For trade: Original WAD 300b push pull interstage amp (Tim de P, revised Andy Grove)

[montesquieu]

The big Tannoys go tomorrow and I'm exploring speakers a bit more room friendly for the smaller space. I'm currently evaluating a set of high-efficiency AN-E's (something of a radical change for a Tannoy lover but they are actually sounding pretty good in this smaller room with plenty of bass and enough scale to fill the space).

These are reputed to sound at their best with good single ended amps ... and I have a little J Soundlab 300b SE (Israeli manufactured with American iron) which is actually making them sing ... but I'm interested to see what else is out there of slightly higher calibre.

So ... offered for possible trade (along with the J Soundlab if need be) is an original WAD 300b push pull interstage amp - bit of a legend in DIY circles. The original design was done by Tim de Paravicini in the early 90s, and modified by a young Andy Grove (who soon afterwards became Audio Note UK's top gun amp designer). It's a zero negative feedback push pull amp, and sounds absolutely wonderful - hear this and never again will anyone say the 300b is lovely and refined but lacks energy and backbone. No less than seven high-quality transformers go into this - a truly massive mains transformer, two chokes, two output transformers, and two interstage transformers (which do away with a driver stage and replace the necessary coupling capacitors with a substantial transformer). Its 28w is overkill for the 98db AN-HE's with the silver voice coils - for these, I need a single ended amp.

Tons and tons of info online on the WAD 300b interstage, needless to say it's head and shoulders above the current kit designs from World Designs or Glasshouse - they a kit like this just wouldn't be affordable to market any more.

This amp was John Caswell's personal amp, he built it for his own use quite a few years ago and only recently replaced it with a KT88 amp to match some fairly power-hungry Proarc speakers. The Andy Grove design was ultra-sensitive (at least, when used in no negative feedback mode) and essentially incompatible for use with active preamps. Rather than introduce NFB, John replaced the WAD input stage with the original one SP Wound (makers of the transformers) recommended, with 6C45P input valves (readily available). It now has a sensitivity of about 1.2mV, works well with active preamps and remains without negative feedback.

John also made an improvement to the HT rectification which is by a pair of 6D22S plus fast diodes in a hybrid Graetz circuit, so warmup is slow (good for tube and component life). According to John, frequency response is flat within 0.5dB from 10Hz to about 34Khz. All valves are auto biasing and there are trim pots to remove hum if the power valves used are not perfectly matched pairs each side.

Comes with all valves including a recently installed quad of JJ 300bs and a spare set of driver and two spare sets of rectifier valves (including a pair of NOS Telefunkens EY500A equivalents). Twin speaker outputs for easy biwiring. The original connections and sockets are all there for restoration to the 5087/6072 double tube input stage if required for any reason - but I doubt anyone would want to after hearing it.

Anyway I'm offering this (and the Soundlab if it's needed) for trade for a nice lowish power single ended amp. Must be someone out there who has a nice single ended amp that's struggling with their speakers, who wants to stick with directly heated triodes, but needs a lot more power and slam to go with them.

No great rush, will just wait and see what comes in.

http://i1181.photobucket.com/albums/x440/montesquieu61/300binterstage_zpsf15dd92f.jpg (http://s1181.photobucket.com/user/montesquieu61/media/300binterstage_zpsf15dd92f.jpg.html)

[NRG]

A very fine amplifier the original WAD 300B PP amp, however, your unit looks nothing like one and in fact bears little resemblance to the TdP prototype. John is a great guy, perhaps it would help if you got his input on the background to this amplifier so as to help your sale...

[montesquieu]

I got it direct from John and background is exactly as I've stated above more or less verbatim from him: all the iron and power circuit is as per the interstage kit (using shrouded upright transformers black powder coated rather than the cheaper bolt-in chromed ones), the input stage prior to the interstage transformer has been modified from two tubes to one to provide lower gain without NFB (sensitivity on the original was something ridiculous like 0.1mv making hard work even of a passive attenuator - just too much gain), the rectifier stage has also been improved to provide slow warmup. It did start life as a kit so layout is something that isn't necessarily a given. John is indeed a good guy and these mods come from very long experience. There were of course major changes between the Tim de P prototype and the kit that went into production so it all depends on which picture you look at anyway ....

[Reffc]

AN/e's Tom? I guess that the Kensington's were a missed oppo or you didn't care for them too much? Good luck with the trade...it's a fine amp.

[dicy]

Knowing John C (and Tom) - this can only be a thing of beauty.

A true craftsman

[montesquieu]

Actually I'm going to put this on hold till I've sorted out what I'm doing with speakers for definite. I should have waited a few more days before putting this up.

[esperowl]

Knowing John C (and Tom) - this can only be a thing of beauty.

A true craftsman

+1 having bought from Tom in the past; buy with confidence :thumbsup:

[NRG]

I got it direct from John and background is exactly as I've stated above more or less verbatim from him: all the iron and power circuit is as per the interstage kit (using shrouded upright transformers black powder coated rather than the cheaper bolt-in chromed ones), the input stage prior to the interstage transformer has been modified from two tubes to one to provide lower gain without NFB (sensitivity on the original was something ridiculous like 0.1mv making hard work even of a passive attenuator - just too much gain), the rectifier stage has also been improved to provide slow warmup. It did start life as a kit so layout is something that isn't necessarily a given. John is indeed a good guy and these mods come from very long experience. There were of course major changes between the Tim de P prototype and the kit that went into production so it all depends on which picture you look at anyway ....

Tom, its not an Original WAD push pull interstage amp as stated in your heading. The kits where different in their layout and circuit layout. The original amp used a GZ34 for slow warmup as standard, yes the input sensitivity was too high as standard. Your amp never started its life as a WAD original 300B PP kit, it's something John C built and modified based on the circuit. There's no dependency on which picture you look at, it bears no resemblance to the kit. One things for sure, its unique and I'm sure a dam fine sounding amplifier. I built and still own one of the originals since 1993...in fact one of the first few kits WAD put together.

[montesquieu]

Tom, its not an Original WAD push pull interstage amp as stated in your heading. The kits where different in their layout and circuit layout. The original amp used a GZ34 for slow warmup as standard, yes the input sensitivity was too high as standard. Your amp never started its life as a WAD original 300B PP kit, it's something John C built and modified based on the circuit. There's no dependency on which picture you look at, it bears no resemblance to the kit. One things for sure, its unique and I'm sure a dam fine sounding amplifier. I built and still own one of the originals since 1993...in fact one of the first few kits WAD put together.

You are actually quote wrong, it did start life as the standard kit and the sockets and heater wiring are still there for it to be put back to original spec if required (John offered this when I bought it but I preferred the lower sensitivity as I prefer an active preamp to a passive pot). I know perfectly well it started life with the GZ34 but as I stated in my listing the rectifier circuit has been modified - part of the benefit here is that the rectifier tubes used are cheap and readily available while good GZ34 tubes are scarily expensive now.

The distinction between 'original' and later refers to the fact it's the interstage amp and not an inferior version without the interstage transformer. The iron and power circuit are as per the original kit, only the driver section and the rectifier have been changed.

These are kits, and kits differ from build to build in execution and layout - this one perhaps more than most but John's knowledge is also far better than most when it comes to what works when laying out an amplifier as he works on them for a day job and knows their foibles inside out. It's still quite fair to say it's the original 300b interstage amp with modified driver stage and rectifier and that's what I've said in the listing. There's no attempt to mislead anyone here (which I guess you are implying). And yes it does sound fantastic, with my old Tannoys, the best of more than 20 power amps of all shapes, sizes and prices up to £10k retail!