Issue of Express headroom

[HiGain]

For my express clone, I followed the Kelly 90 schematic, exactly. I have very little headroom, basically none, with my Heritage Les Paul, and I even have to roll the volume of my tele back to arrive at clean tones.

I tried various preamp tubes, and am using EH EL34s.

Some one who knows a lot more about this amp than me has urged me to experiment with different tubes.

I as thinking of trying some lower gain preamp tubes, together with S.E.D winged C El34s, which are supposed to have a lot of detail, smooth breakup, and higher headroom.

I will report on results.

However, can anyone else think of why this amp, which is known to have decent headroom, does not?

You guys out there that have reported less gain... Could you please describe your circuit/setups?

Thanks,
Jake

[doctord02]

I dont really think the Express is a high headroom amp at all. One certianly wouldnt get that impression from listening to any clips of a Wreck...
It's pretty much a full out, balls to the wall, screaming lead amp.

It does clean up OK with a twist of the guitar volume, but it's not gonna give you Ampeg or BF Twin cleans no matter what.

One thing you can do to make it a little less out of control sounding is to change the cathode resistor of the third stage from a 10k ohm to say a 5k ohm.

[loverocker]

I've done one quick-n-dirty semi-clone and that also had very little headroom. Cleaned up very well though. I changed preamp valves to little effect.

The plan for my second (a proper Express clone) is to try a dual pot, where the second element sits between the 3rd stage and PI. I reckon I'll have to experiment with extra Rs for this voltage divider to fine tune the amp's response over the pot's sweep.

[HiGain]

Well, I know it's a high gain monster. It's just that I'ver heard reports that it actually gets at least some headroom.

I should also mention that I used the preamp and wiring layouts provided by Mark A. (see pic)

Jake

[Jackie Treehorn]

There's not a lot of talk about "gain staging" in the sense of getting the various parts of the amp to distort in the right order. Power amp first, phase inverter, preamp etc. Personally, I think it's far more important than which schematic you follow. If you have the gain staging right, you will get the full clean wattage the amp is capable of producing. If I were you, I'd scope the amp to see where it first clips and take it from there.

However, I suspect that it will be your phase inverter. I'm sorry for slamming A1a, but in my opinion, A1A lacks headroom, bass, and sounds thin because the phase inverter is slammed so hard. Stick a 47k tail, 1.2k cathode resistor on the phase inveter and you will get a huge sounding amp with far more headroom, less gain, etc. I would go down from there, adjusting the values to taste. Another methodology to gain staging would be measuring the AC signal off the phase inverter and comparing it to your DC bias setting. Get them so the grid of the power tube is at or extremely near 0 volts. If none of the preamp triodes are clipping significantly before that point, you've probably got as much headroom as possible.

[Funkalicousgroove]

I find it helpful to change the 150K voltage divider after the second stage to a 100K trimmer and a 56K resistor, that way you can dial in the sweet spot, I built one amp with this control on the front pannel, it sounds great and cleans up nicely.

[macula56]

do you mean like ver. A3 where it's 47K and 1K?

[Elcabong]

I find it helpful to change the 150K voltage divider after the second stage to a 100K trimmer and a 56K resistor, that way you can dial in the sweet spot, I built one amp with this control on the front pannel, it sounds great and cleans up nicely.

Does it affect the tone at the same time?

[Funkalicousgroove]

Let's use "Kelly" as the example, from the plate of V2b there is a.002uf coupling cap and then a 150K voltage divider to ground forming a sort of hi-pass filter. Some of the schematics from blueguitar.org have this value as low as 56K, so I put a 100K trimmer wired as a variable resistor in series with the 56K so that I had a range of 56K to 156K to play with. I seem to like it around 110K, but it does not dramatically change the "Tone" just the amount of distortion when you play hard. on lower settings the amp will clean up more, but it won't break up as much-Tradeoff I s'pose.

[Robert]

Jackie, I'm not familiar with the term "tail", although I have heard it before. Could you specifically describe the location of the changes you refer to:

Stick a 47k tail, 1.2k cathode resistor on the phase inveter

Also, where would one take this measurement:

Another methodology to gain staging would be measuring the AC signal off the phase inverter and comparing it to your DC bias setting.

