thebes Posted January 29, 2018 Author Share Posted January 29, 2018 OK. Never mind. I had the 4ohm output installed instead of the center tap. Switched things around and now the voltages on pins 9 and 7 match up. The colors are so faded on the OPT I had a devil of a time sorting them out and got one wrong. Still have a couple of problems, though. No music for one. Plugged in a portable cd player. Can't find my stereo to mono rca plug, so I just plugged in one side and no sound. Also when I hooked the on/off light and switch back on no power. Bypassed them and I have power. The on/off switch is also the volume pot. Solder lugs in back for on/off (marked "C" (common I presume) and "I' ), then the three lugs for the volume pot. I've tested the switch and only get continuity when it clicks on, and when I twist the volume pot, I get varying values as I turn it, so it looks to be OK. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 29, 2018 Share Posted January 29, 2018 Let's talk about the filament voltages first. The driver/output tubes are running 0.5V higher than they should. So, if you put a 0.3 ohm/2 watt resistor in series with one of the legs from the filament winding that should give you the 6.3V that we want. It seems that you have the hot lead of the power cord going to the fuse first which is fine. The other side of the fuse should go to the on/off switch; and the other side of the switch should go to the pilot lamp and power xfmr primary. The remaining lead from the power xfmr and pilot light should be connected directly to neutral. If the switch shows continuity when activated, and is otherwise open, it should be able to turn the amp on and off. One point, however. It's not a good idea to have the on/off switch on the volume control (yes, I know that Scott, Fisher, et al all did it) as it increases the likelihood of having 60Hz hum coupled into the amp's input stage. But, since you set it up that way, leave it for now until you judge whether it is a problem. Pin 3 of each output tube should have the same voltage since the cathodes share a common resistor. I don't know why you are measuring 8.6V on one and 8.2 on the other. Check for cold solder joints. Don't worry about that voltage being a little lower than specified. If these are not new tubes they may not be drawing the expected amount of current. But, you are close. It's good that you quickly fixed the missing plate voltage issue. With the plates not connected, the screens become the plates and draw way more current than they are designed for which can quickly fry a tube. It seems that you have the hot lead of the power cord going to the fuse first which is fine. The other side of the fuse should go to the on/off switch; and the other side of the switch should go to the pilot lamp and power xfmr primary. The remaining lead from the power xfmr and pilot light should be connected directly to neutral. If the switch shows continuity when activated and is otherwise open it should be able to turn the amp on and off. One point, however. It's not a good idea to have the on/off switch on the volume control (yes, I know that Scott, Fisher, et al all did it) as it increases the likelihood of having 60Hz hum coupled into the amp's input stage. But, since you set it up that way, leave it for now until you judge whether it is a problem. To test why you're not getting any sound, a signal generator would be helpful if you have one (if not, you can plug in your soldering iron and use that as your test probe furnishing a 60 Hz test signal). Connect a crappy test speaker to the amp, turn it on, set the volume control to the middle of its range, ground the appropriate lead from the generator to the amp chassis, set it to any convenient frequency, turn up the output of the generator to a volt give or take, and quickly touch the hot probe to pin 2 of the upper 6BQ5 (the one connected to the wiper of the pot). If the output stage is working, you should hear a tone. If that's good, touch the probe to the end of the pot which connects to C2 from the driver and you should again hear a tone. If that's still good, touch the probe to pin 6 of the 6C4. If you don't hear anything we know that the problem is between the 6C4 and the output stage (be sure that the coupling cap, C2, is well soldered). Will try to check back in later in case you're around and have a chance to check all this stuff. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted January 29, 2018 Author Share Posted January 29, 2018 I appear to have fixed the on/off issue. Pulled the lamp and installed a seperate on/off switch and changed out the dual on/off volume pot for a regular 500k pot. Turns on and off just fine now. Maynard I'm thinking I'll adjust the higher voltages after we get sound. plus I'll take proper measurements when its warmed up for at least a half hour and things have settled in. At this stage I don't want to leave it on for a long time until I'm sure I'm not frying something. I'll run the signal tests and get back to you. I've got a Heathkit RF signal generator but I soldered up the probe years ago so I'm thinking given my track record, with soldering I'll just use the solder gun. I did test my test cd player and it functions and I swapped speakers so that should be working also. I'll let you know. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 29, 2018 Share Posted January 29, 2018 Good news about the switch! An RF generator won't work here. It has to be an audio generator so the soldering iron will do the trick. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted January 29, 2018 Author Share Posted January 29, 2018 Maynard, I did the tests. The pin 2 on the outer 6BQ5 test produced noise at the speaker. The Pin 6 of the 6C4 produced noise However, I'm confused a bit about the second part of the test on the pot for C2. If I touch the middle lug of the volume pot, the one that has the wire that goes to Pin 2 of the outer 6BQ5, I get noise. Is that the one you wanted me to test? If I touch the iron to the lug that has C2 soldered to it I do not get noise. Quote Link to comment Share on other sites More sharing options...
