Fisher 400- I let the smoke out
- Thread starter Ziradog
- Start date
Yeah. When you do you'll notice 2 -220ohm resistors bridged to pin 3 from either Pin 4 or pin 5. These must be removed from Pin 3 and grounded direct. If you don't, you'll have hum in one or both channels. These are R36 and R37 in the heater circuit. Once this is done, remove the ground leads from pin 3 to ground (2 will have wires running to the ground)and run the 10ohm resistors to ground. The 2 with the wire leads, put the resistors in series to the wire. Cover the flying lead side with heatshrink. Flying lead means it's hanging in the air not attached to anything but another lead or part, not a terminal strip or ground point itself.
Well after adding the CL-80 & replacing the rest of the electrolytic caps & the coupling caps, I get -23.5 v on the - side of the rectifier, with 117v going in. But at the output tubes, I am getting 335 volts on pin 7 (should be 365) and 361 - 362 on pin 9 (should be 405), so some more work to do. Sounds ok though, at least through AUX.
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No, that's the voltage on those pins vs. what the schematic lists for voltages. Forgot to say that pin 2 was -15.5ish vs. schematic -17. I haven't installed the 10 ohm resistors yet, as I screwed up & didn't buy the right thing (I did order 12 of them so I have some spares now).
Again if the output tubes are bias'ed too HOT (more toward ZERO VOLTS) they draw too much current and the system voltage drops. You need to get those 10 ohm cathode resistors in to measure current mv and convert to ma's. Match them as close as you can to 10.0 ohms and write down in your notes. Then make the calculations based on those values if they aren't exactly 10.0 ohms.
How is your bias circuit set up? Stock, 10K ohm pot, or IBAM / IBBA boards????????
How is your bias circuit set up? Stock, 10K ohm pot, or IBAM / IBBA boards????????
Stock now, have a 10K ohm pot to install. Plan to put the pot in when I do the resistors. Longer term plan to put a matched set of output tubes, what is in there now is a mix of original Fisher branded and replacements of unknown age.
I understand that the higher current draw can drop the voltage, just did not expect it to be so much (if in fact that is the only issue & not one of several).
I understand that the higher current draw can drop the voltage, just did not expect it to be so much (if in fact that is the only issue & not one of several).
yes, pin3 for 7868
Ok, put in the 10 ohm resistors, and here is what I got with the selector to an empty Aux input & the volume all the way down. This is still with the stock 10K bias circuit set up, haven't put in the 10k pot. Per the schematic, Pin 9 should be 405 & pin 1 or 7 should be 365.
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Tubes are way too cold for the bias voltage. Normal tubes would be somewhere near 42-44ma with -15.5v. ma's work out as 25.5ma, 21.0ma, 21.9ma, 17.5ma. I'd get some new (NEW NEW, not "New old Stock" as those critters are as scarce as hens teeth and cost more per ounce than titanium! The problem with EH7868 (the only new production 7868 tube) is the pins are slightly larger than normal 7868 pins and stretch the socket leaves. So once you put EH's in, you stick with them or re-socket for 7591's. The EH's are good tubes, and sound very nice.
With the matched quad, an IBAM or IBBA board tends to keep them within 1ma of each other for longer periods than if not used. It's a definate plus to the units.
Ok, I have installed the 10K pot in the bias circuit. Do I adjust it to -17v per teh schematic & see what happens?
Also ordered a matched quad of 7868's from Jim McShane, but he is having trouble getting tubes to match from his distributor so it may take a while.
Also ordered a matched quad of 7868's from Jim McShane, but he is having trouble getting tubes to match from his distributor so it may take a while.
i'd probably set it for the most negative voltage it will give, then adjust it until the reading across the 10 ohm cathode resistors is right. I think thats supposed to be 0.32 volts, but it may be 0.34. I'm sure Larry can say for sure.
If its not a matched set, shuffle tubes around to get the tubes per channel as close as you can.
If its not a matched set, shuffle tubes around to get the tubes per channel as close as you can.
32ma (.320v) across the cathode resistor, is a good compromise. Check all 4 tubes as you turn the pot in very small increments. Set for 32ma on the 1st tube that makes it. The rest will all have lower ma's. The lowest should not have a reading lower than 27-28ma which is theoretically the lower end of the sweet spot (distortion rises faster from here on). Don't worry about the -voltages to the tubes as they aren't being measured now. You are measuring current from the tube not the voltages. With the current all the same, the voltages on the tubes will be different. Conversely, if you set the -voltage to -17, then check the cathode current, the current will be all over the place. CURRENT is more important here than voltage except if it closes toward ZERO VOLTS. Not less or closer to ZERO Volts than -10volts.
