Upgrading Paraglows

Squeezing More Out of the Bottle

Well, the standard Paraglows are simply amazing, but the postings on the Bottlehead Forum told me that the bottle wasn't yet empty. So, I trawled around for good ideas and set about tweaking my Paraglows. In the end I felt I had to carry out a ground-up rebuild to facilitate some of the tweaks and to come up with a more professional looking finished article. Compare the picture above with the image of my 'first-build' version and I think you'll agree it looks nicer.

Of course, if you've read my article on my first-build version you will know that I included a number of tweaks into that build, such as silver wiring and Black Gate power supply capacitors, so I won't mention them again here.

Read on to find out about the mechanics of the rebuild and details of some of the 'tweaks' I've incorporated.

The Tweaks

Below is a list of the tweaks I have incorporated into the Paraglow rebuild and which have improved the sound. I think it is important to put these tweaks into some sort of context.

Firstly the stock Paraglow amps are very, very good, especially when the asking price is considered.

The only person I have to please with these changes is me. Others may disagree with my views. It is notoriously difficult to be objective about improvements you make and justifying an outlay by hearing an improvement is hard to avoid. I believe that I have heard the changes mentioned and consider them to be improvements..

Lastly, several of these changes are quite overt, such as the differences between output tubes, but most are quite subtle. Each of the changes is incremental, not an order of magnitude.

The Amplifier

Output Tubes

The standard Paraglow kit comes equipped with Valve Art 2A3 valves. These originate from China. I fairly quickly tried Sovtek 2A3s and of late I have been using TJ Meshplate 2A3s. The following is a brief assessment of these different valves, but it is only my opinion.

Third Place - Valve Art 2A3. These have a nice rich sound. Quite detailed. They make nice music.

Second Place - Sovtek 2A3. When I first tried these after the Valve Arts I didn't like them, something seemed to be missing. Further listening showed that they are more detailed and what was missing was a sort of warm fuzziness that they showed the Valve Arts as having. I would describe them as lean.

First Place - TJ Meshplate 2A3. These are expensive compared with the other 2A3s I've mentioned but well worth the extra. They have loads of detail, more than the Sovteks but more importantly, they allow the music to just ebb and flow so that you are totally drawn into the performance. They are warmer than the Sovteks, without the haziness of the Valve Arts. Highly recommended.

In due course I want to experience what NOS (New Old Stock) 2A3s can offer.

Output Transformers

The first tweak I made that required hard-wired changes was the incorporation of Magnequest cobalt output transformers. I had read a lot of positive comments about the cobalt transformers on the Bottlehead forum. They are not cheap, so there was an element of risk in buying them as they weren't going to be returnable, but I have faith in the views expressed on the forum and took the plunge.

But why Cobalt? Well, Cobalt, is, with respect to transformers, a very special substance that allows you to have your cake and eat it; it has incredibly high permeability, yet great power handling (high flux). In other words, cobalt is a material that is extremely sensitive, yet can handle anything it is called to do.

When they arrived, the first thing I noticed was how much smaller the cobalt transformers are than the standard Magnequest devices, but in this instance size didn't matter. Initially I just installed one of the cobalts and ran some comparisons between the two channels of the amplifier - not a scientific exercise but fun nonetheless. It was immediately obvious that the cobalts were better, the music seemed smoother, yet more dynamic and there was lots of extra detail being revealed. Extended listening (with both channels equipped with cobalt) has consolidated my initial impressions.

Paraglow II

I had also picked up on the Bottlehead forum references to a Paraglow 2 so I set out to find out more. The name Paraglow 2 actually refers to a kit of upgrade parts developed and sold by Exemplar Audio. The man behind Exemplar, John Tucker, has basically redesigned the driver stage of the standard Paraglows. The stock Paraglows use a 5965 driver valve; this is a double triode but only half of the device is used. In the Paraglow 2 another double triode, a 6N1P, replaces the 5965 but the spare half of the 6N1P is used as part of a shunt regulator that builds on the Constant Current source of the stock version.

The Paraglow 2 installation took several hours of careful work, basically entailing the removal of the stock driver stage and replacement with an entirely new one. It wasn't difficult, and anyone able to build a stock Paraglow will be able to achieve the change.

Initial results were disappointing as one of the boards blew on power up. However, after some correspondence with John Tucker a new set of boards arrived and were installed, along with some instructions to ensure that the potentiometer was set to its mid point before power-up. Success, everything powered up just fine and only very minor adjustment of the potentiometers was required to set the correct voltages.

Listening showed this upgrade to be another good step forward. There was a general tightening up of the music, bass seemed snappier and more tuneful and the top end seemed to be a lot cleaner. The amplifiers were really singing.

