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audiowize

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  1. If you're wiring your amplifier with copper wire, the outgassing of that silicone bathroom caulk will cause rapid oxidation.
  2. Here's some of the basic math that governs these relationships. You and Jeff are welcome to take the position that your willful ignorance is just as valid as 60 years of engineering, bIut for everyone else on here or any future readers, it's important to keep in mind. The relationship mu=gm*Rp still holds true for a pentode, but the Rp is the plate impedance of the pentode in parallel with the loading impedance. We will use the KT-88 data, the DS-025 data, and the Klipsch RF-7 II data to do some analysis on what may happen. Jeff is notorious for dismissing any opinions on what he's up to because we don't have his exact speakers, but this is the Klipsch forum right? The RF-7 III is an 8 ohm nominal speaker with a small dip at 4 ohms and a few peaks around 50 ohms. The DS-025 is a 2.5K:8 series feed output transformer, and the KT88 is a pentode with a gm of about 0.011mA/V and an Rp of about 12K. At 8 ohms, the DS-025 will reflect back 2.5K, and in parallel with Rp that is about 2060 ohms. The amplification factor of the output stage will be 22.6x under these conditions. At 4 ohms, the DS-025 will reflect back 1.25K and in parlallel with Rp that is about 1132 ohms. The amplification factor of the output stage will be 12.5x under these conditions. At 50 ohms, the DS-025 will reflect back 15.6K, and in parallel with Rp that is about 6872 ohms. The amplification factor of the output stage will be 75.6x under these conditions. Next we consider that for all of these wanderings of the output, the input signal stays the same. Since you mentioned power, here is the power (and the impedance in question) assuming the amp is putting out 1W at 8 ohms. At 8 ohms, 1W At 4 ohms, 0.6W At 50 ohms, 1.8W You have admitted that the amp is only good for 2.83V at the output, but at 50 ohms we are up around 9.5V of output. This is way, way beyond the "1W" operating level. From your own admission, the amp is only good for 0.16W at those impedance peaks. So again, how does power level go down when load impedance goes up?
  3. Clipping is still clipping. If you're output voltage rises to 20V because the load impedance moves up, this is still clipping behavior.
  4. This says nothing of the damping issues with doing this. Even if Jeff makes 3/4 of a watt at 1kHz, the impedance hump at low frequencies will cause the amp to put out far more power than this. If we presume that the amp will deliver that 0.75W of power into an 8 ohm load, and the load pops up to 50 ohms around the driver resonance, now the amp will try to make 20+W (edit, will rise significantly in terms of voltage). In reality the impedance peaks are probably much higher than 50 ohms, but it's sufficient to explain why this is such a terrible idea. Your only prayer is that the DS-025 will limit how out of control this gets, and the primary inductance will put a cap on how nuts the output gets at low frequencies. On the high end, and amp like this is the kind of amp that should never be run without a speaker connected. You also have a lot of confidence in the amount of power Jeff thinks he can get away with. A 98dB driver will struggle with less than a watt unless your listening level is very, very low. With the topology chosen for this amp, you would also want to make sure there's no LF content!
  5. It is also worth mentioning that the silicone you are using to glue stuff down is corrosive and will oxidize copper rapidly.
  6. Theory is proven and measurable. A builder building your zero feedback pentode amp on the dark side of the moon will run into the same problems that I would building the amp here. That is the distinction, it's not a belief at all! Again, I will ask how you can be an "expert" in tube amplification and not understand how a pentode works? This is a contradiction. On the other hand, a lot of what you posted here is a flat earth argument. You believe many things that are demonstrably false. Your conviction is so strong that you don't care how many forums you get kicked off of, you'll just keep going! There are people who knock on my door with strange books who also hold this same conviction.
  7. If we take your driver mu of 100 and reduce it due to loading (call it 65), then add in the KT-88 pentode loaded, you have a power amp with a gain of 65dB (assuming a resistive load). If your GPA-604 has an impedance peak or two at low frequencies (we'll say 50 ohms for the sake of illustration), then gain will be 82dB where that impedance peak is. This will not be remedied by short wiring runs, a steel chassis, or a proprietary power supply. If you want these results, I'd spend $100 on a car audio amplifier and put a battery on the floor in your listening room, then flip the "bass boost" switch. This of course assumes that you adequately size the cathode bypass cap, which isn't a given all things considered. If you go too small, by say believing that the cathode bypass cap can be sized by using the first order speaker crossover filter calculation, then things can get worse!
  8. Other than the output impedance issue, distortion will be ridiculously high and gain will be off the charts high. You'll get that sound of the AA-5 table radio though, if that's what you're after. Feedback is an acceptable compromise when running a single ended pentode amp. While you are welcome to hate it all you want, that doesn't somehow negate the plethora of reasons that you need to use it in an amp of this type. Being an "expert" in the field, I can't imagine how you don't already know this?
  9. 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.
  10. 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.
  11. 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).
  12. 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.
  13. 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.
  14. 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.
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