Did You Get The Memo?

Dual/Triple Rectos, Maverick, Blue Angel, Nomad, Road King, F-Series, Lone Star & Stiletto

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Re: Did You Get The Memo?

Post by Armando » Wed Jan 13, 2010 3:49 pm

I just came across this old post again and found out through some troubleshooting that the issue was not at all related to the pre amp tube but instead with the speaker A/B selector toggle switch located on the rear of RK head on Ch. 3. I must have bumped it by accident. It caused the exact same issue described by the original poster. It sucks because I went out and purchased new pre amp and power tubes only to discover that the original 6L6 tubes in the RK I heads are no longer sold as Mesa replacement tubes. The newer 6L6s don't have the same bottom end as the older tubes. Hate them!

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Re: Did You Get The Memo?

Post by Addictedtokaos » Thu Jul 07, 2011 9:32 am

Found this info over on a JVM forum. the 11th post down here http://jvmforum.com/phpBB3/viewtopic.php?f=21&t=4409
Here is important information I´ve got from New Sensor in reply to my latest inquiry:
"The Electro-Harmonix 12AX7EH has a spiral filament and should not be used in cathode follower circuits. The Tung-Sol 12AX7, Mullard 12AX7, and Sovtek 12AX7LPS also have spiral filaments."

That would explain reported failures in V3.

The guy from New Sensor recommends Sovtek 12AX7 WA and WB:
"The most rugged 12AX7 types for cathode follower service are the Sovtek 12AX7WA and WB."

I will give the Svetlana a chance and eventually report its failure, incremental noise or any other symptom.

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Re: Did You Get The Memo?

Post by jpdennis » Mon Mar 05, 2012 7:11 pm

I am thankful for this post as the V4 in my Heartbreaker fits this discussion.

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Re: Did You Get The Memo?

Post by talltxguy » Wed Jan 14, 2015 10:31 am

Sorry to revive an old thread, but I have a question about the cathode follower thing.

Mesa says V3 & V5 are the cathode followers in the Roadster. I checked the manual to see tube functions, and V3A is the channel 3 & 4's 4th gain stage, and V3B is channel 3 & 4's the 5th gain stage.

V3A & V3B cannot be a cathode follower and a gain stage, so what gives? Is the cathode follower only active in channels 1 & 2?

Thanks for anybody who can clarify for me.
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Re: Did You Get The Memo?

Post by Authorized Boogie » Wed Jan 14, 2015 7:45 pm

Below is a quote from Randall Aiken ( Aiken Amplification ) that may help.

Q: Are cathode followers gain stages?
A: Yes. However, the problem is that people associate "gain stages" with, well...gain. The cathode follower has a maximum theoretical gain of unity, and typically a gain of around 0.5 to 0.7 or so. To us engineering types, a gain of less than unity is still called a gain (that is, unless it's called a loss or an attenuation, in which case the attenuation is the reciprocal of the gain), so a stage with a gain of unity still has a gain - a gain of 1. The cathode follower *is* an amplifier stage, but not a voltage amplifier in the typical sense. It is used as a "buffer" amplifier, which means it has a high input impedance and a low output impedance. This means it does not appreciably load the previous stage it is connected to, and the very low output impedance allows it to drive low impedance loads without much signal voltage loss. For example, if the previous stage had an output impedance of 100K, and you tried to connect a stage with a 10K input impedance to it, you would only get 9% of your original signal when you connected the second stage, because of the voltage divider formed by the 100K output impedance and the 10K input impedance. If you insert a cathode follower with a 1K output impedance and a 1Meg input impedance, the 10K stage can be driven with 90% of the original signal, because there is now effectively a 1K:10K voltage divider instead of a 100K:10K voltage divider. The cathode follower is basically an active "impedance transformer", in this sense. The reason the cathode follower is used in driving a tone stack is not only because the tone control network impedance is relatively low in comparison to the output impedance of the previous stage, so it would cause a loss of gain, but more importantly, the tone stack is a filter network that is designed to ideally be driven from a zero source impedance to achieve it's proper frequency response. The cathode follower provides a very low source impedance that allows the tone stack to work as designed. If the tone stack is driven from too large a source impedance, not only will there be a loss of gain, but there will be a different frequency response to the network, typically quite a few dB loss of the highs. The cathode follower prevents this loss, allowing the tone stack to retain more of it's theoretical frequency response.
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Re: Did You Get The Memo?

Post by talltxguy » Thu Jan 15, 2015 1:56 pm

Authorized Boogie,

Thank you for the information. So if I'm understanding things correctly, the 5th gain stage of the Roadster's 3rd & 4th channels is actually a cathode follower?
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