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C3g Driven 2A3 Single Ended Valve Amplifier

Introduction

The WE91s are sounding really lovely and I'm thinking that I have the makings of a pretty good system now. Of course, that doesn't mean I'm happy to live with what I've got for the rest of my days when there's so many more valves to try...

But before trying new valves I felt I had a little bit of unfinished business with the 2A3. The Loftin White had some real strengths but the one I built at least was limited at the frequency extremes. Whilst I had been building the WE91s James D had posted on the WAD forum about his 2A3 amp, which had originally started out in life as the WAD 2A3 PSE. Richard Higgins had also built a variant of this amp (but using a C3m pentode) and was very impressed by the sound.

James had altered the amp somewhat by removing one of the output valves and changing the driver valve to a C3g, a German telecom pentode noted for its linearity. A voltage regulator tube on G2 of the pentode was another notable change.

I emailed James to discuss the possibility of building his amp using the bits I had left over in the Loftin White amp. James was very enthusiastic and helpful, drawing up a slightly revised circuit to suit my components, i.e. a Hammond 374BX mains transformer, Hammond 1627SE output transformers and GZ34 rectifier.

The amp requires four chokes, two in the power supply and one in each channel of the audio circuit. I needed at least three more chokes and weighing everything up I decided this was a good opportunity to get some more iron, and AE Europe in the Netherlands had a good reputation for quality transformers at a reasonable price. The only downside was the delivery time, which turned out to be five months. This was largely due to their popularity in selling excellent iron for a good price. Fortunately things are a little better now, taking eight to ten weeks apparently.

So in went an order for four 6 chamber EI chokes and a pair of 2.5k SE c core output transformers, as well as a 100uF+100uF Blackgate WKZ for the power supply and a couple of 1uF Mundorf silver PIOs for coupling. And I waited. And waited. And waited. And after about six months I got a rather heavy carboard box full of beautifully made chokes and handsomely potted output transformers. Fortunately the wait had given me plenty of time to procure the other bits I needed. Blackgate NH caps for the cathode bypass caps, cheap enamelled wirewounds for cathode resistors, and an Alps Blue 250k pot. Alps Blue are really good, aren't they? Commercial amps costing thousands have them in, so they must good, right? Well I got mine from ebay for 8 quid so they're not exactly dear... But it was a good way to get started.

Construction

I wasn't building a previously-prototyped amp this time so it made sense to breadboard. I don't have the space to have one large breadboard, nor separate pieces for the PSU and audio circuits, so I cut a single piece just smaller that the top of the rack, and then made a platform for the output transformers and mains transformer to sit on at the back. A pair of aluminium plates were cut, filed and punched for each channel's C3g, 0A2 and 2A3, plus another plate for the GZ34 rectifier. The plates were elevated above the breadboard such that I could place a "cover" over the guts of the amp and allowed the filament transformers to be mounted on the breadboard beneath the 2A3 sockets.

The plates for input phonos and speaker conectors were attached to the back and then the significant components, chokes, caps and filament transformers were laid out.

Then it was just a question of working through and wiring everything up. I used a number of star earth washers to star earth the amp. One for the power supply, one for each channel, and one for each output stage, all starred to a "master" to which the earth lift resistor is soldered.

Switching On

Always a nervous time the first switch-on. Although nothing went bang my nerves were well founded. The 0A2s which should light a nice purple glow when the voltage reaches its striking voltage instead flashed and clicked, what transpired to be oscillation of the C3g. Connected to the speakers with a little volume set the oscillation caused a rather unpleasant thumping type sound through the speakers. Interestingly, when I tried to check some voltages with my multimeter as the probe approached the valves the multimeter clicked in time with the oscillation of the 0A2s.

This was a little disconcerting as I'd not come across oscillation before. The more I thought about it the more I figured it was oscillation, but I wasn't sure what to do about it. What was needed was a trip to the doctors for a good coat of looking at. Fotunately the good "Dr." Nick Gorham was taking on new patients and an appointment was made to visit his "surgery" in Halifax.

Arriving at Nick's I was greeted by what seemed like a dozen rather friendly lurchers; Nick's wife runs a lurcher sanctuary. Then there were the chickens, rabbits and ferrets - a real conservation area.

Arriving at the "surgery" was another experience - Nick's listening/hi fi room is a haven for all of us who have to use the dining room table to do any soldering and then share any of our creations with the family in a communal living space. Nick had two large-ish James D-style open baffle speakers about a metre from the rear and side walls with a pair of "Russian trawler" 6C33C monoblock amps on the floor next to the speakers. Between these amps was his breadboarded 300B amp with regulated variable power supply, and then against the back wall was his monster 211 breadboaded amp. Nick likes valves and breadboards, and his amps are invariably some of the very best I've ever heard.

