A few weeks after signing up, I just wanted to say THANKS for such an amazing website / resource. I feel like a kid in a candy shop. Everything I've ever wondered about seems to be here (and plenty I never even though about!). I keep starting to read one thread, which then takes me off down 30 other ideas!
Anyway, after many years, I'm considering a new build to help pass the time during these lockdown blues. My original idea was to just build a stock jcm800 50W, but with a f/switch relay for norm / high. But then I thought I'd add 2 different EQ stacks (Marshall & Fender) and switch between them. And then I thought let's just build 2 preamps - jcm800 & Bassman & switch... which has now changed again into jcm800 + Dumble preamp!
The idea is really just to experiment & see what works best. I've got a good test chassis, and I'm contructing the preamps and controls on homemade circuit boards (hand drawn!), with most of the components mounted trackside up, so I can easily change and adjust stuff.
I currently have the hardware to build a 50W El34, but I'm very intrested to experiment with the latest fashion for low wattage / high gain amps. Can I stick with the 50W / El34 hardware but just reduce the ht voltage (half?) using a power resistor??
Anyway - thanks again!
Thank You! / Low Power jcm800?
Moderators: pompeiisneaks, Colossal
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Stevem
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Re: Thank You! / Low Power jcm800?
It’s good your just in the design phase of things!
Two of the best ways of cutting down on volume , but still retaining the needed effect of over driving the output tubes is by means of a inductive/ resistive load box hooked up to the amps speaker output, or by building into your amp a power scaling kit from London amps.
A 50 watt EL34 amp has some 450 volts of V+ in the output stage and dumping enough off that off to trim away 1/2 of that 50 watts would take a very large heat sinked resistor which would dump a lot of extra heat into the amps chassis, plus take up a lot of space.
Another thing you can do to lower your amps output power by 33 percent is to place a speaker load of twice the called for impedance on your output transformers taps,
For example hooking up a 8 ohm load to the transformer’s 4 ohm tap will cut down that 50 watts by 33% leaving you some 34 watts of output.
The other good thing this move does as does the power scaling set up I talk about above is to extend the output tube life a lot, driving your amp into a load box does not provide that benefit!
Another way to get less then 50 watts would be to build the amp with 4 EL84 tubes or four 6V6 tubes.
In this way you can switch out 2 tubes and get a half power setting.
The main down side to this is that in the half power setting you are still running off a power supply that can provide the current needs for a 50 watt amp so you still do not quite get the feel of a 25 watt cranked up amp.
Also make note that due to the way the Human ear precives loudness that cutting 50% of the wattage down does not make for a full 1/2 volume setting, this is why in my opinion the London amps power scaling method is the best way to go since you can get the amp down to 3 watts of output if you care to, or the power scaling method along with a load box is surly the next best thing to just having a amp of the wattage you do need at the time!
Two of the best ways of cutting down on volume , but still retaining the needed effect of over driving the output tubes is by means of a inductive/ resistive load box hooked up to the amps speaker output, or by building into your amp a power scaling kit from London amps.
A 50 watt EL34 amp has some 450 volts of V+ in the output stage and dumping enough off that off to trim away 1/2 of that 50 watts would take a very large heat sinked resistor which would dump a lot of extra heat into the amps chassis, plus take up a lot of space.
Another thing you can do to lower your amps output power by 33 percent is to place a speaker load of twice the called for impedance on your output transformers taps,
For example hooking up a 8 ohm load to the transformer’s 4 ohm tap will cut down that 50 watts by 33% leaving you some 34 watts of output.
The other good thing this move does as does the power scaling set up I talk about above is to extend the output tube life a lot, driving your amp into a load box does not provide that benefit!
Another way to get less then 50 watts would be to build the amp with 4 EL84 tubes or four 6V6 tubes.
In this way you can switch out 2 tubes and get a half power setting.
The main down side to this is that in the half power setting you are still running off a power supply that can provide the current needs for a 50 watt amp so you still do not quite get the feel of a 25 watt cranked up amp.
Also make note that due to the way the Human ear precives loudness that cutting 50% of the wattage down does not make for a full 1/2 volume setting, this is why in my opinion the London amps power scaling method is the best way to go since you can get the amp down to 3 watts of output if you care to, or the power scaling method along with a load box is surly the next best thing to just having a amp of the wattage you do need at the time!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Speedypancake
- Posts: 27
- Joined: Tue Jan 12, 2021 12:19 pm
Re: Thank You! / Low Power jcm800?
Yup! The thinking about it stage is my fav 
And thanks for the advice. I'll keep the attenuator load option in mind, although I'm sure there are whole websites dedicated to just this.
