Traynor YBA-2 (EL84 Version) bad sounding asymmetric distortion... what's up?

[Chuck H]

Another report on the CAD model...

Input signal taken to 100mV performance is excellent reaching maximum clean volume at 5 on all controls (roughly) and about 1V at the input of the PI. Like they were fashioning it after a phono amp or something.

The tone stack is a turd with poor independence between the controls, bands and huge losses.

Signal peaks do indeed clip in the preamp and this is preserved in the power amp at volume settings below about six (assuming a 20% audio taper). This may be where the ugly distortion is coming from, but I can't say for certain. I can say that if this is the case then there is no repair for the problem if the amp is left stock, BUT...

There's an easy fix. Most amps, for clean tones anyway, uber gain preamps not withstanding, always clip the power tubes first. This way maximum clean volume is achieved. With the volume control late in the preamp and the early preamp designed for such small signal sensitivity you could get some preamp clipping artifacts. and this amp IS NOT designed for good preamp clipping So...

Just move the volume control to right after the first triode like 99% of all other guitar amps (as well as the 6v6 yba2). You'll get roughly the same volume levels at volume control settings. But now the ugly preamp artifacts will be masked because the power tubes will be clipping well before it happens. As a bonus this mod should be more compatible with the front panel layout than the circuit that's in there now.

Here's a modified schematic:

[Helmholtz]

and if I plug in .5VAC for an input signal, which I assume for the voltage "component" in spice is 1Vpp???

Yes, spice value is voltage amplitude (= peak value Vp) and Vp = Vpp/2.

[waspclothes]

Another report on the CAD model...

Input signal taken to 100mV performance is excellent reaching maximum clean volume at 5 on all controls (roughly) and about 1V at the input of the PI. Like they were fashioning it after a phono amp or something.

The tone stack is a turd with poor independence between the controls, bands and huge losses.

Signal peaks do indeed clip in the preamp and this is preserved in the power amp at volume settings below about six (assuming a 20% audio taper). This may be where the ugly distortion is coming from, but I can't say for certain. I can say that if this is the case then there is no repair for the problem if the amp is left stock, BUT...

There's an easy fix. Most amps, for clean tones anyway, uber gain preamps not withstanding, always clip the power tubes first. This way maximum clean volume is achieved. With the volume control late in the preamp and the early preamp designed for such small signal sensitivity you could get some preamp clipping artifacts. and this amp IS NOT designed for good preamp clipping So...

Just move the volume control to right after the first triode like 99% of all other guitar amps (as well as the 6v6 yba2). You'll get roughly the same volume levels at volume control settings. But now the ugly preamp artifacts will be masked because the power tubes will be clipping well before it happens. As a bonus this mod should be more compatible with the front panel layout than the circuit that's in there now.

Thanks for that Analysis Chuck! I'm glad to hear someone confirm that this amp design isn't really fantastic for overdriven guitar. It's a shame that it's called a YBA-2 just the same as the version with the cathode biased 6L6's that breaks up like a beautiful 5E3 but for $300

The mod you showed is exactly what I did, except I left the original volume pot there as well for now.

[Chuck H]

The mod you showed is exactly what I did, except I left the original volume pot there as well for now.

IMHO you may as well just replace it with a 1M resistor. I don't think there's any tonal advantage to turning it down at that second volume control.

Hopefully you didn't already drill an extra hole in the face plate? If voicing it for guitar is the goal then there's more that could be done. It's a REALLY bare bones amp that's pretty much wide open for clean tones and no consideration of overdrive character.

[waspclothes]

IMHO you may as well just replace it with a 1M resistor. I don't think there's any tonal advantage to turning it down at that second volume control.

Hopefully you didn't already drill an extra hole in the face plate? If voicing it for guitar is the goal then there's more that could be done. It's a REALLY bare bones amp that's pretty much wide open for clean tones and no consideration of overdrive character.

I put the volume pot in the second input hole. My intention was just to temporarily tweak it until it sounded good and replace it with fixed resistors, but I agree with what you're saying that I may as well just remove the original volume pot. I guess it's sort of like having a pre-PI master volume, but.. no real point in that on this little amp IMO.

If there's any changes you think are advantageous on your simulation, I'm happy to try them on the real amp and let you know the results Chuck. At this point I just told my friend that if he wants the tone of my YBA-2 then to get a 6V6 version, but if there's some changes we can make then I'm all for it.

[Chuck H]

OOOooh... Ok. I'll try to be minimalist about it, but challenge accepted There's only so much LTSpice can tell me, but I do have a good bit of real world experience with this power tube.

First I'd like to know how your friend plays.?. Does he like to run the amp clean and use a pedal board? Does he just crank the amp and play with the volume on his guitar? What's he looking for? What would be hid ideal?

[waspclothes]

OOOooh... Ok. I'll try to be minimalist about it, but challenge accepted There's only so much LTSpice can tell me, but I do have a good bit of real world experience with this power tube.

