parallel preamp tube question

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jezzbo
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parallel preamp tube question

Post by jezzbo »

hi everybody,

A typical (ecc83 etc.) dual triode uses 100K plate resistor and 1.5K cathode + 25uF cap.
I read somewhere that when I want to connect both triodes in parallel (for lower noise) I have to half the value of the plate and cathode resistor.
(so 50K Plate resistor and 750R cathode resistor
But what about the cathode bypass cap? Should that also be half the normal value?

Thank you for your replies.

Jesse
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didit
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Re: parallel preamp tube question

Post by didit »

jezzbo wrote: Sun Sep 10, 2017 6:07 pm But what about the cathode bypass cap? Should that also be half the normal value?
Doubling capacitance best approximates equivalence - http://ampgarage.com/forum/viewtopic.php?f=6&t=11146.

Best .. Ian
jezzbo
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Re: parallel preamp tube question

Post by jezzbo »

Thanks Ian. Reading the linked topic now.
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JazzGuitarGimp
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Re: parallel preamp tube question

Post by JazzGuitarGimp »

Deleted - I didn't see the question had already been answered....
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10thTx
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Re: parallel preamp tube question

Post by 10thTx »

Perhaps I have been doing it wrong for yrs .....................

I've halved the plate resistor and kept the cathode resistor the same. So a single triode of 100k/1.5k becomes a paralleled tube sharing a 56k plate resistor and sharing a 1.5k cathode resistor. This has worked out fine for me.

with respect, 10thtx
jezzbo
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Re: parallel preamp tube question

Post by jezzbo »

Thanks guys.

I would use the parallel tube in the first gain stage.
In this link "http://ampgarage.com/forum/viewtopic.php?f=6&t=11146"
someone says:" 1) Cut value of grid stopper in half (since input capacitance doubles with dual triodes, halving resistance keeps same freq response)."
I already use 33K grid stopper would going lower give me trouble with radio interference etc?

Another (related) question:
I was wondering, for the bypass cap, are there benefits if I use a Tantalum instead of an Electrolytic cap?
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pompeiisneaks
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Re: parallel preamp tube question

Post by pompeiisneaks »

jezzbo wrote: Sun Sep 10, 2017 6:50 pm Thanks guys.

I would use the parallel tube in the first gain stage.
In this link "http://ampgarage.com/forum/viewtopic.php?f=6&t=11146"
someone says:" 1) Cut value of grid stopper in half (since input capacitance doubles with dual triodes, halving resistance keeps same freq response)."
I already use 33K grid stopper would going lower give me trouble with radio interference etc?

Another (related) question:
I was wondering, for the bypass cap, are there benefits if I use a Tantalum instead of an Electrolytic cap?
Tantalums last a very long time, so they're better, they just cost more, usually a lot more, than an equivalent electrolytic. This is why electrolytic are more common.

~Phil
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brewdude
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Re: parallel preamp tube question

Post by brewdude »

I like to use dissimilar biasing on the two parallel triodes and a single plate resistor and coupling cap.
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Colossal
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Re: parallel preamp tube question

Post by Colossal »

brewdude wrote: Sun Sep 10, 2017 8:57 pm I like to use dissimilar biasing on the two parallel triodes and a single plate resistor and coupling cap.
Common anode mixer! 8)
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TUBEDUDE
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Re: parallel preamp tube question

Post by TUBEDUDE »

Never used tants as Ck, but i stopped using electrolytic there. I use metallized polys, usually 4u7 or so. In New designs, or were there is space, I use metallized polys for power filters also. Solen, Audyn, and the Dayton labelled ones are in usable values/voltages.
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brewdude
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Re: parallel preamp tube question

Post by brewdude »

I like the brown putty colored Panasonic 100v poly film caps as cathode bypass caps.
tubeswell
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Re: parallel preamp tube question

Post by tubeswell »

jezzbo wrote: Sun Sep 10, 2017 6:07 pm hi everybody,

A typical (ecc83 etc.) dual triode uses 100K plate resistor and 1.5K cathode + 25uF cap.
I read somewhere that when I want to connect both triodes in parallel (for lower noise) I have to half the value of the plate and cathode resistor.
(so 50K Plate resistor and 750R cathode resistor
But what about the cathode bypass cap? Should that also be half the normal value?

Thank you for your replies.

Jesse
You don't have to halve Ra and Rk, only if you want to keep the gain and bias point the 'same'*. The advantage of 2 x parallel triodes with halve Ra and Rk values, is a lower SN ratio and lower output impedance (better for driving AC loads/tone stacks) and increased transconductance (more plate current swing for a given amount of grid voltage swing). (And yes the Ck would need to double if you halve Rk, in order to keep the same frequency rolloff point.)

*Many amps parallel triodes to get more gain (so they don't bother with reducing the Ra or Rk.
Last edited by tubeswell on Mon Sep 11, 2017 4:09 am, edited 1 time in total.
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jezzbo
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Re: parallel preamp tube question

Post by jezzbo »

Thanks everyone.
What about the grid stopper resistor?
(I already use 33K instead of the traditional 68K)
tubeswell
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Re: parallel preamp tube question

Post by tubeswell »

jezzbo wrote: Mon Sep 11, 2017 2:51 am Thanks everyone.
What about the grid stopper resistor?
(I already use 33K instead of the traditional 68K)
Inter-electrode capacitance is doubled with 2 parallel triodes of the same type. So that means you only need half the grid stopper resistance value to get the same HF rolloff
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jezzbo
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Re: parallel preamp tube question

Post by jezzbo »

Nice. Thanks!
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