[2359glenn]

You are exactly right @Xcalibur255 , the ideal spud tube is high gain, low plate resistance. That is why the EL3N is a very good choice! With a 3K plate resistance and a mu of 20, it makes for a good single stage tube.

There is something I have thought to experiment with though - it just an idea now, have not even simulated it yet - but using a cascode CCS plate load on the parafeed output, you could potentially take the mu output of the CCS as opposed to the plate, lowering the output impedance significantly. This might make it possible to use higher Rp tubes in a spud parafeed design and get a low output impedance to adequately damp headphones. If it works, could open up more possibilities with high Rp / mu tubes. Maybe I will throw together another 700V PS and try it out with the 841, Rp of 25K, mu of 30 just for funsies though, not sending a 700V B+ amplifier out into the world!

The EL3N is a really good sounding tube also. But to really use it in a spud amp really needs a input transformer to boost the input signal to get a decent output.

 

[2359glenn]

This is the one with the 10 driver stage right? I still want to own an integrated single chassis version of this before I die. I have rolled idea after idea around in my head about what my "ultimate" end game amp would be, and time and time again I keep coming back to 10Y >>> 300B.

AKA difficult and expensive.

The 10Y>300B sounds great had them for 16 years now.
But I don't think I would make this direct coupled amp for anyone as the B+ voltage is 850

 

[L0rdGwyn]

I've had a breakthrough in the 801A A2 amp design, a new path forward.

But first, a mistake was made. Remember when I said I was getting a 0.38ohm output impedance? That turned out to be false LOL I thought it was suspiciously low at the time given that I had not drastically altered the feedback fraction, those results were not reproducible, so something was off at the time.

I am more consistently getting output impedances around 2.8ohm with various feedback setups I have tried with the resistively loaded pentode stage. This is borderline high, I am seeking a damping factor of 3 or more for 8ohm speakers, so 2.8ohms on the output ain't really gonna cut it.

So what to do. Altering the degree of NFB via my cathode trimmers improves the output impedance, albeit marginally, but increasing it too much and the stage does not have enough gain to drive the 801A to clipping. As I mentioned, I have left the cathode of the EF37A unbypassed / degenerated, meaning there is internal NFB, which reduces the gain. I decided to try bypassing the cathode with a 500uF electrolytic to see what it got me...I also added a film cap to my feedback loop to take the DC current out of the equation, making the bias process of the EF37A much more straightforward.

Open loop mu of the EF37A stage jumped from 40 to 100 with the cathode bypassed. Closed loop measurements in this setup below.

THD @ 1W 1%
Output Z: 2.8ohms
-2.4dB @ 20kHz



So we have some improvement in THD, output Z is largely the same, rolloff at 20kHz is unchanged but improved at 17kHz.

How about the subjective sound? A step back, no question, as there is now an electrolytic in the signal path less detail, degree of realism and space has diminished, more "smeared".

So now what? I need more gain out of my input stage to increase NFB, lower the output impedance for proper 8ohm damping, and continue to improve bandwidth. At this point though, I am reaching the limits of what can be done with a resistor loaded stage, using 220K on the plate of the EF37A.

I did spend all that time learning about CCS loaded pentodes...LET'S DO IT

One of the difficulties with using one of these stages is dialing in the CCS to provide the anode current for the pentode in addition to the plate load resistor, which is grounded. The benefit is the ability to use a higher resistor value that is typically practical on the plate of the pentode, continuing to flatten the load line and increase gain of the stage beyond the quoted specification. In addition, the CCS provides the same PSRR you could expect in a CCS loaded triode stage (not that it is needed in this amp, with the Maida regulator + CCS, ripple is being crushed to the tune of >200dB PSRR!).

The disadvantage is DC bias point stability. With a nearly flat load line, small changes in cathode voltage result in big swings in bias point, so there is the potential for bias point drift. I figured if I used a larger plate load resistor without going nuts, I could get away with it. I settled on 470K.

So, the CCS stage was simulated in LTSpice to get ballpark figures, and away we went. Here is what it looks like now. Yup it is an absolute disaster, but hey its a prototype on a piece of wood I needed to throw a 1K resistor in series with my CCS trimmers to get the current down to EF37A levels.



