Non-Traditional 15 watt Liverpool with PCB

[martin manning]

So I have a question, since I'm building a 2 x EL84 amp shouldn't the 1k screen dropping resistor be increased to a 2k resistor so the amp responds the same, has the same voltage drop etc.?

No, each tube has its own screen resistor in the 4-tube version so it should stay the same. If you imagine the pair of tubes on each side of the 4-tube version functioning as one with a 1k // 1k = 500R screen resistor, removing one of the tubes and its 1k resistor leaves 1k, so the screen current load has already been doubled.

[rock_mumbles]

So I have a question, since I'm building a 2 x EL84 amp shouldn't the 1k screen dropping resistor be increased to a 2k resistor so the amp responds the same, has the same voltage drop etc.?

No, each tube has its own screen resistor in the 4-tube version so it should stay the same. If you imagine the pair of tubes on each side of the 4-tube version functioning as one with a 1k // 1k = 500R screen resistor, removing one of the tubes and its 1k resistor leaves 1k, so the screen current load has already been doubled.

So you are saying that the 1k 25W power supply dropping resistor (R31 in this Liverpool schematic) should be left at the stock 1k value ... I guess I'll have to think about that ...

I figured that the current draw through R31 would be 1/2 of the real amp, so to get the same voltage drop across R31 the value would have to be doubled ???

[martin manning]

Oh, I was thinking you were asking about the screen resistors at each tube. I think you're right it should be increased, but the current draw through it won't be reduced by half... maybe 1/3? Maybe make it 1k5 as a guess?

[rock_mumbles]

Oh, I was thinking you were asking about the screen resistors at each tube. I think you're right it should be increased, but the current draw through it won't be reduced by half... maybe 1/3? Maybe make it 1k5 as a guess?

Thanks!!

[RJ Guitars]

Hey Guys,

After taking most of a year to sort through personal issues and make an effort to get my body and mind healthy, I have started to do a little amp work again. My prototype PCB Liverpool was about 98% complete when I put it aside last year. Now I am trying to get it finished up and ready for use as a gig ready amp.

I did swap the big power resistor from a 1K to a 2K. I didn't do an A/B comparison and I can't say it sounds amazingly better but it certainly did not hurt things.

The amp is sounding good - very aggressive with very little clean headroom... 15 watts is plenty loud and this thing has the potential to attract unwanted attention if you have neighbors living close to you.

To finish this amp up I've sorted through the various options for adding a master volume... I'll post some more on that later.

rj

[ampgeek]

Right on RJ!
Great to see you back at it once again.
Sure did miss your "smiling" posts!
Cheers,
Dave O.

[vincenz]

yeah !

I look forward to reading more about this amp !
...and I wish I could build one

[RJ Guitars]

On this build I initially I used a leftover faceplate that had a little warp on it since I figured I wouldn't be able to sell it. The faceplate had a master volume on it so I felt obligated to add that feature. I generally think that master volumes reduce tone in conjunction with the volume reduction.... so from the onset I wanted to have a MV circuit that could be entirely dialed out or switched off to preserve the original circuit performance.

Anyway, I started by going to Ken Fischer's Trainwreck pages and choosing my own variant of MV "Type 1" which replaces the PI output resistors with a 250KA dual pot. It worked but I was disappointed that it used very little of the rotation on the pot before it went to full volume. I measured what was happening and found that the volume went up to maximum audible level at about 32K ohms.

I replaced the dual pots with a 6 position rotary switch and selected resistors from 820 Ohms to 32K ohms. I selected 820 ohms as the minimum resistance because as with most master volume circuits I've tried, this one started to sound thin and buzzy as the sound level approached "apartment" levels. It worked and sounded decent but it seemed like a lot of parts and effort and really didn't match up to the front panel. It also seemed to me that I should be able to accomplish a more satisfactory solution using a pot.

