Kennedy On Audio

Things are really hopping around here these days.. Lots to talk about. The ST-70 article I mentioned should be ready for the July/August issue and should be of interest to anyone with a stock ST- 70. It will feature an all triode front end utilizing a single 12AX7A as the first amplifier stage driving a pair of 12AU7A cathode coupled phase inverters which drive the grids of their respective output tubes. The amplifier may be operated in triode mode with a variety of output tubes in addition to the stock EL34/6CA7's. Also discussed are the Chinese KT-88's, other classic era amplifiers in the same league as the Fisher K-1000 and HK Citation II, the Macrotech Reference from Crown - review to follow in a later article, and any other items I feel like throwing in....

On the subject of the ST-70 I want to suggest that you may want to investigate the suitability of Russian KT-66's as well as 6L6GC's in place of the EL34. You will have to decrease the bias voltage to about 25 volts by decreasing the 10K resistor connected to one end of the bias pots and ground to about 6.8K-7.5K (experiment) and in addition low frequency loop stability is likely to be compromised (as the overall loopgain will increase by as much as several dB) and if so increase the 1K resistors connected to eyelets 12 and 13 to about 1.3K or whatever it takes to prevent oscillation. (I haven't tried this) On my advice a friend of mine attempted this modification and found that this mod resulted in a smoother, cleaner, more natural midrange with a somewhat softer bass presentation, and much cleaner highs.....

Those Chinese Golden Dragon KT-88's sound really good triode connected in a pair of ARC M-100's I recently modified. They were clean, detailed and neutral with no sense of glare, brightness or any other significant coloration. Depth was excellent, imaging was precise and stable. The GE 6550's used as references sound pretty dull and lifeless by comparison. These tubes look pretty similar to the cheap ones also made by Shuguang, but seem to have much heavier envelopes that almost appear hand blown when compared to the thin symmetrical envelopes of the cheapies.... These tubes appear to have much more of the silver colored getter coating on the envelopes than the cheap version. Internal components appear to use the same tooling if not the same materials as the cheaper version.... I have not received any for evaluation yet so I can't tell how they will stand up or sound in my heavily modified all triode Citation II with 560 volts on the output stage plates. If they can survive at these voltages in this amplifier they should be rugged enough for most applications. Stay tuned.....

Sadly I can no longer recommend the Tungsram 12AX7A because the last lot I purchased had a disproportionately large percentage of bad tubes - microphonic and/or so badly made that they fail after about 25 hours in any differential amplifier circuits I use. The issue is probably moot in any case as they are no longer widely available anyway....

I am glad to see that the HK Citation II and Fisher K-1000 basic stereo amplifiers are finally receiving the recognition they deserve. In many ways they are the equals of some of the best amplifiers ever made including the Marantz 9's and I think are far better than the three Marantz 8B's I have heard at one time or another... Other amplifiers of this caliber would include the Scott LK-150 (stereo), the Eico HF-50/60 featuring Acrosound transformers (monoblocks), the Acrosound Ultra-linear II (make sure you get two of the same version, either the two pot version or the three pot version), and the Leak TL- 50 Super, which is basically a higher power version of the classic Mullard 5-20 circuit.

A minor but worthwhile tweak for the Fisher K-1000 is to bypass the input bandpass filter circuitry and go directly into the next stage. Remove C16 and C17 which are connected to pin 7 of V2 and V6 (12AX7A's) respectively. Remove R21 and R22 (the 470K grid resistors) solder one end of a 221 ohm resistor to pin 7 of each 12AX7 and then solder a 100K resistor to each unconnected end of the 221 ohm resistors. Ground the other end of each 100K to the points originally used by R21 and R22. Connect the left input to the junction between the 100K ohm resistor and the 221 ohm resistor connected to V2 pin 7. Connect the right input to the junction between the 100K ohm resistor and the 221 ohm resistor connected to V6 pin 7. Replace R49 - R52 (those wire wound resistors connected to the plates of the 6HU8's - V3,V7) with the equivalent series - parallel combination of RN-70 metal film or 2W metal oxide resistors to provide 18K ohms of resistance with at least a 6 watt rating. The reward for this effort will a considerable improvement in detail and a somewhat cleaner, more neutral sounding presentation.

Other vintage items worth another look are the Citation IV pre- amplifier, the Citation I pre-amplifier, the Eico Classic 2200 Stereo tuner, and the Scott 350 A/B/C series of tuners among others. The Citation IV stereo pre-amplifier designed by S. Hegeman has a simple 6 tube design and the earliest example of a tone defeat switch I have ever seen. This pre-amplifier beats the pants off of a lot of respected modern tube pre-amplifiers when operated in the tone bypass mode.

I recently had the opportunity to listen to the Crown Macrotech Reference in a system built around a pair of the Martin - Logan CLS II's, custom Icon Acoustics subwoofers, and custom electronics in a mix of tube (mine) and custom solid state line and phono stages. The amplifier was evaluated against the M-100's mentioned previosly and was found to be comparable or better in most areas except the midrange where the M-100's were a little more open and palpable. There was no question that the Crown was more controlled and extended in the bass when used as a full range amplifier than the M-100's. The highs were clean and extended with no hint that this was amplifier utilizing bipolar transistors in the output stages. The design is quite unconventional in conception as well as execution, few if any premium components are used in the design and yet it manages exemplary sonic performance. This amplifier uses a proprietary floating differential bridge circuit with a huge 3 KVA toroidal transformer in the power supply. Did you know that the same basic technology is available in all of the Microtech/Macrotech amplifiers (the same main pcb is used in several models, and I believe that the output transistors are the same across the whole line) Some of these models like the Microtech 600 are available for around $1000.00 new. They are commonly used for professional pa work. These models are the Microtech 600, and the Macrotech 1200/2400 series and all are available used for a fraction of the price. Improved power supplies should result in a similar if not identical sonic signature.... (See that cheap fanformer and replace it with a decent 25 VA toroid...) More details soon.

I just purchased a Radio Shack Sound Level Meter model 33-2050 that retails for $31.95. It is indispensable for any serious audiophile from the standpoint of measuring room response, setting levels for subjective comparisons between two components in a listening test, and perhaps most importantly to determine that your average listening level does not present a long term hazard to your hearing. If you find that your peak SPL's regularly exceed 90 dB spl at your listening position then you may want to consider turning the volume down. I understand that there is nothing quite so thrilling as an orchestral crescendo reproduced at full tilt, but you might eventually also experience the extreme disappointment of acute hearing loss..... (Almost every audiophile I know likes to play his music at an excessive level, just ask their wives or girl friends.) Note however that the frequency response is stipulated as being flat +/- 3 dB to 10 KHz with "C" weighting, so do not rely on it to very accurately reflect room/speaker response above about 2 KHz based on the furnished curves. It is very useful for doing low frequency measurements and may be very helpful in mitigating the effects of standing waves through careful placement of speakers, furniture, and acoustical treatments. I do not recommend the use of an equalizer as advocated in the instruction manual that comes with the unit.



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©1998 By Kevin R. Kennedy