Triode preamp : Part 2

As promised, here is the current schematic for my tube preamp:

I wrote an (almost) legible version. I'm still making some component changes, so this one isn't completely accurate... but very close. This drawing is for a single channel of course, for stereo operation you must duplicate the design. (Power supply can be shared among both channels).
I also resolved the issue with the lower half of the signal being 'squashed' before the top... I still have a lot to read about tube biasing ;) .


The current output of this design (as seen above) has a fairly constant output across it's frequency spectrum from 20Khz to 20 Hz, so I am pretty happy about that. A 1.5v input gives a consistent 1.5v output. I'm very close to finalizing my first design...

Triode preamp : part 1

My recent purchase of my Harmon Kardon A300 amp has created a serious fascination with vacuum tubes. The sound of this amp is so good and the electronics behind it are so simple. I was able to find the schematic to my amp online, which was very important when I began replacing the caps in it... the labeling on them wasn't the best and it was hard to make out the values on some of them. Looking at the schematic, this amp was so simple! If you took out the electronics necessary for the turntable preamp, and removed unnecessary features such as presence, stereo reverse, and the bass and treble controls (which I always leave flat anyway), there are only a couple dozen components remaining per channel.

I decided I had to start designing my own amp. A few years ago I was fed up with the cheap 1-bit D/A converters found in most CD players and wanted to make my own with nice Burr-Brown 16BIT DACs. I would finish the output stage with a nice tube preamp. Of course I never built it, so I was due. I decided to make a simple preamp with a basic triode.

I first needed some tubes. eBay of course would be my source. The most common triodes such as ones in the 12**7 family are expensive! 12ax7s' go for some serious money, mostly because almost every vintage amp out there uses them and the sources of new old stock 12ax7s' is disappearing. The Chinese are making tubes again, but apparently they sound like ass. My solution is to buy large lots of tubes on eBay for cheap and hope for the best. Besides, there are tons of other triode tubes out there, others will be suited just fine for my purposes.

So several boxes of tubes showed up at my door about a week later showing why drinking a bottle of wine while eBaying always leads to suprises. The good news is everything I bought was only $20 and I had about 150 tubes to work with. In this lot I found a bunch of NOS 5963 triodes. Perfect.

My first design is crude, quickly made, only a single mono channel, but it works!

Yes, it's built on a piece of pine (clear pine that is!). Remember, I already admitted it was crude...

It's totally simple, there are only about 15 passive components. Ill have to upload the schematic once I have a legible version of it that is finalized. I made some component value changes during testing for level adjustments. Here is what the input(top trace) and output(bottom trace) looks like on my scope:


There is some noise in the waveforms, but ignore this as it is from my signal generator which is garbage. The left traces show a 1v peak to peak input and the output of the preamp, which looks very clean... and inverted. The right traces show what happens when the input is increased to 2 volts peak to peak, which drives the tube a little harder. The bottom trace shows the output result which is clipped, or in the case of tubes, more compressed or squashed. This is one of the most interesting properties of tubes. When their headroom is reached, they don't have a hard 'clip' like a transistor, it is a more 'squashed' effect on the waveform, which creates a more pleasing distortion.

Here is another view showing the effect on my scope with an X-Y plot:


I had to turn the scope illumination on to give my camera something to focus on, but you can see the squashed waveform on the bottom right which has the soft curves instead of a hard flat break if this was a transistor clipping.

I still have a lot to play with the circuit, I want to figure out why the bottom of the waveform is squashed a little more than the top. I'm only running the tube plates at about 60V, instead of the usual 120v. (This was a limitation of the transformer I was using for this circuit.) Overall i'm pretty impressed with the sound of it, especially with the complete design and build only taking about 5 hours. Listening to music through it definitely sounded good, and playing my Roland synth through it while overdriving the tube for distortion had some sweet results.

Harmon Kardon A300 capacitor replacement

After almost a month of listening to my new tube amp there was one slight problem I began to notice. The right channel seemed to be a little bit louder than the left. The problem seemed to get worse the more I listened to it. Or maybe I never realized it and now that I have, I listen FOR it which makes it more apparent. Either way... after a lot of checking, swapping channels of signal sources, switching speakers, etc... the right channel was louder than the left.

