Help me fine tune my new Liverpool build...

[mooso]

This is a Liverpool build using the Mark Abbott schematic and the Ceriatone LiverPull layout.

I used what I had laying around, so it is not an official clone, but a re-creation of the circuit.

The chassis is a steel Tweed Hi-Power Twin. The transformers are a Vox AC30 set. I used the AC30 choke in place of the 1K 25W resistor in the power supply, and a GZ34 rectifier tube in place of the bridge rectifier. Everything else was to the original specs.

The amp came out very edgy sounding. Almost diode clipper-like in its distortion characteristics. Not smooth in the least. It comes on volume hard between 2 and 4 and is really dirty from 3 on up with P90s or Humbuckers. To counter this, I made several quick mods:

Changed the input resistor from 15K to 68K.
Changed the treble cap from 500pF to 150pF.
Changed the slope resistor from 68K to 56K.
Unhooked the treble bypass cap from the volume control.
Changed the 12AX7 inverter to a 12AT7.

It's better, tone wise, but still has a real harshness to the distortion and the volume still comes on like a sledgehammer between 2 and 4. Lots of volume, but no real headroom. I prefer an amp that rolls into natural distortion smoothly starting about half way up. This one is hard on it at 3. I don't have any idea what a real Liverpool sounds like, but have heard a couple of Youtube clips and they sure seem to have more headroom and distort smoother than mine.

I know plate voltages can play a big part in the overall tonal character, but I could not find a listing of them to compare. Here are the voltages on my amp:

V1A - 181
V1B - 251
V2A - 246
V3A - 234
V3B - 222
V4 - 7 340

The pre-amp voltages are certainly high enough for headroom...are they too high??

Any info or opinions you have will be greatly appreciated. I'm a big fan of AC 30s and have owned and worked on many originals and reissues and have always admired their clean chime and the smooth, singing tone when pushed hard. I was hoping for Trainwreck magic, but this baby needs some more attention before I can call it done.

Thanks, Mooso.

[funkmeblue]

first off, the liverpool is a high gain amp. A vox on steroids if you will. The rocket was Ken's answer to the ac30. With that said voltages should be....b+1=331v, b+2=318v, b+3=269v, b+4=256v, and b+5=244v. The gain does come on fast as do the great harmonics. Use the volume control on your guitar to get clean sounds. The treble cap should be 50pf, and you could try lowering the 56k resistor to 33k. What kind of speakers are you using, what kind of tubes.....? Any pictures? Lead dress is very important. I usually run mine with the volume about 1 o'clock, bright switch on the 100 pf setting, the treble at 11 o'clock and the rest at noon through a 4x12 with greenbacks with a tele. beautifull cleans with the volume on the tele backed off and great singing leads with it cranked.

[mooso]

I tried it with the 50pF treble cap, but it seemed to render the treble control nearly useless. The 150pF gave me more control

The speaker cab is a closed back Marshall 4x12 with V-30s and basketweave cloth, so it is a pretty dark sounding cab.

Preamp tubes are a good RCA, a Sovtek WB, and a NOS Phillips. The power tubes are the worst...Sovtek EL84Ms. I will opt for better tubes once I get it sounding better.

B1 = 344, B2 = 337, B3 = 281, B4 = 268, B5 = 258. So they are all high, but less than 10%.

I'll change that slope resistor to 33K.

No pictures yet, but I did something funky on the lead dress. The B+ runs are not under the board as usual, they parallel eachother and run down the side of the board on the tube side. I know, not the best choice. There are only 3 of these runs and I could drill access holes and re-run them easily enough if this proved to be a problem with the tone. The amp is not overly noisy as it is.

I'm good with backing off the guitar volume to get to the clean tones, but my problem is, the overdriven tones when the guitar volume is brought back up are really edgy and hard. I'm looking to smooth this harshness.

Do you think I should manipulate the voltage dividers to get nearer the B+ voltages you measured? Could these higher voltages be just over the edge into ice-pick territory?

Thanks for the help, Mooso.

[funkmeblue]

I thinking maybe your not getting enough "sag" out of the power supply due to the choke and lack of 1k resistor. When you hit a pool hard the preamp starves some and copresses real nice smoothing out everything. maybe you could add it after the choke or in place of the 22k resistor like the k60 does.....just a 5w 1k

[surfsup]

Do you mean in addition to the 22k resistor? In place of it would really crank up the voltages on the preamp tubes. I plan to probably put a 560R 3w in line with my choke on my version of the liverpool...my choke is 440R so that works out to be 1k

[zambo]

how big of a difference does the trebel cap make? The Ceriatone liverpull shows a 500pf which i used. I am getting some funky swirl and oscilation from it. would that cause it?

[mooso]

Well, I tried the 33K and it is still not what I'm looking for. I got out the Telly and I'm not having trouble getting clean tones with the guitar volume backed down, but when cranked, it is still real harsh and the tone controls don't seem to be able to defeat it.

