Well, this should be interesting (Dynaco/Eico hybrid)

Sam Cogley

Last of the Time Lords
Subscriber
I have an Eico HF-87 with no OPTs, and a pair of early Dynaco A-470 OPTs with no chassis or PT. Trying to find the Eico iron has been fruitless (at least at a reasonable price), and the basic tap setup of the OPTs is the same (except that the Eico OPTs had 32ohm taps not found on the A-470s). I might as well drop the A-470s onto the Eico chassis to build a working amp. Any thoughts or warnings about the mash-up?
 
My guess is it that it should work fine. I think the driver circuit in the EF-87 is better than the ST-70.

You may have to tweak the NFB loop and also the value of C11 and C12 (750 pf) to prevent ringing and instability.
 
Sam -- the primary impedance and screen tap percent is significantly different between these two transformers. Both work well with EL34s, but the Eico transformer provides loading conditions that work well with cathode bias operation, while the Dynaco transformer is only appropriate for fixed bias operation. The original power transformer in the HF-87 is the same as that used in the HF-89, which is a fixed bias amplifier. In the HF-87, the power transformer leads to provide this bias are simply cut off. If there is enough leads left, you can install the HF-89 bias circuits, and then use the the A-470s to good advantage with your project.

I hope this helps!

Dave
 
Did a bit of reading on the HF-89. It sounds like it ran the EL34s close to meltdown, and I know the iron was heavier than the Dynaco A-470s. It sounds like I should go with the HF-89 fixed bias setup, but tweak it a bit to bring the power levels down to more sane operating regions for the Dyna OPTs.
 
Hmm...the A-470s are just a bit smaller than the original EICO iron.
 

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Yes, those A-470 transformers will work great in that Eico chassis! I would suggest a fixed bias setup, just get a small 12v transformer and hook it up backward to the 6.3v and you will have about a 60vac bias source.

The HF-89 was biased at 63ma per tube, really hot for an EL34. There's no need for that, just bias them at 40ma - 45ma.

In the end your amp will be the same as a ST-70 I built a while back, I used the HF-89 driver circuit and it sounds amazing! There's a thread on it here somewhere.

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ST-70mod.gif
 
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