I figured I had three possible causes of the problem. A single or several bad/weak tubes, an out of tolerance resistor, or a bad capacitor. I started with the tubes. I swapped the 6V6GT power pentodes to see if the louder channel swapped, which it did not. I moved the 12ax7 triodes around as well, with no difference in channels. There was really nothing more i could do without a tube tester (which I find myself needing). But for now I'll assume that the tubes are ok.

Next step will be the capacitors... which I wanted to replace anyway. Lots of browsing websites of tube amp builders and repairers turned up a ridiculous amount of information about the best capacitors to use for an amp. Lots of people recommend things called "orange drops" which provide the best 'classic' sound . Whether this was the technical name of the caps or a description of what they looked like, Digi-Key turned up zero results for 'orange drops'. Now on to the more scientific approach. This site provided me good information on the linearity of different types of capacitors. After looking at this data, it looks like most capacitors would be well suited replacements with the exception of a few(ceramic disc, tantalum, etc...). I purchased high quality Panasonic polyester film capacitors for the signal paths and ordered high quality Panasonic 105 degree C. electrolytics for the power supply filtering while I was ordering parts. The original filtering capacitors were introducing a slight 60hz hum into the audio, and I know a fresh set of electrolytics for the power supply would solve this issue. I have always used these Panasonic high temperature caps for replacements in other devices... always with positive results.

Before:



After:



The results? It sounded better... for sure! The 60hz hum was definitely gone. I always use the Panasonic 105 degree C. electrolytics in all my projects, the extra price is worth it. The overall sound with the new caps in the signal path? Better... definitely. The only problem was my ears were still annoyed at the louder right channel. Time to start checking resistor values. :(

Analog pathway

I have always wanted a real tube based home audio amplifier. The idea has always intrigued me. With today's high end audio equipment and all it's clean transistor based output power capable of producing hundreds or even thousands of watts per channel, why such a fascination with an old obsolete method of amplification? I had to find out. Just the idea of having an amplifier that is 100% analog based with no DSP and no A/D and D/A conversions going on seems so cool. To hear real tube compression and microphonics instead of the usual digital harsh sound we are so accustomed to with todays electronics. To have an amplifier that only produces 12 watts per channel play just as loud as a MOSFET based amp rated hundreds of watts per channel. To amplify such low level signals by using just these mysterious glowing glass tubes, a few large coils of wire, and a handful of passive components... I have to see what they are all about.

I began my search as usual at lots of audiophile pages with owners talking about such and such amp, why it is so awesome, and how much it cost. My search then turned to eBay (my preferred method of purchasing almost anything) to find one I wanted. After looking at a bunch and reading lots of user reviews on the ones available, I decided on a Harmon Kardon A300. It was clean looking and in good shape, inexpensive, and working with all tubes intact. This was important because a set of new tubes (new old stock that is, tubes that were made 40+ years ago that were never used) could easily cost more than the amp itself. Most tubes are still manufactured by a handful of companies today (mostly Chinese), but most people seem to say that they don't sound anywhere near as good as the old Russian, UK, and US tubes made forever ago.


The A300 is a stereo amp with approximately 10-15 watts of output power per channel. It has 9 tubes, 4 6v6 outputs, two per channel in a class A push-pull configuration, 4 12ax7 tubes and 1 12au7 tube in the preamp section. Upon receiving, I hooked it up to my system, replacing my current MOSFET based amp, turned it on and hoped for the best. As for any vintage electronics, there is always a chance for disaster or death with first power on. I looked it over really well, so luckily there was no smoke and there were no sparks... after about 30 seconds I could see all of the tubes glowing so it was time for a listen.

(what's a pcb?)

Now don't get me wrong, digital based equipment sounds good, really good (with the exception of mp3's. An mp3 compared to a well mastered cd on a good system lets us forget how lossy the compression of mp3's actually are). So when I started listening to music through this amp it was amazing. Did it sound better than a non tube amp? Yes... well maybe. It sounded different. So this is the 'tube warmth' everyone talks about, I can't really explain it but it sounded good.

So the next step is to begin upgrading the amp. Replacing all the 40+ year old caps will be a good start... it will be interesting to see if it changes the sound quality at all...