First off, I hope everyone is having a great day.
I designed and posted this amplifier schematic over at AudioKarma and the first reply was it is a total waste of time and money, just set it up for the tubes you want and be done with it. Also, the real knowledgeable people did not get involved. Below is what I wrote over there after I was told this. I am posting it here hoping for a more scientific review
Thank you for the replies. A little background, and my case in theory.
This amp was designed using all Toroidal Transformers, thus the lower cost.
This all started as a training exercise for me and a challenge to myself to design an input stage that could actually roll the input tubes. As I have no one to sit down with and teach me and a friend who is willing to look things over by eyeballing it.
I was already looking at using new issue Tung-Sol 7581 output tubes and read a few places that they are real nice to 85% and 30 watts plate dissipation. I went with 84%. That puts the KT-88/6550 right at 70% plate dissipation using a 6.6K output transformer.
Then I thought, I could do the phase inverter stage like the input stage and ran scenarios for that. The results were just as good as the input stage. Russian tubes were used as an example for using shields and represent a higher amperage draw, 6.3V, 12AX7 input and 6CG7 phase inverter tubes. The 6N6P also has a higher 6.3V amperage draw and is supposed to be a superior sounding tube and, what if it's not or I want to compare tube sound, I can swap it out to a 6SN7. 6CG7 or 12AU7 with the same amplitude with minor adjustments. The same with the input stage and the 6N2P-EV could probably be changed to a 5751 tube with just a voltage change and maybe it would need a couple minor adjustments or any of the other tubes listed on the schematic for that matter.
It was never about the output circuit, that was just a quaint circumstance. It is about the rest of the amp and how one could find a great sounding, low distortion amplifier by rolling tubes. As for sockets wearing out, use socket savers. As for the output stage, set it up how you want.
All this stuff is a waste of money unless you are loaded so you must look at this as an investment in the future in some way. Adding some switches, trimpots and tube sockets to get something more superior is a minor expense in my opinion. Making it with long lasting parts and love is priceless.
I have changed the schematic to 5751 input tubes and 6550A output tubes with 5k 100 watt 40% UL tapped transformers to start with. I left room on in the chassis so Dynaco A431s transformers can be added at a latter date.
If anyone has the time to look this over, thank you. I will be looking forward to replies.
Regards, Tom
I designed and posted this amplifier schematic over at AudioKarma and the first reply was it is a total waste of time and money, just set it up for the tubes you want and be done with it. Also, the real knowledgeable people did not get involved. Below is what I wrote over there after I was told this. I am posting it here hoping for a more scientific review
Thank you for the replies. A little background, and my case in theory.
This amp was designed using all Toroidal Transformers, thus the lower cost.
This all started as a training exercise for me and a challenge to myself to design an input stage that could actually roll the input tubes. As I have no one to sit down with and teach me and a friend who is willing to look things over by eyeballing it.
I was already looking at using new issue Tung-Sol 7581 output tubes and read a few places that they are real nice to 85% and 30 watts plate dissipation. I went with 84%. That puts the KT-88/6550 right at 70% plate dissipation using a 6.6K output transformer.
Then I thought, I could do the phase inverter stage like the input stage and ran scenarios for that. The results were just as good as the input stage. Russian tubes were used as an example for using shields and represent a higher amperage draw, 6.3V, 12AX7 input and 6CG7 phase inverter tubes. The 6N6P also has a higher 6.3V amperage draw and is supposed to be a superior sounding tube and, what if it's not or I want to compare tube sound, I can swap it out to a 6SN7. 6CG7 or 12AU7 with the same amplitude with minor adjustments. The same with the input stage and the 6N2P-EV could probably be changed to a 5751 tube with just a voltage change and maybe it would need a couple minor adjustments or any of the other tubes listed on the schematic for that matter.
It was never about the output circuit, that was just a quaint circumstance. It is about the rest of the amp and how one could find a great sounding, low distortion amplifier by rolling tubes. As for sockets wearing out, use socket savers. As for the output stage, set it up how you want.
