I took up guitar about a year ago, and like any self-respecting DIY audio person, I've put a lot of thought in the gear I use.
Without resorting to software modeling (which actually produces fairly decent results), a significant problem for a guitarist is "authentic" power tube distortion/saturation at sane bedroom-household levels (0.5W or less).
Obviously one can achieve nice non-linear distortion from the pre-amp section (both solid state and tubes) and/or effects pedals, although there are various claims that power tube saturation is the "real" deal..... Is this proposition a myth? I don't know....
In my online travels I have discovered London Power Scaling and other similar products (e.g. here and here). I admit that I purchased the entire TUT series of books - a very worthwhile read - although I find it helps to find perspective from other reputable engineering sources.
London Power claims that Power Scaling(TM) allows low volume playing without sacrificing or altering tone. It's my limited understanding that Power Scaling is simply a variable power regulator (Full disclosure - I have not had time to study TUT4 in depth).
I'm a skeptic at heart..... If we change the operating voltage of the power section (lets say 6L6 tubes), won't this in turn change the saturation characteristics, and thus ultimately the tone? The plate characteristics at 300V, on the 6L6, are different from those at say.... 100V. From the purist perspective, this fact alone would imply different performance at lower voltage -> thus a different "tone" and distortion characteristic from full scale power - which is supposedly the holy grail of guitar distortion to begin with.
Another thought: are the negative tonal effects of a power attenuator less or more desirable than operating the output stage tubes a lower voltage and saturation characteristic than full power?
Without resorting to software modeling (which actually produces fairly decent results), a significant problem for a guitarist is "authentic" power tube distortion/saturation at sane bedroom-household levels (0.5W or less).
Obviously one can achieve nice non-linear distortion from the pre-amp section (both solid state and tubes) and/or effects pedals, although there are various claims that power tube saturation is the "real" deal..... Is this proposition a myth? I don't know....
In my online travels I have discovered London Power Scaling and other similar products (e.g. here and here). I admit that I purchased the entire TUT series of books - a very worthwhile read - although I find it helps to find perspective from other reputable engineering sources.
London Power claims that Power Scaling(TM) allows low volume playing without sacrificing or altering tone. It's my limited understanding that Power Scaling is simply a variable power regulator (Full disclosure - I have not had time to study TUT4 in depth).
I'm a skeptic at heart..... If we change the operating voltage of the power section (lets say 6L6 tubes), won't this in turn change the saturation characteristics, and thus ultimately the tone? The plate characteristics at 300V, on the 6L6, are different from those at say.... 100V. From the purist perspective, this fact alone would imply different performance at lower voltage -> thus a different "tone" and distortion characteristic from full scale power - which is supposedly the holy grail of guitar distortion to begin with.
Another thought: are the negative tonal effects of a power attenuator less or more desirable than operating the output stage tubes a lower voltage and saturation characteristic than full power?
Attachments
Last edited:
I built a "Standard" from TUT5 with Power Scaling. This is the "simple" method scaling not the more sophisticated DC method powerscaling which you realy only need if you want to switch powerscale levels when you switch channels. I don't see the need for that.
I made a few minor mods to the TUT5 schematic. I deleted the "Body" Control (mostly because I ran out of front panel space). I used a quad of 6V6 in to a JTM45 Output Tranny for the power section. The triode/pentode switching and the fixed/cathode bias switching I did on a per pair basis rather than on a per output tube basis.
The VVR circuits I've seen use an identical method of power control, that is mosfet level switching of the raw unfiltered but rectified supply.
If the amp is fixed bias you do need a tracking bias supply regulator.
These are my set to work notes showing voltages:
A quad of 1960's 6V6G for the outputs
Each output tube pair switchable between triode mode and pentode mode, and each pair switchable between cathode bias and fixed bias.
The power scaling adjusted anode and screen voltages to the output tubes ONLY, phase splitter/driver and preamp supplies were not "scaled".
I included the "Compression" or "Sag" control which is just a variable current limit and the Voltage control MOSFET.
The following set to work measurements may help you understand the operation.
Set to Work of the Power Amp. Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).
Initial Setup:
Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts
Set Power Scale Control to 10 Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode
A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.
Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.
Set the 4 bias controls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.
Checks:
B+ = 345 Volts
Idle Current/ Bias Voltages for the 4 output tubes
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V
Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode
EDIT: I actually had these backward, what I thought was triode mode was Pentode Mode and vv.
Set Power Scale Control for B+ = 250V
Checks: B+ = 250V Idle Current / Bias Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V
Set Power Scale Control for B+ =150V
Checks B+ = 150V Idle Current / Bias Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V
Set Power Scale Control back to 10 => B+ back to +345 Volts
Wind Compression Control from 0 to 10 => B+ drops to +105 Volts
Set Compression back to 0 and switch to Cathode Bias.
