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apnneto 27th July 2013 07:11 PM

Need Help - Monoblock Power Amplifier EL 34
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I want to revamp two monoblock amplifiers using four EL34 tubes in the output. I would like to maintain the appearance and arrangement of valves, but use a more modern layout. I need help to identify the scheme that could be used while maintaining the same original tubes and their positions. I'll redo the whole set, including rebuilding the power transformers and output transformes.

Any help is welcome. Below, some photos of the amplifiers:

JonSnell Electronic 27th July 2013 08:30 PM

A nice piece of Dutch workmanship.

Eli Duttman 28th July 2013 12:05 AM

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IMO, you're dealing with a variation on the "classic" Mullard 5-20 theme. The best "vintage" Mullard circuit amp I'm familiar with is the Harman/Kardon Cit. 5. High gm in the small signal complement provides protection against HF error correction signal induced slew limiting, in circuitry that employs a GNFB loop.

You can take the Cit. 5 idea and upgrade it by using an ECC99 as the LTP phase splitter and using a constant current sink (CCS) in the splitter's tail, instead of a resistor. The CCS forces symmetry between the 2 splitter halves.

Among current production 5AR4/GZ34s, the Sovtek is best. However, the Russian tube can get shaky towards the top of the documented voltage range. SS diodes in series with the 5AR4 anodes provides the extra PIV headroom needed. The graphic shows 1N4007s, but they are VERY noisy. Instead, use UF4007s, which are quieter. The vacuum rectifier blocks SS diode switching noise, but less noise, from the very beginning, has to be better.

BTW, the 12AU7/ECC81 is a non-linear POS. If you want to stay close to the original (as best as can be determined) use a 6FQ7/6CG7.

apnneto 28th July 2013 12:21 PM


Thank you for your quick replay. You gave me a great tip and I'll give it a try: mix SS components and tubes to improve the performance of the rectifier circuit.

I will redraw the circuit based on the monoblock components installed. Thus, we can compare the circuits used in this unit with Mullhard 5-20 theme.

apnneto 28th July 2013 12:28 PM

Nice "Look and feel"

A nice piece of Dutch workmanship.
Hello JonSnell,

This amp looks like a tank. Strong and his presence is noticed in any environment. Reminds me of my childhood, when spent hours listening to rock and rool. The more valves had the device, the more I was impressed. For this reason I wish to maintain the original appearance.

Regenpak 28th July 2013 02:42 PM

Why not try to find the original manual and use that as your base? Or even reverse engineer the schematic. It's a Philips amp, in its time top of the bill. The pinnacle of electronic engineering, if you will.

apnneto 28th July 2013 10:50 PM

Before posting here in this forum, I did extensive research on the internet and could not find the schematic of this amplifier. But I will rebuild the schematic based on the installed components. As the amp is very old, it is likely that the monoblocks have changed over the years. Once able to complete the layout, I'll put a copy here, so maybe you can help me to set the project to revamp.

rmyauck 28th July 2013 11:55 PM

Both amps look like they had Output Transformers mounted in between the 4 Octal (big 8 pin) tube sockets spaced far apart in the back left where the EL34's go. You shouldn't have to rebuild any of the transformers normally! Maybe just some cleaning and paint.

Anyway just wondering what they look like as the PS one is sure big.

You actually need a schem for PPP EL34. I think there are some good ones on this site and maybe diytube. They may be for 6550?KT-88 tubes, but should work fine.

Regenpak 29th July 2013 03:56 AM

Yes, there's definitely a void in the grime between the two pairs of EL34. And holes for the wires. That'd be a bummer. @apnneto: why not post a frontal pic of the amp and the innards? I'm sure there's not a lot in it and reverse engineering it would be fairly straightforward. Maybe there's info after all. It for sure isn't manufactured in the Netherlands. But then Philips was a multinational even before second WW. There must have been factories in Brazil in the fifties.

apnneto 30th July 2013 02:01 PM

New Images
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At first, I was thinking about redoing the whole circuit, regardless of what was installed. But now, I will try to improve the existing circuit. I will use the software Eagle to draw the schematic and post it here to see what could be improved.

The output transformers were added in the photos. I removed them to make some measurements, because one of them probably underwent repairs.

The amplifier has tone control and selection for photocell and microphone.

Nowadays, "Philips do Brazil" mainly produces lamps and electrical components. I had no success in contacting them to get the original manuals and schematics of the amplifiers. I believe that they should not have them.

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