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    Building, troubleshooting and testing of these amplifiers should only be
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#26 pre amp

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#5,221
euro21 said:
ISO plate choke is good at series connection (160H, 1160R DCR, 20mA) with 160V B+.
LL1660 is only 100H in Alt Q (4.5:1) configuration, so it's 12k57 impedance at 20Hz. For -about- 3k anode impedance tube it's -just enough, not too much.
Click to expand...
Hi euro21,

The history with my current line preamp is that it started with my best attempt at making Diego Nardi's WOT preamp from Sound Practices 10, using 5687 and LL1660/10mA. (So, I had to keep Ia<10mA and I think I opted for about 8 or 9 mA., probably not the best for 5687, which I think likes to run at higher current.) I was led to add an optional valve as an alternative to the 5687 and I think I tried several, but the most memorable was the 1H4G, a DHT that I used battery for bias and battery for filament heating (since the filament requirement was for 2V@60mA.) Sound was very good, but I gave up on that approach since it was too many batteries too often. So I was/am left with my trusty 5687/LL660/10mA line preamp, and it does sound good. What to do to make it better? Well, I have a pair of LL1660/18mA purchased from the estate of Arthur Loesch at my disposal. I also have a pair of the ISOs, so I thought that the other valve should be the 2P29L.

Det's why I'm here trying to figure out how to incorporate the 2P29L, and I am thinking I would like to have the output option switchable between 5687 and 2P29L, and also between the ISO and the Lundahl.

All for now, Robert
 
#5,222
euro21 said:
ISO plate choke is good at series connection (160H, 1160R DCR, 20mA) with 160V B+.
LL1660 is only 100H in Alt Q (4.5:1) configuration, so it's 12k57 impedance at 20Hz. For -about- 3k anode impedance tube it's -just enough, not too much.
Click to expand...
Hi euro21, I don't understand what you are referring to in mentioning ", so it's 12k57 impedance at 20Hz." about the LL1660. Were you referring to its primary impedance/resistance, which for Alt Q is 1250 Ohm DCR? What about "it's just enough, not too much."? Just enough, not too much what? According to what criteria? Please help me understand what you were trying to say. Best, Robert
 
#5,223
Hi Robert,

Z=2*pi*f*L

100H at 20Hz: 2*pi*20*100=12566 Ohm
This + DCR only 13816 Ohm.

#26 plate resistance 7300..8900 Ohm (depending of operating point).

The rule of the thumb: load must be at least twice, rather 3x..4x than plate resistance.
As you can see, even the first criterion isn't met.

The result: the distortion would be sufficient above about 30-40Hz (depending of operating point), below this will be growing.
 
#5,224
Hi euro21,

Thanks for the reminder about finding inductive reactance and also the rule of thumb about the desireable ratio of anode load to anode resistance of the valve. Okay, not good for the 26 valve, but now if we apply those concepts to the 2P29L, the valve that I will be using, we find that all is O.K. for the LL1660/18mA line output transformer; Load Z/Anode R ~ 14K/3K = 4.67. That ignoring capacitive reactance and how the real and imaginary parts combine...Morgan Jones stopped short of going there in his 686 page Valve Amplifiers, Fourth Ed., so it must be O.K. to ignore that stuff, no?

And, since the ISO RC-160-15W is 160H @ 15mA and 50Hz with series connexion of the windings, there should be no problem on that front. Excessive gain and high output impedance might be problematic with the ISO, but perhaps a Slagle AVC on the output would be a solution (in both cases).

Best, Robert
 
#5,228
euro21 said:
Need to open another one topic for 2P29L preamp, and the appropriate content should be transferred there.
Click to expand...
Hi euro21, et al,

I would be happy to either open a new topic or transfer to FlaCharlie's topic if I knew it was required/ and/if I knew how to transfer selected messages from one topic to another...I don't know how to do that. There have been so many valve types discussed in the 262 pages here that I thought it was acceptable to go beyond the usual politeness of sticking to the OPs topic.

So, if you want to stick to type 26 valve here, do you or does anyone happen to know the primary inductance of an UTC HA-133 for use with a 26? Kevin Kennedy in his Positive Feedback design foray on the 26 indicates that >200H is needed, but, unless I missed it, he did not indicate the actual H value of the transformers he used, and you will not find it specicied in the UTC catalogues.

Cheero! Robert
 
#5,229
Several years ago I tried -good- 100H choke loaded #26.
It was disappointing versus CCS loaded one.

HA-133 is rather input transformer, but primary even accept 8mA DC current..

1662709493311.png
 
#5,230
Hi euro21,

Yes, the UTC catalogue clearly indicates that HA-133 is plate to line, so definitely not an "input transformer." It is a step-down transformer, 5:1 when connected for 15K:600 Ohm.

I suspect that your trial of a "good" 100H anode choke for the 26 violated your ZL > 2Ra criteria...looks like it was more like 1.7., so what would you expect?

Kevin Kennedy in his Positive Feedback article, https://positive-feedback.com/Issue16/kennedy.htm, indicated that the plate choke for 26 should be >200H. He used an HA-133. And he tried unsuccesfully to measure the primary inductance of the HA-133, check the Recommend a good plate choke for 26 tube!! thread.

I'm just trying to figure your stance here. You tried to help me with my 2P29L design for a while, then you said I should take it elsewhere. ? ? ? As I have pointed out, several of your pronouncements are false/incorrect.

Be a good DIYer and double/check what you say.

Oh, and thanks for your help!

Best, Robert

​

 
#5,236
Hi all,
My website was hacked for phishing purposes and got 65k files infected for deletion. My account was suspended by the web hosting and now am in the process to get it up and running with the help from a great buddy as am not that technical on this IT stuff.
No idea how long it will take me to get this back as am pretty busy with work and family these days
Anything please shout or drop me an email as I don’t check this often
Ale
 
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#5,237
Hi guys, I've been following this thread for years and my name is actually in the first post. Until now I have not needed a preamp, but I do now. This is for my desk and will be fed by Qobuz -> 26 preamp -> Compactron tube amp -> DIY speakers built from Minimus 11 speakers.

Last night and today I scrolled through all 262 pages of posts and ended up with one question. I just need simple, so which is the simplest schematic so far?
 
#5,239
I moved on from the 26 a long time ago, loaned the pre-amp that Robert referenced to a friend in nearby Newton. I enjoyed my time with the 26 (something over a decade). They are extremely microphonic and maybe the 2P29 is worth checking out before embarking on a dht line stage project. Also I can't imagine some 20 years since I wrote that article that the supply and quality of the 26 has gotten better. Other choices to consider include the 12A and triode connected 4P1L.
 
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