Peavey vk100 into some bassman like amp

[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.

[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]

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]

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

Mark

[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.

[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. :-)

[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

[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]

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