Cathode follower question

woody · 7th October 2005, 06:33 AM

I saw a tube headphone amp on the Tube Cad web page that
had a feature that I don`t understand. Three triode sections
from 2 6dj8`s were parralled using a common cathode resistor
for the output ok. But these three triode sections also shared
a common 200 ohm plate resistor. What would this resistor
do? would the output impedence change?

Woody

dhaen · 7th October 2005, 08:07 AM

Got a link to the schema?

woody · 7th October 2005, 03:14 PM

I have built this with a few changes I omited the 150 ohm
plate resistor, subsituted a 6c33c-b and a 100ohm for the
3 paralled triode sections. I seem to have a prommising 1-2 watt
otl. I just wonder what that 150 ohm resistor was fore.

Thanks for looking

Woody

Gabevee · 7th October 2005, 03:25 PM

Looks like, maybe, current limiting.

Maybe.

Gabe

SY · 7th October 2005, 04:41 PM

Most likely, it's to help squelch oscillation, analogous to a grid or cathode stopper.

It won't change the output impedance very much; an resistance in the plate is reflected back through the cathode divided by mu- for ECC88, the 150 ohm resistor would only add 5 ohms or so to the output Z.

cerrem · 7th October 2005, 08:09 PM

I don't see how it would squash oscillation in the B+ due to the impedances / plate resistance/ cap involved....
Usually a unbypassed R in the plate circuit of a follower will introduce some harmonics.... This may sound off the wall but the ciruit may have been very sterile sounding and the designer wanted to add some slight coloring and thickness to the sound...

Chris

Johan Potgieter · 10th October 2005, 02:10 AM

Mmmmmm....

Without further information I would say it is a rather pious repetition of what was seen elsewhere. I did not look up, but this seems to be a small percentage of the plate resistance anyway. I am sure that any tendency for instability could have been better addressed elsewhere in the circuit if necessary.

7th October 2005, 06:33 AM	#1
woody diyAudio Member Join Date: Jan 2002 Location: Tyrone Ga. U.S.A.	Cathode follower question I saw a tube headphone amp on the Tube Cad web page that had a feature that I don`t understand. Three triode sections from 2 6dj8`s were parralled using a common cathode resistor for the output ok. But these three triode sections also shared a common 200 ohm plate resistor. What would this resistor do? would the output impedence change? Woody

7th October 2005, 03:25 PM	#4
Gabevee diyAudio Member Join Date: Feb 2002 Location: Richmond, VA.	Looks like, maybe, current limiting. Maybe. Gabe __________________ Gabe CGV Electronics Home of the CGV-300B amplifier on a budget

7th October 2005, 04:41 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Most likely, it's to help squelch oscillation, analogous to a grid or cathode stopper. It won't change the output impedance very much; an resistance in the plate is reflected back through the cathode divided by mu- for ECC88, the 150 ohm resistor would only add 5 ohms or so to the output Z. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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7th October 2005, 08:07 AM	#2
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	Got a link to the schema?

7th October 2005, 08:09 PM	#6
cerrem diyAudio Member Join Date: Jun 2005 Location: San Diego, CA	I don't see how it would squash oscillation in the B+ due to the impedances / plate resistance/ cap involved.... Usually a unbypassed R in the plate circuit of a follower will introduce some harmonics.... This may sound off the wall but the ciruit may have been very sterile sounding and the designer wanted to add some slight coloring and thickness to the sound... Chris

10th October 2005, 02:10 AM	#7
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	Mmmmmm.... Without further information I would say it is a rather pious repetition of what was seen elsewhere. I did not look up, but this seems to be a small percentage of the plate resistance anyway. I am sure that any tendency for instability could have been better addressed elsewhere in the circuit if necessary.

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