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Old 14th March 2006, 09:47 AM   #1
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Location: Austin, TX
Default tube preamp ,tone control problem

hi all , i just built ampeg v3 preamp from this schematic :
http://www.drtube.com/schematics/ampeg/v3pre-jp.gif
( i just built one channel and leave the another channel )
and i use these values from another ampeg preamp schematic for tone control section :
http://www.drtube.com/schematics/ampeg/svtpre-jp.gif
the preamps sounds really great ,but the treble knob is not working ..
any idea ?
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Old 15th March 2006, 10:18 PM   #2
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Hi Ahmad,

No reason as far as component values are concerned. (I trust that you use log controls, but even with linear you should notice a response, just not linear with regard to the spindle movement.)

The +/- 3 dB point for the treble is about 2,5 KHz, with about 12 dB boost/cut at 10 KHz. Perhaps you should check the wiring (again?) or perhaps component values? Do you have access to a signal generator?

Regards
Johan
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Old 16th March 2006, 08:03 AM   #3
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Hi Johan , thnx
yeah i checked wiring and components again and again , i used linear pots cuz there s no log pot here ,,, but still it s not working , in fact the the u-hi switch has a very very small change in output tone too , i donno where s the problem in treble section ...i used regular caps in whole circuit , should i use anything special ?
and yeah i have a signal generator , wat should i do ?
and if it helps here s a pic of my pre amp :
Attached Images
File Type: jpg image(049).jpg (96.0 KB, 696 views)
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Old 16th March 2006, 11:13 PM   #4
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Hi Ahmad,

Now if you were a little bit closer and I could get to you.......

I have a feeling there is something basically the matter since you get little reaction from 2 different parts of your pre-amp.

But first: I presume that "Ultra High" switch is meant for some high treble accentuation (presence?), but it does not do that at all! I simulated your circuit on a computer programme. Firstly, the U-H circuit will be volume control dependent as you are probably aware of. But with the volume control set about half-ways, the treble is +3 dB already at a frequency of 650 Hz, and 8 dB up at 10 KHz. With the volume control further down (15% from 0), the +3 dB point is 400Hz and the response at 10 KHz up by 15 dB. That is not at all "ultra-high", and also quite a substantial treble increase! I would imagine you can do without it at all, but let us at least do things one at a time, and keep that switch off until the rest of the pre-amp is working.

Also then, with the substantial difference that switch should make but does not, may I suggest you first check whether the tubes are working at all, i.e. measure the voltages (with a digital meter if you can). I would place the ECC83 anode voltages at 90V - 150V, with the cathodes at a volt or two. Use the signal generator to determine that you have reasonable gain at all. Do you have a scope?

Further to that you can check the tone control circuit alone, by detaching it from the 2nd ECC83 anode and feeding it directly with your signal generator. (Now you will need a scope.) The treble should give a response between +/- 8 dB or so at 10 KHz.

Two final points, which I should maybe have asked first. You mention only the treble; does the bass control work adequately? Then I see the circuit you use (the first one quoted) feeds out into whatever load you have, without another isolating stage (the circuit giving the values has further stages which I understand you do not use). Be aware that for proper operation of the tone controls the "load" you feed them into must have an impedance of at least 500K or more. (The output impedance of your tone control alone is 140K, which must not be loaded appreciably otherwise the response will suffer.)

This is rather lengthy advice, which you should check first. Do not hesitate to report and come back here - we must get this thing to work!

Regards.
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Old 17th March 2006, 02:01 AM   #5
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I ran a simulation of the circuit using Multisim 8 and the treble control does indeed do almost nothing (treble cut only) I then tried the values shown for the James tone stack in Duncans tone stack simulator and these values worked much better. If you don't already have it download a copy from Duncan's site and use it to try different values I'm sure you will find something that works.
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Old 18th March 2006, 12:21 AM   #6
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Dear oh dear oh dear!

Before we confuse Ahmad; Retailer, I have simulated the tone control section only, on P.Spice, using in serie with the signal generator an input resistance of 120K to the tone circuit proper, to guestimate a rather high rp from the tube (because of current feedback) somewhat shunting the plate load of 150K. (It might well be a little lower than 120K.)

Relative to 1 KHz I found for full treble boost a +3 dB point at 3,2 KHz, with 10 dB boost at 10 KHz and climbing. Going to full treble cut I found a -3 dB point at 5,2 KHz, with treble cut at 10 KHz of 7,3 dB and decreasing. The tone control output was unloaded and the bass control at mid-point. This represents the normal somewhat unsymmetrical response typical of this type of passive tone control. (I also found the bass variation somewhat overpowering and the mid-controls response not quite linear, but let us deal with one thing at a time, since Ahmed inquired about the treble.)

I wonder what the reason for the difference between our findings can be, unless you simulated the whole circuit and there is high frequency attenuation elsewhere (I did not notice such at cursory glance). But if so, the treble control should still have made the expected difference. Can you give an opinion?

Regards.
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Old 18th March 2006, 10:30 AM   #7
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hi Johan and retailer , and thnx for replies ...
unfortunately i don have a scope but thnx for infos johan ,
i downloaded the Duncans tone stack simulator program ( wat a great software ! thnx ) and test a lot w that ..
but suddenly ,i mean i really donno how the treble knob start working ... WITHOUT any changes !!! ... lol , a miracle !
but the very very small changes in treble , in the simulator program , with the ampeg values , it shows more changes than what i here !...
may still have problem ,,,
and two thing else ,
1-there s a lots of noise comin out of it since the treble knob start workin , it becomes too noisy that i cant use it !
2-and how can i use 50k pots and still have long range of changes ?
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Old 18th March 2006, 10:50 AM   #8
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hi Johan ,
yeah , i found diffrent results too, retailer said that he tested the tone control in Duncans tone stack simulator and the treble control do almost nothing , and he said the james original values worked better , but i ran the simulator w ampeg values that i m using , and the results have not much diffrence for ampeg and james values ( i attached the ampeg and original james plot )
but in fact when i m using the the whole preamp , the changes are so small when i m turnin the treble control all the way up or down , unfortunately i couldn t test the circuit w tone generator yet , but i think the changes in treble is limited in very high freqs ( bout 12k or higher if i m not wrong )
i never saw a real ampeg amp to compare my preamp w it , maybe the original ampegs have this small changes in treble too , and the u-hi sw and treble pot are just for cabs w tweeter driver , but with comparin my pre amp w other amps i heared my treble knob is still dead ! , lol
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Old 18th March 2006, 12:45 PM   #9
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the plot
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File Type: jpg plot.jpg (84.7 KB, 544 views)
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Old 18th March 2006, 07:59 PM   #10
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Hi Ahmed,

I think I must first clear up that I have been thinking in terms of the usual hi-fi pre-amp. The Ampeg is obviously for a musical instrument (a guitar?) of which I must embarrassingly admit ignorance, although I once designed such a device for a friend. (I had to do some funny things hi-fi wise, but he was entirely pleased in the end so I presume I was successful.)

Firstly you would see that your second plot is just about the results I obtained. (I must apologise for not being able to present you here with a nice amplitude-frequency plot as you did; my simulator is on another PC.) Thus it would seem that any disagreement is more a question of taste than electronics. That plot is what one normally aims for in a hi-fi- pre-amp, and which I interpret as, well, an acceptable difference for a treble control. The usuall cross-over frequencies are 400 - 500 Hz for bass control and 2-3 KHz for treble, with about +/- 10 dB difference at 10 KHz and slightly more at 40 KHz on the bass side.

If you want more, this circuit will not give it - as said, what you plotted is what it will do. But this can be shifted any way you want by different values or circuits.

To go further, I have also simulated the network round the volume control. With the low-cut, roll-off begins at 106 Hz, and the other switch position provides for a rise below 20 Hz. (Of what use is that? - there is obviously something here I do not know.)

The total amplification superficially guessed will be about 50 - 60, which is not high as such circuits go. There should not be excessive noise, but you need to be careful of external influences. You do not describe the noise - hiss or hum? - but if you tested the built up unit "in the open" as your photo might suggest, you might very well pick up trouble, depending on the surroundings.

To get the same or a larger range of changes with 50K pots, you can scale everything by a factor of 20 - 25, but the required input impedance for the tone circuit then becomes about 5 - 7K, which will mean that you need to feed it with a cathode follower (and a lower rp tube than a 12AX7; 12AT7 or 12AU7), or a semiconductor circuit - if that is not too much of a swear-word!

I would like to help you more if this circuit is not satisfactory for you, but you will then have to mention in greater detail what it is you want. More "aggressive" responses re bass, treble and any form of filter is not difficult, but would then probably involve some sort of feedback circuit, a little more complex than this one.

Hope this helps.

Regards.
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