babadono Posted January 29, 2018 Share Posted January 29, 2018 On 1/28/2018 at 12:41 PM, thebes said: Plug it in and poof, spark smoke at the star ground where everything including the neutral wire for the wall plug comes together. The neutral wire from the wall is going to the chassis(star ground)? I'm confused. Quote Link to comment Share on other sites More sharing options...
thebes Posted January 30, 2018 Author Share Posted January 30, 2018 As well you should be. My bad and I do apologize. The ground wire (green) from the wall plug is going to a grounded terminal strip lug, which is also where all the negative (to ground) wires for the circuit meet. A star ground is where, instead of grounding the wires wherever its convenient, you tie them all together, they meet at one point, and from what I understand, this means less noise in the circuit The neutral wire is used to complete the circuit for the primary power supply and attaches directly to one side of the power supply. In other words there are only two wires soldered together at that point, the neutral and one half of the primary winding. When taken with the hot, or live, wire (black) which goes to things like the on/off (power) switch, fuse etc. creates a closed loop. I suspect you already know most of this but typing it out actually helps me understand it. Who knows, eventually the local fire department won't have to go on alert when I try to build something electrical. Quote Link to comment Share on other sites More sharing options...
babadono Posted January 30, 2018 Share Posted January 30, 2018 Cool, thebes. Cleared that up. Now this is unbalanced equipment, why are you adding a power ground (the 3rd green wire) to it? If it connects to anything else that has a 3 wire cord it MAY (notice the emphasis of MAY) cause a ground loop. Please take what I am saying with a grain of salt, I really know next to nothing about tube amps. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 30, 2018 Share Posted January 30, 2018 14 hours ago, thebes said: Maynard, I did the tests. The pin 2 on the outer 6BQ5 test produced noise at the speaker. The Pin 6 of the 6C4 produced noise However, I'm confused a bit about the second part of the test on the pot for C2. If I touch the middle lug of the volume pot, the one that has the wire that goes to Pin 2 of the outer 6BQ5, I get noise. Is that the one you wanted me to test? If I touch the iron to the lug that has C2 soldered to it I do not get noise. Is the noise you get from pin 6 of the 6C4 louder than when you go to the grid of the upper 6BQ5? For curiosity, turn the volume control all the way up (verify that the wiper is now connected to the lug with C2), and repeat the experiment. I'm not sure why you don't hear anything when touching the lug to which C2 is connected. 12 hours ago, babadono said: Cool, thebes. Cleared that up. Now this is unbalanced equipment, why are you adding a power ground (the 3rd green wire) to it? If it connects to anything else that has a 3 wire cord it MAY (notice the emphasis of MAY) cause a ground loop. Please take what I am saying with a grain of salt, I really know next to nothing about tube amps. While it is possible that using a 3 wire cord can result in some ground loops in certain systems, the additional safety aspect is the overriding consideration in my opinion. You don't want to risk a short to the chassis and not have a path which can route a potentially lethal voltage to ground. For some guys I even go a step further and use a GFI plug as well. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted January 30, 2018 Author Share Posted January 30, 2018 Fixed it! It was the rca input. I kept getting a continuity reading on R1 when it was grounded, but it should have been reading the value of the resistor, like it does on the original amp. Tried putting in a different resistor and even grounded it separately to the chassis but no change, which pretty much left the the rca input. Swapped it out and wallah: sound. It appears Maynard that I was not getting a sound from Pin 6, but the gun was hitting pin 7 or 5 which do put out sound. My bad. But your testing procedure finally let me discover that there was no sound coming from pIn 6. Sorry to be such a pain on this cause it's a really simple build. However, this does provide guidance and learning lesson for others who would undertake these projects. Now I have to put in the bleeder on C4, let it warm up for a half hour and take really good AC measurements and adjust the resistors to the heaters accordingly. Should I also do the same for the 5 volt secondaries? Then its a simple matter of disassembling the original case and building a new second monobloc etc. Quote Link to comment Share on other sites More sharing options...
