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audiowize

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Everything posted by audiowize

  1. That amp will be short on gain for guitar use, and the input impedance might be a bit low. I would put another 12AX7 stage ahead of the first one, with a 1M pot in front of it. You'll need another 10K resistor and 22uF capacitor to decouple this 12AX7 from the next one. Make the 22K feedback resistor into a 50K pot in series with a 10K resistor to give you variable distortion/gain/damping. The schematic above is going to have a hell of a time producing much (if any) class B power. This might be desirable tonally, or it might not. Be prepared to make a voltage tripler/quadrupler off the 6.3V winding to go to fixed bias if necessary. OTOH, if this sound is something you like, put an L-pad between the amp and speaker so that you can get all that yummy crossover distortion at any volume level. There still isn't a lot of room for tone controls, but you can kind of graft that in along with the additional input stage that I mentioned above.
  2. This is indeed likely an RCA made 805 that WE procured and reboxed. The closest actual WE tube was the 331A. Earlier this year, there was a guy on eBay selling "Western Electric" 6SL7's for about $400 a pair that were obviously RCA tubes (the stop sign logos were quite prominent), but he wouldn't give up on the idea that WE made them, and that they were worth millions of dollars. The only way to be totally sure about what you have is to unpack it and take some photos.
  3. There's nothing wrong with the PV-7, it's just a very close cousin with the 60's hifi stuff like a Marantz 7. Your PV-7 will do OK, but will struggle if you have an amplifier with something like a 10K input impedance (subwoofer plate amps are notorious for this).
  4. You'll get better results from that EE-MMX by replacing the electrolytic coupling capacitors with something else. Parts Express Dayton metalized film caps are a good start, try 3.3uF. Try clipping out the 220uF cathode bypass caps on the first stage. This will reduce the gain, but you don't need a preamp with 24dB of gain. Current sources in place of the plate loads on the first stage and cathode loads on the second stage will do wonders for the performance of the circuit, and there are some kits available for this. You actually do not need matched pairs of tubes for this preamp. One tube is the voltage amplifier for both channels and one tube is the current amplifier for both channels. I have 1000-2000 12AU7's sitting around. PM me if you're looking for anything specific.
  5. The "Recorder" jacks may be a line level output for a tape recorder. If you are sending signal into these jacks, then you will hear that signal regardless of which input you use. The idea behind the inputs is that the tuner and the extra are what we would consider "line level". The magnetic input would be a moving magnet phono cartridge, or magnetic tape heads, with Mag Low being the appropriate impedance for a moving magnet phono cartridge, and Mag High being a higher impedance input for tape heads. NAB Tape and RIAA/NAB/Ortho selects an adjustable EQ. The presence of the Mic option would lead me to believe that this selection is for no EQ on the magnetic input. Are you 100% sure that you hear nothing with Mic selected and sending signal through Mag High? On the 299D (the only readable schematic I could find was for the 299D), there is a switch for speakers on/off. Turning the speakers off disconnects the rear panel speaker jack connections, and instead connects a resistor across the output of the amplifier. This will stabilize it with no load. The headphone output is taken from the 4 Ohm tap and fed by a few 150 Ohm resistors. This worked pretty well back in the day when headphone impedances were pretty high, but will give you pretty mediocre performance with today's low impedance headphones. A better choice would be to add a 10 Ohm resistor from ground to ring and ground to tip on the headphone jack, which will clean up bottom end performance and fix the output attenuation a bit better.
  6. If you possibly can, getting a PV-10 or newer is a very good idea, and avoiding the PV-8 is also a good idea. Anything pre PV-8 has a lot of loop feedback around both the phono stage and the linestage, while the PV-10 plays catch up and eliminates all of that. The PV-8 was an attempt at eliminating the loop feedback, but doing so by adding in a bunch of cathode followers all over the place.