Now thinking I might just get an extra transformer with a lower voltage just to power the EL34's... I've seen so much good press about the new PRS 15W (El34 pair) amp I'm keen to experiment.
And thanks for the advice. I'll keep the attenuator load option in mind, although I'm sure there are whole websites dedicated to just this.
Now thinking I might just get an extra transformer with a lower voltage just to power the EL34's... I've seen so much good press about the new PRS 15W (El34 pair) amp I'm keen to experiment.
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
1 others liked this
Re: Thank You! / Low Power jcm800?
Well it’s your money, but still only have 2 different V+ voltages still does not give you as wide of a range of output volume levels as does a power scaling arrangement!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Speedypancake
- Posts: 27
- Joined: Tue Jan 12, 2021 12:19 pm
Re: Thank You! / Low Power jcm800?
Well I don't want to waste money! Is there a particular "power scaling" circuit here that you recommend??
Re: Thank You! / Low Power jcm800?
Steve made a specific reference to this: https://londonpower.com/electronics/pow ... ixed-bias/ Sometimes it is better just to buy than build. Schematics can be found if you really want to try your hand at building from scratch.
I wanted to offer a comment about your desire for lower power. I assume you are going after same tone but something you can use at home. 50W is going to shake your windows. It's for a good sized club. It's too loud for home use, or at least, that's my view of it. Something that many people don't realize is how much you need to reduce power to get a meaningful reduction in volume. It isn't linear. 25W is nearly as loud as 50W. To get a 50% reduction, you'd need to go from 50W to about 5W.
As long as you have the time and resources, you might think about going in another, more versatile direction. Build a rack system with 3 modules. One can be a preamp chassis with two different flavors. Then build your 50W power amp and just allow it to be a 50W amp. Build another module that is a low power amp. Then you can plug your preamp into one or the other. Over time, you can add more modules, allowing you to mix and match.
My favorite low power tube is the 6DL5/EL95. These are out of production but can be found for relatively low prices, so there is low risk in buying them. These will make a 4-5W amp in push pull configuration. You get all the benefits of a push-pull pentode amp at a very civil volume.
http://www.r-type.org/exhib/aaa0406.htm
I wanted to offer a comment about your desire for lower power. I assume you are going after same tone but something you can use at home. 50W is going to shake your windows. It's for a good sized club. It's too loud for home use, or at least, that's my view of it. Something that many people don't realize is how much you need to reduce power to get a meaningful reduction in volume. It isn't linear. 25W is nearly as loud as 50W. To get a 50% reduction, you'd need to go from 50W to about 5W.
As long as you have the time and resources, you might think about going in another, more versatile direction. Build a rack system with 3 modules. One can be a preamp chassis with two different flavors. Then build your 50W power amp and just allow it to be a 50W amp. Build another module that is a low power amp. Then you can plug your preamp into one or the other. Over time, you can add more modules, allowing you to mix and match.
My favorite low power tube is the 6DL5/EL95. These are out of production but can be found for relatively low prices, so there is low risk in buying them. These will make a 4-5W amp in push pull configuration. You get all the benefits of a push-pull pentode amp at a very civil volume.
http://www.r-type.org/exhib/aaa0406.htm
Re: Thank You! / Low Power jcm800?
My two pennies: The Master Volume on the JCM 800 is great. I am VERY happy playing quietly (but gained out!) on my recently built 50w 2204. I tried using my 200w Weber reactive power attenuator, and the result was fine, but no better than just using the MV. I would bet greencashmoney that power scaling would be meh as well. Just use the MV!
Second observation: Cathode-biased power scaling is simple and cheap, requires only a few components and can be built easily by hand. But fixed-bias power scaling is a lot more complicated, and buying a pre-made solution would be a big time and headache saver.
But again, see #1.
Second observation: Cathode-biased power scaling is simple and cheap, requires only a few components and can be built easily by hand. But fixed-bias power scaling is a lot more complicated, and buying a pre-made solution would be a big time and headache saver.
But again, see #1.
I build and repair tube amps. http://amps.monkeymatic.com
Re: Thank You! / Low Power jcm800?
I’ve only been happy with power scaling cathode biased EL84s. Anything other was meh.
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Speedypancake
- Posts: 27
- Joined: Tue Jan 12, 2021 12:19 pm
Re: Thank You! / Low Power jcm800?
OK - thanks all for your thoughts & suggestions. The main idea for this project is to experiment, so looking forward to trying stuff out to see what works (& what doesn't). I'll post some progress once I've started building.
Cheers
Cheers