Cool attenuator idea

[jsn]

http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/

Pretty interesting way to do an attenuator. Never would have thought of that.

jsn

[Matt Wiebe]

Jason,
Phil is putting one together. It is interesting but is noise on light
any different then noise on current? It does get rid of contact
issues. Since Phil normally uses an Autoformer volume control I'm
interested in what he reports.
Matt

[jsn]

I think maybe the slow-ish response time of the photocoupler might act
as a low pass filter, possibly removing the noise almost completely?
neat.

We should do another listening party some time.

jsn

[Michael J. Carter]

We'll have a new house overlooking Deer Trap Mesa to burn-in on
Monday... Might need a few weeks to settle in before we could host a
listening session.

Anyone interested in schlepping furniture is welcome to stop by on
Labor Day. Payment is solely in homebrew...

mjc

On Thu, Sep 3, 2009 at 11:19 AM, jsn<j...@boozhoundlabs.com> wrote:
>
> We should do another listening party some time.
>
> jsn
>
> On Thu, Sep 3, 2009 at 11:12 AM, Matt Wiebe<mwi...@unm.edu> wrote:
>>
>> Jason,
>>        Phil is putting one together. It is interesting but is noise on light
>> any different then noise on current? It does get rid of contact
>> issues. Since Phil normally uses an Autoformer volume control I'm
>> interested in what he reports.
>> Matt
>>
>>
>> On Sep 3, 2009, at 11:01 AM, jsn wrote:
>>
>>>
>>> http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/
>>>
>>> Pretty interesting way to do an attenuator.  Never would have
>>> thought of that.
>>>
>>> jsn
>>>
>>> >
>>
>>
>> >
>>
>
> >
>

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

[Matt Wiebe]

Jason,

I know you have used 6S45P (6C46π) tubes in the past so you
may be interested in another variety that is turning up on Ebay. It’s
a 6S15P (6C15π) that according to the spec’s is identical to a
6S45P, I’m not sure why the different numbers. I got a few from this
seller below that date to 1966.

http://cgi.ebay.com/6S15P-6S45P-WE437-Hi-End-Triode-NOS-Tubes-Lot-of-4_W0QQitemZ280397073200QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item4148f7cb30&_trksid=p3286.c0.m14

Visually the largest difference is much better internal
construction, everything is better aligned than modern production
“Sovtek”s, and the 6S15P looks to have a gold grid. The OTK
inspection number is a “1” which I think is a pretty high grade.

There are also lots of Reflector 6S45P-Es on the bay around
$10 each, OTK “2”. They have halo getters (the Sovteks I have are
conical getters) and have 1990 date codes. Anyway I have three
distinctly different versions of this tube. I cannot comment on the
sound at this point.

Matt

[jsn]

Cool. It is always good to see good new (as in undiscovered) tubes
for audio. One of these days I want to build a 2-way amp for my AR
2-way speakers. It would be fun to do a push-pull spud amp for the
woofers and a single ended spud amp for the tweeters.

jsn

[rjgu...@gmail.com]

Hey guys,

I got excited when I saw Matts note about Russian tubes. It seems there are
so many odd tubes that just might be excellent and it seems the price is
always good on those obscure Russian tubes.

Its been a while back now but I bought a dozen 6N1N pentodes - 9 pin -
supposedly similar to a 6V6 but smaller bottle and lower voltage. I bought
the transformers to build a little guitar amp with them but as you know, I
am often slow to make things happen.

Anyway - transformers for a single ended KT-88 amp? There is something
called a "Dirty Little Monster" that Ken Fischer of Trainwreck amps built.
It is apparently a high gain single ended amp that was rumored to use a
KT-88 power tube and make something like 22 watts... is that possible? I
have a chassis and a power transformer 300-0-300V 250mA Hammond with heater
windings etc.. I need to identify and procure a good output tranny for this
project.

Is there an affordable match up that you can reccomend for a single ended
output tranny? Part of the rumor that goes with this amp is that he used a
push pull tranny and ignored the center tap on the output. What happens when
you do that?

Curious what you think.

rjh

[jsn]

http://www.mif.pg.gda.pl/homepages/frank/sheets/163/k/KT88.pdf

Looks like these JJs can dissipate 42W. The most power you can get
out of a single tube is roughly a quarter of that, so 10W. If he was
getting 22 out of it, he must have been doing something tricky, or
that was RMS power during heavy distortion maybe. Or he was just
hammering the tube - which is totally possible. Forced air cooling,
anyone? :)

SE transformers are designed to handle unbalanced current, where PP
transformers aren't. This means that using PP as SE, the transformer
will saturate quickly, and probably distort. Which might be part of
the "charm" of what you are describing.

The typical operating point of a KT88 is 250V/140mA, so your
transformer should ideally handle that much current.

This amp seems experimental enough, that maybe you just want to find
something "really big" whether PP or SE and call it good :) I'm
thinking eBay.