  7. Dang, $1200 for a single ended EL84? At 4WPC, it's running as a pentode, which isn't what you're looking for.
  8. Oh boy, lots of strange things going on here. Ohm's Law governs wattage pretty clearly, 10 Watts from a tube amp and 10 Watts from a solid state amp into an 8 Ohm resistor is the same voltage in each case (about 9V), but Ohm's Law says nothing about distortion. There's also a sad generality that lumps all tube amps together. I can tell you that mkane's 45 amp will sound absolutely nothing like a Dynaco Stereo 70, and that I could build you an OTL amplifier that would sound a lot like a solid state amp. Distortion both is and isn't on the side of tube amps. In a solid state amp, you generally take the signal at the input, apply an absolutely absurd amount of gain to it, then take some feedback from the output to compensate for the non-linearity inherent in most solid state devices, as well as to flatten out the bandwidth and reduce the output impedance (if necessary). In a lot of tube amps, you do the same thing, but with the output transformer sitting in the way, you end up with stability issues that limit how much feedback you can use. With these high feedback designs, you will tend to get less overall total distortion, but the feedback mechanism tends to create higher order harmonics which are not too pleasant to listen to. In some tube amplifiers, you can operate all the tubes in class A, and get away with no feedback. This will result in reasonably low distortion at low power levels, and the distortion will increase as power increases with the nasty higher order bits only showing up as you clip. In addition, for single ended amplifiers, clipping can be pretty soft, only flattening out half of the waveform, which can let you push an 8W SET amplifier way beyond 8W for brief transients that aren't super noticeable. Regardless of all of this, I'm a subscriber to the idea that you only need enough power to hit about 102dB peaks, and on a 98dB sensitive speaker, that's very little juice. I also have seen 1000W amplifiers hooked up to high efficiency speakers, and have watched someone hot swap a cable and send the cone of a $4000 driver flying across the room. On the CF4's that I had some years ago, 2WPC from a 45 amp would rattle the pictures on the wall without much effort.
  9. You can get some decent mileage out of those 6V6 console amps, provided you are able to undo the EQ'ing they have in them. I just did a pair of 169's for a friend and he's super happy with them, but they were pretty heavily EQ'd for both the speakers and a ceramic phono cartridge IIRC. In these I went ahead and had an ABS front plate made for each one, as I wanted to add a power switch and to cover up the inconsistencies between them. I also added some thin ABS plates internally to cover up some of the holes, as well as to provide a proper mating surface for the binding posts. Internally, I reduced the number of grounding points to two (it would've been one but I forgot the HV CT connection, oops!), I installed a new old stock 9 pin socket in each amp, and all new caps/resistors without the EQ. A big weak link on these amps is the ceramic capacitor at the input to the first stage; just replacing it with a polystyrene does wonders for these little guys. I used a lot of teflon sleeving because I didn't want to add a bunch of terminal strips that would take up any more space in the amp. I dumped the twist-lock and went with a conventional can cap for the high voltage section, and you can see the last power supply electrolytic has been relocated close to the driver stage.
  10. Does your turntable have a plastic arm or metal arm? If you touch one of the screws that holds your cartridge on, does the buzz go away? How loud is the music at "12 O'Clock", if you play music at that volume level and smoke comes out of your speaker, then you need to back the volume down and not worry about this.
  11. Boy, it really depends on what you're using them for. If you have a tube design that has a ton of global negative feedback, I find that the influence of the particular tubes used can be highly minimized. This is going to apply to a lot of the common high power push-pull power amplifiers, as well as a lot of the clones of 1960's linestage/phono stage preamps. In a zero feedback or low feedback design, these tube-to-tube performance and sound differences can come through more audibly, and a fair number of them can even be measured. Some time ago, I think it was Pete Millett who took power and distortion readings of some obscure pentode that had several different variations of internal construction, and there were measurable and consistent differences between each of them.
  12. I'm glad you made it back! I finished constructing the crossovers today and have given them a listen. I just put them into external boxes to manually tweak the attenuation. (Yes, I removed the crossovers from inside the speakers and wired the drivers right to the binding posts) (My "W" and "T" are backwards too, didn't have enough coffee today) ~200Hz could be a resonance from the tall, unbraced sides of the enclosure. I'll see what I can measure tomorrow, and try the less invasive approach of 1/2" dowel if there ends up being a peak there.
  13. Thanks a bunch for posting all of your work. I was about to sit down with my Omnimic and WT-3 to do basically the same process, but now I get to skip ahead a bit! Did you see any enclosure resonances in the impedance plot? What tweeter attenuation ended up producing the response curve you measured?
  14. It's funny, I actually found a guy with stock, so I purcahsed all of his stock. Four K-29-KN's and one K-30-KN. If anybody sees this post and needs a woof, I'll consider selling a couple. -Paul
  15. Does anybody know where I can purchase a woofer for my CF-4 speakers? One of mine went south, badly. -Paul
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