Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on November 03, 2022, 09:16:15 am
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I was looking at some fender schematics that use a pair of 6V6 and the ones that use a resistor instead of a choke vary all the way from 10k down to 470R. What is the reasoning for such wide ranging choices?
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a schematic example would help
so guessing you are referring to the PS dropping R's;
besides the RC filtering factor, they set tap voltages, which are determined by some engineer for some reason, typically not given out to common folk. :icon_biggrin:
occasionally the engineers might listen to a guitarist when they say "IT"S anemic, fix it", so they tweak as needed to placate the artists among us
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Just look at fenders with 6V6. I just googled fender schematics 6v6 and looked at several including tweed and BF deluxe, harvard and a couple others i don't recall. I wanted to try one in place of the huge (20H) choke in my amp but after looking at those i have no idea what value to try. I assume i can just look at it like the other dropping resistors tho i also think large values are gonna really introduce excess sag. So i'm wondering whether to go low value and if so what. Looking at those schematics doesn't help because they are so wide ranging i couldn't get a idea from them what to use.
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Try this:
https://www.duncanamps.com/psud2/index.html (https://www.duncanamps.com/psud2/index.html)
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I was looking at some fender schematics that use a pair of 6V6 and the ones that use a resistor instead of a choke vary all the way from 10k down to 470R. What is the reasoning for such wide ranging choices?
Look at anode characteristics; higher screen grid voltage will allow higher anode current.
Low values of resistance between the OT CT and the screen grid HT nodes will keep screen grid voltage high and so support high anode currents. So chokes, ie very low resistance, tend to get used for designs that are able to achieve a high power output.
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That download link doesn't work.
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Worked fine for me... :dontknow:
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Yeah, the INITIAL link to the page works but the link to the windows version of the app on that page doesn't.
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I would try 1K , 2 watt .
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I would try 1K , 2 watt .
Well, not sure how much of what i am liking is your suggestion or just huge 20H choke vs resistor, but i really like the result. Not sure how it will translate in a band situation but no matter because it may be relegated to home amp anyways. Thanks for the suggestion. Save a bit of weigh too, as the choke is bigger than a lot of small amp OT's ! It was only in there because when i fist built this think it was a EL34 amp and when i changed the OT/PT and started running 6v6 i just left the choke. But it was just hyper stiff feeling. Amp is much more transparent sounding/feeling now. Maybe a small choke like 3H or such might be good but right now i'm liking this and have no desire to do that.
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That download link doesn't work.
There appear to be some issues with the server, I've just been on chat to support.
The files stored at https://www.duncanamps.com/psud2/download.html were just legacy files anyway and are way out of date so I've updated the page to reflect this.
All downloads can now be found at the Files area on this groups.io page which is on the left hand bar if you are accessing this via the web, or at the following link if you are reading this by email: https://groups.io/g/duncanampspsud/files
Regards,
Duncan
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Is there really a "huge" difference? The tubes don't see this resistor. Only voltages and cleanliness.
10k makes better filtering for less money.
470r runs screens hot for lively sound.
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Is there really a "huge" difference? The tubes don't see this resistor. Only voltages and cleanliness.
10k makes better filtering for less money.
470r runs screens hot for lively sound.
Yeah, there really is. I may not b a tech but the difference is not about whats happening theoretically, it just sounds or more importantly feels a lot different. Totally different amp. Screens are 470R and i agree. Big screen resistors have never sounded good to me. Too dull and compressed, even when the overall feel is super stiff due to that big choke. But eliminating the choke did wonders *for this amp*. It's funny because i have had the same choke in a number of amps and never liked the stiffness but it was always the lesser of 2 evils compared to a resistor. Somehow tho in this amp, maybe because it;s the only 6V6 amp that choke has been in, the resistor not only wins but is much better then just the lesser of 2 evils. I've been going round and round trying to get this amp to feel and sound the way i like amps to feel and sound and nothing worked till this. So whatever the technical reasons are, i just now what I experienced.
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Is there really a "huge" difference? ...
Yeah, there really is. ... the difference is not about whats happening theoretically, it just sounds or more importantly feels a lot different. Totally different amp. ...
If you spend a couple-decades, you can experiment enough to make appropriate circuit-changes based only on "feel" and "intuition."
Or you could listen to PRR, spend a couple-weeks discussing "What did you mean when you said ____," and discover the underlying theory that "predicts the feel."
(HINT: PRR has both the technical & musical chops to get to exactly what you want. Over almost 2 decades, I've found the more I learn, the more I understand & agree with what he says)
... The tubes don't see this resistor. Only voltages and cleanliness.
