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Old 12th January 2018, 09:43 PM   #201
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by mjd_tech View Post
Yep, "be kind to your screens and you can punish your plates"
This is one thing that is very nicely covered in Merlin Blencowe's preamp design book. In the section on small-signal pentodes, he makes it very clear that it is the screen grid voltage that controls the current through a pentode, and not the anode voltage.

This was very useful to me when I first read it, because I was coming from the world of semiconductors, and there really is no functional equivalent to a screen grid in any semiconductor device that I know of.

-Gnobuddy
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Old 12th January 2018, 10:37 PM   #202
GeorgK is offline GeorgK  Austria
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Originally Posted by voltwide View Post
Anything that can be discussed about tube amps has been discussed endlessly, there is nothing new on the planet.
You could say this about playing the blues too, but it's still being played.

I fell in love with tubes when I was quite young, I liked the tube glow, the llook, the sound and the smell of old radios. Eventually I learned to repair basic faults.
Later I fell I love with the guitar, and at some point I realized what a good team the three of us could make...

Rock'n'Roll and electric Blues were invented with the gear available at that time, and that's the reason why it sounded like it did. Tubes are definitely the sound of my music.
I do not make a living from music, so I have not that pragmatic approach.
Using transistors or even highly sophisticated algorithms to simulate the tonal properties of technology from the stone age does not really make it feel real to me.

Just as my relation with my guitars went through many ups and downs (and will go through many more), it did and does with my amps. They never sound the same on two days. They sound differently when hot. Tubes wear out and it takes me ages to get used to new ones. Plus a thousand of other things. I admit, they drive me crazy sometimes. But it keeps the challenge alive. They are musical instruments, with all their imperfections.
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Old 12th January 2018, 11:20 PM   #203
voltwide is offline voltwide  Ireland
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Quote:
Originally Posted by GeorgK View Post
You could say this about playing the blues too, but it's still being played.
.
No, I would not say that playing a specific style like blues is bound to a specific technical hardware.
btw, don't get my wrong - I always earned my living from hardware design, not by making music.
And concerning the sound - I know this feeling very well:
One day, the sound is great, the other day it is boring.
The same guitar, same amp, same player.
But in a different mood.
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Old 12th January 2018, 11:22 PM   #204
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by voltwide View Post
Anything that can be discussed about tube amps has been discussed endlessly, there is nothing new on the planet.
It is time for some discussion about SS-guitar amps.
I think that there is actually a fascinating third possibility: mixing solid-state and "vacuum state" components in the same circuit. When done thoughtfully, I think it can allow us to improve on both traditional tube amps and traditional solid-state amps.

A small example: it seems to me a cathodyne phase splitter is a waste of a triode. Because of the heavy local negative feedback, there is no musically attractive distortion from a cathodyne (it's too little to hear). When it does distort, it does very bad things (Merlin Blencowe has a section on this in his book). And the large saturation voltage of a high Ra triode severely limits the maximum output signal available before clipping.

So a cathodyne is a perfect candidate for replacing with a modern high-voltage MOSFET. There is no audible change in sound, a triode is saved for use somewhere more musically interesting, and the MOSFET is better in every way - more drive signal before clip, no heater power or valve socket needed, cheaper, etc. (There is one condition, you have to put in appropriate attenuation ahead of the "Source-O-Dyne" to make sure it never clips; we don't ever want to hear harsh MOSFET clipping!)

Neither Leo nor Jim (nor the RCA engineers who created the circuits they borrowed) ever used a MOSFET in their amps - they couldn't have, because there were no MOSFETs back then.

Another intriguing possibility is to use a powerful solid-state class D power amplifier module, with an all-valve preamp designed to sound as rich as a complete all-valve guitar amplifier (but only producing milliwatts of output power, to drive the SS module). Kinda like the valve amp -> line out -> big P.A. system approach, but this time, building the solid-state P.A. system into your combo amp.

