Hi folks.
About 20 years ago I managed to PU a pair of Bogen MO100A PA booster amps. As many of you may know finding info on Bogen OP trafos(beyond color codes) is pretty much impossible and the 8417 is crazy cost/availability so I originally decided to use my vast selection of EL34s in 4xPPP biased on the mild side(17ma/tube in AB1) and keep the voltage doubler power supply(600VDC) in order to tap the screens at a very safe ~300VDC. I used a 12ax7 LTP as a driver/PI, very similar to that used in Marshall type 100watt guitar amps except the NFB. This worked pretty well. No red plates even at this relatively high B+ and astonishingly, even my Chinese 6CA7s held up well.
The amps sounded great for mids, which I usually have the hardest time keeping under control at relatively high power; amazing for PA, guitar and when used as low mid drivers(80Hz ->3k into 12" 300wt EV drivers) in HiFi 3way electronic C/O. Yes I am power crazy with 600wt SS for bass and a Scott 233(~33wt/side) just for highs, LOL.
However, after more recent research I found many folks highly rate the Bogen OP Trafo for HiFi so I have decided to give them one last upgrade and throw everything I can at them, including an extremely effective and reliable HV regulator design(based on Bi-polar Horizontal OP transistors: Tip50 feeding a stack of ECG238, ECG389/or equivalents) I've been using in other projects since the mid 80s, adding an extra EL34(no worries about extra heater draw, I have some extra 36V CT/1amp trafos I going to parallel the secondary for 18 volts @ 2amp and run 3 EL34s in series) on each side of the PPP circuit to take advantage of the extra tube sockets originally used for remote control and preamp hook-up thereby hopefully allowing a better match for the OP trafo while increasing the power capability, and, maybe using some of the more fancy NOS driver tubes(6CG7, 12BH7, 6267, 5687) I've been saving since forever. The only limit in this build will be the sockets available, there is only two 9pins for the driver/PI so no room sadly for CFs feeding the OPs...
I am curious what some of you may think about my ID so far. If there is enough interest generated I will post a complete proposed schematic.
About 20 years ago I managed to PU a pair of Bogen MO100A PA booster amps. As many of you may know finding info on Bogen OP trafos(beyond color codes) is pretty much impossible and the 8417 is crazy cost/availability so I originally decided to use my vast selection of EL34s in 4xPPP biased on the mild side(17ma/tube in AB1) and keep the voltage doubler power supply(600VDC) in order to tap the screens at a very safe ~300VDC. I used a 12ax7 LTP as a driver/PI, very similar to that used in Marshall type 100watt guitar amps except the NFB. This worked pretty well. No red plates even at this relatively high B+ and astonishingly, even my Chinese 6CA7s held up well.
The amps sounded great for mids, which I usually have the hardest time keeping under control at relatively high power; amazing for PA, guitar and when used as low mid drivers(80Hz ->3k into 12" 300wt EV drivers) in HiFi 3way electronic C/O. Yes I am power crazy with 600wt SS for bass and a Scott 233(~33wt/side) just for highs, LOL.
However, after more recent research I found many folks highly rate the Bogen OP Trafo for HiFi so I have decided to give them one last upgrade and throw everything I can at them, including an extremely effective and reliable HV regulator design(based on Bi-polar Horizontal OP transistors: Tip50 feeding a stack of ECG238, ECG389/or equivalents) I've been using in other projects since the mid 80s, adding an extra EL34(no worries about extra heater draw, I have some extra 36V CT/1amp trafos I going to parallel the secondary for 18 volts @ 2amp and run 3 EL34s in series) on each side of the PPP circuit to take advantage of the extra tube sockets originally used for remote control and preamp hook-up thereby hopefully allowing a better match for the OP trafo while increasing the power capability, and, maybe using some of the more fancy NOS driver tubes(6CG7, 12BH7, 6267, 5687) I've been saving since forever. The only limit in this build will be the sockets available, there is only two 9pins for the driver/PI so no room sadly for CFs feeding the OPs...
I am curious what some of you may think about my ID so far. If there is enough interest generated I will post a complete proposed schematic.
PS: I should note that I would have posted a schematic already but it's a bit of a pain to get a readable post without hooking up a huge scanner I have in reserve, but as mentioned above, I will do so if there is interest.
