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Old 23rd May 2012, 07:21 PM   #1
m3vuv is offline m3vuv  United Kingdom
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Default 872b or 6gj5a for a homebrew bass amp

hi all,can any one point me in the direction of a schematic for a bass amp using a 572b or 6gj5a sweep tube?,i have a built a rf linear amp using 572b valves with a b+ of 2400volts and it works a treat(2x572b in grounded grid),i have a 2400v psu and a dc filament psu plus valves and sockets,thats the reason for wanting to have a go at a bass amp,also just a thought,would it be possible to use a pi network to match the output impedance to the speaker?,this is often done in rf applications but not so sure about audio!,cheers all,Paul m3vuv
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Old 24th May 2012, 02:02 AM   #2
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just a thought,would it be possible to use a pi network to match the output impedance to the speaker?
A PI network like those found in RF amps is designed to work at a single frequency and must be retuned if the frequency is changed. A bass amp must cover 40 Hz to several KHz without retuning.

You need an output transformer to match the high impedance tubes to the low impedance speaker over the audio frequency range. It works like the broadband balun seen in a solid state HF amp, only it is much larger since it operates at a lower frequency.

A bass amp will need a few stages of preamp gain and a suitable output stage. The 572B is better suited to RF amp use since the higher voltages and impedances that these tubes need makes the design of the output transformer much more difficult. It would probably not be an off the shelf part.

Sweep tubes make excellent audio amplifiers. You can lift the preamp schematic from a tube bass amp that you like. Look at this thread for a power amp that works with sweep tubes. I have used it for guitar amp duty.

George KB4LRE
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 24th May 2012, 02:44 AM   #3
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Location: Taxland, New Jersey
George, OT but how about a report on the Hamvention. What did you buy, sell and saw but didn't buy.
1944: 18 year olds storm the beach of Normandy into almost certain death.
2017: 18 year olds need a safe place because words hurt their feelings.
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Old 24th May 2012, 09:33 AM   #4
m3vuv is offline m3vuv  United Kingdom
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cheers for that,i realised about a pi network after i posted the thread!,the psu i have has a voltage doubler so the output from the transformer is 1.2kv at 1amp,the reason i wanted to use sweep tubes or 572b s,was due to me having them around,this is a project for a friend,he has a ss 50w amp but gets drowned out by the drums!,i was thinking about using the ss amp to drive the tube one,if that makes sense,as regards the matching transformer i was thinking of homebrewing one,i have a lathe and plenty of ironwork for one!,i was looking for about 100wats output but wanted to do it with the bits i have in the junk box,any help with component values much needed,thanks for the replys,keeep em comming,cheers all de m3vuv.
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