All this stuff is a waste of money unless you are loaded so you must look at this as an investment in the future in some way. Adding some switches, trimpots and tube sockets to get something more superior is a minor expense in my opinion. Making it with long lasting parts and love is priceless.
I have changed the schematic to 5751 input tubes and 6550A output tubes with 5k 100 watt 40% UL tapped transformers to start with. I left room on in the chassis so Dynaco A431s transformers can be added at a latter date.
If anyone has the time to look this over, thank you. I will be looking forward to replies.
Regards, Tom
Lol, you’re going to get it worse here than you did on AK. I’ll also repeat what I said over there…
There are different ways to enjoy the audio/amp/tube hobby. This kind of amp is specifically designed to see how different tubes sound in the circuit. Why? Because it is fun.
Many say that unless an amp is “optimized” for a specific tube you’ll never get “good” sound. The assumption is that accurate reproduction is the equivalent of good sound. I have always felt that if you want accuracy tubes are the last place you should turn to. Tube amps are distortion machines compared to truly accurate amplifiers. And yet we love our tube amps. Many engineers say we love them despite their inaccuracy but I think it is their inaccuracies that make us love them. And of course the electrical performance of the amplifier is only half of the sonic picture. We never listen to amps, we listen to speakers being driven by amps. The interaction of electrical distortions of a signal with mechanical and acoustic distortions of speakers is complex to say the least.
Have you ever put a well designed amplifier like a Bryston or McIntosh on a set a set of speakers and hated how it sounded only to really enjoy them when you put a tube amp on them? I have. You will never get a tube amp as optimized or of a similar measured performance of a Bryston. Amps with worse performance can give us more enjoyment. Plus, what happens when you get great performance on the bench only to not like the sound when hooked to your speakers? The ability to change the sound of your system is a valuable one.
There are designs that allow you to pop different tube types into them and they work just fine. Many people are only really using a handful of watts while listening. Even if swapping output tube types spikes the harmonic distortion by 15% at full power it will still have harmless amounts at 2 watts.
Some object that this type of sound adjustment is just a tone control. Shrug. I’ll say it is a much more sophisticated tone control, more like a harmonics control. In any case, trying out different tubes in an amp to see what they sound like is no less acceptable way of enjoying audio and audio equipment than buying one amp after another trying to get one that sounds good. Or building your own amp for that matter. I love being able to change the sound of my system just by swapping tubes and re-biasing. It’s a hell of lot cheaper than owning multiple amps.
There are different ways to enjoy the audio/amp/tube hobby. This kind of amp is specifically designed to see how different tubes sound in the circuit. Why? Because it is fun.
Many say that unless an amp is “optimized” for a specific tube you’ll never get “good” sound. The assumption is that accurate reproduction is the equivalent of good sound. I have always felt that if you want accuracy tubes are the last place you should turn to. Tube amps are distortion machines compared to truly accurate amplifiers. And yet we love our tube amps. Many engineers say we love them despite their inaccuracy but I think it is their inaccuracies that make us love them. And of course the electrical performance of the amplifier is only half of the sonic picture. We never listen to amps, we listen to speakers being driven by amps. The interaction of electrical distortions of a signal with mechanical and acoustic distortions of speakers is complex to say the least.
Have you ever put a well designed amplifier like a Bryston or McIntosh on a set a set of speakers and hated how it sounded only to really enjoy them when you put a tube amp on them? I have. You will never get a tube amp as optimized or of a similar measured performance of a Bryston. Amps with worse performance can give us more enjoyment. Plus, what happens when you get great performance on the bench only to not like the sound when hooked to your speakers? The ability to change the sound of your system is a valuable one.
There are designs that allow you to pop different tube types into them and they work just fine. Many people are only really using a handful of watts while listening. Even if swapping output tube types spikes the harmonic distortion by 15% at full power it will still have harmless amounts at 2 watts.
Some object that this type of sound adjustment is just a tone control. Shrug. I’ll say it is a much more sophisticated tone control, more like a harmonics control. In any case, trying out different tubes in an amp to see what they sound like is no less acceptable way of enjoying audio and audio equipment than buying one amp after another trying to get one that sounds good. Or building your own amp for that matter. I love being able to change the sound of my system just by swapping tubes and re-biasing. It’s a hell of lot cheaper than owning multiple amps.