After some tweaking, dialing-in of voltages / currents, we settled on the following EF37A bias with the CCS in place: CCS current 1.56mA / Va 240 / Vg2 95V / Vk 2.77V.

I measured the open loop mu of the stage without a bypass cap, it went from 40 to 125!!! Eff off electrolytic, I don't need you now getting more open loop gain than with the resistively loaded stage WITH bypass. As a result, the input was driving the 801A to clipping and then some, allowing me to further increase the feedback fraction.

So, how about the measurements...

THD @ 1W 0.44%
Output Z 2.44ohm
-1.67dB @ 20kHz



1kHz sine wave at full output, 18.06Vpp into 8ohms = 5.2W



Yeah...big gains across the board. Now will get a >3 damping factor for 8ohm speakers, further HF extension, lower THD, and I got my 5 watts.

The sound is back to its airy, realistic, hyper-detailed state with the electrolytic out of the picture.

I monitored the plate and cathode voltages of the EF37A with music playing at varying volumes, did several shut-downs and start-ups as well - the bias does not drift, it finds its way right back to the same point every time.

So this is the way forward, keeping the CCS, more tweaking and optimization to come, but it is undoubtedly an improvement.

Just to show what further gains could be had measurements wise if the plate load resistor is further increased (which will in turn increase the gain of the stage and NFB), here are the measurements repeated with the cathode bypassed with the 500uF electrolytic with the CCS in place.

THD @ 1W 0.24%
Output Z 2.15ohm
-1.19dB @ 20kHz



Will continue to improve the design, but it's time to call it a day!

 

[L0rdGwyn]

I have one more result to share. For the above, I included a film cap in the feedback loop to block DC from the 801A plate. However, I have seen feedback implemented without a cap.

No doubt including this capacitor degrades the sound, I was aware but didn't realize how significantly until I removed it again and got the original sound back, keeping it out of the loop and creating stable NFB without it is now imperative. What's more, the is the first time it has been removed from the loop and the CCS pentode stage has been implemented. I adjusted the trimmers to account for the DC current from the 801A plate, which is now part of the bias. This rebias resulted in additional gain, and I was able to further adjust the feedback fraction. This is my best result yet, objectively and subjectively speaking, this is the best it has ever measured or ever sounded.

THD @ 1W 0.25%
Output Z 2.14ohm

In my excitement, I didn't measure the rolloff at 20kHz, but this is the widest bandwidth I have seen, looks to me to be about -1dB at 20kHz.



The goal now is to maintain this sound while ensuring amplifier stability, there are now a few semi-exotic circuit elements in this amplifier, have to be sure they are going to play nicely together long term.

 

[Xcalibur255]

I wonder how many people would have put their first iteration into a chassis and listened to it that way for a long time. This has been an inspirational read in so many ways, how you keep going back again and again for more improvement and keep succeeding. In a pretty darn short period of time too I might add!

 

[L0rdGwyn]

Thanks @Xcalibur255 , that's very nice of you to say. It isn't all sunshine and rainbows though unfortunately, even just today I have new issues to troubleshoot. Many times a new change raises more questions than it answers! So it will still be quite a while until this design is done. But I know what it is capable of and that is motivating, just a matter of making all the pieces work in concert. I will be happy and relieved when everything is finalized and I just have to work on the chassis, which will be a challenge in and of itself. Have to admit getting a little fatigued, but I hope all of the posting here has been enjoyable to follow.

 

[johnjen]

The 1st iterations of our 45 amp went thru, too numerous to mention, wiring layout changes and circuit modifications.
And then we scrapped it and tried the 2nd iteration in a whole new chassis which never really gelled.
The latest iteration of the 45 amp will be considerably smaller and less cumbersome all the way around.

And the Purp-Amp 71A/6BQ7A HP amp has also gone thru numerous circuit 'enhancements' and re-wire changes.
It has taken months to dial it all in and find out what does, and does not work, and why.

And once the next generation of gear is up and running, the Purp-Amp will get 're-freshed' with new selected critical parts with even further refinements added to 'complete' the build to make it a 'finished' amp.
Right now it’s a 'hobbyist' amp, meaning it is 'fussy'.
For me that isn't really a problem but for a usual audioist it wouldn't be considered a 'finished product'.

