early 1"/25.4mm stack height PA 060 power transformer will be able to comfortably handle the extra filament current used by the 6SN7 driver tubes!
You can of course upgrade the PA 060 by buying one from one of the many suppliers, this is even larger again with a 2"/51mm stack height and with this model PA 060 you could even use the KT88 family of tubes.
We recommend that you stick with the EL34 family of power tubes, as all our testing etc. was done using the EL34.
The cricuit is based on the classic Mullard design of the 50's with certain improvements to bring it up to 'modern' tube circuits, including an LM334 based constant current source for stable performance.
With this auto bias module, you willl NEVER have to adjust the output tubes bias again. We preset the bias level to suit the EL34 family of power tubes, so that all you need to do is to build and install this kit and enjoy tube music! It has four LED's which are mounted to the main pcb which indicate that bias on all four power tubes is healthy.
Should a tube fail or the auto bias module can no longer maintain the preset bias level, the relevant LED will go out indicating that the tube in question needs to replaced. It is that simple!
The kit comes complete with ALL parts and the AB-Q-ST70/120 auto bias module, including hardware and wires. You will need to splice some of the wires, as the original length on some wires are too short, for that, we supply colored wires and heat shrink for the joints! We also supply a 3 way power cord where the chassis will be grounded to your house ground. This is an important safety upgrade!
The full details are all covered in the comprehensive instruction manual you will get with the kit.
The coupling capacitors supplied with the standard kit are the excellent SOLEN or DAYTON AUDIO film capacitors.
We have made no compromises with the quality of the componets used for this kit.
- Nichicon, United Chemi-Con and other name brand electrolytic capacitors
- 1% high grade film resistors in the signal circuits
- Panasonic Metal Oxide resistors for the power supply
- Solen coupling capacitors, (Russian PIO capacitors as an option)
- top quality FR4-TG standard black printed circuit board, 0.08"/2mm thick, with dual ground planes for ultimate screening
Also to note is that with our kit, the STEREO/MONO switch will no longer work/is disabled and the two front octal sockets are no longer used either.
This kit is not complicated, but your are working with extremely high voltages, in excess of 720VAC & 400VDC, so EXTREME care MUST be taken! These voltages can be LETHAL!
We also stress that good soldering skills are a must have, this can make all the difference for a trouble free project. We strongly recommend that you use a temperature controlled soldering iron, such as a Weller or HAKKO, with small as well as medium sized tips. 98% of all issues our clients have with building a kit is related to bad soldering, so AGAIN we stress, good soldering skills are a MUST!!