Help with UA 610-A DIY

[earthsled]

Hi All!

I'm hoping to revive some interest in this project. A post was started years ago... http://www.groupdiy.com/index.php?topic=13038.0 ...and there's plenty of good info there, but no one seems to be responding anymore. I've uploaded the schematic to the groupdiy docs (the original email seems to have disappeared).

Here's some info I've gathered...

A good option for a high voltage supply seems to be the "practical 300V regulated HT supply" as shown in Morgan Jones' book, Valve Amplifiers... http://www.scribd.com/doc/52148574/62/Specification-of-the-power-supply (page 360 to 362). I'm planning to use MJ's supply for the plates and JLM's AC/DC for the 12.6V heaters plus 48V phantom power. My setup will require two power transformers -- AnTek's AS-05T240 and AS-0515 (respectively) seem to be affordable options.

For audio transformers, there are a few choices from CineMag, Sowter, and possibly others. Right now, I'm leaning towards Cinemag's CMQEE-3440A for the input and the CM-9661A-L for the output. According to David Geren at CineMag, the 3440A will work best with the primary strapped for 150 ohms. Adjusting the secondary load resistance (R26) to 420K will result in an input impedance of around 1.2K. David also mentions that the current production model uses a modified version of the CMMI-7C and that R9 may have been adjusted for increased feedback (reduced gain).

Here's what I need help with...

I'd like to offer a high-impedance input for guitars, basses, etc. My initial thought was to bypass the input transformer and connect the unbalanced signal through a 68K resistor to the grid of V1-A (just like a Fender guitar amp). Will this work?

Also, I'm wondering if the circuit would benefit from a load / termination resistance on the output. Would switching a 620 ohm resistor across pins 2+3 be a good option to have?

Any comments or questions are welcome -- the more the merrier!

 

[funkymonksf]

BAD F%CKing ASS

 

[abechap024]

yea that is friggin sweet!

I want to build a tube console...

 

[0dbfs]

I think we need to rope you into the local gdiy-meet-and-greet.

I think your di approach should be great. May want to experiment with higher values too and see what you like.

Cheers,
jobu

 

[abechap024]

... May want to experiment with higher values too and see what you like...

I agree, I have a 47k resistor switchable with a 2.7M resistor on the input of one of my DI's 2.7M is great for "sucking" out all the tone you possibly can from some bass pickups. But more susceptible to crappy cables....noise etc...

Do you have schems for that?

 

[earthsled]

Thanks to both abechap024 and 0dbfs for your comments about the DI -- it's helpful to know I'm on the right track with this.

Check the groupdiy google docs (Meta-Meta) for the schem. It was uploded this morning.

 

[lassoharp]

I'd like to offer a high-impedance input for guitars, basses, etc. My initial thought was to bypass the input transformer and connect the unbalanced signal through a 68K resistor to the grid of V1-A (just like a Fender guitar amp). Will this work?

Also, I'm wondering if the circuit would benefit from a load / termination resistance on the output. Would switching a 620 ohm resistor across pins 2+3 be a good option to have?

I would omit the 68K series 'grid stopper' - just 1M or > grid leak for the Hi-Z input.  Grid stopper will just cause some loss in hi freq response.

A termination switch is a good option to have.  Sometimes running no termination into Hi-Z input is hardly noticeable, sometimes very noticeable and unpleasant sounding.  Staying around rated secondary Z should give best loading for the output tube.  600-680r is close enough for a "600r" winding. 


Nice score BTW!

 

[earthsled]

I would omit the 68K series 'grid stopper' - just 1M or > grid leak for the Hi-Z input.  Grid stopper will just cause some loss in hi freq response.

A termination switch is a good option to have.  Sometimes running no termination into Hi-Z input is hardly noticeable, sometimes very noticeable and unpleasant sounding.  Staying around rated secondary Z should give best loading for the output tube.  600-680r is close enough for a "600r" winding. 

Thanks for the info -- perhaps the original 1M2 value (R26) could serve as the 'grid leak' for an unbalanced input -- in any case, it seems like some experimentation will be necessary.

 

[TheMagicEight]

Hey guys....I know this is a somewhat older topic, but maybe there's still some interest.

I just built one of these! I got all the info from this site, then after a call to CineMag (who are excellent) I was all set to go (transformers I used are CMMI-7 and CM27101 (with code suffix G1)). I chose a Hammond 269GX for the PT.

I made some modifications based on what I remember from using a Solo 610 a few years back. Basically, I didn't bother with the EQ circuits, leaving them as if the switches were set to 0. I also used the modified input circuit John Hinson had posted, though at some point I'm sure I'll try the "original" variation.

I've got a couple of small bugs to work out still, but so far, it sounds fantastic!  I don't have much to compare it to (previously using Firepod preamps), but so far I'm very happy.

If anyone wants to hear clips or know more, let me know!

 

[tomas1808]

Oh yes we want 

 

[CJ]

does anybody have the UREI 8:2 tube mixer?

it had the 610 preamps,

thanks!

 

[TheMagicEight]

Oh yes we want 

Sounds good! I'm going to work out the final bugs tomorrow, then I'll post details.

 

[My3gger]

Hey guys....I know this is a somewhat older topic, but maybe there's still some interest.

I just built one of these! I got all the info from this site, then after a call to CineMag (who are excellent) I was all set to go (transformers I used are CMMI-7 and CM27101 (with code suffix G1)). I chose a Hammond 269GX for the PT.

I made some modifications based on what I remember from using a Solo 610 a few years back. Basically, I didn't bother with the EQ circuits, leaving them as if the switches were set to 0. I also used the modified input circuit John Hinson had posted, though at some point I'm sure I'll try the "original" variation.

I've got a couple of small bugs to work out still, but so far, it sounds fantastic!  I don't have much to compare it to (previously using Firepod preamps), but so far I'm very happy.

If anyone wants to hear clips or know more, let me know!

Hi,

is your CM27101 gapped or ungapped, i guess code suffix G1 means gapped.
I have one 27101 and it is 50:50Ni/Fe but don't think i've seen #G1 on it, so i suppose it is ungapped. Core is not that big and i'm wondering if it's worth building direct coupled circuit with it.
Eartshled mentioned using 50:50 CM9661 which i also have. It has bigger core and reading old 610 thread it seems like it should work.

I'm wondering what experiences you guys have using 50:50Ni/Fe OT's like this two in direct coupled circuits, when there is not much current, like 4-7mA. I could try them myself but have no means to test them properly. CM27101 and CM2810 are ime cheap and good sounding when cap coupled, i wish we could also use them for the other way.

Cheers,

Mick

 

[mitsos]

The real output trafo is small. If you look at pictures of the1008, and probably more  stuff that used this trafo, you can see the size.    Might be the size of the API 2623, and might be gapped (DAvid at Cinemag thinks it's gapped) but John Hinson thought UA had the trafo already and tried it out of out of convenience  so they wouldn't have a special part for this amp.  Just the 610 didn't use the extra winding for feedback.  I posted a cinemag part number that might be pretty good, it's small too.  I think it's 2092, it should be easy to find, it's in a 610 thread.  I haven't been able to try it yet.