Some 18-watt circuit ideas

[Ron]

I've been staring at the latest version of the 18-watter schematic. I have been rolling my own from the original Marshall diagram, combined with that for the Watkins Dominator. As I stare at the current version on this board, a few ideas pop out:

1. In playing around with a used Komet I just bought, I've come to the conclusion that it's main attraction for me is the guitar-sustain thing it does so well. This is accomplished by having a very stiff power supply with lots of capacitance for reserve energy. Increasing all the filter cap values, especially C20-A might take the 18-watter in this direction. Ken Fisher uses a cute trick to keep the preamp power from being stolen by the output tubes (when the EL84s are using a lot of power, they can drain C20-A, dropping its voltage, then start to suck electrons from C20-B through R5, making the preamp voltage sag too. Ken simply inserts a diode, say between R25 and C20-B, so the current can't be drawn back from the preamp. Pretty simple idea, but effective.

I'm going to attach 40 or 80 uf across C20-A with clip leads and see what happens to the sustain.

2. Use a variable resistor for the phase inverter tail resistor, R17. This resistor can have a pretty dramatic effect, so making it a variable would allow tweaking in place for the tone you want.

3. Use the Tremolo Channel EQ/tone circuit in the Normal Channel also. I don't know why the same circuit is not used in both channels. The Trem Channel variant seems really straightforward, and its implementation is pretty neatly done. I've been doing this for some time.

4. EF86 Normal Channel - Yes! Done it a few times, always like the sound. Gives you a real lead tone, IMO and a separate rhythm tone from the other channel that's quite different. In the standard version, the two channels sound very similar (except one has 3 dB more gain if jumpered per the current schematic).

5. Anyone tried a solid-state rectifier instead of the EZ81? Higher plate voltages mean more headroom and better top-end response. I wonder if there is a plug-in SS replacement for the EZ81.

6. Use a 12AT7 or 12AU7 for the phase inverter. With an EF86 first preamp, there is plenty of gain, so a 12AU7 may give more headroom. I'm going to try this one tomorrow.

7. Use a pair of 300B instead of EL84s. Just kidding. Don't try this without a helmet.

Thanks to everyone who keeps this board going, the information available and ideas flowing.

[loverocker]

Interesting ideas. The WEM I'm rebuilding into an 18Wer has slightly stiffer filtering, 50+50uF (as per WEM standard), followed by a 22uF (because the bits box lacked a 16uF) for the preamp. That diode idea is interesting.

What sort of tone variation would come from a variable resistance on the PI tail?

For mine, I'm doing the 18W Lite II as the first phase (except with only the first single triode stage) and seeing what that sounds like. Once that's working I'll try a post-PI master volume and possibly a conjunctive filter - always wondered why the WEM had one but not the Marshall?

[markh]

>1. I'm going to attach 40 or 80 uf across C20-A with clip leads and see what happens to the sustain.

If you're using the stock (tube) rectifier, remember they have a max cap rating.

2. Use a variable resistor for the phase inverter tail resistor, R17.

Cool idea, but pots tend to get "scratchy" fast when DC current is flowing through them. You may want to identify the "good" settings by experimentation, then use a switch.

3. Use the Tremolo Channel EQ/tone circuit in the Normal Channel also. I don't know why the same circuit is not used in both channels.

The lite is basically the normal channel only but with the trem channel EQ. Most seem to prefer that EQ circuit.

5. Anyone tried a solid-state rectifier instead of the EZ81? Higher plate voltages mean more headroom and better top-end response. I wonder if there is a plug-in SS replacement for the EZ81.

Richie did this way back when, on our very first "variation" circuit. The only problem is it puts even more voltage stress on the EL84s. Weber does make noval (9 pin form factor) Copper Caps.

6. Use a 12AT7 or 12AU7 for the phase inverter. With an EF86 first preamp, there is plenty of gain, so a 12AU7 may give more headroom. I'm going to try this one tomorrow.

Part of the mojo of the overdriven 18watt sound is the PI slamming the EL84s -- decreasing the PI gain will change the overdriven sound character, (for better or for worse).

7. Use a pair of 300B instead of EL84s. Just kidding. Don't try this without a helmet.

...or a hefty 2.5v filament supply.

--mark

[Plexi]

""""Richie did this way back when, on our very first "variation" circuit. The only problem is it puts even more voltage stress on the EL84s. Weber does make noval (9 pin form factor) Copper Caps. """"



Yep Mark is right,i got away with it because i really needed the extra voltage, i was using a different set of transformers. I also have a 6v6 amp here that is SS recto,again because i'm useing transformers that were from an old Premier amp.

As Mark said, with the heyboer trannys, people have used the Weber copper cap. But if you go to SSrecto, it might put the voltage a bit hard to deal with on the EL84s,maybe go with 7189A, I always wondered how those 6973 would work too,but they are getting expensive. If you have a bit less voltage the 6AQ5A or the 60005A These are great sounding little bottles.




Another interesting thing to try is drop the cathode bias to,and put in an adjustable or fixed bias,that was one thing i liked a little better. The amp seemed to have more response to your picking,sounded like it gave a notch more volume,added headroom. Something similar in sound to what you described.As they say, don't knock it till ya try it. I think we even have switch mod too someone posted, to use either or.. This place is getting like my bench, i know i have it, just can't find it..

There all kinds of things that people have tried, even if they aren't listed ,seems someone has tried it. Some turn out good, and others don't.
HA thats the fun of it,also good learning experience.


As for the filtering, in the stock version, it seems to be fine,nothing seems to sag or drop. The only time i added a stage of filtering was useing my TMB treble middle bass circut,then added it for the preamp.

Its good to try your ideas,as you can see from our files, many have changed and addded there circut ideas,and they work.

Richie