Great Clean Sound But Poor Distortion With OD/Distortion Pedals

[HR_Puffinblunts]

Amp is a DIY combo amp that is a cross between a 5F1 and AX84 P1 Extreme. Single ended 6V6 output, single 12AX7 preamp, solid state diode rectifier, no tone stack, switchable cathode by-pass caps, global feedback, and a single Eminence Texas Heat (99.4 dB sensitivity) 12" speaker. I built it about 7 years ago and it has always powered up and worked properly other than the bad distortion with OD/Distortion pedals. I've built several tube amp and this is the only one with this problem

The amp sound great clean but boosted (pedal) distortion sounds terrible. The best way I can describe it is that it sounds to Hi-Fi..unnatural speaker breakup when using an overdrive or distortion pedal at all output levels. I've tried other speaker cabs and same issue arise so, my guess is the issue lies within the circuit and may be a matter of upping input grid resistor values at the cost of losing treble. The current preamp input grid resistor values are 1st stage: 33k , second stage 220k with a paralled 100 pF bypass cap. Output stage input grid resistor is 10K.

Any suggestions, please?

[g1]

I was going to say "why don't you ask Freddy the freakin' magic flute already" but I won't do that.

You say you have other builds without the problem, how does the input sensitivity compare to the other amps? (as in, without pedals, how high can you turn up the volume before it starts to distort)

[mozz]

Even though i can almost picture a ax84 p1 in my head, draw it out so we can see what's there.

[shortcircuit]

Your edit made my reply moot...but I'll keep the "welcome to the forum" ^^^^^^^^^^^schematic would help , pics maybe

[eschertron]

...or a soundclip. I was thinking 'farty' bass, but maybe not? Where's the volume control? Without a tone stack between stage 1 and stage 2, any boost in front is likely to simply "splatter" the waveform in stage 2. Oh, yeah. A schematic would help.

[The Dude]

When I hear "too Hi-Fi", I think too much high frequency. Extra highs can sound good on clean guitar- not so much on distorted guitar.

[Tony Bones]

Are you using the pedals to generate the overdrive signal, or are they providing more of a clean boost in signal level so that the overdrive occurs in the tube amp? I wouldn't expect big grid resistors to help much in the first case, but might be worth trying in the second.

[HR_Puffinblunts]

I was going to say "why don't you ask Freddy the freakin' magic flute already" but I won't do that.

Because, Freddie's Dead

https://www.youtube.com/watch?v=cTLQiIbDN7U

You say you have other builds without the problem, how does the input sensitivity compare to the other amps? (as in, without pedals, how high can you turn up the volume before it starts to distort)

Amp starts to naturally distort at around 60% -70% volume. Natural distortion sounds pretty nice and the sensitivity seems in line with other SE amps I've built.

[Chuck H]

At least part of the problem may be that the clipped waveform symmetry and other power tube behaviors may be getting too far out of typical or even safe operation when the front end is boosted. It's possible something about the design is allowing far too much signal to the grid of the power tube when you use the pedal the same way you have for other amps. As an aside, it may be a good thing that is sounds bad so you won't play the amp that way because if this is what's happening it could be very bad for the screen grid in the power tube.

If you are using the pedal as a boost I might suggest that for this particular amp you don't use the pedal as you normally do. Rather, turn up the pedal distortion a bit and turn down the pedal volume a bit. Repeat until it no longer sounds harsh and ratty. I don't think you'll notice a significant decrease in overall volume in the final settings if you pay attention. It won't be the "more tube distortion" and less pedal distortion that is the goal of a "boost", but if I'm correct then you may actually damage the amp doing what you have been. Maybe not yet. Maybe not tomorrow. But soon enough.

[HR_Puffinblunts]

When I hear "too Hi-Fi", I think too much high frequency. Extra highs can sound good on clean guitar- not so much on distorted guitar.

That is what I have been thinking as well. I also think that with not tone stack and subsequent lowered signal it may be clamping the 2nd stage input grid and causing blocking distortion: http://www.aikenamps.com/index.php/w...ing-distortion

[HR_Puffinblunts]

...or a soundclip. I was thinking 'farty' bass, but maybe not? Where's the volume control? Without a tone stack between stage 1 and stage 2, any boost in front is likely to simply "splatter" the waveform in stage 2. Oh, yeah. A schematic would help.

Not a case of farty bass. No-master volume and the volume control is in-between preamp stages. I agree concerning splattering waveform and I am considering adding resistance to assimilate tone stack loss...any recommendation for value(s)?

I am trying to post a schematic but I don't see 'Photos & Albums' link or a User Control anywhere.

[HR_Puffinblunts]

At least part of the problem may be that the clipped waveform symmetry and other power tube behaviors may be getting too far out of typical or even safe operation when the front end is boosted. It's possible something about the design is allowing far too much signal to the grid of the power tube when you use the pedal the same way you have for other amps. As an aside, it may be a good thing that is sounds bad so you won't play the amp that way because if this is what's happening it could be very bad for the screen grid in the power tube.

Could it possibly be that since I'm using a 6V6 in place of an EL84 that the Hammond 269EX PT is supplying only 255 volts which may operate the tube at a less than optimal range?

[HR_Puffinblunts]

Schematic:



Sound clip: https://www.youtube.com/watch?v=vrp74vkftqo

[tboy]

That can't be right

The schematic doesn't show any ground reference for the 6V6 grid.

[HR_Puffinblunts]

The schematic doesn't show any ground reference for the 6V6 grid.

I think I cathode biased it around 12 or 13 volts.