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#26 pre amp

iko

Ex-Moderator
Joined 2008
Finished a prototype 26 preamp and have a strange problem with hum, which I solved, but can't explain. The setup is as follows:
HT - 220VAC transformer, solid state bridge, CLC filter (470u->6H->470uF), solid state regulator (my own design).
Filament supply - 30VAC, solid state bridge, RCL filter (400R->100uH 2A->33mF), solid state double gyrator (my own design).

At the other side of the gyrator I use a 50W 10R resistor, for biasing.

Star ground for all power stuff, and another star ground for the signal, then both joined to the case.

The 26 tubes are each loaded by a Hammond 156C choke.

At idle, the scope showed a wiggly trace of about 8mV p-p, between choke and the plate, which ends up as audible hum.

Here's the part that puzzles me. If I replace the chokes with 4K resistors, there is no hum. The plate chokes were mounted at the very front of the amp, about 30cm away from the nearest transformer. The transformers and chokes are all in the same case. Any ideas?
 
Finished a prototype 26 preamp and have a strange problem with hum, which I solved, but can't explain. The setup is as follows:
HT - 220VAC transformer, solid state bridge, CLC filter (470u->6H->470uF), solid state regulator (my own design).
Filament supply - 30VAC, solid state bridge, RCL filter (400R->100uH 2A->33mF), solid state double gyrator (my own design).

At the other side of the gyrator I use a 50W 10R resistor, for biasing.

Star ground for all power stuff, and another star ground for the signal, then both joined to the case.

The 26 tubes are each loaded by a Hammond 156C choke.

At idle, the scope showed a wiggly trace of about 8mV p-p, between choke and the plate, which ends up as audible hum.

Here's the part that puzzles me. If I replace the chokes with 4K resistors, there is no hum. The plate chokes were mounted at the very front of the amp, about 30cm away from the nearest transformer. The transformers and chokes are all in the same case. Any ideas?

For headphone amps and preamps with output transformers, a separate chassis for the power transformers is a requirement. Your chokes are no different. You can try turning your chokes 90 degrees, but a separate chassis is best.

Some may get lucky and not hear the hum with thier system, but I can guarantee that a decent soundcard will always show hum pickup in an opt or choke from the power transformers, even when they are separated 12" or more. When dealing with a preamp you are on the level of a 24bit soundcard's noisefloor because of the post amplification, especially if you have sensitive speakers.
 
Hi all,
Just another one who fall into this great preamp. Having had the opportunity to listen to Andy Evans' 26 preamp and finished my 45SET, is now time to embark myself on the DHT preamp quest.
After my conversation with Andy yesterday, I've made the decision to go for filament bias. Also have some DHTs at hand that I want to try: CX112a, 01a, 26, 30, 30sp, 10Y and 4P1L. I know that people may suggest to keep with the 26, but I want this to be a learning experience for me.

Hopefully tomorrow will arrive a pair of Lundahl 1660 I bought from Thomas. So will be breadboarding the preamp soon.

I did a quick simulation in Ltspice. See attached. I'm not entirely sure if I modeled the Lundalhn in 4.5:1 connection in the right way. I biased the 26 at 163V and 5mA with a filament bias voltage of -10.5V. The stage gain is around 1.7 and the frequency response is 15Hz-15kHz. Is the anode inductance responsible for this response? Any way of improving this a bit?

Thanks
Ale
 

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Hi all,
Just another one who fall into this great preamp. Having had the opportunity to listen to Andy Evans' 26 preamp and finished my 45SET, is now time to embark myself on the DHT preamp quest.
After my conversation with Andy yesterday, I've made the decision to go for filament bias. Also have some DHTs at hand that I want to try: CX112a, 01a, 26, 30, 30sp, 10Y and 4P1L. I know that people may suggest to keep with the 26, but I want this to be a learning experience for me.

Hopefully tomorrow will arrive a pair of Lundahl 1660 I bought from Thomas. So will be breadboarding the preamp soon.

I did a quick simulation in Ltspice. See attached. I'm not entirely sure if I modeled the Lundalhn in 4.5:1 connection in the right way. I biased the 26 at 163V and 5mA with a filament bias voltage of -10.5V. The stage gain is around 1.7 and the frequency response is 15Hz-15kHz. Is the anode inductance responsible for this response? Any way of improving this a bit?

Thanks
Ale

It would either be interlayer capacitance or leakage induction, can't imagine spice would be able to predict HF reponse out of an amorphous transformer.

So you are floating the Colemane regulator at -10.5V, using a battery?.
 
Question: Are there any interesting Russian DHT’s for preamp use? I only see DHT pre’s with the well known 01A, 26, 10Y ect., but never Russian types.
Could it be that during the early ages of tube technology the quality of Russian tubes was (far) behind western tubes? Or is the reason that, because almost all of the Russian stuff is from military surplus, these stocks are from later years when IDHT were already common?

Peter
 
Has anyone used the UTC HA133 transformers in their 26 design? After reading the 121+ pages of this thread in a number of settings I cannot recall if there was someone using it or not. I am looking right now at Kevin Kennedys 26 tube schematic and wondering which taps he used. I know its hard to come up with two HA133's so I am looking toward having something made or substitute something else. I'd welcome any ideas.
 
Thanks people I appreciate your willingness to just jump right in there with some input.

My input would be that the UTC transformer is so hard to come by and expensive why risk it when there are guy's like Bud who can build a transformer that you know the specs of. With the #26 you want to be very careful with the primary you chose due to the high Rp.

Now if you have a pair of UTC-HA133's the spec is:

Primary: 15 kohm 8MA DC

Secondary: 500, 333, 200, 125, or 50 ohm

Max level: +22 dbm

30 – 40 kHz (listed as kc) +/- 1db


But they don't tell you the primary inductance, would be worth a try. But no way I would spend $800 obtaining a pair, you probably would want to parafeed the #26 anyway and these are single feed.
 
Hi!

Another alternative which I tried: Tango NP8, but it is not manufactured any more. It was good, but I preferred the LL1660. I'm sure from the current line of ISO Tango there are alternatives available as well. Hashimoto also has a transformer which is suitable. I haven't tried the mentioned UTC, but as the previous poster wrote: depends on the rpimary inductance.

Best regards

Thomas
 
Slowly coming to life. This morning managed to complete the valve board and fitted it in the modular chassis. The board has the two sockets for the 4-pin valve of choice (first one to try will be 26). Also have added the filament bias resistors...
Now will go for the OT board for the LL1660s....

All thanks to Andy Evans who kindly helped me out with some of the parts...
cheers,
Ale
 

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Hi Ale,

looks great! Those black resistors ( I assume filament bias ) Look cool the way they are mounted!

Best regards

Thomas

Hi Thomas,
Thanks! Indeed, those are the filament bias resistors that Andy gave to me. This afternoon managed to complete the second top plate in which I mounted the LL1660 you sold me. See attached.
Just did a simple test with my bench power supply and managed to get Av=3.5 at Va=162V, Vsupply=170V. Ia=5.6mA and Vg=-10.5V

Looking really good. Now need to crack on with the remaining bits and start with the power supply. That will take me some real time...

Thanks
Ale
 

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Has anyone used the UTC HA133 transformers in their 26 design? After reading the 121+ pages of this thread in a number of settings I cannot recall if there was someone using it or not. I am looking right now at Kevin Kennedys 26 tube schematic and wondering which taps he used. I know its hard to come up with two HA133's so I am looking toward having something made or substitute something else. I'd welcome any ideas.


Just saw your post.. The information you need is here: UTC Data

I wired the primaries in series and configured the output taps for 600 ohms.

Jack Elliano can probably make you suitable plate to line transformers, other candidates would be the Hashimoto 20K part. Look at the Lundahl line as well.. 5:1 ratio is near ideal IMHO.. AE, Trafomatic, or Bartolucci could probably make something nice at a price..

I'm still using the much revised original line stage and getting ready to build the latest version..