There has been debate over grounding one side of the speaker leads for years.
One side claims that grounding the secondary winding adds a few pf of capacitance to the transformer. This may be technically true but it shouldn't make any real difference.
The other side claims that the secondary should be grounded for safety reasons. It is possible (and I have seen it happen) for the OPT to short between the primary and secondary windings. If the secondary is grounded the amp should blow its fuse and cease to function. If the secondary is not grounded the amp may continue to function normally but several hundred volts are present on the speaker leads. This is enough to fry someone DEAD if they touched the speaker leads. This is a real possibility in a guitar amp since the commonly used 1/4 inch plugs are often made of metal.
This is correct, You MUST HAVE a ground path from anything metal to the 3rd terminal of the power jack, but there should be only one path. The common ground "bolt" will work. Connect it to the center pin of the power jack and then use it for the star point. The input jack wiring then provided the path for the PC board ground.
I usually use the center pin of the power jack as the star point. I run seperate wires from there to each black speaker jack and the PC board ground. The chassis then gets ground through the input jack if it is mounted in metal.
Ran the next test (hey, it's my lunch break, working at home 
) and checked the output tube grids. With both trimpots turned (almost) all the way down, the voltages were both at -151V, seems right on target. Actually one trimpot was not all the way down, I figured it'd be better to match them than to be as negative as possible, right? With that one all the way down, the voltage was more like -155V or so on that side, so I tweaked it up ever so slightly to make them match. Oh, and after doing this the B- went down a little, to more like -305V.
) and checked the output tube grids. With both trimpots turned (almost) all the way down, the voltages were both at -151V, seems right on target. Actually one trimpot was not all the way down, I figured it'd be better to match them than to be as negative as possible, right? With that one all the way down, the voltage was more like -155V or so on that side, so I tweaked it up ever so slightly to make them match. Oh, and after doing this the B- went down a little, to more like -305V.
Ok, next checkout measurement, with the 5842s in place, now B+ = 421V, B- = -311V, and plate voltages after ~10 min powered on were both around -170V. One channel could go a bit lower than the other - maybe -175V, but the other would go no lower even with trimpot adjustment; I left them matching for now.
Observations: the central ICs (Q1,2 and U2,3) generate a fair bit of heat, even compared to the tubes. This isn't a scientific measurement, but just holding my hand above the components I could feel the heat rising from there. On the other hand, the D1/U1 pair didn't generate any noticeable heat, but there's no driven load yet...
Also the 9-pin sockets from AES were really tight with my raytheon 5842s; I hope I don't have to pull those back out anytime soon!
Observations: the central ICs (Q1,2 and U2,3) generate a fair bit of heat, even compared to the tubes. This isn't a scientific measurement, but just holding my hand above the components I could feel the heat rising from there. On the other hand, the D1/U1 pair didn't generate any noticeable heat, but there's no driven load yet...
Also the 9-pin sockets from AES were really tight with my raytheon 5842s; I hope I don't have to pull those back out anytime soon!
-170V? Are you sure it's not positive 170V when you hook up red (+) lead from your multimeter to the coupling cap lead closest to the 5842 and black (-) lead to the ground connection?PaulyT said:
Yes, they do generate good bit of heat. Just don't touch them. I've heard about someone getting a burn.
That's because it's the first time.
All sounds quite familiar. The 5842s vary quite a bit and I don't think the varying plate voltage is anything to worry about (I assume you had the probes on backwards, as it should be +175V). U1 will get plenty toasty once it has filaments to feed.
Yes, those sockets are very stiff. It's always good to have a junk noval tube around to insert in a new socket a few times.
Yes, those sockets are very stiff. It's always good to have a junk noval tube around to insert in a new socket a few times.
Evenharmonics said:
The sink on the CCSs have B+ on them, unless you took extra steps to insulate them. So it can zap/kill you. The FETs are insulated packages, so no issue there. The sink on U1/D1 should also be insulated, though it is OK not to insulate one of the two. Either way it will only have a few volts on it, but it will burn you.
Duh, sorry, yes I meant +170V on the 5842 plate. I had it wired/measured right, just typed it wrong...
