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Jeffrey D. Medwin

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About Jeffrey D. Medwin

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    Forum Veteran
  • Birthday 11/27/1944

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  • Location
    50 miles East of Kansas City, MO.
  • Interests
    Audio, music, and the stock market, ..............is there anything else ?
  • My System
    Two stage Direct Coupled DIY triode amps, careful wiring, very lightly-modded professional - use speakers.
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  1. In my opinion, on most Klipsch speakers, you should not need to use a 75 WPC amp. Usually, there are better sounding tube amps, of the Push-Pull variety, in the 12 to 20 Watt range, that should readily out-play those Carvers. The task is to FIND them, and give them an audition in your own system. Ask for suggestions from Forum Members, low powered Push Pull amps, hopefully with NO negative feedback loops in them. Does QuickSilver offer such an amp? Hes pretty good at design. Single ended amps are best when they are LOW in power, 1/2 Watt to 3 Watts is ideal, and up to maybe 5 Watts generally. Such SE amps require 100dB + speakers to shine. Jeff Medwin
  2. The input tube. Pin 6 is the tube's plate resistor connection, and the resistor is denoted as R16 on the schematic, 270K Ohms, or 270,000 Ohms. Closest double-the-value stocked MK8 would be 549K. Two in parallel, will become 274,500 Ohms computed. That is about a 1.6% difference in theoretical R values .......surely CLOSE enough IMHO. The original part was likely a 2% or 5% 1/2 Watt part, especially when we compare that part's lack of designation on the schematic to the complete designation of R21 and R20 - of the tube's triode-section / phase splitter, (Pins 1,9,8 ). The second-most significant R -treatment possibility, would be to do something to the four 47K phase splitter Rs ( R21, R22). Alas, no MK8s exist close enough to parallel, and the fabulous Mills from Percy ( MR-200, 200K ) are 2 inches long, and well beyond this amp's need, from what I see. I was thinking about how to meaningfully make up your $25.00 minimum order. I would appreciate it if you did JUST R16 and listened, and share it with others on this Forum. I am sure, for example, that ALL the Dennis Had budget-priced "Inspire" amps would be excellent candidates. You get to be the known-to-all guinea pig, as I am new here, and not fully understood. This is so much better, than just " tube rolling " alone. Tube rolling alone, is like adding using a lovely Rolls Royce, along with a FIAT 500. Lets have ourselves a Rolls and a Ferrari ! If you have additional Qs, you can always PM me. Jeff Medwin
  3. Hey, good post. Go the next step !! Measure and determine what the Ohmic value is of the plate resistor in your first stage. Write that down. Look in Michael Percy's 2-2018 catalog, and find the closest DOUBLE the value Roderstein MK8 ( 2 Watt ) German film resistor. Order four, for both channels. Have Mr. Percy match them into two pairs, NOT to the rated value, but to their respective measured values, to each other ( get 'em at 0.01% or closer). These Rs are only $1.95 each, and the matching is a dollar more. Carefully, neatly, replace the existing stock plate resistor ( of your INPUT TUBE ) with these matched doubled up MK-8s, and LISTEN to what that does for your music enjoyment. You use a modded ST-70. You will hear it. You see folks, when we hear any tube amp, we are not just hearing the tube, but the tube in conjunction with its plate resistor. The INPUT tube is the most sensitive spot in the whole amp. The end result, will probably be far better sonics, than tube rolling alone, which is playing cards with half a deck, my analogy !! Michael is superb to deal with, $25.00 minimum order, easy to fill with all HIS goodies. Please report back to all. Doubled up / very tightly matched MK8s are unbeatable, price VS resultant audio performance, from what I experience. Tell us ALL what you hear !! No substitutions on R s, only use MK8s please !! Jeff Medwin
