I built my first project a few years ago, an EL84 push-pull stereo amplifier (the Tubelab simple push-pull) and have it hooked up to a pair of Celestion ditton 33s. It is 5AR4 rectified and cathode bias, with 12AT7 cathodyne/concertina phase inverter. I’m looking to upgrade it as best I can whilst keeping the same transformers, which are 25W Hammond 1650FA 7k6R, and Hammond 370HX 225v-0-225v 230mA. Ideally I’d get the full 20-25W from those output transformers. I’m happy to buy another power transformer and choke and convert to mono blocks, but don’t want to buy two new power transformers or new output transformers.
Any ideas? Option 1 at the moment is two parallel pairs of EL84s in fixed bias with a solid state rectifier and E88CC long-tailed pair phase inverter. I can use different taps to halve the plate load. Option 2 is running a pair of KT66s in cathode bias (trying to avoid pushing the 25W limit), but I’m concerned about running them at a lowish plate voltage of 350v and into a 7k6 load, with the E88CC long tailed pair inverter.
Are there any better options out there? Design help would be much appreciated.
Any ideas? Option 1 at the moment is two parallel pairs of EL84s in fixed bias with a solid state rectifier and E88CC long-tailed pair phase inverter. I can use different taps to halve the plate load. Option 2 is running a pair of KT66s in cathode bias (trying to avoid pushing the 25W limit), but I’m concerned about running them at a lowish plate voltage of 350v and into a 7k6 load, with the E88CC long tailed pair inverter.
Are there any better options out there? Design help would be much appreciated.
I don't see how you have more than 300v for the output tubes now. By the time you peak 225v and drop it through the rectifier and the OPT it's under 300v. Then you have to subtract the cathode voltage. Are we working with the right numbers here?
That would be great! Most amps seem to run the 6L6 at 400-500v, are there any sonic disadvantages, apart from reduced output power, to running at 350v (or for the KT88, EL34, KT66 for that matter?). Also correct me if I’m wrong but don’t these bigger bottles sound optimal at a lower load of 4K-6k plate to plate?
To obtain your output wattage target, you'd probably want to run them in Pentode mode, 250V screens, 350V plate. Ostensibly, that's about the right loading for a 7.6K primary, PP, AB1. I'd shoot for about 12 dB feedback. Note you're designing a whole new amp from scratch at this point.
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I'm not sure exactly what your goal is. If you need a louder amp, going from about 15W to 20-25W (per channel) isn't worth the bother in my opinion. Keeping the amp and getting more efficient speakers is probably better bang for buck.
If you're looking for another project, save up until you have the funds for the amp you want to build. My $.02
If you're looking for another project, save up until you have the funds for the amp you want to build. My $.02
I can offer a simple experiment using stuff that you already have. It is possible to wire an SPP board for single channel operation using all 4 EL84's in PPP mode. You can try this with the amp you have in mono of course. If you like the way it works, then build a second channel.
I built one of these several years ago then turned it up to 60 WPC by running the EL84's on 425 volts plate and 320 volts screen, but you don't need to do that with your OPT's. At these power levels only JJ EL84's or NOS 6BQ5's live.
To do this you need to disconnect one OPT and run jumper wires from the three connections for the remaining OPT over to the terminals on the board where you just disconnected the OPT. This lets both channels run through one OPT.
The simplest way to drive the amp is to just parallel connect the inputs at the PC board input, but there is a better way.
I removed the 12AT7 for one channel and lifted the coupling cap at its plate connection to the removed 12AT7 and ran a wire over to the plate side of the remaining 12AT7. This lets both output stages run from one driver tube eliminating errors caused by differences between the two 12AT7 tubes.
To get full output from this mode you need to halve the load impedance by connecting an 8 ohm load to the 16 ohm tap, or a 4 ohm load to the 8 ohm tap.
A single Hammond 370HX probably has enough B+ capacity to run two boards on music, not continuous sine waves, but does not have enough filament capability to run two rectifiers or 8 EL84's. I used a big Antek AN-4T360 toroid for my amp but I was running 425 volts. If you like the way it sounds the best option might be to add a second 370HX and make a pair of mono blocks.
I built one of these several years ago then turned it up to 60 WPC by running the EL84's on 425 volts plate and 320 volts screen, but you don't need to do that with your OPT's. At these power levels only JJ EL84's or NOS 6BQ5's live.