If you want to design an amp that is tube ''rollable'', it would be a circuit designed with adjustable supply voltages, adjustable load and cathode resistors, adjustable following stage grid leak resistors and coupling caps, adjustable feedback resistors.... and then you'll have a lab circuit for tube electronics 101 class that will teach you why different tubes require different setups to do what the engineer wants. Then you use a scope, a sweep generator and distortion instrument to document the results so you can understand what you are hearing. You don't take a shot in the dark on a first design idea and then swap tubes to find the perfect sound.
Last edited:
Indeed, you would want some adjustability to the amp but you don‘t need to adjust everything. My main amplifier is a push pull differential amp using a single triode to drive each output tube. It has adjustable b+, separate bias controls for each output tube, can use external supplies for heater voltages, and 8k transformers from Monolithic Magnetics. The input tubes are auto bias. The builder had originally used 6sl7 for the inputs but plenty of other tubes work well in there including 6sn7 and with adapters I use c3m, 6n7, and am currently using 407a. I had been listening to 6ar6 tubes in triode driven by 4 11n7 tubes. Today I have switched over to 407a driving gu50 in triode. I have also used 6aq5, 6v6, 807, and 7027a for output tubes.
My other amp has fewer adjustments but still does a wonderful job with tube rolling. Dennis had designed the KT88 Firebottle specifically for tube rolling. It is a single ended pentode amp that has a regulated screen supply for the output tubes (260v I think) and a regulated b+ for the input double triode. You adjust the output b+ by changing rectifiers. I usually use 6sn7 for the input but have used a wide variety of output tubes like 807, kt88, kt81, and 6v6.
Nobody would blink about me having a SE 300b amp, a stereo 70, a MC240, and some big solid state amp but an amp that allows you to change the sound by rolling tubes makes people grimace and shake their heads. I think that being able to use such a wide variety of tubes is aurally equivalent to owning different amps.
My other amp has fewer adjustments but still does a wonderful job with tube rolling. Dennis had designed the KT88 Firebottle specifically for tube rolling. It is a single ended pentode amp that has a regulated screen supply for the output tubes (260v I think) and a regulated b+ for the input double triode. You adjust the output b+ by changing rectifiers. I usually use 6sn7 for the input but have used a wide variety of output tubes like 807, kt88, kt81, and 6v6.
Nobody would blink about me having a SE 300b amp, a stereo 70, a MC240, and some big solid state amp but an amp that allows you to change the sound by rolling tubes makes people grimace and shake their heads. I think that being able to use such a wide variety of tubes is aurally equivalent to owning different amps.
Because I am new to this site, I am not allowed to post without a mod looking at it first. I did not come here to start anything and not to be treated bad right away on my first post
Neat. IDK why everyone has to be so huffy. This is a fun amp. If you were trying to sell this design as some hifi - state of the art amp, yeah, I would probably be critical of it.
However, for the guy who mostly enjoys messing around on a work bench and wants a cheap fulfilling experience, I don't see anything wrong with this approach.
The only thing I am not super keen on is how the preamp tube is directly coupled to the phase splitter. That just seems like a problem waiting to happen. Fortunately there are plenty of cheap options to make the system a little more fool proof.
However, for the guy who mostly enjoys messing around on a work bench and wants a cheap fulfilling experience, I don't see anything wrong with this approach.
The only thing I am not super keen on is how the preamp tube is directly coupled to the phase splitter. That just seems like a problem waiting to happen. Fortunately there are plenty of cheap options to make the system a little more fool proof.
That is the input tube tied to a long tailed pair phase inverter or as some call it a differential pair phase inverter and was invented for the computer industry. The audio community also benefited from this innovation.
Last edited by a moderator:
Agreed. No sane manufacturer would release an amp like mine. There are too many ways to blow it up! The builder included output fuses for each phase of the output transformers, one on the input tubes, and of course a mains fuse.
I do think that not having a phase splitter on my amp simplifies or at least minimizes the risks of trying different input tubes. Have fun with your amp!
