• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Help Understanding the input circuit on 6505+

I am looking through some mods for 6505+.

It seems mods for that amp are well documented and reposted. Most of them are clearly explained, as far as the reason for them.

Can somebody explain to me what this suggestion would do, theoretically?

Short R27 and C15. The Soldano SLO100 and high-gain Marshalls just use a 68k grid stop and don’t worry about the rest.
Increase R2 to 220K


Here is the link to the schematic.

https://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-5150-II-Schematic.pdf
 
You asked this over on TGP and I had a look for R2. I don't like playing where's waldo so can you point out the approximate location?
Yes, you don't need R27 and C15, probably some engineer added them in with some hypothetical situation in mind, but get rid of them via bypassing them
 
R27 and the C20 form a high-cut network which effectively cuts HF input above 100 kHz or so.

C15 is the DC blocking, and also low-cut input filter to sift out superimposed DC coming from one's guitar that “has a box” attached that isn't well behaved. Nominally, it is both harmless and soundless.

It took awhile to find R2, but its nominal value is 210 kΩ, so changing that UP to 220 kΩ is a waste both of time and good solder. No effect, except for bragging rights.

________________________________________

See … the “problem” with a statement like “I am looking thru some mods for…” is that there is no GOAL(S), except to “make mods”. I'm pretty sure you do have a goal, and criteria that you're trying to achieve. But without so saying, “making mods” is kind of… thin.

Anyway, best of luck.
⋅-=≡ GoatGuy ✓ ≡=-⋅
 
R27 and the C20 form a high-cut network which effectively cuts HF input above 100 kHz or so.

C15 is the DC blocking, and also low-cut input filter to sift out superimposed DC coming from one's guitar that “has a box” attached that isn't well behaved. Nominally, it is both harmless and soundless.

It took awhile to find R2, but its nominal value is 210 kΩ, so changing that UP to 220 kΩ is a waste both of time and good solder. No effect, except for bragging rights.

________________________________________

See … the “problem” with a statement like “I am looking thru some mods for…” is that there is no GOAL(S), except to “make mods”. I'm pretty sure you do have a goal, and criteria that you're trying to achieve. But without so saying, “making mods” is kind of… thin.

Anyway, best of luck.
⋅-=≡ GoatGuy ✓ ≡=-⋅

Thank you for looking. And sorry to not having a screenshot. I have attached one of the input circuit.

My goal is to understand the why somebody is suggesting this circuit change. Many mods already have explanations. I couldnt find the reason somebody would do this, that is why I am asking. I will do some more research based on your response.

attachment.php


Thank you for your time.
 

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R27 and the C20 form a high-cut network which effectively cuts HF input above 100 kHz or so.⋅

Does this network also attenuate broadband or just the HF?

If it is overall attenuation, I guess I could understand removing R27 to potentially add more overall signal to the grid, but if not, then I do not see then benefit of removing it.

As for removing the cap at C15, the only explanation I have ever seen is that in improves "feel." Whatever that means? HAHA

I am inclined to leave that one as well.
 
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Does this network also attenuate broadband or just the HF?… just HF

If it is overall attenuation, I guess I could understand removing R27 to potentially add more overall signal to the grid, but if not, then I do not see then benefit of removing it. … there is no benefit, and some modest risk

As for removing the cap at C15, the only explanation I have ever seen is that in improves "feel." Whatever that means? HAHA HaHa indeed. Leave it be - it will REALLY protect the front-end from a badly behaved guitar, EFX box, or other device attached to the input.

I am inclined to leave that one as well.

To your prior comment about wanting to understand why people-recommending-various-mods made those recommendations, a lot of it comes down to believing in whatever theory of insufficiently sweet design your religion requires.

There are anti-capacitor religious nuts, who will try to remove capacitors at all costs, for any contrive reason known. By and large, under most circumstances they can get away with it. Why then do manufacturers stick in the caps?

SAFETY and RELIABILITY over decades of use, many hands doing naïve things to the front end. That's why. If you are making an amp that is supposed to survive just about any abuse at the input, well … you use DC blocking capacitors; you put in small radio-frequency quenching filters such as the one I talked on earlier. You try to protect against stupidity, and ordinary innocent mishap, bad luck, ground-faults, terrible house wiring, all that.

I cannot conceive of a reason to change a nominally 210 kΩ resistor for a 220 kΩ part (in this circuit). As I said… waste of time and soldering skills. It is about as effective as changing out the original chrome-plated wheel-nuts on one's hot rod car for shiny blue anodized ones. Certainly doesn't make the car go faster. But it is something you can point at, preen about, and get manly recognition for. If that's important to you, LOL.

Ask about other parts, and I am fairly certain I can answer as to why the choices made (on the schematic) were made as shown. I really am not fond of the schematic's designer's value-annotation terminology, but its still somewhat readable. And, at least the resistors are not stupid little long rectangles. Squiggs are better. And squiggs we have.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
 

baudouin0

Member
2019-03-22 5:41 pm
UK
I think 82KP1 means 82 with a point in the middle hence 8k2. Cannot see any advantage of removing the DC block cap and the resistor - looks correctly designed in the first place. The 39pf can change the tone of the guitar has it loads the large inductance of the pickup shaping the frequency response - but this all depends on the volume setting on the guitar too. Its more likely to prevent AM radio getting in. C15 may also reduce the LF response which is quite useful on most pickups - however hvae no clue on its value.
 
