Pro VLA make over

[mvanhelden]

Hi,

Recently I bought an ART Pro VLA compressor in the US. Because I live in the Netherlands, Europe, I have to use a step down converter. This is only a temporary solution for me. The transformer of the Pro VLA can not be set at 230V, so I need to replace it.

There are several (interesting) topics on the site about mods for the Pro VLA. I?ve already removed the input opamps and installed a socket to try out several ICs. And plan on replacing the output IC and critical caps/resistors as well.

But then there?s still a ?major? problem with the unit: low plate voltages. I could go for an ?exact? replacement transformer (2x 8V) and try out different tubes which are designed for lower plate voltages, such as the 6BC8, 6DJ8 and 6BQ7. But according to various people they do not offer the same sonic qualities as ECC83 (etc) series. Also they are much harder to find.

Since I have to replace the transformer I could improve the plate voltages as well. This is where I would like some input. I have enough choice in NOS ECC81, ECC82 and ECC83 type tubes which I would like to swap and see which sounds best. For example: my Sebatron VEQC-2000 sounds way better now I?ve installed Mullard 12AT7s.

But upgrading the plate voltages would also mean several changes to the rectifying circuit. Any ideas, tips, comments?

Kind regards,
Maarten van Helden
www.tubefreak.com

 

[mvanhelden]

Here you go:

http://www.blacklionaudio.com/212schem.pdf
http://www.blacklionaudio.com/212schem-B.pdf

Kind regards,
Maarten

 

[mvanhelden]

No thoughts on transforming it to a higher voltage operation? anyone?

 

[mvanhelden]

OK.
Thanx!

 

[PRR]

> a ?major? problem with the unit: low plate voltages.

Tubes work fine at low voltage, except they make very little signal power. If a 12AT7 working at 300v can make up to 50mW of output, then at 30V it could make 50mW/((30/300)^2)= 0.1mW of signal power.

But so? Here we have a cathode follower buffering the gain stage, an (Darlington) emitter follower buffering the CF, and then a 5532 buffering that. The Darlington outputs as much as 5mW, so its input is less than 5uW. The CF is oddly loaded, 1Meg pull-down, but the NPN transistor always sucks current from the cathode, the pull-down is not needed except for leakage (and fast signals, but I suspect they want those to be colored).

A different question: what is the signal voltage? A rule of thumb for tube work is that the peak audio voltage can be 20% of the supply voltage before THD reaches 5%. This rule is bent at low voltages, but not much for 12AT7 at these impedances. 20% of 45V is 9V peak. The signal flows through a loaded linear pot and then to an output stage with gain of 2 (to balanced load). With pot full-up, we get 18V peak or 12Vrms or +24dBu when the tube bentness gets to 5%. With pot centered, about +14dBu.

We could raise the tube supply to, say, 300V (this would be a LOT of change). Get 20% of 300V or 60V swings out of the tube at 5%, and we sure would need to protect the semiconductors from that. We could only use 9V or 10V of that swing, and THD would be below 1% and very simple, very subtle. Possibly much lower: note that there is feedback around the tube, perhaps to the tune of 18dB. In fact you probably would not hear it as "tube sound" at all.

I've looked at several ART designs and I have a lot of respect for their bread-and-butter chip designs. Nothing too fancy, nothing too simple, works good. Their tube stages are of course under-fed, and I'm sure they know that, and WANT it that way because that is what all the post-1960s kids want to hear: strained tubes. They have to get that 3%-10%THD point down into the normal output range. 45V is ample to do that with standard studio levels. And I suspect, from their chip-work and the complications around that tube, that they breadboarded it all different ways for all different users, and thought this way was best.

I use a pair of ART Tube Channels. I tried all different things inside. I must confess that I lost track of them all. When I had it "happy" (to my ears for my needs), I shut it down to put it in the rack, and was surprized when it turned-on instantly. Hmmmm... I liked it better with the tube stage bypassed. For my "transparent" needs, that makes sense. For someone who wants color, fooling with Tube Drive is very useful (and fun, but not what I wanted for this concert recording rig).

 

[Tubefreak]

That's a lot of good and usefull information. Thank you. I think I'll stick to the standard upgrades then and check which 12AT7 tube sounds best with it.

