6L6GC / 6BG6 x4 Ultralinear Guitar Amp - Page 2 - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 14th April 2012, 06:53 PM   #11
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
This is clearly a musical instrument amplifier discussion, please read the headers on both Tubes / Valves and I & A forums as that makes it clear where this thread belongs. Thanks! It's moving..(Moved!)
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 14th April 2012, 07:03 PM   #12
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
As a trend, older amps have lower value grid stoppers, or none at all.
Older valves tend to be bigger, and have poorer gain at UHF so there is less risk of parasitic oscillation. It could also be that older designers are more likely to understand RF as well as audio, so less likely to make their audio amplifier stages also look like a UHF oscillator.

Grid stoppers (on the first stage) can reduce RF coming in from the outside, but mostly they stop the stages from oscillating.

On a separate issue, ultimately IM and harmonics have the same origin in non-linearity.
  Reply With Quote
Old 14th April 2012, 07:20 PM   #13
nazaroo is offline nazaroo  Canada
Sin Bin
 
Join Date: Feb 2012
Location: http://goldentubes.blogspot.ca/
Quote:
Originally Posted by DF96 View Post
Older valves tend to be bigger, and have poorer gain at UHF so there is less risk of parasitic oscillation.
So, is this directly related to older tubes having higher Miller capacitance?

Quote:

It could also be that older designers are more likely to understand RF as well as audio, so less likely to make their audio amplifier stages also look like a UHF oscillator.
Yes I'd tend to believe that the older engineers were more RF-savvy, although they couldn't have predicted the modern RF-polluted environment.

I still haven't got much input on modern RF issues vs. the 1960s when there wasn't so much transmission at every band.


Quote:
Grid stoppers (on the first stage) can reduce RF coming in from the outside, but mostly they stop the stages from oscillating.
This is interesting, the distinction you are drawing here.
Obviously oscillation can go all the way down to sub-audio (motor-boating).

What are the low-frequency issues and considerations here?


Quote:
On a separate issue, ultimately IM and harmonics have the same origin in non-linearity.
I am intrigued:

Since some designs have different ratios of IM vs. HD,
what are the controlling parameters here?

I am tending to believe at the moment that IM comes mainly from poor interaction between tubes and powersupply hum.

I might believe there is a small component of IM coming from mismatches or imbalances in a Push-Pull topology, (or rather IM falls into visibility and consideration when HD is cancelled out by Push-Pull arrangments!).

But do you think IM is directly related to operating a tube in a non-linear zone? How so?
  Reply With Quote
Old 14th April 2012, 08:04 PM   #14
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by nazaroo
So, is this directly related to older tubes having higher Miller capacitance?
No. Wider spacing means lower UHF gain because of transit time effects. Old valve bases and sockets can be made from materials which are lossy at RF. New valves are mainly of all-glass construction.

Quote:
What are the low-frequency issues and considerations here?
None. Grid stoppers prevent VHF/UHF parasitic oscillation. They have no effect on lower frequencies, unless too big in which case they will affect HF loop instability if there is global NFB.

Quote:
But do you think IM is directly related to operating a tube in a non-linear zone? How so?
If an output contains Fourier components not present in the input (including all inputs such as PSU) then they arise from nonlinearity - this is simple algebra and trigonometry. Valves are not the only source of nonlinearity, but they are a major source. Sometimes harmonics can be cancelled, sometimes IM can be caused by re-entrant effects, so levels are not necessarily equal but they do arise from exactly the same source.

Hum IM is merely one type of IM, and that comes mainly from valve nonlinearity as the PSU hum sweeps the bias point up and down the valve characteristic. All valves work in a "non-linear zone" - the only issue is how much nonlinearity is there?

If you haven't yet read it, try to get hold of a copy of the Radiotron Designers Handbook. That will cover lots of these issues. It is available online, but it is so large that I much prefer the paper version.
  Reply With Quote
Old 14th April 2012, 09:08 PM   #15
diyAudio Member
 
Join Date: Apr 2010
I hope you do not mind my asking, what do you want out of this amp? You seem to be coming at it from a hifi perspective and other than a jazz amp that is not really the goal. You say you want to keep the output clean and use the preamp to generate your color, most want the output to do that job if possible. Are there any amps out there that you like the sound of? The focus on RF with the grid stoppers concerns me as the values chosen for a guitar amp has more to do with driving the tube into overload than with RF.
  Reply With Quote
Old 14th April 2012, 09:40 PM   #16
nazaroo is offline nazaroo  Canada
Sin Bin
 
Join Date: Feb 2012
Location: http://goldentubes.blogspot.ca/
Quote:
Originally Posted by DF96 View Post
No. Wider spacing means lower UHF gain because of transit time effects. Old valve bases and sockets can be made from materials which are lossy at RF. New valves are mainly of all-glass construction.