Checks: B+ = 343V Idle Currents / Bias Voltages
Pair#1 Cathode Volts = +23.9V 25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V 25.2mA, 25.4mA tube currents
Note in fixed bias how the bias tracks the B+ adjustment via the tracking bias regulator.
Hope this is some help.
Cheers,
Ian
I made a few minor mods to the TUT5 schematic. I deleted the "Body" Control (mostly because I ran out of front panel space). I used a quad of 6V6 in to a JTM45 Output Tranny for the power section. The triode/pentode switching and the fixed/cathode bias switching I did on a per pair basis rather than on a per output tube basis.
The VVR circuits I've seen use an identical method of power control, that is mosfet level switching of the raw unfiltered but rectified supply.
If the amp is fixed bias you do need a tracking bias supply regulator.
These are my set to work notes showing voltages:
A quad of 1960's 6V6G for the outputs
Each output tube pair switchable between triode mode and pentode mode, and each pair switchable between cathode bias and fixed bias.
The power scaling adjusted anode and screen voltages to the output tubes ONLY, phase splitter/driver and preamp supplies were not "scaled".
I included the "Compression" or "Sag" control which is just a variable current limit and the Voltage control MOSFET.
The following set to work measurements may help you understand the operation.
Set to Work of the Power Amp. Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).
Initial Setup:
Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts
Set Power Scale Control to 10 Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode
A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.
Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.
Set the 4 bias controls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.
Checks:
B+ = 345 Volts
Idle Current/ Bias Voltages for the 4 output tubes
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V
Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode
EDIT: I actually had these backward, what I thought was triode mode was Pentode Mode and vv.
Set Power Scale Control for B+ = 250V
Checks: B+ = 250V Idle Current / Bias Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V
Set Power Scale Control for B+ =150V
Checks B+ = 150V Idle Current / Bias Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V
Set Power Scale Control back to 10 => B+ back to +345 Volts
Wind Compression Control from 0 to 10 => B+ drops to +105 Volts
Set Compression back to 0 and switch to Cathode Bias.
Checks: B+ = 343V Idle Currents / Bias Voltages
Pair#1 Cathode Volts = +23.9V 25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V 25.2mA, 25.4mA tube currents
Note in fixed bias how the bias tracks the B+ adjustment via the tracking bias regulator.
Hope this is some help.
Cheers,
Ian
Last edited:
Hi,
Replicating full power conditions requires including the
very untrivial output transformer effects and the speaker
characteristics which are not as clean as at low power.
Be skeptical. A power attenuator will include the output
transformer but still can't replicate driving the speaker hard.
rgds, sreten.
Replicating full power conditions requires including the
very untrivial output transformer effects and the speaker
characteristics which are not as clean as at low power.
Be skeptical. A power attenuator will include the output
transformer but still can't replicate driving the speaker hard.
rgds, sreten.
That's a lot of work and thanks for sharing your notes.
That gives us a nice look at the electricals.
What about the tone?
How did the tone sound at those different levels?
If you want a bedroom level amp, then I would suggest
getting a different amp for bedroom use.
There is a spot in every amp where you will hear and
feel it "bloom" as I've heard it described. You find that
by playing and turning up the volume unitl you find it.
Too much and you'll know.
You can ride that wave until you find that max point
right before it gets to be too much....at that point
then you back down on your guitar volume like to 5 - 7.
It should clean up really nice right there.
Then when your playing you have the range to
go from clean to lead depending on your set up
with just your guitar volume.
This "bloom" level is different on different amp configurations.
I have yet to hear an amp you can gig with
sound the same at bedroom level.
I also have yet to hear a bedroom level amp
that you can take out and gig with unless you can
mic it and run it through the PA.
But, one amp that give you the same tone, bloom, feel
across the board?
I either own or have played through all the main line amps
from the main line manufactures and many of the boutique
builders, including their various speaker configurations.
These would include the amps from the 1950s thru 2010.
This if from 1x8 to 8x8s through 4 x 12s, stacks, 15s,
2 x 15, 1 x 18 and everything in between.
I am not saying this to stroke my ego.
It is just to let you know what I've experienced through
my own experiences and those of professional musicians
that I've worked with over the years.
I am all ears and I'd be willing to try one out for evaluation
purposes.
For bedroom amp or recording:
Guitar Amp Kits for Recording or Bedroom Use.
I've known Rich for many moons and he's good people.
Hope this helps.
That gives us a nice look at the electricals.
What about the tone?
How did the tone sound at those different levels?
