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.
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.
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.
Shoog
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.
Shoog
Shoog said:
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!
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?
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?
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.
Shoog
I just ordered tubes for a Fender Champ. I also snagged a 4000 ohm -> 8 ohm output transformer. Seeing as how a 7000 ohm winding is used on the original, this should give me 1.75x what the voltage ouput would be normally?
The output tube is a 6V6. Here's a simple configuration for a parafeed 6v6:
http://www.audioasylum.com/forums/magnequest/messages/318.html
The output tube is a 6V6. Here's a simple configuration for a parafeed 6v6:
http://www.audioasylum.com/forums/magnequest/messages/318.html
You could build something like the moonlight, but single ended with a resistor load rather than the output transformer. Then simply cascade the output into the gainclone (after attenuating it a bit first of course). If you were feeling very adventurous you could add a choke is series with the load resistor for some added pseudo-speaker mojo I suppose.
Merlinb said:
The object of this amp is not to fake a tube amp, but to build a low-power tube amp and buffer it with a GainClone.
I found a 6.7k to 300 ohms interstage transformer on eBay for 10$. Is this suitable for replacing the output transformer in a Fender Champ amplifier? They're usually 7k.
http://cgi.ebay.com/AUDIO-INTERSTAG...ryZ67815QQrdZ1QQssPageNameZWD1VQQcmdZViewItem
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