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Old 25th October 2005, 06:08 PM   #1
Trout is offline Trout  United States
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Question DIY Guitar Amp Advice Needed

Hi Guys,

I just put together a Guitar amp Based Basically On a couple schematics off the web.

The Output stage is pure Fender 5E3 Deluxe, A fairly simple design and easily assembled.

The Pre-stages on the other hand are a derivative of a Modded Plexi I found at Mhuss.com.

Basically the prestage Is where I am in question. Its a clever setup using 1 Hot & 1 Clean Channel.

My Problem is as follows, Channel 1 is the Hot Channel, Its Smoking Good, Great Breakup, VERY VERY Loud.

The Clean channel (2) is the problem. It is very bassy! and produces about 1/2 the output of the hot channel.

I realize I need to fix the tone block as I ran out of 250K pots and Subed a 500K. But this shouldnt affect the output level; to this extreme.

See

Schematic

Any Insight Would be helpful
Thanks!!
Gene
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Old 26th October 2005, 06:27 AM   #2
Enzo is offline Enzo  United States
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The tone stage is common, it doesn't know which input channel you used, so it will not selectively cause one channel to be very low compared to the other.

Your two input stages are identical but for the cathode circuits and the interstage caps values. AND that bright cap on the volume control. Remove the bright cap and see if the two channels are more similar. To me that is a pretty large bright cap and could be the major factor.

Otherwise, I might recheck the values of the mentioned parts. Channel 2 does look like it will have more bottom.
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Old 26th October 2005, 03:25 PM   #3
Trout is offline Trout  United States
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Thanks for The Quick response Enzo.

I removed the brite cap entirely. I actually think its got a better sound without it.

I have checked and rechecked each and every connection, Compared the others to what I have currently running, And they are very much in the ball park to each other.

I revised the schematic and now have exactly as it sits the minute.Revised Schematic

Looking very carefully shows only 2 differences in the 2 channels now, The 1K vs a 2.7k, And the 2 caps feeding the volume control.

I still get about a 65% difference in overall output from the 2 channels.
I followed channel 1 all the way to the volume pot 1 and the connections are correct. VERY Loud Channel !!

I did the same for channel 2, Everythings perfect. at least 65% difference?
At this point, I have no clue other than could it be possibly the pot iteslf?

Those 2 caps feeding the volume control do seem fairly distant from each other, But I amd not sure If thats the issue.

I did swap out the tubes just to confirm that as a non-issue.

Gene

Oh, Theres pics of the unit located in the Tube Section
Click
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Old 27th October 2005, 04:36 AM   #4
Enzo is offline Enzo  United States
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If you turn the pots all the way up, then the differences should go away. Halfway up will show linearity differences between the pots.

You have it down to two resistors and two caps. The cathode resistor does affect gain, and it also affects how easy it is to clip the stage. get a 2.2k resistor and clip it across the 2.7k cathode resistor. That results in about a 1.2k total, close enough to the 1k of the other channel. See what difference it makes.

The duller channel has the larger cap, that seems odd.

Why not be scientific? Apply the same signal to both channels. A clip lead between them or a Y cord will make sure the inputs are identical. 1kHz maybe, though several freqs will tell you more about the amp. High freqs tend to sound louder.

Whatever the input level - I don't really care, 250mv, 500mv, whatever - now check the signal level at the plates of the tube. Now check it at the tops of the pots. Now with the pots maxed, check signal level at the grid. emove signal from one channel then the other to compare what shows up at the grid.

If the signal voltage is different at the plates, it will be so later as well. Once you find the point in the circuit where the gain is reduced, you can then know why.

Have you measured the resistance of the plate load 100ks? And verified the 470k grid mixer resistors? Have yuo verified the resistance of the 68k and 1meg Rs at the inputs? For that matter have you verified the pot values? Have you subbed or measured the two plate caps.

it is one thing to verify the circuit, it is another thing to verify the components themselves.
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Old 27th October 2005, 01:06 PM   #5
Trout is offline Trout  United States
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Understood,

Its very possible that the resistors could be off spec, In fact, Any of these componets could actually be off spec. I did correct the pots in the tone controls, Now they appear to be functioning correctly.

I also uncovered a problem with the rectifier tube and replaced it.

I'll try your ideas on this, They make alot of sense. I figured the dirty channel should be a bit louder, After all it is pushing the tube harder, I suppose I just didnt expect that much difference.

I also thought that cap was a bit larger on the dull channel, But seeing as were down to just a couple posibilities, Im thinking that a solution is very close.

I swapped out the 6V6's to 6L6GC's, Oh My its nice. I feel like this might be my best effort yet, Nother even gets close to warm even after nearly 2 hrs of constant running. Well ok the tubes get warm haha.
Gene
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Old 16th November 2005, 07:21 PM   #6
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Try changing the coupling cap on channel 2, to 0047 instead, the same as channel 1.
As in the chematic channel 2 should be more bassy, the larger cap is passing much more lowfrequency signal, also demanding more current from that stage. Maybe this is what causes the differece in loudness?

Regards
Peter
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Old 16th November 2005, 07:48 PM   #7
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One more thing...
Actually, channel two is pushed a little harder than the other. The smaller 1k cathode resistor makes this triode conduct more current.
Channel 1 triode has more voltage anode-to-cathode, but lower current.
Try a higher value cathode resistor in channel 2.
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Old 16th November 2006, 04:53 PM   #8
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Schematics are dead link
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Old 17th November 2006, 04:31 PM   #9
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Toobes

The toobes may not have the same gain, even if they are the same type and manufacturer. Dual tubes are not matched between sections either.

Consider that.

You can also put a tube in one channel that has higher transconductance than used in the other, especially if they are 12A*7 types...

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