If you want a bedroom level amp, then I would suggest
getting a different amp for bedroom use.
There is a spot in every amp where you will hear and
feel it "bloom" as I've heard it described. You find that
by playing and turning up the volume unitl you find it.
Too much and you'll know.
You can ride that wave until you find that max point
right before it gets to be too much....at that point
then you back down on your guitar volume like to 5 - 7.
It should clean up really nice right there.
Then when your playing you have the range to
go from clean to lead depending on your set up
with just your guitar volume.
This "bloom" level is different on different amp configurations.
I have yet to hear an amp you can gig with
sound the same at bedroom level.
I also have yet to hear a bedroom level amp
that you can take out and gig with unless you can
mic it and run it through the PA.
But, one amp that give you the same tone, bloom, feel
across the board?
I either own or have played through all the main line amps
from the main line manufactures and many of the boutique
builders, including their various speaker configurations.
These would include the amps from the 1950s thru 2010.
This if from 1x8 to 8x8s through 4 x 12s, stacks, 15s,
2 x 15, 1 x 18 and everything in between.
I am not saying this to stroke my ego.
It is just to let you know what I've experienced through
my own experiences and those of professional musicians
that I've worked with over the years.
I am all ears and I'd be willing to try one out for evaluation
purposes.
For bedroom amp or recording:
Guitar Amp Kits for Recording or Bedroom Use.
I've known Rich for many moons and he's good people.
Hope this helps.
I built that amp for a friend Neale.
To go with it I did a KOC "detuned" cab.
That means it was a closed box done to HiFI dimension ratios with cutouts on the baffle board for 2 x 12" speakers BUT a single Weber P12B Alnico "Bulldog" mounted and the other hole left open.
The idea of teh tracking bias is such that the output tubes are working on similar tranfer function curve at all levels. The "Drive Compensation" Control is there to balance preamp overdrive an dpower amp overdrive. When you power scale teh power amp way down in power you obviousley need to reduce the drive into the power amp.
By judicious use of the powerscale control together with the drive compensation control you can get VERY similar sound/tone from bedroom whisper to stadium crusher levels.
Here is something I scribbled up when explaining powerscale and compression (sag) to someone else - based entirely on Kevin O'Connors circuit.
Cheers,
Ian
To go with it I did a KOC "detuned" cab.
That means it was a closed box done to HiFI dimension ratios with cutouts on the baffle board for 2 x 12" speakers BUT a single Weber P12B Alnico "Bulldog" mounted and the other hole left open.
The idea of teh tracking bias is such that the output tubes are working on similar tranfer function curve at all levels. The "Drive Compensation" Control is there to balance preamp overdrive an dpower amp overdrive. When you power scale teh power amp way down in power you obviousley need to reduce the drive into the power amp.
By judicious use of the powerscale control together with the drive compensation control you can get VERY similar sound/tone from bedroom whisper to stadium crusher levels.
Here is something I scribbled up when explaining powerscale and compression (sag) to someone else - based entirely on Kevin O'Connors circuit.
Cheers,
Ian
Attachments
The word I have from virtually every pro who works on amps, and quite a few pro musicians who have tried the various schemes is that (unfortunately) while it does *something* it's just not quite what it is claimed to be.
Most who tried it have said they had it pulled out afterwards.
Otoh, it might work fine for you.
The solution might be a small 1-5 watt amp done up the way you'd like for your sound...
...just reporting what I got back when I inquired about this some time back.
Most who tried it have said they had it pulled out afterwards.
Otoh, it might work fine for you.
The solution might be a small 1-5 watt amp done up the way you'd like for your sound...
...just reporting what I got back when I inquired about this some time back.
Bear,
Many of the schemes do not bother with the tracking bias regulator which is essential to getting close to full power sound at low powers, or with the drive compensation control which is also essential.
But you are quite correct, the best 1 Watt amps are not 50 or 100W Amps powerscaled down to 1 watt but are rather a dedicated 1 W design. PP 12AU7 etc. or single ended amps which have a sound all of their own.
For a practice amp PP 6BM8 (ECL82) or PP 6DX8 (ECL84) are lovely and we have all heard the Marshall PP 6GW8 (ECL86) which great but are beginning to be too loud for a practice amp.
Cheers,
Ian
Many of the schemes do not bother with the tracking bias regulator which is essential to getting close to full power sound at low powers, or with the drive compensation control which is also essential.
But you are quite correct, the best 1 Watt amps are not 50 or 100W Amps powerscaled down to 1 watt but are rather a dedicated 1 W design. PP 12AU7 etc. or single ended amps which have a sound all of their own.
