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Any good #26 line amp designs?

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Frank

Thanks for the fine examples of the 26. Do you have any words of wisdom before I tackle my last line stage? From what I hear the 26 is the worst to work with. I would like to use up my 150H chokes with this until I can get Barcelona to answer my email requesting prices.

Do you have any words of wisdom in regard to plate voltage and Dc supply for the heater?

Joe
 
burnedfingers said:
I would like to use up my 150H chokes with this until I can get Barcelona to answer my email requesting prices.

Email Kevin at K&K as there are some special order LL1667 that can be ordered gapped for 8 or 10mA IIRC and are probably around the 300-350H mark. I beleive they're the same price, but you might have to wait till his next shipment, but do check with him over the logistics. That's basically how the Oz distributor works so I guess he'll be the same.
 
For another twist on 26 line stages you can check out the one I did a couple of years ago. Look in the schematics section down towards the bottom.
Gary P's DIY page

The choke loaded version used 2 150Hy hammond 156C's per channel. The dual chokes extended the frequency response on both ends of the spectrum. The double inductance of the chokes in series increased the bottom end. The capacitance is cut down as the interwinding capacitance is effectivly 2 caps in series for half the capacitance extended the high end.

Filament supply is a OTT dual battery setup. While the preamp is running on one set of batteries, the charger is charging the other pair. The batteries swap about every half hour. Allows long (indefinate) run time on small batteries.

Gary
 
diyAudio Senior Member
Joined 2002
Hi,

Do you have any words of wisdom before I tackle my last line stage?

It's also quite likely going to be the last you'll build...it can be that good.

My word of advice would be:

Watch out for microphony. Use rubber grommet shockmounts on the UX-4 sockets.

Now that Gary mentions it I do recall him using the Hammonds as he describes...somehow it just didn't turn up in my Google search results.

Another weakness is the heater's susceptability to hum...no problem if you go for GPs' clever solution.

With attention to detail in the right places this could be one of the best preamps ever.

Cheers,;)
 
Thanks for the very fine examples of the 26. I will probably build one of each to see which I like the best. I will use grommets to mount the tube sockets as well as the transformer and chokes. In other words take no chances the 26 will pick up any stray vibrations or hum from chassis components.

Does anyone have a favorite plate voltage and current draw for this tube? Gary P have you built a 26? If so would you care to share a picture?

Thanks everyone for the helpfull hints. Brett could you send me a link or something as I haven't heard of K&K

Joe
 
I think I am going to tackle the Choke loaded design from Gary P
diy page. I am going to try this without the battery circuit as I think it can be done with a regulated 1.4 volt supply. I put together several regulated supplies and they are quite clean and certainly deserving of a try. I assume that a well sinked LM317T should be up to the task.

I have a very nice stepped control (value 10K) and was wondering if it was feasable to use it on the tail end of the line stage. Is this a recommended way to do things? Can someone help me with the calculations here? Also, it doesn't list the B+ voltage on the plate. Any ideas here?

Joe
 
burnedfingers said:
I assume that a well sinked LM317T should be up to the task.

It will work, but won't neccessarily sound any good. Tubes work by the difference between the grid and filament (DHTs anyway), so signature of the 317 will be audible, and it's a stinker. Save them for IDHT filaments.

I have a very nice stepped control (value 10K) and was wondering if it was feasable to use it on the tail end of the line stage. Is this a recommended way to do things? Can someone help me with the calculations here?

After the 26, it will change the output Z, and the Z will vary over a large range of the control and will max out at the -6dB point. 10k is a bit too low IMO for a tube with an rp~8k.

Suggested op-point (straight from datasheet) Va=130V, Ia=5.5, Vg=-10. B+ would be 140V + I.R loss across anode choke.
 
Chris Brady used this regulator and had this to say...
Quote:
After finding a suitable transformer I upgraded to a current regulator using an LM317T adjustable regulator. As expected using a current regulator resulted in a warmer, richer sound. ...

I can live with this if the sound isn't compromised too much. It is an alternative to the care and feeding of multiple batterys. It would be nice to tackle the hum problem that has faced others without having to resort to batteries.

I will leave the input control at the input.

The reason I had asked about the B+ at the plate is that I have seen the B+ vary from 115volts to 192volts at the plate. I was wondering what will give good sound. Granted a resistive load will need a higher B+ than a choke loaded plate will. A choke loaded plate will result in less loss according to my brief trial with some before.

Frank... what do you think?


Joe
 
Good for Chris. Nowhere did you mention you were going to current reg (better than a V reg), and I'd still prefer one of the LT108x series, based on my experience.

I've seen 26's running over quite a range of op points, and I bet you find it's not all that critical. The one I suggested looks pretty reasonable for a linestage based on the curves.
 
Good for Chris. Nowhere did you mention you were going to current reg (better than a V reg), and I'd still prefer one of the LT108x series, based on my experience. (Sorry, I should have mentioned my intention)

Would you care to share your experience with the LT108x series? I'm sure that with your experience you have come up with something far better. I would be very interested to see it.

Joe
 
burnedfingers said:
Would you care to share your experience with the LT108x series? I'm sure that with your experience you have come up with something far better. I would be very interested to see it.

Joe,

Everywhere I've used the 317, it's brought a "gritty" sound with a distinct texture, sort of like half way between cruchy and smooth peanut butter. I don't know exactly why this was so, just my experience, as well as that of a few others I know. The LT 108x series seem to be much better in this regard, and as they're not expensive, should be tried. I won't even use a 317 as the CCS in my shunt reg B+ supplies any more, let alone a much more sensitive application like a DHT filament.

There is a recent thread which Saurav was asking about DC for DHT's that has a lot more detail, but basically,

Filament tx - Schottky's - C - R - C - LT108x voltage reg - LT108x current reg - common mode choke (as many mH as you can find) - 10n ceramic across filament pins.

This gives a clean quiet supply, stable, reliable voltage and slow warm up and lots of PS isolation (from current reg). Adjust the R on test and the voltage at the input of the V reg will need to be about 7-7.5V (guess)
 
diyAudio Senior Member
Joined 2002
Hi,

Frank... what do you think?

Chris Brady seems to have picked good operating points for the #26.

One thing I don't understand is that he uses a current regulator for the DHT but then starves it for current...?

He feed it 950 mA but the RCA datasheet states 1.05A.

Other than that it seems fine by me and you all know we both love those VR tubes, right?:)

I think Brett has given very good advice on the construction of a good heater supply.
From experience I can testify you can hear virtually anything in that position so you best feed it from a very good supply.

Cheers,;)
 
I must be blind!

I can't seem to find the power supply voltage on the plate of the #26 tube in Gary Pimm's choke loaded design.

Can you feed the #26 tube 180volts on the plate? What is the maximum voltage you can put on the plate? What should the cathode voltage be? I'm only dropping about 25volts thru the two 150H chokes.

Joe
 
diyAudio Senior Member
Joined 2002
Hi,

I can't seem to find the power supply voltage on the plate of the #26 tube in Gary Pimm's choke loaded design.

The exact voltage isn't indicated on the circuit diagrams.
However a quick look will show that the B+ prior to the series choke load can't be much more than say, 200VDC.

Distract the voltage drop of the 2 chokes; -25VDC and you arrive at what you guessed already: 170-180VDC.

This is very close to the maximum rating of the 26 according the the RCA datasheet but can be used.

Bias is than set by varying the 2k5 pot in the heater/cathode circuit.

Cheers,;)
 
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