Although it has reached a degree of SQ that makes it suitable as a 'reference' for comparison to the up and coming new kids on the block.
Namely a 26 pre-amp, a 45 HP amp, a 300B HP/Spkr amp, and a pair of derivative Western Electric 124 mono block Spkr amps, and more.

In motorcycling it’s the journey, not the destination, that is the portion of the whole experience that is most memorable.
Audio gives us both facets, the journey and the destination, as each yields 'the gud stuff', from the whole experience.

JJ

 

[L0rdGwyn]

Thanks for your thoughts @johnjen , looking forward to seeing the new 45 amp come to fruition! Certainly the experience is just as important as the destination. This amplifier has turned into a test bed for experimenting with several new circuit elements, so no matter the outcome, I will be more knowledgeable on the other side and the better for it

 

[johnjen]

Learning curves are like that, you get to learn what does work, and also what works better, sorta after the fact, which can be applied in v.2.

I have found that learning what NOT to do is more than 50% of the learning curve in terms of being valuable in a build, and most of this isn't found in books nor writeups.

For instance, is it 'better' to ground each component (input connector, input xfmr, volume control etc.) separately?
Or tie them together at a 'key' point for a single 'star' ground connection?
This example along with grounding issues and related specific design implementations have a propensity to be mostly a matter of learn by doing, and in more cases than not, on a case by case basis.

It's all FM (Fricken Magic)…
hahahahahahahahahahahahahahahaha

JJ

 

[L0rdGwyn]

@whirlwind your modified GOTL is done! Sorry for the delay, when you told me the ECC32 was one of your favorites, I had to go back and be sure the new bias point with the CCS load was a subjective improvement. I am happy to say it is, very noticeable gains in soundstage and clarity, so I think you will be pleased. Using the CCS load will get the load line out of the "knee" of the ECC32 curves, so that is a large part of the improvement I am sure. Obviously the amp was not built for ECC32 it is not an exact equivalent to the 6SN7, so balancing the two bias points was the goal with the available B+, very happy with the result.

Here is the interior just before closing.



It will be on its way home tomorrow. Since I no longer have an amp for them, had to have a listen to some very rare tubes in my collection, the Tung Sol 7802. Sounds great with the ECC32

 

[L0rdGwyn]

Some of my powder coating equipment is starting to arrive. Unfortunately, seems the COVID situation has inspired many people to start powder coating, so a significant amount of the gear I purchased is backordered. It will be another couple weeks before I can start working on it.

Here is one important part though, the largest toaster oven on the market! From my research at least. Has to hit 400F to bake the parts after coating.



The interior of the oven will dictate the largest dimension chassis I can coat, the reason it is so big. Here is the top plate of the new 6A5 amplifier chassis inside, it is 15 inches wide. I can probably go as large as 16.5in. Works for me, I'm not very interested in building amplifiers larger than that anyway.



I have the powder as well, I ordered 1lbs of two different finishes, they will go on test parts to see the in person result before a final decision is made.

Stage Pass Bronze
https://www.prismaticpowders.com/shop/powder-coating-colors/PMB-8157/stage-pass-bronze



Yankee Gold
https://www.prismaticpowders.com/shop/powder-coating-colors/UMB-1807/yankee-gold



The Stage Pass Bronze photo doesn't really do it justice, Prismatic Powders has YouTube videos for each finish showing a part being moving around to display how it changes in different lighting, which really makes the bronze pop.

 

[L0rdGwyn]

So, I thought it was going to be a couple weeks until I had the powder coating equipment based on an email I received, turns out it is being delivered tomorrow. But I have to work, and will be out of town this weekend, so I probably won't get to it until late next week.

Regardless, I figured I should go ahead and get the prep work done for the new 6A5 amp chassis. Not trivial, as the layout is changing, so I had to dismantle the old version, figure out exactly how everything was going to fit in the new version, mark up the chassis and do the drilling. Got started yesterday evening, then worked on it another 4-5 hours today.

But it's done! So now when I have the time, I can jump right into the coating process.



I will have to do some final prep work the day of the coating, buffing out some minor scratches in the raw metal, cleaning, then going over the surface with a tack cloth. Holes will be filled with silcon plugs and certain portions masked with high temperature tape. Will have some practice with scrap metal first.

Fortunately, when the coating is done, putting it back together should be relatively quick since a portion of the original wiring was left intact.

