Clone as a bass amp

[heisthl]

Here's a hypothetical - If you had to use one of these clones as a bass amp what would you change? Assume the non-HRM with skyliner stack as the original - I have included a possible reference with a few changes

[Bob-I]

Speakers....

I've played bass through mine with an EVM 15 and it sounds ok. I'd rather have a Fender Bassman tone stack though.

[jelle]

Hi,
I yhink the EVM 15B is a great speaker for this application...the B stands for bass. The EVM15L is the lead speaker.
Have you considered using a Baxandall stack with 1M pots for treble and bass? I'd use a 500p - 1000pf treble cap. And maybe 100k plate with 1.5//50uf cathodes for the preamptubes.

Jelle

[heisthl]

I made some more changes - If I build this the application is for upright bass using 2-way (passive xover 3.5khz & HF Horn) speakers.

[skeezbo]

The stack sure is different on this version! I liked the idea of the switchable midrange, between the Skyliner .01 and the more Fenderish .047. What if you had a third option of a tweedy .022?

Skeez

[heisthl]

Why not - do you really think the .022 would be usable for a bass instrument?

[skeezbo]

Well, it was the mid cap that started it all on the 4x10 bassman, designed I guess for bass, although the same stack was used on the high power tweed twin. If you plot the values with the Duncan Tone Stack Calculator, depending of course on other variables, the .047 seems to dip around 400 cycles per second the .02 at around 700 and the .01 just above 1k. On my solid state Hartke bass head I tend to dip the 500 and 1K bands about 6db. Maybe the .02 is the best choice if you had to pick just one.
Skeezbo

[glasman]

I agree with the .022uf. The smaller you make that cap, the higher the mid dip frequency. The trick is to find the value that will allow enough lower mids without making the acoustic bass too murky.

Gary

[glasman]

One other thing I forgot to mention is that the dip in the Dumble style stack is much less than that say a Fender style. Putting the mid control below the bass control creates a huge dip.

Gary

[skeezbo]

Ever play through an AC-30 top boost? The highs are beautifully bright and chimey because to 50p treble cap and the 1M treble pot slide the mid dip up above 1K. But any attempt to tweak the tone further is limited: there is no mid pot, and the bass jumbles everything all the way up to around 500 cycles--way up into the midrange zone. Kind of strange for guitar; no good for bass.

I'm no engineer, but in the Skyliner it looks like HAD's .001 shows the .1 bass cap to both sides of the bass pot. Is this a tone stack with a .1 bass cap and a .101 mid cap? It seems to set the frequency range of the bass control down low where you need it. Calculate this stack on your Duncan Tone Stack Calculator and you'll see what I mean. Further, the .01 Skyliner mid cap looks like it is not connected in the usual way with the bass pot; it comes from the slope resistor, goes to 250k pot and then to ground (heisthl's revised schematic above seems also to show this unusual three cap setup-- it has a .047 and a .1 on either side of the bass pot like a Fender (but reversed); then there is a separate mid cap and pot that come down to the left. Is the .001 that brings HAD's .1 bass cap to both sides of the bass pot left out? I wonder if maybe it should be).

The region of the audio spectrum between the bass pot and the beginning of the midrange dip, the area, as glasman says, that seems to murk up bass in a normal tone stack when you set the mid dip above 500 cycles, looks to be controlled by the 10k resistor hanging from the bass pot. (I wrote about my experiment replacing the 10k resistor with a 25k pot to see if I could get a low-mid control for a four way tone stack on the all clean Dumble thread and again on the 10k bass resistor thread a few weeks ago. I have since used the Bassman 100 I put the mod into on a gig and it sounded terrific.)

Does anyone have a pink noise generator and a real time analyzer and could check out the frequency ranges of the controls in the Skyliner stack? Rumor is that HAD used them. That would be the best way to find out for sure what is going on.

So, IMHO it seems HAD's Skyliner stack allows the mid control to be moved up into a higher than standard range that sounds good for bass while still limiting the frequency response of the bass pot to true bass. It sounds great for guitar, too, if you are looking for a clean tone. I'll bet a bebopper or swing guitarist would love a simplified Skyliner tone stack in his Twin Reverb.

Skeezbo

[glasman]

I never found the Duncan tool to that great for the dumble stack, none of the models are close enough electrically.

Only AC30 I have played was a AC30CC.

Here is the response plots for the Dumble tone stack.

I am in the finishing steps of a couple of 50 watters, if I have enough time, I will put them on the spectrum analyzer to compare.

This was modelled in LTSpice.

Gary

[heisthl]

The tone stack calculator does illustrate the importance of the treble cap on raising the mid dip. I've decided to build the second schematic with a switch for 330,250 and 100pF on the trable cap. This should allow a wider choice of mid dips.

[skeezbo]

I know what you mean about the Duncan Tone Stack Calculator not being very good for the Dumble stack. You have to put it in a piece at a time and guess what it will sound like. I'll have to get LTSpice.
Thanks for sharing your frequency response plot and for your quick response, glasman. Your schematic shows treble cap values that would be in place if the mid boost was activated, correct? I can't wait to see the plots for the new amps.
Skeez

[glasman]

I know what you mean about the Duncan Tone Stack Calculator not being very good for the Dumble stack. You have to put it in a piece at a time and guess what it will sound like. I'll have to get LTSpice.
Thanks for sharing your frequency response plot and for your quick response, glasman. Your schematic shows treble cap values that would be in place if the mid boost was activated, correct? I can't wait to see the plots for the new amps.
Skeez

The boost is not activated in the drawing. The boost is activated by shorting the 390pf cap.

The 4.7M is there for pop surpession.

Gary

PS> Back to Henry's Thread.

[skeezbo]

Heisthl, I had it wrong. I mistook that .001 network as a substitute for a mid cap; I wasn't paying attention to the fact that it would have to be in parallel, not in series and even then .... I am still on the hunt for a 4 knob bass amp tone stack: bass, low mid, high mid, treble, and I am intrigued by your revised schematic, which does have a bass and mid cap on either side of the bass pot. The 10k resistor below that bass pot should be 10k of midrange, correct? Could your .1 deep control on the left be replaced by a .01 going to the 250k pot for another, higher mid range control?
Skeezo