Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: uki on January 09, 2016, 01:00:12 pm
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Hey guys !
Input jacks, I always wondered why so many in old amps, high impedance, low impedance, then the least known thing about it, the link through a cable from the first to the second channel, I had tube amps for years with 4 input jacks and I've never imagined this link trick, I've come to know it in the last year watching this guy videos.
Here one interesting note on the input jacks, watch from 1:40 to 2:35
http://www.youtube.com/watch?feature=player_detailpage&v=zsjts0I4hOM#t=100 (http://www.youtube.com/watch?feature=player_detailpage&v=zsjts0I4hOM#t=100)
Knowing that the Fender amps jacks are set for strat type (single coils) and Les Paul type (humbucker guitars) if that information is really correct, well happens that my guitar have one of each and onboard pre-amp, plus effects rack Roland GP-16. I have been using the low impedance jack, but the signal isn't as strong as I would like, and in the high impedance jack it is way too strong and go on saturation too easy or it seen like it.
Is there a way to tinker with the input jacks to adjust it to my setup behind the amp? The low having a little more signal and the high less?
Also if the extra jacks are replaced with switches for the low/high and a switch to link the channels instead of a cable, so from 4 jacks the mod would be one jack and 3 switches(not going to plug more than one guitar at once anyway). What kind of switches is needed, how would it be implemented, how the schematic would be for those switches?
Thanks in advance!
PS. Uncle Doug's draw. Thanks.
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An on-on-on double pole switch can give you channel A, B, or both:
http://www.carlingtech.com/basic-circuit-function#17 (http://www.carlingtech.com/basic-circuit-function#17)
As far as diddling the circuit to cut the Hi signal a bit and increase the Lo signal, look at the voltage divider in the Lo Jack. However, remember that with the Hi input, the 1 meg is your grid return resistor. With the Lo input, the grid return resistor is reduced to 68K. That also reduces the gain from the first stage, in addition to the voltage divider.
Page 8 of Sluckey's odds and ends shows how typical input jacks work:
http://sluckeyamps.com/misc/Amp_Scrapbook.pdf (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf)
It is important to understand that the resistors perform different functions depending on which jack you use. The Hi input has a 34K grid stopper and a 1 meg grid return. The Lo input has a 68K grid stopper and a 68K grid return.
I modified the standard Fender input jacks on one project, but my objective was to make the Lo Jack as compatible as possible with a specific harmonica mic in terms of input impedance. It also reduced the volume difference between Hi and Lo.
Hope this helps,
Chip
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Hey Chip thanks for the reply !!
An on-on-on double pole switch can give you channel A, B, or both:
http://www.carlingtech.com/basic-circuit-function#17 (http://www.carlingtech.com/basic-circuit-function#17)
I've been searching for a model to follow, so this is what I found to make the channel NOR/both/VIB switch, https://robrobinette.com/images/Guitar/5E3P_Build/5e3_Channel_Selector_Switch_small.png (https://robrobinette.com/images/Guitar/5E3P_Build/5e3_Channel_Selector_Switch_small.png) This part seen to be easy enough to implement.
Now here is where things get tricky...
As far as diddling the circuit to cut the Hi signal a bit and increase the Lo signal, look at the voltage divider in the Lo Jack. However, remember that with the Hi input, the 1 meg is your grid return resistor. With the Lo input, the grid return resistor is reduced to 68K. That also reduces the gain from the first stage, in addition to the voltage divider.
Page 8 of Sluckey's odds and ends shows how typical input jacks work:
http://sluckeyamps.com/misc/Amp_Scrapbook.pdf (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf)
It is important to understand that the resistors perform different functions depending on which jack you use. The Hi input has a 34K grid stopper and a 1 meg grid return. The Lo input has a 68K grid stopper and a 68K grid return.
I did not find any examples to follow so far.
Ok so if the 68k is increased to lets say to 100k then it will reduce the High input but also will reduce the Low even further, I have no clue how to reduce the High and increase the Low at the same time. Is there a way to change both separately when replacing one jack for a switch, maybe adding extra resistors in the switch before the actual resistors(68,68,1m) without changing them?
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... I always wondered why so many in old amps, high impedance, low impedance, ...
I don't see the High & Low inputs as "High Impedance" and "Low Impedance". I see it as "Higher Gain" and "Lower Gain".
