I am still waiting to get my hands on a set of genuine Golden Dragon KT-88's, but I have heard (reliable) mixed reports about their reliability in some applications and nothing but praise so far for their sonics... Same story with the KT-90 from VTL - I hear they are extremely rugged and really sound dynamic.. (Anyone out there want to send me a few sets for evaluation? Please?) The gauntlet is dropped.. I can promise a real interesting future review of genuine vintage KT-88's, Tungsol 6550's, GE and Philips ECG 6550A's, and whatever modern production I can scare up, (hint, hint...) along with colorful comments from some of our local audiophile community. For those of you who modify ST-70's a small caveat, and yes there will be a construction project based on the ST-70 forthcoming....[No there won't! - Webmaster]
The subject of line conditioning is fraught with controversy and many regard it as an area where only the enlightened few can delve into and comprehend the nebulous quantum physics of ordered electron flow, or alternately as the realm where the charlatans play.... Neither is quite the truth, but a lot of amazingly simple ideas (DIY) can prove to be effective and affordable. I have always felt that some of those expensive line cords aren't particularly cost effective when you consider that most of their performance can be replicated by a few discrete components such as ferrites and good bypass capacitors across the line. Idea one - take three high quality film capacitors (MIT, TRW AC types, or CDE WMF) of say 0.18 - 0.22uF and rated at least 400 VDC and purchase a 2 gang metal junction box, two outlets, a 2 outlet metal face plate, a hospital grade 3 wire plug rated at 15A or better, and the nicest 3 conductor line cord you can find at your local hardware store. Wire the outlets in parallel and connect them to the line cord, making sure the box is grounded. Now take one capacitor and connect it across the line and neutral inside the box, then connect one capacitor from line to ground, and the remaining capacitor from neutral to ground. Make sure that everything is well insulated. Now compare your budget line filter to anything on the market. It's surprisingly good and doesn't defy the laws of physics - It is infact a very simple common mode/differential mode line filter such as found in many industrial applications where commutation and brush noise is rampant. My friend Kwame clued me in, as I had never considered the possibility that this commonly overlooked circuit might be effective for home audio, although I have used it for years in RF, computer and motor control applications. Idea two - buy some of the RF line chokes sold at Radio-Shack and try them on your CD player, pre-amplifier, and power amplifier line cords - they are very effective against RF and other high frequency noise. Several may be more effective than one depending on the application, but they should be installed near the chassis. Idea three - check the line polarity of all your components by disconnecting them completely from your system, and then measure the individual AC leakage currents to ground, and then orient the individual line cords for lowest current to ground. (Alternately you can measure the chassis voltage to ground on the AC position of your DVM and orient the line plug for the lowest voltage.) In most cases this is correct, but in some cases this isn't true, so let your ears be the final arbiter... Idea four - only one component (if any) in the entire system should have an earth ground to avoid potential ground loop and RFI problems, but be aware of potential electrocution hazards. Be careful! Measure leakage current if in any doubt. Measure this with a DVM on the lowest AC voltage range connected in parallel with 1M ohm from chassis to earth ground - if you can measure anything approaching 75 volts (75 uA leakage current) this device is potentially hazardous, and if after replacing the 1M with 10K you get 1V or more this baby is dangerous and must be directly grounded or maybe even removed from use! (UL says that 1 mA leakage current is acceptable in consumer products with adequate grounding, but most applications require currents of less than 100 uA when a grounding circuit is not provided.)
Recently I had the opportunity to try a set of Sound Pucks in a direct comparison with Navcom Silencers, and the difference was most pronounced in my tubed components. The largest improvement was achieved at the pre-amplifier level with a considerable improvement in resolution of low level detail, with much cleaner, less electronic sound, and depth also seemed to increase significantly. Significant improvements were also noted in the power amplifier, and to a lesser extent in my CD player. Unfortunately only one set of Pucks was available for evaluation, but I suspect that the cumulative improvement gained from using them under all amplifying components would be worthwhile. Another logical location for their use would be under a turntable, but set-up problems with my Pink Triangle would have probably made the experiment meaningless.
An acquaintance recently retubed his Jadis Defy 7 with Golden Dragon KT-88's which in this amplifier proved not to be very reliable initially, but the replacement set are doing a good job so far. VAC should get high marks for customer service on this one. These tubes do seem to work quite well in the ARC M-100, infact one owner of a pair of M-100's feels that this tube provides superior performance sonically when compared to any of the other commonly available 6550's. [1996 aside: These tubes subsequently failed causing a lot of damage to the amplifiers in question.]
I recently refurbished a later model Dynaco (tm) ST-70 with Z326 output transformers. These models have a revised PA-060 power transformer that produces elevated plate voltages in excess of 460 VDC as compared to about 435 VDC in the earlier (pre Tyco Corp.) units. I installed a new 450V quad cap. which promptly exploded!! Vendors have occasionally sold caps rated at 450V as being suitable for replacement use in the ST-70, and they certainly work in the older units, but their use is risky at best, and I would recommend the 500V part sold by various vendors, as the only safe replacement.
In addition I am evaluating a set of (Ex. East) German made EL34's I recently purchased for use in this amplifier, they seem to be well made, although they are of rather small and light construction. They don't look like they would withstand very high voltage operation, such as would the original Telefunken or Mullard, which could operate at plate potentials as high as 800 volts. Sonically they are in the same league as the Tungsram EL34 which is no longer widely available, in that they share a somewhat forward, bright, and moderately detailed presentation with good bass extension and control. Warm up time for best performance is about one hour, after which they exhibited no operating point drift at all. I like to operate them at around 80 mA of quiescent current per pair for longer life and lower risk of sudden blow-ups.....
Next time I hope to present an article on some of my modifications to the ST-70/MKIV including the design of a replacement driver circuit. The amplifier in question features an all triode front end, and triode or ultra-linear operation in the output stages, and can drive electrostatic and planar speakers economically with good performance, as well as more conventional dynamic types. Simple adjustments in loop compensation make it sonically compatible with many loudspeaker designs. (The way the amplifier interacts with its load can be tailored to some extent to enhance neutrality.) It is reasonably simple to build and has proven to be quite reliable over the last two years. Till then.....
©1998 By Kevin R. Kennedy