Raspy treble from 2203 amp

[Mark]

I plugged into my friends 2203 and it sounded dreadful. It was overly compressed and the treble is raspy.

The compression was due to the 100K cathode follower resistor measuring 880K. I replaced it and the amp sounded much better. The amp still has raspiness which goes away when I turn the volume up.

I’m wondering if I should consider the Kevin O’Conner mod where you put a 100K resistor in series with the master volume wiper connection.

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http://site.triodestore.com/JCM8002203Stock.pdf

This Triode circuit diagram has a 100pF cap across the first stage and I believe that Dave Friedman does put a 0.002uF across the cathode follower 100K resistor.

Thoughts on raspy treble response?

Thanks for your assistance.

[bepone]

Speaker is v30? Change the speaker?
Some other guitar problem?

[Mark]

Speaker is v30? Change the speaker?
Some other guitar problem?

I used a EVH MIM guitar, and I also used my Strat. The box is a Fender Bassman box with Kendrick Blackframe speakers in it. I have previously used the box with a 1959 and it sounded fine. The speakers aren’t overly bright.

[Littlewyan]

That master volume mod doesn't actually help, I did some tests on this a few years ago with a spectrum analyser and found it made no difference. But YMMV.

Dave Friedman puts a 500pF cap across the cathode follower 100K, lowers the first coupling cap to .0022uF, adds a 22uF to the final stage cathode and lowers the NFB resistor to 47K on 4Ohm tap. This is what he did to the Jake E Lee amp anyway. If you're looking to tame the raspiness I would start with the first and last mods personally, or even try removing the second 500pf treble peaker.

[cdemike]

That master volume mod doesn't actually help, I did some tests on this a few years ago with a spectrum analyser and found it made no difference. But YMMV.

Dave Friedman puts a 500pF cap across the cathode follower 100K, lowers the first coupling cap to .0022uF, adds a 22uF to the final stage cathode and lowers the NFB resistor to 47K on 4Ohm tap. This is what he did to the Jake E Lee amp anyway. If you're looking to tame the raspiness I would start with the first and last mods personally, or even try removing the second 500pf treble peaker.

Agree with these. I also would suggest experimenting with potentially adding a resistor to ground after the second peaker (sort of simulating adjusting the normal volume control when plugged into only the bright channel on a 4-hole amp).

What was the cause of the 880K CF resistor? Was it damaged or was it a mistake? Either way, it might be worth checking other component values also to see if they might be contributing.

[Mark]

That master volume mod doesn't actually help, I did some tests on this a few years ago with a spectrum analyser and found it made no difference. But YMMV.

Dave Friedman puts a 500pF cap across the cathode follower 100K, lowers the first coupling cap to .0022uF, adds a 22uF to the final stage cathode and lowers the NFB resistor to 47K on 4Ohm tap. This is what he did to the Jake E Lee amp anyway. If you're looking to tame the raspiness I would start with the first and last mods personally, or even try removing the second 500pf treble peaker.

Agree with these. I also would suggest experimenting with potentially adding a resistor to ground after the second peaker (sort of simulating adjusting the normal volume control when plugged into only the bright channel on a 4-hole amp).

What was the cause of the 880K CF resistor? Was it damaged or was it a mistake? Either way, it might be worth checking other component values also to see if they might be contributing.

Thanks for taking time to reply to my thread/post.

The 100K went high over time. The 100K on the valve socket is measuring 120K so it’s going too.

I think it’s the old Fender issue where they used 1/2 watt resistors on the anode/plates of the valves when they should have used 1 watt resistors. I replaced the resistors with 1 watt ones.

[GlideOn]

The Dave Friedman trick is to actually use 500pf snubbing caps with a 220k resistor in series to reduce the effect of said cap.

It's a nice balance of high cut without neutering the amp and as a bonus it makes the line-out signal a bit more usable too

[Mark]

Thanks for the advice. The guy is a Strat player so the idea was to get it sounding as smooth as possible. I end up removing the 1000 of bright cap and the first 470pF peaking cap. The original parts are still attached to one side of the resistors so that it can be modded back using the original parts. Lifting the two caps made enough difference to the amp. The guy loved playing through it.

Thank you for all your assistance and support.

[Mark]

The Dave Friedman trick is to actually use 500pf snubbing caps with a 220k resistor in series to reduce the effect of said cap.

It's a nice balance of high cut without neutering the amp and as a bonus it makes the line-out signal a bit more usable too

You have piqued my curiosity and I can’t imagine this mod. Could you please explain it in a little more detail?

Thanks for your help.

[GlideOn]

The Dave Friedman trick is to actually use 500pf snubbing caps with a 220k resistor in series to reduce the effect of said cap.

It's a nice balance of high cut without neutering the amp and as a bonus it makes the line-out signal a bit more usable too

You have piqued my curiosity and I can’t imagine this mod. Could you please explain it in a little more detail?

Thanks for your help.

See Friedman Smallbox schematic of pin 1, V1 plate:

[Mark]

The Dave Friedman trick is to actually use 500pf snubbing caps with a 220k resistor in series to reduce the effect of said cap.

It's a nice balance of high cut without neutering the amp and as a bonus it makes the line-out signal a bit more usable too

You have piqued my curiosity and I can’t imagine this mod. Could you please explain it in a little more detail?

Thanks for your help.

See Friedman Smallbox schematic of pin 1, V1 plate:

Thanks for showing me the circuit, I appreciate it.

[matt1969]

I put a 220k resistor on the volume control's wiper to treble control and it made huge difference. It really smooths out the taper helping to dial in the volume. I highly recommend it! Probably my favorite of all of the mods I tried.

[Mark]

I put a 220k resistor on the volume control's wiper to treble control and it made huge difference. It really smooths out the taper helping to dial in the volume. I highly recommend it! Probably my favorite of all of the mods I tried.

Don’t you mean from the treble controls wiped to the top of the master volume?

[solderhead]

What was the cause of the 880K CF resistor? Was it damaged or was it a mistake? Either way, it might be worth checking other component values also to see if they might be contributing.

All CC resistors tend to age upwards, but I have to admit that a 100k resistor that measured 880k is farther out of spec than anything I've seen as a result of age. Wow.

I think it’s the old Fender issue where they used 1/2 watt resistors on the anode/plates of the valves when they should have used 1 watt resistors. I replaced the resistors with 1 watt ones.

I understand your line of thought, but how much power actually gets dissipated across those plate resistors? My recollection from calculations many yeas ago is that the power dissipation across the resistors in the preamp stages of the supply line is so low as to be negligible. I obviously don't have the rail and plate voltages for your amp to confirm this, but if I assume that there's 100V across the 100k resistor, the math comes out to what? About 100mW? If that's correct then it would seem that a 1/2W resistor provides s 5x margin of protection. I've got to be missing something. I was always under the impression that the decision to use 1W resistors on the plates was intended for the purpose of hiss reduction.

[Roe]

[ how much power actually gets dissipated across those plate resistors? My recollection from calculations many yeas ago is that the power dissipation across the resistors in the preamp stages of the supply line is so low as to be negligible.

rarely that much but the 100k cathode resistor can be worse. Early marshall 100w amps have around 225v across a 1/2w resistor here (see https://www.drtube.com/schematics/marshall/1959t-66.gif). This runs the resistor at 101% at idle. Typically, CC resistors were used, which tend to drift