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Modern Balanced Tube Amp Build
Dec 25, 2017 at 4:49 AM Post #77 of 189
SonicTrance said:
Thanks, baron!
So, when are you starting to build this amp? :wink:
Click to expand...

Next week!!!

Yes it's been good, I never thought I would do anything like this. Maybe this is just the start of an exciting adventure.

Merry Christmas guys,
 
Dec 25, 2017 at 6:12 PM Post #79 of 189
Happy Christmas from me too.

Regarding tube selection for this schematic specifically:

You probably need to change your way of thinking a little bit. Instead of getting a tip of what's hot right now, simply look at some datasheets. If you don't know where to start, look at some eBay stores and start to look up tube datasheets from Frank's for example.

Any tube that has just about the right mu (3 to 20 depending on your needs; input tube mu and source level), just about right rp (800 to 6000 ohms, 1k recommended if no output buffer as Sonic explained) and just about right gm (if the two conditions mentioned before are met, this is met as well).

No need to find "a perfect tube" with regards to anything else besides good, linear curves. All else is just fine if somewhat in the parameters mentioned above.

In short: look at the curves. They tell everything.

Some beam tetrodes and pentodes don't have triode curves listed. Then you can make an educated guess based on a number of variables. This is a longer post and maybe I won't get into this here.

I strongly recommend that you use some sweep tube. There are some with rather big envelopes as well. Make the signal chassis so that if you get the urge, you can fit other socket there later. Also leave 50% of the PSU chassis empty for this same reason.

The DH version is not sensible use of money and effort, at least for a first complicated build. Maybe second build. The 6P31S version sounds just as good, the DH part is just so you have DH, if I'm being a bit rude.
 
Dec 26, 2017 at 8:48 AM Post #80 of 189
Ok, so I've made my first ever power measurement on this amp:

20171226_142732_HDR.jpg


Excuse the blurry pic. This is right before clipping with a 3.9R load @ 1kHz. So 0.48 Wrms of output. So quite low output power, but it's more than enough with this amp! Was planning to test with different loads but broke the output buffer on the first try! Doubt I'll do this again after repair.
 
Dec 26, 2017 at 11:48 AM Post #83 of 189
Hey Sonic, when you have the time are you going to fill us in with more detail on your design, how it differs from your earlier amp, the rationale behind it, what stage you are at etc. I gather you are testing now so let us know how it all goes.
 
Dec 27, 2017 at 6:48 AM Post #84 of 189
baronbeehive said:
Hey Sonic, when you have the time are you going to fill us in with more detail on your design, how it differs from your earlier amp, the rationale behind it, what stage you are at etc. I gather you are testing now so let us know how it all goes.
Click to expand...
The differences between this amp and the previous build are the tubes and layout of the build. Last build had input stage in one chassis and output stage in another with separate psu's. This one has one chassis for psu's and filament supplies. And one signal chassis. Much, much better layout!

The first amp had oscillation problems in the psu's. Got rid of most of it when I took out the rectifier tubes and replaced them with diodes, but still the oscillation came creeping back. So I decided to start from scratch and build a new, more thought out amplifier. Also wanted it to drive speakers, which it does flawlessly! I have a pair of Monitor Audio Bronze 2's hooked up to it in my lab/listening room. Sounds very good!

Tube choices (6C8G + 47) were based on tube characteristics as well as aesthetics. I chose DH output tubes just because I wanted to go "all out" even though same fidelity can be had with IDH tubes. Also I'm a tinkerer and love projects and challenges!

Amp is complete. I've used it daily for a few weeks. I just now got time/energy to do the power output test.
 
Dec 27, 2017 at 7:50 AM Post #85 of 189
SonicTrance said:
The differences between this amp and the previous build are the tubes and layout of the build. Last build had input stage in one chassis and output stage in another with separate psu's. This one has one chassis for psu's and filament supplies. And one signal chassis. Much, much better layout!

The first amp had oscillation problems in the psu's. Got rid of most of it when I took out the rectifier tubes and replaced them with diodes, but still the oscillation came creeping back. So I decided to start from scratch and build a new, more thought out amplifier. Also wanted it to drive speakers, which it does flawlessly! I have a pair of Monitor Audio Bronze 2's hooked up to it in my lab/listening room. Sounds very good!

Tube choices (6C8G + 47) were based on tube characteristics as well as aesthetics. I chose DH output tubes just because I wanted to go "all out" even though same fidelity can be had with IDH tubes. Also I'm a tinkerer and love projects and challenges!

Amp is complete. I've used it daily for a few weeks. I just now got time/energy to do the power output test.
Click to expand...

I see, thanks for that!

I like the layout with separate PSU, sounds ideal.

I don't know about your Monitor Audio Bronzes but I would never change my Silver's, I love the refined sound of them, nice and detailed, balanced with just a hint of warmth.
 
Dec 27, 2017 at 8:51 AM Post #86 of 189
Hey Sonic although your measuring power that is RMS and the unit is wired in triode mode and balanced and is class A right?

I was assuming it should be as powerful as an amp using 2a3 tubes.
Edit:
Never mind 2a3 is a bit more overall power depending...
 
Last edited:
Dec 27, 2017 at 9:11 AM Post #87 of 189
Most 2A3 amps run 60mA or so current. This amp has 15mA per current buffer. In PP a 2A3 amp would have about 120mA of current capability in theory, per channel. This amp has about 30mA per channel theoretical maximum.

Also 2A3 amps usually have Va of 200 or 250V. This amp has Va of about 170V.

Power is made of current and voltage; get more of those and you get more power. However, VOLUME is not necessarily made out of power only. Sonic's amp is probably able to provide the same amount or maybe more of CLEAN listening volume. Volume is mostly not about RMS power, rather it's about transient delivery.

That is what this amp does incredibly well (in my opinion); transient response. I have not heard a faster sounding amp at any price point in any High End event.

If transient response is ok, RMS power is an afterthought.
 
Dec 28, 2017 at 9:41 AM Post #88 of 189
I read in the 47 data sheet that the tube may emit some blue glow.
Is it visible?
SonicTrance it would be great to see some pics of tube glow..
 
Dec 28, 2017 at 10:43 AM Post #90 of 189
Blue glow is gas inside the tube. It glows when it heats up.

It's either trapped there during manufacturing or leaked in gradually after that. In the case of the first, it's rather benign, in the case of the latter, it means the tube is losing vacuum, there is a leak. Gas inside tube is one of the signs of failure.

Gas inside tube can cause problems. It is attracted to the anode and the grid. If enough foreign material attaches to the grid, it may shift the bias point and also impact linearity. To prevent this in later production tubes, some of the heavy duty (such as TV sweep tubes) types had gold plated or solid gold grids. Gold is much more unreactive, so no foreign material gets alloyed with it.

In short, the blue glow is often not a horrible thing, but it is nonetheless unwanted. If your tubes don't glow, be happy.
 
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