Chicago BO-14 100w amp

[lugnut]

Has anyone heard this amp or built one using BO-14 outputs? I have an overbuilt home brew project with 3 chassis using 2-12ax7,1-12BH7, and 1-12az7 as an input stage. Have not fired it up as yet.

[dcgillespie]

That is a rare transformer for sure! Do you actually have a pair?

Dave

[HollowState]

I have a pair that I've owned for years but haven't used yet. They're a "back burner" project waiting to happen. I've been told that they are excellent sounding trannys and they have a tertiary winding for feedback. Either output or driver cathode. A grungy looking single one recently sold on eBay for $273.
MASSIVE CHICAGO BO-14 100W OUTPUT TRANSFORMER RARE! | eBay

[dcgillespie]

This transformer was specifically designed for the very-new-at-the-time 6550, and was the first serious salvo fired in the super powered amplifier realm. Up to that point in time, all that really existed (for any practical purposes) was PPP KT66s, as the KT88 was absent in 1955, and the quad of (typically UL operated) 66s was no match for a pair of 6550s operated in 100 watt pentode mode, power wise. I have looked for these transformers for years with no luck. HollowState -- I hope you have the original diy article describing a Williamson style design for this tube and transformer combination. It is a classic design, and makes some rather lofty claims for new lows in IMD levels.

Dave

[HollowState]

Originally Posted by dcgillespie

HollowState -- I hope you have the original diy article describing a Williamson style design for this tube and transformer combination.

I'm not sure if I have the artical you're refering to or not. I have so many papers and magazine articals stuffed in a file cabinet draw that I haven't looked at in years. I do remember an artical in one of the vintage magazines I have a tall stack of that used the BO-14 with PP 6BG6s. I think it's a "Radio & TV News" from the late 50's or early 60's. But if you have the artical, post it or send it to me through the forum. I'd enjoy seeing it.

Victor

[dcgillespie]

I'm in the same boat as you -- I'll have to dig it up, but I recall the article as in fact being in the magazine your refer to, but dated Nov 1955 as I recall. This article was as much introducing 6550 tubes as it was the BO-14 transformer, as both were very new at the time. In it, they got around the screen regulation problem by using a companion power transformer that had not only the required HV winding, but taps on this winding as well to provide the proper screen voltage for 100 watt operation of 6550 tubes. The HV taps provided this power by way of a separate little 6X4 supply with a choke input filter utilizing dual chokes for effective regulation. Honestly, I don't know if I'd even want to try and lift the thing with this much iron on one chassis. It is undoubtedly heaver than a Heath WM-6, which is plenty heavy as is with just its two transformers!!

Dave

[lugnut]

Originally Posted by dcgillespie

That is a rare transformer for sure! Do you actually have a pair?

Dave

Dave,
I do have a pair of BO-14's along with 1-PCR-300, 1-RC8300,2-RC1585,2 separate fil. xformers,2 additional chokes,and a Triad power xformer(F-18A).It is 3 chassis loaded with tubes, parts and transformers I have pics if you can pm me your email address.

[lugnut]

It looks like an updated circuit of the original Ken Olson amp. The input stage looks similar to what McIntosh used on some of their amps.I haven't traced it out and listened to it yet but everything appears to be working on the bench.It is a monster and weighs a ton!

[kevinkr]

Lugnut please post pix here..

[Tubologic]

Yes, Kenneth Olson's 100 watt power amp was published in the November 1955 issue of RADIO & TV NEWS. (see attached picture) The Chicago Standard "Super-Range" B0-14 was a very high quality transformer especially designed for the 6550 and employing a tertiary cathode feedback winding. There is also a less powerful (30 watts) triode operation version described in the same article. Try to closely duplicate this circuit and you'll get an outstanding amplifier.