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Good driver tube for PX25 / DET5

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Posted on June 8, 2020 at 11:47:24
iodemus
Audiophile

Posts: 167
Joined: October 21, 2010

I have decided to build my last SEDHT-amplifier, with the superior DET5.

It is the prototype of NT40 with hard vacuum to enable operation up to 600 Volts on the anode. The thick glass bulb weight 50% more than PX25.

Must choose a driver, preferably an old tube made for telephony.
I have C3m, C3g, Ca, Ba, Bi, 3A/110A, Cf, KC3, RE134, RE304, 101D and more.

Plate choke, interstage trafo, resistance coupled - nothing decided yet.

Any suggestion?

 

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RE: Good driver tube for PX25 / DET5 , posted on June 8, 2020 at 21:28:32
Cleantimestream
Audiophile

Posts: 7079
Location: Kentucky
Joined: June 30, 2005
Consider the 47... The only directly.heated.pentode I have considered in that position.
The Mind has No Firewall~ U.S. Army War College.

 

RE: Good driver tube for PX25 / DET5 , posted on June 8, 2020 at 23:52:35
Paul Joppa
Industry Professional

Posts: 6994
Location: Seattle, WA
Joined: April 23, 2001
If you want the character of the DET5 to shine, then you want as linear a driver as possible. It's most important to find a tube with a large enough region of uniform mu, and operate it there. I use triodes, because I can figure that out.

I have an admitted fondness for current source plate loads, but a suitable plate choke (or transformer) is still more linear than a resistor.

 

RE: Good driver tube for PX25 / DET5 , posted on June 8, 2020 at 23:55:29
iodemus
Audiophile

Posts: 167
Joined: October 21, 2010

Thanks for suggestion, but I plan to use a triode, preferably one listed.

There are "big holes" in my knowledge about SET amp construction, but all I have built with my 6th sense and ideas from here and there has been working perfect from the first moment. (Look at the "Geiru chan".)

Which DHT can drive a DET5?
I have seen a schematic with Ca as shown. Have some of those masterpieces.

Johnny

 

RE: Good driver tube for PX25 / DET5 , posted on June 9, 2020 at 01:19:54
iodemus
Audiophile

Posts: 167
Joined: October 21, 2010

Linearity is paramount. So is plate choke.

Have these DHT candidates which are very linear:

3A/110B: Mu=12, Ug=-4,5V, Ri=5,5kohms, Ip=20mA, Up=190V.

Ca: S=1,65mA/V(MU?)Ug=-12V, Ri=4,1kohms, Ip=20mA, Up=220V,

Is interstage trafo needed for these or just a capacitor?

Thanks for your assistance!

Johnny

 

RE: Good driver tube for PX25 / DET5 , posted on June 9, 2020 at 07:56:33
Paul Joppa
Industry Professional

Posts: 6994
Location: Seattle, WA
Joined: April 23, 2001
It's a tradeoff. With a plate choke and capacitor you get a high impedance and excellent power supply rejection ratio but you need a really good coupling capacitor; with a transformer you still get the high impedance, don't need a coupling capacitor, but give up PSRR.

High PSRR reduces both noise and the effect of any nonlinearities in the power supply capacitors.

 

RE: Good driver tube for PX25 / DET5 , posted on June 9, 2020 at 12:50:23
iodemus
Audiophile

Posts: 167
Joined: October 21, 2010
I plan to use the best oil caps and Lundahl LL1668 plate choke, or consider to eliminate the caps with Lundahl LL1671 combined plate choke/interstage trafo.

Grid chokes in both stages and driver filament filters C-L-C .
All monobloc SEDHT amps I've made are quiet on Avantgarde Duo (107dB) with shorted inputs.
Are the Ca or 3A/110B tubes capable drivers for DET5?

Thanks, Johnny.

 

RE: Good driver tube for PX25 / DET5 , posted on June 9, 2020 at 14:30:52
Paul Joppa
Industry Professional

Posts: 6994
Location: Seattle, WA
Joined: April 23, 2001
I found some curves for the 3A/110B which look very good; haven't found any for the Ca. Ca has a mu of ~8.

 

RE: Good driver tube for PX25 / DET5 , posted on June 9, 2020 at 22:55:14
iodemus
Audiophile

Posts: 167
Joined: October 21, 2010
Both 3A/110B, Ca, Aa, Ce, Da, Ba, Bh, Ec and similar are made for telephony in war situations and need to be as linear as possible.
For military use it was important to have a good communication and hear clearly what was said in the other end. Even my 102D and 101D tubes in the "Geiru Chan" amps from abt. 1920 are superior regarding linearity, distortion, noise and lifetime. 50.000 hours + are estimated. In 1920!

Johnny.

 

C3m, posted on June 13, 2020 at 12:15:38
PakProtector
Audiophile

Posts: 11765
Joined: May 14, 2002
As pentode. CCS loaded, and the load line tilted just a wee bit with a resistor from plate to ground. 470k ought to do it. From the Tele data, 10 mA idle at 200V plate with v_g2 at 60 will put you in a very linear bit of the tube characteristic.

Use the cascode MOSFET current regulator, and output from the lower fet's source, the so-called mu output. It will drive a 211 easily, output Z on the order of 1-200R depending on how you set up the operation of the lower fet's bias( one of these CCS idling at 20 mA gave me 30R output Z). Battery biased for a larger R-Set.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: C3m/DET5/RE604, posted on June 14, 2020 at 02:50:34
iodemus
Audiophile

Posts: 167
Joined: October 21, 2010
Difficult without a schematic and no experience with pentodes and CCS's.

I have a schematic of "Chanchullo" by Erwin Wiesbauer with C3M/RE604 and Lundahl LL1660 plate choke/interstage trafo.

Think I could use both the RE604 and DET5.
Alternate the Ca or 3A/110B with LL1660 or LL1668 plate choke.

Any suggestions are welcome; thanks for assistance.

Johnny

 

RE: Good driver tube for PX25 / DET5 , posted on June 18, 2020 at 22:04:25
Cleantimestream
Audiophile

Posts: 7079
Location: Kentucky
Joined: June 30, 2005



Bad photo but here is my last build... 8608 driving a 45. Choke loaded with 816 mercury vapor rectifiers.
The Mind has No Firewall~ U.S. Army War College.

 

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