Therapy tube pre

Hi

   have a couple of questions regarding this schematic. HV part of the PSU does not make sense  regarding B1, B2 and B3 to me.
Neither calculating nor simulating I could not get close to the desired voltages for B1 B2 and B3. DC resistance for the choke is 429r.
Could some one confirm that those values for the resistors are OK. I must be missing something.
Also I'd like to add LPF. I was thinking of moving 2k5 resistor before the 50k pot (pot bottom to ground), and from that junction, to add a cap to ground.
Is there a better plan for LPF?
Also i would use this transformers: Input  OEP A262A3E 150:25k. Output Carnhill VTB2290 9.6k:600 gapped. as I have them. Do you think they'll do the job?
Thank you

syn said:

Neither calculating nor simulating I could not get close to the desired voltages for B1 B2 and B3.

Click to expand...

The values are fine. Something is wrong with the way you calculate or use simulator. Ohms law is all you need. I'm lazy to check exact bias so let's just for the sake of this example assume that ECC88 tube B2 line draws about 5mA. That means each 3K9 resistor (for each tube separately) drops about 20volts. Good isolation and smoothing.

http://www.the12volt.com/ohm/ohmslawcalculators.asp

We are definitely in the ballpark, but also notice what mr Knif hints in the schematic: "all values are approximate and non critical". These are tubes afterall. deviations of even 20 volts in your B+ lines in these type of designs can be considered negligible. If a schematic reads 150VDC, and you measure 120-200VDC, you will still have a fine sounding amp and will have a hard time detecting a difference between these two extremes.

Thnx. I somewhat took those values literary and was up or down by ~20V, skiping the "all values are approximate and non critical" part. All well then. How about that LPF and transformers . Any thoughts?

Shouldn't this be called the "therapre"?

Those trannies look possible. It would be useful to see the output trannie inductance, because it will affect low freq performance in pentode mode, especially if no feedback is applied. Try! I suggest you try different primary damping resistor values, those 10k worked ok with LL1671.
For LP I suggest you leave the 2,5k as it is. (The purpose is just to have nice and logical range with well known bottom position.) Use a coil the same way you have the HP cap there to get required LP behavior (as in V76), or, put some (20-50?k) series resistance in front of the pot and ground between through a cap. The down side is that this raises the impedance seen by the output stage and causes treble roll of in triode mode. I spent considerable time trying to find impedance values for a nice compromise there, so I don't suggest big changes.

I finally got back my own pre. It spent most of its time as a mixing amp for almost a year. It is nice to be able to adjust the color to suit the program material.

Therapre, why not

-Jonte

Jonte
 
        thank you very much, Sir . The inductance (2290) is ~ 6H secondary 43H primary @ 120Hz (Tenma 72-960). I'll go with a coil, V76 reference is most useful, thanks again. I'm looking forward to build it. Thank you very much for sharing.

Hi,
I´ve just found this interesting preamp and would like to ask a few question about it. Well, my intention is to imply the feedback loop of the E81L to my REDD to CV test rig. What I have so far: I inserted a 6ak5 tube in between the EF86 and ECC88 valve stages of my REDD preamp. I implied a variable bias for the 6ak5 and the results are fine so far. It´s not really the distortion I had expected, but for the first attempt not bad at all. The 6ak5 runs in triode mode and with proper bias there is a huge amount of gain. I haven´t tested pentode wiring yet, since I would like someone having a look at the schematic before. There is nothing new, basicly it´s the Beatle preamp with overall feedback removed and a 6ak5 inserted. I would also like to make the different tube modes switchable as shown on the schematic. Are there any other modes of wiring the 6ak5 that will result in more desirable distortion? I wonder whether the switchable feedback as used in the therapy pre would give the kind of distortion I´m looking for. Sorry for being so unprecise, but tube theory is quite new to me and honestly, I didn´t understand that much. And what is meant by "UL" as mentioned in the therapy pre schematic (see first posting)

schematic of my R2CV:

http://www.twin-x.com/groupdiy/albums/userpics/My_CV.JPG

kind regards
Bernd

UL stands for ultra linear.

An ideal 200VAC winding gives 282.8V DC rectified and filtered.

Real rect-filters have losses. Transformer, 150 ohms, riple, choke DCR.

Points B1 are noted as 170V. The tube they feed is noted as 15mA.

