Running Champ with 8 ohms instead of 4 ohms
Mark Garvin
used in Champ tube amps (the single 6v6 version). But has
anyone here had trouble running an 8 ohm speaker instead of
the 4 ohm standard?
This should bring the primary impedance pretty high, presuming
Fender operates anywhere close to the normal load for a
6V6. In general, higher primary impedance should operate the
tube with less distortion...not nec. good for a guitar amp.
Any experience or theories welcome.
mg
Mark Amundson
The five watts or so that the champ produces using the stock 5000 to 4
ohm transformer would be reduced. The distortion produced would be similar
to driving a 4 ohm load although the 6V6 would dissipate less since the
amount of AC current swing is less for the effective 10,000 ohm transformer
impedance. The distortion would still be made by clipping the AC voltage
swing which is limited by the amp's supply voltage. I have replaced a few
champ output transformers due to primary winding overheating and open
circuiting, so higher output impedances help a little.
Blackie Pagano
>I realize that there were quite a few different transformers
>used in Champ tube amps (the single 6v6 version). But has
>anyone here had trouble running an 8 ohm speaker instead of
>the 4 ohm standard?
>Any experience or theories welcome.
People do it sometimes, out of ignorance or just plain unavailability of a 4
ohm speaker. At best, you merely lose some of that raging 3-6 watt Champ
output. At worst, the transformer will arc from being run out of spec. I've
seen both scenarios. At this point, the only champ speaker that is tough to
get is the 4 ohm 6" in early circuits.
Mark Amundson
>
>>In article <422p1s$g...@panix2.panix.com> mga...@panix.com (Mark Garv
>in) writes:
>>>I realize that there were quite a few different transformers
>>>used in Champ tube amps (the single 6v6 version). But has
>>>anyone here had trouble running an 8 ohm speaker instead of
>>>the 4 ohm standard?
>>>Any experience or theories welcome.
>
>>People do it sometimes, out of ignorance or just plain unavailability
> of a 4
>>ohm speaker. At best, you merely lose some of that raging 3-6 watt Ch
>amp
>>output. At worst, the transformer will arc from being run out of spec
>. I've
>>seen both scenarios. At this point, the only champ speaker that is to
>ugh to
>>get is the 4 ohm 6" in early circuits.
>
>nder
>used normally-closed speaker jacks to avoid transformer arcing, since
>I can imagine no other reason to do that. And since I don't have spec
>s
>on Fender transformers, I assumed that this may be an indication they
>are
>prone to arcing.
>
>The lack of selectable speaker impedance, and the presence of external
>speaker jacks on most Fender amps also leads me to believe that maybe
>their primaries are spec'd a bit high to begin with (to allow reduced
>speaker impedance without maxing out power tube dissipation).
>
>Does anyone have data on Fender (or other guitar amp) transformer
>primary impedances?
>
>Regards,
>Mark G.
The primary impedance is nominally 5000 ohms. These transformers are pretty
universal here. I have 1959 National student amp with the exact
12AX7/6V6GT/5Y3GT circuit has the same vintage Fender Champ. My National
drives a 3.2 ohm 8 inch Jensen Special Design speaker and the output
transformer has a 3.2 ohm secondary to match (completely stock).
Thordarson makes 4 single plate to voice coil transformers with primary
impedances of 2000, 4000-5000, 5000, and 7000-10,000 ohms. Each is rated at
5 watts into 3.5 ohm voice coil secondaries.
Stancor also offers a 10,000 ohm center tapped at 10 watts with 6-8/3.2-4
ohm secondaries. It is made for push-pull 6V6's but will work for champs.
Hammond also offers 3, 5, 8, and 10 watt universal single plate to voice
coil transformers. Primaries are center tapped and the secondaries have four
taps for a 1200 to 25,000 ohm primary to a 1.5 to 15 ohm secondary.
The reason for shorting secondaries in tube outputs is to present only a
high impedance inductive load if the speakers are present. The output tube
sees only a small inductor here. If the secondary is opened, it acts like a
flyback circuit and may induce arcing.
Mark Garvin
>>I realize that there were quite a few different transformers
>>used in Champ tube amps (the single 6v6 version). But has
>>anyone here had trouble running an 8 ohm speaker instead of
>>the 4 ohm standard?