Very excited to get this amp back up in running, improved, and I can finally try out my Mazda AC/HL drivers.

 

[Xcalibur255]

The list of tubes you can do this with is actually quite large. It just all depends on what you want and how you want to do it.

First step is to figure out what headphones you are trying to drive. Lets look at the HD600/HD650 headphones as an example. I will skip the math and the technicalities and just sort of skip straight to the answers so that you can better understand what is going on without getting confused.

A HD600 has a sensitivity of 105 db at 1 volt. In order to hit 120db peaks, we need the amplifier to provide 6v peak to peak with headroom, so we will probably want to design the amp so that it can output 7v peak to peak. We know that ohms law tells us that the current we need at 7v into a 300ohm headphone is 23.333... ma. Lets just say we need 30ma as a fudge factor.

Since most dacs these days will output 2v RMS which is ~2.83 v, we need an amp with a total gain of about 2.5ish. Again, lets call it 3 for the sake of having plenty of margin to play with.

Now you need to find a combination of a tube and output transformer that will yield a total gain of 3 with a bias current of 30ma (you actually don't need 30ma for a number of reasons, but as you know by know, I like have a ton of margin in my designs).

Keep in mind that a 5K to 300ohm transformer has a voltage ratio of 4, and a 10K to 300ohm transformer has a ratio of about 6. So we need to look at tubes with a gain of somewhere in the mid teen range. A pro tip here is to choose a twin triode tube that can be paralleled. I normally don't like paralleling tubes since it is kind of a pain to find twin triodes that are well matched, but paralleling tubes will actually reduce their self noise by 3db. Considering the low winding ratio of the transformer, we need all the help we can get in order to make the amp quiet.

The ever venerable 5687 would be the perfect tube for this application. In parallel the bias current would be above 40ma, the gain is about 17, and the plate impedance would be reduced to about 1K.

I would use two united SIC Jfets as a CCS for the plate load (about. 15 bucks per channel), and a edcor xsm 15K to 600ohm transformer that you would be using as a 7.5K to 300 ohm transformer which would cost another 15 bucks per side.

Alternatively, you could use tubes like the 6E5P with an unbypassed cathode resistor to reduce gain and increase plate impedance to match the same configuration as the 5687 for even less money.

If you have a few minutes to spare would it be possible to walk us through another scenario like this?

Let's assume we want to build an integrated speaker amplifier with an output impedance targeted for 8 ohm load speakers and input sensitivity appropriate for being driven by a typical 2V output DAC. If we want to use the KT150 as an output tube in this amplifier in a triode connected application (setting aside for the moment whether or not this is a good idea and just assuming it as a fact of the design), what would we be looking for in the ideal driver stage tube assuming we want the amplifier to be *reasonably* affordable and a 2-stage design that would be comfortable driving a speaker anywhere in the 88-98dB sensitivity range to reasonable volumes?

 

[leftside]

So, I thought it was going to be a couple weeks until I had the powder coating equipment based on an email I received, turns out it is being delivered tomorrow. But I have to work, and will be out of town this weekend, so I probably won't get to it until late next week.

Regardless, I figured I should go ahead and get the prep work done for the new 6A5 amp chassis. Not trivial, as the layout is changing, so I had to dismantle the old version, figure out exactly how everything was going to fit in the new version, mark up the chassis and do the drilling. Got started yesterday evening, then worked on it another 4-5 hours today.

But it's done! So now when I have the time, I can jump right into the coating process.



I will have to do some final prep work the day of the coating, buffing out some minor scratches in the raw metal, cleaning, then going over the surface with a tack cloth. Holes will be filled with silcon plugs and certain portions masked with high temperature tape. Will have some practice with scrap metal first.

Fortunately, when the coating is done, putting it back together should be relatively quick since a portion of the original wiring was left intact.

Very excited to get this amp back up in running, improved, and I can finally try out my Mazda AC/HL drivers.

Following along. Mischa and I are also considering different coating/case options. I want my next amp to have a similar coating/case/appearance as my DAC.

 

[Zachik]

Following along. Mischa and I are also considering different coating/case options. I want my next amp to have a similar coating/case/appearance as my DAC.

Can you share some details on said next amp?
(I am sure LG won't mind "hijacking" his thread, especially since we're discussing DIY amps after all...)