Here is a really bad idea: You could use a big 6.3V filament power
transformer as an output transformer! 120V:6.3 would give you 2.9k
impedance into an 8 ohm load. 240V:6.3V twice that. Now THAT would
be a dirty little monster :)

jsn

[Robert J Houlton]

Jason,

Your simple genius never stops amazing me... a 6.3V transformer for an output. I think there are only about 5 million of these at the Black hole, might be an interesting idea for sure! 

O.K. then - if I was to get two identical 6.3V transformers and put them in series and use the connection between them as the center tap, wouldn't I have the super econo push pull tranny? With a few low dollar Russian tubes and one of my econo chassis it might be possible to come up with the $100 guitar amp kit for beginning builders... whats not to like about that?

Matt, how close to a 12AX7 are those Russian tubes you mentioned in your recent post? I saw they were hi mu... anything similar to the standard preamp tubes in a guitar amp?

I re-read what I could find on the DLM amp and I had my numbers wrong. It was 8 watts with the KT-88... maybe still an overly optimistic output but now approaching reality. This allows the cheap EDCOR outputs as well as a ton of others as possibilities.

In reading the spec sheet, it's not entirely clear to me what the output impedance of the KT-88 is. Is it the 12K value or am I on the wrong page?

Thanks,

rjh

[jsn]

If you use a separate transformer as each half of a push-pull config,
you are really just creating 2 single-ended outputs. The beauty of a
push-pull transformer is that the DC current flows in opposing
directions, and cancels out the magnetic flux from the plate current.
But the AC signal from the tubes is in phase. More of that "balanced"
coolness.

So if you could find a big transformer with dual primaries (for
120/220 operation) you could connect them in series and use each half
as one leg of a push-pull output. You would have to get the phase
right, but it should be obvious. If the 6.3V winding has a center
tap, you could use half of it to double the primary impedance - like
you do when you use an 8 ohm speaker on a 4 ohm tap.

And maybe you could use a tube power transformer with a 300-0-300V
winding as the primary. Just leave the 120V winding open? Lots of
stuff to try.

I think there was a "Heavy Metal" article in Valve about using power
transformers as outputs.

Also, back in the early days of the tube renaissance, people used PP
outputs for SE (ignoring the center tap) a lot because that was what
was most scroungeable. Just use something "big" and it should work
just fine.

I REALLY like the idea of a sub-$100 tube guitar amp. Let's do it.
Introducing the RJ Amps Scrounge-Tone 2000!

jsn

[jsn]

http://diyaudioprojects.com/Schematics/Mikael-Abdellah-SE-KT88-Amplifier.htm

Here is a KT88 SE design using 400V B+ and a 5k output transformer.

Math for scrounging: impedance ratio is the square of the voltage
ratio. So, 120V:6.3V has a voltage ratio of 19, so the impedance
ratio is 19^2 = 362. Multiplied by an 8 ohm speaker load, the amp
sees about 3k.

Right?

jsn

[Robert J Houlton]

Jason,

i like the sound of it... Scrounge-Tone 2000

I have a couple relatively small 120 VAC to 240 VAC step up transformers that might work for a power tranny... don't know yet what the current requirements might be.  The black Hole has a bunch of 110V isolation transformers but they are pretty massive. I have a couple EDCOR 175-0-175 power trannies with heater taps but they are also very big... but cheap!

http://www.voltage-converter-transformers.com/step-down-transformer.html

I'll be on the watch for something affordable... might be something with heater taps is already available cheap.

rjh

[Matt Wiebe]

Robert,
I was traveling and I expect most of you questions were answered so I
will be brief.
The 6P6S (6∏6C) is touted as similar to 6V6s, There is a rarer tube
that is all numbers but I'm forgetting that one.
The general rule of thumb about predicting the output of an amp based
on plate dissipation at the chosen operating point is to divide plate
Ws by 4.
I don't know anything about guitar amps, but for stereo KT88s/6550
etc anything 3K to 5K primary impedance should work. Procure from
Edcor, they make good cheap stuff.
Using a P-P output transformer Single End is fine, but there are a
few things to think about. P-P outputs are usually not gapped for DC
across the primary as in P-P applications there is little DC across
the core. If you get too much DC across the core the output will
distort, the core will saturate and bass will drop off and the highs
will sound funny. Some manufacturers build outputs to handle a little
DC offset which is always present because output tube pairs never
match, so these outputs have a chance to sound fine. If you would like
to try this use the largest P-P output in your box and run 30maDC
through it first and if the sound is too you liking step up plate
current.
Matt

[Matt Wiebe]

6N2P is a drop in for a 12AX7, so I would say they are exactly like what is in preamp guitar amps.

If you are interested in building a viable kit I would use Edcors as outputs. They are not "that" much more then filament transformers and you are assured you can easily source them and they will sound good. 

I assume you mean  KT88 plate resistance rather then output impedance. Just remember you will need triode characteristics if that is how you intend to use the tube.

