200AB OT in Gates SA-39B

[Dmichel123]

We recently bought a really rough SA-39B and went through it. The original OT was bad, primary had continuity to case. We didn't have a suitable OT sub around, so I put the OT from an old HP 200AB oscillator in to test the function of the limiter. It works in the circuit, but I think the ratio isn't proper for the tubes.

Will changing the attenuator to a 150/300 Daven put a more suitable load on the 6V6s? Anyone know the ratio of the HP transformer? It's from the oscillator with 6K6 outputs.

Would it be beneficial to use the tertiary FB windings rather than caps from the plates?

 

[emrr]

Original SA-39 used the (then available) tertiary on the transformer.  They did away with that in the 1st revision, and 10 or so years in they made the first output transformer change to one lacking the tertiary.
Primary varied from 5K to 7K7 across the production.  Hardly matters if response looks right.  Changing from 600 to 150 on a Daven ladder may make very little to no difference, you'd have to listen and compare.  I know if you used the cheap Edcor output, you'd need to vary the FB path to make up for the lost low end. 

 

[David Kulka]

That's a fairly valuable unit. Why not have the original transformer rewound?

 

[emrr]

That's a fairly valuable unit. Why not have the original transformer rewound?

There are plenty of original spares out there if you take your time and look.  3 different models. 

 

[Dmichel123]

We are planning on either finding a vintage replacement or rewinding, but wanted to try the limiter in the meantime. It sounds great. We ran some mixes through it today.

 

[PRR]

The 220AB OT is an excellent unit.

A concern is that the nominal primary impedance is very high, to swamp leakage inductance above the audio band; they could do this since it only makes one watt from a pair of tubes good for 8-12 watts. So your maximum output may be down. However this may not matter too much in a limiter.