Tube Amplifier Design and Rolling Tubes

[Bodis-Green]

First off, I hope everyone is having a great day.

I designed and posted this amplifier schematic over at AudioKarma and my first reply was it is a total waste of time and money, just set it up for the tube 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 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

 

[312elements]

I checked out the schematic Unfortunately like most here I assume, I lack the knowledge and education to comment on how well the job was done.

I think it’s a fun project and if you decide to build it, I’d follow the thread with interest. What I would find particularly interesting is the different distortion profiles created by swapping tubes in the same circuit. From what I can tell here with my admittedly limited understanding of reading schematics is that you don’t really have one circuit here that you’re swapping tubes in and out of, but Instead you have multiple different circuits that operate within the same box which would make analyzing the data a little bit different.

At any rate… if you’re looking for a forum to get real feedback on such a circuit, I’d check out diyaudio. There’s a lot less bias against distortion there and a lot more people that I think would be qualified to engage with you at the level you’re looking to be engaged.

 

[DVDdoug]

I know a little about tubes but I've never built an amplifier.

From what I understand, "tube rolling" usually involves switching-in tubes of the same type from different manufactures, or tubes that are similar-enough to be considered a drop-in replacements. Sometimes, you might have to adjust the bias when you replace a tube, but with most tube equipment replacing a tube is DIY and no technical expertise is required.

When I was young (1960s) drug stores and hardware stores sold tubes and they always had a DIY tube tester. You could bring-in all of the tubes from your radio or TV and test them yourself to see which one was bad, replace it, and hopefully repair your radio or TV.

In general... A good design will be highly-tolerant of part variations as long as the parts are in-spec. If the sound changes when you swap-in another compatible tube, that's a bad design, IMO. I'm sure if you swap a tube in a McIntosh amplifier the sound will still be "perfect". Your amp has feedback (negative feedback = corrective feedback) and that's one of the keys, if not THE key, to making a good "tolerant" design. And n general, feedback (along with high open-loop gain) makes "everything better"... Feedback helps to lower noise and distortion and it flattens frequency response.

Transistors and MOSFETs also vary but they aren't easily swapped so the circuit design has to be solid in order to get reliable and repeatable performance.

 

[312elements]

In 2024 if the end goal is to not have the tubes effect the sound of the circuit then we've defeated the purpose of using tubes at all. There are far better ways to create a transparent signal.

 

[sergeauckland]

I've done a fair bit of tube rolling, to see if there's any difference between tubes of the same type but from different manufacturers. My findings are that as long as the tubes are within spec, and I do have a tester, there's no measured difference. The circuits I've done this have had overall feedback, so the results might be different for no overall feedback designs, but I can't see why.

Statements I've seen along the lines that ABC has better bass, or XYZ has more detail, I think are rubbish and wishful thinking.

S

 

[Bodis-Green]

Thanks for the replies,

I was not treated well over at DIYAudio and a person attacked my post right away. Followed me over from AK the way he wrote it like a know it all. It is hard to tolerate such people when you know that what you have designed is right on. More like a computerized tube amplifier using all analog in the amplifier itself and able to roll tubes to a fault, that being using tubes that are unhealthy for the amplifier and not within reason This Amplifier is able to adjust input and phase inverter sections together which are never found to be perfect without adjustment as well as 6.3Vac and 12.6Vac for the heaters.

Regards, Tom

 

[GXAlan]

There is plenty of data on tube rolling and tube burn in. Whether or not it is audible may depend on how hard you are driving the tubes…

Fresh From the Bench: Western Electric's New Production 300B

Renowned tube expert Michael Boele offers his long-term review of a pair of 2021 new production 300B directly heated power triodes. For 10 months, Boele had the opportunity to live with and extensively use a pair of these 300Bs and his impressions and extensive testing could be directly...

audioxpress.com

This is one of my favorite articles to cite.

 

[Jim Taylor]

The owner of this site (Amir Majidimehr) has a thread on tube rolling:

Tube Rolling: Does it Make a Difference?

I recently reviewed the Mhdt Pagoda tube DAC. Owner was kind enough to send me three other tubes to test with it. Here are the set of tubes I received: Let's start again with the tube supplied with the unit (GE 5670) Now let's switch to Tesla 6CC42 (GA): Other than slightly more mains...

www.audiosciencereview.com

Since the last time I used tubes was in the '70s, I wouldn't trust my memory enough to comment.

Jim

 

[dfuller]

Tube rolling within a type in my experience only makes a difference when you take them out of their linear region. As such, it matters much more in a guitar amp (where there is ample intentional distortion) than in hi-fi, where everything is meant to be low distortion.

 

[mhardy6647]

It is fairly straightforward to construct an amplifier circuit that can be sensitive to variation in vacuum tube performance characteristics (i.e., at the individual tube-to-tube level) in a given application (e.g., hooked to a given loudspeaker load).

It will require violating numerous design best practices as we might say nowadays.

 

[EERecordist]

Tubes are like bipolar transistors, power field effect transistors, gain devices. Most amplifiers have feedback. Tube amplifiers use output transformers for gain

 

[MattHooper]

In 2024 if the end goal is to not have the tubes effect the sound of the circuit then we've defeated the purpose of using tubes at all. There are far better ways to create a transparent signal.

For my own purposes, I agree with that.
If I want neutral sound, I would use a solid state amp. I would prefer that a tube amp color the sound in one way or another.

However, I can also see the use of a neutral sounding tube amp. Someone may want neutral sounding amplification, but is nonetheless attracted to the aesthetics and concept of tube amps.

