18 watt lite voltages
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18 watt lite voltages
Hi all.
Just been rebuilding the front end of my lite build based on Ampmakers schematic.
I have checked my voltages against the forums test sheet and the results are as follows. Any ideas and advice?
Jim
Just been rebuilding the front end of my lite build based on Ampmakers schematic.
I have checked my voltages against the forums test sheet and the results are as follows. Any ideas and advice?
Jim
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Re: 18 watt lite voltages
The voltages aren't that far off, you can tweak to match if you want, but the biggest question is your bias. It appears likely too hot. What cathode resistor are you using?
Dissipation:
130 ohms would be 130%
150 ohms would be 113.3%
180 ohms would be 94.2%
200 ohms would be 85%
Those would be with the voltages you posted. Your target is to be between 75 to 85%.
Thanks,
Josh
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Re: 18 watt lite voltages
Thanks Josh .
Its a 150 ohm resistor.
Its a work in progress but I think I am slowly getting there.
Once the bias is sorted I aim to put in your parallel/ cascade gain mod.
Thanks Jim
Its a 150 ohm resistor.
Its a work in progress but I think I am slowly getting there.
Once the bias is sorted I aim to put in your parallel/ cascade gain mod.
Thanks Jim
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Re: 18 watt lite voltages
I have located a 220ohm resistor and i would imagine that will place it somewhere in the ballpark.
I still has the 47uf cap parallel with it, would you recommend I change it over to your suggested 2200uf ?
I have a couple at hand.
Jim
I still has the 47uf cap parallel with it, would you recommend I change it over to your suggested 2200uf ?
I have a couple at hand.
Jim
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Re: 18 watt lite voltages
Yes, I do recommend that. For the record though, it's Zaphod_Phil's Parallel/Cascade mod.
Thanks,
Josh
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Re: 18 watt lite voltages
I have just changed over the power transformer and re checked my voltages.
All are pretty much the same as before apart from the the cathode Bias is now at just under 14 volts.
To get it down, should I change the resistor from its current 220r maybe back down to 150 r ?
Jim
All are pretty much the same as before apart from the the cathode Bias is now at just under 14 volts.
To get it down, should I change the resistor from its current 220r maybe back down to 150 r ?
Jim
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Re: 18 watt lite voltages
Don't arbitrarily reduce it. Check your dissipation first: https://robrobinette.com/Tube_Bias_Calculator.htmJimbpGaff wrote: ↑Sun 07/04/21 7:25 amI have just changed over the power transformer and re checked my voltages.
All are pretty much the same as before apart from the the cathode Bias is now at just under 14 volts.
To get it down, should I change the resistor from its current 220r maybe back down to 150 r ?
Jim
You want to be between 75% to 85% dissipation.
Thanks,
Josh
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Re: 18 watt lite voltages
If B+ is still 360V, the amp is running about 10W dissipation per anode. That's a little hot for a 6BQ5, although some older amps did run them harder. You definitely don't want to install a value lower than 220 ohms.JimbpGaff wrote: ↑Sun 07/04/21 7:25 amI have just changed over the power transformer and re checked my voltages.
All are pretty much the same as before apart from the the cathode Bias is now at just under 14 volts.
To get it down, should I change the resistor from its current 220r maybe back down to 150 r ?
Jim
EDIT: Here's the math... First, determine current through the common cathode resistor. 14V ÷ 220 ohms = 64 mA. Now determine the voltage across the tubes (anode to cathode). 360V - 14V = 346V. Now multiply that voltage times the current to find dissipation. 346V X .064 = 22W. This is for both tubes, so each is dissipating 11W. Now you have to deduct screen dissipation, which is approximately 1W per tube (4 mA X 230V).
Jack
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Re: 18 watt lite voltages
Just want to point out that Rob's calculator appears to be in error. It refers to the El84 as having 12W of "tube dissipation," which is incorrect. The EL84 is rated for 12W anode dissipation plus 2W screen dissipation, or 14W total. Current through the common cathode resistor equals the sum of anode currents and screen currents, so voltage across this resistor doesn't accurately represent anode dissipation. Also, in my calculation above, the 4 mA number for screen current is derived from the data sheet, not the voltages listed by the OP.
Jack
Jack
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Re: 18 watt lite voltages
75% to 85% on Rob's calculator is fine. We're usually off the Wa anyway.
Thanks,
Josh
Thanks,
Josh
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Re: 18 watt lite voltages
Thanks guys.
If I have used the calculator correctly it looks like if I replaced the resistor with a 250r, it may bring it down to just under 80%.
Am I on the right track ?
Thanks Jim
If I have used the calculator correctly it looks like if I replaced the resistor with a 250r, it may bring it down to just under 80%.
Am I on the right track ?
Thanks Jim
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Re: 18 watt lite voltages
Sure, if it works out that way, and that's what you're targeting.
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Re: 18 watt lite voltages
Thanks chaps for all your help so far.
I am quite new to amp building and still only a total novice and completely self taught .
I am getting my head around it a bit now but the theory is still a long way off.
The amp is starting to sound good and I don't want to shorten the life of the power tubes just yet.
Thanks again
Jim
I am quite new to amp building and still only a total novice and completely self taught .
I am getting my head around it a bit now but the theory is still a long way off.
The amp is starting to sound good and I don't want to shorten the life of the power tubes just yet.
Thanks again
Jim
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Re: 18 watt lite voltages
The 220 ohm value you are/were using is good for this. 10W anode dissipation is 83%. Nothing wrong with that, and good tubes will last a long time. This is less dissipation than many other amps like the VOX AC15.
Jack
Jack
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Re: 18 watt lite voltages
I agree with Jack. It's fine where it is. It's also less than a lot of earlier 18 watter builds. I'm typically at 85% on that calculator, and I like the way it sounds.TriodeLuvr wrote: ↑Mon 07/05/21 11:56 amThe 220 ohm value you are/were using is good for this. 10W anode dissipation is 83%. Nothing wrong with that, and good tubes will last a long time. This is less dissipation than many other amps like the VOX AC15.
Jack
Thanks,
Josh
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