Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on February 19, 2025, 08:32:51 am
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let's try this again. hopefully some clarity. I have this circuit with an unused triode on V1. To get a bit more fullness out of this stage i'd like to parallel V1. I dont want it switchable. Would i just jumper the plates and grids together on the socket? For the cathode i'm not clear on what i need to do . Some say tie them together and halve the resistor value. some say double the resistor value. some folks say the cap doesnt need any change as well. What say you?
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You should halve the value on a shared cathode resistor like Leo did on the first stage of the 5E3 (so 820 instead of 1.8k), although he didn't bother on the 5F6A, 6G3, etc., so sirens will not go off in the distance if you don't bother either. Check out this thread for some further guidance: https://el34world.com/Forum/index.php?topic=22434.0
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so like this?
(https://i.ibb.co/C5zt6qNt/Untitled.png)
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Cool idea!
I've built this circuit, and it is pretty aggressive so I would half the plate load resistor, from 220k to 100-120k as well as the cathode resitor.
I hope you will let us know how this turned out :icon_biggrin:
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If you simply want less output impedance, then "halving" the tied cathode resistor value is the "right" answer. By Ohm's Law, since you are doubling the current through the shared cathode resistor, in order to maintain the same voltage drop (eg, you are keeping the cathode bias the same), you need to decrease the resistor in half.
That said, I've built several Hoffman Blues Junior Conversion amps, in which V2 is a shared triode. Doug kept the usual 1.5k cathode resistor, and shared the 100k plate resistor, and it sounds great to me. I did try halving the cathode resistor in one build, but found no difference in tone. I did not plot out the load lines for this shared triode, but that might be fun.
Doubling the value of the shared resistor would quadruple the bias point.
With the SLO series, I would probably build it with a wasted triode half first, and experiment later. These high gain badboys have demanding specs for keeping the distortion in check. You have to get it just right for the amp to sound good -- my experience only, and maybe others have had different experience. When you do get right, these are fun amps!
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thanks all. so the 1uf cap is okay to be shared? what i'm building is a similar circuit not this slo amp but a preamp. I already built one without the parallel'd V1 . Gonna build another and try the parallel V1.
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thanks all. so the 1uf cap is okay to be shared? what i'm building is a similar circuit not this slo amp but a preamp. I already built one without the parallel'd V1 . Gonna build another and try the parallel V1.
You can do that.
Or you can make it more 'interesting' by separating the cathodes and having different biasing on each one. (but keeping the output from both plates going through one plate resistor and coupling cap). That way you might get more 'colour' from how hard you play the guitar or what pedals you use in front.
But really, I think you're missing out if you don't have two inputs, one for each triode, with one of them doing both triodes at once and the other just doing one triode, but when both inputs are plugged in you have a stereo input that works with the stereo out on a pedal board. YMMV