Output Transformer - push pull or SE ???
Joe
I'm building my first guitar amplifier (my first electronics project
actually), a small two-tube single-ended amp. The schematic calls for
a 3 Watt output transformer with a 2K primary and an 8 Ohm secondary.
I have a nice 8 watt transformer (Hammond 125J), but the primary is
center-tapped and the description says it's for use with "push pull"
circuits. They have a separate set of "single ended" transformers
(125SE).
Here's some information from the Hammond site:
Hammond 125J Description:
Designed for general purpose or replacement use in push-pull tube
output circuits.
Hammond 125J Data:
Cat. No. Audio Watts Primary Secondary
Impedance Impedance
(Ohms) (Ohms)
---------- --------------- -------------- ---------------
125J 8 2.5K or 4K 6/8 or 3.2/4
Can such a transformer not be used in a SE circuit? I had thought I
could use one primary lead and the center tap to form a 2.5k primary,
and just ignore the other primary lead.
Am I way of the mark here? Should I use an actual "single ended"
transformer?
Thanks!
Joe
Jon Yaeger
j...@emailserver.net wrote on 11/30/04 8:23 PM:
Joe,
Single ended transformers are designed differently from push-pull types and
except in special cases (e.g. parafeed configurations) they aren't
interchangeable at all.
Unless you want to use a different push-pull circuit, I'd bite the bullet
and get the recommended SE transformer. You can sell the other on eBay or
use it for another project . . .
Jon
Joe
On Tue, 30 Nov 2004 20:46:35 -0500, Jon Yaeger <jon...@bellsouth.net>
wrote:
Rod Keys
Actually you just have to use the transformer at lower power ....
The hidden issue here is "core saturation". Don't run away Joe, it's not
that complicated! If direct current goes through a transformer's primary
winding it puts magnetism in the transformer's iron core. The problem is
that the core is limited as far as how much magnetism it can "hold". If
it's loaded up with magnetism from DC there's no more left to pass the AC
signal. The answer to this problem is; a) a physically bigger core or, b)
another winding with the DC flowing the other way to offsetting the DC in
the first winding.
In a push-pull transformer the primary winding is split so that half the DC
goes one way and half goes the other way and they counter balance each
other. This minimizes "core situation".
A push-pull transformer will work just fine single ended as long as the
power level, especially the DC component in the primary, is so low that you
don't saturate the core. In other words, your eight watt transformer can't
put out eight watts single ended, but I wouldn't be the least surprised if
it worked just fine single ended at two or three watts.
When a core saturates the first thing to go is the bass (because bass eats
up tons of "core"). In a hi-fi amp this is a killer. In a guitar amp it's
less of a killer .. guitars amps usually emphasize treble, not bass. And
guitars only go so low anyway, (The lowest grunt you can get out of your
git-fiddle is about 100 cps and guitar's don't even do that very often).
Finally, you're going to bump into some impedance mis-match issues. But it
won't be deadly. Transformers actually work on a ratio. If you hook an 8
ohm secondary to a 4 ohm speaker you'll just cut the impedance of the
primary impedance (approximately) in half. Your Hammond transformer is
probably rated at higher impedance that the 2k called for in the circuit,
but use a 4 or 3.2 ohm speaker and things should be fine.
If this is your first project watch out for shock hazzards. Don't dink
around with stuff till you're SURE the electrolytics are discharged
(unplugged isn't enough) and remember, AC/DC circuits (usually using tubes
starting with 50 like 50L6 OR 50C5) are bad news for guitars.
Rod
"Joe" <j...@emailserver.net> wrote in message
news:2v9qq0t1dqoqcqhjc...@4ax.com...
Jon Yaeger
wrote on 11/30/04 10:19 PM:
Rod,
Technically, you're right . . . but I took into account the experience of
the builder and the fact that after all, it's a guitar amplifier . . . I
could be wrong, but I don't think he's be happy with 2 watts and the
technical considerations are (normally) far beyond the scope of a first-time
KIT builder.
Jon
Fabio Berutti
(http://store.yahoo.com/triodeel/tfchxfwi48oh.html), and maybe it can be
found at a flea mart or gutted from a broken Fender Champ @ a price even
lower... is it worth the pain? The RIGHT tranny is responsible for most of
the harmonic content (hence for the sound) of a guitar amp. Considering that
it costs 10 euros, ie. 2 pints of beer (at our local fool prices), I'd just
buy it.