Yes, I'm careful not to touch anything when it's on; I just held my hand a few inches above the tubes, and you can sorta tell where the heat's coming from. But I did in fact electrically insulate U2,3 with the same little kit as I used on D1, just to avoid having a big metal sink sticking up with 350V on it.
Yes, I'm careful not to touch anything when it's on; I just held my hand a few inches above the tubes, and you can sorta tell where the heat's coming from. But I did in fact electrically insulate U2,3 with the same little kit as I used on D1, just to avoid having a big metal sink sticking up with 350V on it.
Yes. I would use something closer to 60 if these are new tubes. They tend to drift a bit for the first few hours (especially Chinese ones). Check the bias often during the first few hours of use. If they don't drift around, you can crank them up to about 80 if you want. Every tube / OPT combination will have an optimum operating point. Sometimes it is obvious, sometimes it isn't. The 300B has a reputation for having a big sweet spot. Choose the lowest current that gives you good sound.
Well, after a *long* hiatus for various reasons, I'm finally going to try to push to finish this amp. Hope to get the checkout process completed today, and start in on the enclosure assembly.
Minor question: I have an alps pot that doesn't fit the PCB, I guess I should just run a few short lengths of wire (22?) between the pot pins and board? Or is there a more elegant way to deal with this?
Minor question: I have an alps pot that doesn't fit the PCB, I guess I should just run a few short lengths of wire (22?) between the pot pins and board? Or is there a more elegant way to deal with this?
Woo-hoo! Checkout complete, power tubes in and it makes music! Didn't spend a lot of time playing with the bias current, but it held steady at ~60mA, actually one channel seemed to drift down a bit over time. B- at -285, B+ was about 345, seems maybe a tad low? Dunno how critical that is, but I've only got crappy little RS speakers hooked up now, will wait till I have bigger&better and the amp is enclosed before really trying to optimize the bias to improve sound.
Let it be recorded for posterity that the first music piped through this baby was Jethro Tull's Songs from the Wood.
Let it be recorded for posterity that the first music piped through this baby was Jethro Tull's Songs from the Wood.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Congratulations! You've finally joined the club.
By the way, did it make you say, "It's alive!"?
How long of time?
I think that's about average. Some 300B (Valve Art) is said to be better sounding at over 350V. It may be different for EH.
There was maybe a 2-3 mA downward drift of one channel's bias current over ~45 minutes or so. These are brand new EH (gold grid) tubes first time being powered up, didn't want to push them too hard so left them only at around 60mA. Based on George's instructions, I was aiming for the 5 kOhm OPT and "B+ nearing 400 V" range; my EP OPTs are 5k. Maybe I'll have to look into what to do to increase B+... I will also check the PT and line voltages to see how close to spec these are.
Anyone running EH 300B's? Strat, this is what you've got, right? What B+/bias are you using?
Anyone running EH 300B's? Strat, this is what you've got, right? What B+/bias are you using?
PaulyT,
I'm not sure you will hear a difference between a B+ of 345 to one nearing 400. My SE are running JJ and Valve Art 300Bs and started with a B+ of 290 to the present configuration having 340 B+ and honestly, I cannot discern a degradation or improvement either way. I bias my 300B at 80ma.
Remember, if you are going to increase your B+ to nearing 400v, you need to change R14 and R25 to 36K.
Godspeed.
John Revilla
I'm not sure you will hear a difference between a B+ of 345 to one nearing 400. My SE are running JJ and Valve Art 300Bs and started with a B+ of 290 to the present configuration having 340 B+ and honestly, I cannot discern a degradation or improvement either way. I bias my 300B at 80ma.
Remember, if you are going to increase your B+ to nearing 400v, you need to change R14 and R25 to 36K.
Godspeed.
John Revilla
Thanks, yeah I've got 36k at R14/25 already, since this higher B+ was my aim - but again based purely on my reading of George's instructions and his stated preference. I won't obsess over this... at least not right now. Just wondering what difference it'd make, and sounds like maybe not a lot.
Just wondering what difference it'd make, and sounds like maybe not a lot.- Status
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