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.

 

[ruffrecords]

I get the feeling that the early broadcast market in the USA was wedded to 600 ohms in and out (as was the BBC in the same time frame). Bottom line, every output is expected to drive one 600 ohm load and nothing more. The only question then is how high a level do you want to be able to drive into that 600 ohms?.

So, a paralleled 12AY7 can run comfortably at 4mA. (1W of plate dissipation at 250V HT). This means its theoretical peak output current is 4mA or about 2.8mA rms. Into a 600 ohm load, 2.8mA rms is 4.7 milliwatts which is a tad under +7dBm. A 30K:600 transformer will increase this by nearly 17dB to +24dBm which seems about right. If you use a 9K6:600 transformer, the increase is only 12dB giving a maximum output of +19dBm.

Cheers

Ian

 

[systemtruck]

I get the feeling that the early broadcast market in the USA was wedded to 600 ohms in and out (as was the BBC in the same time frame). Bottom line, every output is expected to drive one 600 ohm load and nothing more. The only question then is how high a level do you want to be able to drive into that 600 ohms?.

So, a paralleled 12AY7 can run comfortably at 4mA. (1W of plate dissipation at 250V HT). This means its theoretical peak output current is 4mA or about 2.8mA rms. Into a 600 ohm load, 2.8mA rms is 4.7 milliwatts which is a tad under +7dBm. A 30K:600 transformer will increase this by nearly 17dB to +24dBm which seems about right. If you use a 9K6:600 transformer, the increase is only 12dB giving a maximum output of +19dBm.

Cheers

Ian

Thanks for the help. I’m a little confused because I’m new to tube stuff..
But to make a clarification first, the CM-9600T doesn’t have a 9k6:600 ratio as the name might suggest, it is 15k:600.

But my confusion is… doesn’t a higher turns ratio mean more gain reduction? Wouldnt the 30k:600 bring signal down further than a 15k:600? I apologize for the basic newbie question. I’m sure I’m completely not taking into account the way load changes gain of the circuit prior to it or something like that.

 

[ruffrecords]

Thanks for the help. I’m a little confused because I’m new to tube stuff..
But to make a clarification first, the CM-9600T doesn’t have a 9k6:600 ratio as the name might suggest, it is 15k:600.

OK, 15K:600 is a ratio of 5:1 so that changes the maximum output level by 14dB to 21dBm

But my confusion is… doesn’t a higher turns ratio mean more gain reduction? Wouldnt the 30k:600 bring signal down further than a 15k:600? I apologize for the basic newbie question. I’m sure I’m completely not taking into account the way load changes gain of the circuit prior to it or something like that.

You are confused because I was talking about maximum output level rather than gain/loss. You are right that a 30K:600 transformer brings the signal voltage down further than a 15K:600, but it increases the current it can feed into a load by the same amount and it is the tube current that is the limiting factor.

Cheers

Ian

 

[systemtruck]

OK, 15K:600 is a ratio of 5:1 so that changes the maximum output level by 14dB to 21dBm

You are confused because I was talking about maximum output level rather than gain/loss. You are right that a 30K:600 transformer brings the signal voltage down further than a 15K:600, but it increases the current it can feed into a load by the same amount and it is the tube current that is the limiting factor.

Cheers

Ian

Ok thanks again for this round of education haha.
“Limiting factor” regarding to what? Not distorting? Or perceived loudness?
I’m curious if you have an opinion about which of the two transformers designs you’d use in the context of this Putnam console… the 30K:600 gapped enough for 7mA (though not 10mA as noted in Putnam), or the 15k:600 transformer that isn’t designed to take DC but has the tertiary winding….. ?

To clarify, let’s say all else is exactly as designed in the Putnam schematics. So this means the Program Amp is as designed.

 

[ruffrecords]

Ok thanks again for this round of education haha.
“Limiting factor” regarding to what? Not distorting? Or perceived loudness?

Headroom = maximum available output into 600 ohms. As discussed above, this is primarily limited by the tube quiescent current and the transformer ratio.

I’m curious if you have an opinion about which of the two transformers designs you’d use in the context of this Putnam console… the 30K:600 gapped enough for 7mA (though not 10mA as noted in Putnam), or the 15k:600 transformer that isn’t designed to take DC but has the tertiary winding….. ?

To clarify, let’s say all else is exactly as designed in the Putnam schematics. So this means the Program Amp is as designed.

I am not a transformer expert, especially when one is carrying dc current. However, I do know that the single ended transformer output such as the Putnam designs does allow the plate to swing from zero to twice the HT voltage. It is the only class A topology capable of achieving a theoretical 50% efficiency (compared to the 25% you normally get with a class A stage). Rupert Neve clearly knew this because the BA283 output stage uses the same basic topology. So my choice would be the gapped 30K:600.

