touch sensitivity

[Pedro Vecino]

Seeing this circuit and valuing the changes over a classic 2203, directly I think that itīs very difficult that it sounds well. Still with the best layout, with gain at max. too many overdrive, very dry, with the lows sounding "brump", not "chump" (or "cank" with low/medium gain setting) and without nuances and control in the pulsation. Is it something similar what happens?

[BackwardsBoB]

I'm not sure what you mean by touch sensitivity? It sounds like it does a good job of gentle stroking. Are you looking for more bite - i.e. more highs on your pick attack, etc., or more punch - i.e. less compression? Your power supply looks about as responsive as you can get with a tube rectifier, but the tube will still cream things up a little.

If things sound a bit muddy, and you want more bite, you might try reducing the capacitance on the cathode of V1B to .22uF or thereabouts. This will roll off the low frequency output of V1B somewhere around 500Hz. You might need to adjust the attenuator after V1B to compensate.

[Russ]

All 50w el34 marshalls have pin 1 connected to pin 8, as well as every other el34 amp I've seen.

[scole]

touch sensitivity

i just finished a 5f4 called mac's boogie. i went to hear how it sounds at a gig. after the set the firt words out of the guys mouth were "the notes happen instantly". i was very proud. i think touch sensitivity comes from very good solder joints, lead dress and probably component orientation.

[daz]

All 50w el34 marshalls have pin 1 connected to pin 8, as well as every other el34 amp I've seen.

yeah, that was my argument too ! But i was convinced at the time by several people swearing thats how i should do it. I'll give it another go tho because at this point the amp is very close to marshall specs. I'll let you know if it changes anything, tho i gotta say i didn't notice anything obvious when i changed it to how it is now.

[tubeswell]

The capacitor to the right of the choke is the filter for the plate supply. That's how it's normally done in guitar amps.

I'd use 50 uf if I were him, but I don't think using 32 is the cause of his lack of touch sensitivity.

Shea

Yep I now how it is normally done in guitar amps, that's why I was wondering why the PII filter seemed to be on the wrong side of the plate supply! Daz's schematic shows it between the plate and the screen, whereas the likes of Jack Darr for example, shows it beween the rectifier and the plate! I wondered whether this was some quirk of this circuit, or whether it was drawn incorrectly.

My understanding is that usually the PII filter is right after the rectifier. You have an input filter cap (which is the reservoir cap for the rectifier), followed by the filter choke, followed by an output filter cap (which is the 'anti-ripple' cap), the point being that this whole caboodle is usually right after the rectifier tube, so as to smooth out the B+ and deliver a constant current. The way it's shown in Daz's schematic, there isn't any filtering between the rectifier and the plate supply (which would deliver a really bumpy signal to the Plates).

Of course there's nothing to stop you also have a choke between the plate and screen supplies, instead of a resistor (although a second choke might seem OTT).

[Paul P]

My understanding is that usually the PII filter is right after the rectifier. You have an input filter cap (which is the reservoir cap for the rectifier), followed by the filter choke, followed by an output filter cap (which is the 'anti-ripple' cap), the point being that this whole caboodle is usually right after the rectifier tube, so as to smooth out the B+ and deliver a constant current.

I think the way daz has drawn things may be confusing you. If you take
the first cap and slide it to the right past the wire to the OT you'll have
your first filter after the rectifier filtering things for the plates. Slide the
second filter cap to the left a bit and the choke will be in between the
plates and screens as is normal.

The choke can't handle the current for the OT/plates.

Paul P

[daz]

yeah, if you look at any EL34 schematic i've seen, or at least all the ones i have, the primary center is right after the rectifier along with the first cap. It's after that cap that the choke resides.

I'd use 50 uf if I were him, but I don't think using 32 is the cause of his lack of touch sensitivity

I don't believe you can use a 50 with a tube rectifier can you? I may be off on the max size for the 5AR4, but i coulda sworn you couldn't use a 50uf with it.

[Chuck H]

Touch sensitivity is a tricky thing. All amps (good ones that is) do something well. Touch sensitivity isn't always one of them. I say that because I don't trouble about it too much as long the amp I built sounds good. Because anything you change will change something else. Not always for the better. But if it's THE goal is to achieve touch sensitivity, there are some things that have helped me.

