I intend to build the one from diyparadise, but substituting PL84's (EL86) and using a 5687 as a driver stage. Just need to track down some output Iron at a reasonable cost. Contemplated using toroidals as there is an option to rebalance the cathode resistance to compensate for mismatching of tubes. Hope its as good as it says. I will compare it to the sound of the LEAK Stereo 20 I used to have. I'am hoping it will better it.
Shoog
Shoog
I'm thinking of designing it by myself. It will be a good learning experience, and for any doubts - well, there's diyaudio!!! 
I'm thinking of keeping it simple: single triode stage to make some gain and to do the phase split, cathode biased output stage, tube rectified psu.
When I've got some time I will draw something to explain what I have in mind, and I will ask about the "doability" of the design.
I'm also thinking of breadboarding a single channel of it on a wooden strip, just to see (and hear) how does it performs. Do you guys do this? I will mount tube sockets on long screws directly in the wood, with nails as convenient component nodes. Well, cheap-fi....
Ps: seen my new avatar? well... that's me working on my one-tube radio. Behind me there's even the 'scope... 
I'm thinking of keeping it simple: single triode stage to make some gain and to do the phase split, cathode biased output stage, tube rectified psu.
When I've got some time I will draw something to explain what I have in mind, and I will ask about the "doability" of the design.
I'm also thinking of breadboarding a single channel of it on a wooden strip, just to see (and hear) how does it performs. Do you guys do this? I will mount tube sockets on long screws directly in the wood, with nails as convenient component nodes. Well, cheap-fi....
Ps: seen my new avatar? well... that's me working on my one-tube radio. Behind me there's even the 'scope...
Simple EL84
I have build the Simple EL84 (from diyparadise) using EL84 in triode mode to drive EL84 in push-pull with Fisher SA100 iron.
I'm realy satisfy with the sound: it's great singing!
Another advantage is that you don't need to use matched power tube :there are no other pp design that let you do this.
I have build the Simple EL84 (from diyparadise) using EL84 in triode mode to drive EL84 in push-pull with Fisher SA100 iron.
I'm realy satisfy with the sound: it's great singing!
Another advantage is that you don't need to use matched power tube :there are no other pp design that let you do this.
Stop! Before you start ripping apart to start all over again try analyzing what you have and maybe build up from there. For example, I found an EL84 PP amp from the flea market thinking that I would salvage the parts and rebuild another "better" more modern amp. Lucky, I found Gabe's site and realized that I had an old Magnavox like the one he had written about with the 6EU7 and all. I did some of the modifications he mentioned and ended up with one of the best sounding amps I have come across recently. This is still with many of the original tubes, caps and resistors! So, my advice is to play with it awhile and see what you can do to make it better. If you aren't happy then rip it apart and start over.
BTW, thanks Gabe for your write up of this amp. It truely is all that you said it was. So what else can I try to make it sound even better? There must be something to dp with the 100 ohm cathode shared resistor. CCS of somekind maybe?
BTW, thanks Gabe for your write up of this amp. It truely is all that you said it was. So what else can I try to make it sound even better? There must be something to dp with the 100 ohm cathode shared resistor. CCS of somekind maybe?
Re: Simple EL84
Hi there...That's an illusion.....tubes in grand-daddy's p-p amps were never matched and deemed not necessary. Anyway, in a well designed amp the nfb should take care of discrepancies.
richj
have' said:
Another advantage is that you don't need to use matched power tube :there are no other pp design that let you do this.
Hi there...That's an illusion.....tubes in grand-daddy's p-p amps were never matched and deemed not necessary. Anyway, in a well designed amp the nfb should take care of discrepancies.
richj
RockysDad said:
Sorry, but I thought about this. I cannot use the original amp, because it's built in a strange way. Typical amps have the tubes and the trannies mounted vertically on a chassis, with components under, easy accessible. My amp has everything mounted horizontally, with components extremely difficult to work on, because they're very near to the trannies. It's difficult to explain without a photo... Think about a normal tube amp, then cut the top chassis in stripes and put them one after another vertically: you have all the stuff horizontal now. I should cut the chassis with a chainsaw even to change a single resistor. So I decided to rip it all apart, and build something easier to work on.
Thanks btw!!! My decision was also dictated by the bad corrosion on most of the connectors, and they were also old-style... DIN plugs and the like...
You are quite welcome. I am blushing!
Aside form replacing all the resistors and caps and tubes, nothing else needs to be done. If it sounds terrific now, since you only did the mods, it will be beyond awesome when you replace the caps and resistors and old tubes.
A friend of mine gave me a mini-console and I listened for awhile with all the original stuff. Amazing that it still worked nicely with all that old stuff, isn't it? Of course, I immediately replaced everything and applied the mods.
As for the 100 ohm common... unless you want to build a regulated bias for the output tubes, nothing.
BTW, this may make me sound dumb... but... what's a CCS???? Common Cathode Source??? You want to use a tube to act as the 100 ohm? Like a SRPP? Or... what? I know in solid state a transistor is used a a current sink... or source... is that what you mean?