To my mind, the disadvantages of the single ended tube output stage are the dc in the transformer and the need for the tube to possibly withstand twice the HT voltage. In my own designs I avoid both of these by using an SRPP output stage running at about 6mA. As this is a push pull stage, the peak output current is twice the quiescent (12mA) wihich is about 8.4mA rms. This translates to just over +16dBm into 600 ohms so I only need a capacitor coupled 2:1 step down to achieve a maximum output of +22dBm. Note that although the quiescent current is nearly twice the value of the 6AY7 stage, each tube only has half the supply voltage across it so the dissipation is the same as the 12AY7 stage running at 3mA and I don't need a gapped transformer.

Cheers

Ian

 

[systemtruck]

Headroom = maximum available output into 600 ohms. As discussed above, this is primarily limited by the tube quiescent current and the transformer ratio.

I am not a transformer expert, especially when one is carrying dc current. However, I do know that the single ended transformer output such as the Putnam designs does allow the plate to swing from zero to twice the HT voltage. It is the only class A topology capable of achieving a theoretical 50% efficiency (compared to the 25% you normally get with a class A stage). Rupert Neve clearly knew this because the BA283 output stage uses the same basic topology. So my choice would be the gapped 30K:600.

To my mind, the disadvantages of the single ended tube output stage are the dc in the transformer and the need for the tube to possibly withstand twice the HT voltage. In my own designs I avoid both of these by using an SRPP output stage running at about 6mA. As this is a push pull stage, the peak output current is twice the quiescent (12mA) wihich is about 8.4mA rms. This translates to just over +16dBm into 600 ohms so I only need a capacitor coupled 2:1 step down to achieve a maximum output of +22dBm. Note that although the quiescent current is nearly twice the value of the 6AY7 stage, each tube only has half the supply voltage across it so the dissipation is the same as the 12AY7 stage running at 3mA and I don't need a gapped transformer.

Cheers

Ian

Amazing info, and this is a lot to process and consider which I will do, thank you. I’ll look into your SRPP designs.
1 more option though:
Can I ask how the ( 15k:600 w/ tertiary winding) CM-9600 might compare to the above, if capacitively coupled to remove the DC factor?

 

[emrr]

I might gently suggest you build and use some various preamp types for awhile and decide what you like before committing on a project of this scale. Define what you want it to sound like and do. Keep the mixer part separate. In fact anything I’d build would be modular so I could use different PRE and PGM types with the mixing system. For certain if I committed to one type I’d find a new favorite the next month. The mixer will work with most anything. Pick something you like the sound of and don’t pick esoteric parts requirements unless they prove an unavoidable value. I mix and match between a dozen+ tube preamp types on a regular basis; they’re cool for different things and there are some I wouldn’t sub for others, I’d use modern SS instead. In fact I mix and match tube and SS on almost every session and I’m not sure that anyone could pick apart what was used for what; it’s about the final picture not the parts. Every session is different, not that you can’t do fine work with one type but be prepared for changing perspectives and wants. Look at Ian’s designs. Look at the NewYorkDave designs (hopefully someone’s put those back up). Transformers in vintage are frequently the predominant sound. Different from modern. Building old circuits with new transformers gives different sonics - may be the circuit wasn’t the cool part. ETC ETC YMMV FWIW

 

[systemtruck]

I might gently suggest you build and use some various preamp types for awhile and decide what you like before committing on a project of this scale. Define what you want it to sound like and do. Keep the mixer part separate. In fact anything I’d build would be modular so I could use different PRE and PGM types with the mixing system. For certain if I committed to one type I’d find a new favorite the next month. The mixer will work with most anything. Pick something you like the sound of and don’t pick esoteric parts requirements unless they prove an unavoidable value. I mix and match between a dozen+ tube preamp types on a regular basis; they’re cool for different things and there are some I wouldn’t sub for others, I’d use modern SS instead. In fact I mix and match tube and SS on almost every session and I’m not sure that anyone could pick apart what was used for what; it’s about the final picture not the parts. Every session is different, not that you can’t do fine work with one type but be prepared for changing perspectives and wants. Look at Ian’s designs. Look at the NewYorkDave designs (hopefully someone’s put those back up). Transformers in vintage are frequently the predominant sound. Different from modern. Building old circuits with new transformers gives different sonics - may be the circuit wasn’t the cool part. ETC ETC YMMV FWIW

I agree, i think it’s time to build and see how things sound! Transformer wise and power wise I’m equipped to build a handful of preamps at once.

I come from the session engineering background so I have experience using some legendary ol’ preamps and boards and outboard gear. Some of the big guns. But a while ago I burned out from that and that’s when a studio friend gave me a Solo 610 as a gift. I’ve been weirdly obsessed ever since, having never found something so dark and wooly and mellow (I’m the opposite of the 1073 headspace haha). It’s all i use for personal stuff, so my goal is to meet or beat that and build 8 of them. Totally eccentric !

For a while I thought why not try building some RCA preamps and summing amps since whole board schematics are readily available and i know it’s all about big iron, but then i figured why should i veer when the “610 world” is already the sound goal. Then… you guys showed me the Putnam console which I didn’t know about. So the alarm bells went off, and i need to see if the “older” 100 series Putnam stuff is superior to the new or old 610, for this dark round wooly sound. I did not ever come across an old 610 to use in session, so I’m just not sure what’s related to what in terms of sound.

Ironically the new 610 is all new iron of course. That’s good news for me. So I know the goal is achievable. I already have two of the 30k:600 output transformers that Cinemag apparently made for UA for the 610, to the best of my understanding per David. He gapped them for 7mA DC, since he knew i was building an original 610 and that’s what’s noted on the schematics going around.

So this leads me back to where i am… what setups to build and compare? The best i can think of right now is…..
A, build an original 610 with it’s 30k:600 single ended output
B, build a 100D with that 30k:600 output and experiment with NFB configurations
C, build a second 100D but with the 15k:600 tertiary output with capacitor coupling to remove the DC.

I think i need to steer in the opposite direction of clean. In both the 610 and the 100D, maybe the heavier 15k load is better for me than the 30k load.

I guess the worst case scenario is they all sound too clean or bright, and i have to reverse engineer the new UA610 by opening it up and just build 8 of them. Those schematics are of course completely unobtainium. I can’t just buy 8 of those units, but even if I could i would have to heavily mod them since they have quite a bit of power transformer hum since it’s in such a tight package.

 

[ruffrecords]

I concur that you have reached the point where the only way to see a clear way forward is to build something and listen to it. Your suggestion A seems like an obvious place to start because you have an original to compare it to. If it turns out your new build version sounds radically different then there is some serious head scratching ahead

Cheers

Ian

 

[ruffrecords]

Amazing info, and this is a lot to process and consider which I will do, thank you. I’ll look into your SRPP designs.
1 more option though:
Can I ask how the ( 15k:600 w/ tertiary winding) CM-9600 might compare to the above, if capacitively coupled to remove the DC factor?

The downside of the SRPP is it creates a lot more distortion than a single ended triode stage and it needs about 20dB of NFB (but if you want readily available "tube tone" an SRPP on its own is a good way to go. I am right now building a 12 channel line level mixer consisting entirely of SRPP stages without NFB)

The problem with capacitively coupling a single ended triode stage to a transformer is that you need to add a plate resistor and so you lose the advantage of the transformer being able to swing the plate up to twice the HT voltage. Essentially this means you lose 6dB of headroom. If you look at the REDD47 preamp its paralleled ECC88 output stage draws 18mA quiescent through its 8K2 plate resistor and the tube dissipates 2.7W and that will drive a 600 ohm load via a9600:600 transformer. Bottom line is a capacitively couple single ended triode stage needs a huge quiescent current in order to have a decent headroom.

Cheers

Ian

 

[systemtruck]

The downside of the SRPP is it creates a lot more distortion than a single ended triode stage and it needs about 20dB of NFB (but if you want readily available "tube tone" an SRPP on its own is a good way to go. I am right now building a 12 channel line level mixer consisting entirely of SRPP stages without NFB)

The problem with capacitively coupling a single ended triode stage to a transformer is that you need to add a plate resistor and so you lose the advantage of the transformer being able to swing the plate up to twice the HT voltage. Essentially this means you lose 6dB of headroom. If you look at the REDD47 preamp its paralleled ECC88 output stage draws 18mA quiescent through its 8K2 plate resistor and the tube dissipates 2.7W and that will drive a 600 ohm load via a9600:600 transformer. Bottom line is a capacitively couple single ended triode stage needs a huge quiescent current in order to have a decent headroom.

Cheers

Ian

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. This 9600T (15K:600) is a pretty enormous build that David had mentioned people achieving +44dBm performance with, so maybe it can actually just handle the current somewhat. David had also mentioned the similar 9661A can take some DC current even though it’s not meant for it, due to sheer beefiness.
It would be SO cool if i could follow the 100D schematic exactly by using the 9600T (15k:600) and just let the DC roll through it and see how it sounds with the tertiary winding and all.

Anyways, yes I’m going to start building the old 610 design first. Then I’ll compare my new UA “solo” 610, which is the sound I’d like to meet or beat, and see if the original design build sounds as good or better. Like i said, the old 610 build will have the modern 30k:600 output transformer from Cinemag, which should in theory be identical to the one in the new “solo” 610 I’m comparing it to. So, that’ll be pretty interesting hearing the circuits side by side with same output transformers.

 

[emrr]

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!

 

[pucho812]

FWIW

I have been doing research on the goldstar console that was responsible for the wall of sound and other great recordings.

From what I have uncovered so far the desk seems to be comprised of 100D modules with accompanying eq module.

 

[systemtruck]

Amzzi

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!

Amazing. Interesting note about RCA stuff and I’ll take a look at that.

So in conclusion, I’ll use the 30k:600 gapped transformer for the 100D test build.
I need a plan first though..
1. The tertiary winding is nixed for the time being unless I have something custom built some day. So.. take the NFB from the top / positive of secondary? And otherwise keep the NFB values the same?
2. The center tap of transformer primary is gone (although it’s never been confirmed that this tap in schematic is actually a center tap). So where to start that line that feeds C10.. Use a resistor (value?) and connect it where? I know this is a stabilizing circuit but don’t really know how it works.

After this it seems I have the plans figured out and can start building.