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Old 12th October 2011, 04:02 PM   #11
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I estimate your amp uses about 1.2A (estimated 105 W, 95% efficiency, 80% power factor). Decreasing the cathode resistor will increase that a bit more...

Transformer secondary should be grounded for safety, could also help stability in some cases by acting as a shield (only likely to be an issue in high gain guitar amps). Also must be grounded if you want to use global negative feedback.

Pin 1 connection can be removed if you don't intend to use EL34, but there's no reason to remove it unless you use metal base 6550/KT88 with shell connected to pin 1.

The LED provides "fixed" bias (constant voltage) due to its low dynamic resistance and eliminates the need for a cathode bypass.
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Old 12th October 2011, 05:22 PM   #12
ChrisA is offline ChrisA  United States
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Originally Posted by hoerath7 View Post
..
I am using a 1amp 250v slow blow fuse and I tried to calculate the draw from the power supply this morning (I know very little about this) and I came up with .9 amps... my next question is whether or not I should just bump up to a 1.5 amp fuse or something???

Last question what would happen if I changed the 560 ohm resistors from pin 8 to ground on the power tubes to a 470 ohm resistor... I never plan on running el34s in this amp...

I'd double the size of the fuse. Use a 2A fuse. You can't damage anything by putting in a to large fuse. The purpose of the fuse is to limit the damage to the amp if something fails. For example if a diode shorts. So you want a fuse small so that it will do a good job of containing any damage after a part fails but not so small that you get "nuisance" fuse blowing. Likey with your 1A fuse there is a slight voltage surge on the AC line and that kills the fuse. Going up to 2A Slow Blow is a good compromise.

About changing the cathode resistor. I like to use pots in amps so I can adjust resistance. I think it is best to use a few resistors in series, First a 1 ohm, simply to make measuring current easy, then a fixed resistor then a 100 ohm pot. Most of the voltage drop is in the pot which takes the heat off the pot. Then I can put a volt meter across the 1R resistor and read amps directly with no math involved (other than divide by 1 which is easy) Turn the pot to get the current I want. Every tube is different even if they are "matched" it is never perfect. Do make sure to use a pot rated for 5W or more and choose the largest value fixed resistor you can. So I gues I say to got even further then simply changing the resistor - make it variable then set it every time you swap a tube
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Old 12th October 2011, 05:30 PM   #13
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Tom Bavis and tim614 pretty much summed up what you need to know.

1.5 to 2 amp fuse would be fine and proved ample protection.

Decreasing the cathode resistor will only cause the output tubes to draw more current. Class A is pretty close to 50%, No need to draw more current than you need.

Regards, Ron
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Old 12th October 2011, 06:01 PM   #14
ChrisA is offline ChrisA  United States
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Originally Posted by spkrman57 View Post
Tom Bavis and tim614 pretty much summed up what you need to know.

1.5 to 2 amp fuse would be fine and proved ample protection.

Decreasing the cathode resistor will only cause the output tubes to draw more current. Class A is pretty close to 50%, No need to draw more current than you need.

Regards, Ron
That is true, but the hard part is in the last four words knowing exactly what you need. I'd think you'd want the no signal quiescent current to be on the f latest part of the tube's characteristic curve. Distortion is lowest at that point. Mostly we simply let the designer guess where this point is. it gets you pretty close. But it will be slightly different for every tube. Tubes are 20% tolerance devices at best.

That said, given a choise I'd use the largest value of resistor that gives good result. This means the current is the minimum that gives good result and that the heat produced is minimum. The parts should last longer if they run cooler.

Awhile back I bought a distortion meter. It measures THD directly and it is a good way to set tube biases. Meters are cheap on eBay. But just as good, maybe better are spectrum analyzer (FFT) software. Some is available for free. These work so well you may not want to look because you can see all the "trash" your amp is creating from a clean input signal. You can set bias so as to minimize the "trash" (as long as you stay within the tube's limits)

I guess what I'm saying is that yes you can change the resistor but it's pointless to do so unless you have some method to measure the result of the change. Like I said, it's those last four words how to know what current it is you need.
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Old 12th October 2011, 10:27 PM   #15
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Thanks so much for all your responses guys...... its awesome to have a network of people so willing to help out. I won't bump this thread for a week or so until I make some changes and do some listening tests.

Ron ... I assume you still remember me ... I visited your "museum" a few years ago.... I'll have to pm you sometime! Hope you are well.


thanks again
7
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