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John Warren

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Everything posted by John Warren

  1. Maron's wife, through an friend of hers, contacted me after his death. She had a long Excel list of hardware he owned. It's not clear to me how she came across my contact info. He either instructed her to contact me or found my contact information in his personal papers. Either way, she asked me to evaluate the list and estimate sale prices so she could sell it without getting short changed. She didn't want to deal with buyers directly and I didn't want to take the task on of being a middle man. I thought about buying the whole lot but I'm past the point of collecting vintage hardware. There were many JBL vintage pieces.
  2. I've been asked about tube amp hum/buzz and how to get rid of it. It's noticeable buzz when the amp gets to heat and doesn't change when the volume pots are adjusted, it's just there. This is what it looks like on an oscilloscope, at 20mV/division. The amp is idling and the load is an 8 Ohm power resistor. The scope probe is across the resistor over a 100ms time window. Below is with the time window reduced to 10ms. It's not a sinewave, it's a 60Hz sawtooth. And with a peak to peak amplitude of 40mW, ignoring high frequency glitches, it's audible especially when you're speakers have high sensitivity tweeters. The 6.3VAC heater secondary tap is prone to picking up wave characteristic (periodic) noise from rectification, line voltage and other sources. The noise is coupled to the tube cathodes. Heater supplies configured below (a Fisher design) are problematic. There's a few ways to improve the situation (see artificial center-tap, Humdinger potentiometer). There's other approaches but these are common. Here I use the artificial center tap concept but replace the "typical" 100 Ohm, 2W resistors with .022uF, 400VAC Orange Drops thus filtering the rails of periodic harmonic garbage and the result (before and after) is below. RMS value of noise is reduced three-fold and, most important, the amp is silent even when the ear is against the grille cloth. Note that the peak to peak readings report glitches which are momentary impulses that are of no significance. So why did the engineers at Fisher, Scott, Pilot and a few others not add a couple of resistors or capacitors to their designs? I would propose that the hum, though noticeable with no program material is of such low magnitude that it made little difference to the sound quality.
  3. The 25W rating is the power dissipation the windings are designed for.
  4. Nothing exciting. I've had inquires to purchase either as a completed unit or a kit. I purchased parts for three units and the third is built but waiting on the 7591s and, as of today, JJ is still down due to material shortages. I ordered the tubes early June. Regardless, I've ordered more hardware to a make a few more amps. The chassis plates will have mounting hole patterns to accept either the Lundahl, Hammond, Transcendar or OEM output transformers. The Lundahl OTs are heavy and the housing, which is nice btw, has four threaded holes for M4 screws at the bottom for mounting to a chassis. M4s are too small. If I was to ship amps with M4 screws holding the transformers down they'd likely break the screws. That said, the next sets of performance tests I'll show will be with the Lundahl output transformers. Here's the wiring on the underside of the chassis (unit #3).
  5. EV published the DIY plans for the Georgian and sold drivers for the enclosure. The woofer should be a 15W K, the K is stamped into the rear badge for Klipsch. The DC resistance is 3.1-3.3Ohms. The issue with these old drivers is the voice coils are fragile and can't take much heat. They're tube era drivers. The Georgian was EV's version of the Klipschorn which they licensed from Klipsch to produce.
  6. And you can install a 1A fast or slow blow fuse in the Variac to limit current draw regardless of what the Amp meter shows. Regarding the amp meter, I use a Fluke that measures in mA AC. Most tube amps idle at about 1000-2000mA. I have a dedicated VAC meter on the setup as well.
  7. I purchased a Citation II. The important bits and pieces are there and it sounds good but it needs a good going thru. The chassis is in good shape and the transformers are fine. Every other piece, including the boards will be replaced. I'll restore the entire thing to museum quality and when I die leave it to one of my kids.
  8. I have no hobbies, I'm just an engineer.
  9. Ignoring the cost, which is considerable for high end audio components, driver entitlement becomes a near reality when the enclosure is rigid. Although the images are from the Magico website, Aluminum enclosures machined from plate stock are nothing new. I heard a pair of JBL L100 clones fabricated from 7/8" Al-plate, milled and assembled using cap screws and Loctite adhesive. It was heavy but easily managed once placed on a dolly. Internal bracing resulted in the overall enclosure being somewhat larger than the OEM version. The difference in the low end performance is hilarious.
  10. Entitlement in the audio reproduction "chain" isn't in the electronics or even the loudspeaker drivers, it's in the enclosure materials. And the folks fabricating enclosures out of Aluminum alloy are on to something. It's a "ffs" moment listening to them.
  11. 99.999999% of audio amplifiers are voltage sources. If transconductance amplifiers were a better mouse trap, the world would have marched to them immediately. Why do you think it did not?
  12. Short of cracking open a 7591 and just looking at G2, the datasheets from all the old suppliers state that Pin 8 and Pin 4, which both make internal connections to G2, are to be externally connected via jumper for "efficient" operation of the screen. The cold resistance between P8 and P4 is 0.2-0.3 Ohm, i.e. a short. So that's got me curious, why the external jumper? I can speculate it's due to the way the grid is made, two sections connected by grid wire that, when operating, develops a hot resistance large enough to sustain a potential difference between p8 and p4, a difference enough to alter the screen grid performance. Anyone actually know?