This would also use the best characteristics of both SS and valve devices, so you'd combine musical valve distortion, and efficient, economical, lightweight solid-state power amplification.

It's absolutely amazing how much clean audio power you can get from a small, light, class D module for surprisingly little money now. Like the recent thread about the 600 watt (!!) TI module for well under $100 US.

Taking this sort of hybrid approach, I think there is the possibility of creating new guitar amp designs with less input noise than all-valve designs, with rich valve distortion, smaller, lighter, cooler-running, and where every active device (solid-state or vacuum-state) is used for its greatest strengths, with its weaknesses minimized.

-Gnobuddy
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Old 12th January 2018, 11:23 PM   #205
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by voltwide View Post
But in a different mood.
Very true, there are even days when David Gilmour's guitar sounds don't cause rainbows and angels to appear inside my head!

-Gnobuddy
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Old 13th January 2018, 12:06 AM   #206
voltwide is offline voltwide  Ireland
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Over the years I designed numerous Class-D-amp pcbs, including TPA3255, TPA3100, TPA3118, TPA3251 and TPA3255. My motivation focused on building 12V battery powered small combos. One of the first things to learn was that Class-D-clipping distortion is real crackling nasty sound, unacceptable if you push the amp into the limits (i.e. not for guitar players)

Yesterday I had to check some old musicbox speaker and I connected it to one of my hifi-TPA3255 boards that lay around. Then I applied a low frequency sine wave around 40Hz and turned up the volume until a crackling sound like some loose voice coil windings came up. After some investigation it became evident that the speaker was fine, but the amp was just reaching its clipping point. The transisition from linear to clipped is noisy, and in case of a low frequency test signal that noise is inevitably audible.
This sound has been captured using my soundcard and as a result I append an audio-snippet of that recording.
Feeding the audio into audicity shows the distortion and the noise during transition.
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Old 13th January 2018, 12:10 AM   #207
voltwide is offline voltwide  Ireland
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problems with upload
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Old 13th January 2018, 12:16 AM   #208
voltwide is offline voltwide  Ireland
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mh, diyaudio rejects my mp3 file as invalid file?!
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Old 13th January 2018, 12:35 AM   #209
voltwide is offline voltwide  Ireland
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Audacity screenshot with better resolution
Attached Images
File Type: png 40Hz_Overdrive_TPA3255PBTL.png (81.2 KB, 62 views)
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Old 13th January 2018, 01:08 AM   #210
Tubelab_com is offline Tubelab_com  United States
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Quote:
Originally Posted by Gnobuddy View Post
felt that that post (and subsequent related ones) should be split off and moved into a new thread. Which, to my surprise, was called "Tube Emulation & EQ".
Solid State anything became a very touchy subject in that thread. It has a lot to do with how the thread got started. A person boasted that he had designed the absolute best "tube guitar amp kit" ever made for a certain price point, and made a statement that nobody here could do it better. I replied with two words "wanna bet." A third party then created the HBAC and set forth some rules. There was a contest to see who could design and build the "best" TUBE guitar amp for less than $100 in parts cost. Unfortunately the words "best" and "tube" were not fully defined up front, and as much as a third of the posts in that thread were arguments, insults, and general nastiness about what was allowed in a "tube" guitar amp. It seems that the originator used solid state diodes in his power supply, so that was OK, but no other forms of silicon were allowed, even though the challenge starter agreed to the use of mosfets.

Since your design went totally against the grain of the originator, that's probably why it got moved.

Quote:
Originally Posted by Gnobuddy View Post
So a cathodyne is a perfect candidate for replacing with a modern high-voltage MOSFET.
That was the item that started a storm somewhere in the early days of the thread. Some of those posts were zapped and therefore no longer part of the thread. The little 4 tube guitar amp from that thread is still the only working guitar amp that I have right now. It suits me just fine. The fetodyne got pulled in favor of a self split output stage that just sucked, and was the first thing that I put back when I rebuilt the amp for usefulness, not rules compliance. I had been working on the mosfet saturator in Florida before the move, so it was number 2 on the list. So the amp today has 4 tubes and 2 LND150's. Is it still a tube amp? I really don't care......it ROCKS.