Try not to overdrive any one or two stages to death. Get the signal to a level that can overdrive the next input a small amount and then try to do the same to the next stage, etc. This also allows you to use fewer trimming type circuits (tiny decoupling caps to trim bass to the next stage and bleeder caps to get rid of fizzy treble) because the overtones generated by each stage aren't so extreme. The amp will have a more open and natural sound. Plus, if each stage is just about to go into clipping at about the same time, The amp will respond more dramatically to pick attack and playing technique.

If sensitivity is the goal, it's best to try and do this with EVERY stage of the circuit, preamp, inverter/driver and power amp. Fine tuning the power tubes for best bias, grid stoppers, etc. is icing on a cake thats already getting very tastey.

Not all Marshalls are touch sensitive, but the good ones are. Changing the voltages in a known circuit can change gain structures down the line and alter touch sensitivity. There were periods in Marshall mfg history where they changed B+ voltage without making any cicuit adjustments other than power tube bias. These amps are generally considered "lesser" Marshalls. Probably because they lack touch sensitivity (among other things).

Your amp is running a lowish voltage compared to any Marshall I know of. If you didn't make any changes to the dropping resistors in the B+ rail then you have altered the responsiveness and gain structure of the circuit. Quite a bit actually because the differences become more profound in a multi stage amp. I would suggest that you look at the schems for the Marshall amps similar to your build. Change the B+ rail dropping resistors to try and duplicate the preamp voltages from the amp your comparing your build to. It's just a couple of resistors in the B+ rail and a reading to check volts. But it could make a huge difference.

Chuck

[daz]

Your amp is running a lowish voltage compared to any Marshall I know of. If you didn't make any changes to the dropping resistors in the B+ rail then you have altered the responsiveness and gain structure of the circuit. Quite a bit actually because the differences become more profound in a multi stage amp. I would suggest that you look at the schems for the Marshall amps similar to your build. Change the B+ rail dropping resistors to try and duplicate the preamp voltages from the amp your comparing your build to. It's just a couple of resistors in the B+ rail and a reading to check volts. But it could make a huge difference.

Chuck

Proably because most marshalls are diode rectified. Mine was for a while but i found the 5AR4 gave better feel even when using a sag with the diode rectifier. Maybe i should try going back to diodes and try some other way of getting that feel back. maybe a larger R or something, i dunno.

[booj]

It seems to me that the 4 ohm tap should be the one closest to the ground, not the 16 ohm tap. That makes me wonder what the effect would be of trying the feedback loop at different output taps. Interesting resonance circuit! Allways wondered how that was done.

[tubeswell]

I think the way daz has drawn things may be confusing you. If you take
the first cap and slide it to the right past the wire to the OT you'll have
your first filter after the rectifier filtering things for the plates. Slide the
second filter cap to the left a bit and the choke will be in between the
plates and screens as is normal.

The choke can't handle the current for the OT/plates.

Paul P

Yep thanks for pointing that out Paul P - I thought I was going crazy for a moment, but I am certain that the Pii filters I have seen do just what I describe, i.e. have an input (reservoir) cap followed by a choke followed by an output (ripple smoothing) cap before the plate supply. See page 72 in the attachment. And I'm sure a large enough choke would handle the plate supply. I've seen pictures of some gianormous chokes on old fenders.

[Enzo]

There are a few amps where there is a whole pi filter before the OT plates, but mostly not. The choke is there to filter the B+ from ripple and provide some regulation. The push pull output tubes will tend to cancel B+ ripple, so the B+ filtration at the plate node can be relaxed a little.

Sure it you need ultra-pure B+ for some application, then use the pi filter befoe everything. But in guitar amps, we usually opt for the pi filter running everything else BUT the plates.

[R ski]

I was looking at the 1st tube on the pre amp, the cathode resistor is normally 1 meg not 820 ohms, that value normally shows up on the 3 rd stage as shown

[pdf64]

I don't know about a 1MOhm cathode resistor being normal.
I checked a few of the earlier Marshall schematics, and none of them seem to show grid stoppers on the power tubes, whereas yours has them at 5k6. Might be worth reducing the value / removing them altogether to see if that helps. I would be looking to move all component values towards the original bassman which Marshall copied, rather than the 70s Marshall typical values which you've used, to improve the dynamic feel.
Have you tried different tubes? The ones you've got might be tired or just not to your taste, or not in sympathy with those lowish voltages. The differences between individual tubes, and then between tube brands, is generally subtle, but it's the subtlies that you're looking for here.
Peter.