You guitar pickup is going to be around 4-12kΩ at the extremes. 68kΩ or 1MΩ are both "high(er) impedance" compared to the source. However, the Low Input on the typical amp is a voltage divider passing 1/2 the signal to the first gain stage, if you don't plug in anything to the High Input.
If you do plug something into both the High and Low Inputs, then they both see the same path to the 1st tube stage, but just have isolation resistors between them (so turning one guitar's volume off doesn't kill the sound from the other guitar).
... Knowing that the Fender amps jacks are set for strat type (single coils) and Les Paul type (humbucker guitars) if that information is really correct ...
While we may think of those inputs jacks like this now, and people may have used them in this way in practice, I strongly doubt Fender designed them with that in mind. Fender was selling guitars as well as amps, and Gruhn has noted that Fender guitars probably sold very well because they had great amps to mate with the guitars. I therefore doubt Fender cared if their amps worked well with Gibson's product.
Besides, if your guitar is too hot going into the input, you just turn your guitar volume down. I myself didn't find much useful difference between the High & Low Jacks. These days if I build something I just use a single input jack.
Look at the diagram you drew in your 1st post: consider how each are used as voltage dividers.
For the #1 input, there is a 1MΩ to ground, and a 68kΩ between the jack and the tube grid. The grid is like an open circuit, and all the signal applied at the jack (and across the 1MΩ) goes to the grid unattenuated.
For the #2 input, The signal at the jack sees a 68kΩ resistor in series with another 68kΩ resistor, with ground on the other side. The tube grid is connected to the middle of these resistors, and that gives an attenuation of 1/2 (or 50% reduction, or -6dB, whatever you prefer). It's not the low total compared to the 1MΩ that gives the lowered gain, but the voltage division set by the ratio of the resistors.
Further, Dave Funk pointed out that Fender pickups tended to be 6-7kΩ, and that the best transfer of voltage from a source circuit to a load circuit happens when the load impedance is 10x the source impedance, or more. 6-7kΩ times 10 is 60-70kΩ. Look familiar?
Further, the 68kΩ resistors are used to isolate and mix the signals from the 2 jacks when both are being used. Again, the 10x value is suitable for this purpose with 6-7kΩ pickups.
And 34k-68kΩ works with the Miller Capacitance of the input stage to attenuate radio frequencies to reduce noise & interference. So the parts values have been selected and arranged to perform 3 or 4 different functions.
... I have been using the low impedance jack, but the signal isn't as strong as I would like, and in the high impedance jack it is way too strong and go on saturation too easy or it seen like it.
Is there a way to tinker with the input jacks to adjust it to my setup behind the amp? The low having a little more signal and the high less? ...
For now, let's set aside the desire to do everything under the sun, and focus only on getting a good level into your amp.
Can you not turn down the signal at your guitar or at the output of the Roland? If this just cannot be done (for whatever reason), look back at the 68kΩ resistors at Jack #2. If you make the resistor from the tube grid to Jack #1 bigger, you will pass more signal from Jack #2 to the tube grid. Try raising that resistor's value to boost your signal.
EXCEPT: Making that resistor bigger will roll off highs when you plug into Jack #1. Only you can decide if this is a problem or not. If it is a problem for you, then instead of (or along with) making the 68kΩ resistor between Jack #1 & grid bigger, try making the 68kΩ resistor at Jack #2 to grid smaller. If you go too small, you'll eventually have poor isolation when playing through both Jacks 1 & 2. Again, only you can decide if this is a problem.
I'd caution against trying to have every option possible available, at least at first. Most players can have 15 options available, but they soon settle on one (or maybe 2) which work best for them then don't use any of the others.
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As an example, if you replaced the upper resistor in your drawing with a 33K and the lower resistor with 100K, the voltage divider on the Lo input yields 75% instead of 50%. The grid stopper on the Hi input is about 25K - fine. The grid stopper on the Lo input is the 33K - also fine.
(1/33)+(1/100)=(1/24.8)
33/(33+100)=75
Note that I deliberately picked resistors with aggregate value of about 2x68K. See HBP's post.
However, a 25% signal drop probably isn't worth the extra jack. Just use the guitar's volume control as HBP said.
That help?
Chip
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I agree with everything that has been said. My solution is just turn the guitar volume down. Very convenient. Very easy.
