thebes Posted January 13, 2020 Share Posted January 13, 2020 This is a little integrated with four 12ax7's, four 6BQ5's and one 6ca4 rectifier. Un-restored it's always had very low volume. You need to have it set to around 2PM to get decent output. Below 9AM there is no discernible sound. it has an on/off switch and the volume pot appears to be dual 500k with two 100k resistors running off the outer leg to the balance pot. I've tested the pot and it appears to be working. This unit does have bass and treble controls and puts out about 28watts. I've replaced several resistors, all coupling caps and the power supply caps. I've tried it with two different sets of tubes, including the rectifier, with no change in the volume. Can't remember how to test the actual output, but basic testing shows a bit over 10 volts on the heaters, 320vdc on the 6BQ5's anode and 300vdc on the screen. Same readings on the 12ax7's show 90 and 100vdc on anode and 2nd anode (pin6) on first two and 253 and 136dc on the second set. I presume these voltage differences are related to the phono output versus line outputs. A schematic seems to be unobtainable so I'm guessing as to the circuit. I have two spare dual pots here. One is 250k and the other is 2000k. I know there are two types of pots audio taper (?) and linear taper (?) I think they are called, but I'm not sure what the original is or the two spares I have. I'm wondering if swapping them in would change things and which one I should swap in. I'm also wondering if changing the value on those two 10k resistor between the volume and balance pot would have an effect on things. Your collective thoughts would be appreciated. I did find this attached DYI Forum thread which startsout trying to address the low output, goes on four 12 pages and at the end of it there is no change in the output volume. However, they never changed out the pot so that's why I'm thinking that is where the problem might lie. https://www.diyaudio.com/forums/tubes-valves/120307-low-output-knight-kn928-amp.html Here's a pic of the pot: Quote Link to comment Share on other sites More sharing options...
thebes Posted January 13, 2020 Author Share Posted January 13, 2020 Here's a picture of the underside showing it unrestored. I did not change out any of the ceramic caps. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 13, 2020 Share Posted January 13, 2020 Hey Marty— measure the voltage from pin 3 to ground on the 6BQ5s. The volume pot should not be the issue. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted January 13, 2020 Author Share Posted January 13, 2020 Hi Maynard, thanks for chiming in. I let it warm up for a few minutes and I'm getting 10.26vdc on pin 3 on all four tubes. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 13, 2020 Share Posted January 13, 2020 Well, the bias is reasonable. Have you tried phones and looked at the phone jack? Let me know. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted January 13, 2020 Author Share Posted January 13, 2020 Will do. All I have here are the small earphones from my old walkman but I'll root around and see if I have an adapter. I'll also what the output is from the jack. Quote Link to comment Share on other sites More sharing options...
thebes Posted January 14, 2020 Author Share Posted January 14, 2020 Got to rewire a pair of ears to take the bigger plug, but first I'll hook it up to some Heresies, just using my test bench speakers right now, check out the line stage and phono stage and hear what I hear. Quote Link to comment Share on other sites More sharing options...
thebes Posted January 15, 2020 Author Share Posted January 15, 2020 Well I've put about four hours listening on it over the last two days, plus I changed out some more out of spec resistors. For it's price, $58 brand new 60 years ago, it's amazing. Playing it with all controls flat it sounds much better than it should, and the phono section is flat out excellent. Linestage volume and phono volume are pretty much equal, a rarity because despite the curves and circuits an such they rarely match up. Been playing it through some Heresies, both line and phono. As it's broken in the volume has gone up. 80db at eight feet is about 1PM on the volume pot. Too high, but doable. Also, if I turn it all the way down, I can still hear a teeny bit of the music coming through, so that seems wrong to me because I should be getting dead silence. No hot tubes and the op's and power xformers are not even close to overheating. I am getting some hum but I haven't tackled replacing the cord with a three prong and installing a fuse yet. So should I swap out the 500k volume pot, and if so taper or straight or I can't remember the correct terminology. Quote Link to comment Share on other sites More sharing options...
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