Original UA console - 100D preamp, EQ, 101D program amp

[systemtruck]

The 610 seems to me to be a 'cost reduced' version of the 100 series. The 610 has both tubes set up as two stage CC with NFB which allows the channel fader to be placed between the two stages. The limited 610 EQ looks like a very cut down version of the 100 series implemented in the NFB of the second tube and because there is only one tube driving the output transformer it has a 30K primary which throws away 17dB of gain and may go some way to explaining anecdotal reports of distortion in the 610 under some conditions.

Cheers

Ian

Old thread!

Wondering about these older 100D Putnam preamps VS the original 610 preamps, since I was about to build a 610 or two but learned that maybe these sound better.

1. The 610 uses a 29k/600 output transformer, to be best of my knowledge. So I guess this 100D would use something in the ballpark of half that so a 15k/600 ? … and something that can handle DC since it’s single ended? Any ideas of an off the shelf replacement from Cinemag / Lundahl / Jensen / Carnhill / Sowter?

2. The 100D preamp has no gain / input adjustment where as the 610 has a volume pot between the two isolated NFB stages. The Putnam console then follows the preamp with a volume pot soon after. But this of course doesn’t allow adjustment of the gain/drive/overload of the preamp itself if that is needed. So I was wondering.. could maybe a pot between mic transformer secondary and input of first tube work cleanly and not throw anything off? I’ve seen examples of this in Collins and Fairchild designs. Interestingly, this document here https://funkwerkes.com/web/wp-content/techdocs/MixedProAudio/UniversalAudio/UA100D.pdf has a note about modifying the 100D preamp for more gain. The #2 option of a shunt resistor at this very point is removed where as in the unmodified schematic there is an option there for loading the transformer when using pre in booster mode. That value is 82K. Do you suppose putting a 1M pot there would work to provide some input level control without reducing overall possible gain of the circuit?

 

[systemtruck]

Still going over this preamp design and considering how to implement an output transformer.
Aside from going custom for the transformer, it seems that using off the shelf versions there will be compromises..

1, The parallel 12AY7 has a plate resistance of 12.5k. Every output transformer that I’ve found, at least from Cinemag, Carnhill, and Sowter, the primary is designed for around 12k to 15k. So, if I used the 15K, I guess that may work ok. That’s the Cinemag 9600T.

2, I haven’t found a single transformer with tertiary that is designed to handle DC current. So I guess this circuit, if planning to use a tertiary winding, will need to be parafed / cap-coupled.

3, It’s obviously much much easier to find a single ended transformer that is the appropriate impedance rating that can handle the DC current. I suppose the circuit could still sound ok without the feedback, OR the feedback could be fed from a leg of the secondary and voltage divided back into things. Certainly both of these options would perform differently than the original.

So, I could check with a company about gapping a tertiary PP transformer, and about increasing its primary impedance.

 

[ruffrecords]

The big problem for the 100D is the 12AY7 is not really an output tube, even if the two halves are connected in parallel. Its 22k plate resistance is the real issue. The ECC88, for example, has a plate resistance of around 2K5 and even then, EMI paralleled the two in the REDD47 output stage. Unfortunately the ECC88 did not come out until 1958.

Cheers

Ian

 

[emrr]

It is an odd choice. Perhaps looking at the RCA BC-2B preamps with 12AY7 and 50kish output primary. Or BA-21A, PP 12AY7 into 30kish.

My guess would be current/heat reduction inside the console.

 

[systemtruck]

Maybe there some transformer selection / compromise that can make this preamp function well enough to sound pretty original?

If the parallel plate resistance of two 12AY7’s totals just above 10k, then maybe a transformer meant for 15K but slugged a little high on the secondary will suffice ? To bring it up to 20K we’d only need to bring that secondary slug up to 800 ohms. Incorporating any permanent circuits after this of course, to total an 800 load.

The Cinemag 9600T has the highest primary impedance of any tertiary wound transformer I could find. All the rest seem to be 12.5 K ish (which I suppose are all mimicking the Peerless 217D?)

The comment from @ruffrecords near the beginning of this thread about wasting gain from the required 30K : 600 ratio transformer of the old 610 seems relevant now. ( For what it’s worth, i know the new Solo 610 also uses the 30K:600 ratio because I bought two of the transformers from Cinemag that I believe is the OEM design for the Solo, the CM-27101. He gapped mine for 7mA ) ………

…. so what turns ratio does this original Putnam preamp design appear to call for?

I could try this circuit with that 30K:600 but I am guessing it will sound dramatically different than the 9600T which has the tertiary winding AND it requires the transformer circuit section to be capacitor coupled.

Neither seem to be great solutions but one has a tertiary so I’m leaning that direction.

Or I could check with Cinemag about custom making some 9600T’s with enough gap for 10mA of this Putnam pre.