10k makes better filtering for less money.
470r runs screens hot for lively sound.
... Big screen resistors ... Too dull and compressed ... But eliminating the choke did wonders ... the resistor not only wins but is much better ...
Theory Predicts "Feel":
Circuit 1: Choke ---> Cap ---> (Smaller) Resistor ---> Tube
Circuit 2: (Bigger) Resistor ---> Cap ---> Tube
Circuit 1: Provides the most output power and fastest, least-compressed response if when there is no-resistor between the Cap and Tube. But we put a resistor between Cap & Tube because if the supply voltage gets high enough, the Tube's screen can run over-hot. This is where PRR is saying "runs the screens hot for lively sound."
Circuit 1 Modified: However, you may have experimented with something much bigger than 470Ω between Cap and Tube in Circuit 1. Screen gets a lower voltage, and some more protection against being over-hot. But compression can go way up (when using 2k-4.7kΩ rather than 470Ω), and the player might feel this sounds "too choked."
Circuit 2: The tube's screen will amplify an AC signal, so we need to clean hum off the voltage applied to the screen. Circuit 1 did this with a choke, but these cost more than a resistor, so Circuit 2 uses a large-value resistor to clean the ripple out cheaply. The Resistor works against the capacitance of the Cap to filter ripple/hum from the power supply, so PRR says, "10k makes better filtering for less money."
Circuit 2 also doesn't (normally) have a resistor between Cap and Tube. Normally this means "less compression" because there is nor Resistor right before the tube to drop screen voltage (a plate current capability) when the Tube tries to make max power. But the (10kΩ) Resistor in the power supply is a large value, and it limits the current to the Cap somewhat when the Tube's screen tries to make max power & draw lots of screen current.
Higher Screen Current x Larger Resistor = More Volts Dropped ---> Less Volts at Cap ---> Less Volts to Tube Screen
There is still compression with Circuit 1, but less because the Cap is averaging the Voltage Reduction.
This means the Cap is also Averaging the Compression (to a smaller amount)
Bottom Line: you get "some compression" but perhaps "not Too Much compression" with the big resistor in the power supply versus having a big resistor between the Cap & Tube.
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... huge 20H choke vs resistor ... when I ... started running 6v6 i just left the choke. But it was just hyper stiff feeling. ...
It matters enormously how loud you play the amp vs its overall power capability.
- People who play their amps quietly at home often want a looser power supply with more compression/give.
- Guys who play at/beyond the amp's limit of clean power (onstage or via attenuator/re-amper/etc) will sometimes object to intermodulated-hum and demand a cleaner power supply.
- And what seemed like a "stiff power supply" at low volume is pushed to its limit at high volume. AC30 or (non-master) 100w Marshall may sound stiff at low volume & much more compliant when distorting.
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- People who play their amps quietly at home often want a looser power supply with more compression/give.
- Guys who play at/beyond the amp's limit of clean power (onstage or via attenuator/re-amper/etc) will sometimes object to intermodulated-hum and demand a cleaner power supply.
- And what seemed like a "stiff power supply" at low volume is pushed to its limit at high volume. AC30 or (non-master) 100w Marshall may sound stiff at low volume & much more compliant when distorting.
I think you may be right. Now that i think about it the first time i used it in a band setting after pulling it out of storage and redesigning it was last saturday, and thinking about it i don't recall it feeling too stiff at all. That was WITH the choke. I love the tone i am getting now w/o the choke but truth is it DOES feel loose to the point where in the band i may feel a loss of punch and/or definition. So I may put the choke back and make it switchable for either way then see how it sounds next time out in about 3 weeks. However, i need to relocate it due to putting a different speaker in where the choke would be in the way where it was. Can anyone tell me if there is a part of the circuit to try and keep the choke away from that might cause issues? Or is that even an issue? Not a lot of space but if need be i could mount it inside the combo cab and extend the wires. At least for the purpose of seeing which works best in the band.
Edit: Fixed mangled Quote. -- HotBluePlates
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... Can anyone tell me if there is a part of the circuit to try and keep the choke away from that might cause issues? ...
When a choke is near input circuitry it can be problematic unless there is a shield added. Also, the choke should be rotated compared to the output transformer if they are near, so that it doesn't couple hum into the output transformer.
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It would have to be fairly close to the input so i think i will pass till after the next outing in a few weeks and see how it fares. I should have a good idea at that point. I must say tho, i cranked it a bit ago (have to wait till neighbors aren't round :icon_biggrin: ) and was surprised that it still seems to have a lot of punch loud so i have a feeling it'll be fine. Time will tell.