DIY Guitar Amp Comments Thread

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DIY Guitar Amp Comments Thread

Postby macrae11 » Tue May 24, 2011 7:18 am

Well I figured I might as well be the first one to start this thread, and see if we can dig some more info.

When you get some time, could you post a schematic or block diagram of where you think this is going to be when it's finished? It would be cool for me to kind of follow along on paper, both to enhance the project, but also to learn a little more about amp design along the way.

Also I've already chatted with you about it, but a little history to bring everyone else up to speed about what your goals are, what the design is based on etc. might be helpful.
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Tue May 24, 2011 8:34 pm

I really will not know where the schematic will be at design wise until I'm finished, least of all what values where.
I will say this though, nearly all tube guitar amps owe their lineage to the Fender Bassman.

With that said below is a look at the first stage.
Image

Can anyone tell me what the plate current would be if the load resistor was 220k the cathode resistor was 1k8 and B+ was 360V?
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Tue May 24, 2011 8:49 pm

macrae11 wrote:
Also I've already chatted with you about it, but a little history to bring everyone else up to speed about what your goals are, what the design is based on etc. might be helpful.


Modern high gain two channel guitar amplifier.
100W, Class A/B.
Lowest achievable noise floor.
Highest achievable reliability.
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Tue May 31, 2011 6:22 pm

Alain Benoit wrote:Can anyone tell me what the plate current would be if the load resistor was 220k the cathode resistor was 1k8 and B+ was 360V?


Anyone at all?
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Re: DIY Guitar Amp Comments Thread

Postby Malcolm Boyce » Tue May 31, 2011 6:24 pm

Alain Benoit wrote:
Alain Benoit wrote:Can anyone tell me what the plate current would be if the load resistor was 220k the cathode resistor was 1k8 and B+ was 360V?


Anyone at all?

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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Tue May 31, 2011 6:49 pm

Hint, I=E/R.
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Wed Jun 01, 2011 2:43 am

Major milestone on this project this evening.
The power section is officially going to be running on KT77's.
I knew that they drew 1400mA each for the filaments as opposed to 900mA for the 6L6GC's but with an underrated 10A on the 6.3V taps, even with the five dual triodes, this is not a problem.
I also knew that the 2k2 primary impedance of the OT was optimized for the 6L6's Ra of 22k5 but what i hadn't known was that the KT77 has an Ra of 23k so its a near perfect match, no need to misalign the output loads to reflect a more ideal primary, which I wasn't prepared to do anyways.
Last but not least, unlike the EL34's, g3 is internally connected to the cathode as in the 6L6, leaving pin 1 free as a tie point which was part of my lead dressing plan.
It should be mentioned that apart from the tonal characteristics of the KT77 being in the vain of what I was aiming for, its electrical characteristics make it a far superior choice than the usual suspects.

Thanks S. Boyer for lighting a fire under my ass tonight.
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Re: DIY Guitar Amp Comments Thread

Postby sean.boyer » Thu Jun 02, 2011 10:35 pm

Hey man, glad I could encourage the use of KT's. A lot of people are too scared to use them, because they're so clean and awesome (and, well... rather pricey too).

One thing I would recommend, is NOT using the spare pins as tie points, just in case you decided down the road to try other tubes with different pin configurations in there. Maybe you won't want to, but maybe you will.

By the way, for your plate current calculation, is the cathode bypassed, and are we talking quiescent current?
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Fri Jun 03, 2011 1:01 am

sean.boyer wrote:Hey man, glad I could encourage the use of KT's. A lot of people are too scared to use them, because they're so clean and awesome (and, well... rather pricey too).


I was always debating whether I wanted a 6L6, EL34, 6550, or KT88 power section. Lately I had been leaning towards the KT77 and definitely away from the EL34 familly. I just hadnt bothered to do the definitive spec check and confirmed whether or not 77's would be a go given my transformers. Now I'm good. Just gotta figure out my bias feed (bias to grid) and grid resistor values.

sean.boyer wrote:One thing I would recommend, is NOT using the spare pins as tie points, just in case you decided down the road to try other tubes with different pin configurations in there. Maybe you won't want to, but maybe you will.


I wont make that final decision until the day that I actually do it, but, if I use pin 1 as a tie point I willl basically cutting out the EL34/6550 familly which would need other circuit mods as well anyways.

sean.boyer wrote:By the way, for your plate current calculation, is the cathode bypassed, and are we talking quiescent current?


The cathode is bypassed with a 1uF cap which we can ignore and yes quiescent. We're basically asking for the DC load line, I wanna keep things relatively simple around these parts, then again maybe not so much with you in these parts now.
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Re: DIY Guitar Amp Comments Thread

Postby Jef » Tue Jun 28, 2011 7:26 am

When you connected the ground wire to the chassis, is it necessary to scrape off the powder coating to get a metal to metal contact, or is there enough contact between the screw and the edge of the hole to give it a proper ground?
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Tue Jun 28, 2011 2:51 pm

Good question. It is paramount to get a good clean low resistance ground connection. Scraping the powder coating is always a good idea, which i did. I also mentioned the use of star washers, they really work. Once I was done I measured the DC resistance from the ground pin of the IEC inlet to the sleeve of the slave out jack (furthest points) and had less resistance than my ohmeter could measure, <100mΩ.
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Tue Jun 28, 2011 5:34 pm

Alain Benoit wrote:Can anyone tell me what the plate current would be if the load resistor was 220k the cathode resistor was 1k8 and B+ was 360V?


