There are some opinions about the GB becoming less sensitive while the battery charge drops down. I could not believe that a company like Fisher would do such a stupid mistake, so I decided to ask Fisher directly. I sent an email and received an answer from Dave Johnson in person. Here is the message, I think it could be interesting for the GB users.
"I can't make an ironclad guarantee that 11 volts won't cause damage, but I regard it as very unlikely. We know some customers use NiMH and we've never seen a machine problem where the suspected cause was a fresh NiMH.
In as much as everything having to do with basic performance is powered through a voltage regulator, there should be no variation in basic performance from about 6.3 volts all the way to fresh battery voltage. The regulator we use has been in widespread use in industry for 30 years and is regarded as bulletproof, no concern over that. Sensitivity won't be affected.
The audio (speaker) circuit is powered from the battery, not through the regulator, so you may hear a noticeable difference in loudness between higher and lower voltages. If something were to blow, it would be the speaker circuit, but that's highly unlikely.
The 9.6 volters are the most popular because most customers think that "more is better". Some manufacturers offer lower voltages (7.2 and 8.4 V) with higher mAH capacity, and if you can get those mAH capacity batteries they're actually a better choice for most applications.
There are also some newer chemistries (both primary and rechargeable) available now in the nominal 9 volt rectangular package. Although we haven't tested them, I have no reason to think they'd cause any problems. The batteries I specifically un-recommend are ordinary zinc-carbon (low capacity, high internal resistance may result in unstable operation), so-called "heavy duty" which are highly variable in quality, and NiCad because they're inferior to NiMH and because they're banned or tightly regulated in some jurisdictions due to their toxicity.
With NiMH, some brands will deliver short battery life, so pay close attention to the mAH rating. Good alkaline 9-volters run around 600 -700 mAH, 500 mAH or less in alkaline would be regarded as junk. I've seen NiMH as low as about 150 mAH; I think the best ones nowadays run around 300 mAH but haven't checked recently. So battery life will probably be on the order of half what you'd get from a good alkaline, the difference of course being that you can recharge the NiMH.
--Dave J."
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