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It is currently 10 May 2021, 23:17

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PostPosted: 11 Mar 2021, 19:57 
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Joined: 24 Nov 2010, 14:39
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Location: California
One thing that I keep thinking about is something that Jeff said in his first post - that when he happened to probe pin 5 of the 555 with a multimeter, the relay closed and the amp powered up. Maybe that acted to shunt some noise out of the 555? There have been a few posts where builders had faulty LR8 regulators that were noisy / oscillating and causing some illusive problems. Just a thought...


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PostPosted: 11 Mar 2021, 20:30 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I suppose anything is possible. But when wired correctly I have never had a LR8 misbehave. I use them all the time and they are used in my commercial kits without any issues. They can misbehave if the first capacitor after the output terminal is not nearby. I would check the 12 VDC circuit. Verify or replace the bypass capacitor on the electrolytic. Also check the capacitor in the DC lift circuit for the heaters.

Good listening
Bruce

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PostPosted: 15 Apr 2021, 16:16 
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Hi Bruce,

Happy to report I have corrected the issue. Turns out my low and high voltage traces passed over each other on the circuit board and your suspicion regarding induction was correct. I rerouted the heater power supply lines using wires and cut the traces on the board. Relay works every time now. Guess I learned a bit about circuit board design! Not sure why my other block never experienced the same issue, but I'll be making the same change just incase. They sound great! Thank you and everyone for your feedback, insight and help!


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PostPosted: 15 Apr 2021, 20:40 
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Hi, That can do it. 8-)

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Bruce

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PostPosted: 01 May 2021, 16:54 
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Joined: 12 Jun 2010, 19:38
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Location: Canada
Hi Bruce. Your designs are very cool, have decided to make them my first attempt at tube gear.
Will use my stash of Russian tubes (6n9s metalbase/6sl7 & 6p3s-e coinbases/6l6).
While I wait for Edcor to build the trannies, I'm reading through the thread again.
Had question about resistor ratings.
What wattage should the 1ohm's in the output stage be ?
and
Is it only the paralleled 100ohm pair in the psu that's recommended 5 W ?
Thank you


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PostPosted: 01 May 2021, 21:32 
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Hi, The one ohm ones need only be 1/2 watt. I squared x R = about 8 milliwatts. Yes they are the only ones paired. Use 20% tolerance if possible as they will not be really well matched. It is a subtle safety feature. If there is a problem with the output stage and it draws too much current then one will fail first then the other and protect the amp. You could use a 47 and a 51 if you can find some that are available....just as long as the tolerances don't make them too close together. A 43 and 56 would be fine as well. Normal dissipation is about 1 watt in any of them so if things go bad in the output stage ....like major current draw the resistors will fail. Usually the fuse is slower to respond to such issues. A fast blow won't work either as it will trip often on the start up surge. Standard blow fuses are recommended.

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Bruce

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PostPosted: 02 May 2021, 08:58 
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Hi, There are days when I think that I am in too much of a hurry and screw up. The resistors are 100 ohms not 50s. :blush: In some design prototypes I did use 50 ohm ones. But the best performance was with 100 ohm pairs. The same thinking is applicable though. Use 20% and try not to get them too close together in value. A pair of 91 and 120 (or 110) is really probably the best but hard to come by. I just get a batch (yes I know you all can't buy big quantities of them) and measure a few to get values that don't match well.

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Bruce

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PostPosted: 02 May 2021, 13:11 
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Hi, thanks for explaining, I mastered scewing up sometime in my childhood :(
As this 100ohm pair acts as a kind of fuse, should they each be 1/2W or 1W rated.
I assume if you use too high a wattage it defeats the purpose.
Dale metal films come in a huge range of values so people may not need a bunch of resistors to achieve the 10/20% discrepancy.
Thx again.


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PostPosted: 02 May 2021, 21:28 
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Hi, They should be 5 watts each (three watts is optimun but they are harder to find). Each will dissipate just about 1 watt when the amp is running. The reason for so big is two fold. One you don't want them to fail all the time in normal operation. Two any failure in the B+ going to the output tubes is going to be huge. At least 500 ma or whatever your power trannie will deliver. The most likely reason for the huge current is that the types of failures are loss of bias (the LM317 or control gets shorted out) and actual internal tube short. In either case the tube will conduct everything. If it is the LM317 circuit then the tubes will share the load and be fine for several minutes....enough for the resistors to fail. If it is a tube failure then it is toast anyhow.

Good listening
Bruce

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PostPosted: 07 May 2021, 10:03 
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Joined: 15 Feb 2020, 18:53
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Would that make these a good choice for those 100R resistors in the power supply?

https://www.mouser.com/ProductDetail/Vi ... DjaiLbiMPf

Or would it be better to go with some 5w resistors that have %20 tolerance so that the values can be off set more?

Another question I have on this project: On the most up to date schematic at the bottom of the page, there are film caps in the heater section which look to me like .1uf but everywhere else in the schematic the notation is 0.1
Are those two caps 0.1 or 1 uf?

Thanks,


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