Fairchild 660/670 control amp transformers

[beatnik]

I am gathering parts and info about a Fairchild 660 project.

Found these transformers made by a company here in Italy , they are for the control amp output.

https://shop.piemme-elektra.it/Categories/339/Products/8134
https://shop.piemme-elektra.it/Products/7261

Two different ratios available, 5K/100 7:1 and 10K/600 4:1
I was wondering which one would be more appropriate.
Planning to use 6AQ5 tubes which should be very close to the 6V6 in the original.

There is an alternative from Sowter which is also 10K/600 4:1 but it's 4x times the price !!

Sowter also have a model for the control amp input which has a very high ratio 600/170K 1:17.
I remember seeing an older post where I believe CJ stated that a 1:10 would be fine, but I was wondering what could be the reason for Sowter choosing such a high step up ratio, is it because the extra gain from the transformer would put the threshold control in a more usable range ?

 

[CJ]

Here is a print from an original,

Can't vouch for why Sowter did that,
I can vouch for Shep Proudfoot,

 

[beatnik]

Thanks I remember seeing that exactly.

I am comparing this to the blueprint you released for the A-10 , they seem rather similar so maybe a current production AMI UT-10 could be a good choice ?

UTC A-10 blueprint

AMI UT-10 specs

I will try get some info from Brian Sowter as to why they have gone for the higher ratio.

What about the control amp output ?
I have not found a datasheet for the original Chicago output, but some postings seem to confirm it's a 10K / 600 ratio.
From what I understand for the Fairchild it's important to achieve very low impedance in the control amp output, so was wondering if the higher 7:1 ratio could be an improvement over the original 4: 1 ?

 

[thomasdf]

If you end up pulling the trigger on the Piemme transformers, I am very curious about them... Not for a 660 but for another tube limiter. They look beautiful (for lack of other tests) and the price seems very competitive !
This + not import taxes to another EU country... Seems too good to be true

 

[beatnik]

Check this out as well

https://shop.piemme-elektra.it/Products/7103

Should be good for Sta-level, BA-6A, etc.

 

[thomasdf]

I know, 39€ including VAT seems absolutely insane for this type of transformer ... Just unclear how much level it can handle !
I have a pair of BA6As in mind.

 

[beatnik]

Perhaps it's more insane what other manufacturers have been charging ?

It's a 20W transformer

From the docs I have it looks like Sta Level runs at about 14.8W and BA-6A 19.6W

 

[thomasdf]

I have an original General Electrics BA9 set of transformers, virtually the same as a STA Level and the output transformer is a effin heavy massive brick than can handle a lot more than 15W, these pieces of gear were overdesigned and overspec'd, than contributes to their sound a lot, especially regarding headroom and low end behavior. I am pretty sure the output of the BA6A is largely overspec'd as well.

That being said, in a modern setup there is very little chance that they run at +20dB output so that might not be a problem... I don't know. I guess I'll have to give them a try some day !