18 watt PI resistors

[andrew]

Hi, this will be my first 18 Watt build and I was wondering why the resistor values in the PI are different from the usually seen values. Instead of 1M on pins 2 and 7 there is a 470k. Instead of 220ks on the B+ there are 470ks. Just wondering.
Thanks.

[Structo]

The 18 watt Marshall is probably the most built and modded amp there is.

I think the main difference is that it uses EL84 power tubes so the phase inverter has to be set up to drive those tubes.

[Phil_S]

When you lower the 470K resistors to 220K, you take a lot of bite off the amp and raise the clean headroom. The purists will say you are neutering the amp by lowering the value of those resistors.

After building one, lowering the value to 220K was one of the first things I did. I guess I'm not as much of a Marshall fan as I thought I was.

[katopan]

The 470k gives you more drive as the others have said. 1M grid leaks are kinda standard for preamp stages so also common in the PI. But a LTP PI kinda doubles that resistance as the effective input resistance, so dropping to 470k still gives you almost 1M input resistance but a bit less noise. You could even go lower without loading up the preceding stage by too much. Merlin talks about the input resistance on his AC coupled LTP page. http://www.freewebs.com/valvewizard/acltp.html

[Gee]

The 470k vs 220k is key to understanding the difference between the 18 Watt Marshall derived designs and other designs.

The 470k as pointed out provides more drive to the power section than would exist if 220k was used.

The 18 Watt LiteIIb for example has a clean low gain pre amp that drives the power amp very hard resulting in "that" Marshall power amp sound. The downside is that the 18 Watt designs can sound too drive, fuzzy, fizzy or whatever you call it. Sometimes this can be tamed with the Zener solution.

On the other hand if you compare the Matchless Spitfire (uses 220k) with the LiteIIb, you can also get a raunchy amp from a similar design. Just a bit more pre amp gain and less power amp gain in the case of the Spitfire.

It's a good place to have a switch to offer both 470k and 220k.

[Phil_S]

It's a good place to have a switch to offer both 470k and 220k.

Funny you should say this. After I wrote, above, I got to thinking how simple it would be to add a DP switch and use two pairs of 470K. The real trouble comes when trying to name the settings. Earth and Dirt? Mom and Apple Pie? What? Not On and Off. Not Drive and More Drive.

[Colossal]

It's a good place to have a switch to offer both 470k and 220k.

Funny you should say this. After I wrote, above, I got to thinking how simple it would be to add a DP switch and use two pairs of 470K. The real trouble comes when trying to name the settings. Earth and Dirt? Mom and Apple Pie? What? Not On and Off. Not Drive and More Drive.

For those wanting a bit more flexibility, a dual-gang 1MA pot with 1M fixed resistors in parallel is used in the grid leak position and called Drive by 18watt enthusiasts. It is simply a post-phase inverter master volume variant. This varies the degree of power amp distortion and can increase the clean headroom of the amp slightly giving a bit more tonal variety.

The OP asked about the 470k/470k grid bias resistors in the phase inverter of the 18W design as compared with the normally seen 1M/1M grid bias resistors. The input impedance of the traditional setup is 2M while 470k/470k gives 940k, a slight reduction in noise. As others have noted the 18W's preamp stays largely clean but hits the output stage with a large signal developing the signature Marshall power tube distortion characteristic. On the 18W Lite IIb design, the plate load resistor on (paralleled) V1 is increased to 120k.

Done right, 18W (Marshall) amps have killer chime and an almost 3D quality to them. Some however experience rather nasty crossover distortion artifacts in their builds and Paul Ruby came up with a zener clamping mod to deal with this. Google up Paul Ruby Mod. Speaker choice is subjective but an important part of the equation. G12H Heritage, G12H Anniversary, and G12M Greenback (both Heritage and re-issue), and Celestion Blue are the most popular choices. In a 2x12, a Blue and G12H Anniversary are killer.

[Structo]

I'm kind of guessing here but on the 18 watt amp with tremolo, idon't the 470K grid resistors match up the impedance better from the tremolo?

Then on the non tremolo amps, they just kept the 470K?

[MysteryFever]

On the topic of 18watt power sections, I noticed a on a couple schematics the coupling caps to the power tubes are .01uf. I brought it up to .047 to match with a rocket (which has twice the power tube compliment). I got high static distortion when I hit anything hard, so I reduced the caps to .022 and it went away... what did I just experience?

[plexitone]

On the topic of 18watt power sections, I noticed a on a couple schematics the coupling caps to the power tubes are .01uf. I brought it up to .047 to match with a rocket (which has twice the power tube compliment). I got high static distortion when I hit anything hard, so I reduced the caps to .022 and it went away... what did I just experience?

Maybe blocking distortion. Larger value coupling caps are one cause.

more info: http://www.aikenamps.com/BlockingDistortion.html

[andrew]

I have a 4k OT and the chassis is already wired for four EL-84s. Is this going to change the "18 watt" sound? Do PI changes need to be made? I do not want to buy another output transformer to run only two EL-84s. Thanks.

[katopan]

A 36W with a quad of EL84s runs the same 18W standard PI.