I do think that not having a phase splitter on my amp simplifies or at least minimizes the risks of trying different input tubes. Have fun with your amp!
The best tube ''rolling''... buzz'' I get is from being able to swap in various vast 6BQ5's I've acquired to a Magnavox amp I use a lot. From a multitude of sources I have Mullard's (Magnavox lable) , Zenith, JJ, RCA, Mitsubishi, Westinghouse, GE, used, NOS, new, all checked for emission quality and low voltage heater (age) grading, and I can swap any of those tubes into any position in that amp and get the same high quality, no noise output and sweet deep dynamic range. What more can anyone want for an amp when it comes to the end goal of having great sound from the source material? If you roll tubes to find the best, are you ever going to swap sub-par tubes back in? Why? Then, if you get to that point in your experience you've figured out why that's the best setup and understand then what to do in the next amp you build, ''perfectly.''
Last edited:
It's not the topology that concerns me. It's the fact that you have a direct coupled tube that has switchable plate loads that concerns me. If you plug in a tube and don't toggle the switches right, you could have an issue on your hands in a hurry.
I have multiple sets of tubes that I think give equally good sounds. They are different, not better or worse. There seems to be a perception that there is a “correct” way the amplifier should be reproducing the signal and anything else is incorrect. If you are willing to live with the inadequacies of tube amps you have already lost the accuracy argument IMO.
When Stereophile reviewed the first Cary 300b amp John Atkinson dismissed it as “…a tone control, and an unpredictable one at that.” Then he hooked it to his speakers and was shocked at how much he liked the sound. Measured performance is not a good predictor of satisfaction. There are many well engineered amps out there and they typically give you different sounds on a speaker. Which one is correct? Yours? If someone can enjoy using different amps (I mentioned McIntosh and Bryston earlier) and getting different sounds then surely getting different sounds out of the same amp is fair game. The fact(?) that those amps are somehow optimized is irrelevant to my ears.
When someone says they're ''shocked'', that should put your skeptic ears on alert. It usually means they have (by design) underestimated or misjudged with their highly skilled credentials...right? ...the object, ... only to be shocked by the obvious truth they now illuminate us all to... to be 'Oh so wrong'' at first, is to elevate the subject to the top of the mountain with their fatefull revelation. Hurry before they are all gone. It means that tube amps can be very sloppy around the mean and still sound good, and that blows away the folks who claim that a $40 coupling cap as opposed to an original $2 cap made a world of sonic change in their amp. What is your assessment of those types of common claims? Bad original, undetected?
That isn’t Atkinson‘s game. He is (was?) the measurement guy. He reviews the measurements for the most part. He wasn’t afraid to publish bad stuff. Nobody reads Sterophile for that though. The regular listening review of the amp was very positive, they almost always are.
Even if he was a complete shill my main point still stands. Different amps sound different regardless of how well they are designed. And yes, changing coupling capacitors can certainly change the sound, is that controversial? I’m suspicious of anyone claiming that any part of a circuit, including the tubes, is always an upgrade. The way some people talk about tubes, capacitors, transformers, etc. makes it sound like you can just keep accumulating goodness. Every tweak is upward, surely at some point you will hit pure nirvana and leave this plane of existence.
No, you can change the sound by changing things but what is better or worse is purely subjective in the end. Like I mentioned earlier, I have multiple combinations that I find equally good but sound different.
Even if he was a complete shill my main point still stands. Different amps sound different regardless of how well they are designed. And yes, changing coupling capacitors can certainly change the sound, is that controversial? I’m suspicious of anyone claiming that any part of a circuit, including the tubes, is always an upgrade. The way some people talk about tubes, capacitors, transformers, etc. makes it sound like you can just keep accumulating goodness. Every tweak is upward, surely at some point you will hit pure nirvana and leave this plane of existence.
No, you can change the sound by changing things but what is better or worse is purely subjective in the end. Like I mentioned earlier, I have multiple combinations that I find equally good but sound different.
Tom Johnson,
Welcome to Tubes / Valves threads!
I only looked briefly at your amplifier schematic, so I only saw a few things to comment on.