Then to the side there is his Technics SP10 with Hadcock arm and Denon DL103R cartridge, his own LCR phono stage (which is still the best phono I've heard) and a stash of vinyl. Then there's a narrow strip of carpet to tiptoe through to the listening seat, or beyond to the workbench. Yes, this is a functional room.

So, the patient was placed on the workbench and the scope attached. Nick diagnosed the oscillation pretty quickly and the addition of 300R? carbon resistors to G1 of the C3g pentode sorted the really high frequency oscillation.

But there was still some low frequency oscillation which was manifesting itself as oscillation of the speaker cones. I'd guess it had a frequency of about 50Hz and although the oscillation wasn't causing any sound to come from the speakers it shouldn't be there.

Nick had a pretty good idea what was causing the oscillation, suspecting that the 22uF cap in the feedback loop next to the C3g needed increasing. Clipleading a pair of 120uF caps, one in parallel with each channel to give 142uF stopped the remaining oscillation in its tracks. Playing further at home I found that 54uF on each channel was enough.

Listening

I like this amp. I like it a lot. And after the C3g heaters were referenced to ground it's completely quiet through my 97dB Fostex. The sound is detailed and well balanced, and the improvement from switching from the monoplate Sovteks to the dualplate RCAs was even more discernable than with the Loftin White.

Comparing to my other amps I would say that this amp is a little better than the Loftin White overall, but not quite as good as the WE91s with the KRs.

Improvements

Well, the RCAs are very nice. A margin better than the Sovteks.

Nick did suggest an alternative. When he had been playing around with this front end Nick found that he preferred taking the 0A2 to ground rather than the top of the cathose resistor. I tried it but couldn't hear any difference.

Nick also suggested trying one choke instead of two in the power supply. Playing around with different values of capacitance I found that 10H and a large 30uF motor run cap was enough to give a ripple small enough that I couldn't hear any hum.

Sticking with the WKZ I could hear a small difference between a single choke and two. Whereas the two chokes had a firmer, more beefy, bass the single choke seemed to remove a veil: it was more detailed with a lighter touch. So I wired 10H 100uF up and lived with it for a while. After about a month I went back to 10H 100uF 10H 100uF and thought I preferred the extra beefyness of the second choke. I think it's probably a suck-it-and-see thing from amp to amp. More valve welly might be better suited to a single choke perhaps?

I did add some choke snubbers to the first choke, a pair of 220nF 1250V Wima PPs from Maplins. These had a subtle effect on the sound, removing a veil and allowing more detail through. I got a similar improvement by removing the C3g gridleak resistor and just using the volume pot.

And that Alps Blue pot that's used in all those high end amps? I replaced it with an altogether better one. I was looking for a Panasonic pot as these are supposed to sound rather nice. Benny at Aquablue in Belgium used to supply them but he only had 50k pots and I didn't want to go as low as this. He did have a 100k pot which he reckoned was on a par with the Panasonic, though he didn't remember who made it. So I gave it a whirl, and sure enough it's a lot better than the Alps.

A Diversion Into 45s

I have a few old 45s I bought on ebay for the day I get round to building Gordon Rankine's Bugle amp; a well regarded amp which uses a 6072 or 5951 in SRPP to drive a 45. But thinking about the C3g driver stage I thought it would probably drive a 45. Checking with James he said give it a go, so I did, changing the cathode resistor to 1k5R to suit the 45. Whereas 2A3s like a 2k5 impedance in the output transformers 45s prefer 5k. Unfortunately I didn't have any 5k opts so I used the 4ohm tap instead of the 8ohm. Then all I needed was a little tweak to the first power supply cap to bring the HT voltage down a touch, and that was it.

This was the first time I'd heard a 45 amp at home so I was really interested to hear how they sounded. The first thing that struck me was that the 45 sounded rather warm and a little coloured in the bass after the 2A3. Initially I found the 45 a little uncomfortable to listen to such was its apparent colour. But I decided to live with it for a while to give me chance to get accustomed.

Over the space of a month or so I grew to appreciate what a 45 can do. It has a magical top that a 2A3 can't match, nor many others for that matter. But the bass is just a little woolly, in this amp at least. Maybe the PSU needs some tweaking to get the best out of the 45? But somehow I can't help feeling that the 45 sounds rather like a tabletop radio, the very application it was intended for. Anyway, I definitely need to build a Bugle to compare.

Interestingly, when I wired up for 2A3s again I had problems with the low frequency instability again, even though I had only reconnected what I had used without problems earlier. Anyway, adding 200uF to the 54uF cap of one channel solved the problem.

Conclusion

Overall, this has been a very interesting amp to build, and I've learned a great deal. The high transconductance C3g pentode has not been without problems, and the low frequency instability has been frustrating at times too. But I played with a few new things and have a great sounding amp to listen to my 2A3s. I think the next thing to try is a nice triode or two in place of the C3g.

February 2008.

 

 

 

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WAD PHONO II and PSU II