  4. Interesting learning experience, last few weeks and days. A vacuum tube Passive RIAA EQ circuit, that is feed forward, and not using negative feedback, typically has many thousands of Ohms of series resistances added to their plate-driven circuit. This is done by almost everyone, as " conventional EE wisdom " as it assists in tube aging, and in primarily - getting the circuit to very closely follow the RIAA EQ. Here are some passive RIAA circuits that come to my mind, with their added series resistances : Thomas Meyer ( 6SF5-6J5) 200,000 Ohms added Original RCA phono EQ ( 12AX7-12AX7) 470,000 Ohms added Eli Duttman's modified RCA ( 12AX7-12AX7) 255,000 to 265,000 Ohms added Jeremy Epstein's ( 6C45Pi-6C45Pi) phono stage, 28,670 Ohms added. Bruce Rozenblit's ( 6C45Pi, 12AX7 ) phono stage 91.000 Ohms added. Andrae Ciuffoli ( D3A-D3A) 220,000 Ohms added. Salectric ( D3A-5687) 21,500 Ohms added Whenever you take the audio signal in a passive phono EQ, and pass it through a any such series resistor, ( as above ) you have a sonic degradation, VS the resistor not being there at all. KISS sometimes rules !! I discovered, over the last three weeks, that perhaps three high end phono Manufacturers, ( or more - that I am unaware of ), might be using NO added series resistances at all, for the signal to pass through. They all believe an L.P. music-source, played back through such phono stages, SOUNDS BETTER to them !! How VERY interesting, the decades-long conventional strict adherence to the RIAA curve, is possibly totally trumped by play back, sounding better, with zero series resistances added !!!! Well, as part of this thread, and as part of my tube phono preamp's design, I have decided to have NO added series resistances in my upcoming phono build. I also decided to not use the lovely 6C45Pi ( highly variable, and it can oscillate ). Rather, I'll design with the 12AX7, or one of its variants, as a more conservative possibility, including perhaps even consider the use of a 12BZ7. I have several really good thoughts on how to precisely implement a 12AX7, 'takes extra care. B+ supplies in the new design will ( uniquely / heavily ) be double-series-shunt fed, for each B+ use-point. If the audio circuit draws 70 milliamperes, the shunted supply may draw an additional 110 milliampreres, so it becomes a 180 milliampere Phono stage ..... like a small power amp, in current requirement. Two tube rectifiers, not one, and three series B+ chokes, all three chokes, each under 7 Ohms in DCR. Henry Pasternak's "Flywheel" input filter. 15 Ohms total - for all three, is the series choke DCR. Phono preamp will use a 10 AWG m22759/11 Mil Spec ( stranded copper-silver plated ) ground buss . Am PSUD-optimizing this supply, doing many iterations of simulations these days. Fun stuff. Jeffrey Medwin
  5. Did everyone notice the Specimen web site ? Refreshingly honest : Terms : 50% deposit, non refundable. Delivery : 6 months to 2 years.
  6. Nice looking 2A3 amp. I've been working with SE 2A3 amps powering efficient speakers for the last 13 years, and with wire for audio for the last 41 years. My best conservative no-risk advice in 2019 is to use three parallel runs of Mil Spec cable, specifically, M22759/11, for each amp-speaker polarity. Two of the three should be 12 AWG, and the third 14 AWG. Minimum lengths should be precisely 9.52 feet, which is 57.125 inches times two. If you need longer cables than that, go to some multiple of 57.125 inches, usually times three ( 14.28 feet ) will do. Keep the descriptive lettering, on the wire's outside, going in the same direction, for all three wires in any trio. Connect them, descriptive lettering - amp to speaker, uniformly also. Terminate both ends into spade lugs sold by Michael Percy, budget priced gold plated and, I believe, capable of accepting 8 AWG wire. Use a Weller D-650 or D-550 - 300 Watt or 325 Watt soldering gun, to solder the spades and wire. Do not allow the two wire polarities to touch each other, on its way from the amplifier to the speaker terminals, nor touch the floor's carpeting, etc. This above, will insure the best possible linear and wide-band transfer efficiency, 2A3 amps to speakers, on a cost versus performance basis. Look up what M22759/11 wire is !! It is multi-stranded copper , with each copper strand silver plated, in a teflon outer jacket. The silver content is important ! Readily available, surplus and Steve at Apex Jr, on line, is a trouble-free great source. If you have any additional questions, feel free to ask. Happy Listening. " An inch of bad wire can ruin the musical experience ". Its wonderful that you inquire - on how to do it optimally. Nothing is perfect, but this is easily the best "I" am aware of in 2019, and what I use. Jeffrey Medwin
  7. Nice post Maynard !! I'd like to add, always remember this folks : the transfer efficiency of the wire used, between the amp and all-the-way to the drivers' voice coils, is also a factor in how much amplifier power is needed. Irrespective of the speaker's " dB " at-one-watt rating . Jeff
  8. Something new : 'Am thinking of doing a phono stage using Russian 6C45Pi tubes, and a 12B4A Finals stage, in lieu of 12AX7s for the EQ. Any of you tube listeners have experience with the 6C45pI? How does it strike you ? Comments welcomed. ( Bruce Rosenblit's Phono stage uses a 6C45Pi in the first stage, and a 12AX7 in the second ). I've never heard it. Have you ?? I think, use of a 6C45Pi in both EQ stages, is commercially unavailable, and limited to a DIY build effort, I am talking to Jeremy presently. http://home.earthlink.net/~jeremyepstein/6c45riaa.html IF I do this, I designed a brute-force power supply, tube rectified, ' got it all PSUD simulated - for it. Jeff Medwin
  9. Thats easy. When both choices ( anode-drive, cathode-drive ) are properly executed, and generally speaking, a cathode follower does not sound quite as good - as the music's signal being anode-driven. In more familiar-to-audiophiles' terms : this is perhaps why some prefer the sonics of a simple, modest two tube DYNA PAS-3 RIAA EQ, when it is well modified, over the three tube ARC SP3A-1 RIAA EQ, having a cathode follower. In my home system, I purposely use no line stage preamp, to avoid the extra circuitry. I just have a DIY 10K System Attenuator, Ladder type, with source-switching ( selection ) , to feed my SE amp(s). A solid state Blu-Ray player (Pioneer Elite BDP-09FD ) in use, can easily drive 10K. But a tube RIAA stage, normally can not. Jim Dowdy ( " The Dowdy Lama " ) came up with the idea this year of using a high current triode, to DRIVE 10K from the anode, to avoid a cathode follower. This takes over 33 mA. of idle current, a whopping amount - to do this !! So, I eliminated the Coronet's 12AU7 Cathode Follower third stage. I figured out how to directly couple the 12B4A to the preceding two Coronet RIAA EQ stages. The direct couple, avoids a cap in the circuit path, between the two stages. Good !! Its hilarious in a way, the stock Coronet's four 12AX7 RIAA EQ sections draw 3/4s of a mA. each, ( right " on the money " ) or 3 mA. total, but the two 12B4A tubes will use a total current draw of 66.66 mA, ....to drive 10K from the plate. The 12B4A triode is a linear tube, decent sounding. Jeff Medwin 12B4A.pdf ------------------------------------------Click on this to see the interesting G.E. datasheet on a 12B4A. Next is a comment "The Dowdy Lama" just made to me in a email this morning, on 11-7, re Cathode Follower avoidance :
  10. What are your thoughts of it? How does it compare to other tube phono stages ? I have been thinking of using the 12AX7 stages' circuit, my power supply, and adding an anode-driven 12B4 output stage, instead of Jim Hagerman's Cathode Follower. Jeff
  11. These Mono 60s, have two NICE advantages : besides being MONOBLOCKS, they also use DUAL 5U4GB rectifiers. Because of this , a pair of 60s out-performs the other McIntosh amps of this era, I've been told, and in particular, all McIntosh 275s of that era . If you need power, this should be long-term-excellent to own. 'Just my understanding , just expressing an opinion, subjective and technical reasons stated.
