On the bench today.......

[dave]

The rats nest has been running for over 2 hours, sucking around 130 watts from the mains. The extra large vintage transformer is up to 40oC, I am confident enough to cut some metal tomorrow. yipee.
The excess current was due to a failed grid capacitor, I wonder if that's where that clicking noise was coming from, the slightly strange wobblies could well be RF oscillations. seemed to go away when I put scope probe on it. The push pull drive was not very symmetrical amplitudes either, so trimming that out helps and it is easily giving me 15 watts. Have overcome my Lundahl transformer wiring crisis of confidence, it's all fine. Generally quite happy.

 

[pmcuk]

Dave - I have some nice E80CC tubes if you need any. Andy

 

[dave]

Dave - I have some nice E80CC tubes if you need any. Andy

Oo that would be interesting, I haven't got any of those. I was thinking of trying out some 12BH7s and 6n6ps, and the other usual suspects. am wondering if a fair amount of grunt required to maintain the HF.

 

[pmcuk]

I like the E80CC, one of my favourites. I think you have my email? Sling me an email and I'll tell you what I have. I also have some E180CC which are quite nice and some 12AV7. Plus plenty of 6P1L.

 

[AudioSatori]

One from the past, but who can guess..? ;-)

 

[toprepairman]

Teac/ tascam? not JVC

 

[AudioSatori]

Teac/ tascam? not JVC

close but not quite

Nak 600 series, a wonderful machine, so nice to work on.

 

[dave]

Surprisingly competent those old Nak's, even the really base variants.

 

[pmcuk]

I'm baffled. I've just put together a 2a3 amp which sounds marvellous - much better than I expected. I just love listening to it. I ought to sell it but I'm getting a bit attached to it. It started life as a really well-made amp by Phil at Bluebell Audio with big Hammond transformers and choke, but it was awfully heavy and a pain to lift, so I took out the big OPTs and sold them. I substituted a pair of Techno 5K OPTs which were much smaller and lighter. The PSU was designed for "book" operating conditions like 250V, -45V, 60mA, though the cathode resistor was 900 ohms not 750 ohms, so 50mA current. Strange with such massive OPTs. Anyway, I left the 900 Ohm resistors in and that only gave 12 watts. I'm going to change them to 750 ohms.

When I got the amp it had no input stage so I thought "what the hell, I'll just put in something simple and sell the amp". So I added a basic 6P1L stage with resistor load. I did however use very nice parts - Holco resistors, teflon FT-2 coupling caps and DC Link bypasses. The amp already had a very nice output stage with good filtering and Black Gate cathode bypasses. So all premium parts. A simple 6P1L - cheap and plentiful. It had no right to sound this good but it did. I read that the 1960s production was better than later versions, but no idea when the Edicron I put in was made. I have a selection of 6P1Ls so I can do some tube rolling to see what that gives. I did read that a lot of other people including the folks at Bottlehead liked a 6P1L input stage with 300b or 2a3.

So here I am listening to an amp I expected nothing special from, and due to some quirk of fate all the parts work together beautifully. I'll be building another amp pretty much identical to this one next. Let's see if the magic can be reproduced! Then I can put the amp on sale, since I have too many 2a3 amps already. It's been my amp of choice for a while now.

Photo shows the amp before modification with a 6SN7 posing for the camera and a couple of redundant sockets.

 

[dave]

I occasional hear rumours that small transformers sound better than big ones, but I have not proved that to myself. I lived with a surprisingly small pair of output transformers in an ECL86 SE record player amp, putting out about 2 watts, for several years.

 

[dave]

The PP triode slowly proceeds, a bit of a battle between sitting around on my arse doing nothing (well watching my new (to me) 60" plasma, cause I feel worn out, and drilling holes and wiring etc, quality time by myself in the garage, which seems to have similar to better re energising qualities to tv viewing. The holes and wiring are winning at the moment. But it is getting to be a bit of a squeeze. Was originally planing to use smaller transformers, but they ended up large.
While I work on it I have been enjoying my 859 homage recently in "eco" (4 watts) mode, running in to NS1000s, drives them very well.

