The Octal Preamplifier

Hi!

since I mostly write about my separate phono and linestages, many people are not aware that I also have a full function preamp in the offering which has a phono stage integrated. The Octal Preamplifier.

The Octal Preamp has a transformer coupled lien stage with auto former volume control. In this Mk2 version the linestage uses the 6AH4 triode.

The phono section is a LC coupled circuit with passive split RC RIAA.

6SC7 is used as input tube and 6N7 in the second stage.

Signal section and power supply are in separate chassis.

In this case the preamp is built in the classic landscape chassis but could be done in portrait stye as well.

A lower cost alternative to my D3A and 10Y combo.

Best regards

Thomas

The Octal Preamplifier Mk2

Hi!

Since I just finished another build of an Octal Preamplifier in the Mk2 version, I thought this is a good opportunity to write again about my Octal Preamplifier series.

The Development of the first Octal Preamplifier started back in 2008. After I published the 6CB5A amplifier concept on a german tube forum, the question about a matching preamp came up. Since the 6CB5A amp got built in various incarnations including some no compromise versions, The question about an improved Octal Preamplifier came up as well. This led to the Mk2. The details about the design can be found here.

After all the line stages with directly heated tubes and LCR phono stages, it was refreshing to work on this design again. The circuit is unchanged from previous versions. Also the chassis design is virtually the same classic landscape layout. With just the exception that the tubes on the signal section are now on a suspended plate to reduce microphonics.

The use of such high mu and low transconductance tubes as the 6SL7 or 6SC7 as in this version, as first tube in the phono section gets unjustified criticism by some tube amplifier builders recently. The claim being that only high transconductance tubes will yield a low noise design. But in real life the few dB difference in noise performance which can be gained by using high transconductance tubes is rather minor. The 6SC7 performs nice and quiet in this design. But it doesn't have the diva like behaviour which tubes like the D3a or WE417 can show. If not taken care of they can burst into self oscillation which can cause rather harsh sound.

The 6SC7 is complemented well by the 6N7 which is used as second stage in the phono section. RIAA equalisation is done by split passive RC networks. The beefy 6AH4 is performing in the line stage. It drives a Slagle/Intactaudio AVC through a step down line output transformer.


The volume control is done by a 24 position Elma rotary switch which selects the taps on the AVC and provides 2dB steps. There are three line inputs and one phono input for MM cartridges. MCs can be connected through an external step up transformer. The MC transformer can be integrated as well but the person who is going to use this preamp already has a selection of step up transformers.

Power is provided to the signal section from a PSU in it's own chassis. As a special touch the power supply has a hybrid rectifier bridge using two 816 mercury vapour rectifiers in conjunction with two 6AX4 TV damper tubes.

The 816 is the 'small sister' of the 866A and provides a similar eerie blue glow in operation. More about this tube in the upcoming tube of the month post.

Due to the 6AX4s controlled warm up, no separate filament and high voltage switches are needed. The high voltage comes up gradually as the heaters of the 6AX4s warm up. This has the nice visual effect that the blue glow appears gradually. This is illustrated in this short video clip:

How about the sound of this preamp? As it is playing right now I am enjoying nice vocals, beautiful tone colors and a smooth sound. The 6SC7 and 6N7 combo in the phono section are performing without any harshness. No listening fatigue. The overall representation tends a very slight bit to the caramel side compared to other preamps. But in a very subtle and nice way. The line section brings all the advantages of a low impedance transformer volume control. High resolution and transparency. The low output impedance can drive any cable lengths and pretty much any power amp.

So why bother with separate phono and line stages, LCR EQ and directly heated triodes if this design performs so well? Of course such separates bring yet another level of performance. A D3a or EC8020 phono does have a more quiet back ground and improved articulation of sibilants. A direct heated line stage does give a higher level of resolution and stronger tone colors. But all that at 2.5 - 3 times the cost or more depending on some choices and twice the rack space needed. If the budget does not allow for a full fledged LCR phono and DHT line stage, the Octal preamplifier is a smarter choice, rather than trying to take short cuts and choosing a cheap implementation with LCR and DHTs.

The photos above do not show the final wooden enclosures. These are still in the making. The wooden frames on the photos are just temporary and the chassis will teals get some feet to rest on. This preamp will be used with a 6CB5A amp for which it was originally designed. The 6CB5A amp is also finished und currently undergoing testing. It will be shown in an upcoming post. Stay tuned!

Best regards

Thomas

The Octal Preamplifier Mk2: Circuit Update

Hi!

Here is a minor update to the circuit of the MK2 version of the Octal preamp:

This is only a minor change. The high voltage for the phono section is better picked directly from the HV input from the PSU, rather than the linestage decoupling cap. The latter connection can result in some low frequency oscillations, depending on the DC resistance of the chokes in the filter chain. The scheme as pictured above is free from that. Of course the PSU needs to deliver an adequately smoothed supply voltage. I use a LCLC filtered supply with high inductance chokes. The last cap on the PSU output can be left in the PSU chassis, but can also be placed in the preamp section after the PSU input.

The optional bypass caps are left out in this version of the schematic. I don't use them. With the inductance values of the plate chokes as shown, they are not needed. Of course experimentation with this is encouraged!

Best regards

Thomas

Tube of the Month: The 6SC7

Hi!

This months tube is a indirectly heated small signal triode which I chose as the input tube of the phono section of my new version of the Octal Preamplifer. The 6SC7.

The pictures above show 6SC7GT tubes which are branded Western Electric on the box. But the tubes inside were manufactured by Tung-Sol. In this case WE didn't even bother to put their own label on the tubes themselves. Rebranding was common practice. In this case WE probably had a contract to deliver this tube type to the military. They bought the tubes in bulk from another manufacturer and put them in their own boxes.

