100W 6L6 amp sounds harsh at high Volume - diyAudio
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Old 1st July 2009, 05:32 PM   #1
Jim5150 is offline Jim5150  United Kingdom
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Default 100W 6L6 amp sounds harsh at high Volume

Hi all , I 've recently built myself a 100W combo power amp built on a fender type power amp design. The combo is basically used to amplify my Line 6 PodXT Live floor pedal which provides the distortion/effects etc...
The amp sounds great at lower volume , coupled with my distotion settings from the line 6 I get a wonderfull rich full tight tone comming from the amp which is great.
However when I turn the amp up to the volumes i need to play in my metal band everything gets really harsh and the sound looses its tightness.
Anybody have any ideas why the sound is getting ugly at high volume is this a common problem , Is there a particular section I should be focussing on.
I have some hand drawn schematics that I'll scan and post when I can. But the amp is basically made up of 1 12AT7 gain stage (just to raise the pedalboard to line levels) a 12AT7 phase splitter going into 4 6L6GC tubes. There are no tone controls just a volume which is coupled to the phase splitter section and a prescence control which I was going to use to control the negative feedback from the 16ohm tap on the OT. But for some reason this doesn't work, something that I need to look into.

Hopefully you guys can help me out or point me in the right direction of where to look..

cheers

Jim
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Old 2nd July 2009, 08:45 PM   #2
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If you know what parts values you used I would suggest you give us a schematic with some numbers. That would help us determine your operating points and see what might be wrong.
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Old 2nd July 2009, 09:33 PM   #3
Fenris is offline Fenris  United States
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Not having seen the schematic, I'm going to guess that it might be related to the volume control or depending on how you're running the 12at7, you might not have enough oomph to drive the four 6L6GCs. Or you could be saturating the transformer and getting some weird feedback from this to the "presence". But it's hard to tell what's going on without the schematic.
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Old 3rd July 2009, 11:34 AM   #4
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Bias and iron make a BIG difference here.

Not only a schematic, but OPT and power supply info would be great.

Cheers!
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Old 3rd July 2009, 11:47 AM   #5
Jim5150 is offline Jim5150  United Kingdom
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Hi all , I realise it's hard to make comments without a schematic am in the process of drawing them will post ASAP.

Jim
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Old 3rd July 2009, 11:47 AM   #6
Jim5150 is offline Jim5150  United Kingdom
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Hi all at last I've scanned the schematics in. Apologies that they are hand drawn and are on separate pages but I don't have any software for doing schematics I hope this is OK.

so here goes

This is the Bias supply
Click the image to open in full size.

This is the Power Supply
Click the image to open in full size.

This is the gain stage using a 12AT7 did want to use a 12AX7 but with the distorted signal coming from the POD it sounded over distorted tinny and basically awfull!!
Click the image to open in full size.

This is the Phase Splitter using a 12AT7
Click the image to open in full size.

And this is the 6L6 power section
Click the image to open in full size.

I hope this helps I've basically copied these schematics from various designs and altered values from a 40W 6L6 power amp I built about a year ago. I read alot about the designs in various books and websites but I'm not particularly good at electronics but am OK with a soldering iron. Basically what I was after with this amp was to just amplify my POD to gigging levels so I was after flat frequency response. But as mentioned before at low volumes the amp sounds great but at high volumes looses the tightness especially in the bass. I'd like to get that tight sound at low volume at high volume if that makes sense. But the amp has to be loud as we have a very loud drummer.

There is one last thing thats not on the schematics I've put in a prescence pot (10K) between the 16ohm speaker output and the 820ohm resistor that feeds negative feedback into the Phase splitter. Basically so I can vary the negative feedback. But strangly this doesn't work. Although it's fine on the other 40W amp I built which has the same design. I was hoping to tighten things up using this control (but usually at the expense of dramatic loss in volume)

Any help you guys can give me will be greatly appreciated. I've spent alot of time and money on this amp (Parts are hard to get in the UK) . It would be a shame if I can't get it to sound the way I want.

Again apologies for the scruffy drawings.

thanks

Jim

PS will be including some Photos soon as well.
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Old 3rd July 2009, 12:13 PM   #7
Jim5150 is offline Jim5150  United Kingdom
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I got the output transformer & Mains transformer from Torres UK. I explained what I was trying to do and they suggested

a Vyse VYLM022B 240V 100W Mains Transformer
Output at 750V @ 150mA, 50V Bias Tap and a 6.3V @ 6A Heater Tap.

As for the OTP its a VYL007A 100W Output Transformer 4,8,16 ohm outputs
And thats pretty much all I know about them.

Cheers

Jim
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Old 3rd July 2009, 12:25 PM   #8
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There is a basic design flaw.
You are injecting the NFB before the master volume.
When you increase the volume, you are also increasing the NFB.
Perhaps there is too much NFB when the volume is turned up towards maximum. This can cause instability.
Remove the NFB and see if the distortion goes down at high levels.
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Old 3rd July 2009, 02:27 PM   #9
Jim5150 is offline Jim5150  United Kingdom
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Hi Frank , I disconnected the NFB and it hasn't made a difference in the sound. Mild distortion is present when I turn the volume up.I'm wondering if I have too much gain on the first stage. But this shouldn't be affected by volume??
What I have noticed is that if I just plug a guitar in the amp with no POD as I increase the volume the bass gets very loud with the mids and the highs quiet. I wonder if this + the distortion of the POD is providing too much bass and turning the sound harsh.


Jim
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Old 3rd July 2009, 04:13 PM   #10
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That's possible.
Bass frequencies eat up far more amplifier power than mid/high frequencies.
Is it possible to move the location of your master volume control to the input of the phase splitter? That will keep the NFB loop in the circuit at all times. At the same time, you can eliminate the two .047uf caps.
The increase in bass and decrease in mid/highs may be caused by the interaction of the master volume control, the .047 caps and the 220k resistors from the bias supply.
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