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In Reply to: RE: DIY 45 amp posted by etch on August 21, 2007 at 12:07:01
I've built a number of amp with the 5842 and NEVER had any trouble what so ever. So, hold the phone before you rip it out.
Is it possible to remove all but one of the grid pins in the tube socket for the 5842?
What 5842s are you using?
What does it look like from the RCA phono jack to your 5842?
Got a picture?
Follow Ups:
Thanks for that, the tube is actually the WE 417a, although I've put amperex 5842 in as well and same problem.The 5842 is connected to the white (left) channel.
Well, that could very well be your problem right there. If possible, please send a pic that shows from the rca to the tube. You really should have some shielded cable tranversing from the rca to the tube. That and the method of grounding is important too. From the looks of it the rac is electrically isolated from the chassis, no???
I often use Mogami which has 4 inner conductors, 2 blue and 2 clear, with an outer shield.
Maybe a visual example for you:
Look in Issue #3 on my website, www.ultrafitimes.com, on page 11 at the top right hand pic. There you'll see the 2 blue wires used for the center conductor or +, and connector to the 100k grid resistor, if you will, and the 2 clear wires used for the outer conductor or ground, -, chassis ground the tube socket, which is in electrical contact with the top plate.
Now look on the page, page 12, at the pic in the upper left. Again, the 2 blue wires are used for the center (+) of the rca and the 2 clear are used for the (-) or outer shield of the rca. Now, look closer. The outer shield is also connected to the outer shield of the rca.
It is not on the tube socket end. It is electrically isolated, being under the outer jacket.
Also, the whole ground scheme is best grounded in real close proximity to the input tube socket, and electrically isolated everywhere else.
Again, look at the Sebring. The big copper ground buss runs from the input tube socket, electrically in contact with the top plate again, to the IEC ground, picking up the B+ caps, the output tube cathode resistor, and the center tap of the B+ winding along the way.
Send me close up of the 5842 tube socket too.
We'll fix this bad boy!
One more iteration and we'll have it, I just need some eyes....
BTW, nice amps and cool magazines Larry.
Chad
I think both channels play on his amp, but he has an 0D3 providing his cathode bias voltage, and then a plate resisor coming off that 0D3 for the driver tube B+. The 0D3 on one channel keeps blowing out, so could high frequency oscillations be increasing the current the tube is sucking down?
-Paul
Is there a schematic for this thing anywhere? Don't think so, as to your suggestion. Pretty easy to pull the OD3 and substitute a resistor for purposes of troubleshooting though. I guess that means this is a direct coupled two stage amp?
Here's the schematic...
Placing a gas regulator tube in the cathode of the 45 is a really bad idea. The current through the 45 will change with the signal applied to the grid - it will not stay constant at 36 mA. If you run the amp at it's limits, the cathode current of the 45 will swing from nealy zero to nearly twice the 'no signal' value, or from say 5 to 70 mA. A OD3 will only maintain regulation within a range of 5 to 40 mA, so you are in danger of both exceeding the maximum current rating of the 0D3 (45 ma) and 'straving' it (less than 5 mA), at which point it will extinguish. The fact that the plate resistor of the 5842 is connected to the OD3 will mitigate the affect somewhat (as the 5582 draws more current, the 45 will drwa less and vice versa), but (without doing the math) I doubt that it is enough to avoid the problem.
Caucasian Blackplates suggestion is much better, since the OD3 is no longer in the cathode of the 45.
Ok, thanks, assuming the construction is good, which I am obviously doubting...
1. Check the wiring as I mentioned previously.
2. Get ride of the battery bias on the input stage for purposes of trouble shooting. that could be it too. Just use a resistor. The value is the batteyr voltage, say 1.5 divide by 10 mA, or 150 ohms. No real power issues here just a 1/2 watter will do.
3. Get rid of the voltage reg tube. Here the resistor value is 150 volts divided by 25mA. Call it 6k, but it needs to be 150 times 25mA, or 3.75 watts. I'd shoot for a good 12 watter here. But, gab anything that fits the bill.
Also, ya might try moving the rca right up to the input tube too. Just disconnect it and shorten the wires and move it right up there. Sit the amp on some blocks so the interconnect can reach under the chassis.
This might be a decent way to go, and the 0D3 can still be used. The 12k resistor could probably be 15k or higher...but I think 12 should be fine.
-Paul
Larry and Paul,
If I follow the post as laid out by Larry, I would:
1. Rewire all of the signal circuitry.
2. Use shielded wire for the channels coming from the inputs.
3. Omit the OD3, use resistors instead, but in what position?
4. Omit the battery bias and use a resistor there as well.
5. Take every precaution to ensure that the ground wiring is as close to the amp as possible, using the shortest wire lengths possible between positions.
My wholehearted thanks!
No the wiring work is not up to snuff, I will remedy this!
Either you put the 0D3 between the center tap and ground of your 2.5V transformer, or you have a 50-100 ohm hum pot between the heater pins on the tube. Either way, the 6k resistor will go either from the CT to ground, or the center of the hum pot to ground. I would also bypass it with a 100uf 250v capacitor.
-Paul
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