Ciao
Fabio
"Jon Yaeger" <jon...@bellsouth.net> ha scritto nel messaggio
news:BDD2AF1F.EE70%jon...@bellsouth.net...
Ian Iveson
> it costs 10 euros, ie. 2 pints of beer (at our local fool prices)
10 euros would get me 6 pints at my local pub, or 8 during happy
hour. Shame I don't drink.
That's why there are few valves in Bradford. They cost too many
pints.
cheers, Ian
robert casey
An issue is that SE transformers have a gap in the
laminations to reduce the effects of the quiescent
DC current thru the primary. P-P transformers don't
have this gap, as the pair of DC currents cancel out.
But nothing will blow up, but you'll get distortion
before you get to 8 watts. As power = current squared*R,
1/2 the current would get you 1/4 the power. In a
guitar amp, the distortion you may get might be interesting
anyway. Give it a try. But be prepared to be able to
easily change the transformer if it sounds bad.
mick
>
> That's why there are few valves in Bradford. They cost too many pints.
>
I can't get over all these people that seem to find trannies & things at
car boot sales. Certainly none turn up around here! They are all full of
non-amplifier-related junk.... :-(
--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info
Web: http://projectedsound.tk
Engineer
> Hi guys,
>
> I'm building my first guitar amplifier (my first electronics project
> actually), a small two-tube single-ended amp. The schematic calls
> for
> a 3 Watt output transformer with a 2K primary and an 8 Ohm
> secondary.
(snip)
Try a scrapped "wallwart" transformer of 120 VAC (primary) to 7.6 VAC
(secondary) at 1 amp or so, i.e. 15.8:1. You could "unwind" a 12 VAC
one (see below.) This should get you by until you can find the proper
transformer (or you might even like it!). Let's recognize that this
in not "Hi-Fi", it's a tiny practice guitar amp! Don't worry about
the DC magnetic saturation, either.
I've used the above approach in tube radio restoration (a 6K6 plate,
at about 8000, ohms to 4 ohms but I had to unwind the secondary to 2.7
VAC.)
Caution and disclaimer: don't do anything until you understand the
dangers of vacuum tube operating voltages!
Cheers,
Roger
Lou deGonzague
Jon Yaeger
wrote on 12/1/04 7:00 PM:
Don't confuse the possible with the practical. Remember, this poor guy is
building his 1st KIT . . . (After he gets some confidence, he may want to
rewind wall warts, but I doubt it. I wouldn't!)
;-)
J
John Stewart
Joe wrote:
The Hammond 125 Series PP Output Transformers does work
"A OK" in a SE circuit. I've done that not once but many times
beginning more than 40 years ago.
If you like you can even use the center tap to form an ultralinear
mode connexion. Done that, too!!
If you want to take the time you can even read about how to do it
complete with comprehensive test results in two of my published
articles on SE amplifiers as follows-
AN AFFORDABLE SE TRIODE AMP-
Uses a 6EA7/6EM7 Double Triode into a 125E
Glass Audio Magazine Issue 4, 2000
MORE POWER FOR THE AFFORDABLE SE AMP-
Uses a 6LU8 in Triode or UltraLinear Connexion into a 125E
Glass Audio Projects Book
See www.audioXpress.com for these publications.
There are several other unpublished amps, both SE & PP
using the Hammond 125 Series.
The description you give of your proposed amp leads me to
believe that it is a "transformerless" design, that is, there is
no power transformer. If that is the case you should immediately
abandon that circuit because of safety concerns. Whatever
you do should include a power transformer to get you off
the power line to avoid grounding & electrical shock problems.
Good Luck (And be careful) with your project, John Stewart
Engineer
news:BDD412EE.F09F%jon...@bellsouth.net...
Good point, Jon, and I agree! As an avid reuser and recycler of old
electronic stuff this sort of thing is both a challenge and fun.
Cheers,
Roger
Jon Yaeger
>
> The Hammond 125 Series PP Output Transformers does work
> "A OK" in a SE circuit. I've done that not once but many times
> beginning more than 40 years ago.
>
> If you like you can even use the center tap to form an ultralinear
> mode connexion. Done that, too!!