The 6550 tubes are in Fixed Bias (or Fixed Adjustable Bias). The resistors, Rg to the grids from the bias, are 100k Ohms.
The maximum resistance for 6550 in Fixed/Fixed Adjustable Bias is 50k Ohms. There is danger of Thermal Runaway. OUCH!
Just changing the resistors to 50k Ohms, creates another problem . . . the driver tube now has to drive 50k, not 100k.
A KT88 would work far better in that spot, no need to change the 100k Ohm Rg's.
There are so many pots. Yes, I understand, that is so you can tube roll just about any tube type that fits the wide range.
For those who have never fine adjusted a pot to Null-out the 2nd Harmonic Distortion, just like all Mathematical Nulls, the point where the null occurs . . .
is Extremely Sharp!
Null Sharpness is in the Math, the sharpness is not in the topology or circuit configuration.
the wipers on pots sometimes become intermittent, and can wear out when constantly an repeatedly adjusted.
The amplifier uses Global Negative Feedback. Starting at the output transformer secondary, it goes all the way back to the input tube Cathode.
A characteristics of global negative feedback . . . it covers a multitude of errors; so it Tends to minimize the differences of the "sounds" of different parts and different tubes.
Adjust the amplifier pots without the global negative feedback, and then apply the global negative feedback ??? Perhaps.
But the gain is extremely high without the global negative feedback.
Global negative feedback makes output transformer saturation worse, it does not make saturation better.
Right or wrong, Toroids may saturate early, with or without global negative feedback.
Even given the above ideas, a push pull amplifier, global negative feedback, and so on, can sound very very good.
Output Tubes:
I prefer to use Individual Self Bias for every output tube.
Then I can listen to 6L6GC, KT66, 5881, all in the same socket, and 807 in a socket adapter, or real 807 socket, and not have to adjust a pot(s).
Plug and Play (not Plug and PRAY)
As long as you connect pin 1 to pin 8, using individual self bias . . . you can tube roll an EL34 and KT77. No need to adjust a pot(s).
Plug and Play
EL84 and 6BQ5 tubes also work with individual self bias, and no pot(s) needed.
Plug and Play
In each of these plug and play examples, one tube is a Pentode, and the other tube is a Beam Power tube.
(well, some tubes are not built like they originally were, so those tube types can be either pentode or beam power).
Have Fun!
Recently one of the amplifiers I designed was Ultra Linear, a form of local negative feedback; I changed it to Triode Wired mode.
Now, the only forms of negative feedback I use are:
Triode Wired beam power tubes (plate to screen is negative feedback).
Cathode to cathode feedback for CCS LTP phase splitters.
A single common self bias resistor to cathodes of driver tubes (un-bypassed), and a single common self bias resistor to cathodes of output tubes (un-bypassed).
These forms of negative are as short of a feedback loop as is possible. No transformers involved in the negative feedback, and no capacitors involved in the negative feedback.
Just my versions and choices: they are essentially "Forward Facing" signal paths.
There are so many other topologies and circuits that work very well too; when they are designed and implemented properly.
$0.03
Welcome to Tubes / Valves threads!
I only looked briefly at your amplifier schematic, so I only saw a few things to comment on.
The 6550 tubes are in Fixed Bias (or Fixed Adjustable Bias). The resistors, Rg to the grids from the bias, are 100k Ohms.
The maximum resistance for 6550 in Fixed/Fixed Adjustable Bias is 50k Ohms. There is danger of Thermal Runaway. OUCH!
Just changing the resistors to 50k Ohms, creates another problem . . . the driver tube now has to drive 50k, not 100k.
A KT88 would work far better in that spot, no need to change the 100k Ohm Rg's.
There are so many pots. Yes, I understand, that is so you can tube roll just about any tube type that fits the wide range.
For those who have never fine adjusted a pot to Null-out the 2nd Harmonic Distortion, just like all Mathematical Nulls, the point where the null occurs . . .
is Extremely Sharp!
Null Sharpness is in the Math, the sharpness is not in the topology or circuit configuration.
the wipers on pots sometimes become intermittent, and can wear out when constantly an repeatedly adjusted.
