Re: Bias ckt schematic is posted


"gil" <gilbert@xxxxxxxxxxxx> wrote in message
news:6spd73p8fr1pbp3gcbmdlp5eggddatpa5e@xxxxxxxxxx
On Mon, 18 Jun 2007 10:24:56 -0400, "Phil S."
<psymonds_no_spam@xxxxxxxxxxx> wrote:

Flipper:
Schematic of the bias circuit is up a the link. The rest of the amp
really
is the AA764 VibroChamp available widely on the internet.
http://home.comcast.net/~psymonds/AA764/AA764.htm

On the second picture, I would be grateful for some recommendations for
the
various R's. Note that R3 is the grid resistor and R4 is the plate load
resistor for V1B, which I assume stays at 100K.

As far as pots go, I've got that 48K that is in there now and a 5K in the
parts box. I have a feeling you are going to tell me I should get a 25K
pot. I'd appreciate your recommendations all around as you really have a
firm grasp on how this all hangs together.

I have to comment that your request to for a schematic turned out to be a
real eye opener for me. When I drew it out, a great deal of what you have
been telling me fell into place. This turned out to be much more than a
simple exercise.

Many thanks.

Phil


Phil, I've read just a little of your past posts on this subject but
haven't replied. Here's some general info you might find useful.
Again, this is GENERAL. In regards to you 'driver' stage, V1B, R4 will
usually be 2 X rp, or two times the dynamic plate resistance. The
dynamic load for R4 is the path of R3, VR1, R5. R2 will be in
parallel to R5 assuming you have a cap on the other end of R2. This
dynamic load should be at least 4 X R4. Dynamic load being the AC
signal load, not the DC load. Since it appears that the combo of R3,
VR1, R5, R2 is smaller than R4, you'll probably have distortion in the
drive signal. Sometimes this is ok if the peak to peak drive signal
needed is not too great, i.e. with the 6AQ5. Driving larger power
tubes like this will cause distortion since the peak to peak drive
needed is greater. You should check for distortion caused by excessive
loading by the bias network.

I'm sure you've noticed the grid resistance limits stated on a given
tubes datasheet, depending on if it's cathode or fixed bias. The
reason for this is charge build up on the grid of the power tube. If
the grid resistance on the power tube is too large, then this build up
will cause the power tube to shift it's bias and possibly run away. If
there is cathode resistance, then runaway is prevented by the 'self
biasing' nature of the cathode resistor, and that's way they have a
bigger grid resistance listed for cathode biasing.

If using fixed bias for the 6AQ5, then your grid bias network should
be 100k max. and R4 should be around 25k for best results.

Gil,
Thank you for your reply. I think I'm confused again. You've thrown a lot
at me and I was pretty good until I got the last paragraph where you suggest
R4=25K. R4 is the plate load resistor for V1B and is 100K. Va on V1B is
already 250v. According to the schematic for the AA764, Va should be
around 200v. I think you are telling me, if I lift C3 out of circuit, Va
for V1B will drop 50V or there abouts to what the schematic shows?

BTW, in fixed bias, the grid R limit is 100K. Living within this seems to
be one of the challenges.
Phil


.

Relevant Pages

  • Re: Bias ckt schematic is posted
    ... Schematic of the bias circuit is up a the link. ... Note that R3 is the grid resistor and R4 is the plate load ... I'm sure you've noticed the grid resistance limits stated on a given ... reason for this is charge build up on the grid of the power tube. ...
    (alt.guitar.amps)
  • Re: -mm seems significanty slower than mainline on kernbench
    ... I think that I got that part wrong and you can end up with a bias load to be moved which is less than any of the bias_prio values for any queued tasks. ... I think that this analysis is a strong argument for my original patch being the cause of the problem so I'll go ahead and generate a fix. ... This indicates that, on average, 98.9% of the total available CPU was used by the build. ...
    (Linux-Kernel)
  • Re: Brook sliding bias operation.
    ... Thus the bias clipping will become "harder". ... So as the Ek signal rises when the amp moves into class AB, ... cathode biased PP amps when a low load provokes a serious rise in Ek ... So probably its a simpler idea to use two OP tubes and fixed or cathode ...
    (rec.audio.tubes)
  • Re: feedback & stability (long)
    ... did not expect the lower frequency outputs to be so dependent on bias ... the load before plate goes to zero. ... current across the lower load. ... I think it's the effect of the loadline (load plotted on ...
    (rec.audio.tubes)
  • Re: Ping Prof Turner: Amplifier questions
    ... >>and then you can draw in the load line to the best position. ... The anode load on an SET amp tube is always considered to be straight line. ... >>> Fixed bias did get the 4.6 watts out of hard clipping though. ...
    (rec.audio.tubes)