The Mods thread.

[leadfootdriver]

Wasn't there a sticky with all' the cool mods to do to these amps?

I put a resonance control back on the amp and it really helped to warm things up on the low end. It's not that the express is super bright, it's more of a lack of low end IMO. With the tone stack right after the 1st gain stage, you're limited on the amount of bass you can run before the sound gets washy. Even still, the low end doesn't get that big.

With the Resonance control, you can pass more bass after the gain to keep the sound tighter. I'm totally diggin' the sound I get from this amp with this mod. I think it's a modern addition that it needed.

Post your mods.

[zambo]

how do you add the resonance control? Can you keep the presence as well? Sorry, saw the pic after I logged in to reply. Guess its self explanitory when you see the pic. I must try this!

[Colossal]

how do you add the resonance control? Can you keep the presence as well? Sorry, saw the pic after I logged in to reply. Guess its self explanitory when you see the pic. I must try this!

Hey Zambo,

Yeah, Presence is to highs as Resonance is to lows. You can keep your presence pot, as usual. Both controls work inside the negative feedback loop and independent of one another (although there are other ways).

I love the Resonance mod. Just a must-have for me.

[leadfootdriver]

I'm also back on JJ KT77's. They have more headroom than an EL34, so the tone is tighter and less compressed.

The bottom end is stout!

[roberto]

About the depth mod, I prefer a fixed depth (100k//4n7) instead of the 1Mpot. I feel that that configuration works good with a wide range of NFB amounts and different circuits, and keeps the whole amp simpler, that I think is quite important (my amps have two channels, one single eq, two MVs and one presence, and by now nobody said he needs another eq).

[eneru]

About the depth mod, I prefer a fixed depth (100k//4n7) instead of the 1Mpot.

It's what I did on my amp and I like it a lot, simple but useful!!!

[Colossal]

About the depth mod, I prefer a fixed depth (100k//4n7) instead of the 1Mpot.

I plan to use the fixed version for a Superlead I am finishing for someone. No more knobs on the front of back.

[roberto]

I think it's a good idea. It's something you really need when you go to hi-gain preamps, but also for lower gain amps it helps to obtain a rounder tone at lower volume levels. It could be helpful to add the shelving bass eq to the stage right before the PI, just because this will leave the dampening as is in the entyre audio range, and tightens the bass.

I also usually prefer less NFB. In my 100W amps I go from 68 to 100k on 4ohm tap with standard 4k7 to ground. If you manage the bass content in the preamp (EG the coupling cap after the 3rd stage) to fit with the power amp needs, and control higher harmonics in the preamp, this is a nice solution that gives you way more dynamics and presence in the band mix.

[Mark]

Hi

I'm a little late on this thread, but I found the third stage doesn't have a lot of gain, but it does do a lot of tone shaping.

If you are after more bass response, perhaps you should consider increasing the coupling cap to the third stage per the Trudy amp or to look at the Komet amp for more bass.

I suppose the important thing is, does it sound good?

I've looked at some of the Dumble circuits and I wonder if the PI balance control would be beneficial, I've also noticed the cathode follower in the Dumbleator thickens the tone of the amp. Fischer's Concorde amp also used a cathode follower.

I also found there was a significant difference using 250K pots in the tone stack (per the Komet) than the Express' 1 Meg pots.

[roberto]

the third stage doesn't have a lot of gain, but it does do a lot of tone shaping.

The simple 100k 10k stage at those voltages, loaded with two bypassed stages (and a tonestack in between) adds tons of harmonics, also if the gain is quite low. But it's certain that the small cap, so the higher output load at lower frequency, together with the real low output load at higher frequency has a huge influence on harmonics of the 3rd stage when overdriven.

If you are after more bass response, perhaps you should consider increasing the coupling cap to the third stage

You mean after the 3rd stage, isn't it? The problem with the added bass there is that you have to rearrange the voltage divider to the PI, otherwise it could sound flubby (don't forget that PI is a gain stage too (well, two gain stages), and that too much lows means an harsh fuzzy overdriven sound.

[Mark]

http://ampgarage.com/forum/files/Expres ... iation.jpg

Here is the Komet pre-amp I was referring to.

The 220K will drop the highs off a little and a small cap can be used to restore some of the highs. Values of 100pF to 500pF would work fine.

[roberto]

Thanks, now I got what you mean.

With that configuration it will be useful to reconfig the switch this way:
- 100n plus 220k in series when split loaded
- 2n2 (up to 22n) straight to 330k on full gain

This will permit you to have a fuller "clean" and a raw "crunch".
You need a 1M cap across the switch to keep the 2n2 cap grounded (avoiding pop), or you can ground the 2n2 cap by a large resistor to obtain the total value (in parallel with the 330k) needed: if you ground the 2n2 cap with a 330k, you'll obtain a 165k resistor.

[mobisimo]

I also found there was a significant difference using 250K pots in the tone stack (per the Komet) than the Express' 1 Meg pots.

Hi Mark, I'm curious about this and wonder if you would mind elaborating a bit.

Many thanks,
Mo

[Mark]

Hi Mo

I did this in 2004 I would find it hard to describe the difference. I do recall the amp was noticeably different with the different value pots.

I think it would be best to try this for yourself. I would only change the treble pot and work out how far you can turn the bass pot up till you reach 250K.

[mobisimo]

Hi Mark, thanks for your reply. I'll try it out in a wreck type build and report back.