I know I promised an article on the ST-70, but once again my exciting life got in the way..... HA!!!! Anyway I have a lot of interesting things to discuss and I will have a construction article in the near future.
I'll start this time with some more modifications for the Citation II since everyone else has already beat me to the punch... But seriously this is one of the best tube amplifiers ever designed. Thankyou Stewart Hegeman.... You will need an original HK schematic to follow my list of modifications.
Use extreme caution when working on one of these amplifiers as lethal voltages are present in the chassis. Carelessness or inexperience could result in DEATH. I assume no responsibility for any consequences resulting from the use or misuse of this information.
I am assuming that all the original resistors and capacitirs have been replaced with the appropriate quality modern equivalents. Half watt resistors can be replaced with RN-60 types by Dale, Resista and Holco. Power resistors in the plate circuits of V1-V6 were mostly inductive wirewound resistors and must be replaced with a combination of RN70 resistors (or at least metal oxide 2 watt resistors)[1996 aside: Mil non inductive wire wound resistors available form Michael Percy, Audio Consultant in Inverness, Ca. can be used also] in series-parallel in order to get the resistance and power handling required. Replace all the ceramic capacitors with Mallory SX series or MIAL (polystyrene) capacitors with a voltage rating of 600 V or better.
First replace J1 and J2 with your favorite brand of high quality RCA jack and replace the original wiring with 22 gauge Cardas chassis wire or a good quality teflon jacketed silver plated OFC copper. Substitute 100K 1% resistors for R2 and R37.
Remove the jumpers from the RC networks consisting of R8, R17, R43, and R52, and you may also want to remove these components along with the associated capacitors. This modification removes one of the nested feedback loops. Do not remove the other loop consisting of R9,C5, and R15,C8 and R44,C21 and R50,C24 as I cannot attest to the stability of the loop under these conditions.
Connect a 0.1uF 600V from each wiper of R20 and R57 to ground (the AC balance pots), and in addition connect 0.33uF 200V capacitors to each of the wipers of R25 and R60 (the bias pots - four in all).
If you triode connect the output I would recommend the use of 330 ohm resistors between pins 3 and 4 of the output tubes as I have observed parasitic and subsonic oscillations with some output tubes when I used 100 ohm resistors in this location.
Install a pair of 6A 200V bridge rectifiers and 10000uF 25V capacitiors for DC to the filaments of V1-V3 and V4-V6.
Make sure your 12BY7A's are fresh, same brand and that V2/V3, V5/V6 are well matched. Make certain that none of them are very microphonic, and particularly V1 and V4 are crucial in this regard if you want a quiet amplifier!
Upgrade the bias supply with low ESR 680uF/250V (2012 edit) capacitors in place of C17A and C17B.
Upgrades to the power supply are important and consist of increasing the amount of capacitance in the supply to at least 200uF on each side of the chokes and adding small shunt capacitors across all the electrolytics. If you are slightly braver you may want to boost the B+ to the output stage to about +560 V by adding a 2A 25V transformer in series with the high voltage secondary such that it boosts the secondary. The primary of this transformer is connected across the main primary. You must decouple the driver stages from the increased voltage such that each driver circuit receives about 450 Vdc as before, the best way being a dropping resistor and large decoupling capacitor for V1, V2 and V3 as well as V4, V5, and V6. You might even consider using a string of 5 watt zeners to regulate to 400 - 450V for each channel's driver stages. These mods are best performed by those with experience tweaking these sorts of modifications.
Please replace the rectifiers SR1-SR4 with PTC-205 or equivalent 1KVPIV 2.5A rectifiers. [Also consider Hexfred rectifiers, and the many others since this article was originally written. 2012 edit]
These modifications should remove the last vestiges of grain and enhance all the strengths of the original design without deviating very significantly from the original intent of the design. Imaging, depth, and detail were all significantly improved by these changes.
©1998, 2012 By Kevin R. Kennedy