Advice needed...

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TriodeLuvr
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Re: Advice needed...

Post by TriodeLuvr »

The4thWatcher13 wrote:
Tue 04/06/21 11:22 pm
By the way the impedance of the cathode and plate windings are on the original schematic, but we've been using a truncated, pre-amp only version lately. Therefore I've included an updated schematic of the entire amp with this post.
Can I suggest that you modify the document so it refers to the windings as DCR (DC resistance), not impedance? That's what's being shown by those numbers. Good info to have!
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Re: Advice needed...

Post by JMPGuitars »

The4thWatcher13 wrote:
Tue 04/06/21 11:38 pm
Josh, I've updated the current working version schematic and included it with the previous post. I've added 1K 1/4W resistors to the screens of the 6CU5's. The maximum output is right about where it was when I started all this, albeit maybe a little cleaner. :x
The supply section C3-B is +123.2
Screen voltages at zero signal are V4: +118 / V3: +120
V4: down 5V and V3 down 3V.
Seems like you and Jack are in a Sc-R / No Sc-R tug of war! :lol:
I'm pretty pragmatic; I'll try anything as long as I think I can get away with not blowing anything up. But screen R's did bring the screens down a little. The protection and sound improvement that you mentioned may become more evident once another pre-amp tube is added. Thanks Josh
You should be using 2W or 3W screen resistors. Jack and I having a difference of opinion is irrelevant. Try searching this site, or google, and you'll see it's a lot more than any one opinion. There's some of that science stuff behind it.

And for the record, having properly rated screen resistors will help reduce red plating, and help avoid things like blowing up. Not having a screen resistor does...nothing for you.

Thanks,
Josh
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Re: Advice needed...

Post by JMPGuitars »

TriodeLuvr wrote:
Tue 04/06/21 11:19 pm
There's nothing wrong with adding screen resistors as Josh is suggesting, but it's just one thing you could put on a rather lengthy list of mods and tweaks to make this sound more like a guitar amp.
LOL! If he's not trying to make this sound like a guitar amp, he's on the wrong site! Hahahahah
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Re: Advice needed...

Post by TriodeLuvr »

JMPGuitars wrote:
Wed 04/07/21 5:54 am

You should be using 2W or 3W screen resistors. Jack and I having a difference of opinion is irrelevant. Try searching this site, or google, and you'll see it's a lot more than any one opinion. There's some of that science stuff behind it.

And for the record, having properly rated screen resistors will help reduce red plating, and help avoid things like blowing up. Not having a screen resistor does...nothing for you.

Thanks,
Josh
Josh, it's not a Marshall. It doesn't have +400V on the anodes and screens. In fact, I'll bet nothing in this amp is anything like what you've worked with in the past. A 1K screen resistor w/ 8.5 mA dissipates less than 80mW. Please also note that this amp is more than 50 years old, and it hasn't blown up yet. How long does it take for a lack of screen resistors to become a problem? If you want to apply science, you'll need to start by using use the charts published for these tubes.

Jack
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Re: Advice needed...

Post by JMPGuitars »

TriodeLuvr wrote:
Wed 04/07/21 7:52 am
Josh, it's not a Marshall. It doesn't have +400V on the anodes and screens. In fact, I'll bet nothing in this amp is anything like what you've worked with in the past. A 1K screen resistor w/ 8.5 mA dissipates less than 80mW. Please also note that this amp is more than 50 years old, and it hasn't blown up yet. How long does it take for a lack of screen resistors to become a problem? If you want to apply science, you'll need to start by using use the charts published for these tubes.
I linked the datasheet, and in every instance, it lists the screen voltage lower than the plate. I also mentioned above, this is a guitar amp website, and I've never heard a guitar amp that sounded better with the screen voltage above the plate voltage. We don't have to agree on the subject, it's okay. ;)
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Re: Advice needed...

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JMPGuitars wrote:
Wed 04/07/21 5:58 am
TriodeLuvr wrote:
Tue 04/06/21 11:19 pm
There's nothing wrong with adding screen resistors as Josh is suggesting, but it's just one thing you could put on a rather lengthy list of mods and tweaks to make this sound more like a guitar amp.
LOL! If he's not trying to make this sound like a guitar amp, he's on the wrong site! Hahahahah
Watch out!!! I think an 18W can make a mean bagpipe amp too 😄😄😄
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Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Wed 04/07/21 12:07 am
Can I suggest that you modify the document so it refers to the windings as DCR (DC resistance), not impedance? That's what's being shown by those numbers. Good info to have!
Schematic changed. Thanks, ...I should have known better.
Jack: I also figured the wattage as quite minor, hence the choice in 1/4 W resistors.
Josh: I have been reading up on screen resistors and I'd agree that I haven't seen a guitar amp yet without them and I'm a better safe than sorry kind of guy.....usually. So I think I'll try 1.5K, or so to bring them down a little more.
And yes, it is for guitar! :lol:
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Re: Advice needed...