360 / (220k + 1.8k) = 1.6mA

Does everyone see that?
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Re: DIY Guitar Amp Comments Thread

Postby Mathieu Benoit » Mon Jul 04, 2011 8:01 pm

Alain Benoit wrote:
Alain Benoit wrote:Can anyone tell me what the plate current would be if the load resistor was 220k the cathode resistor was 1k8 and B+ was 360V?


360 / (220k + 1.8k) = 1.6mA

Does everyone see that?

I do now, I didn't know to simply add the resistance up.

Question... What is the depth (resonance) knob. What does it actually do?
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Mon Jul 04, 2011 9:02 pm

The knob is simply a mechanical HUI to adapt an opposable digit type grip onto an otherwise difficult to manipulate .250" nickel plated shaft.
Seriously though, the depth control will behave much like the resonance control on many current production amplifiers. Basically a VLF control.
Stay tuned to the latest post for further explanations.
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Re: DIY Guitar Amp Comments Thread

Postby Malcolm Boyce » Mon Jul 04, 2011 9:05 pm

So.... Lottery to see who gets to use the thing first once it's all through it's final testing? :idea:
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Mon Jul 04, 2011 9:07 pm

It's not like I'm building an ultralite or anything! :mrgreen:
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Re: DIY Guitar Amp Comments Thread

Postby Mathieu Benoit » Mon Jul 04, 2011 9:47 pm

Alain Benoit wrote:The knob is simply a mechanical HUI to adapt an opposable digit type grip onto an otherwise difficult to manipulate .250" nickel plated shaft.
:-|

Alain Benoit wrote:Seriously though, the depth control will behave much like the resonance control on many current production amplifiers. Basically a VLF control.
Stay tuned to the latest post for further explanations.

Ok, I'll wait until you get to it.
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Thu Jul 07, 2011 1:48 am

Got to it.
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Re: DIY Guitar Amp Comments Thread

Postby macrae11 » Thu Jul 07, 2011 8:33 am

Question re: negative feedback loops. I know you said it can tighten up the response of the amp by increasing the damping factor, but does it also cause any phase issues? Also This may be a stupid question, but how does damping factor relate to slew rate? Do they affect each other at all? Is Slew rate even an issue in guitar amps, or modern amp design?
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Thu Jul 07, 2011 12:31 pm

macrae11 wrote:Question re: negative feedback loops. I know you said it can tighten up the response of the amp by increasing the damping factor, but does it also cause any phase issues?


Not that I am aware of. Do you mean because the NFB is out of phase with the incoming signal? Because the PI takes care of that.

macrae11 wrote:Also This may be a stupid question, but how does damping factor relate to slew rate? Do they affect each other at all? Is Slew rate even an issue in guitar amps, or modern amp design?


No stupid questions only stupid Fredericton based PT operators.
Slew rate and damping factor are not related to the best of my knowledge but, slew rate is basically a speed spec, how high of a voltage swing can the amplifier do in a μS.
This as you know would be more relevant the lower the frequency, not because the wavelength is longer but because the amplitude is much higher. This spec is important is signal electronics and in power amplifiers, I would also consider it for bass guitar applications. I have never otherwise heard it discussed when talking about guitar amps.
Consider this though, a great slew rate with a poor damping factor is certainly counter intuitive.

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Re: DIY Guitar Amp Comments Thread

Postby macrae11 » Thu Jul 07, 2011 12:48 pm

Makes sense and confirms what I was thinking. One note with your technical writing Al. Your gratuitous use of acronyms is a bit hard to follow, at least if not prefaced by the full word earlier in the post. By PI, I'm pretty sure you mean the phase inverter, but this might not be obvious to all. And come to think of it, I could be bloody well wrong! :-P
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Thu Jul 07, 2011 12:59 pm

Sorry that's years of shop talk with other techs, hard rabbit to break. PI actually stands for Private Investigator as in Magnum!
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Re: DIY Guitar Amp Comments Thread

Postby macrae11 » Thu Jul 07, 2011 1:13 pm

Do you think we can get Tom Selleck to post in this thread??!!!?!

Or at least his mustache.
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Re: DIY Guitar Amp Comments Thread

Postby macrae11 » Fri Jul 08, 2011 3:22 pm

So I'm following through some of your calculations, just to try to keep the old noodle fresh, and while most of it makes sense, I do have a couple of questions.

1. What do Cgk and Cgp stand for? Combined, they're the overall capacitance of the tube? Is this something that tube manufactures would reference for you, or do you have to test for it? Does it vary greatly from various tube types? Say 12ax7 to 12at7?

2. What is gain factor, like what is the scale it's on?

3. What effect does going through multiple tubes have on this whole scenario.

4. Any RC effect in the power section?

Ok more than a couple.
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Re: DIY Guitar Amp Comments Thread

Postby Alain Benoit » Fri Jul 08, 2011 3:34 pm

macrae11 wrote:
1. What do Cgk and Cgp stand for? Combined, they're the overall capacitance of the tube? Is this something that tube manufactures would reference for you, or do you have to test for it? Does it vary greatly from various tube types? Say 12ax7 to 12at7?


1. Cgk = Capacitance grid to cathode, Cgp or Cga = Capacitance grid to anode. Along with stray capacitance they make up the overall input capacitance. Yes this information should be available on a spec sheet. A typical 12AU7 for example has a Cgk of 1.9pF and a Cgp of 1.63 pF. A proper datasheet should differentiate between the values of system 1 and system 2 for a dual triode.
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