>>Any experience or theories welcome.
In <blackie.7...@interport.net> bla...@interport.net (Blackie Pagano) writes:
>People do it sometimes, out of ignorance or just plain unavailability of a 4
>ohm speaker. At best, you merely lose some of that raging 3-6 watt Champ
>output. At worst, the transformer will arc from being run out of spec. I've
>seen both scenarios. At this point, the only champ speaker that is tough to
Mark Garvin
>In article <422p1s$g...@panix2.panix.com>, mga...@panix.com says...
>>
>>I realize that there were quite a few different transformers
>>used in Champ tube amps (the single 6v6 version). But has
>>anyone here had trouble running an 8 ohm speaker instead of
>>the 4 ohm standard?
>The five watts or so that the champ produces using the stock 5000 to 4
>ohm transformer would be reduced. The distortion produced would be similar
I was not sure that the primary was 4-5k. That's good to know.
>to driving a 4 ohm load although the 6V6 would dissipate less since the
>amount of AC current swing is less for the effective 10,000 ohm transformer
>impedance. The distortion would still be made by clipping the AC voltage
>swing which is limited by the amp's supply voltage. I have replaced a few
>champ output transformers due to primary winding overheating and open
>circuiting, so higher output impedances help a little.
The Champ transformer does not look very rugged. Seems there's a
relatively narrow band of usable speaker impedances. I would have
guessed that 8 ohms would not be too far off, and would possibly
help keep the primary cool. But Blackie's mention of arc'ing
(adjacent messages) is a realistic concern.
I have tried to contact Fender and some parts vendors to find out
about failure modes for Champ (and other) output transformers, but
no response yet.
Regards,
Mark G.
Dr Distortion
: Does anyone have data on Fender (or other guitar amp) transformer
: primary impedances?
The only "spec" I have is something Dave Funk told me once; he said that
the 50-watt Fender output xfmrs are around 4.5K plate-to-plate. I've
never taken measurements, but I have the necessary equipment so I suppose
that I should...
mgsam
Rejsa) wrote:
> : Thordarson makes 4 single plate to voice coil transformers with primary
> : Stancor also offers a 10,000 ohm center tapped at 10 watts with 6-8/3.2-4
> : Hammond also offers 3, 5, 8, and 10 watt universal single plate to voice
>
> Where do we find these guys? Anybody got a number handy?
>
> Related question - why did they make the Champ 4 ohms to begin with?
> It would seem that 8 ohm speakers are more available (at least today)
> and when you are transformer driven there is not the same power/load
> limitation you get with a direct coupled transistor amp, so it isn't
> a question of wanting more output.
>
> Are there any single 6L6 amps out there?
>
>
> --
> ------------------------------------------------------------------------
> /* mi...@primenet.com "Less is more..." */
> ------------------------------------------------------------------------
My guess with the original TWEED champs is that Leo simply designed the
champ around whatever cheep eight inch speaker was available. That
happened to be a Jensen at four ohms. Interestingly enough, CBS in the
late sixties, early seventies did come out with a "better" champ speaker,
a 6.6 ohm replacement that has a bigger magnet then the 4 0hm Jensen.
Guess what? It sounds wimpy compared to the original Jensen. Oh yeah, it
stays cleaner but no beautiful champ distortion.
The reason why this amp has become a cult item was not necesarrily in the
minds of the designers.
On another subject, I have a couple of champs in for repair right now and
I got ahold of a Fender replacement part power transformer. The wiring
doesn't match up with the original and they sent no technical bulletin.
GRRRRRR. Anybody have any ideas?
Best,
Kevin
Mike Rejsa
Dave Cigna
The Champ 12 was the new Fender Company's first attempt at an all tube
amp. They sold them from around 1986 to 1992. It had a single 6L6 power
tube, but the rest of the amp was a rather unusual design that I still
haven't figured out completely. They sounded OK, but lacked the warmth
and sweetness of the original Champ's 6V6 and extremely simple circuit.
The original Champ was basically unchanged from '55 until '82 except for
voltage increases and a change in the tone controls. There wasn't much
to change!
-- Dave Cigna
ESolo65940
would love to find a source for 6" 4-ohmers. I paid 35 bucks for the amp
so I can't really complain. The thing needs a cap job, and the 6SJ7 preamp
tube is marginal at best, but it still sounds cool when the vacuum gnomes
go on holiday. Anyone know where to find 6" 4-ohm speakers, short of
checking out every junked AM radio from the 50's?