Post by The4thWatcher13 »

So I've continued to change the value of the screen resistors and have settled on 5.6K. I've also switched the 6CU5 pair with another pair that were more balanced. Which brings the 6CU5 voltages to:
V4: Pl +118 / Scr +111.7
V3: Pl +118 / Scr +111.8
I played through it again and it seems improved in clarity and maybe even a little amplitude (subjective?). At this point I think that's a screen voltage I can live with. Jack's observation that R7 (390 Ohm) and other things could be fiddled with to introduce more distortion has not gone unnoticed. So for now let's go with a 6V offset.
Is the surge voltage of +135 to the screens not a problem? It only lasts about 5-6 seconds then drops to +111 over another 10 sec or so.
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Re: Advice needed...

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The4thWatcher13 wrote:
Wed 04/07/21 2:44 pm
it seems improved in clarity
Unbypassed screen resistors don't improve clarity, they increase the levels of harmonic distortion and IMD. This is why I said it's only one technique that could be useful on your path to converting this to a guitar amplifier. It's not what every player wants. The resistors otherwise serve no purpose, are entirely unnecessary, and their previous absence presented no risk whatsoever of damage to the tubes. That is fully evidenced both by the longevity of the amplifier in its original configuration and by the data published by the manufacturers of the tubes.

As for the screen voltage surge, I'll let someone else answer that. My interpretations of the data don't seem to carry much weight here, and I have several projects of my own to work through at the moment. If it was me, I'd be concentrating on adding the preamp stage and maybe looking for simple ways to make the tone control(s) more versatile. Good luck with the project, I think you're almost done!

Jack
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Re: Advice needed...

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TriodeLuvr wrote:
Thu 04/08/21 2:54 pm
The4thWatcher13 wrote:
Wed 04/07/21 2:44 pm
it seems improved in clarity
Unbypassed screen resistors don't improve clarity, they increase the levels of harmonic distortion and IMD. This is why I said it's only one technique that could be useful on your path to converting this to a guitar amplifier. It's not what every player wants. The resistors otherwise serve no purpose, are entirely unnecessary, and their previous absence presented no risk whatsoever of damage to the tubes. That is fully evidenced both by the longevity of the amplifier in its original configuration and by the data published by the manufacturers of the tubes.

As for the screen voltage surge, I'll let someone else answer that. My interpretations of the data don't seem to carry much weight here, and I have several projects of my own to work through at the moment. If it was me, I'd be concentrating on adding the preamp stage and maybe looking for simple ways to make the tone control(s) more versatile. Good luck with the project, I think you're almost done!

Jack
Wow Jack!!! Don't be discouraged posting your opinions or knowledge. I think there is always room for disagreements and interpretation. The more windows we can look into, the richer the knowledge in the end. Different people have a different focus.. doesn't mean one is more right than the other!!
I for one am always intrigued by your opinions or point of view.
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Re: Advice needed...

Post by JMPGuitars »

TriodeLuvr wrote:
Thu 04/08/21 2:54 pm
Unbypassed screen resistors don't improve clarity, they increase the levels of harmonic distortion and IMD. This is why I said it's only one technique that could be useful on your path to converting this to a guitar amplifier. It's not what every player wants. The resistors otherwise serve no purpose, are entirely unnecessary, and their previous absence presented no risk whatsoever of damage to the tubes. That is fully evidenced both by the longevity of the amplifier in its original configuration and by the data published by the manufacturers of the tubes.
Clarity is a funny word, and not always so clear. The screen resistors, through the effect of dropping the screen voltage below the plate voltage, change the characteristics of the distortion (as you mentioned). I've found distortion to be muddier with the screen voltage too high, so maybe he's saying the distortion sounds better, with more definition.

At some point you have to look past the paperwork and listen to how the thing sounds.

