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ReinoutdV 6th April 2020 07:06 PM

Philips MRO PT8 preamplifier
9 Attachment(s)
Good evening DIY Audio,

my new preamp is just finished and i'm really happy with the results.
Some years ago i had 2x EL82 poweramp monoblocks but although good sounding they were not to remain. One of the monoblocks was rebuild to a 6C45PP SPUD-amp and the remaining chassis/monoblock was the basis for the new preamp.

This new preamp is in essence a Kimmel-/Mu-stage preamp with a Philips MRO PT8 as "bottom tube" (voltage amplifier) and a Amperex 8608 penthode as "upper tube".
The volume control are a couple of step attenuators from scrapped (Rohde & Schwarz) measuring equipment. Each channel has 2 attenuators:
- one of the attenuators has 6 steps of 10 dB;
- the other attenuator has 9 steps of 1 dB and 5 steps of 0,2 dB.
More than enough options.

The power supply is completely in a separate housing but fixed to the audio-housing. One of the connecting tubes is hollow; here is the cabling between power supply and audio part of the chassis.

In one of the pictures you can see the PT8 preamp next to the 6C45PP SPUD-amp. Same chassis but the one with the bigger tubes is the preampů

You don't see MRO PT8's in abundance but many manufacturers produced equivalents as these repeater triodes were quite common decades ago. The 4 volt 0,25 filament already indicates that it will take some time to heat up. Correct ! It functions within half an hour; it performs in a hour.
STC 3A/107A, Standard Electric 4019A, WE 101L and Philips 4609 are equivalents.

What does this preamp bring: amazing soundstage ! My trusty 20 years old Gryphon XT-MC has seen many competition last years but always came out as the better performer. Till now.
From sweetspot to sweetroom ?!

Although i have some PT8's in store it's time to look for 4019A's, 3A/107A's, WE101L' if you have those i'm interested.

Keep showing all your interesting projects on this forum. So many interesting items to see.

VuGiaA9 15th April 2020 02:42 AM

I hear alot about the MRO PT8. But it's very hard to find.
Nice build :D

ReinoutdV 15th April 2020 04:39 AM

Good morning VuGiaA9,

thank you for the comment.
You're right about the availability of that triode; not easy to be sourced. And if you find them (on Ebay) they are "funny-priced". I noted that after i described my preamp on WWW the prices suddenly went up a notch ?!

Using the Tube Vade-Mecum (great reference book) it's possible to source equivalents. There are more 4V/0,25A filament triodes to be found with near equivalent characteristics. Most of the time it is just a matter of another socket but that can be dealt with using converter-sockets.
WE101L (please use the "L" if you want to have have a near equivalent), STC 3A/107a, 4019A, etc....all good. If you can use B4-sockets/convertors there are options from Philips, Telefunken, Marconi, etc. These triodes were made by almost all tube manufacturors pre-World War II.

If your amp can provide 4V/1A then you don't have a problem at all because you can use the WE101D which is currently produced again by a number of companies ! I noted that there is even a modern version available with an UX4 socket instead of the standard UV4 socket !

Regards, Reinout

dady 15th April 2020 01:11 PM

which feastrex do you use?
and the woofer one, is biamp?

ReinoutdV 15th April 2020 02:46 PM

1 Attachment(s)
Good afternoon Esteban,

The mid/high is done by a Feastrex-unit: the D9NF.
The low is done by a Fertin unit: 38EX (field coil)
Description of the speaker-project is to be found here: FAST / WAW with Feastrex D9NF and Fertin 38EX
Very happy with the result. Currently i ran these with my 35TG amplifier as the really big amplifiers were swapped for an interesting turntable. The 35TG pushes out 10 watt which is more then enough.

Regards, Reinout

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