pdf64
Wholenote
Posts: 557
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Post by pdf64 on Mar 29, 2023 5:19:47 GMT -5
I sussed out a few of the puzzling issues (all minor), checked the bias, did some math and figured this is dissipating about 12.5 watts The heater voltage was too high so I paralleled a pair of opposing diodes on one leg of the heater circuit and got it down to 6.8 vAC. That's still a bit hot and will shorten the life of the tubes. I'm guessing I might stick the EZ81 rectifier in there...dunno yet; it's a work in progress. No red plating on the EL84s, but they are running hot. I think this is probably normal for EL84s that are up at the top end of their power spec... It’s fine to use a resistor to bring heater voltage into line. A metal clad type bolted to the chassis can be useful. But another option is a mains bucker, especially as you might like the HT voltage also brought down a touch. I suspect you might just be noticing a weird characteristic of the Gibson/Vox tonestack. Think of it as if there’s a mid control built in the bass. With the bass set control from 0 to 95%, it’s effectively on full (10k) but that last nudge of the bass also adjusts the mids to 0, shorts out that 10k mid resistor, introducing a significant mid scoop. To see what I think you’re hearing, have a play with www.guitarscience.net/tsc/vox.htm#RIN=717&R1=100k&R2=10k&RT=1M&RB=1M&RL=600k&C1=47p&C2=22n&C3=22n&RB_pot=LogA&RT_pot=LogAI find it more useful in a bass amp than guitar. If you like it, a linear taper bass control may help to avoid it being a hair trigger, all or nothing effect. If you hate it, try removing the ground connection from the bass control track end. That surprises me a bit, my thinking was that it would be best exactly in the middle, and degrade as it’s turned in either direction away from that? You’re welcome
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Post by Peegoo 🏁 on Mar 29, 2023 11:34:55 GMT -5
I sussed out a few of the puzzling issues (all minor), checked the bias, did some math and figured this is dissipating about 12.5 watts The heater voltage was too high so I paralleled a pair of opposing diodes on one leg of the heater circuit and got it down to 6.8 vAC. That's still a bit hot and will shorten the life of the tubes. I'm guessing I might stick the EZ81 rectifier in there...dunno yet; it's a work in progress. No red plating on the EL84s, but they are running hot. I think this is probably normal for EL84s that are up at the top end of their power spec... It’s fine to use a resistor to bring heater voltage into line. A metal clad type bolted to the chassis can be useful. But another option is a mains bucker, especially as you might like the HT voltage also brought down a touch. I suspect you might just be noticing a weird characteristic of the Gibson/Vox tonestack. Think of it as if there’s a mid control built in the bass. With the bass set control from 0 to 95%, it’s effectively on full (10k) but that last nudge of the bass also adjusts the mids to 0, shorts out that 10k mid resistor, introducing a significant mid scoop. To see what I think you’re hearing, have a play with www.guitarscience.net/tsc/vox.htm#RIN=717&R1=100k&R2=10k&RT=1M&RB=1M&RL=600k&C1=47p&C2=22n&C3=22n&RB_pot=LogA&RT_pot=LogAI find it more useful in a bass amp than guitar. If you like it, a linear taper bass control may help to avoid it being a hair trigger, all or nothing effect. If you hate it, try removing the ground connection from the bass control track end. That surprises me a bit, my thinking was that it would be best exactly in the middle, and degrade as it’s turned in either direction away from that? You’re welcome I will do some homework on the mains bucker; I'm assuming it's a buck regulator? Further study is needed! The bass control dropping out the mids is a really good description of what's happening here. I described it as a volume drop, but the mids disappearing is what's happening. Maybe I'll just leave it as is, because I don't see me playing the amp with the bass control dialed up that far anyway. The amp sounds best with the bass rolled off a bit, actually. That may be due to the speaker cab. I'll lift the ground on the bass pot and see what that does. The humdinger is near the center of its travel, but not exactly in the middle. I used it because the heater supply in this PT has the center tap (no need for an artificial CT). If I wanted to eliminate the humdinger I guess I could try a 50 Ohm/1W resistor per heater leg to the bias circuit. But if it's working fine as is, should I even mess with it? I'm just thinking out loud on this because I know the more wire length you have in a chassis, the more opportunity there is to introduce noise in the circuit. The pot is a wire rheostat type. You could easily teach a uni-level course on this stuff. Cheers and thanks.
