• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

# triode 31 line pre amp

Status
Not open for further replies.
Jump to Latest
#2
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?
 
#4
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.
 

Attachments

  • 31-Line-Preamp-fil-Bias.jpg
    31-Line-Preamp-fil-Bias.jpg
    187.3 KB · Views: 978
#6
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.
Click to expand...
Why? -Only the different ratios of both opt?
 
Last edited:
#7
ilikedht said:
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?
Click to expand...

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
 
#9
Vinylsavor said:
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
Click to expand...

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.....
 
Last edited:
#20
Vinylsavor said:
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
Click to expand...

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?
 
Status
Not open for further replies.
Top Bottom