Matt

[Matt Wiebe]

[rjgu...@gmail.com]

Matt,

Thank you for the help with the tubes. It would seem that with the Russian
supply there will be plenty of good tubes available in the future. I can't
help but think a $2 preamp tube would be key in our efforts to make the
budget guitar amp... possibly making a good quality amp at the same time.

Attached is the 9 pin tubes that I mentioned previously. I have a dozen if
you have something with 9 pin sockets that you might want to plug these
into. I'm hoping to get the guitar using them up and running sometime this
fall/winter.

Thanks again.

rjh


----- Original Message -----
From: "Matt Wiebe" <mwi...@unm.edu>
To: <audio...@googlegroups.com>
Sent: Friday, September 25, 2009 10:17 PM
Subject: The other russian 6V6 is the 1515 <nt>


>
>
>
> >

[Matt Wiebe]

One thing I ran into with a P-P 6005 amp (a "6V6" type in a 7-pin
bottle) is that the heat of flowing 80 ma of 250V plate volts through
the tiny 7-pin socket and pin connector, was not reliable. Too much
heat, even with the cathode resistor 1.5 inches away from the cathode
pin. I ended up with a mil-spec ceramic 7-pin and greatly oversized my
cathode resistor to lower heat and it seems to be holding. But big
power tubes in small pin bases can have problems.
I don't know how hard you have run your 6P1P but I expect a 9-pin
sockets ability to dump heat is not that much better then a 7-pin. I
have socket and pin problems running 6P14P-EP (russian EL84) higher
then spec, but not nearly as bad as 7-pins. But if you run your tubes
a 80 percent max dissipation which you probably should for a kit, the
9-piners should be fine.
If size is not a concern for your kit I would suggest using a 6P3S-E
(the coin-base version not the full-base version) which is an octal
6L6 type. Octal sockets can take much more abuse then the smaller
guys, and the larger glass envelope helps dissipate heat better. The
1515 is a nice tube but it's kind of pricy at $9 or so delivered, my
6P3S-Es were less then half that.
Putting a kit together sounds fun but it requires you make
conservative choices, like using tubes readily available, easy to find
transformers and output iron, and spacious layout so novice kit
builders don't run out of room. I make none of these choices with the
stuff I build, I like to get things as small as possible, rely on
bench power suppliers and use the most odd ball tubes I can find. I
don't have 24 varieties of subminiature tubes on octal sockets for
looks, I love plugging them then wherever they work.
Matt

> <Russian 61P1.jpg>

[Robert J Houlton]

Jason,

I know you sent me something one time about using a 6.3V heater
transformer for an output transformer, but I can't find anything except
the final values --- 110V to 6.3V = 2.9K to 8 ohms....wait... I found it.

Now I have your notes but what is the rest of the equation? I am trying
to end up with closer to 6K or 8K.

Here is my idea - I'll use two 6.3V transformers to make a 110V
isolation transformer, tapping the 6.3 from between them to feed the
heaters. then use a third 6.3V as an output tranny. I am trying to get
the impedance higher to better match the Russian 6V6 tube... I suppose i
could do push pull using an interstage transformer and still keep the
part count low, but simple is the goal.

Ultimately, I am trying to figure out if I should look for a 12V
transformer or a 3V for the output? Intuition says 3V or a 240V to 6.3V
filament transformer, but I'll admit I'm not sure.

thanks,

rjh

[jsn]

Hey bob. I like it!

So the math is like this:

voltage ratio = turns ratio
impedance ratio = turns ratio squared

110V:6.3V -> turns ratio is 17.5:1 -> impedance ratio is 305:1

If you have an 8 ohm speaker on one side, then the other side sees 305
times 8 ohms = roughly 2.4k.

Of course the frequency response and inductance and whatnot come into
play, but at least the selection of available 6.3V transformers will
let us play around with this some. And the amount of DC current you
can run through the primary might be limited.

The old Bottlehead kits used one of those 70V distribution
transformers as an output, in parallel feed configuration. That could
be another option for cheap outputs, but you would have to parallel
feed those.

jsn

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[Robert J Houlton]

jsn wrote:

...The old Bottlehead kits used one of those 70V distribution transformers as an output...

I have three of those I bought off EBAY for like $6 each. I forgot about them but went and checked this morning and I still have them. The boxes are actually labeled as 70V line transformers with an optional wiring diagram to use them as output transformers. They have an 8K tap (as well as 1K, 2K, and 4K) and are rated at 10 watts - perfect!

Even using Edcor iron and new Mouser parts I think this can be done for under $100


$14.00  --  4 each 6.3V transformers and 4 each 250V Sprague filter caps  --  The Black Hole
$  6.00  --  Output transformer -- EBAY
$  6.00  --  Russian preamp and poweramp tubes  --  EBAY
$10.00  --  Econo chassis  --  from my stock
$10.00  --  Misc switches, pots, resistors, caps etc. -- Black Hole
$10.00  --  Other (shipping and stuff)
---------
$56.00  --  Estimated cost for the "Scrounge Tone 2000"