But if you still just want to play around, this mod will work with the typical hi/lo jacks found on many Fender amps. When the pot is turned max CCW, the circuit is just like the unmodded jacks. When the pot is turned max CW, the hi input will be less hot and the Lo jack will be more hot. Set the pot somewhere in the middle to 'tune to what you want. I doubt there will be one setting that is optimum for both conditions, so tweak the pot for one or the other and re-tweak if needed.
Play with it a while. I bet you'll probably end up putting the circuit back stock.
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> my guitar have ...onboard pre-amp, plus effects rack Roland
Then there should be no problem turning-down somewhere in your added electronics.
The hi/lo was for the first hi-output pickups which maybe did not have volume controls. That was over 50 years ago.
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Hey guys thanks for all the answers, very helpful ! :smiley:
You guitar pickup is going to be around 4-12kΩ at the extremes. 68kΩ or 1MΩ are both "high(er) impedance" compared to the source. However, the Low Input on the typical amp is a voltage divider passing 1/2 the signal to the first gain stage, if you don't plug in anything to the High Input.
My guitar pickups read without the pre-amp meter at 20kOhms: single coil+dummy coil for noise cancellation does: 8.2kOhms; Humbucker 12.7kOhms this used to be a little smaller before the addition of the dummy coil, 3.3kOhms and 10kOhms respectively, the dummy coils is about 2.5kOhms.
With the pre-amp readings get really crazy, had to change the settings in the metter to 2000kOhms to have some reading and it does around 1800kOhm(what?) and keep bumping arount it. :dontknow:
The rack output reads 102.2kOhms, on or off, meter at 200kOhms, dunno if this is the way to do it.
For the #1 input, there is a 1MΩ to ground, and a 68kΩ between the jack and the tube grid. The grid is like an open circuit, and all the signal applied at the jack (and across the 1MΩ) goes to the grid unattenuated.
For the #2 input, The signal at the jack sees a 68kΩ resistor in series with another 68kΩ resistor, with ground on the other side. The tube grid is connected to the middle of these resistors, and that gives an attenuation of 1/2 (or 50% reduction, or -6dB, whatever you prefer). It's not the low total compared to the 1MΩ that gives the lowered gain, but the voltage division set by the ratio of the resistors.
Further, Dave Funk pointed out that Fender pickups tended to be 6-7kΩ, and that the best transfer of voltage from a source circuit to a load circuit happens when the load impedance is 10x the source impedance, or more. 6-7kΩ times 10 is 60-70kΩ. Look familiar?
Yes is does sound familiar and now I have a much better understanding about the input circuit in the amp, thanks for this!! :thumbsup: Still I have lots more to learn.
... I have been using the low impedance jack, but the signal isn't as strong as I would like, and in the high impedance jack it is way too strong and go on saturation too easy or it seen like it.
For now, let's set aside the desire to do everything under the sun, and focus only on getting a good level into your amp.
Can you not turn down the signal at your guitar or at the output of the Roland? If this just cannot be done (for whatever reason)
> my guitar have ...onboard pre-amp, plus effects rack Roland
Then there should be no problem turning-down somewhere in your added electronics.
The hi/lo was for the first hi-output pickups which maybe did not have volume controls. That was over 50 years ago.
In my other amp there are 2 inputs only, but it is normal and bright, the gain doesn't change, it is a way different amp and have about 40-50watts output. This jump in gain doesn't happen. I had other tube amps before(100w ones) but it was long ago, as far as I remember, the inputs did work differently this 2 amps and there was a bright/normal switch.
The GP-16 doesn't have a output pot control, the volume output can be set differently for every memory slot, can have lvl 1 output in one slot and 100 in the next. Usually I have it at 90-100 bellow it the volume goes bellow the guitar volume, unless there are things adding up like compressor, EQ and drive or distortion, other effects doesn't add volume/gain(w/e it is called) to the signal but just mix it to the signal.
So I have to edit several memory slots to turn down the output. I rarely use the guitar volume, it is always in the max, it is something I don't use, plus there is a glitch with my pre-amp, when it is engaged the ends of the pots in the guitar make a noise, this doesn't happen with pre-amp off.
I did follow the amp build to the letter, so it is set to work in the 1960-1963 era or almost, not 2016, does this count ?
I will find a workaround for the setup before the amp. But how to replace 2 jacks with 1 jack and one switch for high/low without messing with its values?
Thanks for the mod Sluckey! Thanks for the equations Chip!
