[powerdoc]

Is this by design, I see it on the schematic though, or has there been
some post-production fiddling with the unit? Are there any components
that are prone to failure? Can someone walk me through the bias
adjustment for KT88's, I don't have the manual? Thanks!

[Peter Wieck]

powerdoc wrote:
Is this by design, I see it on the schematic though, or has there been
some post-production fiddling with the unit? Are there any components
that are prone to failure? Can someone walk me through the bias
adjustment for KT88's, I don't have the manual? Thanks!

Got a link to the schematic?

Peter Wieck
Wyncote, PA

[powerdoc]

http://www.drtube.com/audioamp.htm#Jolida

[Sander deWaal]

"powerdoc" said:

Is this by design, I see it on the schematic though, or has there been
some post-production fiddling with the unit? Are there any components
that are prone to failure? Can someone walk me through the bias
adjustment for KT88's, I don't have the manual? Thanks!


Where is it on the schematic?

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[Sander deWaal]

"powerdoc" said:


http://www.drtube.com/audioamp.htm#Jolida


OK, 0.1 uF/630V, in the power supply.
Omitting it won't blow up your amp.

Now, for the biasing: from the schematic, I'd expect some 500V on the
CT of the output transformer.
Let's say 480V on the anode.
With KT88s, Pa max is 42 watts, so Ia max should be 87.5 mA.
Ignoring Ig2 for a moment, the voltage over each 1 ohm cathode
resistor should then be 87.5 mV at most.

EL34 can't handle that, Pa max is 25 watts in that case, so Ia max =
52 mA.
Ignoring Ig2 again, this is 52 mV over Rk again.

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[Peter Wieck]

powerdoc wrote:
Is this by design, I see it on the schematic though, or has there been
some post-production fiddling with the unit? Are there any components
that are prone to failure? Can someone walk me through the bias
adjustment for KT88's, I don't have the manual? Thanks!

C7 is of dubious necessity as shown. I doubt it would make any
difference in or out.

Sander brings you to the voltages you will need for bias. From my
reading of the schematic, I am guessing that the test-points are as
shown (TP), the trim-pots RU2 & RU3 are where you make the adjustments.
You will be adjusting for each individual output tube where RU2 will
adjust V3 and RU3 will adjust V4. The other channel will be similar.

So, you will put your VOM probes one on the common (center) of the five
(5) test points, and one on each of the other four in turn, adjusting
the trim-pot to the appropriate voltage for the tubes in place. Go
through all four tubes, the start all over again until all of them are
where they should be. Do not even start the procedure until the unit
has been on for at least 15 minutes. Make sure you use the most
sensitive DC voltage scale on your VOM. In this case an analog meter is
most useful as the dither on the last digits is not noticable as it
might be on a digital meter.

Usual cautions: Lethal voltages under the hood. Hot surfaces, use
caution, and so forth. In point of fact, until you are sure you are on
the right test-points, use the highest scale on the VOM to avoid damage
(to it or the unit).

Peter Wieck
Wyncote, PA

[powerdoc]

Thanks all for the warnings but I've grounded enough capacitors through
my fingers in the past to have some respect for the voltages! To avoid
seeing the tubes glow like neon during the warmup, is there any way to
pre-set the bias voltages without the tubes in place? I think I got
this one because the PO had a Fourth-of-July experience with it and
didn't want to f with it anymore.

[Sander deWaal]

"powerdoc" said:


Thanks all for the warnings but I've grounded enough capacitors through
my fingers in the past to have some respect for the voltages! To avoid
seeing the tubes glow like neon during the warmup, is there any way to
pre-set the bias voltages without the tubes in place? I think I got
this one because the PO had a Fourth-of-July experience with it and
didn't want to f with it anymore.


Might have been a bad batch of Chinese tubes (or early JJs).

Anyway, turning the pots towards the junction of R10/R11 will give you
the most negative grid voltage, and the least current through the
tubes.
That's RV2 down, and RV3 up in the provided schematic.

After adjustment of the tubes, as a final balance adjustment, you may
put your DMM across both cathodes in each channel, and try to get the
readings even by adjusting only 1 bias pot in the direction of
R10/R11.
This is a static balance adjustment, so it will only be valid in that
one operating point, at idle.
It doesn't say nything about balance of operation in working
conditions.

Repeat the entire exercise after an hour, and every few months.

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[powerdoc]

It may be a roundabout question but does the bias voltage affect sound
or just power?
I biased at 50mv which is what others I subsequently read did and it's
a bit bland, but then again I'm not used to tubes.

[Peter Wieck]

powerdoc wrote:
It may be a roundabout question but does the bias voltage affect sound
or just power?
I biased at 50mv which is what others I subsequently read did and it's
a bit bland, but then again I'm not used to tubes.

In rank order:

a) Tube stability
b) Tube life
c) sound

If the tubes go into runaway, they could eat your output transformers
(and possibly the tubes) in short order. If they are on the hairy edge,
then they could fail early. THEN, there is the sound.

Proper bias insures the third, enhances the second and prevents the
perhaps-catastrophic consequences of the first.

Peter Wieck
Wyncote, PA

[Sander deWaal]

"powerdoc" said:


It may be a roundabout question but does the bias voltage affect sound
or just power?
I biased at 50mv which is what others I subsequently read did and it's
a bit bland, but then again I'm not used to tubes.


I bias my KT88 trioded PP amps as hot as possible, meaning most of the
delivered power is in class A.

That means a lower supply voltage, I only use 380V on the anodes and
screen grids.
That also accomodates the mostly lowish primary impedance, in the
order of 5 k or even lower.

In triode, higher Ra-a gives lower THD, but, what's even more, the 3H
is attenuated more than the residual 2H.

As I have taps of 2.83 ohms, a connected 4 ohm load (Maggies, very
nice resistive loads) gives about 7 kohm Ra-a.

Sounds wonderful, despite the power power.

--
"Due knot trussed yore spell chequer two fined awl miss steaks."