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Old 7th October 2006, 11:03 PM   #21
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Yoooo...

Sorry that i jumped on this thread so late. I have built several hybrid gainclone (overture series chip) guitar amps, as well as several parafeed guitar amps (RC, LC and CCS in SE and PP modes) including several of the AX84 SE designs, PP tube differential splitter-into-differential gainclone output section.

Anyhow... in layman's terms, resistive loaded parafeed is just like a regular grounded cathode RC coupled gain stage.. like in the preamp of a guitar amp. The only difference in YOUR case is that you would be RC loading a El84 or the like. I built up a one a while ago... it put out about 1-2+ watts before clipping and sounded pretty good. I also made a version of an AX84 Hi-Octane with a LC parafeed output section using a cheapo Hammond choke as th load, whihc i did to simulate the low end rolloff of an old guitar amp transformer.


That is all to say that it can be done, you just need to use an alternate arrangement for the output stage.. as was mentioned above.
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Old 7th October 2006, 11:37 PM   #22
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Can you please explain that in stupid people's terms?
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Old 8th October 2006, 02:19 AM   #23
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Quote:
Originally posted by Spasticteapot
Can you please explain that in stupid people's terms?

Explain what?
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Old 8th October 2006, 02:32 AM   #24
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Quote:
Originally posted by aletheian



Explain what?

The "alternate arrangement"

What were you referring to?

I'm new to tube amps, so I figured I should go for a low-power amplifier. 1/2 a watt is as low as it gets!

(Plus, distortion is a GOOD thing!)
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Old 8th October 2006, 08:47 AM   #25
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I would expect that the output voltage from the step down transformer should be about 2V. Of course this is entirely selectable by the choice of step down transformer. You could get anything from 0.5V to 10V depending on your choice.
The schematic is basically a simplified version of a Marshall 200W Pig front end, without the phase splitting and presence control. There are two inputs - one clean and one dirty. The Treble,middle and bass Tone stack. If you place another volume pot after the interstage tranfsormer you get a Gain control (first pot) and a master volume control (second pot).
Its about as simplified as it can be whilst maintaining the functionality. You can take off an output for headphones after the interstage transformer.

I expect to build my version in the next month or so, so will have more to report then. I can't really see any reason why it wouldn't work as drawn, but if it doesn't it should be a simple matter of adjusting resistors to get it working.

Hope that helps.

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Old 8th October 2006, 03:10 PM   #26
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Gotcha. Let me know how it goes!
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Old 8th October 2006, 05:54 PM   #27
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Quote:
Originally posted by Shoog

The schematic is basically a simplified version of a Marshall 200W Pig front end, without the phase splitting and presence control...

HA! A mini Marshall Major? That's funny stuff. I was running sound in a small 2nd story club one time, and the room was about 20'x40', and band pulled out a pair of Majors and four 4x12" stacks! I almost had a heart attack. 10,000 watts of PA was no match for those pigs... and they had the audacity to complain about the monitor mix not being loud enough!
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Old 8th October 2006, 06:04 PM   #28
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Hey Shoog.

I am going to assume by the use of the term "mains" that you are not located in the US. So that tranny that you used for the OPT is a 240:6 stepdown... meaning that for a US tranny, it would be a 120:3 stepdown... or whatever makes a 40:1 ratio. That presents a 12.8k load with a 8r speaker attatched.

Was that a purposeful load, or arbitrary? IF it was purposeful, then how did you arrive at that number?
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Old 8th October 2006, 08:07 PM   #29
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Quote:
Was that a purposeful load, or arbitrary? IF it was purposeful, then how did you arrive at that number?
You are right on the ratios. The load will be dominated by the 3K5 load resistor so in absolute terms it makes very little difference what load the transformer actually presents.
The design is for a Gainclone front end, so the reflected load would be more like 1Meg depending on the input resistance of the gainclone. The actual choice of transformer is arbitary as you guessed and should be adjusted when the actual gain of the front end is revealed by real world testing.

I repaired a Marshall major for a friend. Personally I think its an amp without a purpose in this day and age. You are either going to amp up a quitar amp in most situations, or play in small venues where 20Watts of tube power is more than enough. How can you overdrive a 200W output stage in anything less than a concert hall. Also there was the small matter of the cost of replacement KT88's which makes it a very impractical amp indeed.

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Old 8th October 2006, 08:33 PM   #30
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Quote:
Originally posted by Shoog


You are right on the ratios. The load will be dominated by the 3K5 load resistor so in absolute terms it makes very little difference what load the transformer actually presents.
The design is for a Gainclone front end, so the reflected load would be more like 1Meg depending on the input resistance of the gainclone. The actual choice of transformer is arbitary as you guessed and should be adjusted when the actual gain of the front end is revealed by real world testing...

Ah yes, that makes sense. I forgot that we were talking about gainclones here and though that there would be a speaker hanging off that mains transformer.
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