Low gain options for the 18 watt normal input channel.

[Mark]

I was playing the 18 watter yesterday and it bugged me that the amp hits full power when set to 9:00 o'clock.

This wouldn't be a great issue if there were stacks of useable gain on tap, but I found from 1:00 o'clock onwards it was just much as muchness.

The initial thought is lowering the plate resistor value to 47K or 56K and possibly splitting the cathodes to get different bass response.

Not wishing to reinvent the wheel, what have you guys tried.

[katopan]

Same observation. I'm so used to using my guitar volume to clean it up I haven't experimented with anything. But I agree with your statement about 1 o'clock onwards and would very rarely turn up past that no matter what guitar I'm using.

I haven't tried it, but given the tone sucking nature of putting a resistor in front of the tone pot, I'd be more inclined to keep the first stage values (to keep the character) but do a split plate setup to set the gain where you want it. I and many others have found that to sound fairly transparent but give the result you're after, but that was in other amps. But it should work the same in an 18 Watt.

[echuta13]

I've always been under the impression that the PI drives the EL84's into clipping pretty quick. How about adding a "Drive" master volume to it?

http://www.trinityamps.com/ForumGallery ... wattMV.gif

[Colossal]

How about adding a "Drive" master volume to it?

This!

[Mark]

I'm inclined to think reducing the plate resistor to 47K would be similar to have a regular 100K plate resistor for a regular triode circuit.

It seems to me this circuit would be the equivalent to having two parallel circuits with a 1.5K cathode resistor and a 220K plate resistor. Given that the EL-84 don't need a big signal I can't but help think it is overkill.

[Sazafraz]

I prefer to use just low gain input (if you have a high and low input) and volume pot on guitar.

Just been wishing for an easy way to switch back to the high gain input

[Mark]

I did the usual two mods to the normal channel. I didn't like the tone control so I end up using the same one that is on the tremolo channel. I also did the parallel triode mod. As I've said before I never use it. I use the single triode jack.

I haven't had time to try lowering the plate resistor yet. I will report back when I do it.

Another thought I had was to use a EF-86 for the tremolo (12AX7 or FET for the oscillator). It should have enough gain to filter out the thump. The idea came from the Vox AC-10.

[Mark]

Reporting back, I did try putting another 100K across the existing 100K to bring the gain down. This did reduce the gain, but not as much as I hoped.

It would be like the amp breaking up on "4" instead of "3", I'm also thinking of splitting the cathodes to get greater variety from both triodes, in a SLP sort of way of course.

I'm also thinking the Matchless/Vox switchable coupling caps might be a better option than the regular tone control. I just don't think it does enough to justify it's position. When the amp is cranked it can sound a bit too "woolly" the switchable tone caps should help reduce the bass response under such conditions.

A permanently wired in bright cap would probably be the ticket.

[Colossal]

Mark,

EL84s are very easily driven, bias is about -11V fixed, IIRC. Reducing the grid leak resistors from 470k/470k with a PPIMV will get you a cleaner sound. The 18W front end stays pretty clean. If you've got 5n for your tone cap, try increasing to 10n or even a little more to roll off some of the wooly at higher volume.

What are you PI values (82k/100k or matched 100k/100k)?

[Mark]

Since my previous post I have given the amp a run using a Weber Mass (and the Les Paul with Timbuckers), by and large the tone delivered by the Mass isn't wonderful but if I take into consideration the short falls (reduced bass response and a buzzy quality to the distortion) I can determine if the amp is doing what I want it to (without being deafened).

I think the small 50K resistor does make the amp react in a way that is more like the larger Marshall amps. There is a good range of distorted tones from "3" to "11". Funnily enough I haven't tried it with the paralleled inputs though.

Hi Colossal, thanks you for answering my thread. I don't think the PPIMV is the ticket with this amp, no science involved just a gut feeling.

I know there are many ways to achieve the end I've described and I don't doubt for a minute the PPIMV has worked very effectively for you. As stated above I like what I'm hearing from the change in value.

[Reeltarded]

Don't PPIMV it. Use simple master just before the PI instead. That is how you stage gain into the PI and get the volume you want.

Works!

[Greg Smith]

Another option to soak up some of the gain from the input stage would be to insert a BAX tone control network after the input stage coupling cap. The Bass side of the network uses up a lot of available gain, giving you more clean range on the volume control.

This is a pretty dramatic departure from the classic 18 watt design, and it's possible the at the BAX network would use up too much gain. Might be worth experimenting, though.

Greg

[Mark]

That is something to consider, I hadn't thought of that option.

I think I'd still like to play with the cathode resistors and see what result I get.

[Mark]

Weird double post issue.

[rock_mumbles]

Mark,

EL84s are very easily driven, bias is about -11V fixed, IIRC. Reducing the grid leak resistors from 470k/470k with a PPIMV will get you a cleaner sound. The 18W front end stays pretty clean. If you've got 5n for your tone cap, try increasing to 10n or even a little more to roll off some of the wooly at higher volume.

What are you PI values (82k/100k or matched 100k/100k)?

I agree with Colossal ... the 18 watt preamp is basically clean reducing the preamp gain won't do a whole lot ... the signal getting to the EL84's is what needs to be addressed.

I also agree that the tone control is better with the 10n bass cap than the 5n stock cap.

I have a PPIMV and Cut control in my 18 watt and like them a lot.
The PPIMV helps clean up the output section quite a bit ...