  12. Yes, I think the O'Netics are worth their original price, today, as are any Magnequest MQ DS-025s or MQ ES -025s.
  13. Wolfbane, Your referenced article deals with DC heating of tube filaments, typically done with DHTs ( directly heated triodes ). I TRY to totally avoid DHT Finals ( output ) tubes that require DC heating, because AC heating sounds better to me. This means, in DHT tube amps, use of a lower voltage rated AC filament - 2.5 V-rated filament Finals tubes, rather than use of 5 V-rated (or higher ) filament Finals tubes. This means starting a DHT design with a 2A3 as the Finals tube choice, or maybe a 45 if you like them, not a 300B, or a 6B4G, etc etc. This above, is not a concern with INdirectly heated 6.3 VAC Finals tubes ( 6BQ5, 6AQ5, KT88, etc. ) The 2A3 DHT's lower rated voltage, applied to the filament, allows one to get away with AC heating, on 100+ dB speakers, with very acceptable hum levels. AC heating ( VS: DC ) of a 2A3 amp will sound better to most experienced builders, who experiment with good equipment, sources, speakers, wire, etc. For a tube power amplifier's front end tubes, typically indirectly heated, most will use AC heating. - - - - - - - - - - - - - - -- - - - - In 2019, I have changed my designs to use totally separate Filament Transformers, for the front end - and ALSO for the rectifier tube's filament supply. When you use a single power transformer in a power amp, that is doing duty for the High Voltage B+ / Plate supply to the Finals. This INTERMODULATES all the amp's other powered filament supplies. The intermodulation, or "buckling" on transients, of PULSED musical information is very audible to me. This occurs, even when the front end tubes are (usually) INDIRECTLY heated ( have a cathode). Amazingly to me, we hear the filament's intermodulation "through" the cathode. Tube preamps have different filament heating requirements ! Of course. Lower signal levels prevail. People have found, the use of a separate Filament transformer for amplifier's rectifier tube ( only ) always plays music better, more dynamic , more FUN to hear - than if it was powered by the same Transformer that powers the B+/Plates of the Finals. The rectifier is the first tube that demands a separate Filament Transformer, IMHO. Hammond makes em, 3 A. rated ( 167M5 ) and 6 A. rated ( 167Q5 ). Small, reasonably priced. Next in line for a " separate" treatment is the Input/ Front end, the very FIRST tube in the circuit. Hammond again - for those Filament Transformers. - - - - - - - - - - - - - - - - - Another important design pointer: It is VERY important, when ever one adds separate transformers to an amp, that ALL leads, on the transformers' Primaries, and the Secondaries be PHASE ALIGNED, to each other,........ and uniformly connected to the tube socket positions, and...... also, channel to channel. Very important !! A dual trace scope and two probes is the simplest way to commence, and determine relative phases. All these things discussed are audible, to me, and to others. In my opinion, separating the two Filament Transformers from each other, and both from the main B+/ Plate Transformer, produces the difference between any good tube amp...... and a great tube amp !! Jeffrey Medwin
  14. This belongs to a friend of mine, and I have it listed for him. Its got a week to go, on eBay, and its only at a dollar bid as I type this !!! Very high quality pair, simply read the ad and do some research. https://www.ebay.com/itm/293289849327 Thanks for looking. Jeff Medwin
  15. The highly revered Malantz Model Nine monoblock !! Here is an interesting and true tidbit on its design. In the 1980s, I had befriended an avid audiophile, who was also the DEAN of E.E. at the University of Alabama His name was Charles A. Halijak, PhD. During his career, he published 20 papers at the University, and received 27 different Citations ( not for speeding, gents !! ). His published paper record can be seen on-line here : https://www.researchgate.net/scientific-contributions/2017834553_Charles_A_Halijak Here is what I want to tell you all. Dr. Halijak's worst-liked amplifier, from a E.E. design point of view, was the Marantz Number Nine. He felt it had so many design errors, that yearly he would have the students in his class, critique the design, and offer improvements to submit to Dr. Halijak. This was his own requirement, before they would be able to pass his E.E. course !!!!!! If any E.E. schooled by Dr. Halijak at the University of Alabama in the 1980s is reading this, I would enjoy your confirmation, of what I am reporting herein, and believe to be true. Dr. Halijak was also a big believer of low as possible / practical DCR ( Direct Current Resistance ) power supplies. I wrote about that on page 8 of a KT88 build thread, earlier this year. His favorite value power supply choke, for his own personal tube amp builds, had to be custom made back then. He had Stancor build for him to his spec : 1 HY at 8 Ohm DCR power supply chokes. Isn't that interesting, because it does not necessarily meet critical inductance, as is always taught the last maybe 100 years in all the conventional EE courses. Dr. Halijak was brilliant, look at his papers. Today, Hammond has a off-the-shelf 159ZA choke, six ohms, and it requires 75 hours of music - playing to break in. One would always use at least two of these, in each tube amp's supply, as a L1/C1/L2/C2 B+ filter to feed the Finals. Two 159ZAs, connected in series, is only 12 Ohms total DCR feeding the Finals tube, from inductors . Jeffrey Medwin
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