Plexi 50watt build, life after bright cap

[audiosalvage]

hello, I've recently built a 1987 type amp with a lowered plate voltage.  It sounds good but a bit brittle in the highs, so I removed the vol bright cap.  All of a sudden I have very ample bass now, which gives me a nice warm clean tone and I love the linear vol pot response.  I've had to jack the presense and treble so I'm looking to bump up the highs without using the vol cap.     
Specs are:
820/250uf
820/.68uf
2k7/.68
33k/500p/.022/.022 stack
100k snubber
47k on 8ohm NFB

[sluckey]

I suggest putting the bright cap back and jump the channel inputs together. Then use the two volume pots to blend the dark channel with the bright channel for the tone you want.

[stratomaster]

56k in place of the 33k slope in the tone stack.  Has the benefit of taming some boominess of the normal channel too. 

[PRR]

.

[audiosalvage]

I started out with channels jumped with bright cap, didn't like the staticky hair all the way past 2:00.  I just want a little wiggle room in the high register

[stratomaster]

I started out with channels jumped with bright cap, didn't like the staticky hair all the way past 2:00.  I just want a little wiggle room in the high register

You can try adding the bright cap back and remove the 470pf across the 470k just downstream.

Or replace the .005uF with a .001uF or lower instead of removing it entirely. 

I think playing with the slope resistor as recommended above can also get you there. 

[audiosalvage]

I'll try that

[audiosalvage]

Ok, I put a 56k in the slope and .001 on the pot.  I notice a difference and I
'll see how I like it over time.  I probably put the bright cap on a switch so I can A/B it in different situations.  I also removed the 470pf on the mixer and seemed to do nothing, so I put it back.  Thanks for the help guys.

[AlNewman]

It's kind of an interesting circuit.

It looks like there's 2 separate channels, the one you've modified would be the guitar channel?  But it seems to have a relationship with the bass channel through the 2 low value capacitors, one which you've removed. 

Had you tried plugging into the low impedance input on the guitar channel?  Might help solve some of your tinny sounding response.

If you would really like to remove the bright cap, you may also try experimenting with your cathode bypass caps, or experimenting with the 470k resistor/500pf cap between the channels, and see what happens.  But I'm just throwing around ideas, I'd be curious what would happen if you were to increase the 470k resistor to 1M, but there's smarter people than me to make those suggestions.

There's also opportunity to create a high pass filter to ground in your NFB loop to bring out highs, ( or not cancel them), or perhaps a low pass in series, but those ideas could create other issues.  You could also experiment with changing your presence cap to a lower value maybe?  I dunno.  Sounds like fun either way.

[audiosalvage]

One thing I need to point out the schematic posted (not by me) is more or less a generic 1987 mine is slightly different as it has a cathode biased 34s.  Other that that, it's  close

[stratomaster]

One thing I need to point out the schematic posted (not by me) is more or less a generic 1987 mine is slightly different as it has a cathode biased 34s.  Other that that, it's  close

Can you post a schematic and voltages of what you built? 

I'm particularly interested in plate, screen, and cathode voltages on the EL34s and what resistor/cap network is used on the cathode.

Also, it's common practice to eliminate negative feedback when employing  cathode bypass.  I want to make sure you are still using negative feedback, the Presence control is still in the NFB loop, and the component values and OT tap will give the right amount of NFB with the correct frequency response to the Presence control.

If biased very cold with a small bypass cap, non optimal negative feedback, and the wrong frequency to the Presence control, you'll always have a bit of hash to the sound regardless of what you do in the preamp circuit. 

[audiosalvage]

I'll post a schematic and jot dog's voltages when I get home

[Bieworm]

What kind of bright cap is in there? The silver mica type can be harsh. Maybe try a good quality ceramic cap? They tend to sweeten the highs to some degree

[audiosalvage]

yellow poly film

[audiosalvage]

OK El34 plate to ground 344v
                        screen  334v
                        cathode 22v
using a shared 150R 10w resistor and 100mfd cap

[TitaniumValhalla]

I had a Marshall SV20, which you've posted the schematic of. The stock bright cap was ridiculously harsh and bright. I could not believe that they sold an amp that way, even if the standard procedure with a Plexi is to blend the channels. It was just way wrong. I don't remember what value I settled on but I tested two or three different values of bright cap until I found the right one and that made the amp sound like a Plexi should. I would spend a few bucks on a range of cap values and you'll find the right one.

[audiosalvage]

Currently I have a .001 on a switch to see if I get along with it, I'm torn.

[stratomaster]

You can piggyback  2w 1k resistor across the 150 ohm for testing purposes.  This will give ~130 ohms Rk and bring the dissipation closer to 100%.  Recheck voltages to confirm.
 You can clip this in just for testing to see if it gets you anywhere sonically.

I've often found larger bypass cap values also seem to smooth the tone of cathode bypassed power sections.  In terms of frequency response, on paper there should be no difference between 100uF and say 220uF in the guitar range, but in practice I hear one.  So that's another avenue to explore.  The guys that build 18w EL84 amps with various preamp circuits would traditionally precede fixed bias power sections claim a suitably large bypass cap can mimic a fixed bias response and minimize the sonic difference between the two.  I can't confirm this, but can confirm a sonic difference by doubling the cathode cap.

[Jennings]

Just throwing it out there…but you could put a rotary switch on your front panel and have multiple bright cap values, plus a no cap option, all there on one switch. Same thing perhaps with your NFB values.

[audiosalvage]

I put the switch on top of the chassis as not to drill any new hole in the panel.  So far I'm digging the no cap Malcolm Young tone I get.  I will try the 220uf cathode bias cap the see what difference I get.  Thanks