Peavey vk100 into some bassman like amp

[MarkusBass]

I was fixing HiWatt with 100k pot installed as the Volume pot (there was also some resistor in series with the pot). I was surprised with the low resistance of the pot - I thought that this was a mistake. I wanted to replace the pot with 470k but after testing both versions, the owner of the amp decided to keep the lower value - it sounded better that 470k.

Mark

[bea]

Bea is right (as always). I know that in Hiwatt there is 100k pot used as Volume pot (and the amp sounds great).

Slight correction: the vol pots in my Hiwatt clone (Mywatt 200) are: 470k gain and 220 k master after the tone stack +22k in series with it. The tonestack itself might reach a pretty small impedance, in some of the possible positions it might reach the order of 50 k or even less (roughly estimated from the values, not computed).

So the effective load on the previous stage is even smaller than those 100 k.

It is no problem to present an ECC83 with such a heavy load (but the rail voltage should not be too high), its effect is mostly that the gain will decrease and distortions will increase a bit.

[MarkusBass]

Bea is right (as always). I know that in Hiwatt there is 100k pot used as Volume pot (and the amp sounds great). I checked that in this preamp 500k pot works exactly in the same way as 1Meg (or even better - at some frequencies the gain is slightly higher ).
I spotted few problems:
- the MIDDLE pot should be most probably 10k. With 50k the preamp des not work as expected (especially bass frequencies regulation).
- the BRIGHT cap (120p) is in my opinion slightly to high. I suggest 68pF - to be verified when the amp is build.
- the 1000p capacitors before the grid stopper resistors do not have any influence on the amp. They should be either removed, or moved after the resistors.
- 100n capacitors in the power amp can be decreased to 47n (or even to 33n).
- I suggest that the DEEP switch is implemented by means of additional capacitor in the cathode of the first tube and the switch should alter from 1uF to 25uF - of course by shorting 1Meg resistor. This also can be verified when the amp is up and running.
- the input resistor should be decreased to 33k (or 22k - to be tested).
I haven't had time to tackle the AMPEG preamp - I don't have much time in the next few days. I'll check it maybe next week.

Mark

[überfuzz]

The 500k pot is an artefact from when I did the initial set up. I tend to use 2 resistors that I link. Seems I forgot to change the value, i.e. initially a 1M potentiometer is 2 500k resistors. When I link the volume resistors into a "pot" I forgot the alter the resistance values. Eh... the that message get through..? The 500k volume pot is suppose to be 1M pot. :-)

[bea]

- I would decrease input resistors to 33k,
- I'm not sure if the 500k Volume pot isn't to low. I would try 1Meg.

ECC83s perform quite well with a load of 500 kOhm. They do, for example, in Hiwatts. Even at a load of only 100 kOhms they will give a gain of about 50 if the anode resistor is large enough.
Reducing the input resistors to 33 k is a really good idea. It would reduce the high frequency losses of the cable. Maybe it can be reduced even a bit further. This would actually be more a matter of trial and error.

- I'm not sure about switching C8 and C22 to get less bass frequencies. This is inconvenient (due to high voltage on the caps).

Of course i would not simply switch them.
The problem with the high voltages can be easily overcome by using 2 caps in series, the smaller value always active in order to block the high voltage and the cap in series bridged by the switch. This would make it possible to add additional resistors to reduce switching noise if necessary.
Similar approaches can of course be applied in the alternate positions suggested by You.

[MarkusBass]

I was refering to the schematic file only (asc extension). PM sent.

Mark

[überfuzz]

Thanks for the interesting input Markus! I'll try it as soon as I get half an hour, probably during the week end. If you want me to I can send the spice file to you, rather than bloat this thread with a bunch of bode-plots. :-)

[MarkusBass]

Here's a first draft, please comment on faults or just weak spots in general.

I don't see any mistakes on the schematic. I would only change few component values:
- I would decrease input resistors to 33k,
- I'm not sure if the 500k Volume pot isn't to low. I would try 1Meg.
- C62 and C63 (100nF) seem to me to high. I would try here 47nF (or even 22nF). Having LTSpice simulation you can easily verify it.
- I'm not sure about switching C8 and C22 to get less bass frequencies. This is inconvenient (due to high voltage on the caps). I would try switching C1 and C21 to lower values instead (something like 4.7uF , or even 1 uF). This can be also easily verified with the simulation.
- I think that in similar amp the power supply is higher than in your amp (415V). I would try at least 450V, or even 470V.

BTW, can you post the simulation file just to verify my ideas?

Mark

[überfuzz]

Off topic:

I more or less always use cheep head phones somewhere down the road while mix and mastering. Nowadays I also reference listen in my phone, at the moment a iPhone 4. This is with the same reasoning in mind as Enzo's car speaker here above in the thread.

[km6xz]

We started using small 5 in speakers in mix for balance but the most important reason was have small essentially point source speakers at arms length so stereo image could be heard with minimal impact by the room acoustics and reflections. Two cheap speakers within 2 feet of you gives a better representation of the image than any large room monitors. So when tracking we used big speakers that were often built in to the walls over or just to the sides of the window into the main room. Those were used for full tone range and imagining was not important, and would not have been good any for it any way. The small speaker also gave a better accounting of the rhythm section since just upper harmonics of the bass and kit were heard but that was fine since those additional harmonics could be heard even at low volumes very distinctly. Nowadays people with project studios usually have a small 2-3 way monitor mounted close but imaging is not good because of the multiple drivers, and they are to small for full spectrum. They add a sub often which in turns creates mixes that are too light in the bottom. Subs are almost never adjusted so they fit into the system well, nor are time aligned.

[Enzo]

I still have a pair of L100s.

Recording studios always have a pair of small speakers to listen to, because no matter how wide ranging the original material is, it also must sound good on typical car radio speakers, and there are no 15" woofers there. Guys put big subwoofer systems in cars to listen to the artificial low end on rap music, but now we are getting away from any sense of tone.

[km6xz]

Bea makes a very good suggestion.

What sort of sound are you looking for? The HP filtering is a great way to generate tight articulate bass that is easier on speakers and listeners....plus is a lot louder. As G1 noted, in the radio and vinyl days, the low fundamentals were filtered out from the cutting head or modulator and no one every complained there was not enough bass. In fact it was easier to dance to because the peaking at 70 or so hertz, emphasising the second harmonic gave all the brain needed to know the note bases on the harmonic series and our brain's ability to complete sequences given any portion of it. 32hz will rob any headroom, tax speakers and muddy the tone but it became popular with the advent of the first disco era in the 70s. Musicality is not diminished, but enhanced by rolling off the lowest portion of the spectrum. Did you ever try dancing to older live shows or records and have a problem hooking up to the rhythm section? It was easier than when large systems focused on thundering subsonics came into vogue.
So it depends on your intent, but don't underestimate how much tighter and more impact every note has by emphasizing the second harmonic.
Remember how JBL was really popular in home stereo in the era of separate components, 1970s? Salesmen always commented that customers said the JBL speakers had the best, tightest bass. Bass, "tight", sorry, they do not go together. Doing a measurement of the speakers all theirs had poor deep bass, and all those models that sold so well like the L100 had drivers with a lower free air resonance than the cabinet. So they had less bottom but a peak in the 65-75 range that emphasized the second or third. The brain synthesized the fundamental from the harmonic series but since the speaker, amp, listener's ears and room was better suited to 60-100 hz than 30hz the impression was tight powerful bass, each note being articulate, even if not very accurate in reproduction. All depends on your goals.If you want the bass line to be heard distinctly, a bass player would normally set the bass response to not have so much gain in deep bass. Besides, with as inefficient rooms and speakers are at 32 hz, you probably do not have enough power to reproduce it well anyway. 1000 watts at 32hz has a much lower perceived loudness than 1000 watts at 64 hz, you would never need that power at that frequency in fact, but never have enough for 32.

[überfuzz]

Correction
According to the all knowing internet the frequency of a standard tuned A-string is 55 Hz...

Thanks for the input bea, seems internet was right all along, according to you. :-)

Enzo - I know how filters affect the frequency sweep. More particular in this case where bea addressed a first order filter. However, thanks for reminding me. :-)

Edit - PS
If this set up result in wonky bass tone, i.e. bad low frequency behaviour, I think I might tinker with the bypass capacitors of the pre amp gain stages. This would be to sort of nip it in the bud. Next step would be the high-pass filters suggested by you bea.

[g1]

I like the bass sound I hear from an old AM radio in a car with a small in-dash speaker. It's coherent and intelligible, punchy, and it stands out for me. But to a certain extent I have no use for sub-woofers.
So this is the difference between having all the fundamentals there or not, as I think Bea is suggesting. I believe the same holds true for the popularity of 8 inch drivers with bass players.

[potatofarmer]

Indeed, Enzo, and indeed, my suggestion explicitly and intentionally means to reduce the level of the deep fundamentals a bit (6dB / octave is a weak reduction, even at 30 Hz).

Reasons:

a) the tone of an electric bass guitar is dominated by it 1st harmonic, not by its fundamental. This holds even for extremely bassy instruments like the Gibson EB-0. Too much energy in the lowest registers will lead to mudiness and to conflicts with the bass drum.

b) most bass cabs are tuned to something around 55Hz. The majority of the bass drivers are not suitable for lower tunings, except of course, specially designed drivers. But those drivers are reduced in efficiency (unavoidably - the lower a speaker can go the lower its efficiency).
With a relatively small amp like this it is (IMO) preferable to use drivers with a good sensitivity.

c) the lowest registers eat up a lot of energy - with many bass cabs that energy is simply wasted to silently move air around (vented cabs driven below their Helmholtz resonance). Not good for the speaker and it reduces the headroom of the amp.

If You d not believe me - simply try it out. It'll cost You a few ct for one or two capacitors of a few nF.

To add to this:

d) The Peavey OT was designed for guitar and it's a fair bet it lacks the primary inductance necessary to push significant power at those ultra-low frequencies, further limiting your headroom.

The Matamp/Orange "FAC" control is very useful for variable bass cut.

Also I just noticed that the schematic is missing a reservoir cap and the screen supply filter cap is on the low side.

I'd move C64 and C65 to the grids, "after" the grid stoppers.