The more important transformer secondary voltage to measure is the heater voltage, as that needs to be close to 6.3Vac with valves in.
The HT voltage will measure highish with no load - its the winding resistances that matter more, along with having a nominal heater voltage. Using a simple load resistor for the HT only gives an approximate indication of voltage regulation, as a capacitor input filter power supply forces a very different loading on the transformer. That is the advantage of PSUD2, as it takes the no load voltage, and the winding resistances, in order to assess regulation and Vdc.
The HT voltage will measure highish with no load - its the winding resistances that matter more, along with having a nominal heater voltage. Using a simple load resistor for the HT only gives an approximate indication of voltage regulation, as a capacitor input filter power supply forces a very different loading on the transformer. That is the advantage of PSUD2, as it takes the no load voltage, and the winding resistances, in order to assess regulation and Vdc.
I'm getting ready to order in some supplies for this project, and was going to get some new tubes as well, as the condition of the old ones is uncertain.... I notice that there are tubes which are strictly labeled 5881, which others are 5881/6L6WGC. I'm assuming the two variants have slightly different construction, is there a general preference for one or the other? What are the differences?
Thanks again! Dave
Thanks again! Dave
The 6L6WGC is a Russian tube that is basically pretty solid, some say it will tolerate more current than the 5881. Either one would be fine. There's a Russian 6P3S-E (the "E" is important) that folks use in place of a 6L6GC. They are pretty affordable on eBay if you're on a budget. Personally I would get the new Tung-Sol 5881, it's a very nice tube. And you can always go with a 6L6GC or KT66. The Chinese KT66 and Tung-Sol KT66 are not good tubes, Gold Lion is far far better.
It... Is... Alive! Finally getting back to this project! I replaced all the supply related electrolytics, and a few of the resistors that were tangled around the multi-section cap, and powered it up this morning, for the first time in a couple of decades. The smoke all stayed contained, and the amp is working! Next step is to perform Gillespie's mods, as Grover suggested, but first I'll plug in a speaker and see how it sounds! Thanks for the pointers and suggestions guys! Cheers, Dave
Not so lucky today 🙁 After my successful power up last weekend, I went ahead and made all of the Gillespie mods... Maybe I should have done them in steps.... In any event, after checking my wiring a couple of times, I powered the girl up once more, and discovered that I've made a nice 93 kHz oscillator! Well not that nice, it isn't a perfect sine wave, but you get my meaning! I'll start double/triple checking things tomorrow, in particular around the feedback network mods, but I'll take any other suggestions you might have on the troubleshooting front. FYI, I'm testing this with the input shorted, and a 9 ohm resistor on the 8 ohm output (it's my "close enough" 50 watt test load).
Any risks I should watch for, if I fire it up without the feedback connected? I guess the output stage shouldn't pull to one rail?
The other thing I should mention, is that I moved the feedback components from their original location near the input jack, closer to the output terminals. It simplified some things to do it that way, but it does mean the wire running to the input stage is longer than it used to be....
The other thing I should mention, is that I moved the feedback components from their original location near the input jack, closer to the output terminals. It simplified some things to do it that way, but it does mean the wire running to the input stage is longer than it used to be....
Feedback disconnected shouldn't hurt anything. The input sensitivity will be much higher without the feedback so keep the input signal low until the amp is on. I did DaveG's full treatment on my pair including the change to triode connected EL34's. Sounds very nice.
For mono chassis Williamson UL amps, I have used CL-140's with excellent results while keeping the power dissipation low as the 140 is near 100% of it rated current capacity.
Never let an old dude with bad eye's read schematics.... Looking things over for the fourth time, I noticed the tiny red dot next to the second set of inter-stage coupling caps.... What I had read as 15 uFd, is in fact supposed to be 0.15 uFd (I hate people that drop the leading zero!) The decimal is there, but it's a bit hard to see in red, and I didn't double check the text of the mods, where it's a bit clearer....
So I'm off to buy a couple more caps, and am hoping this turns into a "Wull thar's yer problem" moment! Stay tuned!
So I'm off to buy a couple more caps, and am hoping this turns into a "Wull thar's yer problem" moment! Stay tuned!
15uf is a little large for a coupling cap.
A 0.15 should work better and be less expensive.
Also, you're in for a real treat with those to300s.
I built some custom TO-300 monoblocks and I can't audibly tell the difference between them and the Mcintosh 30s.
They came out really nice.
I built 4 of them also. Two for me, two for a friend.
A 0.15 should work better and be less expensive.
Also, you're in for a real treat with those to300s.
I built some custom TO-300 monoblocks and I can't audibly tell the difference between them and the Mcintosh 30s.
They came out really nice.
I built 4 of them also. Two for me, two for a friend.
I have multiple pairs of reading glasses throughout the house so they can be at my dispose.
No joy in mud-ville.... I replaced the second pair of coupling caps with the correct 0.15 uFd ones, and I still have an oscillator. As Midnight suggested, I disconnected the feedback network from the input triode, but that didn't change anything. Poking around with my scope, the oscillation is clearly at the plate of the phase splitter portion of the first 6SN7 tube. Once the caps have had a chance to discharge, I'm going to pull the tubes, and measure all the resistors around this part of the circuit, to make sure they're in spec. Can I fire it up with the output tubes, and the middle 6SN7 (tube C in the schematic) pulled, to just look at the input/phase splitter portion of the circuit? Or would the lack of load cause the supply to rise too high? I can dial back my variac, but that would drop the heater voltages as well....
Bummer again! You can power it up with the driver removed, it's not that much of a current draw, maybe 10mA.
Double check your work. When I built mine I had one lead on the wrong pin and it oscillated and had other problems until I found and corrected my error.
Best of luck
Best of luck