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#26 pre amp

Having this thought here.... :bulb:

What about using a cascode ccs right after B+, then a single ccs to the plate, but instead of connecting the plate to the coupling cap, get the output at the source of the FET (the mu follower), that way, the output z will be small enough that there is no need for an output transformer (?) (see, I am trying to avoid one here) :D

And by this way it preserves the isolation of the output signal path from the power supply path, and it becomes an SRPP amp but with nice PSRR? :eek:

Any comment?:spin:
 
I've been juggling around with glow tubes and voltages, and the nearest I've come is the following:
HT = 360v
Glow tubes - 150+105 = 260v
IXYS sense resistor = 485 ohms
Plate voltage = 168v
Cathode resistor = 1.91k
Cathode voltage = 10.6
Current = 5.6mA

No doubt it could be tweaked to get 6mA out of it, but it took me a while to arrive at all this!

andy
 
I am Alvin.

Hi all, here is my 27 not 26 :)

this is my olderly project when I was crazy for tube ;)

Umm...Parafeed config(cascode DN2540) with tamradio(tamura) 10K : 600 & 1.33uf*2, LED cathode for biasing.

power : 270V*2, 80 retifier, LCRCRCRC, and about 25V inject to the filament to elminate AC filament noise. :headshot:

Good sounding, still love it much:D

Cheers.:spin:
 
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Oh, pic..
 

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Hi all, here is my 27 not 26 :)

this is my olderly project when I was crazy for tube ;)

Umm...Parafeed config(cascode DN2540) with tamradio(tamura) 10K : 600 & 1.33uf*2, LED cathode for biasing.

power : 270V*2, 80 retifier, LCRCRCRC, and about 25V inject to the filament to elminate AC filament noise. :headshot:

Good sounding, still love it much:D

Cheers.:spin:

Very nice! Did you ever try battery grid bias? (I suppose 9V would work fine.)
 
Nice 27 fingerx! :D

A tube veteran indeed! :spin:

Your Symmetry DCB1 on the Pass thread is something I will long for next, say perhaps as a buffer to the RC coupled 26 version. (Together with Lightspeed too, you bet ;)

Yes, I am getting a pair of 20k:600 this week for the transformer loaded settings.

Will order some IXYSs too for putting together that Gary Pimms-like circuit as well. That could be a flexible platform to also play around with 01A and PTT2s, PTTx, etc.... at the mu follower or high Z outputs....

Many options to have fun :spin:
 
Hi Guys,

I’ve been following this thread with great interest for a while as I’m building my 26 preamp. I have the separate power supply built and just have to wire up the preamp this weekend.

Power supply is 285CT – AZ1 – CLC split into right and left, then LC – VR tube regulation. VR tubes are 190V (0C3 and 0B3).

For the preamp I’m initially using 2 Hammond 150H chokes in series as plate load, with cathode bias, but have some Hashimoto 20k:600 transformers to try.

I would like some advice regarding grounding, I have followed Kevin’s suggestions in his 26 article, using star grounding in the power supply chassis, but I’m not so sure about the preamp chassis.

I have the VR tubes in the preamp chassis and all CLCLC parts in the psu chassis. Should I:
a) join all grounds, (RCA in/out, cathode R & bypass cap and VR tubes to a common star ground an then return this to the psu star ground? Or
b) separately return i)VR tube grounds and ii) RCA in/out, cathode R & bypass cap grounds to the psu chassis star groung?

Any advice would be appreciated

Cheers,

Rich
 
OK, getting a little bit more understanding now, please correct me if wrong...

With CCS, what the tube sees is a load that comprises the inductance of the primary coil of the output tranny in parallel with the reflected impedance from the secondary.....

Assuming the input impedance of the connected amp is 10k, if the ratio of the output tranny is 4:1, then the tube is essentially seeing the output tranny's inductance at primary in parallel with the reflected impedance of 160k. The rule of thumb is that this overall impedance should be no less than about 3 times the plate resistance. (for 26 it is 8kx3=24k)

So I really don't care about the indicated impedance of the output tranny but it's turn ratio, right? : ) But to certain extend that the inductance of the tranny can't be too low, and the capacitance can't be high....

So I should be able to use a tranny with nice size of primary inductance, say connected as 600 ohm: 125 ohm for ccs-parafeeding the 26 tube, yielding an output impedance of about 0.5k (or 8k/16) and a gain close to mu/4 which is about 2, true?
 
Hi 45,

I have a couple of stepups to try, including a UTC KB143. I'm even considering the Silk STA 522A which does a + 6db . Can I influence the primary impedence by resistor loading the secondary?

Rich out

Yes you can.
I think that 20 Kohm is supposed to be both the source (i.e. the tube) internal resistance and primary load with open secondary. In fact the equivalent resistance is the parallel of the two: 10 Kohm. With 100H primary inductance you get 16 Hz at -3dB, as shown in the frequency response graphic!
If you use a set-up transformer make sure your source (CD player, phono amp or tuner) is able to drive heavy capacitive loads. The step-up transformer can be quite tricky.....

