UA 610 Preamp - Transformer Substitute

[Winston OBoogie]

I don't understand what the point of connecting the 82 pF cap between the two cathodes is, other than for convenience's sake, because on the original, it was easier than finding a nearby ground connection?

Since the triode before the gain control has a cap bypassing it's cathode resistor, the small cap is going to see ground anyhow... correct?

Hey Max,
Bypassing the triode before the gain control still leaves the cathode at it's DC operating point which isn't ground.
It's been over 20 years since I analyzed the original 610 circuit but, a guess right now regarding the "point of connecting the 82pF between the two cathodes" would be that it's to feed a small amount of high frequency signal forward to the 1st 6072 stage to compensate for a bit of a high end droop around that fairly high impedance 250K level control. In other words, to counteract the Miller of that 6072
At 6dB down, a 250K pot by itself has a 62K5 output impedance. But we have the output impedance of the previous 12AX7 stage (which is fairly high) into the mix too.

I'm open to other suggestions as, like I said, it's been 20 years since I euthanized and gave an original 610 module an autopsy

 

[Winston OBoogie]

Ltn. Columbo (exiting the room): "Just one more thing"

I've noticed in a couple of inside shots of UA 610 units: 2-610, Solo 610, LA-610 etc., that the last tube section (12AT7 in some, but also earlier 6072 equipped units) looks to be quite under biased - if I'm reading my resistor colors correctly.
It should have about 6mA standing current, but, if I'm not mistaken, seems to be idling at about 1 - 2mA.
Don't know if that was an engineered decision for a "Revision" which seems an odd one to me, or if it's a slip up in the BOM etc.

Anyway, as you were gentlefolk, I'll go look for some "Solo 610" internal guts pics...

 

[MaxDM]

Hey Max,
Bypassing the triode before the gain control still leaves the cathode at it's DC operating point which isn't ground.
It's been over 20 years since I analyzed the original 610 circuit but, a guess right now regarding the "point of connecting the 82pF between the two cathodes" would be that it's to feed a small amount of high frequency signal forward to the 1st 6072 stage to compensate for a bit of a high end droop around that fairly high impedance 250K level control. In other words, to counteract the Miller of that 6072
At 6dB down, a 250K pot by itself has a 62K5 output impedance. But we have the output impedance of the previous 12AX7 stage (which is fairly high) into the mix too.

I'm open to other suggestions as, like I said, it's been 20 years since I euthanized and gave an original 610 module an autopsy

At first thought I thought it might be as you mentioned, but looking at where the 82 pF cap is connected, there's a .001 cap to ground, so effectively it's shunting any high frequency to ground.

If the 82pF cap had been connected from the cathode of the post-volume-pot stage, directly to ground, nothing would have changed functionally.

They look like high-frequency boosts, which, I am imagining, would compensate for HF loss in some other parts of the circuit.

 

[MaxDM]

Ltn. Columbo (exiting the room): "Just one more thing"

I've noticed in a couple of inside shots of UA 610 units: 2-610, Solo 610, LA-610 etc., that the last tube section (12AT7 in some, but also earlier 6072 equipped units) looks to be quite under biased - if I'm reading my resistor colors correctly.
It should have about 6mA standing current, but, if I'm not mistaken, seems to be idling at about 1 - 2mA.
Don't know if that was an engineered decision for a "Revision" which seems an odd one to me, or if it's a slip up in the BOM etc.

Anyway, as you were gentlefolk, I'll go look for some "Solo 610" internal guts pics...

Are the tube types marked on the chassis? Could the tubes have been of a different type when new?

 

[Winston OBoogie]

At first thought I thought it might be as you mentioned, but looking at where the 82 pF cap is connected, there's a .001 cap to ground, so effectively it's shunting any high frequency to ground.

If the 82pF cap had been connected from the cathode of the post-volume-pot stage, directly to ground, nothing would have changed functionally.

They look like high-frequency boosts, which, I am imagining, would compensate for HF loss in some other parts of the circuit.

I'm only speculating my thoughts from memory of the circuit and, as I said, it's been 2 decades and lots of brain cells lost since I analyzed it
So you're guess is just as valid as mine.

If, as you say, it's compensating for HF loss in some other part of the circuit, would that not include the high z interstage source against the 6072's input capacitance/Miller as I wrote? I'd need to look at all the elements of the feedback and the feedforward going on, along with the frequency of the boost and cut from that 12AX7 cathode junction into the 6072 cathode, along with the changing impedance of that 250K interstage and the shifting of the roll off point due to that and...
But for now, I do know that the circuit as drawn works, so at least I've got that going for me

Are the tube types marked on the chassis? Could the tubes have been of a different type when new?

In regard to the tubes type, I've seen the issue with both types they've used. They're similar enough (12AT7 & !2AY7 (6072)) that there wouldn't be the need for such a drastic change in how they are biased.

Again, no idea? Not my problem any more anyway

Cheers.

 

[grid_stopper]

Is it noise from the H.T. supply? That V2B is the first section that sees the main H.T after the filters, all other sections go through additional RC dropping and filtering sections so any noise will be reduced for those.