  13. Took some time but the Lundahl units have arrived. They're 25W RMS, 6K primary 4/8/16 Ohm secondary, C-core transformers. Here's the primary side of the output transformer, eight solder pins. And here's the secondary side, sixteen tinned wires. The enclosures are optional, total cost with enclosures is $625 for a pair. The secondary is a bit of a pain to wire. To measure the primary impedance, I temporarily soldered it for an 8 ohm load per the schematic below. GREEN is the primary impedance of the Lundahl with non-inductive, 8 Ohm load and, for comparison, the Hammond 1650PA PURPLE and the Scott OEM output transformer, GOLD. The Lundahl is a bit flatter than the other two. Output tubes are 7591s in push-pull arrangement.
  14. I have no doubt that vacuum tubes today can be made vastly better than what the OEMs could provide. Vacuum systems, glass technology, materials supply chain are all better. And there still is, today, a large presence in the high power, vacuum tube industry (radar and communications). There are plenty of highly skilled technologist in the vacuum tube industry. Look up, Travelling wave tubes, cross-field amplifiers, Klystrons, Magnetrons. But, is there a business case for such an investment in audio grade vacuum tubes? I would propose no, there is none, too costly and the returns are not there.
  15. New Mom's use it in baby formula too.
  16. is the only water you should use to saturate your solder tool sponges with. Don't use tap water to wipe your soldering tool tips clean, the Chlorine, Fluorine and salts will corrode the plating on the tip and shorten tip life by half. It's literally free at our supermarket (0.99/gallon) with 3 year shelf life. To be called distilled, the total solids content can't exceed 10ppm which is pretty spectacular given your paying about a buck for a gallon. Don't use Di-ionized water either, it will attack the tip.
  17. Spent Friday last listening to the Transcendar and Hammond output transformer versions of the amplifier. I have a well broken in set of Sylvania 7591s that I was using, swapping them from one to the other. Speakers are JBL L200s. I also revised the feedback on the Hammond to provide a bit smoother response above 50kHz. Here's the distortion curves for both channels with the Transcendar outputs and Sylvania tubes, virtually identical. There's a leveling off as the power approaches 8W RMS or so then there's a significant rise past 15W RMS. The plates can dissipate 19W RMS. Same tubes with the Hammond outputs. The Hammond is rated for 60W, a much larger core than the Transcendar which is a 22W transformer. And here's the amps How did the sound? Hard to discern a significant difference between the two but I did like the Hammond version better. The L200 isn't that good in the mid range and the bass is "tubby" but, that said, both amps sounded good to my 62 y/o ears. I was plugging a 24-bit CD player directly into the RCAs, no preamp. The distortion curves were taken after my listening so I wasn't entirely biased based on measurements.
  18. I'll be giving a pair of Lundahl output tramsformers a go in this amplifier. I'm curious to measure how a C-core transformer measures against the conventional units. The LL1620/PP has the correct primary impedance (6k Ohms), wattage (25W) and max plate current (150mA) into an 8 Ohm secondary load. There's a nice powder coated metal enclosure that can be purchased too. It's almost an exact replacement for the OEM version but, theoretically, a better version. It will require a third set of mounting holes be added to the chassis plate. The advantage of a C-core transformer is the magnetic field is well contained and lower stray flux, no sharp corners in the iron circuit. The coupling between the primary and secondary should be better. But can it be heard? and can it be measured in %THD? .
  19. Yes, he contacted me a few weeks ago asking if he could purchase it so I sent it away. I've serviced a couple of his tube amps in the past. I'm curious to get his take, he has many vintage tube amps including some rare pieces. I've have a second Beta built and parts for a third. PC boards and the machined chassis plate have a small price break at three pieces. The amp will produce full power into an 8Ohm load (~12VRMS) and <0.5%THD with an input signal of ~1.6VRMS which allows for DVDs, CDs, tuners are other "line level" outputs including pro hardware. With sensitive speakers the output would be painfully high with a direct plugin of CD player and listener a few feet away. It will require a preamp for a phono inputs. There's a separate level adjust for each channel and a stereo balance potentiometer. There's also a line level center-channel output RCA jack with level adjust too. I always wanted a center-channel in my setup and now I've have one. I have the preamp board on order but have yet to consider how it will be packaged. I'm less excited about tube preamps. I've designed solid state preamp boards with greater than 100dB distortion free output.
  20. Here's the beta board set. The boards are designed using a package that links the schematic to the layout. So you first layout the schematic and all the painful details associated with the size of the leads, component body, wattage, pin identification and then commence with the layout. Dual traces (top and redundant bottom trace) are dark shadowed in the photo. Here's closeups of each board
  21. Some performance data comparing the two designs. Here, the analyzer was calibrated and left alone, the amps then measured sequentially. Same tube types and manufacturers (JnJ) but the alpha tubes have more time on them so not entirely an apples 2 apples comparison. DC balance was checked and adjusted before the test. 1kHz %THD. Both channels in the beta are similar in performance. FFT of the alpha unit, both channels (CHA is top): FFT of the beta (CHA is top): So some differences with beta being a bit better but not dramatic differences. Both sound great and are fun to use.
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