This was something, which I didn't even know if it was even going to work, at all...
I've been working on an amp build with parallel output transformers, for the very first time. As can be deduced, these potentially can be fraught with issues- oscillation and other misbehavior being very high on the list... there's all sorts of opportunity for paralleled components to interfere with each other!!
Well, apparently, I dodged a LOT of bullets. I did do a LOT of thought, on preventing common sources of interference- to the point of using separate phase inverter triode stages for each pair of output tubes (there's a total of four phase inverters). As a result, this one actually worked on the second try (the first try required changing polarity in the circuit to get the feedback loop to work correctly- a simple wiring "oops"), with no oscillation whatsoever, under my favorite range of feedback for these smaller amps (about 12dB).
The amp is using the somewhat-popular Leslie 100-12 output transformers- which are usually 9K to 16 ohms, for EL84 output tubes. So, my take was- parallel two per channel, and that becomes an EIGHT ohm output system (two 16 ohm OPTs paralleled equals 8 ohms output matching, just like paralleling two 16 ohm speakers gives 8 ohms).
This, frankly to my surprise, is one of the MOST POWERFUL things I've had on my test bench, PERIOD. On paper, it should be about 25 watts per channel- but it's the first amp I've ever had on the bench, that made my bench CD player SKIP from the bass from the bench speakers (and I can absolutely just overdrive the poor bench speakers, on low bass, too)!! And I've had Dynaco ST70s, Scott 299s, Sherwood S5500-IVs, and even Dynaco Mk II amps on there before...
Here's a picture of the layout diagram of the amp, as would be seen from underneath:
It's using 12AU7s for preamp, driver and phase inverter, and pairs of EL84s (one pair per OPT) for output. Hence the "Dual Quad" moniker- there's four EL84s for each channel (in a square layout).
This is also the first time I've scratch-built with a toroidal power transformer- I was sure to put the requisite inrush limiter, to keep from blowing fuses. But, the benefit is an EXTREMELY RIGID power supply- even without active regulation- it drops only 1% in voltage (from about 297 to about 294.5v) from zero to HARD CLIPPING output level on both channels!
It's also got a volume control and four-source selector (it is a full function integrated amp, with four line inputs). It's "medium" gain- you do need to crank the volume control to close to max to get it to clip on a standard line source- but it does make it to full volume, fine. IMHO, that's a lot better than one that maxes out before you get to 10:00 on the volume control!
I will be getting pictures of the amp as it nears completion now- I still have to finish fabricating the bottom panel and doing a little clean-up here and there... but this one is a little MONSTER. And the sound, so far, is fantastic- I don't think I've heard that much image depth on my crappy workbench before...
The great thing about this- is that if you are a person who loves EL84s, but needs more power- THIS is one good way to do it, apparently...
Regards,
Gordon.
I've been working on an amp build with parallel output transformers, for the very first time. As can be deduced, these potentially can be fraught with issues- oscillation and other misbehavior being very high on the list... there's all sorts of opportunity for paralleled components to interfere with each other!!
Well, apparently, I dodged a LOT of bullets. I did do a LOT of thought, on preventing common sources of interference- to the point of using separate phase inverter triode stages for each pair of output tubes (there's a total of four phase inverters). As a result, this one actually worked on the second try (the first try required changing polarity in the circuit to get the feedback loop to work correctly- a simple wiring "oops"), with no oscillation whatsoever, under my favorite range of feedback for these smaller amps (about 12dB).
The amp is using the somewhat-popular Leslie 100-12 output transformers- which are usually 9K to 16 ohms, for EL84 output tubes. So, my take was- parallel two per channel, and that becomes an EIGHT ohm output system (two 16 ohm OPTs paralleled equals 8 ohms output matching, just like paralleling two 16 ohm speakers gives 8 ohms).
This, frankly to my surprise, is one of the MOST POWERFUL things I've had on my test bench, PERIOD. On paper, it should be about 25 watts per channel- but it's the first amp I've ever had on the bench, that made my bench CD player SKIP from the bass from the bench speakers (and I can absolutely just overdrive the poor bench speakers, on low bass, too)!! And I've had Dynaco ST70s, Scott 299s, Sherwood S5500-IVs, and even Dynaco Mk II amps on there before...
Here's a picture of the layout diagram of the amp, as would be seen from underneath:
It's using 12AU7s for preamp, driver and phase inverter, and pairs of EL84s (one pair per OPT) for output. Hence the "Dual Quad" moniker- there's four EL84s for each channel (in a square layout).
This is also the first time I've scratch-built with a toroidal power transformer- I was sure to put the requisite inrush limiter, to keep from blowing fuses. But, the benefit is an EXTREMELY RIGID power supply- even without active regulation- it drops only 1% in voltage (from about 297 to about 294.5v) from zero to HARD CLIPPING output level on both channels!
It's also got a volume control and four-source selector (it is a full function integrated amp, with four line inputs). It's "medium" gain- you do need to crank the volume control to close to max to get it to clip on a standard line source- but it does make it to full volume, fine. IMHO, that's a lot better than one that maxes out before you get to 10:00 on the volume control!
I will be getting pictures of the amp as it nears completion now- I still have to finish fabricating the bottom panel and doing a little clean-up here and there... but this one is a little MONSTER. And the sound, so far, is fantastic- I don't think I've heard that much image depth on my crappy workbench before...
The great thing about this- is that if you are a person who loves EL84s, but needs more power- THIS is one good way to do it, apparently...
Regards,
Gordon.
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