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

[Brian Roth]

Best case it works and you have much higher distortion. Worst case it burns out the transformer wiring. 10mA is a lot to ask of an ungapped transformer, do the comparisons on the expected current as designed, consider the primary wire size and the primary resistance.....ask the designer.....

It's rated 6W = 37dBM, that suggests you might get away with it but I'd rather order the right custom part or pick another design versus burning down the part. There's no provided datasheet with info for the 9600T that I've ever found. This is an Ampex 351 output replacement right? I read elsewhere in that circuit it's PP 12AU7 drawing 13mA. So you're wanting it to draw almost as much current across twice the wire resistance, before considering imbalance. If it's like the Sowter it's 1500 ohms, they tend to use big wire and have 'lower than' DCR's, that suggests the wire can take it but it's also mighty low for a non-power tube output stage. I suspect it's actually slightly gapped to account for imbalance at it's current/power levels anyway.

44dBm is 25W. That's gotta be with reduced low end and it needs a current measurement to go with it.

All the RCA stuff is direct coupled output transformers for higher signal levels with lower current requirements. Keeps consoles cooler and running longer. OP-6 remote amp is an outlier having a parallel feed approach, same result.


RANT - holy cow the number of things autocorrect wouldn't let me type in this post, and the number of others I had to find on review after posting, and correct! The robots are out of control!

Oddly, I was recently looking at the 351 schemo for a different reason ....answering a question for someone. Here it is, in all it's glory!

Bri

 

[systemtruck]

Well....change those things, it's a different preamp. No point copying the rest, really. The existing NFB and C10 is more about the EQ than anything else. You really only pull NFB from the secondary when it's a speaker.

I suppose I could give it a shot with just no NFB at all, for fun. I’d expect the current draw to jump and tax the power supply, and for maybe those poor little 12AY7’s to glow up. But if it’s stable, maybe it’ll sound terrific after some attenuation on the output. I’ve had great luck with that in another project here.

I don’t know what I’m doing regarding calculating dB loss/gain with the NFB in relation to the rest of the gain, but if it operates as a standard voltage divider, i can say that the signal going back into the input in reverse phase is about 10dB less than the signal it’s pulling from the tertiary. That comes from a 8300 / 3790 voltage divider. Then, with the modded schematic here, which brings the top of the divider way up to 24000 by adding two 12k resistors in the EQ jump path, the NFB signal should be an additional 7.5dB less. So in my pretend understanding that says the signal will jump 17.5dB without NFB. There are also a couple capacitors strapped in there and i don’t know the impact on NFB.

Anyways, maybe I’ll reach out to David at Cinemag about a custom output transformer that is gapped, and has center tap of primary, and has a tertiary winding. I’m guessing he’d just take the 9600T and add the gap and center tap. That might be helpful for anyone wanting to do 100D builds going forward. However, I’m really not sure what the turns ratio and impedance of the TT 5471 is.

 

[ruffrecords]

Very interesting stuff.
Makes me want to ask a meat-head questino…. Other than saturating the transformer and losing audio performance like low to high frequencies, what happens with SAFETY / DAMAGE when i go right ahead and toss 10mA of a HV DC current through a non-gapped 15k:600 transformer running in single ended mode? Do i burn the house down? Do i destroy the transformer? Do Tesla lightning bolts shoot my eyes out? Just wondering in case somehow it sounds good.

It is a conventionally biased class A stage. Essentially this means the power it draws over time is constant most of which will be emitted from the plates. You are unlikely to get that stage to draw 10mA unless you change its bias and then you will simply cook the tube. The only power dissipated in the transformer will be due to the plate current and the transformer primary dcr. Even if the dcr is 3K, then 10mA running through it will only generate 300mW of heat

Cheers

Ian

 

[systemtruck]

An interesting and pertinent update about the transformers for the 100D…
I spoke with David on the phone, great guy.
Firstly he thinks that 9600T can handle the up to 10mA DC current of the 100D without batting an eye. However, the ratio of just the tertiary winding of the “stock” 9600T is 26.5 : 1, so pretty small, which he says he can’t always guarantee will be strong enough to provide enough NFB for any given task.
He did make a custom 9600T for someone who wanted a much stronger NFB signal, so they made it 5.5 : 1 : 1. The secondary and tertiary are just as strong as each other. He saved this design as an actual Cinemag part# so that others can order it, and named aptly the CM-351. I’m assuming this is THE transformer that is shown in the example schematic on the Cinemag website, but i didn’t confirm that with him. So if someone wants to order that specific transformer, it’s probably the CM-351.

Now, I’m not certain that the 15K:600 impedance setup of the 9600T is best for the 100D, but i suppose i can’t hurt anything by trying. They are around twice as expensive as the 30K:600 CM-27101, which doesn’t have the tertiary. Certainly this will be an influence on using 8 of them.

I’m looking forward to getting this stuff active in test builds…

 

[emrr]

Given R5/R13 values, one could probably sort the Z range of the tertiary based on what it takes to make the EQ do what it's quoted at. 220K is pretty big also, suggesting room to play, possibly. Or simplify, and change the 220K until the gain is as it should be.

The 'rule' on NFB most followed is none or 20dB+, anything in between is noted for making additional harmonics without attenuating them sufficiently. There are outliers out there.

My biggest problem the the 100D is a cascode front end with pentode-like gain, and no interstage gain control. You will virtually always have a 20dB 'U' pad in front of this amp unless you only use ribbons with moderate volume sources.

 

[systemtruck]

Given R5/R13 values, one could probably sort the Z range of the tertiary based on what it takes to make the EQ do what it's quoted at. 220K is pretty big also, suggesting room to play, possibly. Or simplify, and change the 220K until the gain is as it should be.

The 'rule' on NFB most followed is none or 20dB+, anything in between is noted for making additional harmonics without attenuating them sufficiently. There are outliers out there.

My biggest problem the the 100D is a cascode front end with pentode-like gain, and no interstage gain control. You will virtually always have a 20dB 'U' pad in front of this amp unless you only use ribbons with moderate volume sources.

The lack of gain control is a killer. Pad would be needed, and the idea of being a good recording engineer using proper “gain staging” in sessions is not an option.
What if we took R7 470K and turned it into a pot? Feed both pins 2 and 7 with the one wiper. Does that screw up, or accomplish anything?
Or, what if R7 is left alone and a 1M dual pot is placed just before pins 2 and 7, so that both inputs to the output triodes are attenuated together.

 

[emrr]

NFB 101 - can’t put a gain pot (voltage divider) in the middle of a NFB loop without changing the NFB per knob setting. It’s self defeating. It makes the knob throw super weird.

There’s pretty much no time I ideally want pentode-like gain after a high gain input transformer, even if there’s a voltage divider pot right after it. It’s almost always too much. Transformer/triode/pot is the greatest flexibility.