LA-610 Mod

[sr1200]

Can anyone with vastly more knowledge than me (basically everyone on this forum) verify that this is legit?
http://www.foxaudioresearch.ca/LA610.htm
I've heard of just doing the pretube, but didn't recall ever seeing anything about the compressor section.  Also not too sure what this guy is doing wrong, but I dont hear my LA610 being bright at all, in fact i usually wind up boosting a lil top end doing vocals thru it.  But i do notice a little grit when i get past 0. (not necessarly a bad thing, just dirtier than my solid state TX based pres)

 

[pucho812]

o.k. well the tubes are compatible so I don't see you blowing up anything or smoke out a component or two but there are gain differences between the 2, the original tubes, the 12at7 and the one he suggests you replace it with the 12au7. The 12at7 has a higher gain so I dunno. give it a shot and at worst you won't like it. But if ask me, If your happy with the sound now, why change it.

 

[sr1200]

Good point.  A touch more headroom would be nice, but then again, maybe i should just turn the damned thing down a bit lol.

 

[squib]

for such a detailed investigation, there is one simple thing missing ... check the voltage UA decided to run the heaters on....fixing this is a significant improvement.

 

[0dbfs]

Thanks for this!
Will check it out soon.

Ciao,
j

 

[EEMO1]

i have a mk1. the heaters we're pretty low... the adjusting resistor on the adj leg of the regulator brought it around 5,8v.


the r76 on my mk1 unit is NOT the same as in the mod notes. It would be nice to have the schematics with mod info...

 

[Rossi]

I have a MK I, too, which I've hardly ever used because it always sounded fuzzy to me.

I just measured the heater voltage: 5V. Oh boy!

So the resistor that needs to be upped is R12?

 

[sr1200]

Im gonna ask them about that.

 

[Harpo]

I just measured the heater voltage: 5V. Oh boy!
So the resistor that needs to be upped is R12?

Seems so from 1.25V*(1+220R/75R)+0.0001*220R=4.94V.
R12 300R would get you in the 6.3V ballpark. (I'd use 2*150R an additional 82R in series to the 220R or 680R||560R)

 

[gemini86]

Throw a 500r trim pot in there, wired as a rheostat of course.

 

[pucho812]

anyone got before and after audio  samples?

 

[EEMO1]

rossi,

yeah r12. I lifted one leg and put in a 500ohm'ish resistor in series = 5,8v or something in that ballpark. just because that was the only resistor that I could find that I had lying around.

 

[Rossi]

Thanks guys! 

Well, here's "Oh Boy! Part II":

R12 is 2.2K (not 220 ohms), so, lazy as I am, I cut one leg and inserted a 620 ohms resistor (so didn't have to disassemble the whole PCB). Measured the voltage: 5.8V. Hmm. Inserted a 750 ohms resistor - still 5.8V. What's the problem? The input voltage is only 7V, so there's very little voltage for the regulator.  So 5.8V is pretty much the maximum heater voltage. 

I did some measurements with RMAA and an RME Fireface 800. From what I can tell so far, the performance difference is marginal - but then again, the heaters are still somewhat underpowered.

I also tried some tube substitutions. As I said, this is a MK I unit, so the stuff in the modding article doesn't apply. I replaced V2 and V3 (the input tubes; the numbering is weird), which are ECC83S, with a ECC81 (one at a time). Couldn't see much of an improvement; the high end seems to become a bit more linear, but you also lose a some gain. Distortion remains relatively high. Also tried an ECC82 instead of the 6072A output tube (V1). Doesn't look like an improvement, either, but I might give it another shot.

So far, no real improvements. 

Perhaps I better sell the box to someone who thinks a tube unit ought sound like that.

 

[CJ]

you could rectify that 5.8 up to 6.3 DC but i do not think that a low heater voltage is the problem, 5.8 should not cause distortion that could be heard,

maybe try an octal tube in there on the front end, a 6SL7 sounds great in that spot,

 

[mitsos]

John hinson said the output stage (of the 610) was changed to use a "cheaper" non-gapped output transformer. Looks like it might apply here too.  I remember him posting a slightly different input section too.  He's the guy that traced the 610 for UA from the putnam's originals.  Hinson's 610 schematic should be in the tech docs section.  FWIW, in another thread a while back, I posted an output transformer model by Cinemag that was told to me by David Bock, that "might" be based on the original output trafo, but I never got around to buying one. It was cheap, around $25 a piece, but comes with a lead time.

 

[mjrippe]

you could rectify that 5.8 up to 6.3 DC but i do not think that a low heater voltage is the problem, 5.8 should not cause distortion that could be heard

If the 5.8V is measured after a voltage regulator it had better be DC already ;-)  I agree that it shouldn't be causing significant distortion though.

 

[sr1200]

Does the mki have the switching ps like the 2 does?

 

[Rossi]

It's a linear PSU, and, of course the post regulator voltage is DC. I'm leaving it with the additional 620 ohms resistor now. I guess the only way to get about 6.3V would be to re-design the heater voltage part, perhaps using a dropping diode to get from 7V to 6.3 - but then the voltage wouldn't be regulated anymore and depend on the actual voltage coming out of the outlet.

Too bad we don't have a schematic. Perhaps the box could be improved significantly by just changing a few resistors. I guess we'll never know.

I found out that most of the distortion comes from the input stage. The easiest way to get more usable audio from this box is to turn up the output pot and not use too much input gain. That of course diminishes S/N somewhat (about 6 dB to get from 0.5% to 0.2% THD). Those were just measurements, I didn't have time for real listening tests, so far. But that confirms my previous observations, from the rare cases that I actually used this box, that you really need a 6-10 dB gain stage behind the LA-610.

 

[mitsos]

zener regulation?

 

[andyfromdenver]

For your consideration:
http://www.valvewizard.co.uk/heater.html


"Voltage considerations: The heater voltage specified in the data sheet is the optimum value (usually specified as +/- 10%). Running them at higher voltages will considerably reduce valve lifespan and should be avoided. Running them at lower voltages, within reason, will increase their lifespan and reduce hum but also reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced. Running heaters under-voltage is therefore perfectly acceptable, and provided the voltage isn't too low there will usually be no noticeable difference in sound. Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage since they usually operate very close to saturation.
It is not uncommon for the mains voltage to be slightly high, resulting in a heater voltage that is also proportionately high. If this is the case where you live then the following may be useful: A pair of ordinary high-current silicon diodes can be added to the heater chain to drop the heater voltage by about 0.7V. (If using a DC heater supply, only one diode will be needed.) This method can also be used for a heater standby switch."

Merlin Blencowe's page is a good valve resource.