Here I go again…

gpw5150 · July 21

So, Modulus 5e3 - fully rewired with cloth cable using the shortest runs I could. Lowered the bias resistor on v1 to heat up the preamp and lose a bit of headroom and completed the Paul C Pi mod A - it absolutely sings and I could not be happier.

my 5e5a (Fender 5e5a 57 Custom shop) has the same preamp mod, but also has the local feedback loop removed, switchable global negative feedback and the paul C mod - totally sings and drives super hard, yet with great clarity.

next thing I am gonna try is removing the choke in the 5e5. It measures at about 100ohm, but as an inductance device I guess that changes depending on the current/voltage pull and it probably removes some hum due to the same dynamic. The 5e5 layout (not 5e5a) as a none choke amp suggests a 2k5 resistor in place of the choke - but any recommendations/ideas here. I am experimenting a bit to see how it sounds - my ultimate amp would be a vintage 5e5, if one comes up in the Uk…!

i am also looking at the Marsh Amps 5e5 kit as an option, so, if anyone has built this please do let me know.

ecc83 · July 22

Looking at a schematic, the 5E5A, unusually for guitar amps, puts the choke directly after the 5U4 rect cathode and thus the whole amp's HT current flows through it. Most amps that use a choke put it post the transformer centre tap and just feed the G2s and the pre amp valves.

Thus if you fit a 2k5 resistor in place of the choke a) you will drop quite a lot of HT and the resistor will run very hot and probably need to be at least a 10W component. And b) say the choke is 10 H it will have an impedance at 100Hz of about 6k and stop a lot of hum! You would need to at least double up on the 16mfd caps to quieten things down. You could of course double the caps but put the resistor only post traff tap.

But why remove the choke? Some idea of "sag"? If so put some resistors in the 5U4G anodes and give that an easier life!

Dave.

ecc83 · July 22

I also see that the design does not have any stoppers in the G2 circuits? Not a good idea, screen grids should always run a volt or two below anodes. I would fit 1k 1W MFs. Yes, ultimate power output will be marginally reduced but reliability and valve life much improved...and as we all know, valves are getting very expensive!

Dave.

gpw5150 · July 22

ecc83 said:

Looking at a schematic, the 5E5A, unusually for guitar amps, puts the choke directly after the 5U4 rect cathode and thus the whole amp's HT current flows through it. Most amps that use a choke put it post the transformer centre tap and just feed the G2s and the pre amp valves.

Thus if you fit a 2k5 resistor in place of the choke a) you will drop quite a lot of HT and the resistor will run very hot and probably need to be at least a 10W component. And b) say the choke is 10 H it will have an impedance at 100Hz of about 6k and stop a lot of hum! You would need to at least double up on the 16mfd caps to quieten things down. You could of course double the caps but put the resistor only post traff tap.

But why remove the choke? Some idea of "sag"? If so put some resistors in the 5U4G anodes and give that an easier life!

Dave.

Yeah, this is what I suspected. The Fender 57 5e5a is not quite the same as the online schematics - it has stoppers fitted and is ‘safer’ shall we say.

My reason is that I love the basic 5e5 sound - the 5e5 does not have a choke, it has a 2k5 resistor in place, i wondered if it contributed significantly to the 5e5 amp bloom. My 5e5a has been modified for cathode bias and runs a 5u4 rectifier - it is a hybrid of 5e5 and 5e5a.

ecc83 · July 22

Cathode bias? Careful or you will end up with an AC15! I did not get the schematic off line. I have the Dave Funk Tube amp Workbook Ed One.

Yes you could swap the choke for a resistor for +B supplies and feed the transformer directly from the rect' cathode but as I say, I think you will need to up the filter caps.

Dave.

gpw5150 · July 22

In terms of voltage or capacitance - both 5e5 and 5e5a layouts have 16uF 450v.

ecc83 · July 22

gpw5150 said:

In terms of voltage or capacitance - both 5e5 and 5e5a layouts have 16uF 450v.

Yes but on my circuits the 5E5 has TWO 16mfds in parallel on the 5U4 cathode, the 5e5-a just one because then there is the choke.

Chokes were common in post WW2 valve kit but as electrolytic caps got cheaper and more reliable they were gradually phased out.

What impact LC smoothing over RC smoothing for a guitar amp has in terms of sound I have no idea but I suspect very little to bugger all! That of course depends on both techniques being implemented properly.

Dave.

gpw5150 · July 22

Gah shoot, you are right Dave. As an experiment would you advocate if I whack in a power resistor to see if there is a sound/dynamic difference?

ecc83 · July 23

gpw5150 said:

Gah shoot, you are right Dave. As an experiment would you advocate if I whack in a power resistor to see if there is a sound/dynamic difference?

No, I wouldn't because as I say all the HT current goes through that choke. You could try shorting out the choke but then feeding the G2s and rest of the circuit from the 2k5. I suspect it will hum a bit but then I am an old hi fi buff and DETEST hum in any amp! (I modded son's HT-20)

How exactly you will get any kind of meaningful comparison I cannot see? Record the amp before and after?

Dave.

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