And of course, all and any input will be much appreciated.
Thanks ahead.
And of course, all and any input will be much appreciated.
Thanks ahead.
Wow, well over 100 looks and only one reply?
I suppose many of you are wondering why I don't just copy a QS. Well first off, I see very little if any improvement over the original Bogen design(IMO) and when they used half a 12ax7 and half a 12au7 to replace the 7247, well maybe the same results yes, but seems like a waste of 2 halves of the dual triodes. Maybe it's just me, but using a 6av6 and a 6c4 would make more sense(but then I have bags of them) if you wanted to copy the Bogen design, although a 6SN7 or two would have been way cool and if I had the extra space for octal sockets, that's prolly the way I'd go. As it is I'm holding to a 6267/EF86 front end, and a 12AT7, 6CG7, or 12BH7 as a PI/driver, very similar to the LEAK model TL/50plus, EICO HF50 and Peerless/Altec High Stability FB amp originally posted in Audio, October 1962.
Further, when converted to EL34, I don't see how 4xEL34(like every mod or update I've seen) could possibly replace 4x8417. I ain't talking mu, although that is a consideration I already delt with by changing the PI/driver, and the max OP from a 8417 is 35 + 5 watts, compared to 25(plate)+8(g2) for EL34, close but the big consideration is the proper load. It seems EL34 requires almost twice the p to p resistance at HV to achieve the same power(looking at the Mullard specs for EL34 and Sylvania for 8417). This is the main reason I want to go with an extra tube on each side, to match the load better.
Lastly, I know you can drive the heck outta 4 EL34s and get 100wts at some crazy low P to P load and moderate voltage, but 6 will do it without pushing the limits, resulting in longer tube life and greater reliability.
Anyhow, I have to work within the amps I have with their huge American made iron that I want to save/use.
Thanks at least for all the lookie Loo's, be nice to see some more posties too
I suppose many of you are wondering why I don't just copy a QS. Well first off, I see very little if any improvement over the original Bogen design(IMO) and when they used half a 12ax7 and half a 12au7 to replace the 7247, well maybe the same results yes, but seems like a waste of 2 halves of the dual triodes. Maybe it's just me, but using a 6av6 and a 6c4 would make more sense(but then I have bags of them) if you wanted to copy the Bogen design, although a 6SN7 or two would have been way cool and if I had the extra space for octal sockets, that's prolly the way I'd go. As it is I'm holding to a 6267/EF86 front end, and a 12AT7, 6CG7, or 12BH7 as a PI/driver, very similar to the LEAK model TL/50plus, EICO HF50 and Peerless/Altec High Stability FB amp originally posted in Audio, October 1962.
Further, when converted to EL34, I don't see how 4xEL34(like every mod or update I've seen) could possibly replace 4x8417. I ain't talking mu, although that is a consideration I already delt with by changing the PI/driver, and the max OP from a 8417 is 35 + 5 watts, compared to 25(plate)+8(g2) for EL34, close but the big consideration is the proper load. It seems EL34 requires almost twice the p to p resistance at HV to achieve the same power(looking at the Mullard specs for EL34 and Sylvania for 8417). This is the main reason I want to go with an extra tube on each side, to match the load better.
Lastly, I know you can drive the heck outta 4 EL34s and get 100wts at some crazy low P to P load and moderate voltage, but 6 will do it without pushing the limits, resulting in longer tube life and greater reliability.
Anyhow, I have to work within the amps I have with their huge American made iron that I want to save/use.
Thanks at least for all the lookie Loo's, be nice to see some more posties too
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Here is an example.
Patrick Turner is fairly "cluey" so there would be worse schematics to copy, peruse for ideas.
100w-monobloc2-2004
I have occassionally thought about a 6 x EL34 per channel HiFi Amp to use some Hammond 1650T Output Trannies I have on the shelf.
Cheers,
Ian
Patrick Turner is fairly "cluey" so there would be worse schematics to copy, peruse for ideas.
100w-monobloc2-2004
I have occassionally thought about a 6 x EL34 per channel HiFi Amp to use some Hammond 1650T Output Trannies I have on the shelf.
Cheers,
Ian
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