Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on December 30, 2011, 10:33:32 am
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Does a champ use a 7K:3.2 or a 7K:4 OT?
Now I know that a 4 ohm speaker will read ~3.2 and I've read thet tha champ schematics say 3.2 for the output and some say it says that because the speaker reads 3.2 and that's why, but this is not a question about that.
A couple of things make me wonder this.
1) A 7K:3.2 OT is not the same as a 7K:4 OT. A 7K:3.2 OT is a 8750:4 OT.
2) Hammond sells 7K:3.2 OTs for a champ replacement.
Looking at the Hammond site I noticed that the champ was the only OT that shad a 3.2 tap. All the other OTs said 4. This got me wondering. I wouldn't think that a transformer company like Hammond would just call it a 3.2 if it's really a 4(because fender did, as the story goes) so I looked at the spec sheet.
It reads:
Primary impedance 7000Ohm(187.1V)
Secondary impedance 3.2/8/16Ohm(4/6.325/8.944V)
Taking out my calc and doing the math:
(187.1V/4)2 X 3.2 = 7001.28
(187.1V/6.325)2 X 8 = 7000.29
(187.1V/8.944)2 X 16 = 7001.7
BUT (187.1V/4)2 X 4 = 8751.6
So the Hammond OT IS a 7K:3.2/8/16 not a 7K:4/8/16. That means that it is a 8750:4/7K:8/7K:16. So two 8 ohm speakers in series on the 16 ohm tap would present a 7K load but in parallel on the other tap would be 8750?????
Why wouldn't you want the same load for any speaker configuration?????
Just wondering if Champs actually used a 7K:3.2 OT(8750:4) or actually used a 3.2 speaker?
I could see that if Hammond is selling this as a replacement for a champ and if a champ did use a 3.2 speaker not equivelent to todays 4 ohm speaker, this would give you the option of using the original speaker or a 8 or 16 ohm speaker and still present the tube with the same load. But as it stands, according to the sheet, this is a 8750:4/7K:8/7K:16 OT which seems weird to me.
I did the math on the transformer I'm using now and it's a 7K:4. So using the same speaker with this transformer or a true 7K:3.2 would present a different load to the tube, 7K VS 8750.
3) I pulled an OT from the audio of an old vacuum tube T.V. set that used a 6V6 and it measured ~8500:4
If I were to build an amp from scratch using a 6V6 at 330V dis 12W would it be better to use a 7K load or a 8750? I thought rule of thumb was V2/Watt dis, or V X I. For this example you'd get 9075(closer to 8750 than 7K). Which is why I never understood why a Champ would use a 7K OT.
I thought the one I pulled from the T.V. was high(8750:4) compared to a Champ and wasn't going to use it, but now I'm thinking the one I'm using now may be low(7K:4) compared to a Champ.
Maybe it don't make a hill of beans but it got me thinking.
Is a Champ OT truly 7K:3.2.(8750:4)? :dontknow:
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Doug, Just out of curiosity what's the voltage ratio and impedance ratio of your Champ transformers?
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according to mine, turn ratio squared = 1.5k . that is 6k with a 4 ohm load.
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Is your OT from a champ or a replacement?
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> Maybe it don't make a hill of beans
The color affects the sound more than 7K/8K/9K.
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a replacement Doug sells.
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"The color affects the sound more than 7K/8K/9K. "
Mine's silver, the Hammond one's black. Maybe I should get it. The amp's too bright maybe the black one will give my amp a darker tone.
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"The color affects the sound more than 7K/8K/9K. "
Mine's silver, the Hammond one's black. Maybe I should get it. The amp's too bright maybe the black one will give my amp a darker tone.
Black bells for blackface tone, silver bells for silverface tone ;-)
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that's what i was about to say.....................the little f......silver o.t. is so harsh and treble I will try a push pull hammond instead, to see if it is worst........or better ( can't be worst hey hey )
Happy new year
Colas ( no coca cola tonight believe me ).
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Remember Jeff,
All those charts and graphs are more or less design notes for Hi Fidelity, optimum performance, lowest distortion and the like.
Guitar amps do little of those things (not great sounding ones at least)
Guitar amps purposely distort, over drive , cut and boost band frequencies and all other unimaginable things that would make a audiophile cringe.
mismatching a OT then coupling it to a speaker with horrible qualities (in an audiophiles eyes ears) is part of what makes guitar amps sound good.
You don't hear of guys praising the way that their LesPaul shreds through a Dynaco ST-70.
***EDIT**** notes the grateful dead and their wall of macintosh MC-2300's
Most guitar amps have little response on the low end and are small for their rated output (making them saturate quicker)
Most vintage Fenders (sic OT's) do not have an "optimal" match as stated in tube data sheets and many vary widely in the 20% +- tolerance the engineers spec'd them to. If you read the Merc Mag hype, they look for the best sounding amps and dissected the trannys of those unique ones. this leads one to believe that there is quite a bit of leeway in an OT.
Just my opinions here
Ray