All rights of photos and text reserved. Usage of photos or text from my blog on other websites or for any other purpose only with prior permission. If you want to use any material from my blog please contact me by email.

Tuesday, June 26, 2012

Making of a 6AH4 Line Stage: Assembly


As with the 6CB5A amp, which was recently presented, this second part of the 'Making of the 6AH4 Line Stage' will show the assembly process. First a photo of the finished preamp:

The circuit was already shown and discussed in the first part. This post will show how the components are arranged and wired. In the first step, the metal plate which holds all the parts is fixed in the assembly rig and the first parts get mounted: power transformer, capacitors, connectors and tube sockets:

The rig allows handling of the preamp during construction without the danger to scratch the varnished caps or damaging the plate. It can be turned at either side for easy access. The next photo shows the wiring of the power transformer to the rectifier sockets and the heater wiring to the signal tubes:

Next the inputs are wired to the input selector switch and from there to the 6AH4 tubes:

Chokes, line out transformers and transformer volume controls are mounted on a second tier:

Everything wired up, tested and complete, so the bottom lid can be put in place:

Before the bottom lid is attached the cover is mounted over the power transformer.

The preamp in operation:


The preamp together with the 6CB5A power amp which it is intended to drive:

Best regards


Saturday, June 23, 2012

Making of a Field Coil Speaker, Part 4: LV FC PSU


Finally I found some time to continue with the work on the speaker system. Up to now the woofer field coils were powered form an ordinary lab power supply. This got replaced now by something more appropriate.

A dedicated power supply for the woofers in a similar style as the high voltage field coil supply  of the horn drivers. The woofers are running at 7-9V / 3A. So no tube rectification. The PSU provides two independant voltages for the left and right woofers. Schottky diodes are used for the rectification. The filter is choke input using Lundahl LL2733 filament chokes.

As power transformers two of my 5A filament transformers are used for each side. Since each transformer only delivers up to 10V when all secondaries are wired in series, two of them needed to be cascaded to reach the desired output voltage range.

The supply is configurable with switches to adjust output voltage. The filter can be selected between pure choke input, or cap input with two different capacitance values:

Some pics of the assembly process, power transformers and some caps mounted:

Some wiring:

The completed supply before the bottom lid is attached:

The chokes provide a well filtered low noise output voltage. Smoother and more delicate sound compared to the ordinary lab supply!

Both the high voltage horn driver supply and the  new one besides each other:

The voltage indicators are still missing on the new one, these will be added later.

Next the system will get a crossover in the same style as the one I built for Twogoodear's Gotorama system. Stay tuned!

Best regards


Thursday, June 21, 2012

Tube of the Month: The 6DQ4


This article covers another TV Damper tube: The 6DQ4. Although I covered already several different TV dampers, these are worthwhile the attention since they are fabulous for rectifier duty.

The 6DQ4 is pretty much identical to the 6AX4 electrically. Please refer to the 6AX4 Tube of the Month article of last year for the technical paramters and application of the tube in audio amplifier power supplies. In this article I will show the construction details of this tube.

Like most TV dampers, the 6DQ4 was manufactured by many different companies, here just a few brands:

The tube was made in a various different shapes:

Let's have a closer look at one with the small 'coin base':

The side with the getter on the glass:

In the next photo it can be seen that the plate is made of copper:

A close up of the top:

Now let's have a closer look, by dissecting a tube. First the glass is removed.

On the other side the ring can be seen which held the getter material before it got flashed and condensed on the glass as a silvery mirror:

A close up of the part between base and electrodes, which shows how they are connected to the pins:

The heater wire extending out of the cathode tube can be nicely seen, it is surrounded by a spiral of insulating material. The next photo shows a close up of the cathode at the other end of the tube:

The insulation spiral extends well beyond the cathode sleeve to ensure the very high heater to cathode voltage rating these tubes have. In the next picture the heater wire inside can be seen:

Let's have a closer look at the cathode. Here the cathode is removed from the plate and the heater wire is pulled out:

A close up of the top end of the cathode. The coated  white part is the actual emissive area:

A peek inside the cathode from the bottom:

The heater:

A close up:

A close up photo of the plate structure:

Looking into the plate opening from the top:

The copper plate, opened up:

This is how the cathode would be placed inside the plate. Of course in a working tube the cathode would be held in place by the top and bottom mica supports, to ensure correct distance:

The construction of this tube might not be as impressive as that of the EC8020 which I presented last month. But still a remarkable piece of vacuum tube technology!

Luckily TV damper tubes are still available in abundance at low prices. Occasionally they can be found in bulk packs:

I hope you enjoyed this Tube of the Month article. As a last goodie a close up picture of a 6DQ4 shot in the dark with the tube turned on:

Best regards


Tuesday, June 19, 2012

How Does it Sound ? (Part 2)


In last years How Does it Sound? Article I already wrote about what I look for in terms of sound reproduction and about the loop holes in listening tests. There had been some recent discussions on internet forums about how certain things sound, in which I participated. I'm also very often asked about my opinion about the sound of certain parts or design aspects. So I thought I summarize my thoughts about this in another article.

Most often such questions are related to a specific tube. People would like to know how a certain tube sounds, probably compared to certain other types. Very understandable because the output tube is the most obvious part in a tube amp and most often the single aspect based on which an amp or a DIY project is chosen.

Unfortunately there is no generally valid answer to this. Sure there are many opinions floating around how, say a 2A3, sounds. If you ask about the sound of a certain tube on a forum, you will get many answers. Probably some of them contradicting.

