Express Tweaking....

[paulruby]

Since I just typed this in an email, figured it could all go here, too.... This is not gospel. It's simply what I do, plain and simple.
_____

Start with the A1A schematic in wreckxpr.pdf attached.

Use a 1M bass pot but leave the caps the same value. And, wire the 22nf caps in the tone stack to the wipers of the bass and mid pots (as opposed to the shown method of wiring them like variable resistors). Look at the Marshall tone stack in Duncan's tone stack calc and wire both the mid and bass pots just like the mid pot is wired there.

Be sure to us a switch to select the bright cap values on the volume pot. On mine, I hard wire a 100pf on the pot and use an SPST to add in another 250pf or so. To get EQ balanced, start with 100pf there and you can choose the add-in value later. Alternatively you can use an on-off-on switch to select between two different caps or neither. But, no bright cap is pretty dark.

Also, the wreck intentionally limits bass content between the 2nd and 3rd stages via the coupling cap. If your amp is still too bright (which also means lacking low end) after the above changes, increase the value of that cap. Try doubling it.

In short: A1A schematic. 100pf bright cap. 1M bass pot.

Tweak the coupling cap between 2nd & 3rd stages until you have the right amount of bass.

Tweak the 100pf bright cap value to get the right amount of sparkle at low volume setting (cleaner tone).

Tweak the add-in bright cap until you like the mid-range grind that is added by that cap at higher volume settings.

Tweak the plate resistor value on the 2nd stage until you like the balance of pre to power amp break up (or add the "attack" control which adds a selectable tap off that resistor).

Do not remove the presence control if using EL34 (or any big bottle tube). You could remove it if using 6V6 (and never using anything else) and tweak the hard-wired settings to taste.

And, the 100k resistor *IS* your NFB and is required to enable the presence function, which is nothing more than an EQ on the NFB.

- Paul


-------------- Original message ----------------------
From: ####################
>
> Hi Paul,
> Sorry to bother you with such a trivial question but after reading quite a
> few of your posts in the Amp Garage I figured you'd be able to make me see
> the error of my ways.
>
> Here's the deal, I built a TW clone which sounds fantastic, except for one
> small nagging thing, it had too much "highs" for my taste (even with the
> treble turned down and the presence fully counter clockwise).
>
> So, I down loaded the Duncan tone stack program and began tweaking. I think
> the tone stack could benefit from swapping out the (2) 0.022µf caps and
> replacing them with (2) 0.01µf caps. At least it seems to work on paper, it
> appears to give more mids.
>
> Secondly I was going to remove the presence control completely and replace
> it with a 0.0047µf cap and a 250KA pot (similar to a cut control on a
> rocket).
>
> Thirdly, what would happen if I removed the 100K resistor which is
> connected to the tail resistor of the PI & the 8 ohm tap? Is it necessary?
> Could it be replaced with a 50K value?
>
> Hope I didn't take too much of your time,
>
> Respectfully,
>
> -Greg
>

[Fischerman]

Paul,
Unless I'm not seeing it right or misunderstanding you...it looks like the bass pot in Duncan's TSC is wired as a variable resistor. The 'top' lug and wiper are connected to each other and then 'bottom' lug is connected to the mid pot. It looks different than the way the mid pot is wired. Just want to make sure I'm clear on this.

EDIT: OK, I didn't read carefully enough....you said to wire them both like the mid pot is wired in the TSC. Sorry for the confusion.

I'm also curious about the split-load thing on the second stage. I can't speak for others but from my reading it appears that many of us have a similar problem in that there is some sort of blocking (or something) going on. The attack is too 'fuzz-like', the amp doesn't sing/sustain the way it should, and when trying to get the lower gain 'in-between' tones the amp just barks way too much and just isn't fun to play at all (it's almost like there's too much distortion...yet not enough at the same time...hard to describe). My Express clone sounded best at full-on rage and only OK at that...it sounded pretty bad at most other settings (I'm being pretty critical here...I don't like building 'just OK' sounding amps ) . I think we've tried all sorts of stuff but I haven't tried split-loading the plate load on the second stage. Is that what this mod is supposed to cure?

[harryk]

Hi Paul! I just tested your proposal to modify Express, but I did it to my Liverpool 2xEL84 version. I tried a 4,7 nF cap between stage 2 and 3 and come back to value of 2,2nF. I made also those modifications to bass and mid pots but it was not a remedy for this treblish sound illness. My amp lost that touch responsivness after these modifications. This my experience so far. Have you any ideas how to continue with tweaking? Regards Harry

[paulruby]

Just some thoughts...