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... The GP-16 doesn't have a output pot control, the volume output can be set differently for every memory slot, can have lvl 1 output in one slot and 100 in the next. ... I have to edit several memory slots to turn down the output. ...
Yep, that's a challenge of rack gear.
You guitar pickup is going to be around 4-12kΩ at the extremes. 68kΩ or 1MΩ are both "high(er) impedance" compared to the source. However, the Low Input on the typical amp is a voltage divider passing 1/2 the signal to the first gain stage, if you don't plug in anything to the High Input.
My guitar pickups read without the pre-amp meter at 20kOhms: single coil+dummy coil for noise cancellation does: 8.2kOhms; Humbucker 12.7kOhms this used to be a little smaller before the addition of the dummy coil, 3.3kOhms and 10kOhms respectively, the dummy coils is about 2.5kOhms.
With the pre-amp readings get really crazy ...
You're using a preamp in your guitar (or it should be in your guitar). The output of that preamp is/should be very low impedance compared to any amp input. Measuring resistance may not be helpful, as the preamp's opamps sit within feedback loops which lower their effective output impedance.
I would neither worry about impedance levels after the preamp (because your preamp output impedance will be a plenty-low source), nor "matching" impedance levels (because matched impedance is only important for power transfer, not voltage transfer).
... I rarely use the guitar volume, it is always in the max, it is something I don't use ...
I'm 60 years out of date; I almost only use my guitar's volume & tone controls and rarely even have a pedal between the guitar & amp. :dontknow: There's a youtube video of Joe Bonamassa demonstrating how to get 5 or 6 unique tones from a single guitar & amp channel solely by adjusting the volume & tone controls along with the pickup selector.
... But how to replace 2 jacks with 1 jack and one switch for high/low without messing with its values? ...
Do what Sluckey showed first. Does this give you a workable setting?
I'm mainly trying to talk you out of extra switches & settings if they aren't really needed. If you're just trying to mate the amp to your present arrangement, it seems easiest to figure out what works best for that guitar/preamp/rack effect setup.
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But how to replace 2 jacks with 1 jack and one switch for high/low without messing with its values?
Here's a solution for that question...
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You're using a preamp in your guitar (or it should be in your guitar).
Yes the preamp is inside the guitar, just after the guitar circuit, before the output jack, remember the thread about the preamp (http://el34world.com/Forum/index.php?topic=19379.0) under effects section.
... I rarely use the guitar volume, it is always in the max, it is something I don't use ...
I'm 60 years out of date; I almost only use my guitar's volume & tone controls and rarely even have a pedal between the guitar & amp. :dontknow: There's a youtube video of Joe Bonamassa demonstrating how to get 5 or 6 unique tones from a single guitar & amp channel solely by adjusting the volume & tone controls along with the pickup selector.
Although I do not use the volume control, I do use a lot the cap selector, the tone control, and a couple more switches in the guitar, the cap selector is modified and have 4 positions(the new 5 position ones doesn't fit in the guitar, wood would have to be removed that is a nono) so there are 4 tones, then with the flip of one switch(phase inverter), 3 different tones appear, the single coil is always on, it acts in 3 positions with the humbucker, and alone in the neck position, there is a switch for it to have the humbucker alone as well. The tone control isn't really a tone control, it does something else, bright cut and atenuator, 16 tones just with the guitar total, then the effects on top of it...one can get lost with all of it. :laugh: It is been a long process and time to get to it.
Do what Sluckey showed first. Does this give you a workable setting?
I'm mainly trying to talk you out of extra switches & settings if they aren't really needed. If you're just trying to mate the amp to your present arrangement, it seems easiest to figure out what works best for that guitar/preamp/rack effect setup.
I will do and will find out what works best, I appreciate the advice very much sir, and will listen to the experience you share
the plan is to have all the input types with just one jack plus 3 switchs(I'm still studying it(bah I still have to fix that hum problem with V5 circuit)) and free some jacks for next build haha, I'm hooked.(Switchcraft ones are expensive down here over 10 bucks, chinese ones are cheap and terrible) ;) To build amps is awesome. Maybe a small one next a Champ perhaps !
One thing I also realize here, I must play directly with the amp, no preamp or effects to feel the amp better, haven't done that enough... :think1:
But how to replace 2 jacks with 1 jack and one switch for high/low without messing with its values?
Here's a solution for that question...
Oh this is much easier than I though... Thank you so much Sluckey you're the man !!! :happy1: