Sag in Express

[MHProd]

My Express sounds great with more playing time on it. It's now well broken in.

The only thing I'd like to tweak is the sag. I find the amp to be a bit stiff and would like something more mellow/vintage.

What would you recommend to achieve this? The only thing I can think of is increasing the value of the 20W resistor to 1K5 or maybe even 2K2.

Another option would to get a PT rated for 150ma instead of 300ma but that's a lot of undertaking...

What do you think?

[katopan]

What PT have you got in there? The Express PT is somewhat undersized compared to its nameplate and contributes significant sag.

[MHProd]

The usual Pacific 300ma PT from RJ. I certainly wouldn't call it undersized...

[strelok]

You could try lowering the filtering a bit. That might give the amp a slightly looser feel to it. Two 80uf or 64uf in series for 40uf or 32uf respectively for the mains and 32uf or 16uf for the screens like the old Marshalls. Though you'll probably run into ghosting/power supply noise the lower you go.

You could increase that resistor, but keep in mind the higher the resistance you go for the more power its going to dissipate and the hotter it will get.

Care to elaborate more on what you're looking for when you say a more mellow/vintage sound?

[geetarpicker]

What output tubes are you running?

How much is your B+ on the output tubes?

What speakers are you running?

On my original Express amps (there are many YouTube videos posted) there is no doubt that my speakers also add some compression. The old '71 era 25 watt G12Ms in my 4x12s have plenty of clarity but they do compress a bit especially in the high end and that helps smooth out the sound. The compression also adds to the amp's clean to mean range and helps even out the net volume when working the guitar knobs. That said, it's not all the amp!

GK

[MHProd]

XF2s, around 390V on the plates.

Don't get me wrong, the amp sounds good. It's just a personal preference... I'd like the attack to be a bit less fast. Think JTM45. I'm worried about upping the 20w resistor if it means it's going to get hotter. It's already very hot at 1k. I made sure it's not touching any wire but still...

[Tone Lover]

Just mimick the jtm 45 filtering in the first 2 stages. I havnt looked at what the filtering is. But sounds like your talking about filtering, when people talk about fast or slow responce.
Bill

[geetarpicker]

Just as an experiment it would be quite easy to test the circuit with a higher ohm resistor. Then if it truly sounds better you could later
work out the details of heat and mounting.

[rooster]

Geez, I run the XF2s as well and this is a pretty creamy amp. I bias my pair at 28mAs, how about you? If you tell me 40mAs I am going to have a questioning moment... ..Anyway, 1K is huge IMO. However, if you want more compression, just swap the screen resistors to 2.2K and leave the choke circuit alone.

(Also, as a reference, remember that the JCM900 circuit uses 2.2K screens, easily obtainable in 5 watt values.)