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Post by Tinkerer on Mar 29, 2023 11:56:27 GMT -5
Peegoo and Pdf64, I have been greatly enjoying and awed by the exchanges between the two of you. I understand, at best, 1/10 of 1% of what you are discussing, but your dialogue represents that very best of what this forum is all about - people with a shared passion generously sharing their expertise and interests. I am so grateful to have this incredible resource and I thank you both for what you, and so many others, bring to it!!!!
Pdf64, may I ask where/how you have come to know what you know - career, hobby, etc?
Thanks again guys!!
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Post by Peegoo 🏁 on Mar 29, 2023 13:53:03 GMT -5
Pdf64, may I ask where/how you have come to know what you know - career, hobby, etc? Now that I think about it...I never checked his credentials Pete's been a Known Quantity for many years online, along with Vintage Jon (RIP), Bill "BillM" Machrone (RIP), Wally, Rob "Robrob" Robinette, Jon Snell, and several others.
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Post by Peegoo 🏁 on Mar 30, 2023 5:21:25 GMT -5
I lifted the ground from the bass control and it did smooth out that sharp mid cut. I like this better!
I'm going to swap in the EZ81 rectifier tube to bring the HT down because tube data shows it provides a 28-volt drop. I'd like a little more than that, but it's a start in the right direction. I read up on bucking transformers and while it would do what I want, I'd like to find a way to make this self-contained and not rely on an external unit for power reduction.
I'm also going with an aluminum-cased power resistor to corral the excessive heater voltage. A five-watt type will work, but a 50-watt type is less than a dollar more and will run a lot cooler.
More to follow. Right now I'm thinking it's around 70% done because the tubes will not last very long running this hot.
The amp sounds fantastic. But if you've spent any time at the track or the strip with a wrench in your paws, you've heard the phrase "it always sounds best right before it blows up."
I'll do a vid when I get this thing to about 95%. This amp is as much a learning tool for me as it is something to play through. And it's been cheap, too; all in, probably $40-$50 in materials including the repurposed transformers, chassis, and several other durable parts for this build.
The troubling thing is that I have four or five more old derelict PA amps (6V6 and 6L6 powered) I've collected over the years at hamfests that are just begging to be turned into guitar amps. Just like building guitars, this really is a disease.
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Post by Peegoo 🏁 on Mar 31, 2023 8:40:27 GMT -5
NOW we're getting somewhere Switching to a tube rectifier pretty much fixed the elevated voltage issues; here's a comparison of the B+ rail. With silicon rectifier B+1: 375v B+2: 372v B+3: 323v B+4: 211v B+5: 176v With tube rectifier B+1: 345v (-30) B+2: 342v (-30) B+3: 296v (-27) B+4: 189.5v (-21.5) B+5: 157v (-19) Heater voltage came down to 6.6v. That's still a tiny bit high so I'll see if I can land it at 6.3v. Right now the only issue is it sometimes makes a whistle when handling the unplugged end of the guitar cable. I'm guessing this might be due to using a 33K grid stopper on V1. I may try adding negative feedback to see what that does. Not that this amp needs it because it does the really clean/really loud thing well with no farty low end, and as the gain (vol 1) is dialed up it's a nice smooth transition from clean to crunch. Adding NFB may change those for the worse.
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Post by Peegoo 🏁 on Mar 31, 2023 15:43:33 GMT -5
Here's a tube chart. I'll get a resistor on that heater circuit and I'll be close to done with version 1 of this amp.
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Post by Pinetree on Apr 17, 2023 9:10:31 GMT -5
We're waiting.
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