Finally I decided to look at the other types of master volume circuits that would give me simplicity and function. The MV "Type-3" in the TW pages was really simple and gave me a good sound in my bench tests. I added a switch so that I could take the MV out of the circuit when I wanted the original circuit performance. I'll add some drawings and photos so you can see what I did.

I wired the pot in reverse so that dialed to the left you had max volume with no MV function. As soon as the switch is closed I have a drop of a couple DB. As you dial the pot to the right the volume goes down smooth and evenly until you get to the minimum resistance... 1.5K gave me a nice minimum volume that still sounded decent. Curious if anyone else has had similar ideas or results?

rj

[rp]

RJ very glad you are doing well and I knew both your posting and the store would be back - it's obvious you can't get enough of this stuff.

My first observation on this amp is that it goes from clean to mean more abruptly than the Express and also I think it is too mean when you dial it above noon. My first inclination would be that the output section is getting hit too hard...

After reading this, I'm convinced the day I build a LP it will be a 4 X EL84 with and MV or power scaling. I'm glad I decided on making my Rocket 4X. I don't think you can just drop 2 tubes and keep the tone, tone and headroom are too intertwined.

Love the discrete resistor MV - make the taper anyway you want, get rid of that sudden rise. Need finer gradations, here you go

Actually that image came from an interesting tutorial:
http://www.hificollective.co.uk/compone ... uator.html

[RJ Guitars]

Yup, that's some crazy stuff but no doubt you can dial it to the sweet spot(s) of perfection to your own ears. I have been wondering about jumpering the resistors with bypass caps to do a little tone compensation as you change the volume levels.

[rp]

I have been wondering about jumpering the resistors with bypass caps to do a little tone compensation as you change the volume levels.

Like the old 'Loudness' switch on old stereos that would give you a bass boost at low volumes where the sound thinned out. You might be onto something. I'd get patent!

[RJ Guitars]

After adding the MV I decided that I liked this amp enough to put it in a cabinet and have it gig ready. I borrowed some furniture technology from the late 70's waterbed business and made myself a quick and dirty cabinet out of pine and stained it with dark stain and then put a few coats of clear coat over it. If time is money it was still expensive but it really doesn't look like it was... it supposed to look earthy and rustic?

it's done!

[vibratoking]

RJ - nice build. I bet it's fun to play. I like the idea of PCBs and vearing away from the standard board approach. PCBs make for a much easier build IMO and definitely more repeatable. Kudos on your work.

I hope you don't take offense, but that cab looks like a sleeper...like a rat bike. I like that approach. From the looks, nobody is expecting much and then wham you hit'em hard with some killer tone.

[dave g]

The Liverpool DOES in fact drive the EL84s WAY too hard. The swing coming from the LTP PI plates is almost as large as in the Express, but while the Express' EL34s are biased around -30 to -35 volts on the grid, the EL84s in the liverpool are going to self bias around -10 to -12 volts (with respect to their cathodes). That's a difference of almost 10 dB!

I think a PPIMV (not just VVR) goes a long, long way in a Liverpool.

[RJ Guitars]

RJ - nice build. I bet it's fun to play. I like the idea of PCBs and vearing away from the standard board approach. PCBs make for a much easier build IMO and definitely more repeatable. Kudos on your work.

I've found a few "norms" in building Wrecks - one of them is consistency is tough and another is that any piece of metal in the signal path seems to invite noise. The PCB helps with both of these issues. It's likely that the layout could be improved but relatively speaking it is well behaved and uses way less metal than my other traditional wire and turret builds...

I hope you don't take offense, but that cab looks like a sleeper...like a rat bike. I like that approach. From the looks, nobody is expecting much and then wham you hit'em hard with some killer tone.

No offense at all - i really enjoy that aspect of this amp. I wanted to come up with some ideas for cabinets that allow me to use my prototypes as gig amps. Not all of them are worthy of of a true Wreck hardwood cabinet.

This particular Liverpool is quickly becoming my personal favorite amp - relatively low noise, great Wreck sound, not afraid to carry it around in the back of the car and even though it's crazy how loud it is, it only weighs half what my Express does.