Henry van Cleef
>
>On another subject, I have a couple of champs in for repair right now and
>I got ahold of a Fender replacement part power transformer. The wiring
>doesn't match up with the original and they sent no technical bulletin.
>GRRRRRR. Anybody have any ideas?
>
voltage winding will the the highest resistance and will have a center
tap. The primary will have the second highest resistance and no center
tap. If it is a 115/230 volt setup, there will be two windings
virtually identical ohms.
The heater windings (and rectifier filament) will look like almost dead
shorts.
Once you have the primary identified and the other windings figured
out, put 110 volts on the primary and measure the other voltages. If
it is a two-primary transformer, connect one end of the second primary
to one of the terminals on the first, and measure the voltage across
the other two terminals. Zero volts, you've got them phased correctly,
just parallel them for 110 volt operation. If you see around 230
volts, you have them backward from each other. Reverse the wires on
the second primary and run them in parallel. For 220 volt operation,
connect the two primaries in series rather than parallel, after phasing
them as above.
--
***********************************************************
Hank van Cleef vanc...@bga.com vanc...@tmn.com
***********************************************************
mgsam
(ESolo65940) wrote:
Two good new eight inch, four ohm speakers are available for the champ.
The Kendrick and the Celestion. Call Nate at Mojo music. 1-800-927-mojo.
Personally, for a tweed, I like the Kendrick. Sounds more like the
original Jensen. But for a hopped up silverface Amp with a Torres mod,
the Celestion has a nice upper midrange scream.
Best,
Kevin
mgsam
> In article <mgsam-01099...@198.68.31.41>, mgsam <mg...@wave.net>
wrote:
> >
> >On another subject, I have a couple of champs in for repair right now and
> >I got ahold of a Fender replacement part power transformer. The wiring
> >doesn't match up with the original and they sent no technical bulletin.
> >GRRRRRR. Anybody have any ideas?
> >
> Get out your trusty Simpson meter and ohm the thing out. The high
> voltage winding will the the highest resistance and will have a center
> tap. The primary will have the second highest resistance and no center
> tap. If it is a 115/230 volt setup, there will be two windings
> virtually identical ohms.
>
> The heater windings (and rectifier filament) will look like almost dead
> shorts.
>
> Once you have the primary identified and the other windings figured
> out, put 110 volts on the primary and measure the other voltages. If
> it is a two-primary transformer, connect one end of the second primary
> to one of the terminals on the first, and measure the voltage across
> the other two terminals. Zero volts, you've got them phased correctly,
> just parallel them for 110 volt operation. If you see around 230
> volts, you have them backward from each other. Reverse the wires on
> the second primary and run them in parallel. For 220 volt operation,
> connect the two primaries in series rather than parallel, after phasing
> them as above.
>
Thanks Hank, for the transformer lesson. Will do as instructed and let
you know if I figure this out.
Ain't the internet grand!
Best,
Kevin
Joseph Pampel
>On another subject, I have a couple of champs in for repair right now
and
>I got ahold of a Fender replacement part power transformer. The wiring
>doesn't match up with the original and they sent no technical bulletin.
>GRRRRRR. Anybody have any ideas?
Here's a shot at it: When I started building amps (about 5 years ago)
Fender was still selling the Champ/Princeton PT as a replacement part.
This may be what you have. It's part# 014077 (stamped on the end bell)
The secondary winding of this PT follows standard wiring code:
Yellow: Rectifier Filiments
Red: Rectifier plates
Yellow/Red: center-tap of B+ winding
Green: 6.3V Filaments
The tricky part is the primary, as there are 7 different leads! This is
because the unit is an export unit as well, and can be run on all sorts
of volages, 90V to 220V I beleive.. (I can't find my original notes on
it but am looking at am amp I built with one to post you.. such
dedication. :- > ) The proper leads for 120V (USA) operation are the
black wire and the orange wire with the black stripe. The 6.3V fil
winding has no centertap, so you either have to grund one side (what they
did with Champs) or give it a center with a pair of 100-ohm resistors to
ground (lower noise than grounding one side..)
Hope this does it!
Joe