To do this you need to disconnect one OPT and run jumper wires from the three connections for the remaining OPT over to the terminals on the board where you just disconnected the OPT. This lets both channels run through one OPT.
The simplest way to drive the amp is to just parallel connect the inputs at the PC board input, but there is a better way.
I removed the 12AT7 for one channel and lifted the coupling cap at its plate connection to the removed 12AT7 and ran a wire over to the plate side of the remaining 12AT7. This lets both output stages run from one driver tube eliminating errors caused by differences between the two 12AT7 tubes.
To get full output from this mode you need to halve the load impedance by connecting an 8 ohm load to the 16 ohm tap, or a 4 ohm load to the 8 ohm tap.
A single Hammond 370HX probably has enough B+ capacity to run two boards on music, not continuous sine waves, but does not have enough filament capability to run two rectifiers or 8 EL84's. I used a big Antek AN-4T360 toroid for my amp but I was running 425 volts. If you like the way it sounds the best option might be to add a second 370HX and make a pair of mono blocks.
I found some notes I kept for an 807 type PP AB1 amp I built some time back. It used Hammond 1650H outputs (6.6K) and a custom wound Heyboer power transformer. The power supply delivered 400V under quiescent conditions. Output stage was UL connected, stereo chassis build with one power supply driving both channels. The amp delivered 20 watts output to a dummy speaker load, where both channels were driven.
Later I converted that same amp to pentode outputs and added a 284V screen supply. It then delivered 25 watts output, both channels driven.
So there are a few data points to make your comparisons. With your higher primary OPTs, and lower 350V power supply, you will see less power than what I quoted above.
I'd guess with 7.6K primaries, to hit 25 watts output in UL mode with 6L6 tubes, you'd need close to a 450V power supply if you run both channels from it. If you go monoblocks, 425V should get you there.
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Thanks for your help everyone. It looks like parallel push-pull EL84s is the way to go unless I want to buy new power transformers to get the plate voltages up high enough for a pair of kt66/el34/6L6. What would be the best possible phase inverter and driver configuration to get an input sensitivity of around 1V? We can use up to two bottles per channel, and I’m a fan of the E88CC/6922 and the EF86 pentode but happy to take advice.
Is the E88CC cathodyne/concertina phase inverter from Morgan Jones’ Bevois Valley also appropriate/optimal for the parallel arrangement? Or would a pair of EF86 like the Quad II work better? What effect does an extra pair of output valves have in terms of increased capacitance and load on the phase inverter?
Is the E88CC cathodyne/concertina phase inverter from Morgan Jones’ Bevois Valley also appropriate/optimal for the parallel arrangement? Or would a pair of EF86 like the Quad II work better? What effect does an extra pair of output valves have in terms of increased capacitance and load on the phase inverter?
I was look through the datasheets and saw a spec on the GEC KT88 datasheet for 30W for an ultralinear cathode-biased push-pull pair with 375v Va and a 5k output transformer.
My hammond 370HX is 550Vac so I should get 390Vdc diode-rectified, and the opt are rated at 25W 7k6 so I think they will manage, the increased load should bring down the output power a bit. The bias point is -35v so the phase inverter needs to swing 70v per tube. I would only need to buy an extra power transformer and choke to make monoblocks.
I've drawn up a design using a DC-coupled cathodyne 6SN7 (like the tubelab simple pp) but it might be a little over-stretched. I'd like an input sensitivity of 2v. Perhaps a long-tailed pair with a CCS would be better, they seem popular. Running the KT88 at a lower plate voltage seems uncommon and there aren't many designs out there.
Might this work?
My hammond 370HX is 550Vac so I should get 390Vdc diode-rectified, and the opt are rated at 25W 7k6 so I think they will manage, the increased load should bring down the output power a bit. The bias point is -35v so the phase inverter needs to swing 70v per tube. I would only need to buy an extra power transformer and choke to make monoblocks.
I've drawn up a design using a DC-coupled cathodyne 6SN7 (like the tubelab simple pp) but it might be a little over-stretched. I'd like an input sensitivity of 2v. Perhaps a long-tailed pair with a CCS would be better, they seem popular. Running the KT88 at a lower plate voltage seems uncommon and there aren't many designs out there.
Might this work?
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