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Saturday, May 10, 2014

300B Amplifier Modifications

300B amplifier modifications turned 300B repair!

We've received a case from customer on upgrading his 300B SET - changing Alps Blue Velvet to stepped attenuator, change stock output transformer to Precision Wind Z11 OPT, and upgrade some of the components.

Upon opening the chassis, it turned out to be a nightmare! At the end, as the design was too bad, owner preferred to just repair the amplifier and sell it. Original funds will be used to build a brand new J&K 300B SET instead (Like!).


300B SET

Vintage GEC U52 Rectifier
Looks fine from far but is a nightmare!
This design is flawed. Putting the tuning factor, there are several design rules violated.

It uses capacitor input rectification but the 1st capacitor connected to the rectifier is at least 470uf x 2 in series, resulting a whopping 235uf for the GEC U52! Maximum value for capacitor input is 47uf - 60uf. Please say hello to the GEC U52 tube before it says bye bye.

DC is used for indirectly heated tubes - unnecessary. AC is fine and is dead quiet. AC sounds better too. Do not hide the design flaws by using DC filament supply. 300B filament supply is just using a bridge + 10,000uf capacitor - high ripple and bad for 300B filament longevity. Such DC supply does not sound good too.

Cheap source selector is used. Period.

Wrong design of how the 5692/6SN7 is connected. It uses only 1 triode from the double-triode but it is wired the same for both sockets. It should be alternated so that the tubes can be swapped to utilize the unused side of the double triode.

I better stop here before everybody stops buying this amplifier.

Trios that need to be changed as they leak and bloated - going to explode soon.

Change Blue Velvet to J&K precision matched stepped attenuator with SMT resistors for shortest signal path.
One of the cold solder joints (tonnes of them!) - touched and it just snapped off.

Original condition - coupling cap is mounted with double sided tape!
What kind of job is that - sticking the capacitor onto the chassis with a normal strength double sided tape? Crappy job!
Good parts are used but 1st modder workmanship is pure rubbish!

Original: wires are "glued" using solder onto the IEC socket. What sort of rubbish this is...

Parallel rectifier - bad design.
Bad design turned savior? 1 of the GEC U52 has already got 1 of its' filament burnt. It could still work since the rectifier is wired in parallel.

Bloated trios.
Left and right capacitors are leaky and bloated. Right capacitor still looks fine but it was changed too as it might have aged. Fresh capacitors are installed.

10th May 2014
J&K Audio Design

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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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