LA-610 Mod

[0dbfs]

I've got two MKI units (#'s 990 & 1016) which do not have relays nor an SMPS and they have 5V heaters on all the tubes.
V1 is a JAN 6072A while V2,3,4 are ECC83S's and V5 is a JAN 6005W. The 83s's are JJ's except a couple are unbranded.

More to follow!

Cheers,
jb

 

[emrr]

in the MK II, they use lots of relays, these are 5V relays and get their supply from the heater voltage.

is this the reason they chose 5V for the heaters?

if I take a tube and put it in my tube tester and measure its emission. I get a very different result on 5v compared to 6.3 V on the heater. How does this translate in a practical circuit?

Could be why, probably is.  Though, the relays could be dropped to 5 from 6, if wanted too. 

Tube tester uses a different set of parameters for indication, and likely does not reflect real world events.  Emission testers are generally not well regarded for comprehensive testing. 

We have no proof there is a problem with 5V operation, we need measured apples-to-apples differences shown, to have a smoking gun.  It's kinda funny to me that everyone accepts under-heating in tube mics, but revolts in every other instance.  It depends on the tube and the service it's placed into.  We have Gates Radio manuals that talk about running 6.3V tubes as low as 3V with no operational differences noted outside of warm up time.  I have noted many situations in which this is the case; very little difference when powering tube equipment with a variac, and having the entire system at 75% of expected line voltage.  Most things power on and run at 50%. 

The point about the gain versus input controls is huge, and inline with my earlier comments about philosophical operational differences. 

 

[Rossi]

My MK I unit has no relais, either, but has the 5V heaters, so that's probably unrelated.

As I said, rising the heater voltage to 5.8V didn't make a huge difference. It certainly didn't cure all that's wrong with this model.

I think underheating a tube mic and underheating a preamp/channel strip are not the same thing, as the latter has to deal with higher signal levels at least in the output stage. Also, 5.8V is about the voltage Gefell run their tube mics, but not 5V.

If you've read Pip's comment: he has a Solo 610 and an LA-610, and he confirms my observations: The Solo 610 sounds good, the LA-610 doesn't. Actually, I bought the LA-610, because I thought it would sound as good as the Solo 610. Well, it doesn't. (BTW I liked the Solo 110 even better)

So I don't think it's a philosophical question. I'm not a newbie, either, I've been recording for 20+ years, and I own some (real) vintage stuff, and I know how to use my gear, new and old. Granted, you can make the LA-610 sound real nasty if you don't know its workings, but you can't make it sound really good no matter how you use it.

One more thing: The distortion characteristics don't change much with the gain switch. In my measurements the distortion spectrum remained basically the same. You can manipulate the distortion level (not the spectral distribution!) a bit by using less input gain and more output gain, but not much. Using an additional gain stage behind the LA-610 is a bit more effective. In both cases you lose some S/N, obviously.

 

[emrr]

Seems clear now the two units are different.  Maybe one just sucks.  I know neither one, so....

Filament voltage only has to do with temperature.  It has very little to do with signal level, it has more to do with current demand.  I don't perceive this as a unit with high current demand, these aren't high current tubes.  If we were talking about a power amp, I'd find it more likely to see difference. 

Do they even have the same in/out transformer set? 

I'm done, unless someone can take some real gain block level measurements and post some stats.  Post measurable observations at different filament voltages.  We are talking in circles until that time.  Anyone want to send me one, I'll put it on the Audio Precision, do some tube type swaps, raise the filaments, etc etc, file a report.

 

[EEMO1]

I'd love to send you mine, but im in europe...