Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dscottguitars on March 04, 2013, 01:22:04 am
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I'm thinking about using this for a boost. Essentially it is the high/low input circuit I see on most amps that have two inputs.
Any thoughts??
Thanks,
Daniel
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Isn't there another place to go for a boost other than the input?
I would not wire a relay like that...
What amp? Schematic?
I personally would look for somewhere in the chain of the preamp,
and place a switchable cathode bypass cap in there, or do the tone stack
lift as in the Dumble stuff... IE PAB. No noise. You can tweak the tone in any case,
for the kind of boost/tone you seek.
I'm not a fan of putting a relay on the V1 input. Noise.
There are many places to relay switch a boost...
Extra half Triode in there anywhere?
In the diagram, a typical Fender input circuit is shown.
R1 and R2 have been 68K forever... but can be whatever. Some older Boogies did not use the 68K at all,
so the tube would get hit pretty hard. Might make a good radio, if that isn't addressed somewhere...
R3 needs to be there (1Meg) for a ground reference for the grid when input is unplugged.
You could replace R2 with a 10K, and R3 with a 2.2meg, then input one... would give a boost.
Since you are not dealing with any current to speak of here a 1/4 watt resistor size is plenty.
Personally, I would find a better way for a boost.
I'm sure others will chime in with ideas.
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Any amp. And why not the V1 input? What do you mean wire a relay like that? There is no place to have a boost for both channels.
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If you would put a schematic, or at least which amp you are thinking of doing this to...
we could show how to get a boost on both channels.
You could always use an A/B box and set the 2 channels for a boost on one of them...
You have worked on/built amps before... right?
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here
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Looks like a AB 763 set up for the most part.
A Triode calculator shows a DB gain of 24 unbypassed, on the first initial stage, and 32DB bypassed.
This is also a place you can switch in and out for a boost, for either channel... which ever one is active, will do it.
The Ck on the first tube on a switch or relay.
So you have the capability for numerous boosts, and channel switching with this architecture.
Split mixer resistor values keep the gain down, unlike
the 220K one usually sees there.
This could be a place to put a relay to channel switch,
with the unused channel grounded out, also by the relay.
Either side of the channel 1 and 2 Volume pot.
You may not need the 820K's after the caps, and before the volume pots. You are dumping gain with the volume pots anyway. Or at least matching levels between the channels. I would ditch the 820k's.
I guess what you are looking for in tone and flexibility is in there... I would experiment, and see what your ears prefer.
The PI looks like you are going for Clean Clean, too.
Should be a nice sounding amp.
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Why 4W resistors ?
K
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Why 4W resistors ?
K
Yes, they would handle a few hundred volts going in there :w2:
The pre tubes would be like... yikes!
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There's one (overriding, IMHO) issue about this idea, and I bring it up in the context of the noise heartburn you've been having with your other build.
*ANY*thing you place in the signal path at this tender and very sensitive point in the signal chain is amplified something like 500-1000 times by the time it hits the speaker wires. It is a place where ambient noise is, unless aggressively controlled, perfectly capable of being just about as strong as the geetar signal you are trying to shove into the amp. The more wire and parts and anything else you elect to hang on that particular circuit section, the more it is likely to act as an antenna.
The odds, quite frankly, are against you achieving anything that does not force you to pay an awful price. You really want this part of any amp circuit to be short, sweet, to the point, and get it over to that first grid with the least amount of nonsense you can. This comes from the "school of hard knocks" department.
Because what is very important in amp design is that I don't care about your middle control and your presence control and your pre-gain sub-master "meat" control and your "dirt" control....if the overall amp can't operate in a quiet "don't know it's on" fashion, then it will not be an attractive thing. So while you are designing for those goodies and knobs and options, more than what can be placed in to any given amp, just remember, the execution has to be reasonably low noise. Futzing around with that input circuit just doesn't give good odds of that.
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There's one (overriding, IMHO) issue about this idea, and I bring it up in the context of the noise heartburn...
You are absolutely correct.
The only thing near the front of the amp I would do is a Ck switch.
I prefer the channel switching go further down the chain.
Everything else, short and sweet.
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High voltages didn't mean high currents
you really don't need 4W resistors in a preamp input
K
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Why 4 watts? That's what I have. I got a bulk of them cheap.
The 820K resistors are there because the gain is too high. Those keep the amp stable. Without them it's too much.
I cannot use the first triode as a bypass boost- there's noise there, I've tried it, even with shielded wire going to the footswitch. The other triodes for each channel don't offer enough of a boost. That's why I thought of attenuating the incoming and increasing the output by changing the 820K's to a smaller value.
It has channel switching and is very quiet. No hum or buzz in this amp. It sounds great too.
Since most amps already have this input circuit I see no reason why a relay switching between the two would add any noise. And here's a better version with less.
Also these triodes are wired together in parallel so the values attached to the plates and cathodes are adjusted to reflect that...
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And here's a better version with less.