The 3K9 resistors feeding B1 must drop about 4K*15mA or 60V.

The post-choke raw B+ must then be 170V+60V= 230V.

Some guesswork on B2 also gives somewhat over 200V post-choke.

The rect-filter seems to be losing 282.8V-230V= 50V. Is that reasonable?

The 150 ohms carries full ripple and drops more than you think. I get 5.6V peak-peak ripple at about 245V average DC. Nearly 40V lost right there.

The remaining 245V-230V or 15V drop can be accounted with 300 ohms in the choke.

Thank you PRR.
Hope I'll find some time to finish the pre soon.

mmm... getting back to DIY.

What's that pentode on the output?

E81L.

I have a keen interest in tube mic pres so I was I was very interested in this circuit because I hade not seen it before.

At first I was not terribly impressed because I expected there would be a lot of difficulty with the NFB loop around the ECC88 since it was attempting to vary the gain (and hence the amount of NFB) by 50dB which is notoriously difficult to do in tube circuits simply because they mostly have no NFB at dc (something which is very easy with transistors)

However, after a lot of simulations, the very clever subtleties of this circuit became apparent to me and I am now very impressed with its design. The problem with tube NFB designs is mainly at the low frequency end where, because of the series capacitor in the NFB loop, the closed loop gain rises at very low frequencies which can easily cause instability. This limits the amount of NFB that can be used and hence limits the minimum gain you can achieve. Yes, you can use a very large capacitor in the NFB loop but all this does is push the instability down to a lower frequency. So I was surprised that this design could achieve a gain of only 5dB from a circuit that could also achieve 55dB gain and indeed early simulations confirmed that instability resulted at low gains. Then I simulated the circuit more precisely, and especially the gain setting attenuator/NFB network and that is where I discovered the subtlety of this circuit. What happens is that the gain setting switch simultaneously alters both the closed loop gain and the open loop gain. When a lot of NFB is required for lower gain the open loop gain is automatically reduced and when less NFB is required the open loop gain is correspondingly increased.

This is actually very reminiscent of the technique used in the early Neve three transistor gain blocks used in the classic Neve mic pres.

Cheers

Ian

I've looked over the schematic and I'd like to incorporate some of the simple things in something I'm working on.  But I'm wondering about some of the components and why they're there.  For example the DI input.  There's the 10M load, and the 100nF coupling cap, but what does the additional 2.2M resistor and 3.3uF cap to ground accomplish?

On the input, what's the PH switch? 

millzners said:

I've looked over the schematic and I'd like to incorporate some of the simple things in something I'm working on.  But I'm wondering about some of the components and why they're there.  For example the DI input.  There's the 10M load, and the 100nF coupling cap, but what does the additional 2.2M resistor and 3.3uF cap to ground accomplish?

Click to expand...

The 3.3uF cap is part of the dc biasing circuit. dc bias is created by the 680 ohm resistor and the bottom of this resistor is effectively the ground input for the first triode. The 10K resistor is part of the ac NFB. So the dc bias is fed to the grid vias the 200K resistor and decoupled at ac to ground via the 3.3uF.

On the input, what's the PH switch? 

Click to expand...

Phase change switch.

Cheers

Ian

more info in the original thread, by the designer Jonte Knif: http://www.groupdiy.com/index.php?topic=38252.0

Kingston said:

more info in the original thread, by the designer Jonte Knif: http://www.groupdiy.com/index.php?topic=38252.0

Click to expand...

Thanks for that. It seems the front end is based on the V76. I have never really looked at the V76 in any detail, thinking it only to be an overly complicated version of the V73 but in fact its basic front end topology is quite different and very instructive. Just proves once again that the ancients solved all these problems aeons ago.

Cheers

Ian

Sorry I'm late.
The switch labeled PH is phantom on/off switch, that also switches mic and line inputs (with second deck).
When "phantom on" no "line in" connectivity and vice versa.

Best

ruffrecords said:

This is actually very reminiscent of the technique used in the early Neve three transistor gain blocks used in the classic Neve mic pres.

Cheers

Ian

Click to expand...

Ian, thanks, I'd like to hear more about that.  Are you referring to the BA283 card used in all the two-stage 1272 and three-stage 1073 family?  Could you elaborate, explain? 

(I've shown the 283 circuit to several tube designers, but they didn't quite understand transistors well enough to know how the Neves worked.)