>
> If you want to take the time you can even read about how to do it
> complete with comprehensive test results in two of my published
> articles on SE amplifiers as follows-
>
> AN AFFORDABLE SE TRIODE AMP-
> Uses a 6EA7/6EM7 Double Triode into a 125E
> Glass Audio Magazine Issue 4, 2000
>
> MORE POWER FOR THE AFFORDABLE SE AMP-
> Uses a 6LU8 in Triode or UltraLinear Connexion into a 125E
> Glass Audio Projects Book
>
> See www.audioXpress.com for these publications.
>
> There are several other unpublished amps, both SE & PP
> using the Hammond 125 Series.
>
> The description you give of your proposed amp leads me to
> believe that it is a "transformerless" design, that is, there is
> no power transformer. If that is the case you should immediately
> abandon that circuit because of safety concerns. Whatever
> you do should include a power transformer to get you off
> the power line to avoid grounding & electrical shock problems.
>
> Good Luck (And be careful) with your project, John Stewart
>
Thanks, John, I stand corrected relative to the advice that I offered this
poster.
Is it something about the construction of this particular PP transformer
that makes it particularly suitable for SE? In general, is it not true that
most PP transformers tend to saturate when used in SE mode?
No doubt you've tried other PP XFMRs in SE mode. What's been your
experience?
TIA,
Jon
John Stewart
are many ways one may go toward a successful amp.
Is it something about the construction of this particular PP transformer
that makes it particularly suitable for SE? In general, is it not true that
most PP transformers tend to saturate when used in SE mode?No doubt you've tried other PP XFMRs in SE mode. What's been your
experience?TIA, Jon
Williamson & PP were king. There was little interest at all in SE by
the hifi crowd. Unlike the present, you would have had trouble
giving away an SET amp.
As a result, output transformers for quality SE applications were for
the most part nonexistent. I don't recall ever seeing one.
OTOH, there were many replacement type SE output transformers
& some universals such as the Hammond 125 Series. At the time the
largest universal from Hammond was their 125D, still in production.
Jensen also had some universals such as their 2430 which looks like
a direct copy of the 125D, although it could be the other way around.
I never used any of their SE transformers but found many
applications for the 125D.
The daily driver amp in my workshop uses a 6AU6/6AQ5 FB pair,
fully integrated with Baxandall tone controls & mic/phono preamp.
It uses one of those very old Jensen 2430 universals.
All inside a Hammond 8 x 12 x 3 chassis.
Over time I managed to get much more than passable results from
the Hammond 125D's in a variety of both SE & PP amps. That
would include PP 6AQ5's, 6K6GT's, 6F6GT's, 6V6GT's,
25L6GT's, 35L6GT's & 50L6GT's. The SE amps were using
as outputs the 6AQ5, 6V6GT, 25L6GT & 50L6GT.
All of these with varying degrees of NFB.
By the time I finally got back to tubes a few years ago Hammond
had gone a little further with their 125 line with the 125E. That
made quite an improvement at bass frequencies, both SE & PP.
I used two of the 125E's in my version of Norman Crowhurst's
Twin Coupled Amp of the late 50's. Each channel needs two
output transformers. You can read about it in AudioXpress
magazine, Aug 2004 issue. The results in terms
of IMD are outstanding.
When I built the SET & SEUL amps mentioned in an earlier post
I again used the Hammond 125E's. Shortly after that Hammond
introduced their low cost output transformers for SE operation.
That would be p/n's such as the 125ESE. Needless to say, these
would definitely give better performance in an SE amp. One of
these days I intend to try that in an existing SE amp here
which now is using a 125E.
Some reading this may be familiar with Joe Roberts magazine
"Sound Practices". After something like 16 issues Joe had to get out
since it was impossible for him to make a living at that. I personally
miss his efforts since I feel he did contribute quite a bit to folks
like us.
Anyway, there is an article in Issue 15 (Winter 1997) that perhaps
gives us some hint of just how good these low cost Hammonds are.
The amp in question is stereo, using in each channel a 6SF5 direct
coupled to a 2A3. The purchaser is given a choice of Hammond
output transformers or a pair of purpose built devices from a very
well known specialty transformer house.
Joe makes the comment that the Hammonds are $75 for the pair
if you go out to buy them while the alternative adds $280 on top
of that to the overall cost. In the photo you can barely see the
Hammonds but from the price mentioned & what I could see in
the photo they sure look like 125E's. AFAIK, Hammond did not
have a small bracket type SE output transformer at that time.
