Tube preamp, tone control problem

[retailer]

I reran the simulation in Multisim 8 using the ampeg circuit values and as I played with it I realised that the ampeg pots were not marked log or lin so I INCORRECTLY assumed that they were linear. Duncans tone stack calc shows log pots for the James tone stack. Running the sim at a treble pot setting of 0% and then 100% gave about a 7db range at 5khz and 16db at 10khz. There is no log option in Multisim for virtual pots (or if it has then I can't find it). Using linear pots gives no treble boost ( if I can call it that) from the 12 o'clock position If you are using log pots and your circuit is wired correctly you should get some treble boost and cut. My son 's first speaker cab was a no name 4x12 It had a response that fell off sharply after about 4 khz so that even though his treble control was working the effect appeared to be minimal.

[Ahmad_tbp]

hi retailer , Johan ,, and thnx
i don think usin linear or log pots change the overall boost/cut function , but in same position they have diffrent change in boost/cut ..
i still have trouble with this , as u guess it s a bass guitar preamp ,ampeg is the best amp maker for bass , they just build bass amps and cabs and so they r the greatest , i m sure that this values is the best for a bass guitar ,and i m forgettin bout changin values , i donno where s the problem is in the treble line , cuz as i said u just can here the changes in very high freqs and so small , infact if u hear it for the first time u may not be attentive to it ...
and noise .. i still have them , i solder all plates and caps , pots ,sws , and everythin to ground , it decreased so much , but still i have it , i have hum ( around 60hz i think ) and hiss too , and also it seems a big hornet is flyin in my preamp !!!!!!!!!
i m goin to use this pre amp as a direct box for recording i have to make treble knob work and also completely kill the noise ....
and i m thinkin about adding marshall tone control to it w a 6av6 as a buffer ,,,, wat do u think ???

[retailer]

If you have an audio oscillator and an o'scope do a check of the frequency response with full treble boost and cut. If you don't have an oscillator or scope try some programme material ( CD or radio) through the amp, also try some ordinary hifi speakers as well. Maybe its working but its out of the range of a bass guitar. Increasing the value of R4 C3 abd C4 in the james tone stack of duncan's simulator seems to move the response of the treble control toward the low frequency end. Might be worth a try. I am building a guitar amp for my son its almost finished. I am using circuit for orange ad30tc , my son complains that there is not enough response from the treble control so I have some work to do to increase mid and treble. I had BAD hum and squeal, (both are fixed now) there are plenty of articles on the net for star grounding and how to do it Do a google search I'm sure you will find lots of info. Tonite we will try a hifi type graphic eq after the first gain stage to find out if we need to boost mids or highs or both.

[Ahmad_tbp]

i will trying change the caps tonight , hope it help me ...

[retailer]

Refering to Duncan's tone stack simulator

Increasing the value of R4 shifts the operating point of the treble control back towards the low frequency end. I have tried values up to 470k Increasing the value of C3 and decreasing the value of C4 gives a mid boost. I am going to experiment with a 6 position 2 pole rotary switch which will act like a switched mid control. I tried a C3=820p and C4= 1.2n, this give a useful mid boost. Going the opposite way (decreasing the value of C3 and increasing C4) gives a mid scoop. I'll arrange the values on the switch so that position 3 gives a flat response; 4,5, and 6 = mid boost with 1 and 2 = mid scoop. I may still go back to a Fender type tone stack, but since the James type tone stack has a much greater range of adjustment its worth a few dollars woth of componets to investigate.

[Ahmad_tbp]

,, i changed the treble section caps ... but nothin happend ! not that much change that i saw in tone stack simulator , still some minor changes in hi freqs ...
and that s a good idea bout addin switch for mids , i think i ll do it

[Bob Richards]

I've been messin' with the James type tone controls as well. I put it in a guitar amp I built and like it a lot. Now I'm trying to design and build a high end hi-fi preamp with four James tone controls. The problem I'm having is that the pots need to be LOG taper, and existing log taper pots have curves other than the 2 choices in the Duncan simulation program. Duncan offers 10% or 30% when the pot is centered. Alpha pots measured 17%, Nobles were around 40%, and pots with plastic resistive elements have taps with additional fixed R's meeting up at one end, which needs to be common to input and output when used as a volume control, and these types can not be used in a James circuit because the resistance to one end from the wiper actually goes 25% higher than the stated value of the pot, when the wiper is between the stops at some point. The circuit elements will react to that in a hard to predict way. I just had to return TKD pots for this reason, and Partsconnexion will only give me store credit rather than a refund ($180 for 5 pots).

The thing about the Ampeg circuit is that it will be noisy because the resistances are all so high, and that's because the driving tube takes it's output off the plate, which is a pretty high impedance. If you're building your own amp, I'd use a follower buffer so the driving Z is around 500 ohms instead of around 40K ohms, then scale down the entire James circuit by at least 10 times. Any resistors that are over 25K should also be metal film IMO, if you want minimal noise.

[Bob Richards]

The other thing I wanted to mention is that guitar speakers with low power ratings (25 to 40 watt) and small diameter voice coils usually have better treble response up to 7-8kHZ, while many of the higher power speakers (100watt) roll off around 4-5kHZ, and have more of a "honk" type sound that I personally hate. Better to use multiple 40 watt (such as Jensen P10Q) drivers than one 100 watt Jensen. The Celestion Vintage 30, rated at 65 watts is very good IMO, but it's several pounds heavier due to the magnet type.