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01-19-2024, 02:00 PM
Hi Guys
Arc voltage is the limit for plate voltage in a tube and this is rarely listed. Many tubes have other notes in their spec sheets that suggest the true capability. For the EL-84 / 6BQ5A, note that the pins to either side of A are left empty to provide isolation for the voltages that may be present on the anode.
Get rid of the dropping resistors and go fixed-biased as the original schematic shows and you will have 20W.
EL-84s are heater current hogs for sure, pulling 760mA each compared to a 6V6 at 450mA and 6L6 at 900mA., and a pair of them is already near the stock tube complement's full heater load (1850mA; 2x700 + 300 +150)). A cathode-biased amp tends to be garbled at and near clip, where fixed-biased amps tend to have some definition left.
You could save current by switching to 6V6s, or just change one tube to 6V6. The mix of the '84 brightness and the V's cream is quite nice. This mix would require separate Rk+Ck or fixed-bias with individual controls.
Every tech has his own style and deals with customers differently, accepting or rejecting certain customers and/or their projects. As I mentioned elsewhere, I would ask a player what the goal was tonally and/or functionally, and then tell him the cost of different options. It is up to me to interpret his goal into tech. I would choose components and circuits based on what I know to be reliable and that suit my beliefs and experience. Once the player starts demanding the use of yellow caps and green resistors and red transformers we are done. There are other techs willing to cater to those compromises.
Your friend is obviously reading forums or blogs and assuming what he reads is accurate. He may not even know what his own goal actually is? In electronics there are many ways to achieve the same goal. The 50W amp is a better choice if he wants something that will stay clean to use pedals with. The 20W may be closer to the loudness he wants with inefficient cabinets and a cranked sound. He is getting a bargain having you do the work for him. You are doing it for the experience and to help your friend and he is just paying for parts. You will end up going around in circles although in the end if he is not happy with the result it is only because he dictated what you should do rather than properly defining the goal and let you loose. It will be "your fault" either way.
I suppose the original tubes are history? You could easily make a nice amp using those with the output stage and PSU left in their stock form, either rewire the splitter and PA front-end or leave as is, and revoice the preamp.This applies to either amp. The stock power tubes are a similar construction to the '84 and will sound about the same when driven hard. The key PA point is to not have a feedback loop to the secondary - which most old PAs did not have anyway.
Did the PA work at the beginning? Did your friend assess the sound? Open the NFB and the tone would likely be how he wants. 6EU7 is like a 12AX7 with a different pinout, so it should get the same tones. The 6GW8 triode is identical to 12AX7 and the pentode is remarkably similar to the '84.
Yes, the 2A slow-blow mains fuse is not protecting the PT at all. TUT3 explains how to select fuse values.
You see Schmitt splitters in most of the TUT3,5 projects simply because that is what the iconic circuit had and what hobbyists expect. I haven't used them since 1985, and then only for a hifi amp. Concertina is king for versatility, stability and tone.
Arc voltage is the limit for plate voltage in a tube and this is rarely listed. Many tubes have other notes in their spec sheets that suggest the true capability. For the EL-84 / 6BQ5A, note that the pins to either side of A are left empty to provide isolation for the voltages that may be present on the anode.
Get rid of the dropping resistors and go fixed-biased as the original schematic shows and you will have 20W.
EL-84s are heater current hogs for sure, pulling 760mA each compared to a 6V6 at 450mA and 6L6 at 900mA., and a pair of them is already near the stock tube complement's full heater load (1850mA; 2x700 + 300 +150)). A cathode-biased amp tends to be garbled at and near clip, where fixed-biased amps tend to have some definition left.
You could save current by switching to 6V6s, or just change one tube to 6V6. The mix of the '84 brightness and the V's cream is quite nice. This mix would require separate Rk+Ck or fixed-bias with individual controls.
Every tech has his own style and deals with customers differently, accepting or rejecting certain customers and/or their projects. As I mentioned elsewhere, I would ask a player what the goal was tonally and/or functionally, and then tell him the cost of different options. It is up to me to interpret his goal into tech. I would choose components and circuits based on what I know to be reliable and that suit my beliefs and experience. Once the player starts demanding the use of yellow caps and green resistors and red transformers we are done. There are other techs willing to cater to those compromises.
Your friend is obviously reading forums or blogs and assuming what he reads is accurate. He may not even know what his own goal actually is? In electronics there are many ways to achieve the same goal. The 50W amp is a better choice if he wants something that will stay clean to use pedals with. The 20W may be closer to the loudness he wants with inefficient cabinets and a cranked sound. He is getting a bargain having you do the work for him. You are doing it for the experience and to help your friend and he is just paying for parts. You will end up going around in circles although in the end if he is not happy with the result it is only because he dictated what you should do rather than properly defining the goal and let you loose. It will be "your fault" either way.
I suppose the original tubes are history? You could easily make a nice amp using those with the output stage and PSU left in their stock form, either rewire the splitter and PA front-end or leave as is, and revoice the preamp.This applies to either amp. The stock power tubes are a similar construction to the '84 and will sound about the same when driven hard. The key PA point is to not have a feedback loop to the secondary - which most old PAs did not have anyway.
Did the PA work at the beginning? Did your friend assess the sound? Open the NFB and the tone would likely be how he wants. 6EU7 is like a 12AX7 with a different pinout, so it should get the same tones. The 6GW8 triode is identical to 12AX7 and the pentode is remarkably similar to the '84.
Yes, the 2A slow-blow mains fuse is not protecting the PT at all. TUT3 explains how to select fuse values.
You see Schmitt splitters in most of the TUT3,5 projects simply because that is what the iconic circuit had and what hobbyists expect. I haven't used them since 1985, and then only for a hifi amp. Concertina is king for versatility, stability and tone.