It is very common among tube enthusiasts to arrange shootouts to find the answer to such a question. We want to know which tube sounds best? Let's set up a system and hook up 211, 300B, EL34, 2A3 and a 45 amp and shoot it out. Only, it is not that easy. If you really want to nail it down how a certain tube sounds compared to another one, this is going to be very difficult. It is always the entire amp which you are listening to. And not even that alone, it is also the way this amp interacts with the speaker used and even the preamplifier. What works well in one system,.might not be so good in another.

Now let's assume you have two amps which share the same parts and circuit topology, only use different output tubes. I actually did that several times. For example a triode strapped 6CB5A and a 45. Both in a transformer coupled amp, using the same driver tube (6N7) and the same interstage and output transformers. Also the same capacitors and similar power supplies. Tube rectified in both cases with full wave TV damper bridge feeding a choke input filter. Even the same power transformer, only using diffierent taps since the 45 needs a different B+ voltage.

Now this is really a valid comparison and all we are comparing are the two output tubes, right? To a large degree yes, but there are still other factors playing a role. Both tubes have different plate resistances so they require different loading. The output transformer used (Tango XE20S) provides tapping for different impedances, so it can be adapted. Still both amps will not have the same output impedance (damping factor). Some speakers react sensitively to that, others less. So this is one aspect which has an impact on the sound. It is not a qualitative aspect as such. With one speaker a higher output impedance can be beneficial with another one it will cause a slightly worse sound.

Next is the interaction with the driver. The tubes have different grid drive requirements, different input capacitances, different tendency for grid current and need different voltage swings for the same output power. So the combination with the specific driver used is also at play. The driver might be adequate for one of the tubes, but at the verge of being stressed with the other.

Then both amps have different max. output power (by a factor of 4). The more powerful amp will be usable with a wider range of speakers. But let's assume we use a sensitive speaker and listen only at power levels below the maximum of the weaker amp. Both amps will require a different input voltage for the same output power, since the tubes have different amplification factors and the output transformers are wired for different step down ratios. So one will need more voltage swing from the preamp than the other. Of course we adjust the volume control on the preamp for the same output level between both amps. But that means we play each amp at a different setting of the volume control. Many preamps sound different at different volume settings. often it is the volume control itself. Especially resistive potentiometers tend to sound different at various settings. Also in one of the settings the distortion spectrum of the preamp might be different, this can be quite dependant on the output voltage. So we are not comparing just the 2 tubes, but we also listen to the differences of the preamp at different volume settings.

All this might sound extreme and render such listening comparisons worthless. No, that's not what I want to say. I just want you to be aware what it is you are listening too. Don't jump to conclusions too quickly. Such tests will still tell you something and the more you know the preamp, speaker and amplifier circuit and their impact on the overall sound, the more you can judge the result.

Ok, here is the answer you are waiting for: I prefer the 45, but take this with all the grains of salt mentioned above.

Just be cautious when you hear about listening test results from others, or read about them on the internet. Always take it with a grain of salt. Try to understand the set up, what was used, how the comparison was done. Otherwise findings like '300B has a beatuiful midrange but somewhat loose bass, while EL34 sounds more sterile but has a tight bass' are useless.

Recently there was a question asked on a forum about power supplies: What is better a common stereo PSU, or separate PSUs for each channel? Or rather separate supplies for each stage? At first sight this seems a very valid question. But there are many possibilities how power supplies can be implemented. A common supply can have decoupling between channels and stages or not. And most importantly: what kind of circuit is meant to be powered? Some of these aspects will have a far greater impact on the overall result than the mere decision to split the supply or not. Such questions are like asking which are the best tires for a car with 4 doors. Well, it depends.

Often people get a bit frustrated when they ask such questions to me and the answer is 'it depends' or I demand more details to be able to give a good answer. Often it just is not that simple. There is no good answer to such a general question. Because it is a poor question. I don't like to give bad answers so I either try to get the question asked in a more consice way or rather don't give an answer. Unfortunately this sometimes can be interpreted as arrogance or unwillingness to help. Be assured it is not.

I hope this post helps to understand why I'm sometimes reluctant with sound descriptions. It is because they might not be valid for other environments and most importantly they would be based on my personal subjective preferences. People are seeking simple and general answers to the question how to get good sound. If it would only be that simple. The only way to get the answer for yourself is listening and trying many things. This will enable you to build up experience and understanding in how things interact and work together. It will help also to get some intuition how to find the path in the confusing jungle of many different tube types, transformers, capacitors, circuit topologies and power supplies.

Best regards


Sunday, June 17, 2012

Making of a SE 6CB5A Amplifier: Assembly


In this article, the assembly process of the SE 6CB5A amp will be shown.

First the metal plate which holds all the parts gets mounted in an assembly rig:

Here the caps, power transformer and tube sockets are already mounted. Some initial wiring to the power transformer and rectifier sockets is done:

Heater wiring to the signal tubes and some wiring to the caps:

Output transformers, two of the chokes and the resistors mounted and wired up:

The last missing pieces, interstage transformers and two more filter chokes:

Everything assembled and ready for the first measurements and listening test:

After measurements and listening test results are as expected, the power transformer cover can be mounted and the amp inserted into it's wooden frame:

Another shot taken in the dark, with the amp switched on:

The assembly of the matching line stage is almost finished and will be shown in an upcoming article.

Best regards