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Bogen HO50 1949 AB2 vintage amp

Posted by ivan_terrible (P) on October 19, 2020 at 00:00:46

As you can see the results of the rebuild of the now beautiful H50 here
https://www.audioasylum.com/cgi/vt.mpl?f=tubes&m=277691

I figured it was also time to give an update of the results of the identical pair of HO50 which were rebuilt in 2019.

In that case I decided to leave them with their original 110V transformers, and buy in a superb USSR (Polish) made 230-110V convertor to power both of them, housing that in a nice little box, ordered in specially.

The transformer is rated at 0.95KW...and terminated in those gorgeous little Bulgin plugs - one for each channel,to make it easy.

The tops were blasted and powder coated, then some special transfers made to put the DB insignia back on them.

The bases were also blasted and powder coated, but warped a little with the force of the blast process. They only sit flat, once screwed back on the cases.
There's a reason to have the tops and bottoms all neatly screwed on.

You don't want your kids going anywhere near that exposed 500V internally!

Of course the mods to the original circuit are really numerous.
The -42V negative bias lines being derived via a stack of 5W zeners, lurking in the HT supply transformer CT return.

This was a tough trick to execute properly, as the decision to change the entire amp to rare matched pairs of genuine GEC KT8C also turned out to be a struggle.

(Basically this converts the 807 amp to NOS KT66, on an ancient B5 base, but an affordable price instead of the insane prices now worldwide for the octal base genuine GEC KT66.)

Of course the KT8C takes 30% more heater power than the 807, as it has a much fatter larger cathode. 1.27A instead of the 807.

Compare the 807 versions with the KT8C

How does this ancient perform, you may ask?

Well, it was directly compared against a fully modd'ed MO200,a Bogen DB110 and several other brand new "hi end" amps costing lots of money.

We also ran it into Vandersteen brand new speakers & Amphion.

The result was quite a surprise, being as they have an output power and dynamic range which exceeds most moderns we tried.

A 70yr old amplifier that works as well as this with balanced line input too?
Surely not?

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

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