I'll try adding a resistor to the choke as you suggested. My choke measures pretty close to 440 ohms as well. Because of the layout and limited room in my chassis, it will be hard to put the resistor after the choke, but will be much easier to fit it in before the choke. Would the placement of this non-inductive load affect the sag we are looking for?

Thanks for the help guys, Mooso.

[Gaz]

Seems like this particular amp may benefit from a cut control ala AC30. Hi-gain EL84 amps aren't for everyone, and I'm thinking the amp may just have a little too much gain for your particular taste. You can easily add a voltage divider before the PI, or even a pre-phase inverter master volume. This may be a good experiment anyway to see if the harshness you're hearing is being generated in the preamp or power amp. It's good you're not to worried about deviating from the stock schematic, because it seems like all the little differences have added up and you just need to make some judicious tweaks to get it sounding right to you! Good luck!

[jaysg]

...you could try lowering the 56k resistor to 33k.

You have to change the cap value there too, to keep the RC rolloff at around 350 to 400Hz.

[fishy]

+1

With a Liverpool I generally prefer 68k / .0047uF here. Gain is still abundant.

My preference would be to go back towards a 50pF for the treble cap and adjust R7/C8 initially.

If the tone stack doesn't seem to respond well, may be something is amiss with it.

[mooso]

OK, I've been studying. You all probably know this, but here goes.

This is basically a Blackface tone stack (not my favorite to start) followed by a tuned "universal gain stage" as layed out in the "Trainwreck Pages".

This gives me a basis for modifying the gain stage. First I'm going to bypass it completely. If I like the basic tone of the pre-amp and power section, then I'm going to try to reduce the gain in the gain stage to see if it fattens it up.

I've looked at a bunch of EL84 circuits and it is clear to me now that my B+ voltages are just fine where they are. Nothing is being overdriven into harshness by high voltage.

If none of this works, then, this is not the circuit for me. I'll rewire it to the Tweed Bassman / JMP Marshall / AC30 style tone stack.

Zambo, Ken Fisher suggested that you add a supression cap in parallel with the 100K plate resistor of V2 to combat oscillation in the gain stage. 250pf, 500pf, to .001mf if necessary. You might try this.

Anyway, I'm not giving up yet! Still have work to do.

Thanks, Mooso.

[Ken Moon]

One easy way to experiment with that 3rd stage is to replace the 150k resistor (between the 3rd stage grid and ground) with a more traditional voltage divider setup (after the coupling cap, put two resistors in series to ground, and connect the junction of the two resistors to the grid of the third stage).

Even before that, try reducing the 150k to 100k - this lowers the gain by placing a heavier load on the second stage.

I've seen 150k/150k (on the Trex amp), but another way you could do it would be to tack in a pot (I'd probably try 100k, 250k and 500k pots, since I have them on hand), and adjust to taste, then replace with fixed resistors. I've played with dumping over 90% of the signal here, and it does clean the amp up more and more as you lower the input to the 3rd stage, but at the expense of the 'edge-of-feedback' feel and a lot of the blooming effect. You can use a combo of resistors and a pot to fine-tune it even more (keep leads a short as possible, though, since this node is very sensitive to noise.

And of course the coupling cap needs to be adjusted along with the pot to get the sound you like (a .0022uf in parallel with a .0018uF, for .004uF, sounds good to my ear with the single 150k resistor),but going even higher may make the amp sound 'smoother'. One way to know the cap value is too low is when the Bass pot no longer has any effect.

And some folks poo-poo this one, but a 500pf-.001uF cap across the plate resistor on the second stage helps on these amps with the brittleness.

A similar effect to the voltage divider could be achieved with a split load on the second stage plate (see the Komet amps with the fast/gradual 'touch' switch).

so many things to play with...

[mooso]

Never trust the new guy! You guys assumed I was not an idiot!

Fishy was on to me. The tone controls were operating, but they were not linear and had peaks and valleys and the 50pf treble cap just killed the treble control. I went over the tone stack and sure enough, I had the bass control hooked up wrong. Sheesh!

Now it sounds like something built on purpose. Still a little edgy, but in a good way. Lots of gain. No pedals needed here! The tone controls have enough sweep to dial things in.

The good thing is, I really went in depth studying this thing and following your leads. It was a new circuit to me and the error made me look at it closely, rather than just throw it together. There are a few more things to try. I have a couple extra holes in the chassis so I want to put in a Master Volume and try the Poor Man's Power Scaling mod. Is there a MV type that works better with the PMPS mod?

In the end, it always turns out to be something simple, doesn't it?!!

Thanks everybody, Mooso.

[Ken Moon]

Cool! Glad you found the problem

And you're obviously no idiot - those are the guys who post "hey does the color of the handle rob tone lol".

There's something evil about tone pots and input jacks - I think they re-wire themselves while I'm sleeping just to mess with my head

[zambo]

Glad you got it dialed in Mooso! I actualt took out the cathode bypass on the power amp section and it solved all my problems. I am using scavenged blues jr pt and ot so my volts are a little off as well but lower. I went back to the 500 pf eq cap as i prefered the sound more. Thanks for the help though!