Hi,
I've recently upgraded my BHEL34 in order to test a special NFB loop I'm workin on.
I started by the last working point suggested on Mullard datasheet ( https://frank.pocnet.net/sheets/129/e/EL34.pdf ) for maximum power, slightly adapted.
I now run them at 460V and 44mA, on a Raa of 6.6kOhm and 23% ultralinear taps.
To improve the loadline of the phase splitter (so making it flatter) still being able to keep a good local feedback on the EL34, with the plate voltages below 300V and the cathode voltages above 0.9V even at maximum power, I've modified the 220k + 47k + 39k combo into a 330k + 33k + 27k combo to supply the two triodes.
The reduced UL tap increase the available power, lowers the driving requirements and also the input capacitance of the EL34, but the higher swing compared to the EL84 pushed me to increase the current flowing through the power drive, so to reduce the bias resistors of the PD CCS down to 100 Ohm. This allows to have more current to be able to swing more volts on the grids of tghe EL34.
I wont' repeat the calculations here, you can find them in my EL84 thread.
I have old boards, with PSU onboard like for EL84, so I worked on the dropping resistors to set the voltage of the powerdrive and phase inverter CCS.
I set them to 100V and 30V for EL34 and 120V and 25V for 6550.
I've played the EL34 amp (without the new feedback) all saturday afternoon and I liked it alot.
It has alot of definition and it performs very well on many kind of music.
It pushed my Klipsch to very high levels, far beyond previos versions.
I hope to have one evening during the week to test the new feedback and report the results.
The new feedback will use an OPAMP to amplify the difference between a scaled down version of the output of the amp minus the input signal, so basically only the distortion. Simulations give ppm distortion at 1 Wrms and a Rout of 0,09 with that specific feedback, and a maximum power above 50 Wrms.
I need to find a load to test the full power in real conditions.
6550 version has only been simulated until now, and with an Raa of 4k with UL at 23%, with the same 460V B+, will give around 80Wrms.
Here below you can find the modified schematics.
I've recently upgraded my BHEL34 in order to test a special NFB loop I'm workin on.
I started by the last working point suggested on Mullard datasheet ( https://frank.pocnet.net/sheets/129/e/EL34.pdf ) for maximum power, slightly adapted.
I now run them at 460V and 44mA, on a Raa of 6.6kOhm and 23% ultralinear taps.
To improve the loadline of the phase splitter (so making it flatter) still being able to keep a good local feedback on the EL34, with the plate voltages below 300V and the cathode voltages above 0.9V even at maximum power, I've modified the 220k + 47k + 39k combo into a 330k + 33k + 27k combo to supply the two triodes.
The reduced UL tap increase the available power, lowers the driving requirements and also the input capacitance of the EL34, but the higher swing compared to the EL84 pushed me to increase the current flowing through the power drive, so to reduce the bias resistors of the PD CCS down to 100 Ohm. This allows to have more current to be able to swing more volts on the grids of tghe EL34.
I wont' repeat the calculations here, you can find them in my EL84 thread.
I have old boards, with PSU onboard like for EL84, so I worked on the dropping resistors to set the voltage of the powerdrive and phase inverter CCS.
I set them to 100V and 30V for EL34 and 120V and 25V for 6550.
I've played the EL34 amp (without the new feedback) all saturday afternoon and I liked it alot.
It has alot of definition and it performs very well on many kind of music.
It pushed my Klipsch to very high levels, far beyond previos versions.
I hope to have one evening during the week to test the new feedback and report the results.
The new feedback will use an OPAMP to amplify the difference between a scaled down version of the output of the amp minus the input signal, so basically only the distortion. Simulations give ppm distortion at 1 Wrms and a Rout of 0,09 with that specific feedback, and a maximum power above 50 Wrms.
I need to find a load to test the full power in real conditions.
6550 version has only been simulated until now, and with an Raa of 4k with UL at 23%, with the same 460V B+, will give around 80Wrms.
Here below you can find the modified schematics.
Hi zintolo,
Thanks for sharing. I've part way though a EL34/6550 build at the moment so your post is useful.
Andrew
Thanks for sharing. I've part way though a EL34/6550 build at the moment so your post is useful.
Andrew
@Drewaudio
Sorry for this very late reply Andrew, I lost your message.
Thanks, keep us updated on your amp!
Sorry for this very late reply Andrew, I lost your message.
Thanks, keep us updated on your amp!