First I'd like to know how your friend plays.?. Does he like to run the amp clean and use a pedal board? Does he just crank the amp and play with the volume on his guitar? What's he looking for? What would be hid ideal?

He wants all of his dirt to come from the amp. I think that's why he liked my 6V6 YBA-2 so much, that thing is just some very nice distortion that reminds me of a cranked up Princeton Reverb. He plays with humbuckers and the speaker he wants to pair this with is a greenback, so I think he's going for a more british vibe.

[Chuck H]

And how much gain? Genre? Blues/rock, hard rock, metal? Gritty or saturated?

[waspclothes]

And how much gain? Genre? Blues/rock, hard rock, metal? Gritty or saturated?

He plays classic rock but likes distortion. I can't comment on gritty/saturated.. but probably nothing too aggressive, I think that greenback is already going to add some grit.

[eschertron]

He wants all of his dirt to come from the amp. I think that's why he liked my 6V6 YBA-2 so much, that thing is just some very nice distortion that reminds me of a cranked up Princeton Reverb. He plays with humbuckers and the speaker he wants to pair this with is a greenback, so I think he's going for a more british vibe.

Didn't the AC15 originally have a very high-gain input stage with an EF86? How close to a very British AC15 could this thing be nudged?

off topic: I purchased Hammond's replacement OT (10k primary) for the YBA-2 to build a 2x EL84 (and alternately 2x EL844) a while back. Haven't yet figured out what the rest of the circuit will look like, so this thread has my utmost interest. Keep up the good work!

[Dave H]

At this point I just told my friend that if he wants the tone of my YBA-2 then to get a 6V6 version, but if there's some changes we can make then I'm all for it.

It sounds like he doesn't like the pre-amp distortion produced by the cascaded gain stages in the EL84 version but he likes the power amp distortion produced by the single gain stage 6V6 version. You could try eliminating the second gain stage from his amp as follows - Centre bias the input stage by changing the cathode resistor to 1k5 bypassed by 22u. Remove C1, disconnect C3 from V1b plate and connect it to V1a plate. Its a simple easily reversible mod.

[Chuck H]

Ok, here's what I have for a "guitar amp" improvement for this amp.

The changes may not be what you expected, but since this amp will likely be overdriven a lot and the power tubes are at a high voltage I think measures need to be taken to keep them safe and improve the clipped wave form.

This is not an easy topology to work with. This is the minimally invasive alterations and additions. A tear down and rebuild as a different amp could be better, and a lot more work.

Component value changes and additions in red.

This will have a bit more gain than the stock amp because you mentioned your friend wanted to get his distortion from the amp. You might wish to put the first stage cathode bypass cap on a "boost switch" just to have a mode where it's easier to get a cleaner tone at a "normal" volume setting. The bypass cap value should be experimented with to see what sounds "right". And please do away with the "second" volume control at the end of the tone stack you have now. It can only make the amp sound wrong.

And since you're doing this for someone other than yourself I think it would be irresponsible not to add a proper three prong, grounded AC power cord.

[printer2]

Ok, here's what I have for a "guitar amp" improvement for this amp.

The changes may not be what you expected, but since this amp will likely be overdriven a lot and the power tubes are at a high voltage I think measures need to be taken to keep them safe and improve the clipped wave form.

This is not an easy topology to work with. This is the minimally invasive alterations and additions. A tear down and rebuild as a different amp could be better, and a lot more work.

Component value changes and additions in red.

This will have a bit more gain than the stock amp because you mentioned your friend wanted to get his distortion from the amp. You might wish to put the first stage cathode bypass cap on a "boost switch" just to have a mode where it's easier to get a cleaner tone at a "normal" volume setting. The bypass cap value should be experimented with to see what sounds "right". And please do away with the "second" volume control at the end of the tone stack you have now. It can only make the amp sound wrong.

And since you're doing this for someone other than yourself I think it would be irresponsible not to add a proper three prong, grounded AC power cord.

Would the change from 1k to 150 ohms in the power supply increase the plate voltages on the EK84's?

[Chuck H]

Would the change from 1k to 150 ohms in the power supply increase the plate voltages on the EK84's?

I'm confused. There was no 1k resistor that I changed. The 1k resistor to the screen supply remains as it was (though I might be inclined to up it's value to 2.2k.?.). I ADDED the 150 ohm resistor where there was none before. It's purpose is to add a little power supply sag to soften attack and add a small amount of compression to the overdriven tone.

[printer2]

I'm confused. There was no 1k resistor that I changed. The 1k resistor to the screen supply remains as it was (though I might be inclined to up it's value to 2.2k.?.). I ADDED the 150 ohm resistor where there was none before. It's purpose is to add a little power supply sag to soften attack and add a small amount of compression to the overdriven tone.

Sorry, you added the resistor not replaced it as I thought. I think I will go hang my head in shame and go stand in the corner.