This circuit doesn't do anything. The common and normally open contacts are jumpered together.
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Correction...
And the relay will be mounted less than an inch away.
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Correction...
That circuit won't give you any boost either. Try this...
The two 47Ks form a 2:1 voltage divider when the relay is de-energized. Only half the guitar signal gets to the tube. The voltage divider is removed when the relay is energized, allowing all the guitar signal to get to the tube. Increase the value of the shunt resistor for less boost.
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I'm confused because that's exactly how a 2 input high/low amp like Fender works. I thought about what you did and realized the 820K and 47K are now in parallel significantly changing the grid resistor value.
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I'm confused because that's exactly how a 2 input high/low amp like Fender works.
Your circuit does not do what the Fender hi/lo jacks do. Look at page 6 of this pdf for an explanation of the Fender jacks.
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf)
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I see, thanks for that link. So as I understand it, the high input is full because of the 68K/1M divider which is not really any cut and the low is cut in half due to the equal balance of shunt and load resistors, yes?
Now to get the amount of attenuation I want (or boost depending on how you look at it) I would want to raise the value of the load resistor. I'm guessing 50% is probably too much difference. Or lower the shunt as you suggested too, but I think having a bit more on the load side is better.
Thanks again...
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the high input is full because of the 68K/1M divider which is not really any cut and the low is cut in half due to the equal balance of shunt and load resistors, yes?
That's mostly right, except there ain't no voltage divider when using the hi jack. It's all about where the 1M resistor is placed. Fender places the 1M directly across the hi input jack so all the signal is developed across the 1M. Then the two paralleled 68Ks just act as a single 34K grid stopper. If the 1M were connected to the grid (like your 820K) then it would always be part of a voltage divider. 68K:1000K ain't much of a divider though.
Temporarily put a 500K pot in place of the 47K shunt. Dial in the amount of 'cut' you want then measure the pot and replace with a fixed resistor. Just remember, this really isn't a boost circuit. It's a cut circuit. I really don't think you will be happy with it, but if you don't try it, you'll never know.
So, what's going on with this amp? Last I remember you started a thread where you were about ready to "take a sledge hammer to this $650 project"? Even had some replies. I can't find that thread now. Did you get all that fixed? If so, what did you do?
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Yes I know it's a cut, but I am going to remove some of the cut that I have in place on the volume pots. Then adding the high input will give me a boost.
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So, what's going on with this amp? Last I remember you started a thread where you were about ready to "take a sledge hammer to this $650 project"? Even had some replies. I can't find that thread now. Did you get all that fixed? If so, what did you do?
I got sick of people being 'concerned' about the size of the chassis and noise issues when there were none. It happens quite a bit with people telling something won't work when it already does and no one addresses the original questions. I found two of the problems, the light popping noise when I touched the treble knob only happened when in channel one, I discovered the output wire's insulation was pressing up against the treble cap. I moved it and that was it.
Another issue was very little boost with the bypass cap engaged. That's why I came up with this idea. Last was the loud pop when turning off the amp even though I put a 0.01 cap across the switch. I put that on another post instead...
It seems people want to criticize what I'm doing rather than help. I glad you and the other senior moderators are here to learn from.
Thanks for your help.
Daniel
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Why 4 watts? That's what I have. I got a bulk of them cheap.
That is the best reason to use 4W resistors every where :grin:
K
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I haven't seen criticism, as much as folks wanting to help you with your issues.
I like the drawing you did, very nice...but gut shots always help everyone to see what is there.
Posts seen regularly here and TAG, with gut shot pix, have a great chance that someone will
see something amiss quickly.
Until everyone here has an understanding of all of individual backgrounds, and sense of humor...
it is difficult to teach/learn these skills, without putting a dent in someone else's feelings.
It is also hard to determine somebody's "tone" in their writing too.
Please don't be offended by my attempt to help you.
Constructive criticism is never an attack. We all try to help each other... irregardless.
I'm glad you hashed out some of your difficulties.
Funny how a single wire, or bad or leaky part, can drive one bananas.
Also, if you have a gain issue on any of the stages, and need to reduce;
you may reduce the value of Rg to taste and/or increase the value of the grid stoppers as well.
It is also a good idea to have a Rg (grid leak) on all preamp tubes, for a bias/reference for the next stage.
Again, values to taste.
Peace
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Well said guitardude57.
Brad :icon_biggrin:
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Constructive criticism is never an attack. We all try to help each other... irregardless.
Peace
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It would take a putz to bring this up, but maybe some "constructive criticism" is in order. Irregardless is not an accepted use of the word. The word is regardless. The prefix ir and suffix less cancel each other and make the word synonymous with regard. Even though I understand what you intend to say.
Try a little constructive criticism on your wife by telling her if she cleaned the house twice as often, it would be a nicer place to live and see if your comment is considered "never an attack".
Please keep in mind I am just having fun and have broken 2 rules in proper online communication. No insult intended.
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You are correct in all respect... :laugh: