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Rolling 12AX7 to 12AU7?

pure sound

Wammer
Wammer
Sep 10, 2011
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West Devon
HiFi Trade?
  1. No
If the eq filter has been designed to be driven from a certain output impedance, then installing a valve that has a very different output impedance in that position will change the characteristic of the filter ie it might no longer follow the RIAA spec.

 
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Professor Chaos

Wammer
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Jun 6, 2007
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Kelley
So, I can suck it and see without doing any damage?

I don't have my heart set on using the 12AU7s. It's just that upgraditis is setting in, combined with lack of funds. I'll be changing the tubes, output caps, and op amps. I have several sets of 12AU7s about, so that would save some money if I can use them. If I can't, it's not like 12AX7s are terribly expensive:)

 

Valvebloke

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Dec 3, 2009
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Graeme
HiFi Trade?
  1. No
I don't know the circuit for the JD9 but Jolida say the valves are used in the output stage (all the tone stuff is done with op-amps) and elsewhere on the interweb I read that the output stage is a cathode follower. If this is true then you probably won't notice any gain difference since all well-designed cathode followers have close to unity voltage gain no matter what valve is used http://www.valvewizard.co.uk/accf.html. If you were starting from scratch then it would be easier to design an effective cathode follower using a 12AU7 than a 12AX7 since the former has higher current capacity and the latter was specifically designed for high gain. The cathode follower circuit has no need for the high gain feature. Make of that what you will ...

So, I can suck it and see without doing any damage?
EDIT: That'll depend on the circuit and component details, but the answer is probably yes. I'd ask them.

VB

 
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graham67

Wammer
Wammer
May 19, 2010
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Hi KelleyTo reduce the gain in the JD9 drop in 5751 tubes. No issue what so ever.

Regards
Absolutely agree this is the sensible path if you are trying to reduce gain. NOS 5751 or 5751WA are usually better value too.

 
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