Originally Posted by Andy40
Yes, you can make a special bleeder-probe to discharge the supply filter caps after you turn the amp off and completely disconnect it from the mains by un-plugging it, you can make a bleeder by attaching a resistor to a piece of wooden dowel, and then solder a piece of wire with an alligator-clip to one end of the resistor, the other end can either be attached to a small nail driven into the end of the wooden dowel or used as is, when you use the bleeder-probe to discharge the caps, the alligator-clip is clipped onto a convenient place on the chassis of the amp, which is normally securely tied to ground, the other end of the bleeder-resistor is touched to the positive terminals of the caps, I need to make a bleeder-probe since I'm going to be doing some amp-building work next year, I just found a piece of 8mm Oak doweling that I could use to make a bleeder-probe, dry-wood is a good electrical insulator, but you can also use a piece of PVC piping cut to a suitable length, a bleeder-probe is a handy piece of equipment to have when it comes to servicing a valve amp.
What I like to do while I'm discharging supply filter caps is to use a digital multimeter to monitor the voltage as I discharge the caps so I know that there aren't any dangerous voltages lurking in the amp circuits I'm working on.
Maybe I could start a thread about making a bleeder-probe, as I think it would prove to be quite useful.
When you choose a resistor for bleeding supply filter caps, you should pick a resistance value that's high enough so that you don't get a big spark when you touch it to the positive terminal of the cap, but low enough so that it bleeds-off the voltage quickly enough, you could probably use any value from about 10k to maybe 220k, and a power rating of maybe 1 Watt to 5 Watts, Marcel might be able to suggest a suitable resistance and power-rating for a bleeder resistor.
I've heard that JJ's are supposed to be a good brand of valves, although I've never tried them....yet, I've got a couple of Genelec E83CC Gold Lion gold-pin preamp valves installed in my Marshall MA100C amp and I think they are pretty good, although they did cost me about Au$50.00 each, I guess it really depends on how much you are willing to spend on valves, I personally wouldn't buy cheap valves since you never know when they'll go bad, and usually with no warning, Electroharmonix are supposed to be pretty decent, as are Sovteks, I read somewhere that Mullards are supposed to be some of the best valves you can buy but they tend to be pretty expensive, Shugangs are supposed to be good too, new old-stock RCA's are supposed to be pretty good too, generally, pretty much all the brands of valves that Evatco stock are good.
I've got a couple of Trigon 6SN7GT octal-base twin-triode valves that seem to be pretty good, and I just remembered that i happen to have one American-made General Electric 6V6GT valve in my stash of valves, I might use it to make a Fender Champ one day.
Anyway, I personally would try and buy the best quality brand of valves I could afford for any of my valve amps, depending on my budget, the better the quality of the tube/valve, the more expensive it's going to be, some audiophile-grade valves, like a new old-stock vintage 300B audio power-triode can cost quite a bit of money.
There's nothing wrong with doing what's called "Tube/Valve-rolling" which is basically trying tubes or valves of different brands in your amp till you find a brand that works for you.
Some of the valves in my stash of valves were scrounged from some defunct electronic equipment, I remember one time when I was studying electronics at uni in the mid 90's, they were getting rid of some unwanted stuff, and I managed to score myself a big box of miscellaneous valves for nothing but my time, including quite a few AWA brand 6CA7 valves, which are equivalent to EL34s, they were still in their original cardboard boxes, unused, wish I still had them now, there were also quite a few 12AU7As and 12AT7s, as well as 12AX7s, really kicking myself now that I didn't bring them with me when I moved to Darwin in 2000.
Going back to the bleeder-resistor, some valve amps, like Marshalls and Fenders, have a bleeder resistor permanently soldered in, in parallel with the power supply filter caps, this does two things I can think of, firstly, the resistors equalize the voltage across two electrolytic caps if the caps are placed in series to double the working-voltage of the cap, have a look at the power supply circuit of a Fender Twin and see if you can spot the two caps in series with the bleeder resistors connected in parallel, the resistors will be something like 220k in value, the two series-connected supply filter caps will usually be placed after the standby switch, the second thing that the bleeder resistors do is to bleed-off the voltage when the amp is powered-down as part of a safety feature, but it's always a good idea to check that there aren't any dangerous voltages remaining in the amp after it has been powered-down and completely disconnected from the mains supply, since bleeder resistors have been known to fail.
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