Fizzy distortion seems to have a lot to do with OT choice. I use heyboer iron exclusively and never have this problem.

I also know there are lots of speakers that can be fizzy. I always seem to return to the G12H30 or green backs every time stray away.

I would make sure the OT and speaker choice are correct first. Then start considering the amp.

A larger value tail resistor on the PI can help a little. This will reduce the swing of the PI so it's less prone to inducing blocking in the following coupling caps. But, going to far with this starts to sound very different as the PI gets cold and begins to be the dominant distortion for the whole amp.

I have built a couple where I reduced the 10k cathode on the 3rd stage a little to bias it a bit farther from cutoff. Try 8.2k or 6.8k.

[harryk]

I have Hammond 125E in my amp and primary is some 6,8 kohm/8ohm. I have tested my amp with a closed back 1x12" Celestion G12H30 and a semi open 1x12" Red Fang. Did you find any lack of touch responsivness after your modifications? Next I try to reduce 3rd stage cathode resistor value according your advice. Regards Harry

[Darkbluemurder]

Do not remove the presence control if using EL34 (or any big bottle tube). You could remove it if using 6V6 (and never using anything else) and tweak the hard-wired settings to taste.

And, the 100k resistor *IS* your NFB and is required to enable the presence function, which is nothing more than an EQ on the NFB.

- Paul


-------------- Original message ----------------------
From: ####################
>
> Hi Paul,
> Sorry to bother you with such a trivial question but after reading quite a
> few of your posts in the Amp Garage I figured you'd be able to make me see
> the error of my ways.
>
> Here's the deal, I built a TW clone which sounds fantastic, except for one
> small nagging thing, it had too much "highs" for my taste (even with the
> treble turned down and the presence fully counter clockwise).
>
> So, I down loaded the Duncan tone stack program and began tweaking. I think
> the tone stack could benefit from swapping out the (2) 0.022µf caps and
> replacing them with (2) 0.01µf caps. At least it seems to work on paper, it
> appears to give more mids.
>
> Secondly I was going to remove the presence control completely and replace
> it with a 0.0047µf cap and a 250KA pot (similar to a cut control on a
> rocket).
>
> Thirdly, what would happen if I removed the 100K resistor which is
> connected to the tail resistor of the PI & the 8 ohm tap? Is it necessary?
> Could it be replaced with a 50K value?
>
> Hope I didn't take too much of your time,
>
> Respectfully,
>
> -Greg
>

Paul,

Why should you not remove the presence with EL 34s but with 6V6s? Is this a recommendation which is specific to Wrecks or all amps? I used the cut control with very good results in my EL 34 Marshall 100W.

Stephan

[Fischerman]

Fizzy distortion seems to have a lot to do with OT choice. I use heyboer iron exclusively and never have this problem.

I also know there are lots of speakers that can be fizzy. I always seem to return to the G12H30 or green backs every time stray away.

I would make sure the OT and speaker choice are correct first. Then start considering the amp.

Thanks, I used Heyboer iron too. I used the 300v PT taps and the 5k2 OT primary...my OT doesn't have the 6K6 primary, just the 5K2.

I'm using 30W H75 Scumbacks with that thick wheat colored grill cloth.

I did have to go up to 22k on the PI tail resistor...helped a lot but still not right. My amp originally had 91k/100k for the PI plates but swapping an 82k for the 91k helped too (Nik has since changed his kits/amps to use the 82k there). But still not there.

Regarding the Presence control, I removed it but put in a 4k7 resistor in place of the Presence pot and kept the NFB (IIRC I used an 82k NFB resistor). My Presence control never (as in...not ever once ever) went above '0'...the last thing my amp needed was more brightness/gain/rawness. I used a dual-pot post PI 'tone' control...but I think that was just a band-aid for...whatever is wrong with it. Just not a great tone...very fatiguing to the ear.

[paulruby]

Paul,

Why should you not remove the presence with EL 34s but with 6V6s? Is this a recommendation which is specific to Wrecks or all amps? I used the cut control with very good results in my EL 34 Marshall 100W.

Stephan

That was specific advice to the specific email... Not Gospel... A wreck circuit is already tweaked to have proper bass response with NFB and presence control. Removing presence and/or NFB will mean other tweaking needed and will likely mean sticking with a particular tube type. My email response is mainly targeted at the specific question "my express is too trebly so should I ...?"

