BASS AMP: KT88 PP (or PPP) 6SN7 CF vs CC driver - diyAudio
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Old 7th March 2013, 10:26 AM   #1
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Default BASS AMP: KT88 PP (or PPP) 6SN7 CF vs CC driver

Comming from tube pre amps (bass guitar), I'm taking the plunge into tube power amps. One question that rises after reading and studying multiple schematics is the driver configuration used to drive the power pentodes, KT88 PP or PPP ultra linear in my case. I'd expect to see more CF's due to the low output impedance advantages, yet I mostly see 6SN7 in CC configuration. Can someone elaborate on the how and why for CC vs. CF? The drivers themselves will be provided with signal from a 12AX7 cathodyne with 56K plate and cathode resistors, comming from a 32dB pre amp. Any insight is very welcome.
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Old 7th March 2013, 11:26 AM   #2
DF96 is offline DF96  England
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Should be in Instruments and Amps?

Provided you are not running in AB2 with grid current you should not need a CF to drive KT88s, especially in a guitar amplifier where there won't be much HF. If you are using AB2 then a CF might be insufficient; use sand or a transformer instead.
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Old 7th March 2013, 04:46 PM   #3
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I'm into learning about tube power amps in general, so this section seemed ok, but feel free to move if it's better placed somewhere else.

The amp I have in mind will be AB1, so you're right, no grid current.
I guess, but please correct me if I'm wrong, the CC configuration is a bonus when it comes to a bigger signal swing presented tot the power tubes, so for a given input signal, either more power is put out, or more feedback can be used while getting the same output.
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Old 7th March 2013, 05:54 PM   #4
kevinkr is offline kevinkr  United States
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Moved to Instruments and Amps since this thread is about bass guitar amplifiers. All musical instrument amplifier threads really ought to be posted to Instrument and Amps.
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Old 7th March 2013, 07:37 PM   #5
JMFahey is offline JMFahey  Argentina
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Quote:
mostly see 6SN7
Where are you looking?
6SN7 haven't been popular in Guitar Amps since the 50's.
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Old 7th March 2013, 09:49 PM   #6
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Quote:
Originally Posted by JMFahey View Post
Where are you looking?
6SN7 haven't been popular in Guitar Amps since the 50's.
I'm not set on the 6SN7. It's just an example of a widely used low mu tube found in many (hi-fi and classic) power amps. I'm learning the basics of tube power amps at the moment, so hi-fi designs come by as well. I might go for a 12au7. The 6SN7 however has two advantages; high max plate voltage and linearity. For bass, linearity is not a bad thing in my book. Many awesome clean bass power amps out there. But whatever tube makes the cut, the common cathode vs cathode follower and understanding the (dis)advantages of both configuration was my initial reason for this thread.

Last edited by funk1980; 7th March 2013 at 09:51 PM.
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Old 7th March 2013, 11:41 PM   #7
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A nos 6sn7 sounds real nice too, I put one in my new harmonica amp followed by a 6sc7, it just sounds fuller than a 12au7 and a 12ax7. but harder to get.
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Old 8th March 2013, 12:38 AM   #8
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You use cathode followers for 2 reasons:
1) To drive the Miller Capacitance of the Output Tubes at higher frequencies - for Pentode connected output tubes the Miller capacitance is not that much of a problem and for bass guitar you don't need to go all that high in frequency anyway. With Ultralinear connection the Miller Capacitance will be a more significant issue particularly for parallel push pull.
2) To keep low grid 1 to 0V resistances such that you can push the output tubes harder for higher output power (bass amps need grunt). THis can also be don using a 6SN7 or 12AU7 (or ECC99) CC Splitter with say 22K anode load resistors.

Some fellow HiFi crowd "cork sniffer" blasphemy.
Being from the HiFi crowd, might I suggest that you use some solid state.
Use a mosfet source follower direct coupled to grid1 for each output tube (just a grid stop in between). For best results current source load the source follower but a resistor load is OK. Ring of Two Transistor Current Sources work well, a small signal high beta transistor on the bottom and a MJE340 or similar on the top for some voltage withstand. Apply individual bias for each tube to the gates of the mosfet via 1M or 2M2 resistors. AC couple from a Standard 12AX7 diff amp Phase Splitter. 10nF caps should be ample due to teh high impedance seen at the mosfet gates. Don't forgrt protection zeners from gate to source or gate stoppers of 1K.

This will allow some AB2 action and you will be impressed with powerful and how quiet this arrangement is as a result of the low drive impedance taking firm control of output tube grid1 AND shunting output tube grid current noise to ground.

Cheers,
Ian
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Old 14th March 2013, 07:27 AM   #9
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On a different note, check out the new(ish) KT-120 tubes. 6 of them in AB2 PP ought to put out what, like maybe 400+ watts? And for a lot less money than a Mesa 400+.
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