I think that using two direct-coupled 26's, the first resistor loaded and the second transformer loaded could work well if you don't have problems with microphonic (I do not recommend mounting the tube directly on the chassis, use a perforated board like this with 2-3 turrets: http://zsyabai.win.mofcom.gov.cn/www/9/zsyabai/img/200762711033.jpg ). You could get exceptionally low distortion at low levels because of harmonic cancellation ( this technique should work at its best because the two tubes are the "same" if you work a little bit on the plate voltages and anode currents). I mean something like the input/driver stage of the Sun Audio 2A3/300B amplifier where you have a direct coupling of the two halves of a 6SN7.
In your case the only difference would be the transformer in place of the plate resistor for the second tube. In this way, using your Hashimoto, you would get approx. 15 dB gain and low Zout.
You need more or less 300V supply.

Cheers,
45
 
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OK, getting a little bit more understanding now, please correct me if wrong...

<snip>

So I really don't care about the indicated impedance of the output tranny but it's turn ratio, right? : ) But to certain extend that the inductance of the tranny can't be too low, and the capacitance can't be high....

So I should be able to use a tranny with nice size of primary inductance, say connected as 600 ohm: 125 ohm for ccs-parafeeding the 26 tube, yielding an output impedance of about 0.5k (or 8k/16) and a gain close to mu/4 which is about 2, true?


It's not quite that simple, the primary impedance of the transformer is important because it implies that there is enough primary inductance to maintain the frequency response within the defined limits under the stated conditions. A transformer with a primary Z of 600 ohms is not likely to have anywhere near the primary inductance of a transformer with a primary Z of 15K even though they might in fact have the same numerical transformation ratios.

Your assumption in the next paragraph is wrong, the transformation ratio scales as the square root of the impedance ratio, so although the impedance ratio is ~1/4 primary to secondary the voltage transformation ratio is going to be ~1/2. Your output impedance is going to a somewhat > 2K.

Gain will be about 1/2 of mu into an open circuit load, and could be substantially less if the load impedance is not at least 10X the reflected rp.

A 600 ohm transformer probably is not going to cut it, insufficient primary L will result in a substantial LF roll-off, worst case you may not have a useful response much below 300Hz, and even if you do this is an excellent recipe for increasing distortion as the frequency decreases. IMHO To guarantee really good performance the xl should be about 10X greater than the load impedance at the lowest frequency of interest. You need a transformer with a primary L of at least 150 - 200H to meet my criteria.
 
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Yes you can.
<snip>
If you use a set-up transformer make sure your source (CD player, phono amp or tuner) is able to drive heavy capacitive loads. The step-up transformer can be quite tricky.....


Cheers,
45

45 has an excellent point here, if you apply a 1:2 step up transformer at the input you will effectively quadruple the capacitance seen by the stage driving it. This may not be much of a problem in practice if the source impedance and matching impedances are low - and assuming that the device in question can drive the transformer.

Some people place the gain control after the transformer and before the tube, this does have the advantage of reflecting only a % of the capacitance back to the source, but it does impose a variable rc roll-off dependent on the position of the volume control pot. (this is normally the case anyway)

Placing a pot (sometimes a constant impedance L pad) before the transformer is frequently done when the source and transformer are 500/600 ohm devices, but this is a bad practice with higher impedances because of the variable source impedance implied and the effect on frequency response as a function of pot position becomes a significant issue.

Using a 20K:600 ohm transformer will give you almost no gain into an open circuit and an appreciable gain loss into a matched load. Certainly with most digital sources there is no need for significant voltage gain in a line stage. I'm using 15K:600 ohm UTC HA-133 in my 26 pre-amp and get ~4dB of gain in this configuration, and this actually more than enough with my digital sources which all produce about 3Vrms FS. My phono stage is designed with almost 50dB of gain to provide sufficient signal level to this line stage.

You might want to read my Positive Feedback Online 26 DHT pre-amplifier article which you can access through my website. (I talk about some of these issues in the article.)
 
As I now plan a 26 pre with LL1660/10mA/4,5:1 I did a working simmodel of it. Ug ca -6V and Ia ca 8mA. Dmitrys 26-model follows the published curves perfect.

This one shows a gain of 5dB with a Zout of ca 500ohm. Due to the unbypassed 5.6ohm with 6VDC over it, this one takes well over 10V in before clipping. -3dB is well below 10Hz and 20Hz will be -0,6dB. 165H is a little on the low side and will cause some phase shift in the low end(ca 10degrees at 50Hz).

The simmed fourier-analysis shows extremely low distortion but shouldn´t be taken to seriously.

Would be fun to see an IRL analysis.

The right one is an idea of a prafeed with a voltagecontrolled gyrator instead of inductor/CCS. By playing with the two capacitors it can be made to have a very low Fc.
 

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<snip>

The right one is an idea of a prafeed with a voltage controlled gyrator instead of inductor/CCS. By playing with the two capacitors it can be made to have a very low Fc.

I use a very similar approach based on voltage controlled gyrators to load the amplifier stages in my latest phono stage. All tube based though with 5687s used in the gyrator circuits. It works pretty well, not constant current of course, but there are instances where having entirely predictable plate voltages is a benefit - from an ac perspective it is possible to synthesize fairly high load resistances without the voltage drop associated with a real resistor. (I'm not actually simulating an inductor with this thing, but that is obviously possible.) In my phono stage in order to use a comparable resistive load I would have to have a >1kV supply versus the 240V supply I actually use.