babadono Posted January 30, 2018 Share Posted January 30, 2018 6 hours ago, tube fanatic said: While it is possible that using a 3 wire cord can result in some ground loops in certain systems, the additional safety aspect is the overriding consideration in my opinion. You don't want to risk a short to the chassis and not have a path which can route a potentially lethal voltage to ground. For some guys I even go a step further and use a GFI plug as well. My concern is connecting this now "power grounded" piece of equipment to something that is not. Say for instance hooked up to a tube pre amp that only has a 2 wire power cord. If there is a "fault" in the pre amp the current travels down the shields of the interconnects to get to this now grounded piece of equipment. I like the GFI idea much more. Quote Link to comment Share on other sites More sharing options...
thebes Posted January 30, 2018 Author Share Posted January 30, 2018 Anybody got any thoughts on weather the topside of the builds should be side-to-side (in another words identical), or bookend? Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 31, 2018 Share Posted January 31, 2018 It is certainly reasonable to reduce the rectifier filament voltage to 5 if it's more than 5% high. As to mirror image construction, it's a matter of your personal preference. Many of the guys I know use a vertical equipment rack so I tend to build them identically since they look nicer when stacked. Also, layouts which are really terrific in one configuration may not be as good when flipped to the opposite side of the chassis. Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted February 1, 2018 Author Share Posted February 1, 2018 Too late! I decided to go with a bookend build, before I saw your post and have already done the drilling and am about a third of the way on building the 2nd bloc. I let the amp warm up for a half hour and took proper measurements and I have 6.7vac on the heaters and 5.5 vac on the rectifier. That's with the .1uf wirewound already installed. Normal line voltage in this area is around 122 so I'll adjust accordingly if I have sufficient wirewounds on hand. If not I can use a variac until I order up some new resistors. By the way, the voltage on PIn 3 (sorry forget which part of the tube that is) has steadied up with 9.1 vdc and 9.2vdc respectively on the two El34's . I presume that's probably due to minor differences in the tubes. With some luck I will have these built, tested and running in my main system over the weekend. and will, of course be providing a detailed listening report once they have a few hours on them. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted February 1, 2018 Share Posted February 1, 2018 For the filament droppers, metal oxide power resistors do a wonderful job and are quite small. I'm anxious to see the final results and get your listening impressions. Very exciting! Maynard Quote Link to comment Share on other sites More sharing options...
thebes Posted February 1, 2018 Author Share Posted February 1, 2018 No doubt but I'm using what I have on hand, and in the needed values I only have the wirewounds. 2nd one is just about done. Will let it rest overnight and look at it afresh. Need fresh eyes cause I think I have cataracts and can't get into see the doctor for another month. If all goes well it will be in main system tomorrow night with listening impressions forthcoming. Quote Link to comment Share on other sites More sharing options...
thebes Posted February 2, 2018 Author Share Posted February 2, 2018 Hooked into my main system, playing away with no magic smoke! Can a build by Thebes with no fires, alarms and panic attacks be possible? We'll have to wait and see. Initial impression is it needs some break in and fine tuning, playing with that treble filter etc. I'm using the 4ohm tap at present but will probably switch that over to 8 ohm for tonight's listening session. Amazing what 60 year old metal with a little freshening up can do. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted February 2, 2018 Share Posted February 2, 2018 I thought I saw the lights get very dim around the time you fired up the amps! Congrats Marty- I'm sure you have a wonderful feeling of satisfaction at creating a musical instrument with your own hands!!! Definitely assess what the use of the 8 ohm tap does to the sound. And, you will find that the treble filter can make quite a difference in shaping the top end to suit your listening preferences. Maynard Quote Link to comment Share on other sites More sharing options...
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