Need help: Marshall jcm 800 2204 build lacks balls

[marshall_law]

I have a 2204 build I did from scratch. Carbon film resisters. Sozo caps. Classic tone iron. I have eh el34s and tungsols 12ax7s. I've made a couple mods from the original circuit to beef it up. Removed the 10k resistor on v1a and replaced it with a 2k7/.68 combination and also put a .68 on the 820k in v2a. Clipped out the treble peaking circuit as well. In my estimation it still lacks the low end punch or balls I am looking for. I've considered changing the out put transformer to something like a MM but not sure if that will get it done. Anyone with any ideas?

[mixn4him]

Those changes are not going to help the "punch" or lowend of the amp, best bet is some type of resonance circuit. Either fixed or variable on a pot will do. Also possibly 6CA7's instead of EL34's..

[danman]

I had to add a fixed resonance cap to my 2204 build to get the extra low end I wanted. The right speakers will help alot also. What are you currently using?

[Carbia]

fixed resonance with a 220k resistor and a 2n2 cap in parallel on the NFB line

[Haze13]

I have a JCM800 2204 build too + PPMV. NO other mods, and it have the low end and the punch. Build your self a tube overdrive like GTO supercharger ( I have some pics of it in the Pedals And Gear) and you are will be just fine. All these mods that you are doing are filtering out the low end. The Peak (no attenuation of the frequency) of a JCM800 is some were at 2.5 kHz. Above and below are being attenuated. How can you check this out? Easy. Connect a scope to the out of the amp. Download a Frequency generator on your phone (For free ) and connect it to the amp. It will be much better if your amp will be connected to the resistor as a load an not to the speaker - better for you and neighbours. Look for the uncliped signal on the scope on the max volume on your amp. And start changing the frequency! You'll see that the amplitude of the signal is changing on the output but it's a constant at the input. The max will be some where in the 2.3-2.7 kHz.
SO! If you need more bass (Low End) you need much larger by-pass caps. The corner frequency of the first stage (Rl - 100k, Rk - 2.7k and By-pass cap - 0.68uF) is some where around 230Hz. 1/(2*Pi*(output resistance of a tube in parallel with Rk resistor)*0.68uF) = 229.9535.... Hz <----- No LOWS!

[darkbluemurder]

fixed resonance with a 220k resistor and a 2n2 cap in parallel on the NFB line

That's a good one. This goes between the connection of the speaker tap and the feedback resistor.

Alternatively you can use a 0.01uf cap instead of the 220k II 2n2 cap combination for a similar effect.

Cheers Stephan

[DG45]

Forgive the hi-jack but can someone explain, without getting too technical, what the resonance circuit is/does and how it works? I see that it's part of the neg. feedback circuit so how does that effect the NFB and the presence?

[darkbluemurder]

The resonance control or circuit blocks the low frequencies in the feedback circuit, which means that they are not "fed back" to reduce the gain. As a result, the gain on those frequencies is "open loop", i.e. as if the feedback loop was not there, whereas the gain at the other frequencies is "closed loop", i.e. attentuated by the feedback circuit.

The presence does the same thing - only for high frequencies.

Cheers Stephan

[DG45]

The resonance control or circuit blocks the low frequencies in the feedback circuit, which means that they are not "fed back" to reduce the gain. As a result, the gain on those frequencies is "open loop", i.e. as if the feedback loop was not there, whereas the gain at the other frequencies is "closed loop", i.e. attentuated by the feedback circuit.

The presence does the same thing - only for high frequencies.

Cheers Stephan

Thanks Stephan. Well presented so that even I understood it.

[syscokid]

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