Quote:
Originally Posted by Gnobuddy View Post
I wondered when you would work your magic on some 6V6s.
My first "guitar amp" was a Magnavox monophonic HiFi that used a 12AX7, a 5Y3 and a pair of 6V6GT's. The first DIY guitar amp I ever made was a Champ 5C1 clone, again a 6V6. I was about 12 years old then, with zero budget, so I used whatever tubes that I could pull out of the trash. 6V6's were a bit rare, but 6BQ6 TV sweep tubes were common, so I used them a lot. I have revisited the 6V6 several times in life, and again just recently.

The project in question is a Universal Driver board intended for HiFi amplification. It will drive just about anything, and I have been rolling every output tube that I can find through it from 6V6's, 6W6's 6L6's to military tubes and TV sweep tubes. I plan to use this board to make a few HiFi amps, from 25 WPC to the "big one" at 500 WPC. The details are being presented here:

Tubelab Universal Driver Board, 2015 version

The 6V6's in that experiment were operated in conventional G1 drive with the screen grids all wired directly to a variable power supply (no resistor). The idle current was arbitrarily set at 25 mA. The 30 WPC was extracted with 250 volts on G2 and 350 volts on the plates through a 3300 ohm OPT. I let it play some streamed prog rock (Pineapple Thief) at a volume level just touching clipping on peaks for most of the day Wednesday. It sounded nice and nothing unusual happened.

Quote:
does the recipe above work for guitar as well
I haven't actually plugged my guitar into it yet, but there is no reason that it wouldn't work. The schematic is posted in the thread, its one LTP driving another with mosfet followers to drive the output tube grids.

Quote:
Originally Posted by mjd_tech View Post
be kind to your screens and you can punish your plates
That's true to a varying extent on a lot of different tubes.
Quote:
Originally Posted by Gnobuddy View Post
Merlin Blencowe's preamp design book. In the section on small-signal pentodes, he makes it very clear that it is the screen grid voltage that controls the current through a pentode, and not the anode voltage.
In a textbook "ideal pentode" Only the screen and control grid voltages affect the plate current. The plate voltage does not. In some small signal tubes that's almost true. Many larger pentodes have less than flat plate curves. This means that the plate voltage does affect the plate current, but to a far less degree than the screen voltage.

Quote:
no functional equivalent to a screen grid in any semiconductor device that I know of.
There were some dual gate mosfets in the 70's that were used in TV and FM radio tuners. The second gate almost worked like a screen grid. The 3N211 was my favorite, but it's been extinct for 25 years or so. It was good for audio compressors and synthesizer VCA's.

Quote:
Originally Posted by Gnobuddy View Post
Like the recent thread about the 600 watt (!!) TI module for well under $100 US.
I got one of those monsters. It did a good job of making my DIY blow proof speakers sound like I had actually blown the drivers, but autopsy revealed that I had rattled the drivers loose, half the nuts were in the bottom of the cabinet. The speakers live on.......haven't plugged a guitar into that one yet either.

I made a DIY guitar preamp board several years ago. I'm still looking for it, and running out of boxes to open. It was a typical pair of 12AX7's, Fender style tone stack........

Quote:
Originally Posted by voltwide View Post
The same guitar, same amp, same player. But in a different mood.
I had a friend in high school who played guitar in a local band. He had an Acoustic solid state guitar amp. Some days he sounded good, some days he sounded just OK, and some days..... We were at an outdoor gig in a strip mall parking lot and everybody sounded particularly nasty. It took me a while, but I figured it out when I could SEE the bass players notes in the pilot light on the rhythm players Kustom SS guitar amp.......LINE VOLTAGE, and IMD created by it's fluctuations. The whole band was playing on ONE extension cord. The line voltage was low and everybody sounded BAD!
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