increase filter caps stages

[diddymix]

Hey guys. I wanted to give something a try after looking at a Dumble schematic, tweaking my Marsall.. and I wanted to get closer to the feel and dynamic response of a Dumble amp..Ive never been too much of a fan of the Marshall.. 'sponginess'... I realise theres more to it than just the filter caps.. but Ive noticed they are quite high in the 102/HRM design and this surely does contribute to the feel. In my amp I have replaced the big main one with a 120uf.. from 100uf and I like it.. In the dumble Ive noticed there is 20uf per tube.. but mine is 10uf. I was hoping I may ask someone with power supply expertise to look at my schematic and see what they think... I ask because even though IM sure I could replace all those 10uf caps with 20's.. and it may work.. I don't know if this would be stressful on the power supply (PT + Diodes, etc etc) and if it wouldnt be able to handle that increase. .. or if its not actually an increase at all because the large 120uf is the one that matters..

ANy expertise and advice most welcome! All I know is a trust my ears and know what I like and would like to try it out.. however in this situation I wouldnt necessarily know if this is a bad idea for the stability of the amp.

Thanks so much!

[pompeiisneaks]

Increasing filter capacitors in pretty much any amp does make the power supply stiffer. The only time I know of that you need to be wary of the filter capacitor values used is in tube rectified amps, the tube rectifiers have a max capacitance they can feed without stressing the tube. Yours is solid state diode rectified so nothing to fear there. You also could carefully tack on or alligator clip a 10uF to the other 10uF in parallel to see how it impacts the tone of the amp. (Just don't shock yourself hehe).

The thing is, that actually will change the overall tone of the amp too, as the higher the capacitance in the filtering, the more bass response the amp gets, as the higher capacitance filters less bass out of the amp. Also the sponginess you mentioned will be less present, but some people will argue this is what defines a marshall. So some of the tone that you may like in the marshall may also disappear or at least diminish.

~Phil

[xtian]

Agree with your response, Phil, except this bit:

as the higher capacitance filters less bass out of the amp

We're talking about the filter caps for the HT supply, and this part of the circuit is not in the signal path. It cannot directly affect the tone of the amp...EXCEPT when the amp draws so much current (like when you hit a loud bass note or thump) and then the HT supply can't keep up with demand, so the amplification sags under the load, and thus the bass response is less than if you had a very stiff power supply (higher capacitance).

[tubeswell]

Agree with your response, Phil, except this bit:

as the higher capacitance filters less bass out of the amp

We're talking about the filter caps for the HT supply, and this part of the circuit is not in the signal path. It cannot directly affect the tone of the amp...EXCEPT when the amp draws so much current (like when you hit a loud bass note or thump) and then the HT supply can't keep up with demand, so the amplification sags under the load, and thus the bass response is less than if you had a very stiff power supply (higher capacitance).

Increasing capacitance on the screen supply node (particularly where the screen supply node is a smoothing cap in a CRC filter), shunts more bass frequencies out of the HT rail - and therefore reduces bass frequencies in the screen current (which otherwise feedback into the signal and reduces those same bass frequencies).

[pompeiisneaks]

Agree with your response, Phil, except this bit:

as the higher capacitance filters less bass out of the amp

We're talking about the filter caps for the HT supply, and this part of the circuit is not in the signal path. It cannot directly affect the tone of the amp...EXCEPT when the amp draws so much current (like when you hit a loud bass note or thump) and then the HT supply can't keep up with demand, so the amplification sags under the load, and thus the bass response is less than if you had a very stiff power supply (higher capacitance).

Unless I'm mistaken, and my memory does seem to get worse with time, when I was learning this exact topic, sluckey et al on hoffman forums told me this exactly. That filter capacitors still do allow some signal shunting to ground, as they're directly connected to the anodes and signal can either go to the next stage via the coupling cap OR to ground via the decoupling/filter caps. My general understanding of filter capacitors to earth is that as the capacitance goes up, the low frequency cutoff that is blocked decreases to lower frequencies . Thus why bassman's had such high filter capacitance on the first cathode instead of the more common 5-25 uF they're like what 125uF or something (Going from memory). It's a more subtle impact on power supply filtering but it still 'works' that way.

I definitely would appreciate someone clarifying what I misunderstood there.

~Phil

[wpaulvogel]

Some of the Marshall power transformers are saggy. They have higher DCR

[JD0x0]

Some of the Marshall power transformers are saggy. They have higher DCR

They also have less current rating. IIRC a typical marshall 50watt PT has something like 150mA rating on the HT, while Schumacher/Fender type iron was rated at 200-240mA at similar HT voltages, but Marshalls have always seemed tighter to me, because of their voicing and the circuits I've played had plenty of filtering.