May 27, 2024

Sylvania 6A5G conversion

A few months ago, I built a 41MXP amp with the intention of using it with my tweeters along with an active crossover. The results were not as expected. Not to say this amp sounds bad, the 41MXP is a clean and clear triode with an enlightening treble. It’s more about the sonic results I had with multi-amplification. The stage appeared thinner, and the overall presentation was less convincing and natural than with my passive Xover.

After careful consideration, I decided to take it apart and use transformers, chokes, and outputs for another project.

It didn’t take long for me to get a nice pair of 6A5G triodes out of my stash. These tubes are said to be fine-sounding triodes, sometimes compared to a 2A3, with a unique internal construction. It’s an indirect-heated tube with a cathode tied around the heaters. What an oddity! I believe it was very hard to build. The tube has been engineered that way with the goal of reducing hum while keeping all the characteristics of its relative, the 6B4G. And it works. According to some people, it’s a delicate and detailed tube. It is a good reason to give it a try, especially since I wonder why I did not use it sooner. In fact, I remember buying them after reading good reports about the Peerless A100A amplifier. But this is another story.

I have completely redesigned the driver. Replaced the ML4 with a 6SN7 in a shared current stage that proved to be an excellent solution, providing high gain under low impedance.

By experience, I found driving a power triode under low impedance to be a good solution for better bass restitution and a more relaxed sound. At least when CF is properly designed to prevent overdrive by the preceding stage.

I kept the Tango anode choke and added a resistor to the circuit. This combined load ensures proper DC voltage while keeping AC gain higher than with a resistor alone. Transients are much better, too. Current transfer in such a complex time constant depends upon the choke/resistor ratio.

τ = L/R

To make a long story short, the smaller τ, the faster the steady state of the circuit of the circuit will be established. Means that changing R will affect the sonic behavior of the amplifier.

It took a lot of time to find the best value, spending hours listening to different setups with loads ranking from 27K to 68K. I finally stopped at 40.2K. Any 39K will do the job, but I had the 40.2K on hand, and these vintage metal films sound very good.

To get the necessary voltages, I replaced the PT220 power transformer with a PT260 and recalculated a few resistors in the HV line to keep time constants as low as usual.

I didn’t exactly bias the 6A5G according to the datasheet. I chose another point with a lower anode voltage and a 750 ohm bias resistor.

The reason is that I wanted to test the French R120, as I have a few samples on the shelf. A similar triode that bears an excellent reputation among tube lovers. Despite a completely different construction, the characteristics are quite similar. A simple bias change will allow me to enjoy these superb triodes, originally intended to fit professional equipment. The recommended value is 600 ohm. I implemented a switch to connect a 3K resistor in parallel with the 750 ohm one.

Filaments are floating due to a center tap connection on the 6A5G.

Wiring is kept as compact as possible using parts I had on hand: Aerovox V161, a longtime favorite; Sprague Koolohm; Allen Bradley's; and Sic Safco low ESR electrolytics. The power supply is fitted with ICAR polypropylene capacitors along with SEL metalized paper.

The 6A5G is renowned for its noise-free restitution, but it is a somewhat intriguing tube, depending on the manufacturer. I have a few Visseaux on hand, and there is a persistent hum, apparently for no reason. Different construction? It’s a pity; they sound better than Sylvania. Mysterious issue.

I made some measurements to see how these tubes perform. The squares are good, and the slight overshoot at 10 KHz is due to transformer ringing. The vintage Tektronix 453 I just bought (what an incredible construction) has a very fine trace and is very pleasant to use despite its small cathode ray tube. Up to the task for this use and much more compact than my imposing 7834.

Maximum power is 3.5W and 3.2W, respectively (6A5G_R120). The harmonic distortion versus power is exactly as expected.

At first glance, they both share good sonic qualities, but after a long time of listening, they show quite different personalities. It depends on music and mood. The R120 is crisp and punchy compared to the 6A5G, which appears a bit mellow. Thus, despite a magnificent medium, the 6A5G proves to be less attractive than the R120 on human voices; the message is slightly confused, and I prefer to keep it for piano or violin, where it adds softness and romanticism. In my opinion, the R120 is better suited for complex music with its clear and detailed reproduction. The 6A5G plays in a more relaxed atmosphere, ideal for ambient music, easy to listen to for hours, but globally lacks punch and vivacity. Conversely, the R120 is more nervous and lifelike, an attention-grabber whatever I spin on my Garrard. This is the tube to go on small jazz formations, vocals, big band, orchestral fact on any kinf of music.

Despite higher harmonic distortion, the R120 sounds better to my ears and will remain the tube of choice in this amplifier.

Driver choice

I tried half a dozen 6SN7/VT231 tubes to finally end on their predecessor. A friend of mine gave me a nice NOS pair of Tung Sol VT99 with adapters. Once plugged in, it became evident that the R120 had found its counterpart, and the search was over. These double triodes excel in any aspect of music restitution: balance, delicacy, precision, naturalness, etc. Only the Ken Rad VT231 outperforms the 6F8G in the low register by a slight margin.

I guess one day I'll have to rewire the sockets to accommodate the different pinout, but I feel a bit lazy. I prefer spending time with music for pure enjoyment.


  1. PWK aka MrKlipsch designed a 6a5g PP amplifier in 1946 with built in electronic crossover. We use it on the 1st Khorn with WE555 on mid and a Jensen 12 in the khorn LF. Too much fun.

    1. Hi Ian, you are absolutely right about Paul Klipsch speakers and unfortunately lesser knowned amplifier. This man built some of the most glorious speakers ever. Thanks for your comment and interest. Fred