UA 100 Preamp Output Transformer

[StarTrucker]

I'm starting a UA 100 Preamp project and would like to use an old Chicago style transformer that I have laying around for the output. It's not gapped for DC and my question is can I couple the output of the two triodes to the OT with a cap, ground the other end of the primary, and power the plates off a single plate resistor from the B+ supply.


Thank you for your sage wisdom; first-time poster, long-time lurker.

 

[ruffrecords]

Cap coupling to the transformer and using a plate resistor is exactly how it is done in the RED47 preamp. That preamp also uses negative feedback from the output stage plate back to its grid as does the UA100. So in theory what you suggest should be possible. The difference is the REDD47 uses an ECC88 output tube rather than a 12AY7. The ECC88 has a higher gm, similar mu and hence a lower plate resistance than the 12AY7 so in that sense it makes a 'better' output tube. In practice this means that in order not to lose a lot of gain, the 12AY7 plate resistor needs to be higher in value than for an ECC88 which means the plate current will be lower which means the output drive capability is lower. Having said that, as long as you are not planning on driving 600 ohm loads it should be fine.

Cheers

Ian

 

[emrr]

Max output will be lower. The other option is to use a choke.

 

[dmp]

There are also taps for negative feedback.

 

[CJ]

the 12ay7 plates are in parallel so you will have half the plate R as one section,

which chi town xfmr?

if it is big enuff, you might get away with 7 ma DC without the gap,

 

[StarTrucker]

These are the mystery transformers, one of which I'd like to use as the output - maybe old Chicago or early Stancor? Primaries are around 50 ohms and secondary measure around 200 ohms. Do these ring a bell with anyone - they're about the same size as a UTC A-11

The only reason I think they're non-gapped is because that's what the gentleman who sold them to me told me...pretty good huh?

What is the advantage of using a choke?

 

[ruffrecords]

These are the mystery transformers, one of which I'd like to use as the output - maybe old Chicago or early Stancor? Primaries are around 50 ohms and secondary measure around 200 ohms. Do these ring a bell with anyone - they're about the same size as a UTC A-11

I think you need a step doen transformer so you would expect the secondary dcr to be less than the primary.

The only reason I think they're non-gapped is because that's what the gentleman who sold them to me told me...pretty good huh?

What is the advantage of using a choke?

The advantage of a choke or transformer plate load is that a) the plate voltage is close to the HT supply voltage and b) the plate can swing above the HT. Basically the plate can swing twice as far as with a resistive plate load.

Cheers

Ian

 

[My3gger]

Cap coupling to the transformer and using a plate resistor is exactly how it is done in the RED47 preamp. That preamp also uses negative feedback from the output stage plate back to its grid as does the UA100. So in theory what you suggest should be possible. The difference is the REDD47 uses an ECC88 output tube rather than a 12AY7. The ECC88 has a higher gm, similar mu and hence a lower plate resistance than the 12AY7 so in that sense it makes a 'better' output tube. In practice this means that in order not to lose a lot of gain, the 12AY7 plate resistor needs to be higher in value than for an ECC88 which means the plate current will be lower which means the output drive capability is lower. Having said that, as long as you are not planning on driving 600 ohm loads it should be fine.

Cheers

Ian

Is red/wh connection on the output transformer made at "certain percent" of primary winding we see in pp power amps, maybe ct for some reason despite not being connected pp? Tertiary tap is obvious but i have never seen gapped trafo connected this way.

 

[emrr]

The resistances measure much too low for the job, implies inductance won’t be sufficient either.

I would bet $ on the orig being a center tap pri. Very few transformers ever with other ratio taps, but it’s custom, so…..the actual clue is whether or not they used the same in another amp.

Another tip - there are cases of PP 6L6 10W transformers being used SE with parallel 6SN7’s at 25mA for 1/4W output in pro broadcast audio, good specs.

 

[dmp]

The 100D had 40dB fixed gain with 33dB of negative feedback. Taking the negative feedback from the transformer taps 'cancels' transformer effects with the negative feedback (both the primary tap and the tertiary winding). If you skip any negative feedback you'll have higher gain.
This may not be the best tube preamp to try to make without the right transformer. Maybe better to aim for a design that isn't using a custom transformer for negative feedback and inductive coupled.

 

[Walter66]

Single ended output won't work with those units, use a cathode follower output stage for this low impedance, parafeed. That might eventually work.

 

[StarTrucker]

Thank you all for your responses. Sounds like I will need to order the correct OT for the UA100 project and find another project for these mystery transformers. Parafeed output section will be new to me but I like the idea of it.

Will upload freq response curves once I finish this one up.

Thanks!

 

[Walter66]

Parafeed is, technically speaking, an improved variant of transformer output. With its high inductance choke, the coupling capacitor and a completely bias- free output trans, it has better technical data and less iron is needed.
Especially in the UA100D, I wonder if a grid leak biased upper cathode is the best method for a cascoded tube input stage of high quality?

 

[emrr]

Western Electric and RCA used parafeed a lot, as did many prewar manufacturers.

 

[Walter66]

Really? I studied their tube preamps but mostly they were of the conventional SE or PP type of outputs. That type of parafeed preamp output was more likely to be seen in europe. For e. g., the EMI Redd series (Beatles, Abbey Road Studios) used parafeed output. Same with german studio micamps.

 

[emrr]

was more likely to be seen in europe

...post-war, maybe, like I said. Very common in American pre-war. Choke was an expensive part to ditch with post-war expanded commercial competition.

RCA 12-A, 24-A, 41-C, OP-6, BN-2A to name a few. Many in various early power amps.
Collins 12H console line and monitor amps.
Lots of Western Electric, Amertran, Remler, others.

 

[Walter66]

Thank you!