 

[dave]

I can't seem to settle to any one thing at the moment, I have been working on my BS1000 speakers. I decided that the 2" dome mid wasn't really happy, it resonates at about 500 ish Hz, so need ot roll off about 1k, but the B250s not so happy, by my standards, going that high, so I rummaged around and found faital pro small mids, I ended up getting 3" driver, as the curves looked quite nice and 91dB efficient, so grafted that in:

Now, I like to believe that each driver has a character of its own, the 3" papercone mid does seem to work quite well with some on going crossover tweaks, quite listenable, but, I do detect a little bit of a transistory radio quality, and thinking about it, the driver is pretty much the same as that fitted in to many radios today. Maybe I should have held out for a more grown up 5".

 

[dave]

Life is full of little distractions, and for some reason I got fixated on the little Pye black box record player amplifiers, again. They are so cute. I think itis because of the pp triode amp got me thinking, the first black box amp was triode connected, so I thought I would try my luck and score one on eBay. It wasn't, it was a mk2 with ultra linear and EL42s.

It came with one of the newer ones, also ultra linear, but with PCL83s. I was a bit sniffy about it to start with and was originally planning to sell it on, and convert the mk 2 in to a triode, but there is a major problem with all of these amplifiers, take note anyone else thinking of "domesticating" them. They are designed for a 1.5 ohm load!!!

I have developed a cunning plan. I found a transformer which is about the right size (see above) and I could arm twist into working with an 8 ohm load. So my plan is to try and graft in the new transformer, and tidy up the remarkably complicated circuit.

 

[toprepairman]

Them little brown caps will most likely need replacing.

 

[dave]

Well that's the easy bit done. The new output transformer conveniently screws into some existing holes in the chassis, and a new cap for the mains transformer, in the style of the earlier ones.

 

[dave]

Has been an interesting adventure with the black box, which we feel is really Wee Bill, it is actually a Williamson! Especially if you strip away all the slight oddness and tone controls, it's behaviour gets more and more civilised. Replacing the 4 hunts 47ns that had turned into resistors made it work, but the first 1/2 ECC82 not happy with too high a voltage on the anode, causing the following direct coupled concertina phase splitter to not have enough room to work. I doubled the anode resistor to 200k and that burst in to life with nice linear behavior. Reading around, I am not the only person who has had this problem, and the data sheet says the situation is wrong, but there seems no obvious reason for it.



Putting back in the PCL83s but with the output transformer disconnected, showed to me the following long tailed pair, the anodes were a bit low (but right according to data sheet), so paralleled some 100k with the 220ks and that seemed to work much better with room to swing. Hooking up the transformer, a fairly well behaved output. The amp has a weird 10Meg positive feedback resistor, taking that out cleared up some peculiar waveform behaviour, left it running for a while and the output died, one of the PCL83s running away, bit exciting... Swapped them for some NOS 30PL1s which are supposed to be equivalents, and carried on.

I can only get about 3 watts out of it no matter what I try, a bit disappointing as the original quoted as 5. Then I remembered the original OTX, the ultra liner tap is only about 10%, compared to the probably >50% of the new one. Tried pentode wired output, pushing >5 watts easily. So I think I am going to settle for 3 watts, as it seems so happy in that place. Wondering about grafting in a gas regulator, for my peace of mind, and the bouncy mains in my house.
Now for some tidying up and decide what output terminals to use.

 

[rabski]

There's some oddities in that circuit for sure. If you leave the global FB in, could there be potential phase issues that the local FB was intended to compensate for?

 

[dave]

There's some oddities in that circuit for sure. If you leave the global FB in, could there be potential phase issues that the local FB was intended to compensate for?

Yes, I think the transformer I am using, even though its a Chinese cheapie is a lot better than the original, so the original problem probably gone away.

 

[rabski]

Yes, I think the transformer I am using, even though its a Chinese cheapie is a lot better than the original, so the original problem probably gone away.

Makes sense.