Losing sparkle with Dumbleator...

[butwhatif]

If u have experience w/ a sig gen and scope, it is easier than u think. A sweep will show it to u, if u know what you're lookin---otherwise a square wave is so easy because it mol applies the equivalent of all harmonics of the fundamental at the same time, sorta like a real signal, so you can easily see the interaction.

[Tonegeek]

A sweep will show it to u, if u know what you're lookin---

thats the magic phrase. Guess I will have to drag out the scope tonight and see if I can apply what you guys are trying to teach me.

[Tonegeek]

Put my loop on a scope, and fed it a 10k square wave. If I ran it at the level where I play, the square wave input turned into a triange, even on the clean channel. I inverted the signal coming off the return level pot and superimposed it on the input which came off the follower grid. The input and output waveforms looked the same to me except at very low levels where the wave was still square. So, even though I can hear a slight loss in highs, the scope is not picking it up. Must say my skills with a scope are minimal so I am probably doing something wrong here.

[butwhatif]

If u used a 10k SW it prob did look like a rolled off signal. But if u try something more realistic like 1k it should provide an indication of what might be happenin. As u know the fundamental freq of hi e string A at the 17th fret is only 880hz, and a good speaker like a 12L rolls off around 6k - the sparkle you refer to is the sharp front edge of more like a 1kz SW. See if there's any loss of the integrity of that thru the loop.

[Tonegeek]

If u used a 10k SW it prob did look like a rolled off signal. But if u try something more realistic like 1k it should provide an indication of what might be happenin. As u know the fundamental freq of hi e string A at the 17th fret is only 880hz, and a good speaker like a 12L rolls off around 6k - the sparkle you refer to is the sharp front edge of more like a 1kz SW. See if there's any loss of the integrity of that thru the loop.

Thanks for the guidance.
Here it is at 1k. The smaller of the 2 signals is the input (taken from the .05 cap). the other trace is from the output (taken off the input to the level pot, recovery stage). I have inverted the output at the scope to make it easier to compare to the input. They do look slightly different, but I could not tell you what it means. I know my ears hear attenuated highs. Also looks like some DC on the output, but my scope my not be calibrated, or rather it was until I switched the volts/div control to a higher scale.

[butwhatif]

If u used a 10k SW it prob did look like a rolled off signal. But if u try something more realistic like 1k it should provide an indication of what might be happenin. As u know the fundamental freq of hi e string A at the 17th fret is only 880hz, and a good speaker like a 12L rolls off around 6k - the sparkle you refer to is the sharp front edge of more like a 1kz SW. See if there's any loss of the integrity of that thru the loop.

Thanks for the guidance.
Here it is at 1k. The smaller of the 2 signals is the input (taken from the .05 cap). the other trace is from the output (taken off the input to the level pot, recovery stage). I have inverted the output at the scope to make it easier to compare to the input. They do look slightly different, but I could not tell you what it means. I know my ears hear attenuated highs. Also looks like some DC on the output, but my scope my not be calibrated, or rather it was until I switched the volts/div control to a higher scale.

It does indeed look like there is some rolloff, but hard to tell with this pic--hard to discern which trace is which. Looks like the smaller sig has a more square leading edge. Also looks like the recovery signal also lost bass. If you really want to know what the loop is doing, insert the signal into the input to the send stage, not the front end of the preamp. and compare to the recovery-- is that what you're doing?

[dogears]

Bright cap on master.

You need to use a small bright cap on the master. Not the Dumbleator. This will restore order to the world. Start with 15pf and go up to 47pf. I will warn that if you go to 47pf, you will require appropriate sized cables. When you get that sweet spot, the sum of the parts is superior to the amp with no loop.

[wjdunham]

What do you mean when you say "appropriate sized cables"? Is there some matching that has to happen here, or is it just more important to use a higher quality (lower loss) cable to hear the effect?

In my short experience building these type of amps, the bright cap on the master has had the biggest effect (even though still subtle) on the overall tone of the amp, especially in OD. I have a 30pf silver mica in there now and put just the right edge back in there. Knock the top off with the treble bleed and then put it back with the master cap... never would have tried that if I hadn't read it here

[dogears]

I mean the appropriate length. If the cable has 30 pf/ft then you can choose the length to get you to the right amount of treble bleed. For example, 8 feet of such cable has 240 pf/ft. This, btw, is not too much depending on the size of the bright cap and the rest of the circuit. The brighter the amp, the more singing it is. Use the loop to tame.

What do you mean when you say "appropriate sized cables"? Is there some matching that has to happen here, or is it just more important to use a higher quality (lower loss) cable to hear the effect?

In my short experience building these type of amps, the bright cap on the master has had the biggest effect (even though still subtle) on the overall tone of the amp, especially in OD. I have a 30pf silver mica in there now and put just the right edge back in there. Knock the top off with the treble bleed and then put it back with the master cap... never would have tried that if I hadn't read it here

[butwhatif]

Was this thread about the master bright cap, or your loop's eq integrity? I guess it got hijacked a bit. If you don't have any bright cap on the master, i would recommend it, if the loop is lossy, u will be chasing your tail.

[Zippy]

Was this thread about the master bright cap, or your loop's eq integrity? I guess it got hijacked a bit. If you don't have any bright cap on the master, i would recommend it, if the loop is lossy, u will be chasing your tail.

The way I read the subject line, it say's "Losing sparkle with a D'lator..."

What's wrong with restoring the sparkle to the whole amp if indeed the whole amp is what is the signal path?

Where's the hijack? Got some issues of your own?

Sheeeeeeeeeeeeeeeeeeeeesh...

[dogears]

The big man used the bright cap and loop as an integral part of the system. Just saying since this is the Dumble forum. If you build a clone and use the loop, use it or at least try it as it was intended.

Peace out....

[butwhatif]

Was this thread about the master bright cap, or your loop's eq integrity? I guess it got hijacked a bit. If you don't have any bright cap on the master, i would recommend it, if the loop is lossy, u will be chasing your tail.

The way I read the subject line, it say's "Losing sparkle with a D'lator..."

What's wrong with restoring the sparkle to the whole amp if indeed the whole amp is what is the signal path?

Where's the hijack? Got some issues of your own?

Sheeeeeeeeeeeeeeeeeeeeesh...

Did he say the whole amp had lost sparkle? I took it to mean the loop had lost it for him. I have two amps of my own that have loops, neither of which loses anything. Just trying to help the gentleman t'shoot his loop, not change his whole amp first. The tone of these amps is what's important, no? There are many references and and threads discussing the effect of the master bright cap, I use them. It's a critical item. Is that how u fix the sound if the loop is lossy? sorry i mistook it, insult me all you want

[Tonegeek]

Everybody is a winner. I agree with everybody on the bright cap recommendation and always appreciate dogears penchant for historical accuracy. I may ad the bright cap on my master(s) at some point. For purposes of this thread though, I am just trying to learn more about the loop. Just trying to understand every circuit in this amp.

As butwhatif suggested, I injected the signal directly to the dator input. Got a nice square wave at the send level and the rounded one at the return level pot. the next photo is the same thing but I shorted the grid leak network to the recovery tube. Looks like we got square corners back but something else is now going on. I need some help interpreting the data. I also disabled the feedback network on these photos, but the only thing that changed is the recovery level dropped very slightly a few mv. the wave looks exactly the same with the feedback as without it.

[butwhatif]

Cool, do u have a cable or anything in the loop, or is it just direct? and, is your scope on the 'hot' term of the return level pot? I would keep a grid input r/c but you can mess with the elements in the stage to try to get the recovery amp square or close. As u know, any element in the loop has a capacitance, and also the return level setting can affect this. I have a trimmer in one of my amps as a return pot, which I have found the sweet spot on, and i leave it alone.