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# triode 31 line pre amp

Triodenklaus

Member
2015-09-26 3:01 pm
Hello Magnus,
... interesting question.
I think as a preamp tube tube type 31 is not a bad choice. But very rare :(

I have no experience with this tubes. But i have 2 31 Philcos in private stock.:)
I think a good line output transformer with low output impedance is necessary. look into the datasheet of type 31 :Lundahl 1660 / 10ma 4.5: 1 Alt Q or LL1692A / 10mA 3.5: 1 would be suitible. But the LL1692 has slightly more output impedance, but more gain.
Potentiometer must be an induct. stepped attenuator for good perfomance (after OPT. )
Cathode or filament bias?
Anyone other ideas?
 

Rod Coleman

Member
Paid Member
2004-07-25 6:55 pm
UK
The Ra of the 31 is lower than the 26, and its anode current a little higher, but you should be able to use a similar output transformer.

If you are interested in trying filament bias, this is my starting-point schematic, revised for the 31.
 

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hi all,

thanks for all interesting answers.

i think the small filament current ou 0,13A ist perfect for battery supply, isnt it?
What are your opinions for battries? (low noise und hum)...
Lundahl 1660 / 10ma 4.5: 1 Alt Q or LL1692A / 10mA 3.5: 1 would be suitible. But the LL1692 has slightly more output impedance, but more gain.
Why? -Only the different ratios of both opt?
 
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Triodenklaus

Member
2015-09-26 3:01 pm
hi all,

thanks for all interesting answers.

i think the small filament current ou 0,13A ist perfect for battery supply, isnt it?
What are your opinions for battries? (low noise und hum)...
Why? -Only the different ratios of both opt?

hi

the resitance is transformed to the square of the gear ratio:)

higher gear ratio: lower resistance and gain...


@ 31 preamp:
my first test: tube 31 is a very very good preamp performer. WOW
no microphonic problems like 30sp
 

Triodenklaus

Member
2015-09-26 3:01 pm
Hi!

I agree, the 31 makes an excellent preamp tube. I was positively surprised. I still prefer the 10Y overall but the 31 comes close. I prefer it over the 26

BR

Thomas

hi, very nice amp 31 in our blog, Thomas.:)
Iam testing now in cathode bias...( my filament-test-transformer isnt suitible for filament bias now ....)
...in future i hope to find/hear differences between cathode and heater biasing.....
 
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hi

here my shematic now:
for information: its only a testing circuit!
Ive changed cathode bypass cap to ultrapath,- i thinks its better.
also i turned from voltage regulator to lcl-filter for filament. its incredible but it sounds much better....

[IMGDEAD]http://i66.tinypic.com/2qcdrtx.jpg[/IMGDEAD]
 
I have used several 31s in the preamp I built, no issues with microphonics. With clean filament and B+ supplies there will be no hum. The tube is not more or less susceptible to hum than others.

BR

Thomas

i go to mount 10Y preamplifier with similar topography of Triodenklaus, only few db needed, but i ordered hammond 126C for output
silicon M29 EI core bifiliar 100H 10K/10K 15mA
but instead of input transformer LL1670 grid choke
what do you tink?