Brimar 25P1

[OldHector]

I have the valves for the Brimar 25P1 amplifier. This appears to be a well known and appreciated design, so I was a little surprised I could find a reference in this forum.
Brimar 25P1
Has anyone built this? I’d like to know what impedance OPTs were used. The article (and the book from where the article is taken) only refer to the Partridge and Savage part nos.
Also, has anyone used a 6BS7 in place of a 6BR7? I’d like to know how one adapts for a top cap to the grid, and whether it is a worthwhile swap.

 

[32860]

Some info on the Partridge P5352.

 

[OldHector]

That’s great, thanks!

 

[trobbins]

The 6BS7 and 6BR7 would only show up as different in an amp due to ability of constructor to design out hum issues (or inadvertently design hum in). The top hat connection pretty much only benefits the user if you have some seriously low grid current requirements - and that is very specialist area, where lots of other techniques are deployed as well. Most people would use an EF86 as it is common and likely to exceed most peoples capability to reduce hum from all other sources.

 

[Diabolical Artificer]

The 6BS7 is used in a lot of old valve voltmeters, often for it's very low grid leakage. The 6BR7 is a good valve and still available for a reasonable price.

JLH reckoned the 25P1 a very good design, it's in one of his books.

Andy.

 

[OldHector]

The 6BS7 is used in a lot of old valve voltmeters, often for it's very low grid leakage.

Would you use a 6BS7 in this application? I also have the 6BR7.

The 6BR7 is a good valve and still available for a reasonable price.

That was part of the motivation. Likewise the 5B/255M can be found for a reasonable price, unlike it’s contemporary the KT66.

JLH reckoned the 25P1 a very good design, it's in one of his books.

Andy.

That’s what prompted me to look closer, after finding a link when looking up information about the output valves.

 

[Diabolical Artificer]

"Would you use a 6BS7 in this application? I also have the 6BR7" Why not, if you have them, use em, but as you have 6BR7 anyway... You could try both, it's only one extra hole in the chassis and one connection to change. To be honest in this application there's not much to chose between them.

Yeh, KT66's are daft price wise, why they sell for more I suppose is because they have a mythos and look nice.

Andy.