The 6SC7 is a dual triode with a common cathode. With it's amplification factor of 70 it classifies as a high mu triode. Plate resistance is 53kOhm. I use it with both halves in parallel, which yields half the plate resistance or 26.5kOhms. This makes the tube very suitable for a phono input tube. Detailed information can be found in the datasheet.

The tube comes in several versions, either with a metal enclosure, or in a glass tube. The latter version is called 6SC7GT. It is also available with a 12.6V heater as 12SC7.

Unfortuntaley the 6SC7 is quite rare and thus expensive. Fender used it in some of their guitar amps. As far as I know no 6SC7s are currently beeing manufactured.

Below a metal 12SC7 from Ken-Rad:

The 12V version is more commonly available. Luckily I could score a carton full of them:

In my Mk2 version of the Octal Preamplifier the 6SC7 proved to be perfectly suitable for phonostages. It replaces the ubiquitous 6SL7 which I used in the Mk1. It gives a very detailed yet smooth sound. Exactly what I'm looking for. Lot's of tone and color. No problems whatsoever with microphonics, which is important in a phono stage. Both halves paralleled give a low enough plate resistance so that inductive loading is possible. And that's how I'm using it, with a 800Hy plate choke. This yields it's full mu, so it provides lot's of voltage amplification. Exactly what you want as the first stage in a preamp.

The plate curves give an indication why it sounds so good. They are very evenly spaced which promises low distortion. The photo below shows the plate curves which I took from one of my samples with a curve tracer:

If you are looking for a high mu tube and want to use something else as the 6SL7 tube or if you look for an Octal based alternative to a 12AX7, the 6SC7 might be the choice for you. If you can't find any, check for the 12V heater cousin. They are just as well performing.

Best regards

Thomas

The Octal Preamplifier Mk2: Assembly

Hi!

As promised in the last post with the circuit, here some photos of the assembly. Let's start with a picture of the finished preamp:

Construction starts with the predrilled and engraved plate which will carry all parts:

Mounting of sockets and connectors:

Capacitors attached to the plate as well as input selector. Some initial wiring, heaters and input signals to the selector switch:

More wiring, adding of ground bus and some resistors:

Transformer volume controls added and wired. Also the first plate choke. Since I didn't have the intended Lundahl chokes at hand, I used 3 small chokes in series which yields the same inductance. MC step up transformers are also mounted already:

Everything completed and tested. Due to the use of 3 small chokes for each plate load, things got quite crowded. RIAA circuit is underneth the chokes:

Preamp inserted into the wooden frame, it just fits in:

Finished preamp, PSU is in an external chassis, not shown:

After some burn in, the preamp opened up a lot. In fact it is remarkably better compared to the Mk1 version. Especially the phono section, but also with line level inputs it is improved. The 6SC7 proved to be perfect for the phono input. It is very quiet. Also the 6AH4 turned out to be an excellent performer. I like it much more than the 6SN7. The preamp sounds very detailed, yet musical. I'm especially critical about the articulation of s-sounds in the phono section. It performs very well there, approaching the level of a LCR phono. Of course a well executed LCR phono will be better yet, but the result is truely excellent for a RC RIAA. It turned out to be better than I expected.

Best regards

Thomas

The Octal Preamplifier Mk2: Circuit

Hi!

In the first post about my Octal Preamp I already mentioned that I have an improved circuit in mind. In the meantime the circuit has been worked out and is already tested. The main reason for a change was the gain in the line section which is a bit high in the Mk1 for the ususal line sources. So a different tube was chosen instead of the 6SN7. The Mk2 uses a 6AH4 there. The phono section is completely reworked, going to LC coupling rather than RC coupling.

Here is the schematic of the Octal Preamplifier Mk2:

Quite a radical change, with lot's of iron! Since the line tube was changed to a less common tube, why not also change the phono input tube to something less often seen in audio? So I picked the 6SC7 instead of the 6SL7. Still all octal tubes, so the name of the preamp can remain.

Let's start with the phono section. Why go to LC-coupling? The 6AH4 line section would require a lower B+ compared to the 6SN7. This would offer too little headroom for the Phono section to operate with good overhead and reasonable operating points, so LC is the way to go. Lundahl has some nice plate chokes, the LL1667 which offers very high inductance. When chosen with a 5mA gap, it offers about 800  Hy of inductance. Enough for the rather high rp of the phono tubes. Even enough to skip the cathode bypass capacitors. Still the caps are drawn in in grey as an option. My first version of the preamp left them out. Works nicely. To keep the influence of the missing bypass caps low, rather lowish cathode resistors are chosen. The RIAA circuit is the same as in the MK1, split RIAA, but the component values needed some adaption. The second phono tube is still a 6N7. The triode section of both 6SC7 and 6N7 are paralleled.

The separate chokes in the B+ lines for the two phonosection are a bit overkill. I just threw them in for the fun of it and since I had some small suitable chokes available. They can be replaced with some resistors as well, about 5k to the 6N7 and 10k to the 6SC7 will do as well. The first choke should be kept for a low ripple B+ and decoupling per channel.

The preamp is playing right now and sounds very beautiful. Good resolution, musical flow, colors in tone. All is there. Not at the level as the LCR  phono which I showed in an earlier post but that was expected. I equipped it with a built in MC step up transformer, the Lundahl LL1933 wired 1:16.

The line section uses the Lundahl LL1692A which fits better to the 6AH4. Volume control is a AVC from Dave Slagle as in the Mk1 version.

The preamp is quite a bit more elaborated as the Mk1, so it won't replace it but is rather a higher quality option of the Octal Preamplifier series.

I will post another article with some photos of the building process in an upcoming post. Stay tuned!

Best regards

Thomas