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Old 10th February 2012, 03:57 PM   #1
JoshuaK is offline JoshuaK  United States
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Join Date: Jan 2012
Question Guitar Amp EQ Control Problems

Hello again,

Seeing as how my first question/thread garnered adequate attention and qualified help, I'll try another. I did a light search for threads throughout this forum and did not find anything of much help to my current situation.

For my clavinet project [see thread: Pickup wiring/inductance/output questions] to go further, I needed an amp to properly test the circuit. I ended up finding a used solid-state guitar amp that had enough control [and high enough input impedance] to use with the clav's circuit when it's finished - for the time being, I've got a cheap guitar to plug into it and noodle with until the clavinet is ready.

It is a Rocktron R50C - solid-state 50W, 2x8", stereo chorus, stereo ext-outs [8 Ohm Min.]. Pretty run-of-the-mill, until the stereo chorus is switched on, which [obviously] provides the sound with pleasant depth and breadth - anything from basic stereo audio effect to a warbling vibrato. Overall very neat, and worth more than what I put down for it. Essentially brand-new, and most of it works without error - very much a keeper IMO. In fact, with the stereo extension-speaker outputs, I may eventually construct separate 'stereo' speaker cabinets to take advantage of that capability [with adequate preliminary reading/forewarning, of course].

Anyways, here's my problem: the only components that don't operate as they should would be the entire EQ section - the basic rotary-pot 4-control "Low-Mid-High-Presence" array. The signal path is not interrupted in any way, but the specific band that its respective pot should control has been 'scrambled' or 'reassigned' to another, I guess. This is what is occurring [control name, followed by result]:
  • Low: nothing happens. From '0' to '10', no perceptible difference in signal.
  • Mid: controls the lows. No midrange difference.
  • High: very vague, almost imperceptible difference in super-high-frequency range [as if it were the 'Presence' control]. Normal treble range unaffected.
  • Presence [super-highs]: dramatic reduction of treble overall, no midrange difference, with bass retained - entire signal muffled on zero, as if it were a 'Tone' control on a guitar circuit.
I've looked all over the 'net, and have not found a schematic for this amp. I'm tempted to inquire with Rocktron's tech support representatives to see if they'll provide one, but I highly doubt it. At least with a schematic, I can take a look at it and follow the path [and even then, I'm still not adept with schematic reading], and have a decent heads-up overall.

Is there a simple explanation for the aforementioned EQ behavior? Something that I can pinpoint to begin with, and see if it makes a difference? I would REALLY love to get this amp working completely. For the record, I have [since my last question] looked into basic precautions and safety-first guidelines [including filter cap drain] concerning the insides of an amplifier.

Thanks-in-advance for all of your comments, questions, criticisms, and assistance. Also, I can take detailed pictures of the circuit board inside and post/link them, if necessary.
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Old 10th February 2012, 09:03 PM   #2
Bone is offline Bone  United Kingdom
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Location: Dorset, UK
I think the problem is that this is a GUITAR amplifier. You need a linear amp for a clavinet, similar to a HI-FI amp. Guitars do not have a linear frequency response and guitar amp equalisers are made to reflect that.
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Old 10th February 2012, 09:42 PM   #3
tuhkam is offline tuhkam  Estonia
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I agree with Bone, an electric guitar has usually a relatively loud mid-range, 400Hz may be 9 to 12dB louder than 100Hz. The amps response is tailored to make this flat again (with the EQ centered).
There may also be a low pass filter in place (10-14kHz), that you can not influence with EQ.
In addition the amps speaker may go only to about 12-16kHz.

Using aux input could improve things, but it might go straight to poweramp, bypassing the effects section.
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Old 10th February 2012, 11:23 PM   #4
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And on a different note, weren't the original guitar amplifiers used to reproduce voice along with accordion and guitar? They could not be all that out of whack. Also what real frequency content is there above 14 kHz? And pushing 6 kHz is a lot for a guitar speaker. And as far as a guitar having the 400 Hz region being 10dB louder than the fundamental of the open strings, well where does all that flubby overdrive sound come from when using a output transformer that does not have adequate low frequency bandwidth? And even if the bandwidth was limited on a guitar amp why would the tone controls have no effect?

Not having a schematic to look at but seeing that the amp has a passive TMB tone control I would say you have a open connection at the base of the tone stack. That is how we effectively take the tone stack out of the signal chain, by lifting the ground on the mid pot.
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Old 11th February 2012, 01:11 AM   #5
Enzo is offline Enzo  United States
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Tempted?

WHy wait at all? Of course call their tech support. IF they say "no" oh well. Many companies are perfectly willing to send schematics.

Never think up reasons not to check something.
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Old 11th February 2012, 03:57 AM   #6
JoshuaK is offline JoshuaK  United States
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Quote:
Originally Posted by Printer2
Not having a schematic to look at but seeing that the amp has a passive TMB tone control I would say you have a open connection at the base of the tone stack. That is how we effectively take the tone stack out of the signal chain, by lifting the ground on the mid pot.
Thank you.

Quote:
Originally Posted by Enzo
Tempted?

WHy wait at all? Of course call their tech support. IF they say "no" oh well. Many companies are perfectly willing to send schematics.

Never think up reasons not to check something.
And thank you.

Done deal - thanks again, folks, for your help and time.
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Old 11th February 2012, 04:41 PM   #7
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Quote:
Originally Posted by Printer2 View Post
And on a different note, weren't the original guitar amplifiers used to reproduce voice along with accordion and guitar? They could not be all that out of whack.
You might be surprised. Check out this neat little app:

TSC

To get "flat" response from most guitar amps, you have to turn the bass and treble down all the way.
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Old 11th February 2012, 11:44 PM   #8
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Quote:
Originally Posted by Keriwena View Post
You might be surprised. Check out this neat little app:

TSC

To get "flat" response from most guitar amps, you have to turn the bass and treble down all the way.
I use it all the time. The thing I was disputing was that
Quote:
an electric guitar has usually a relatively loud mid-range, 400Hz may be 9 to 12dB louder than 100Hz.
And the Marshall only has to be turned down to '4' to get a mainly flat response. The James is flat at '5' (as used by Traynor and others), The 'E' stack which is mainly flat with bass at '5' and treble at '6') although this tone stack is not used much other than in some vintage gear and some pedals), And the Big Muff type of control is normally used in pedals. And there are those who tweak it increasing the high pass section capacitor to 9nF removes the mid dip with a response more suited to the 18 Watt (with values scaled for tube circuits). And then there are the Tweed type of amps that are not represented here.

Which does not change the OP's question, 'Why does my tone controls have no effect on the sound?' Even if the frequency response is not flat (and who says that is desirable?) in an amp, the tone controls should effect the sound.
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Old 14th February 2012, 03:42 PM   #9
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Quote:
Originally Posted by JoshuaK View Post
[*]Presence [super-highs]: dramatic reduction of treble overall, no midrange difference, with bass retained - entire signal muffled on zero, as if it were a 'Tone' control on a guitar circuit.[/LIST]
Presence in tone controls is usually the upper-mid range (or "low treble")
FWIW, My old Marshall JCM800 and Fender Princeton guitar amps are "far from flat" when all tone pots are at 12 o'clock.....


YMMV
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Old 14th February 2012, 11:20 PM   #10
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Presence controls are usually the result of reducing the feedback of the power amp by rolling off the treble in the feedback signal. Because there is less voltage to reduce the treble signal coming into the power section there is more treble at the output. Has more effect when running the power amp clean than when in clipping.
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