The amplifier uses Global Negative Feedback. Starting at the output transformer secondary, it goes all the way back to the input tube Cathode.
A characteristics of global negative feedback . . . it covers a multitude of errors; so it Tends to minimize the differences of the "sounds" of different parts and different tubes.
Adjust the amplifier pots without the global negative feedback, and then apply the global negative feedback ??? Perhaps.
But the gain is extremely high without the global negative feedback.
Global negative feedback makes output transformer saturation worse, it does not make saturation better.
Right or wrong, Toroids may saturate early, with or without global negative feedback.
Even given the above ideas, a push pull amplifier, global negative feedback, and so on, can sound very very good.
Output Tubes:
I prefer to use Individual Self Bias for every output tube.
Then I can listen to 6L6GC, KT66, 5881, all in the same socket, and 807 in a socket adapter, or real 807 socket, and not have to adjust a pot(s).
Plug and Play (not Plug and PRAY)
As long as you connect pin 1 to pin 8, using individual self bias . . . you can tube roll an EL34 and KT77. No need to adjust a pot(s).
Plug and Play
EL84 and 6BQ5 tubes also work with individual self bias, and no pot(s) needed.
Plug and Play
In each of these plug and play examples, one tube is a Pentode, and the other tube is a Beam Power tube.
(well, some tubes are not built like they originally were, so those tube types can be either pentode or beam power).
Have Fun!
Recently one of the amplifiers I designed was Ultra Linear, a form of local negative feedback; I changed it to Triode Wired mode.
Now, the only forms of negative feedback I use are:
Triode Wired beam power tubes (plate to screen is negative feedback).
Cathode to cathode feedback for CCS LTP phase splitters.
A single common self bias resistor to cathodes of driver tubes (un-bypassed), and a single common self bias resistor to cathodes of output tubes (un-bypassed).
These forms of negative are as short of a feedback loop as is possible. No transformers involved in the negative feedback, and no capacitors involved in the negative feedback.
Just my versions and choices: they are essentially "Forward Facing" signal paths.
There are so many other topologies and circuits that work very well too; when they are designed and implemented properly.
$0.03
Last edited:
Tj226 made a point but didn't get specific enough. If someone drops in a different phase inverter and it had different internal resistance, it's possible that the cathode voltage could be lower than the grid voltage. And alternately if a different driver tube is installed with an internal resistance that creates a higher plate voltage that goes over to the PI grid, it could also be higher than the cathode voltage there. How long would it run like that before the experimenter does the needed voltage checks and adjustments on which pot? Plate voltage or cathode pot on which tube first? Ooooops, fried PI tube grid. Gonna need a preflight check list for every tube change to put the circuit into a safe zone. It will take someone with excellent design experience to set up different tubes and then read and set the pots first, expecting the right voltages, and knowing also how the stage gains will be affected. Input sensitivity and feedback factor will need to adjusted.. check. No problem... but the amp will ''sound'' different, but is it the tube or the setup? Ask the Magic 8 Ball.
Last edited:
I was being plenty specific. What I said was a nice way of saying that the input needs a redesign. I was leaving it up to him to carefully examine the problem and come up with his own solution, or to ask for help/explanation.
Direct coupling tubes comes with a lot of inherent risks. It's not "possible" that the cathode voltage will change. The cathode and plate voltages for both the input and phase inverter tubes WILL change. Not only from tube to tube, but the voltage will change overtime with tube degradation. Hell, even fluctuations in the line voltage is a potential issue.
Thankfully, it's not an overly difficult problem to solve. Hell, you can always brute force it with a microcontroller. 30 cents will even get you a really nice little stm32 chip with a built in ADC : )
Direct coupling tubes comes with a lot of inherent risks. It's not "possible" that the cathode voltage will change. The cathode and plate voltages for both the input and phase inverter tubes WILL change. Not only from tube to tube, but the voltage will change overtime with tube degradation. Hell, even fluctuations in the line voltage is a potential issue.
Thankfully, it's not an overly difficult problem to solve. Hell, you can always brute force it with a microcontroller. 30 cents will even get you a really nice little stm32 chip with a built in ADC : )