From blacki–(at)–nterport.net Thu Aug 31 22:05:16 CDT 1995
Article: 691 of rec.audio.tubes
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From: blacki–(at)–nterport.net (Blackie Pagano)
Newsgroups: rec.audio.tubes,alt.guitar.amps
Subject: Re: Running Champ with 8 ohms instead of 4 ohms
Date: Thu, 31 Aug 1995 09:03:12 UNDEFINED
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In article <422p1s$g3--(at)--anix2.panix.com> mgarvi–(at)–anix.com (Mark Garvin) writes:
>I realize that there were quite a few different transformers
>used in Champ tube amps (the single 6v6 version). But has
>anyone here had trouble running an 8 ohm speaker instead of
>the 4 ohm standard?
>Any experience or theories welcome.
People do it sometimes, out of ignorance or just plain unavailability of a 4
ohm speaker. At best, you merely lose some of that raging 3-6 watt Champ
output. At worst, the transformer will arc from being run out of spec. I’ve
seen both scenarios. At this point, the only champ speaker that is tough to
get is the 4 ohm 6″ in early circuits.
From mamunds–(at)–tk.com Thu Aug 31 22:06:01 CDT 1995
Article: 722 of rec.audio.tubes
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From: mamunds–(at)–tk.com (Mark Amundson)
Subject: Re: Running Champ with 8 ohms instead of 4 ohms
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In article <424gvn$ka--(at)--anix2.panix.com>, mgarvi–(at)–anix.com says…
>
>>In article <422p1s$g3--(at)--anix2.panix.com> mgarvi–(at)–anix.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.
>
>In
>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 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.
>
>Transformer arc is exactly what I’m concerned about. It seems that Fe
>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.
From mamunds–(at)–tk.com Thu Aug 31 22:06:50 CDT 1995
Article: 686 of rec.audio.tubes
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From: mamunds–(at)–tk.com (Mark Amundson)
Subject: Re: Running Champ with 8 ohms instead of 4 ohms
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In article <422p1s$g3--(at)--anix2.panix.com>, mgarvi–(at)–anix.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?
>
>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
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.
From mgarvi–(at)–anix.com Thu Aug 31 22:07:09 CDT 1995
Article: 698 of rec.audio.tubes
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: rec.audio.tubes,alt.guitar.amps
Subject: Re: Running Champ with 8 ohms instead of 4 ohms
Date: 31 Aug 1995 10:39:09 -0400
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In
>In article <422p1s$g3--(at)--anix2.panix.com>, mgarvi–(at)–anix.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.
From rfrie–(at)–etcom.com Mon Sep 4 17:14:50 CDT 1995
Article: 3347 of alt.guitar.amps
Newsgroups: alt.guitar.amps
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From: rfrie–(at)–etcom.com (Robert Fries)
Subject: Re: Running Champ with 8 ohms instead of 4 ohms
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stephen.delf–(at)–elcom.gen.nz (Stephen Delft) wrote:
>Glass Audio 2/92 page 8 ( article on modifying a Fender Champ)
>refers to a Fender #0024038 OPTX ; 5.1k to 7.5 Ohms.
>In the article it is suggested for a Champ modified to take a 6L6, but
>also seems to suggest that this would be a more robust alternative to
>the regular 6V6 Champ OPTX, and has more bottom end.
>Anyway, this might be one way of running a Champ into an 8 ohm
>load with minimal loss of output.
>What puzzles me is……this Fender stock part appears to be a
>possible *alternative* for a regular Champ, but is not an exact
>replacement. So what is it originally intended for ?
>Is it an intended Champ upgrade, or did Fender at some time
>make another “single-output-tube” amp which used this larger OPTX ?
>Fender TX # 0024038 might be available from Magic Parts, but I haven’t
>received their new cat yet.
>Cheers, Stephen.
024038 is the output transformer for the ‘Champ 12’; which used a
single 6L6. A couple of years ago, I ordered two of these from Fender.
They sent the regular VIbroChamp transformer, 022905. They took so
long to arrive, I jsut didn’t bother with trying to straighten it out,
and the Glass audio project got put on permanent hold for me.
Which reminds me – does anyone need a 022905? I have two,
I’ll sell for $20 each.
Robert Fries
