I bought a VOX NT15H secondhand, for a price I deemed suitable for the enclosure and transformers. The idea was to use the hardware to build an amplifier.
As received, it appears original, apart from valves. Despite the initial Internet raves, I do not like the sounds. First thing I noticed is an irritating fizz/fuzz/woosh sustain on note decay, it would probably not bother at faster high gain playing, but as soon as you let a note hang, and slowly decay, the fizzy decay spoils all the fun. One way to describe it would be, take a cheap spring reverb tank, open up the volume, go to max reverb, and knock on the tank - that ocean surf woosh fizzsh sound?
The amplifier has a “Bright / Thick” channel switch, on “Thick” the tone stack is lifted from ground, and extra cathode resistors are parallelled in at the first and third stage cathodes. The circuit then also couples the first and third stages’ cathodes. I suspect this “feedback” results in this unwelcome sound. Besides this, the gain sounded harsh and “brittle”.
The signal path is: Input - V3A (12AX7) – tone stack fed off plate – V3B – V4A (12AX7) – V4B (cathodyne splitter) – 2 x EL84 power valves. “Gain” pot after the tone stack, dual gang “Master Volume” pot after the splitter. Oh, “Triode / Pentode” switch too. Probing the circuit to look for the possible cause of the fizzy decay, and to draw load lines, with a wall Voltage of 230 VAC, revealed that the first pre-amp valve plates are at 102 V and 106 V. The B+ rail for this valve is at 244 V.
The second pre-amp valve plates are at 170 V and 225 V, the B+ rail is at 256 V for the first half, 275 V for the second half (cathodyne splitter).
Comparing to a schematic I have, I found a few numbering errors. The PCB has some foam goop glue all over which covers up some of the resistors and PCB numbers - one internet poster said he found that moisture collected under this goop, resulting in noise. Getting the goop off is not easy. The function of the goop is unclear to me.
After reading a lot of information on this amp, I did the following:
Bridge the channel switch legs 4 & 5 – This earths the tone stack in “Thick” mode as well. Also reduces gain.
Cut the bright cap on Gain pot – C27 ( 47 pF). To reduce harshness.
Remove R38 (10K) at third stage cathode – This removes the connection to the first stage cathode, as well as not reducing third stage cathode bias when switched to “Thick” – Reduces gain as well.
Remove C25 – connects third stage grid to earth? Function of this, exactly?
Remove C28 from the “thick” tone bypass circuit.
R9 – first stage - parallel in a 230 kOhmonto the 220 kOhm plate resistor, to get 110 kOhm as plate resistor.
R26 – second stage – replace the 220 kOhm with a 100 kOhm plate resistor.
R24 – first stage cathode resistor– measures 6.8 kOhm – parallel on a 6.8 kOhm to get 3.4 kOhm. This alters the first stage carthode resistor in “Thick” mode to 1.02 kOhm (was 1.2 kOhm)?
The internet says one should put a grid stop resistor in front of the Cathodyne splitter, this will require cutting PCB traces. Posters say the dual-gang post splitter master volume is dof, but I can probably not alter that.
Initial testing indicated that the irritating “FizzWoosh” is much better. I find the amplifier to be rather uninspiring, though, a bit “bland”. I tried a few guitars, a few speaker cabinets. The amp does not ooze “Fun!”.
It looks as if a lot of component changes will result in a Fender Princeton-ish pre-amp. Could look into that before I rip it out and turret board it into something else.
