How to improve tube amplifier tone by changing one part

Tube or solid-state, this section goes to eleven!
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jubal81
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Post by jubal81 »

FiveseveN wrote: 24 Jun 2020, 07:57 See, I think the secret of vintage tube amp tone lies in the mains cable! I just swapped the cord on an Epiphone Valve Junior with a Black Rhodium Fusion and the difference is DRAMATIC:
- sound is overall more focused and Hi-Fi
- warm, haunting mids
- fragile harmonics survive better
Are you going to try my SUGGESTION? Would it help if I show you a hundred people who believe in the tonal impact of mains cables?
Welp, you've convinced me. Sound quality is all in the brain. Gonna sell all my gear and get a 15 watt Gorilla amp and just decide it sounds better. It's so simple, I don't know why I didn't think of this years ago!

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george giblet
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Post by george giblet »

Google "speaker cable burn in" if you want to see crazy :D
I remember HiFi people saying they could hear the improvement in sound quality when a line filter was placed on the mains side.

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bajaman
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Post by bajaman »

resurrecting a dead thread: I recently had a MkIV version of the Fender Blues Junior in the workshop for repair - one of the EL84 output tubes had shorted internally.
After replacing the tube I was puzzled why I could not measure a negative grid bias, until i realised that Fender had changed the output stage from fixed to cathode bias - doh.
There is no "official' schematic available for this MkIV version but some kind soul on another forum has altered the MkIII schematic to read as a MkIV - many thanks ;-)
Anyway, when the tube shorted it took out one of the cathode bias resistors (Don't ask why fender used a separate bias resistor and electro for each tube !) I replaced the tube and resistor and once again the amplifier worked fine, or so I thought ;-)
It worked but now there was a nasty earth buzz at certain positions of the gain and master volume controls - even with both turned fully counter clockwise ! There was a point about 40% of the gain where the buzz was slightly lower and almost acceptable. What could be causing this ? - bad earthing ? pcb construction ? bad design ?
Well, often the simple answer is the right one ;-) - I removed the first 12AX7 but the buzz persisted, I removed the second 12AX7 and the buzz was reduced to almost nothing.
Can you guess what was wrong with this amplifier ?
I could keep you all guessing, but I very much doubt anyone will know what was wrong (well maybe one of our members will)
There are two small 47 ohm resistors connected from each side of the 6.3vAC heater wiring to ground - Fender in there wisdom ??? had decided to make these resistors fusible types instead of the usual practice of using 1W devices - both these resistors are on the main circuit board and visually there was no sign of overheating / burning, because fusible resistor do not give you those clues ! Even removing all the tubes and putting an ohmeter across the heater windings would not tell you anything because it is a very low impedance - much lower than the value of the resistors - the only way you could possibly tell if one or both resistors had gone open circuit in situ would be to measure the resistance from one side of the heater wire to ground.
I decided to remove both resistors and both of them measured open circuit. i replaced them with 1W metal film types and voila - nice and quiet now, with no hum or buzz
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mozz
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Post by mozz »

I usually swap in 100 ohm. That's the old standard to use for that. The best thing to use is a 250ohm pot, that way you can balance it out.

How hot are they running the tubes in cathode bias?

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bajaman
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Post by bajaman »

Fender-Blues-Junior-IV-Rev-A-Schematic_V1.1.pdf
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Caleb
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Post by Caleb »

lol did anyone ever try your suggestion yet Bajaman?

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Post by RuptorGB »

bajaman wrote: 05 May 2020, 02:18Okay, now what you have been waiting for - the most important capacitor to change in ANY tube guitar amplifier is the output coupling capacitor connected to the plate of the tube performing the very first stage of amplification :!: Do not ask me why - just try replacing it with a "mustard" capacitor it and see for yourself
I have found on many occasions, that simply replacing this first coupling capacitor has a HUGE impact on the perceived tone of the amplifier -clearer defined high end response and warmer rounded midrange tone. In some cases I have noticed an almost natural reverb and increased sustain of plucked notes by simply changing this ONE capacitor.
I haven't read the whole thread after running out of steam on the second page but here is my tuppence worth of interpretation. I am sure you notice a difference that is most likely due to the capacitor quality dictated over the years by designed lifetimes. The capacitors from 1940s to 1950s were bitumen wax sealed with much higher voltage tolerance than required and I have some that are still working today but as we all know modern caps are now down to a 2 year lifetime. There is even a website called Badcaps where people find fixes for their modern equipment due to blown caps. The insulation of the bitumen wax type is from the days of spark transmitters for tens of Kilo Volts (+10KV) but now if a 6.3V cap sees 7V it is toast or degraded. My guess is the later caps are degrading by design from the first day the amp is turned on so replacing them makes the tube amp work like it was new but the best way to put the question to bed is analyse the caps by testing their frequency response measuring their capacitance, resistance, inductance & any resonances at the operating voltage compared to a new existing type & the replaced one. Unfortunately it requires specialist equipment most of us don't have so we will never know for sure. :roll: It is like :beatdeadhorse:
Keep up the good work mate.

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