Capacitors

The only significant caacitor in the signal part of the Paraglows is the Parafeed coupling capacitor. The stock version is a 630V 3.3uF Solen Polypropylene. I felt this could be improved and postings on the Bottlehead forum suggested their might be benefits in substituting a better quality capacitor and by usng a higher voltage rating.

I first used Auricap capacitors (4uF 1500V) in the parafeed coupling position, which were an improvement on the Solens, but have now substituted Mundorf Silver/Oil capacitors (3.3uF 800V) with very satisfactory results; I perceive a general cleaning up of the sound, with a bit more detail and openness.

Resistors

My original Paraglow build had a passive attenuator based on Phillips, then Holco, resistors but I substituted Vishay bulk foil resistors for these in the rebuild. I also used Vishays for the signal resistors around the driver tube. Using the Vishays opened up the sound, making it seem more relaxed yet not in any way bland.

The Power Supply

Schottky Diodes

The stock Paraglow uses solid state rectification in its power supply; two UF4007 diodes performing the duties. There is a lot of information on the Bottlehead forum about the 'noise' that diodes put onto the power supply along with ways of addressing the problem. Now don't read this wrong, we are not talking about overtly audible noise but small spikes on the power supply, produced by the diodes each time they switch. One possible solution is to use snubbers (search for 'snubber' on the Bottlehead forum) which substantially remove the spikes. I decided to use an alternative solution; schottky diodes produce an order of magnitude less noise. The schottkys I use are 5A 1200V devices.

I broke one of the rules of DIY hi-fi with the schottkys as I didn't actually A/B them against the standard diodes, but the outlay is not too great and the rebuilt Paraglow 2 amp is better than the original with all of the bolt-on tweaks.

DC Heaters

The standard Paraglows use all AC heaters but these can contribute to hum from the amplifier; I was experiencing enough hum for it to be just audible in the listening position between tracks. With direct-heated triodes, such as 2A3s using AC heater supplies can contribute to hum so I decided to implement DC heater supplies to quieten things down.

While it is possible to use a very simple bridge rectifier/capacitor approach to providing the 2A3 heaters with a DC supply the common wisdom seems to be that this represents quite a compromise and regulated supplies are a much better proposition. I opted to purchase a pair of DC heater supply modules from Guido Tent (http://www.tentlabs.com/) but it is possible to find DIY circuits on the Internet.

Here is a picture of one of the modules installed under the chassis top plate.

When installing the 2A3 heater modules I also dispensed with the hum-bucker potentiometer, simply connecting the 2A3 cathode resistor/capacitor to the ground side of Tentlabs module.

Applying DC heating to the 2A3s significantly reduced hum, such that it is not audible in the listening position and you need to get your ears very close to the speakers to really detect it - and sound quality doesn't seem to have been compromised at all.

I also applied DC heating to the 6N1P driver tubes using a circuit I borrowed from a World Audio Design amplifier circuit.

Here is a sketch of the revised power supply for my Paraglows. The sketch assumes a single mains transformer to fit my approach of building my Paraglows into a stereo chassis.

The Rebuild

What became clear very early on was that I needed to build a new chassis for the rebuild as many of the upgrade components are physically much larger. I also felt that I could make the rebuild look a lot more professional - I think you'll agree that the finished item in the picture at the top of this page looks pretty good.

The image below shows some of the capacitors that were to go into the rebuild. That's a CD in the foreground for scale.

The four silver caps are 100uF 630V Solen Polypropylenes for the power supply, the yellow caps are 4uF 1500V Auricaps for the Parafeed coupling and the two black caps are 220uF 400V Solen Polypropylenes for the 2A3 cathode position.

The Chassis

My experience is that for the DIYer it is quite difficult to achieve truly good looking metalwork as to do it well you need some specialised tools. I was aware of a company offering a service to produce limited batches of metalwork using CNC technology and decided to use them to make a chassis plate for the Paraglow rebuild. To design the metalwork you use easy to use software that is downloadable from the company's website, which is at;

http://www.schaeffer-apparatebau.de/English/index.htm

The software calculates the cost of making your design and has an on-line ordering function.

I laid out all of the components and sketched out a new chassis top plate, initially with pen and graph paper, then transferred this to the software and finally, to check the layout I printed the design from the software and laid out the main components again.

When laying out the main components I separated the power supply from the amplifier circuitry as much as possible and kept any wiring carrying mains frequency AC well away from the amplifier. In the finished layout the power supply occupies the back two thirds of the chassis and the amplifier the front third.

Having completed the layout it was off with the order and wait patiently. In only a few days (including shipping from Germany to the UK) the finished chassis plate arrived - fantastic service and a great quality piece of metalwork. Here's the finished item.