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Light at the end of the tunnel...

11/23/2000 6:14 PM
Ian Anderson
Light at the end of the tunnel...
Okay, I've got it working now with the help of a few people via email (you know who you are), so here goes...  
B+ = 593  
Vp = 593  
Vg1 = -55  
Vg2 = 592  
The resistor between the rectifier diodes and first filter cap was burnt out, so I've put a 22ohm/17W one in there as suggested. Would the value of this resistor cause these high voltages, or not?  
This amp is for *bass use* so tight, clean bass is the goal. Would 6550's be a good option?  
It sounds great as it is with EL34's, but I wanted to run it all by you guys first as I'm still a novice at this game.  
... Ian :D
11/24/2000 5:43 PM
Eric H

Ian, take a look at Bruces reply to "ohms law ques." a couple of threads up --it might help. FWIW I think 22 ohms is too small.  
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11/24/2000 6:03 PM
Ian Anderson

The original 'burnt' resistor measured 24.5ohms - a value of 22ohms was suggested by someone else, so I thought that it was more than a coincidence that they would be so similar.  
Have you looked at my drawing to see where the resistor is in the circuit?  
What value do you think is more apropriate?  
It certainly couldn't harm to drop the voltages a bit... 592v on the screens.  
Cheers... Ian :)
11/25/2000 2:07 PM

Trouble with going "big" on this resistor is as follows. Lets say with the amp flat out it draws 300 mA at 590 volts from the HT supply. Then you say you want to drop about 100 volts. That means you need a resistor of 333 ohms. However this resistor needs to dissipate P = I x V = 0.3 x 100 = 30 watts. That's a big resistor.  
But then consider what happens at idle. The HT supply may only be drawing 130 mA. That means the resistor only drops 43 volts, so you end up with a huge variation in B+ (meaning the bias point will walk all over the place while you're playing), as well as what will basically be a 30 watt heater inside your amp, which is something you can do without, especially so close to the filter caps.  
If you want to drop some B+ I believe there are some Zener diode tricks that work, but I can't find them! Anyone?:)  
11/25/2000 4:03 PM
Eric H

"The original 'burnt' resistor measured 24.5ohms - a value of 22ohms was suggested by someone else, so I thought that it was more than a coincidence that they would be so similar."
Ian, this part of the thread appears to be gone.  
I was guessing the resistor was a kludge to drop B+ --which is why I suggested a higher value. A 5cm resistor might handle 30 watts (as pointed out by liam). I have a 220ohm 50 watt that's about that long (one of those gold-anodized aluminum jobs). I think Liams correct, that it isn't the way to go --in any event, and a string of zeners makes more sense.  
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