PTP Tube Opto Comp (modified LA2A) - Need help to finish the design

[thomasdf]

Hello everyone

I am not new to DIY, but I am to tube DIY! I used to tinker and learn with an old wise man that knew a whole lot about tube designs, SRPPs... But he sadly passed away a few months ago and I now have to learn by myself... And that is always a bit more difficult.

We were working on a tube opto comp based on the LA2A because I had told him that I wanted to learn the basics of point to point tube wiring. I got a dead tube radio from the 50's and decided to reuse the chassis and wooden box. The transformer is working fine, and I have a lot of NOS tubes at home. The idea was to build something from scratch with low budget in mind, and recycling parts.

We ended designing this... Even tho it is not finished. I was taking care of the design and he was correcting my mistakes and guiding me with advices and constructive criticism.

Basically we got rid of the input transformer, replacing it with a SRPP tube, and we redesigned the PSU section and sidechain section too.

Schematics:


Direct link to the image: http://imageshack.us/download/742/KoqoBx.jpg

As far as the Vactrol is concerned, that man gave me 2 vintage VTL 1B5 (neon) that I would like to use because he would have liked me to so, but if they are complicated to use I will be fine with Led driver ones like. Here is the datasheet of the VTL 1B5.


Direct link to the image: http://imageshack.us/download/538/crpeYz.jpg
And for the fun, some picture of the chassis, case and some vintage meters that I gathered in yard sales or such and would love to use for the comp. I have kickass vintage bakelite knobs too


Direct link to the image: http://imageshack.us/download/905/lJ8Hmk.jpg

Direct link to the image: http://imageshack.us/download/661/mvmqlT.jpg

Direct link to the image: http://imageshack.us/download/537/b4cVit.jpg

Any help is very welcome!

Cheers

Thomas

 

[joaquins]

HI,

For what I can see using a led vactrol may bring some problems to the design, first using caps for driving an led will charge the caps and that's not good, using a reversed diode in parallel will help, or the neon lamps as you pointed. Then the time response, the cell on the LA2A is really slow, the ones you posted are quite fast for the task, below 5ms for attack at neon ones, and 60ms decay, too fast for this comp, you should try to look in something much slower or using an SS active part with a rectifier and attack release controls as usual.

Other than that, the signal path is pretty straight forward, I guess you took it close to LA2A original one so, there should be no problems there, the input stage looks fine, simple, unbalanced though but you surely know the limits of that, and electronically balanced input in tube may not worth it.

JS

 

[thomasdf]

Thanks for your answer!

the ones you posted are quite fast for the task, below 5ms for attack at neon ones, and 60ms decay, too fast for this comp, you should try to look in something much slower or using an SS active part with a rectifier and attack release controls as usual.

I haven't noticed that the attack and decay time where so fast... Indeed it is not going to work! Can please explain the "SS active part"? Do you mean an active sidechain with attack and decay controls? Because, I was also thinking of using an opamp SC with a LED Vactrol, but the PSU section was getting a bit more complicated and I wanted a simple design.

Other than that, the signal path is pretty straight forward, I guess you took it close to LA2A original one so, there should be no problems there, the input stage looks fine, simple, unbalanced though but you surely know the limits of that, and electronically balanced input in tube may not worth it.

I thought it could be less expensive to take out the input Xformer and that would make another twist on the LA2A so it is not a complete clone except the SC. Plus I have lots of tubes to try.

 

[joaquins]

The heart of the LA2A and it's character is in fact the T4B cell, which is the only responsible for the response times, you could manage to get the parts and build a similar one much cheaper than a NOS T4B, if not doing this the other option is to go as you said, with an opamp, make a buffer, rectifier, attack, release and led driver for a led vactrol, from your 12V winding you could make a doubler and use either ±12V or 24V (not precise, just apron) and work with it, ±12V seems easier, but single PS with opamps is not as hard, just a couple of extra resistors. To make a ref voltage, and you don't even need that because you are using a 12V PS which could be your reference for that part, it doesn't need to be too clean either, since it's only for side chain, so not even a regulator needed, just good filtering and you are done, it will be cheaper than a tube, but you already have the tube, so save it for another occasion.

For your first project I'd recommend a mic preamp, or even simpler a guitar preamp since you don't need the I/P transformer. Then you can go with a comp, there is other cool project which only needs one 12AU7 and one interstage transformer, and all the rest are opamps, to make a vary-mu compressor, the only high voltage you need for it is 100V, there should be around here somewhere.

Going back to the LA2A, there are some designs for SS opto comps, some of which doesn't even has makeup stage, leaving you to choose the makeup you want, go for one of this, once you get it working you add the make up amp as the one you draw, you don't really need the input stage, input impedance it's already high enough, 68K worst case, and anyway you are going unbalanced so I don't really see the point in doing your way, save that tube for other project too. I'd said to make the makeup stage for one side and the compressor in another side and then get them together and see how they are as a comp. I just remember I had this schematic of what I'm talking about, never tried it out, it should work, it's half wave rectifier, but not a big problem in such simple design, and you don't need to touch your PS. There are a couple of things to look on this, maybe another diode to discharge the first cap, in parallel with the 33k resistor from ground to the cap, maybe I'd add another BJT as a emitter follower after the first jFET. Then you could add some changes in the signal path (the resistor and LDR) choosing the values for that resistor as the ones for your's, but scaled properly for the LDR used, but in order to have the comp/limit switch, or you could use a pot or more steps for ratio. A cap should be added in series with the bypass switch in order to avoid problems with the PS and signal polarity, value is not critical, only driving 100k resistance, you may use it with really low value caps to add a HPF if you like.

I don't know about the PS, I'd use +12V for heaters instead of -12V as you are using (the ground to the other pin) to avoid problems, more negative the heaters to the cathodes more noise you have, if you have some noise on heaters even after the regulator it's much more likely to leak into cathode as it is than would be having it higher.

JS

 

[thomasdf]

Ok thanks for your feedback. I really want to keep going with this unit as I know I am not very far from finishing it, in terms of general designs. (Thanks to the LA2A also).
Plus, I like the idea of having a straightforward 2 knobs colored compressor. I was toying with the idea of copying the D-OAC which has 3 knobs (input, threshold and output gain) and that would my maximum number of functions, but its PSU looks quite big so for the moment, I'm sticking to the LA2A.

From what I understand reading you, my main problem is the GR technology. T4B like > tube sidechain, Neon Vactrol > hybrid, LED Vactrol > Opamp.
I do not want to go for hybrid, I'll stick to a DIY T4B or something similar, or will do an opamp SC with a slow Vactrol (Vactec VTL5C2 ?).
Need to dig a little deeper on that.

As far as the input of the unit is concerned, I should use a tube or a transformer for the sound of it, if not for electronic reasons don't you think?

-12V to heat the tubes must have been a mistake! Thanks for noticing it!

Regards,

Thomas

 

[ruffrecords]

Hi Thomas,

A couple of observations:

You have an ECC88 SRPP before the gain reduction element. You don't say how sensitive you want the input to be but this SRPP stage will have about 30dB of gain.

You have an ECC83 SRPP stage driving the Vactrol. If this is a neon  type then this is probably OK but an ECC83 will not drive an LED Vactrol. For that you need to use another ECC88.

Cheers

Ian

 

[joaquins]

If you could go with transformer input is much better, balanced input, much better noisy environments, and you have yet another flavor added and more close to the original, so both, technically and taste. In case not using a transformer you don't need to put anything, just the input going to the 68k resistor, it's unbalanced but as unbalanced as the tube input and you don't get that gain which may be a problem as Ian pointed.

You could build a T4B, there are some kits and parts around, but I'd start as I said with the SS circuit and a common to get vactrol, almost any will work as you can control the time constants, then you can let them fixed for something close to the original design response.

JS

 

[thomasdf]

Thanks to both of you for your input. I appreciate it!
An ECC88 driving a LED Vactrol could be a good in between solution, do I just need to adapt my schematic to it or is there other tweaks that needs to be done? Then what Vactrol? Any as long as it's pretty slow? What should I use around it (resistor to limit current etc or anything else?)

I am fine with both getting rid of the SRPP input (then unbalanced) or adding a input Xformer (probably better).

In the meantime I will do research on diy T4Bs but I am not necessarly looking for this GR part, any opto is fine as long as it's pretty slow. But T4Bs are cool, indeed

 

[ruffrecords]

Here's a design for a poor man's complim I sketched out a couple of years ago. There's an error in the schematic - the LED should be connected to the other side of the 1K resistor but otherwise all the principles are in place.  The 1K resistor acts as a crude current d source for the Vactrol and operates it at up to a few mA. It should easily provide a 20dB gain reducrion range. Of course there is no metering shown but you could include that easily by using the dual version of the 5C3 which, if memory serves, has characteristics  similar to the T4B. I never got round to building it but it is based on a design I added to an existing power amp for a customer.

Cheers

Ian

 

[thomasdf]

Looks like an interesting design Ian. If you don't mind, I'll redraw it in Eagle, correct that mistake, add PSU and VU section at post it here.
The dual VTL 5C3 is easy to source here. Response time is supposed to be around 150uS, sounds fast to me?
Anyway, since I am not trying to copy the LA2A response time, I should give this a try. I just hope it's not too fast

6922 tubes are E88CC, but I cannot find any good informations about how it compares to ECC88 (because I have a few of these in my stuff, vintage ones)

 

[ruffrecords]

Looks like an interesting design Ian. If you don't mind, I'll redraw it in Eagle, correct that mistake, add PSU and VU section at post it here.
The dual VTL 5C3 is easy to source here. Response time is supposed to be around 150uS, sounds fast to me?
Anyway, since I am not trying to copy the LA2A response time, I should give this a try. I just hope it's not too fast

6922 tubes are E88CC, but I cannot find any good informations about how it compares to ECC88 (because I have a few of these in my stuff, vintage ones)

Redrawing is fine by me.

Not sure where you got the 150uS time from but this data sheet has the attack as 3mS and the decay as 50ms:

http://www.sdiy.org/colbecklabs/spec_sheets/Vactrol%205C3-2.pdf

6992 and E88CC are the same. The main difference from the plain ECC88 is the heater cathode specification (Vhk) which is 200V for the E88CC/6992. This is needed for the SRPP where the top cathode sits at just above half the HT voltage. For the regular ECC88 it is just 50V for one triode and 150V for the other. I usually elevate the heaters in an SRPP to about 75V anyway.

Cheers

Ian