Do you have a circuit diagram of your supply we can look at?

I'm almost certain its coming from the HT supply at this point. Ive posted a schematic below. the only thing i couldthink of is that perhaps either C11, or C12 is leaky. Once the localelectronics shop opens back up tomorrow, im going to grab some replacements and see if that does the trick, but id happily welcome any other recommendations!

 

[Winston OBoogie]

I might have put one or two RC filters before your Mosfet but, see what happens if you put a fairly large value resistor between the top of R6 (1M5) and C12 (22uF). I'd even go as high as a couple Meg or more. This will give you slow ramp up on your H.T. initial power on voltage (in line with your heaters warming up), time constant being your new 2 - 3 - 4M and your 22uF. But it also takes advantage of the Mosfet being a capacitance multiplier.
Your circuit is class A so, that C12 22uF is being asked to supply a steady current so, it'll just sit there, quietly minding its own business while it's constantly being trickle fed by the new 4M or whatever value you have on hand.

Edit on this: Do you have a filter cap on the pre amp section, right before the H.T, enters the transformer winding?

 

[Winston OBoogie]

I have wondered, with my UA610 'solo' box, if it's possible to mid the DI/Thru aspect, to make this an isolated Thru,

...

Keen to hear if there's something I can add our modify, or wheat else have people done in this situation ?

CHEERS
Evan

Hey Evan,
I looked at internal pics of the Solo online, none showed a view of the rear of the 1/4" jack.
Nonetheless, the info I was checking for was clear enough.

As is, I don't see that it'd be an easy mod to get you that isolated thru jack within the Solo 610 chassis.

I'm open to others' suggestions but, I think, by far the easier solution is to use an external isolation transformer box.
You could still use the thru jack on the Solo, but you'd take that out to the transformer isolator and then to the power amp.

 

[grid_stopper]

I might have put one or two RC filters before your Mosfet but, see what happens if you put a fairly large value resistor between the top of R6 (1M5) and C12 (22uF). I'd even go as high as a couple Meg or more. This will give you slow ramp up on your H.T. initial power on voltage (in line with your heaters warming up), time constant being your new 2 - 3 - 4M and your 22uF. But it also takes advantage of the Mosfet being a capacitance multiplier.
Your circuit is class A so, that C12 22uF is being asked to supply a steady current so, it'll just sit there, quietly minding its own business while it's constantly being trickle fed by the new 4M or whatever value you have on hand.

Edit on this: Do you have a filter cap on the pre amp section, right before the H.T, enters the transformer winding?

just heading to the shop now, to grab some bits, and ill try adding the resistor

also, no cap between the PSU and transformer winding, but it seems like that could be worth adding...

 

[Winston OBoogie]

**Edited out because I misunderstood your post Grid.

Instead:

Yes, sounds good, lemme know what happens.

Just to be clear, I don't mean for you to replace the top 47K (above the 1M5) with a 4M or so resistor. Leave the 47K and 1M5 as is.
It's the junction of those two that you'll connect the new resistor to with the other end of the new resistor at 22uF C12.
Cheers

 

[grid_stopper]

success! a bit more filtering between the PSU and the transformer secondary and the noise is MUCH better, managed to push it down another 25 db or so. I also added a 3M resistor as suggested, and everything seems to be running nice and smooth

thank you so much for the help, really helped me wrap my head around this

 

[Winston OBoogie]

Hey Grid,
just saw this. That's awesome.
If you're getting 25dB noise improvement that's MASSIVE. Even if it were half that I'd be considering the mods a great success. Well done.

That 3M resistor is nice to have there, not only as a big filter with the 22uF gate cap, but as a ramp up of your B+ when you first switch on.

Cool beans

 

[grid_stopper]

Hey Grid,
just saw this. That's awesome.
If you're getting 25dB noise improvement that's MASSIVE. Even if it were half that I'd be considering the mods a great success. Well done.

That 3M resistor is nice to have there, not only as a big filter with the 22uF gate cap, but as a ramp up of your B+ when you first switch on.

Cool beans

yeah! a very pleasant surprise that i was able to get the noise floor so far down.

my only regret is that i didnt make a whole bunch of them, this thing sounds so good.

for anyone curious, this is the end result:

 

[Winston OBoogie]

Thanks for the pic. Nice job there.
Enjoy

 

[grid_stopper]

Realized i have most of the parts on hand to make a second one of these, with one caveat-that being that the OPT i have isnt gapped. would utilizing that transformer be as as simple as not sending the HT through the OPT primary, and adding a DC blocking cap, or am i missing something important?

 

[Winston OBoogie]

Missed this reply, sorry.
You could certainly resistor load the last 6072 anode and then cap couple to the output transformer.
You'll have up to 6dB less headroom but, depending on how you use the pre, it may not be an issue for you.


Aim for dropping about half your B+ voltage over the load resistor. It's been decades since I played with it and I forget exact details, but circa 15K anode R, 2.2uF coupling cap comes to mind.

 

[Dimon]

Hi
Please tell me what input and output transformers are used in the UA610? It would be awesome if you posted a photo.