Thanks!
Gabe
Oh.... so yes, I was right... No need to reinforce my stupidity! ;-)
Just kidding. Just never paid attention to many abbreviations.
No, A CCS would not be good for an output stage. We want variable current. A CCS is good where one desires a close to pure voltage only signal, as in the input stages of an amp. Oh, and maximum gain. As far as I understand it.
For an output stage, fixed bias is different from CCS. We need the high variable current to make the high variable voltage. Fixed bias makes sure that the stage does not vary from its "ideal" or engineered bias for all frequencies and amplitudes. So for a signal that is at say 35 volts, the tube remains at the same parameters as for a simultanious signal at 5 volts. This helps linearity. This is why bass comes out sounding deeper than for a cathode bias amp.
In the case of the amp in question, the fixed bias is created by the three other tubes and the 100 ohm resistor acting as a voltage divider fixed bias for the fourth tube. Theoretically. However, as you can attest to first hand... ;-)
Gabe
Just kidding. Just never paid attention to many abbreviations.
No, A CCS would not be good for an output stage. We want variable current. A CCS is good where one desires a close to pure voltage only signal, as in the input stages of an amp. Oh, and maximum gain. As far as I understand it.
For an output stage, fixed bias is different from CCS. We need the high variable current to make the high variable voltage. Fixed bias makes sure that the stage does not vary from its "ideal" or engineered bias for all frequencies and amplitudes. So for a signal that is at say 35 volts, the tube remains at the same parameters as for a simultanious signal at 5 volts. This helps linearity. This is why bass comes out sounding deeper than for a cathode bias amp.
In the case of the amp in question, the fixed bias is created by the three other tubes and the 100 ohm resistor acting as a voltage divider fixed bias for the fourth tube. Theoretically. However, as you can attest to first hand... ;-)
Gabe
Sorry for hijacking this thread but last question for Gabe..
What were the final values for the four blocking caps that you used on this amp? Were they all .04 or two .0012 and two .04? Humm... .04 are not standard values, .047 will have to do.
Might as well start replacing components...
Thanks!
What were the final values for the four blocking caps that you used on this amp? Were they all .04 or two .0012 and two .04? Humm... .04 are not standard values, .047 will have to do.
Might as well start replacing components...
Thanks!
news!
I've started to rip down my amp, I've now extracted the transformers. But... here's the sad thing... one of the OT is open!!!
Yes, one of them measures 200VAC (with 12V on the secondary from an AC wall wart) from CT to one end, and 200 to the other end. So, 400VAC in total which leads me to an impedance ratio of 10k-8ohm. But when I connect the other one in the same way, I get much lower secondary voltage, about 6/7VAC (due to the wall wart sagging because of loading - there could be a short somewhere), and I measure 110VAC from the CT to one end of the primary, and 0V to the other end.
A dead tranny you may think. In fact, measuring DC resistance from the CT to one end, you get 170ohm, to the other end you get infinite. But the strange thing is... even the good one measures like this!!! (the same DC resistance readings). So even the good one has the primary open but it gets the proper voltages on it...
It's strange. Someone can help me???
I'm so sad now... I can't build a stereo amp
(do I have to build a guitar amp? )
I've started to rip down my amp, I've now extracted the transformers. But... here's the sad thing... one of the OT is open!!!
Yes, one of them measures 200VAC (with 12V on the secondary from an AC wall wart) from CT to one end, and 200 to the other end. So, 400VAC in total which leads me to an impedance ratio of 10k-8ohm. But when I connect the other one in the same way, I get much lower secondary voltage, about 6/7VAC (due to the wall wart sagging because of loading - there could be a short somewhere), and I measure 110VAC from the CT to one end of the primary, and 0V to the other end.
A dead tranny you may think. In fact, measuring DC resistance from the CT to one end, you get 170ohm, to the other end you get infinite. But the strange thing is... even the good one measures like this!!! (the same DC resistance readings). So even the good one has the primary open but it gets the proper voltages on it...
It's strange. Someone can help me???
I'm so sad now... I can't build a stereo amp
(do I have to build a guitar amp?
)
Re: news!
Hi there.....when measuring DC primary resistance of o/p trannies (and mains trannies)with a digital V:M.....the autoranging aspect of some meters can conflict with the high primary inductance to give duff readings. Simply s/c (one of) the sec windings and the problem will disappear.
With an analogue needle meter type....the rise of current will eventually settle without having to do the above.
richj
A dead tranny you may think. In fact, measuring DC resistance from the CT to one end, you get 170ohm, to the other end you get infinite. But the strange thing is... even the good one measures like this!!! (the same DC resistance readings). So even the good one has the primary open but it gets the proper voltages on it...
[/B]
Hi there.....when measuring DC primary resistance of o/p trannies (and mains trannies)with a digital V:M.....the autoranging aspect of some meters can conflict with the high primary inductance to give duff readings. Simply s/c (one of) the sec windings and the problem will disappear.
With an analogue needle meter type....the rise of current will eventually settle without having to do the above.
richj
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