Musical Paradise MP-428 300B Amplifier

[Rodango]

I recently picked up what I consider to be a steal of a deal in 300B amps... the Musical Paradise MP-428. All top-notch discrete point-to-point components and wiring. Output iron is big, heavy and supposedly Japanese.

But more importantly, it sounds great.

My question is tube choice. It uses 6AC7s and 6SJ7 preamp tubes. Why would these tubes be chosen over, say, 6J5s? Also it seems that 99.9% of all 6AC7s these days are metal. I'm a glass tube kinda guy and sure would like to replace the metal 6AC7s with glass. But I understand there is no exact substitute for the 6AC7. Is that true for the audio range? Would 6SJ7WGTs work? 6J5s? Something else?

Thanks,

[Rodango]

Wouldn't glass look a bit better?

[rotaspec]

6AC7 and 6SJ7 are pentodes, and the 6J5 is a triode so you would have to redesign the stages to accommodate them.
Simplest answer is to look for some 6AC7/GT and 6SJ7/GT. The GT suffix indicates glass tube.
ESRC sells both.

Gary

[Rodango]

I pulled the 6AC7s and replaced them with 6SJ7WGTs. Now I have glass all around and it still sounds great.

But does anyone know the downside of substituting 6SJ7s for 6AC7s? What changes would be expected?

[jaderust]

I also bought the MP428 recently. It is a great SET amp fpr the price. I had communicated with Garry, the designer and he informed me that the best replacement for both the preamp 6AC7 ( in the front) and driver 6J8P is the RCA red 5693. I realized you said you preferred glass, but according to him, this is the best.

[FullRangeMan]

Hi,
Nice amp, the building quality are very good unfortunately 300B tubes are expensive in my country.
I wonder if the face plate and sides are wood or thin steel??

I look forward if MusicalParadise could made a 6C33 15W SET monoblock Integrated amp to fit audiophiles that like this Russian tube.

OBS.> MP site inform the output Trafos are made with Z11 japanese laminated steel.

[jaderust]

The sides are steel. It is well made, the transformers are heavy, and the components are not cheap stuff normally associated with some Chinese amps. Regarding the cost of 300Bs, there is a switch on the amp which allows you to change to 2A3 output tubes instead.

[FullRangeMan]

Thanks for informing. It is a great amp for the money asked.
Surely it had a good sound quality, by what I see on the under side photos on MP site.

[ClefChef]

I wish I could get the schematics of this amp...
I am interested in this amp but I wonder about the specifics of its architecture. The musical paradise site gives this description: "Filaments are powered by DC regulated circuit for cleaner background and longer tube life". There is a picture of the circuit on the bottom of the product page which indeed shows two separate circuits powering tube filaments. Even though it says "DC circuit", it looks rather like high frequency AC circuit (or is that just a relay in the plastic housing?). Anyway, regulated filament circuit is a feature rarely found on tube amps at such price. DC circuit is difficult to implement and sometimes doesn't provide desired results. I was wondering about the specifics of this particular design. Hope someone can give me an answer.

Also if I buy this amp the first thing to go is the internal signal wiring from RCA to the board - it looks like a real signal choke...

[astouffer]

Looks like a normal relay to me. There's nothing magical about a regulated filament supply, probably uses a LT1083 or similar chip.