5E3 sounding dark and dull

Gene Ballzz

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I'm not an amp expert, but I know I like my 5E3 for what it does. Don't know why or how, I just like it.

I totally and emphatically agree! :dude: The only things not bone stock on mine are the Pre-Rola Greenback and the completely passive effects loop between V1B & V2A. The makes a good way to add a nice reverb and delay to this amp. Mines a clone, so who really cares about how "vintage correct" it is, except for those glorious tones!

Just Tweedin'
Gene
 

2L man

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I query that regular valve guitar amps with regular OTs exhibit OT saturation. ie output pentodes simply can’t deliver power into low impedance loads, and a saturated OT would lose its primary inductance, and hence would present a low impedance load.
So very onset of saturation is the best that could feasibly be achieved. Attempting to increase the excitation current any further will just cause it to self limit. So the OT’s BH curve can’t be pushed into the horizontal areas.
Why do you think OT saturation is occurring, eg what’s the evidence that’s what’s happening, and why would 5E3 obviously exhibit it to a greater degree?

My take is that 5E3 flub would more likely be due to bias shift, caused by the long time constants of its RC coupling circuits.

I have Hammond 1760E on my 5E3 and it is very small. I have looked signals using an oscilloscope inputting sine wave and low frequencies get screwed a lot. Athode coupling capacitor seems too small to offer better operation but that too might be needed for Tweed Deluxe sound?

Does too small power tube coupling capacitor shift bias on long notes?
 

Hebaton

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Just finished Installing the NFB mod, no time to play with it yet, but very rapid test shows significant headroom augmentation in at least one setting. Still can get the original setting as well so, win win here
 

Pete Farrington

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I have Hammond 1760E on my 5E3 and it is very small. I have looked signals using an oscilloscope inputting sine wave and low frequencies get screwed a lot
I don’t see that as evidence of OT saturation per se. Maybe its primary inductance is too low to support the signal level - frequency combination you were using?
https://www.hammfg.com/files/parts/pdf/1760E.pdf

Athode coupling capacitor seems too small to offer better operation
The cathode bypass cap would normally be described as a decoupling cap. With a shared pair of cathodes in push pull, it doesn’t really do any decoupling though until the sgnal level pushes operation beyond the class A area. Within the class A area, signals at the cathodes will largely cancel out.
How does a bigger cathode decoupling cap, lower signal level or higher test frequency affect things in your tests?

Does too small power tube coupling capacitor shift bias on long notes?
Bias shift occurs due to rectification at an overdriven grid, ie when the instantaneous vg1-k>0V, a forward biased diode is formed and the g1-k resistance falls to a low level. With regard to RC coupling to the grid, long time constants will hold the rectified voltage for longer than shorter time constants. So long periods of heavy overdrive combined with a long RC time constant will cause bias shift to build up.
Aiken describes the effect http://aikenamps.com/index.php/what-is-blocking-distortion
 
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