For a practice amp PP 6BM8 (ECL82) or PP 6DX8 (ECL84) are lovely and we have all heard the Marshall PP 6GW8 (ECL86) which great but are beginning to be too loud for a practice amp.
Cheers,
Ian
Sync,
I originally posted this stuff at Powerscaling.com which has since disappeared. There was a whole build thread there.
I used Kevin's TUT5 schematics and board layouts and one of his powerscaling kits with just minor changes. Those changes were just my preferences. I've been tube audio hobbying for quite a while (particularly in HiFi Tube Amps) and am an Electronic Design Engineer in the day job so I'm confident and relaxed about making changes.
One of my HiFi tube amp designs over at DIYAudio has been built by 50 or more guys around the world.
It made sense to me to divide the output tube quad into 2 pairs and provide cathode/fixed bias switching and triode pentode/switching for each pair. The original standard provided this switching on each output tube. Since I was doing that the "Body" control which effectively puts a post phase inverter (post PI) "master" control on one side of the push pull to "pan" between push puull and a pseudo single ended was also left out.
Attached is a during the build shot, shows the output tube sockets, the bias adjustments and monitoring points and teh fixed/cathode bias switches and teh triode pentode switches. You can also see the powerscaling board. The blue things poking out from teh rear panel are 10 turn Bournes bias adjustment pots.
I used the same "Standard" preamp for my own guitar amp which is a single pair of 6V6 and no powerscaling. Also use all 6SL7 instead of 12AX7 for it.
Cheers,
Ian
I originally posted this stuff at Powerscaling.com which has since disappeared. There was a whole build thread there.
I used Kevin's TUT5 schematics and board layouts and one of his powerscaling kits with just minor changes. Those changes were just my preferences. I've been tube audio hobbying for quite a while (particularly in HiFi Tube Amps) and am an Electronic Design Engineer in the day job so I'm confident and relaxed about making changes.
One of my HiFi tube amp designs over at DIYAudio has been built by 50 or more guys around the world.
It made sense to me to divide the output tube quad into 2 pairs and provide cathode/fixed bias switching and triode pentode/switching for each pair. The original standard provided this switching on each output tube. Since I was doing that the "Body" control which effectively puts a post phase inverter (post PI) "master" control on one side of the push pull to "pan" between push puull and a pseudo single ended was also left out.
Attached is a during the build shot, shows the output tube sockets, the bias adjustments and monitoring points and teh fixed/cathode bias switches and teh triode pentode switches. You can also see the powerscaling board. The blue things poking out from teh rear panel are 10 turn Bournes bias adjustment pots.
I used the same "Standard" preamp for my own guitar amp which is a single pair of 6V6 and no powerscaling. Also use all 6SL7 instead of 12AX7 for it.
Cheers,
Ian
Attachments
Last edited:
Ian,
Thank you for your patience and explanation. I was concerned because
you mentioned "recall I am using the 372 power tranny"...I'm like, okay wait
a minute, where are you copying that from.
And
Now I understand. That is a lot of switching, etc to rmemember,
set tweak and then once you dial it in and laster change it, some
times it a real pain again to figure out what you did to get the
tone.
KOC is a big Hammond guy, Hammond makes a lot of tranny's
some folks like em and yeah, their okay.
The next pint's on me.
Ian, thank you very much for posting your experience in regard to power scaling. I will use the information as a valuable reference if I choose it try it myself; hopefully others will find the information useful as well.
Would you be willing to share details in regard to your non-powerscaled amp, or are details posted elsewhere?
I realize that designs such as the Dumble Overdrive Special rely predominantly on pre-amp distortion, and may be adequately controlled with a good master volume. This is in contrast to an amplifier such as the Trainwreck Express that relies on power stage distortion for its characteristic tone; in this application power scaling would be an ideal match for playing a lower volumes (1W or less).
Very interesting discussion here - thanks everyone.
Um!
I ONLY have untidy schematics but here they are:
The Power Amp has some experimental stuff on it which was a failure, that is, current sources in the output tube cathodes for biasing, this worked but had the worst overload characteristics of any bias method I've ever tried. I ended up using fixed bias. Links are drawn as if that bias method was still being used.
Mods I would do today would be bigger grid stops on the output tubes (say 22K) and a 470K grid stop on the concertina triode.
Cheers,
Ian
I ONLY have untidy schematics but here they are:
The Power Amp has some experimental stuff on it which was a failure, that is, current sources in the output tube cathodes for biasing, this worked but had the worst overload characteristics of any bias method I've ever tried. I ended up using fixed bias. Links are drawn as if that bias method was still being used.
Mods I would do today would be bigger grid stops on the output tubes (say 22K) and a 470K grid stop on the concertina triode.
Cheers,
Ian
Attachments
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.