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
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
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.
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
This is the Power Supply
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!!
This is the Phase Splitter using a 12AT7
And this is the 6L6 power section
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.
so here goes
This is the Bias supply
This is the Power Supply
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!!
This is the Phase Splitter using a 12AT7
And this is the 6L6 power section
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.
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
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
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.
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.
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
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
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.
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.
That pot would be better between the gain stage and the phase splitter, And doing so eliminates a pair of capacitors. Try that first and see what happens to the distortion. It may also be a tracking mismatch in your pots channels, IE, they attenuate dissimilarily at at high settings, which could muck with the feedback and phase, affecting the sound.
OK time to get the soldering iron out me thinks.
There is one mistake on the schematics the coupling cap out of the gain stage is actually a .2 not a .1 would this make a difference to the frequency response?
I will move the volume pot and let you all know how it works out.
thanks all appreciated
Jim
There is one mistake on the schematics the coupling cap out of the gain stage is actually a .2 not a .1 would this make a difference to the frequency response?
I will move the volume pot and let you all know how it works out.
thanks all appreciated
Jim
Apologies again the coupling cap out of the first gain stage is actually a .02uF not a .1 as in the schematic or a .2 in my previous posting! I wonder if this is producing too much bass.
Can anyone comment on a suitable coupling cap that would get me a flat frequency response or close too it?
Can anyone comment on a suitable coupling cap that would get me a flat frequency response or close too it?
Hi all , Made the changes to the amp so that the volume pot now sits between the gain stage and the Phase Splitter. This has altered the sound dramatically if anything the amp is now even more harsh at high volumes and the bass has turned into a farting noise similar to a blown speaker noise. I've checked out the speakers with a different amp and they are fine.
There is still alot of distortion evident at high volumes and the Presence pot does still not work.
I'm seriously looking at changing the coupling capacitor to a .1 instead of a .02 to see if that makes a difference.
I'm also thinking of upping the gain of the first stage. When I use the Line 6 pedal I have to have the level on the pedal quite high to get the volume I need from the amp. I'm wondering if it's too much for that gain stage and it's over distorting. If I can increase the gain for the first stage I can have the volume on the line6 lower and see if that helps. I'm currently using a 120K and a 1.3K for the plate and cathode resistors. Looking at some other schematics for poweramps (e.g mesa 290) . A 100K and a 1K is used so I think I may try that. Also on these schematics .1uf is used for the coupling cap.
Anyone have any comments on what I'm trying to do , or am I barking up the wrong tree.
Jim
There is still alot of distortion evident at high volumes and the Presence pot does still not work.
I'm seriously looking at changing the coupling capacitor to a .1 instead of a .02 to see if that makes a difference.
I'm also thinking of upping the gain of the first stage. When I use the Line 6 pedal I have to have the level on the pedal quite high to get the volume I need from the amp. I'm wondering if it's too much for that gain stage and it's over distorting. If I can increase the gain for the first stage I can have the volume on the line6 lower and see if that helps. I'm currently using a 120K and a 1.3K for the plate and cathode resistors. Looking at some other schematics for poweramps (e.g mesa 290) . A 100K and a 1K is used so I think I may try that. Also on these schematics .1uf is used for the coupling cap.
Anyone have any comments on what I'm trying to do , or am I barking up the wrong tree.
Jim
Hello,
Sorry to jump in on this. I am new to the forum but I think I may be able to help.
Your power transformer seems too small. Is it really rated for 150mA? if it is then it would be fine in a 40W amp. You probably cant even bias you output valves enough.
I dont know if you have measured the bias when you built it?
I guess you can see if the supply is drooping loads if you get a friend to play loud whilst you measure the supply voltage. Be careful, take your time, the voltages are lethal.
Cheers
Matt.
Sorry to jump in on this. I am new to the forum but I think I may be able to help.
Your power transformer seems too small. Is it really rated for 150mA? if it is then it would be fine in a 40W amp. You probably cant even bias you output valves enough.
I dont know if you have measured the bias when you built it?
I guess you can see if the supply is drooping loads if you get a friend to play loud whilst you measure the supply voltage. Be careful, take your time, the voltages are lethal.
Cheers
Matt.
Also,
What is the voltage after the choke, I am guesing (cant spell to save my life) that you are taking your screen supply from here (B2 point 2 on diag.) I dont think you will have a probem with screen dissipation with the power tranny you have but do be careful not to exceed the specs for screen dis.
Cheers
Matt.
What is the voltage after the choke, I am guesing (cant spell to save my life) that you are taking your screen supply from here (B2 point 2 on diag.) I dont think you will have a probem with screen dissipation with the power tranny you have but do be careful not to exceed the specs for screen dis.
Cheers
Matt.
Hi Matt, Please feel free to jump in any help anyone can give me is most appreciated.I don't think the whole transformer is rated at 150MA just the HT part which provides 750V @150mA which I'm sure is enough to run 4 6L6's (about 30mA each I think) . There is a separate 50V tap for the bias and also a separate heater supply tap.
I have measured the Bias coming out of the supply circuit and using the 10K pot to adjust I can get about -70Volts to about -30.
In any case I seem to be able to bias the power tubes fine.
I have also measured the voltage after the choke and from memory getting about 500V. But will check it again when I can.
Are we saying the voltage is too high and exceeding the spec for screen dissapation?
thanks
James
I have measured the Bias coming out of the supply circuit and using the 10K pot to adjust I can get about -70Volts to about -30.
In any case I seem to be able to bias the power tubes fine.
I have also measured the voltage after the choke and from memory getting about 500V. But will check it again when I can.
Are we saying the voltage is too high and exceeding the spec for screen dissapation?
thanks
James
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