 

[MattHooper]

I wouldn’t try and convince someone else tube rolling makes a difference. But I quite enjoy it, because I think tubes are cool, including different types of tubes, and it’s fun to play with.

I have a pair of Conrad Johnson premiere 12 tube monoblocks. At one point I bought another secondhand pair that were somewhat newer and had been serviced more recently. So for quite a while I had both pairs of amps to compare. This was before I ever tried tube rolling.

I felt some shock when I hooked up the new pair and played the first tracks. It was like I got bigger loudspeakers. It seemed like fuller deeper, tighter base had been added, as well as a more expanded airy sound, making the whole presentation feel larger. I certainly wasn’t expecting that (though of course you don’t need to expect something for bias to kick in). Anyway, this was such a persistent impression when I switched between the new and the old amps, I realize the main difference was that the new amps have been supplied with the larger newer KT120 tubes, versus the standard 6550 power tubes in my old amp. So I switched those KT 120 tubes into my old amp and… there it was! The same qualities of deeper tighter bass, change in the highs and sense of a larger presentation. That was driving my Thiel 2.7 speakers.

I got the same results on my Joseph speakers. In fact, using the 6550 tubes, I had played with the idea of adding a subwoofer.
But once I put the K120 tubes in, I felt like I didn’t need a subwoofer anymore. It felt rich enough.

It was after my experience playing around myself with these tubes that I started to look into tube rolling and was fascinated to find that there was a consistent consensus that the K-120 tubes tended to bring just the type of qualities. I thought I was hearing - especially deeper tighter bass. So that was another surprise. (even if it frankly doesn’t make particular sense.)

Before any ASR member blows a gasket, I am certainly not presenting this as any sort of good evidence. And EVEN IF the KT 120 tubes in my system slightly altered the sound, I highly doubt that they actually extended the bass any deeper. IF they altered the sound, I would guess it would be some subtle dip or bulge here or there in the base that simply gave the impression of what I was hearing.

But in any case, I don’t have any objective evidence from my impressions. But I’m OK going along with my impressions because they are very strong and quite satisfying, so if a bias effect is doing the work, it’s doing the work very well, and I like the fun aspect of playing with tubes.

I’ve tried rolling the smaller input tubes as well, with some interesting results that I like as well.

 

[LTig]

Tubes are like bipolar transistors, power field effect transistors, gain devices. Most amplifiers have feedback. Tube amplifiers use output transformers for gain

Nope. The output transformer actually reduces the voltage gain, because tubes cannot deliver much current but high voltage. The transformer transforms it into low voltage with high current, suitable to drive low ohm speakers.

 

[DMill]

I wouldn’t try and convince someone else tube rolling makes a difference. But I quite enjoy it, because I think tubes are cool, including different types of tubes, and it’s fun to play with.

They are super fun to play with. And if you have the money and a great amp like a CJ or a Cary (like mine) that can let you fiddle with rolling tubes it’s a blast. Before I get roasted, I know my other system is better. But I have to invite anyone over to my house to hear what a tube amp can do. It’s pretty damned hard to hear a difference.

 

[SIY]

First off, I hope everyone is having a great day.

I designed and posted this amplifier schematic over at AudioKarma and my first reply was it is a total waste of time and money, just set it up for the tube 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 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

Serious and possibly delicate question: do you want affirmation or do you want an actual critique?

I don't know who hurt your feelings at diy, but if it was EC8010 or PRR, I'd listen to them carefully. If not and you want an actual critique, put on your asbestos shorts.

 

[IEales]

As far as the amp design, it's very versatile, but I wouldn't use it with all the pots and switches in the circuit. If I was building such a device, I have screw terminal block so I could insert the parts I wanted. More tedious to use but far more reliable and repeatable. Another problem with the adjustable parts is track wear. Pots have different thermal properties. Switches are never as reliable as crimped or screw terminal connections. Additionally, once determinations have been made, getting fixed parts to match the adjusted values may be well-nigh impossible.

Tube rolling should be discouraged as repeated insertions and removals create microfractures in the glass which may let in gas.

If rolling NOS tubes, they need to be run on the heaters ONLY for at least 24 hours [or longer] to degas. Some will never degas completely and re-gas rather quickly.

Tubes are not identical and unless very well matched, results will differ depending on their position in the amp and on what is driving them and what they are driving. A tube may be dulcet in one channel and ho-hum in the other.


See tube rolling for examples of new and NOS tube measurements and some other tube facts.

 

[MattHooper]

Tube rolling should be discouraged as repeated insertions and removals create microfractures in the glass which may let in gas.

What’s your view as to how to best remove and insert tubes? I’ve heard a number of different ideas, for instance, giving a very gentle twisting motion in one direction while lifting.

I tried every which way, but to me the method that seemed to be the least hard on the tube sockets with gently pulling the tube directly upward, and if necessarily giving very teeny shaking wiggles if necessary to dislodge.

 

[mhardy6647]

I wiggle 'em a wee bit as and if needed. I certainly wouldn't twist 'em.

 

[SSS]

To my experience it is almost not feasible to remove a glass ground plate tube without little wiggle. Same to the octal power tubes, but these sockets are not sensible since the pins are not directly in the glass tube. Since tubes have a relatively wide tolerance new and due to aging sound can be altered by tube rolling. Especially the power tubes are the key. Front end depends on the circuit whether these tubes have local feedback which stabilizes for a certain operating point.