I think I have mentioned before, the Fast Gradual Switch(see Tranflex schematic) knocks down a lot of the distortion and increases clean headroom. A nice side effect is it complety eliminates the background hiss of the amp at idle. I recently experimented with A/B switching it in and out and found it to also decrease sensitivity and touch response and decided I like that much better so I removed it. I also installed the TR MV. That allows me to decrease the preamp gain/volume while increasing the overall volume. I can't imagine needing more clean volume than that puts out.

[Elcabong]

I think I have mentioned before, the Fast Gradual Switch(see Tranflex schematic) knocks down a lot of the distortion and increases clean headroom. A nice side effect is it complety eliminates the background hiss of the amp at idle. I recently experimented with A/B switching it in and out and found it to also decrease sensitivity and touch response and decided I like that much better so I removed it.

I am not sure to follow you here, Robert. So finally, do you have the Fast/Gradual Switch removed from your built?

I would think that when the switch is on the lower position (using 100k plate and .002uF cap), it should sounds the same as a circuit not using this feature.

[HiGain]

Thanks for the replies!

So:

Introduce the split load.
Increase the tail/cathode resistors in the phase inverter.
Install another volume pot.
Lower the 150K resistor (possibly using a pot) in the HPF heading into the third stage.
Reduce the cathode R of the third stage for a less wild feel.

I suppose I could also reduce some cathode bypass caps, or add a voltage divider ahead of one of the stages, like in the Komet, or version A7.

What I'd like to do is whatever is simple and has the least impact on the feel and tone.

I think I will make the 150K variable. In my other wreck clone (two rock) I installed a 150K center detent pot in series with a 27K resistor for the 150K R position. Center is 150K and I can go in either direction. I use this a lot, but notice it affects low end response as well as gain.

I will try increase the phase splitter resitor to 22k or 47k and see what that does (I'm really intrigued based on what Jackie said), keeping in mind Jackie's comment about gain staging. I think you can try to do some gain staging with tube selection as well.

Question to Robert: Which TR master volume did you use?

Thanks again for all the terrific replies!!

Jake

[Robert]

I would try the voltage divider off of 2nd gain stage, that is the Fast Gradual switch. I have tried both Mark Abbotts Express Var. No.2 with no caps and the Trainflex1.1 version with caps. That is the version I tried most recently. Rather simple to implement even with a switch. I do think using 22k and 82k it reduced the touch sensitivity of the amp and so I chose to remove the Fast/Gradual switch. I preferred the sensitivity and will adjust the distortion by guitar volume and pick attack. In hindsight I should have experimented with different values of resistors to try to retain the decrease in hiss and increased clean headroom while minimizing the decrease touch sensitivity.

Hi Gain, the TR MV I refer to was on the layout that came with their kit. If you didn't get one I will try scanning it for you or redrawing the MV circuit and send that. Let me know your email.

[Robert]

Elcabong, sorry for the confusion. You are correct, when selecting the normal circuit values with the switch, the amp sounded normal. However my problem was the loss of touch response when engaging the 22k-0.1uF side of the circuit. I did not think I would use it enough to justify installing it as a permanent switch on the front of my amp and removed the mod. I now think it would have been worth experimenting with different resistor values to retain the desireable aspects and decrease the negative.

[HiGain]

OK, here's an update.

I did not have much luck varying tubes to reduce gain. I tried different 12AX7 tubes, as well as a couple 12AU7 and 12AT7 tubes. The latter were lower gain, but I did like the sound as much. I don't have any 12AY7s.

Next:

I tried Jackie Treehorn's suggestion to increase the K resistors in the PI to 47k/1k2. A little less gain, but still not enough headroom for me. It also seems to make the amp sound a tad bassier, or darker, which I liked.

Next:

Reduced the .002/150k HPF ahead of third stage to .0057 (.001 in par. with .0047) / 22k. This reduced the gain just the right amount, without altering the frequency response of the filter.

Now I have a fair amount of clean headroom, a nice bluesy breakup for teles and strats (sounds like a really ballsy bf amp), and it can still go into high gain territory (infinite harmonics and sustain). The interesting thing is that with medium crunch settings, the amp has this throaty impulse to tear (best word I can come up with) into harmonics. It's strangely good.

Last:

You can maximize the headroom from an exprees with the bright switch. Use a 47pF-82pF cap. Turn your gain to very low, turn your treble all the way off, and turn your mids down, and bass up. With the bright switch engaged you have plenty of sheen, and a good amount of volume. Sounds very nice, and sort of fendery.

Jake