None. Grid stoppers prevent VHF/UHF parasitic oscillation. They have no effect on lower frequencies, unless too big in which case they will affect HF loop instability if there is global NFB.


If an output contains Fourier components not present in the input (including all inputs such as PSU) then they arise from nonlinearity - this is simple algebra and trigonometry. Valves are not the only source of nonlinearity, but they are a major source. Sometimes harmonics can be cancelled, sometimes IM can be caused by re-entrant effects, so levels are not necessarily equal but they do arise from exactly the same source.
Thanks for your input.
I guess what I mean is that any "IM" which is harmonically (musically) related to the music being amplified is a lot more tolerable than non-related IM (e.g. frequencies having no relation to the main musical material).

So, IM which is a result of new harmonics and/or interaction of legitimate harmonics, or which can be tolerated because of 'harmony' with the music is of no real concern.

It is more like the fact that for example, Power Supply hum, which is a fixed frequency and so almost NEVER musically related to the program material is the kind of IM which actually grossly interferes with musical enjoyment.

I have certainly listened to many 'poor quality' stereos with much enjoyment, because the distortion was mainly harmonic distortion and/or IM that was musically related to the music.

But I am constantly reminded of the annoyance of powersupply hum (60, 120, + harmonics + IM with program) even when a lot of 'masking' is occurring via loud playing.

Quote:
Hum IM is merely one type of IM, and that comes mainly from valve nonlinearity as the PSU hum sweeps the bias point up and down the valve characteristic. All valves work in a "non-linear zone" - the only issue is how much nonlinearity is there?

If you haven't yet read it, try to get hold of a copy of the Radiotron Designers Handbook. That will cover lots of these issues. It is available online, but it is so large that I much prefer the paper version.
I have a copy here, along with just about every published book on tubes circulating from 1940-1980.

Its probably more to do with my memory retention in my old age.
  Reply With Quote
Old 14th April 2012, 09:46 PM   #17
nazaroo is offline nazaroo  Canada
Sin Bin
 
Join Date: Feb 2012
Location: http://goldentubes.blogspot.ca/
Quote:
Originally Posted by Printer2 View Post
I hope you do not mind my asking, what do you want out of this amp? You seem to be coming at it from a hifi perspective and other than a jazz amp that is not really the goal.
Well, yep I am coming from a 'hifi perspective'. And I love Jazz.
The goal for me is a really musical guitar amp.
If that means it ends up "outside the box" as far as previous guitar amps and experience goes, oh well, I won't be heartbroken.


Quote:
You say you want to keep the output clean and use the preamp to generate your color, most want the output to do that job if possible.
I'm sorry to hear that, because I know from my own experience that the kind of distortion that comes from overloading output transformers and crappy unregulated power supplies is not musical.

A Push-pull stage is not the place to introduce 'harmonic distortion'.
That should be left to single-ended (pre-amp) stages.

If you're talking about 'soft-clipping' that is another matter,
but one which can again be solved and better controlled in an input stage.

Quote:
Are there any amps out there that you like the sound of?
Short answer: no.
I'm a perfectionist, and I can't help but mess around and improve things.
Quote:
The focus on RF with the grid stoppers concerns me as the values chosen for a guitar amp has more to do with driving the tube into overload than with RF.
I think we agree here.
I actually AM more concerned with how the driver stage will be affected than by RF worries. What I see in popular designs is over-compensation for RF, and little concern for grid current effects during performance.
  Reply With Quote
Old 14th April 2012, 10:06 PM   #18
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by nazaroo
I guess what I mean is that any "IM" which is harmonically (musically) related to the music being amplified is a lot more tolerable than non-related IM (e.g. frequencies having no relation to the main musical material).

So, IM which is a result of new harmonics and/or interaction of legitimate harmonics, or which can be tolerated because of 'harmony' with the music is of no real concern.

