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01-19-2024, 05:55 AM
(This post was last modified: 01-19-2024, 06:11 AM by physics.
Edit Reason: Remembered a relevant forum post.
)
I was wondering about the dropping resistors myself, I'd guess that the person before me wanted to bring the plate voltages closer to EL84 datasheets. I'd read in the TUTs that the actual allowable plate voltages are higher than the data sheets, if I recall correctly it was because the rating system used was often not "max values before destruction" but instead "headroom for application variations", in particular the design center system which was eventually update to the design maximum system. I tried looking in the TUTs for what EL84's can actually take on their plates when I first started work on this amp, however I didn't find anything for those particular tubes. Maybe I missed it. I didn't find a method for determining actual maximum voltages for tubes either, is this something you find by digging around for data sheets that give "absolute maximum" ratings or can it be calculated from the design center/maximum values?
Edit: Actually, I think something was mentioned in passing on this forum in another thread, about Seymour Duncan knowing how to read datasheets and protect EL84's while running them at 450V on the plate using high screen resistors (I assume 1k at least?).
Regarding the screen resistors, I had thought I'd seen you state either on this forum on in one of the TUTs that for smaller tubes like the EL84 that 1k is not enough and it should be 2k2 or something like that instead. If 1k is fine though then that is less parts to replace.
As to the overall goal, it is to get maximum power on a budget. There are actually two amps my friend has the choice of being rebuilt. One, this CHB-20 which is already well on it's way to being a guitar amp, and two, a CHB-50 that is still in it's PA form. Which one he takes depends on which one is cheaper to bring up to snuff, which includes being in the ballpark of at least 20W. The CHB-50 should be loud enough without a doubt, though it has all the original carbon comp resistors, old electrolytics, wacky input/output jacks, etc. The CHB-20, since it's already been rebuilt with new 'lytics, resistors, choke, etc. could end up being cheaper if it doesn't take many components to get it pushing around 20W, hence why I've been screwing with the cathode bias arrangement I found it with before switching to fixed bias. I was hoping a resistor swap or two would bring it into Class A2, with about 17W output, but that hasn't been the case so far.
In the name of reliability and max output, I think switching to fixed bias would be a good choice, as you mentioned. However, I'm largely bound by what my friend wants here unless I start investing my own money into the project. I'm mostly just doing my best to present him the options and risks and provide suggestions on what I think should be done for reliability, repairability, etc., but if he wants to run the tubes at 12W each after being made aware of all the risks in the name of not blowing the budget now and just paying for it later when tubes die prematurely, transformers need replacing, etc, then that's likely what will be done unfortunately. I think I can convince him to go fixed bias though.
I'll have to keep the Body control in mind, though I think he'll choose a colder tone over more parts. We'll see though, thanks for suggesting it.
With regards to splitters, I didn't know you felt so strongly about the Schmitt. From reading TUT, I got the impression that you favored the Schmitt for your designs.
As for the numbers, I agree that they look an awful lot like class-A numbers, however my issue is that those particular numbers were obtained /with/ a bypass cap. Removing the bypass cap gives me even less, as shown in the second post. Per TUT2 and the tung sol data sheet I'd expected somewhere around 12*1.414 = 16.9W with the cap, though admittedly the bias voltage isn't quite right per TUT2's recommendation.
Regarding the CHB 20's output, yes my understanding is that it normally would output around 20W, though I have not gone through all the details to verify that based on the original schematics, I'm only going off the word of other techs who have worked on them before.
I don't have much of an idea of the power transformer specs unfortunately. I can say that without any tubes plugged in, the voltage doubler outputs about 480V, and that the original tube complement plus pilot light for the amp drew about 1.85A from the heater winding compared to the 2.5A of the current tube set (with pilot light removed). Back of the amp is labeled for 90W and a 1A Slo-Blo, though the previous owner installed a 2A Slo-Blo, presumable because of the extra heater current but I have not verified if that is too large yet.
I'm thinking that later today I'll swing by the electronics shop and pick up parts to convert to fixed bias, worst case is I uninstall it at my friend's request and get to use the parts in my own builds.
Edit: Actually, I think something was mentioned in passing on this forum in another thread, about Seymour Duncan knowing how to read datasheets and protect EL84's while running them at 450V on the plate using high screen resistors (I assume 1k at least?).
Regarding the screen resistors, I had thought I'd seen you state either on this forum on in one of the TUTs that for smaller tubes like the EL84 that 1k is not enough and it should be 2k2 or something like that instead. If 1k is fine though then that is less parts to replace.
As to the overall goal, it is to get maximum power on a budget. There are actually two amps my friend has the choice of being rebuilt. One, this CHB-20 which is already well on it's way to being a guitar amp, and two, a CHB-50 that is still in it's PA form. Which one he takes depends on which one is cheaper to bring up to snuff, which includes being in the ballpark of at least 20W. The CHB-50 should be loud enough without a doubt, though it has all the original carbon comp resistors, old electrolytics, wacky input/output jacks, etc. The CHB-20, since it's already been rebuilt with new 'lytics, resistors, choke, etc. could end up being cheaper if it doesn't take many components to get it pushing around 20W, hence why I've been screwing with the cathode bias arrangement I found it with before switching to fixed bias. I was hoping a resistor swap or two would bring it into Class A2, with about 17W output, but that hasn't been the case so far.
In the name of reliability and max output, I think switching to fixed bias would be a good choice, as you mentioned. However, I'm largely bound by what my friend wants here unless I start investing my own money into the project. I'm mostly just doing my best to present him the options and risks and provide suggestions on what I think should be done for reliability, repairability, etc., but if he wants to run the tubes at 12W each after being made aware of all the risks in the name of not blowing the budget now and just paying for it later when tubes die prematurely, transformers need replacing, etc, then that's likely what will be done unfortunately. I think I can convince him to go fixed bias though.
I'll have to keep the Body control in mind, though I think he'll choose a colder tone over more parts. We'll see though, thanks for suggesting it.
With regards to splitters, I didn't know you felt so strongly about the Schmitt. From reading TUT, I got the impression that you favored the Schmitt for your designs.
As for the numbers, I agree that they look an awful lot like class-A numbers, however my issue is that those particular numbers were obtained /with/ a bypass cap. Removing the bypass cap gives me even less, as shown in the second post. Per TUT2 and the tung sol data sheet I'd expected somewhere around 12*1.414 = 16.9W with the cap, though admittedly the bias voltage isn't quite right per TUT2's recommendation.
Regarding the CHB 20's output, yes my understanding is that it normally would output around 20W, though I have not gone through all the details to verify that based on the original schematics, I'm only going off the word of other techs who have worked on them before.
I don't have much of an idea of the power transformer specs unfortunately. I can say that without any tubes plugged in, the voltage doubler outputs about 480V, and that the original tube complement plus pilot light for the amp drew about 1.85A from the heater winding compared to the 2.5A of the current tube set (with pilot light removed). Back of the amp is labeled for 90W and a 1A Slo-Blo, though the previous owner installed a 2A Slo-Blo, presumable because of the extra heater current but I have not verified if that is too large yet.
I'm thinking that later today I'll swing by the electronics shop and pick up parts to convert to fixed bias, worst case is I uninstall it at my friend's request and get to use the parts in my own builds.
