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Old 24th April 2008, 11:45 AM   #1
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Lightbulb crude, simple and powerful PP-amplifier

got the idea of a REALLY basic and simple design for a high power amp. I have just started to play my bass guitar (bought it probably 2 years ago), and now I need an amplifier for it. I have already built a preamp with an EF86 and a BUF634 as output buffer to drive headphones etc. I havent measuered output voltage yet but i will do that eventually. the buf634 is supplied with +-15VDC so in theory i can get almost 24V P-P maximum signal voltage from my preamp. I thought I could build a power amp with just 4 tubes (probably 6l56gc or 6550) and a custom input transforer to provide phase splitting as well as gain to get the tubes near or maybe even over clip. I now i will probably get a lot of crossover distorsion but i don't think it will be that critical considering the application. am i missing something or can it really be this simple? i have never built push-pull before and I am a bit curious. I plan to use the hammond 1650R or 1650T to give me somewhere 100-120W. any other suggestions for transformers or tubes? i am open to suggestions.
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Old 24th April 2008, 11:52 PM   #2
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Mikael,

If I were doing it, it would probably be something along the lines of the following schematic, although I would use 6BG6GAs, the 35 watt ones, rather than those expensive 6L6GCs. I would use a 12AT7 as the schematic shows. Of course you can adjust the feedback, etc. to suit yourself. As for the power supply, I would aim for a B+ of 450 volts and a bias supply of about 60 volts for plenty of adjustment range. I would also configure the bias adjustment pot such that if the wiper ever went open the output valves would receive the full bias potential rather than no bias at all.

Crossover distortion is a Class B artifact that results when one valve switches off before the other one switches on, and should be a non issue in an AB amplifier.

One small issue to consider is that of output transformer primary impedance. My recommendation would be about 2500 to 3000 ohms p to p for a quad of 6L6 family valves but Hammond does not offer one in your power range. However, matching plate impedance to output transformer primary is not really as critical as one might think, and as long as you are fairly close it should work just fine. I can't make a recommendation as to whether you would prefer the sound of the 1650R or the 1650T.

Just my opinions and hope it is helpful.

Best, Wade
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Old 24th April 2008, 11:54 PM   #3
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A possible power supply...
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Old 25th April 2008, 03:04 AM   #4
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Try running Edcors at 5K:16 into 8 ohm speakers. Or if you only need 60W or so, do the same thing with the 6K6:16 one.

http://edcorusa.com/products/transfo...xpp/index.html
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Old 25th April 2008, 04:44 AM   #5
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thank you very much (both of you) for great input. the power stage is exactly what i was looking for, but i was thinking about losing the initial gain stage completely and replace it all with a phase splitting transformer to make it even more simple. would that work?

Quote:
Originally posted by tubewade
Mikael,

If I were doing it, it would probably be something along the lines of the following schematic, although I would use 6BG6GAs, the 35 watt ones, rather than those expensive 6L6GCs. I would use a 12AT7 as the schematic shows. Of course you can adjust the feedback, etc. to suit yourself. As for the power supply, I would aim for a B+ of 450 volts and a bias supply of about 60 volts for plenty of adjustment range. I would also configure the bias adjustment pot such that if the wiper ever went open the output valves would receive the full bias potential rather than no bias at all.

Crossover distortion is a Class B artifact that results when one valve switches off before the other one switches on, and should be a non issue in an AB amplifier.

One small issue to consider is that of output transformer primary impedance. My recommendation would be about 2500 to 3000 ohms p to p for a quad of 6L6 family valves but Hammond does not offer one in your power range. However, matching plate impedance to output transformer primary is not really as critical as one might think, and as long as you are fairly close it should work just fine. I can't make a recommendation as to whether you would prefer the sound of the 1650R or the 1650T.

Just my opinions and hope it is helpful.

Best, Wade
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Old 25th April 2008, 05:49 AM   #6
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I can't see an issue with the transformer, as long as the previous stage can drive it.

But then, I'm still a newb.
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Old 25th April 2008, 05:54 AM   #7
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great! then all i have to do is figure out what tubes to use, calculate the gain and impedance and all for the phase splitting transformer, and choose an output transformer and I'm on my way! can't wait. I took a look at the suggested 6BG6GAs and although they are cheaper than the 6L6GC, they are not as widely available.
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Old 25th April 2008, 09:07 AM   #8
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i am leaning heavily towards hammond 1650T (120W) and 4 pcs 6l6gc. i can get the transformer fairly cheap and there are tons of 6l6gc's available out there. should provide me with plenty of power.
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Old 25th April 2008, 12:18 PM   #9
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Sounds like your plan will work. You need to find out for sure how much output swing you have from the preamp buffer. I expect the bias voltage on the output valves to be around -50 volts, so you will need a total swing in excess of 100 volts. This you will need to know when calculating the required turns ratio of the input transformer.

Do you have a feedback scheme in mind? You would not necessarily have to use feedback but the output impedance will be rather high (probably 70 ohms or higher) using tetrodes with no feedback. Of course, you might prefer the sound that way.
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Old 25th April 2008, 12:55 PM   #10
Sherman is offline Sherman  United States
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Quote:
Originally posted by Mikael Abdellah
great! then all i have to do is figure out what tubes to use, calculate the gain and impedance and all for the phase splitting transformer, and choose an output transformer and I'm on my way! can't wait. I took a look at the suggested 6BG6GAs and although they are cheaper than the 6L6GC, they are not as widely available.

Mikael,

Be sure to check the specs on the phase splitting trafo you plan to use. Some, designed primarily as input trafos can't handle the signal level you will get from your input stage (or preamp in your case). If the trafos can handle a couple watts they should work.
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