Thanks,
Josh
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Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Thu 04/08/21 2:54 pm
The4thWatcher13 wrote:
Wed 04/07/21 2:44 pm
...it seems improved in clarity
Unbypassed screen resistors don't improve clarity, they increase the levels of harmonic distortion and IMD. This is why I said it's only one technique that could be useful on your path to converting this to a guitar amplifier. It's not what every player wants. The resistors otherwise serve no purpose, are entirely unnecessary, and their previous absence presented no risk whatsoever of damage to the tubes. That is fully evidenced both by the longevity of the amplifier in its original configuration and by the data published by the manufacturers of the tubes.
As for the screen voltage surge, I'll let someone else answer that.
My interpretations of the data don't seem to carry much weight here, and I have several projects of my own to work through at the moment. If it was me, I'd be concentrating on adding the preamp stage and maybe looking for simple ways to make the tone control(s) more versatile. Good luck with the project, I think you're almost done! Jack
Jack, I can't agree with "Bieworm" more, you opinion along with Josh's have been of more help than I can say. I started this project on another thread before I found 18Watt and it didn't take long before support there kind of fell off, for whatever reason. So I'm indebted to you, Josh, 18W and anyone else watching this thread and chiming in. The differences in opinion here are only a benefit from my perspective. And I know everyone of you guys is busy with your own stuff and that's one thing that makes me appreciate the time you take to help me that much more, so please continue to do so if you can. And when I get better I'll do the same for others here. It's a great community. In regard to your statement that I should be moving on to the addition of another preamp tube, that is exactly what I've been doing for the past day. I've been looking at a lot of the 18W and other small amp schematics. I've started modifying my own schematic and reviewing theory of operation relevant to guitar amp design. Essentials like grid leak and/or grid stopper R's, plate R's, bypass caps, decoupling caps and tone circuit pretty much make up the pre circuit and determining values relative to what one wants to achieve is where the magic lies. And that is where your opinions and experience really shine. I'll be reviewing today but, again, your thoughts and opinions are greatly appreciated.
Note: as experienced right now there is no distortion when run at max volume so I believe that will have to be taken into consideration for the preamp component values and the tweaks to come.
Mark->
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Re: Advice needed...

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Bieworm, I participate in forums like this for several reasons. First, I benefit from the experience of others who work in special areas of interest outside my own. Everyone here at 18 Watt was extremely helpful in my recent amplifier build, and the advice from folks like Josh and yourself (and others) was instrumental in saving time and aggravation during the process. Second, I enjoy contributing what I can in terms of technical expertise. I’m a retired engineer, and I’ve been working with tubes and audio professionally or otherwise nearly my entire life. I understand that different experiences and goals create differing opinions and perspectives. However, what’s happened here is that my own contributions of technical data have been repeatedly met with push back. I’m not referring to matters of opinion, but rather reflections of the data published by manufacturers and the application of basic rules of electrical engineering. Post after post attempting to present basic concepts relative to this discussion have been challenged and criticized, and to be blunt, it just isn’t fun anymore. I’m not leaving the group, but it’s clear that the hours I spend analyzing tube charts and simulating circuits in SPICE might be better spent elsewhere. And like I said earlier, I do have a number of projects of my own that need ground-up work and testing right now. I’ll continue to contribute when I can, but it’s time to focus on other pursuits for the time being.

Jack
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Re: Advice needed...

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I'm sorry you feel that way. I've experienced this too but I'm a novice, so I took the advice the way it came... soft or hard. The bonehead in me sometimes went for my own gut feeling.. but in the end I was always proven wrong... or less right. And that was by experiencing it while doing it.
Unfortunately there's math and there's practice. Sometimes the practice wins over the math.. not always excpainable by numbers. That's all there's to it sometimes. You can't explain etheric stuff with maths, but that doesn't mean it's not there 😉
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Re: Advice needed...

Post by zaphod_phil »

TriodeLuvr wrote:
Fri 04/09/21 11:51 am
Bieworm, I participate in forums like this for several reasons. First, I benefit from the experience of others who work in special areas of interest outside my own. Everyone here at 18 Watt was extremely helpful in my recent amplifier build, and the advice from folks like Josh and yourself (and others) was instrumental in saving time and aggravation during the process. Second, I enjoy contributing what I can in terms of technical expertise. I’m a retired engineer, and I’ve been working with tubes and audio professionally or otherwise nearly my entire life. I understand that different experiences and goals create differing opinions and perspectives. However, what’s happened here is that my own contributions of technical data have been repeatedly met with push back. I’m not referring to matters of opinion, but rather reflections of the data published by manufacturers and the application of basic rules of electrical engineering. Post after post attempting to present basic concepts relative to this discussion have been challenged and criticized, and to be blunt, it just isn’t fun anymore. I’m not leaving the group, but it’s clear that the hours I spend analyzing tube charts and simulating circuits in SPICE might be better spent elsewhere. And like I said earlier, I do have a number of projects of my own that need ground-up work and testing right now. I’ll continue to contribute when I can, but it’s time to focus on other pursuits for the time being.

Jack
I'm also an EE, although not yet retired. I've spent many years working with tubes, since building my first tube radio at the age of 12. :)

So, my advice is don't be afraid of the pushback and say what you need to say. Your input is important to this community. Just always be courteous and remember that many of the usual EE rules don't always apply with guitar amps.

Keep on rocking! :-)
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