When Joe asked if the upgrade to the purpose built output
transformers was worth the extra $280 the reply was
'not as different as
some people would like to think'.
A concern I have with the original posters proposed amp is the spec.
Two watts into 2K is a dead giveaway that the amp is probably
running something like a 50C5 right off the power line. Some guitar
amps of this kind can be found in "The Tube Amp Book" by Aspen
Pittman. They may have got a pass by UL in the 50's but I wonder
if they still would qualify? I doubt it for safety reasons.
Not a good project at all for the inexperienced.
Cheers, John Stewart (And please excuse my verbosity)
Wbittle
An SE transformer is wound different then a push pull type. Although
it will produce a signal and load the output tube, it will not be
your best option. Hammond makes a couple transformers that will work. I
have built several SE guitar amps using the EL34 output tube and have
found that they work quite well. Try one of their universal ones like
the 125ESE. This transformer allows you to use a wide variety of plate
impedance's to match the particular output tube you plan to use. This
transformer is rated at 80 ma and 15 watts. It has a frequency response
that is more then adequate for a guitar amp. Antique Electronic Supply
has them for all of $38.00. For a guitar amp you do not need an
expensive hi-fi single ended transformer. Experiment witht he load
impedance you put to your output tube to see which offers the best tonal
qualities. It will take a bit of experimentation to get the biasing
correct and the load impedance correct so that the amp sounds good and
robust clean and breaks up smooth without 'farting out' when it moves
into overload. What output tube are you planning on using? This
transformer can handle anything from a single EL84 to a single 6550.
Personally, I prefer the EL34. Oh, unless you are using something like a
50C5 as your output tube, you will need a larger transformer then some 3
watt job.
Bill B.
Jon Yaeger
* * * *
John,
Thanks for your lengthy and informative reply. Looks like you have had good success using PP xfmrs in SE applications.
I’m particularly interested in the criteria used to “draft” a PP xfmr for a SE app. The (generally accepted??) dogma — as I understand it — is that SE iron has a gap to reduce the likelihood of saturation. PP iron, lacking that feature, would tend to saturate at some levels vs. the SE iron. So I’ll conclude that in order to use a random PP xfmr (not the Hammond 125 for example) you’d have to design the output to be some fraction of the normal wattage. Is this not so, and are there any “rules of thumb”?
Have you tried PP iron in a SE application with a parafeed circuit?
TIA,
Jon
Joe
able to pick up a used Hammond 125SE 3Watt transformer for $10 from a
buddy of mine.
I finished connecting the components and powered up the amp for the
first time last night (initially through a current limiter until I was
sure nothing catastrophic would occur).
All node voltages were within 12%, and the sound wasn't bad really
considering it was playing through an old car stereo speaker. I was
glad to hear no hum or static.
I'm going to tinker with this a bit before moving on to the next
project. The preamp stages do not include bypass capacitors on the
cathodes, so I'm going to add a couple and observe the difference.
Thanks again for all the suggestions!
Joe
Hi John, I'm familiar with the transformerless designs, and even with
my limited experience had questioned their safety. I've since read
numerous warnings about them, mostly online. The Tube Amp Book is one
I've considered buying but don't actually own yet.
This design uses a power transformer which brings the 120 line voltage
down to 60, and powers a 12AX7 in the preamp and a 50L6 power tube.
Safety is definitely my number-one concern - the amps aren't any fun
if we're not around to enjoy them. I'm thankful that we have forums
such as these where important information like this can be shared.
Thank you!
On Sat, 04 Dec 2004 17:23:52 -0500, John Stewart
<jh.st...@sympatico.ca> wrote:
>
<snip>
John Stewart
Joe- If you are out there send me an email with your correct address.
I have some articles on transformers, both SE & PP that I think you
will find interesting. Your address above did not work for me.
Cheers, John Stewart
kseely
Joe wrote:
> Wow, thanks for all the responses and wonderful information! I was
> able to pick up a used Hammond 125SE 3Watt transformer for $10 from a
> buddy of mine.
Good call Joe. Hammond's 125 series is available in both SE and PP.
Just look for the SE on the end of the part number. Here's a link to
the entire series : http://www.hammondmfg.com/125SE.htm