[gcenker]

"Regarding the Presence control, I removed it but put in a 4k7 resistor in place of the Presence pot and kept the NFB (IIRC I used an 82k NFB resistor). My Presence control never (as in...not ever once ever) went above '0'...the last thing my amp needed was more brightness/gain/rawness. I used a dual-pot post PI 'tone' control...but I think that was just a band-aid for...whatever is wrong with it. Just not a great tone...very fatiguing to the ear."

Fischerman, I couldn't agree more. The ear fatigue really gets to me after a short while. I've been thinking about replacing the presence control function with the above circuit (still keeping the 100K resistor in place) and replacing all the related caps with SoZo Classics to see if that helps any. Also thinking about replacing the 0.022 µf caps in the tone stack with 0.01µf replacements.

Anyone?

-Greg

[paulruby]

Fizzy distortion seems to have a lot to do with OT choice. I use heyboer iron exclusively and never have this problem.

I also know there are lots of speakers that can be fizzy. I always seem to return to the G12H30 or green backs every time stray away.

I would make sure the OT and speaker choice are correct first. Then start considering the amp.

Thanks, I used Heyboer iron too. I used the 300v PT taps and the 5k2 OT primary...my OT doesn't have the 6K6 primary, just the 5K2.

I'm using 30W H75 Scumbacks with that thick wheat colored grill cloth.

I did have to go up to 22k on the PI tail resistor...helped a lot but still not right. My amp originally had 91k/100k for the PI plates but swapping an 82k for the 91k helped too (Nik has since changed his kits/amps to use the 82k there). But still not there.

Regarding the Presence control, I removed it but put in a 4k7 resistor in place of the Presence pot and kept the NFB (IIRC I used an 82k NFB resistor). My Presence control never (as in...not ever once ever) went above '0'...the last thing my amp needed was more brightness/gain/rawness. I used a dual-pot post PI 'tone' control...but I think that was just a band-aid for...whatever is wrong with it. Just not a great tone...very fatiguing to the ear.

If your presence control never goes above zero, then the preamp may not be balanced right. Perhaps needs more bass or less top end coming in. For example, if I use KT66, I don't need a bright cap on the volume pot and use a larger coupling cap after the 2nd stage. Try to get it sounding nice with the presence set to 12 o'clock. Else, you are linearizing the power section too much and it will loose the open, airy tone of power tubes running free at the higher frequencies. Part of the great tone of an express (or most any big bottle tube amp) is allowing the power tubes free-wheel in the mids and highs but keeping them in check on the low end with NFB. That's what the presence control does for you when set to ~12 o'clock or higher.

And, try reducing the overall gain a bit by reducing the 2nd sage plate resistor and reduce distortion by reducing the 3rd stage cathode resistor.

[JimiB]

I like the both the presence control and the cut control myself. I run em both about half most of the time.

[Fischerman]

Thanks again Paul. Just to make sure I'm understanding you...split-loading/reducing the second stage plate resistor reduces the gain but then lowering the cathode resistor on the third stage increases it again. So we're adjusting the structure of the gain to hit the 3rd stage a little less but hit the PI the same (or more)? Is that correct?

That seems to make sense...I felt I had pretty much tweaked all I could out of the PI without straying too far.

[paulruby]

Thanks again Paul. Just to make sure I'm understanding you...split-loading/reducing the second stage plate resistor reduces the gain but then lowering the cathode resistor on the third stage increases it again. So we're adjusting the structure of the gain to hit the 3rd stage a little less but hit the PI the same (or more)? Is that correct?

That seems to make sense...I felt I had pretty much tweaked all I could out of the PI without straying too far.

Think of the 2nd stage plate resistor and the hard-wired MV. That's effectively what that (and the 150k load after the coupling cap) is doing.

The 10k cathode resistor on the 3rd stage biases cold and makes the 3rd stage susceptible to cut-off. which can sound fizzy. Reducing the resistor value *a little* can help a fizz problem *a little*.

In general, I have never needed to deviate much from the A1A schematic to get a great sounding express.

[gearhead]

Speaking of that third stage cathode resistor; why isn't there a bypass cap? Without it, doesn't it make the bias swing with the grid signal?

[paulruby]

Speaking of that third stage cathode resistor; why isn't there a bypass cap? Without it, doesn't it make the bias swing with the grid signal?

That just means it has local NFB to keep it's gain low. It's just another aspect of the gain structure of the amp...