Colour does the unit have enough. I noticed that it cuts highs when it?s not bypassed by it self. Which leads me to a totally different question (sorry!): If I would like to hard bypass the tube stage (and make it selectable with a switch), where would I place the bypass best? Between C30 and C43? And would this lead to much volume differences?

Kind regards
Maarten

 

[Tubefreak]

Hey guys,

I'm working on the unit and came across resistors/pots VR4 and VR8. They can be found in the schematic after the two large jumper cables from the rear pannel break away.

One of the pots measures 12k49 and the other 11k61. For good stereo operation I guess they should be exactly the same, right? Or are they used to "bias" the differences between the channels due to the inacuracy of the parts (Vactrol, tubes, IC)?

Is there a way to measure what the right amount of resistance for this pot should be?

And I'm still thinking about a tube bypass switch.... shouldn't be too hard. Placing the switch after C30 and after C43 (where it goes to the jumper cable) would be a nice place. Any toughts on this?

But I'm wondering what it would do to the gain/vulme when the tubes are bypassed. I could calculate the amount of gain of the tube stage, but it would take loads of time for me :wink: PRR do you have any idea?

Kind regards
Maarten

 

[PRR]

> One of the pots measures 12k49 and the other 11k61. For good stereo operation I guess they should be exactly the same, right?

NO!

They match the poorly-matched photo-resistors. A 10% difference is really quite small. Unless you know what you are doing, leave them alone.

The tube has gain of 4 or 5. You could cut-up all the traces around A2A A10A and change them from unity-gain to gain stages, or build-up a sub-board with a TL072 (or the unused A3B A8B?).

> I noticed that it cuts highs

Should be down 1dB at 20KHz. But the tube and opto flavor may be tricking your ears; it should be "less harsh" than a pure-chip amp.

 

[Tubefreak]

Hy PRR,

Thanx for your input. I was thinking. I could ofcourse use an IC to boost the non-tube setting, when I install a bypass switch for the tube. On the other hand, the tube stage goes directly to a 10k volume pot.

So maybe it's just easier to add another 10k pot inline after the tube stage which I can adjust to match the levels between tube and it's bypass level. Essentially I would drop the volume of the tube stage a bit.

Are there any disadvantages to this like the impedance load on the tube or signal/noise ratio? Are the sonic advantages of an extra gain stage for the solid state setting better then a volume pot for the tube stage?

Kind regards
Maarten

 

[Tubefreak]

Yeah :grin:

Changed the transformer, IC's and tubes and it sounds way better now. Before it sounded more muddy with rolled off top end, even without any compression. Now it sounds a lot clearer/defined and the top end is really restored.

Made a true bypass switch for the tube stage, but doesn't give the result I want yet. The gain the tube adds is very noticable, drops aprox 6dB in bypass. Could build an extra IC gain stage when the tube is bypassed, but the sonic difference doesn't really make it necessary!

For those want to know:
Voltage Xformer: 35W 115/230V prim. 2x 9V sec
ICs for inputs OPA2604
ICs for ouput OPA2134
ICs for buffer MC33078N
Tubes RFT ECC81

All resistors where already within 1% of each other on the in/outputs, so I left them untouched. Like it a lot now.

Maarten

 

[Tubefreak]

mmhhh starts to look like a conversation with my self :green:

But for those who are interested: if you can't beat them, join ' em. I tried bypassing the tube, but without much result. So I went the other way with an easy mod (thanx to PRR).

There's a negative feedback circuit in the tube stage, which is set by the 100k resistors R64 and R96. I placed an external 1M pot to change the amount of feedback. Higher feedback=less gain=less distortion. By increasing the resistor you decrease the negative feedback which allows you to change the amount of tube drive.

With "normal" levels and ECC81 tubes it goes from normal/clean to overdrive. With hotter tubes (ECC83) it goes even into heavy distortion without any trouble. Normally you don't want to add massive distortion on a 2Bus, but it works great on organs, electric piano's, smashing up drums, etc. It's an easy mod, but one wich adds massive potential to the Pro VLA.

Because the gain stage is placed after the compression circuit, the mod does not change the operation of the compression it self, nor does it distort the ICs, since the output volume is placed after it. So you really have a tube drive control!

:!: :idea: :!: