Audio/video cables...

[Michael J. Carter]

I need to order a 2' HDMI cable from bluejeanscable.com. Does anyone need anything?

mjc

--
Michael J. Carter
mca...@pobox.com

[jsn]

Are you ordering from there because they have those "better than the
flood of cheap Chinese cables"? Is the difference substantial?

jsn

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[Michael J. Carter]

I actually don't have any "cheap Chinese cables" to compare to... I ordered some 5.1 audio cables from bluejeanscable.com a few years back and was impressed with the parts and build quality and price - especially since they're made domestically- cheap enough that I didn't feel it was worth building my own. I've since decided I much prefer EL34 powered 2.0 home theater to 5.1 (or 7.1 (or 9.1)), but that's a different thread...

When I needed to buy a HDMI cable for my Roku Netflix player, I noticed that bluejeans had cables made with Belden stock for a very reasonable price (<$17 for 2m). For long runs, the Belden is obviously better (http://www.bluejeanscable.com/articles/belden-hdmi-info.htm). For short runs, it probably doesn't matter.

Plus they stood up to a ridiculous patent infringement from Monster:

http://gizmodo.com/380055/blue-jeans-cable-calls-bs-on-monster-cable-patent-suit-vows-to-fight-to-bloody-death

mjc

[Houlton, Robert J]

Hey guys,

I've been working on the dwarf tube projects here lately and have succeeded with my first PCB based amp. By using all the resistance values in a full sized amp with a big power tranny, I end up with very low voltage on the plates of V1 and V2 (36-60 volts) and it sounds crappy.

So I started reducing resistance on the dropping string and plate resistors until I had over 90 volts on the plates and it sounds better... but I don't know what the other consequences are for doing this. Do I need to make complimentary changes elsewhere or maybe achieve things differently?

Thanks,

rjh

[John Galbraith]

On Tuesday 02 March 2010 8:20:39 am Houlton, Robert J wrote:
> I've been working on the dwarf tube projects here lately and have succeeded
> with my first PCB based amp. By using all the resistance values in a full
> sized amp with a big power tranny, I end up with very low voltage on the
> plates of V1 and V2 (36-60 volts) and it sounds crappy.

Bob, I am assuming that V1 is a input stage, and V2 is a voltage gain stage?

> So I started reducing resistance on the dropping string and plate resistors
> until I had over 90 volts on the plates and it sounds better... but I
> don't know what the other consequences are for doing this. Do I need to
> make complimentary changes elsewhere or maybe achieve things differently?

The plate resistors establish gain, output impedance, and frequency response
in addition to the DC operating point which you are considering now. The load
for both tubes are probably grids, which are high input impedance, unless they
are big power tubes. Smaller resistors will generally improve the frequency
response of the stage, as the output impedance is lower. If this is a
micropowered guitar amp, it's probably not a big deal to reduce the stage's
output impedance.

The *gain*, however, you might care about a lot for a guitar amp. This is
where I would look out. It won't hurt you or the amp, but it might not be the
sound you want.

I would try to leave the plate resistors high (although you really should
figure out what the plate currents are to calculate the right DC operating
point) and figure out why your supply is low. Check the voltage at the filter
capacitors, and at all nodes after any dropping resistors. Figure out where
you are burning the voltage and fix it there. Make sure that you can account
for *all* the current being drawn from your supply. If there is any extra, it
will drop extra voltage across the PT resistance and the rest of the power
supply.

My monoblock project going on right now was stymied for a while for a similar
reason because I did not know how to read the datasheet for the power
transformer. I was pulling 800mA from it, which was the spec, but for *AC*.
The manufacturer straightened me out, because the PT can really only source
500mA DC. That extra current crashed the PT voltage completely; it sounds
very similar to what you have going on now. Sounds like you have an oversized
PT, so it's probably not dropping extra voltage there, but if you are using
more current than you think it could be dropping volts somewhere else.

Also, if you are using a variac like I always do, check the input AC voltage.
8-)

John

[Houlton, Robert J]

John,

thanks for the help. The ultimate voltage the power tubes will take has defined the upper limit of my B+ node. The guys using these 5902 tubes in the guitar amps have stated that these tubes don't last too long with the plates above 175V DC.

in this case I have used a new tranny with a substantially lower voltage Secondary (120-0-120) to keep the power tubes under 180V DC. I started with the same dropping string as is used with a 260-0-260 tranny in the original full sized amp. I've since modified the dropping string by jumpering the same value resistor across each dropping resistor. That kept the ratio of drop similar across each node.

I have five B+ nodes:

B+1 = 160V
B+2 = 158V
B+3 = 143V
B+4 = 122V
B+5 = 118V

I am not sure if there is some magic ratio that I want to keep among the voltages along the dropping string.

It seems the amp is happiest when the voltage on the plates comes up over 90V. I am not sure how my implementation has impacted things performance wise but Johns note gives me a key.

If I don't want to Monkey with the plate resistors, can I reduce things further along the dropping string or are there still more alternatives?

Thanks,

rjh

John

--

[John Galbraith]

On Tuesday 02 March 2010 9:53:13 am Houlton, Robert J wrote:

> original full sized amp. I've since modified the dropping string by
> jumpering the same value resistor across each dropping resistor. That kept

The resistors are not chosen by voltage alone, it matters how much current
your amp draws, too. You definitely want to be aware of how much current each
tube draws; it may be way off from the high power design you copied. I would
be surprised if you wanted to keep the same values, and random guessing is
problematic as well...

> I am not sure if there is some magic ratio that I want to keep among the
> voltages along the dropping string.

Unfortunately, the resistors also create a network of low pass filters (with
the filter caps) in addition to establishing the DC operating point which you
are considering. Higher resistance --> more filtering. Is your amp humming?

> It seems the amp is happiest when the voltage on the plates comes up over
> 90V. I am not sure how my implementation has impacted things performance
> wise but Johns note gives me a key.

The tube will definitely have a sweet spot for current vs. linearity, as
revealed on the curve tracer plots on the datasheet. It's not clear to me
that you know what the plate current is yet...

> If I don't want to Monkey with the plate resistors, can I reduce things
> further along the dropping string or are there still more alternatives?

You can use a choke. It offers a better filter than the resistors (two poles
instead of one, working with the filter cap) and probably has less series
resistance than the resistor as well. The result for you is that you get the
filtering of two capacitors and a resistor, with only one capacitor and a much
smaller resistor. In practice, I would use two filter capacitors and a choke
(the PI filter). That should leave you with more B+ to play with, to keep your
plate resistors higher.

I've not actually looked at your schematic, so I am talking "in general"; I
don't mean to mislead if I am misunderstanding what you actualy have...

John

[rjgu...@gmail.com]

John,

Your info is quite useful. I have built a lot of amps now using other folks
designs and schematics but this particular one doesn't borrow much from
anybody so I am really getting a first lesson in the fundamentals of
tweaking an amp as I go. I need to learn something about tubes and load
lines, how much current they draw, all of that stuff.

I'll attach the original schematic and be at work in creating the as built
schematic in the meantime.

The original Rocket amp is basically the top boost section of an AC-30 with
a few tweaks. It uses the typical Trainwreck power supply.

I am now pleased with the circuit board, everything is in the right place
and it's very quiet... not that hard to do with a couple watts I suppose. It
sounds good but I still think I can get more clean headroom out of it, or at
least I want to prove that I can't. As I brought the voltage up it just
seemed to sound better and better. It has me curious how far to take that.

thanks again,

rjh

[in...@rjguitars.net]

Hey Guys,

Maybe this has been all in reverse order... or in need of a restatement of
what I want to accomplish.

If I work backwards from here maybe it will all make more sense.

http://www.nj7p.org/Tube4.php?tube=6112

This is what I am using for the preamp tubes. I'll start mapping out things
to accomodate this tube as a first and second gain stage.

Can anyone help with with breaking this down into baby sized chunks for
operating this tube at voltages less than 100?

thanks,

rjh

[Matt Wiebe]

Bob,
There is no tube characteristics shared by the 6112 and the 12AX7.
The most critical characteristics are Mu, 70 for 6112 and 100 for
12AX7, Rp, 28-30K for 6112 and 60K to 80K 12AX7, VaMax 165V for 6112
and 300V for 12AX7, PaMax, .5W for 6112 and 1W for 12AX7. As you can
see there is really nothing in common between the tubes from an
engineering perspective. Likewise a power supply designed for the
12AX7 will not work with the 6112.
That the 6112 worked in a circuit designed around the 12AX7 is not
surprising, that's a nice thing about tubes, they don't let the smoke
out very easily. But if your goal is to get the 6112 to work like the
12AX7 in that circuit than you will have to redesign the amp to match
the tube.
As John said load the plate of a 6112 with a 12AX7 plate load and
expect the highs to be attenuated. I have found these submini tubes to
take over voltage fairly well but they sound best at book operating
points.
Matt

[jsn]

Hi Bob,

Want to sit down with the data sheets and design this thing from
scratch in terms of operating points, plate/cathode resistors, etc?
That way we will know the current draw for each tube and can work out
the resistor values in the power supply.

I'm glad to hear that the tubes sound best at book operating points -
that definitely makes it easier.

I have some time between 12:00 and 4:00 today. Maybe we could grab
some lunch or something.

jsn

[Houlton, Robert J]

Jason,

I'd enjoy looking these over. I have meetings at 11:00 and 1:00 today. Can I call you after 2:00 and see what your schedule looks like?

Thanks,

rjh

-----Original Message-----
From: audio...@googlegroups.com [mailto:audio...@googlegroups.com] On Behalf Of jsn
Sent: Wednesday, March 03, 2010 8:34 AM
To: audio...@googlegroups.com
Subject: Re: B+ Dropping string and plate Resistance Values - question

[jsn]

Sounds good.

[jsn]

Looking at the datasheet, the 6112 tubes want around 180V at the power
supply node (B+3/4/5). Using a "string" like this will not give you
that much if the power tubes are held to 180V.

Options:

1) live with it. Using all 9k or 10k resistors in the string, you
will be down to about 118V at the input tube node. This might sound
just fine.

2) use smaller resistors in the string. Maybe 5k or even 1k. See if
that gives you too much noise.

3) decrease the plate resistor value, which will lower the gain.

4) choose a cathode resistor/plate resistor combination that will give
you a lower current operating point. Right now these are under 1mA,
and that feels as low as you would want to go.

5) Actively load the plates (and maybe cathodes) with those wacky
constant current diodes. You can get them in values around 1mA and
they are 2 lead devices, so they can solder in where the resistors
were. I *think* that will drop less voltage between the power supply
and the plate than a resistor. You will get more gain and less noise.
I have been wanting to try these out for a while. They are a buck or
2 each.

jsn