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Is R12 really 82 K?

Yes. … it is unusual to be sure, but there's a defensible reason. This puts the front end in a 'µA' conduction region instead of milliamps. The result is that it then has a wider acceptable input signal headroom. If, for instance, one opted for a more nominal 2.4 kΩ, capacitor bypassed cathode bias, with the 12AX7 valve, the cathode would float at what, 1.2 volts?

Well, at 1.2 V, when an input signal's VPEAK-PEAK exceeds ±1.2 V, then the grid goes into conduction, changing from a nominally near–1-MΩ front facing impedance to something quite nonlinear. This is normally not-a-good-idea.

However, running an 82 kΩ resistor as the cathode bias allows the tube to run with a bias of maybe 4 volts or so (its hard to determine from the available 12AX7 charts! But let's assume that the 82 kΩ was carefully determined thru empirical means.). This increases the available input swing headroom accordingly.

It also puts the nominal power dissipation of the valve at a VERY low value. So, if some 'dumb device' that happens to have power-mains on the tip of the plug is jacked in, the 100+ VAC of swing isn't going to burn out the valve.

Its a compromise, but one wrought — I would imagine — from decades of having to fix returned amps that had some idiotic bad-device or bad-wiring plugged in. Wah, wah, wah, I bought this and after 3 days it broke! Wah, wah, wah.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
 
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baudouin₀;6081642 said:
I think 82KP1 means 82 with a point in the middle hence 8k2. Cannot see any advantage of removing the DC block cap and the resistor - looks correctly designed in the first place. The 39pf can change the tone of the guitar has it loads the large inductance of the pickup shaping the frequency response - but this all depends on the volume setting on the guitar too. Its more likely to prevent AM radio getting in. C15 may also reduce the LF response which is quite useful on most pickups - however hvae no clue on its value.

I don't. I think it means, 82 kΩ, 1 W. Which would be about right from a device sizing design-point perspective. I could be full-of-beans, but given the paragraph I just penned above, I think this all fits just fine. Terribly protective against the gremlins that'll someday be plugged in, but that's what a good amp-for-musicians should have.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
 
To your prior comment about wanting to understand why people-recommending-various-mods made those recommendations, a lot of it comes down to believing in whatever theory of insufficiently sweet design your religion requires.

There are anti-capacitor religious nuts, who will try to remove capacitors at all costs, for any contrive reason known. By and large, under most circumstances they can get away with it. Why then do manufacturers stick in the caps?

SAFETY and RELIABILITY over decades of use, many hands doing naïve things to the front end. That's why. If you are making an amp that is supposed to survive just about any abuse at the input, well … you use DC blocking capacitors; you put in small radio-frequency quenching filters such as the one I talked on earlier. You try to protect against stupidity, and ordinary innocent mishap, bad luck, ground-faults, terrible house wiring, all that.

I cannot conceive of a reason to change a nominally 210 kΩ resistor for a 220 kΩ part (in this circuit). As I said… waste of time and soldering skills. It is about as effective as changing out the original chrome-plated wheel-nuts on one's hot rod car for shiny blue anodized ones. Certainly doesn't make the car go faster. But it is something you can point at, preen about, and get manly recognition for. If that's important to you, LOL.

Ask about other parts, and I am fairly certain I can answer as to why the choices made (on the schematic) were made as shown. I really am not fond of the schematic's designer's value-annotation terminology, but its still somewhat readable. And, at least the resistors are not stupid little long rectangles. Squiggs are better. And squiggs we have.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅

Thank you for the explanations. The other mods I am considering are mostly in tone shaping and are explained...."adds a little more low end...etc, etc." This was the only one that I couldnt get a grasp on.

I just built a 2203 clone and it turned out great. It has really inspired me to understand the whys of circuit design as opposed to just following the schematic.

I am looking forward to learning more.

:)
 
Well, at 1.2 V, when an input signal's VPEAK-PEAK exceeds ±1.2 V, then the grid goes into conduction, changing from a nominally near–1-MΩ front facing impedance to something quite nonlinear. This is normally not-a-good-idea.
I think you'd clip the stage long before you'd run out of room at the input, though that's not so easy to determine without knowing the B+ voltage. Running super low current, the clipping would be very asymmetrical. Maybe that's what they are after in the first place?
 

baudouin0

Member
2019-03-22 5:41 pm
UK
You would be hard pushed to get 1.2v on a guitar pickup - only when the note is first plucked hard.
Input looks fine to me.

Asymmetric clipping is what you want in a guitar amp - but its usually the later stages that do this.
You also don't usually want frequencies < 100Hz.

DC block in case somebody plugs in a nasty effects box.
Cap to ground to prevent AM broadcasts getting in.
1Mohm input impedance - correct.

Guitar pickups don't respond to lower impedance say 10-100k. They tend to high a high Q in the 2-6KHz region which gives the correct tone. Loading them tends to loose that and make them lifeless and bass.
 
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