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Old 4th May 2010, 04:00 PM   #1
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Default Tone control problems

Hi,
I have built a tube amp for a friend who plays Lap Steel.
He supplied me with transformers and a circuit from an old Stromberg Carlson HiFi amp Model AP 437.

I recreated the circuit in a new chassis and added a new front end to the circuit to give the amp a tone stack.

From the input through to the Phase Inverter the circuit is a direct lift from a Fender Delux Reverb Amp Model 763, the Normal channel.
There is no Presence or Bright switch on the circuit I used although I've seen them on other diagrams for this amp.

Here is my issue,
The amp works fine, has guts and tons of gain but, the high end seems somewhat rolled off and the tone controls don't have a lot of effect.

I'd like to tweak this and get a better high end range as well as better response from the tone controls but I'm stuck on where to start with this. I'm hoping someone here will have some suggestions on how to tackle this.

Here is the current circuit I've used.

Thanks much,
Chris
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Old 4th May 2010, 08:03 PM   #2
Merlinb is offline Merlinb  United Kingdom
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You have probably just wired the tone stack wrong- it catches people out every time, no matter how many times you think you've checked it! Either that or some of the components simply have the wrong values.
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Old 4th May 2010, 08:37 PM   #3
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What he said^^^ and not just caps and resistors but the pot values too.
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Old 4th May 2010, 10:59 PM   #4
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Got it, I'll check everything again but I'm pretty sure I have it right, at least wired right. The stack does work but the range on each tone pot seems to be much less than it should be. The volume works fine. I've been wondering if the capacitance for the whole stack is wrong, too high, and it's rolling off the highs.

Thanks for the suggestions.
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Old 4th May 2010, 11:02 PM   #5
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You have 2 potential problems that I can see:

1. The tone stack and volume control are within the global negative FB loop, which will cause the NFB to try to cancel their effect.

2. The input to the tone stack would be better taken from a low impedance source, IMHO. A grounded cathode 12AX7 does not have a low OP impedance.

The thing might work better if you can add a cathode follower front end and insert the tone stack and volume controls between the cf and the first 12AX7 gain stage.
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Old 4th May 2010, 11:20 PM   #6
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Thank you Ray_Moth. I appreciate your observations. I'm hoping to avoid adding another tube so I'll try to address this using your two suggestions.
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Old 5th May 2010, 08:04 AM   #7
Merlinb is offline Merlinb  United Kingdom
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Ray, I don't think the global NFB is actually doing anything- the input cathode is bypassed with a 22uF cap! Seems to be a redundant bit of fiddling that the designer has done.

Just noticed, the slope resistor in the tone stack is not labelled. What value are you using? I'm guessing not less than 100k.
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Old 5th May 2010, 08:38 AM   #8
Ian444 is offline Ian444  Australia
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There are 2 x 100K anode resistors in parallel on the tube after the vol pot, might make the DC a bit high on the input pin 2 to the phase splitter? The 0.5uF cap on pin 7 is returned to B+, is this intended?

Ian.
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Old 5th May 2010, 08:59 PM   #9
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Merlinb, you're right. I didn't spot that!
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Old 6th May 2010, 08:26 PM   #10
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The input cap is sure working to cancel out the NFB, but the unknown element here is how much feedback is actually being supplied by the tertiary FB winding. Dedicated windings like that are usually of a higher level than the VC winding is, so there may just be enough NFB to limit the tone control range -- although that's a pretty high FB resistor. Either way, that needs to be reworked, and the values for the second gain stage are off from typical Fender values. The plate load is usually 100K, which means the cathode resistor should be 1500 ohms since it's not doing double duty for the vibrato channel in this case.

Dave
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