It is more like the fact that for example, Power Supply hum, which is a fixed frequency and so almost NEVER musically related to the program material is the kind of IM which actually grossly interferes with musical enjoyment.
IM is rarely harmonically related to the music. That is why IM is often more annoying than a similar amount of harmonics. However, some IM may be in rational ratios with the music so could sound like a chord.

Hum IM can be an issue with push-pull stages which cancel direct hum but not IM. I suppose some people might prefer some hum IM - fast vibrato?

Large grid stoppers may have a small effect on blocking distortion caused by grid current in an overdriven stage, but unless the grid stopper is large compared to the grid bias resistor it won't help much because the coupling cap still has a much larger resistance to discharge into than was present to charge it. The output impedance of the previous stage is relevant too.
  Reply With Quote
Old 14th April 2012, 10:30 PM   #19
nazaroo is offline nazaroo  Canada
Sin Bin
 
Join Date: Feb 2012
Location: http://goldentubes.blogspot.ca/
Quote:
Originally Posted by DF96 View Post
Large grid stoppers may have a small effect on blocking distortion caused by grid current in an overdriven stage, but unless the grid stopper is large compared to the grid bias resistor it won't help much because the coupling cap still has a much larger resistance to discharge into than was present to charge it. The output impedance of the previous stage is relevant too.
This is interesting, because its the opposite of what I have thought:

I supposed that lower resistance meant a higher driving voltage at a given preamp/driver level (and smaller accumulated distortion from previous stages) and less strain on the first half of the amp.

I assumed that high resistances would cause not only voltage loss requiring higher voltages in previous stages for a given loudness (and more distortion), but also a much less linear behaviour by the output tubes, because of the 'voltage-divider' effect of the resistor in the circuit with the grid current.

Of course I have never seen any proper detailed explanation of the interaction of the components as the output tubes are driven into grid-current conditions.

The little I could gather comes from the strange curves (when makers bother to provide them) as grids are driven positive.

Click the image to open in full size.

Last edited by nazaroo; 14th April 2012 at 10:32 PM.
  Reply With Quote
Old 14th April 2012, 10:32 PM   #20
diyAudio Member
 
Join Date: Apr 2010
Quote:
Originally Posted by nazaroo View Post
Well, yep I am coming from a 'hifi perspective'. And I love Jazz.
The goal for me is a really musical guitar amp.
If that means it ends up "outside the box" as far as previous guitar amps and experience goes, oh well, I won't be heartbroken.
And a Fender, Marshall, Vox, or any of their offshoots are not musical?

Quote:
I'm sorry to hear that, because I know from my own experience that the kind of distortion that comes from overloading output transformers and crappy unregulated power supplies is not musical.
Not talking about overloading output transformers, its the tubes where the action is. Don't like sag? Guess you fall in line with the high gain metal guys.

Quote:
A Push-pull stage is not the place to introduce 'harmonic distortion'.
That should be left to single-ended (pre-amp) stages.
Well if you like thin preamp distortion.

Quote:
If you're talking about 'soft-clipping' that is another matter,
but one which can again be solved and better controlled in an input stage.
So it is a clean amp you want. Why not go SS?

Quote:
Short answer: no.
I'm a perfectionist, and I can't help but mess around and improve things.
As said above, maybe tubes are not for you.

Quote:
I think we agree here.
I actually AM more concerned with how the driver stage will be affected than by RF worries. What I see in popular designs is over-compensation for RF, and little concern for grid current effects during performance.
Most designs now days take overloading the grids inputs in consideration. What may look like RF mitigation is more tone shaping and nonlinear operation control.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
FS: B&G Neo10 x4 Tenson Swap Meet 1 16th February 2011 11:42 PM
6bg6 and 8k opt power amp project Yaho Tubes / Valves 30 15th June 2010 12:42 AM
6BG6 = 6L6GC with top cap - cheap! ray_moth Tubes / Valves 6 31st May 2007 05:46 PM
Has anybody built 6BG6 or similar OTL amp? rnx69 Tubes / Valves 2 14th April 2004 01:25 PM
40WTriode/60Watt Ultralinear amp ashok Tubes / Valves 1 30th June 2003 11:17 PM


New To Site? Need Help?

All times are GMT. The time now is 08:11 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2