18W TRex build - dark sounding
Posted: Tue 05/04/21 4:11 pm
Hey everyone,
I've just put together a TRex as per Mark Huss' schematic, found here:
https://mhuss.com/18watt/schematics/18wTRex.pdf
I'm having issues with the Normal channel sounding very dark, even with tone control all the way up. We're not talking a little bit dark, but "blanket over the speaker" dark. Another way to see it is it's like the tone pot on the guitar is rolled back to around 5, which to my ears I would never use. I've tried with a Strat and an LP. The Normal channel is also not very gainy, only reaching crunch at around 8 on the volume. It sounds like there is some noticeable treble roll off, but I can't work out where it could be coming from. I've heard these amps are normally too bright if anything!
The Wreck channel sounds great to me (a fair amount of bass coming through which makes it mushy at higher volume, but I can tweak that with some component changes). It actually sounds fantastic at lower gain, especially with a Strat neck pickup. It has significantly more treble available than the Normal channel (which is unusable as things stand). In case this is relevant, I am getting a kind of "whoosh" sound on held notes when I have the volume up on the Wreck channel only - I don't mind the effect but haven't heard this before. I don't know if this is part of the amp's character (this is the only EL84 cathode biased amp I have) or if this indicates a problem somewhere.
It's worth saying that I'm using stock values on the components, and transformers from AmpMaker. The PT is non-centre tapped (so I'm using a bridge rectifier) but I don't see why this would matter.
Here's what I have tried, to no avail so far:
- Bypassing the Volume and Tone pots (and related components). I did this by desoldering the ground connections on both pots, then putting a jumper over from the plate of V1a to the coupling cap before the PI. No difference. (This should sound as though both controls are all the way up, right?)
- Removing the optional high pass filter around the tone stack (asterisked on the schematic). This shouldn't really affect the tone, since the knee is at 68Hz? I wanted to see if gain increased, but it's not that noticeable. Still lower than it should be.
- Changing speakers. I have two cabs available, a cheap Marshall speaker (from an MG50DFX, which is what I've been converting to make this build) - which is a bad speaker and may have some treble roll off. And an Egnater Rebel 112x cab. It COULD be that both speakers are bass heavy, but it's quite an extreme effect to be caused by speakers.
- Output Transformer. I'm using the 18W Push Pull offering from AmpMaker. I haven't swapped it, but I have another one of these in another build (the AX84 Lead preamp into 6V6 poweramp), and this amp has treble on tap when I need it. Admittedly the NFB with Presence control helps with that. I don't think it's the OT but it's possible.
- Lowering the coupling cap between V1 and the PI down to 5n (was 22n). I couldn't hear any difference.
- Removing the cathode bypass cap on the power tubes. Again, I couldn't hear any difference.
- Swapping preamp tubes. No effect.
- Checking voltages. They all look normal enough to me, but I'd appreciate another eye cast over these. This is my second amp build so I only know what I've read on the forum - particularly I don't really know what to expect from the PI. My B+ is a touch low because I've got a 200 ohm sag resistor in place for now (I might take this out - I kinda like how it feels, but it drops too much B+ from what I've read). Here are the voltages:
B+ (off the rectifier, after the sag resistor): 322 (without the sag resistor this is 355)
Screen supply: 306
PI plates supply: 270
Preamp tubes supply: 263
V1 (input stage of Normal + Wreck channels)
Pin 1 (Plate): 175
Pin 3 (Cathode): 1.30
Pin 6 (Plate): 169
Pin 8 (Cathode): 1.39
V2 (Wreck channel stages 2 and 3 - probably not relevant)
Pin 1 (Plate): 236 (this is a cold clipper stage)
Pin 3 (Cathode): 2.46
Pin 6 (Plate): 172
Pin 8 (Cathode): 1.35
V3 (PI)
Pin 1: 198
Pin 2: 50ish (forgot to take an accurate reading)
Pin 3: 71.2
Pin 6: 207 (note unbalanced PI so this is higher than pin 1)
Pin 7: 50ish
Pin 8: 71.2
V4 (power tube)
Pin 3 (Cathode): 10.5
Pin 7 (Plate): 319
Pin 9 (Screen): 299
V5 (power tube)
Pin 3 (Cathode): 10.5
Pin 7 (Plate): 320
Pin 9 (Screen): 298
Could this kind of thing be caused by a bad solder joint? I've reflowed most of them, and have a better soldering iron coming tomorrow.
If I can't find a problem, my current solution would be to increase the plate resistor to ~220k for more gain, then add a high pass filter / lower some coupling caps. But I'd rather work out why this is happening and do a proper job
Obligatory gutshot: https://drive.google.com/file/d/1hOF72_ ... sp=sharing
Any help is appreciated!
Regards,
Simon
I've just put together a TRex as per Mark Huss' schematic, found here:
https://mhuss.com/18watt/schematics/18wTRex.pdf
I'm having issues with the Normal channel sounding very dark, even with tone control all the way up. We're not talking a little bit dark, but "blanket over the speaker" dark. Another way to see it is it's like the tone pot on the guitar is rolled back to around 5, which to my ears I would never use. I've tried with a Strat and an LP. The Normal channel is also not very gainy, only reaching crunch at around 8 on the volume. It sounds like there is some noticeable treble roll off, but I can't work out where it could be coming from. I've heard these amps are normally too bright if anything!
The Wreck channel sounds great to me (a fair amount of bass coming through which makes it mushy at higher volume, but I can tweak that with some component changes). It actually sounds fantastic at lower gain, especially with a Strat neck pickup. It has significantly more treble available than the Normal channel (which is unusable as things stand). In case this is relevant, I am getting a kind of "whoosh" sound on held notes when I have the volume up on the Wreck channel only - I don't mind the effect but haven't heard this before. I don't know if this is part of the amp's character (this is the only EL84 cathode biased amp I have) or if this indicates a problem somewhere.
It's worth saying that I'm using stock values on the components, and transformers from AmpMaker. The PT is non-centre tapped (so I'm using a bridge rectifier) but I don't see why this would matter.
Here's what I have tried, to no avail so far:
- Bypassing the Volume and Tone pots (and related components). I did this by desoldering the ground connections on both pots, then putting a jumper over from the plate of V1a to the coupling cap before the PI. No difference. (This should sound as though both controls are all the way up, right?)
- Removing the optional high pass filter around the tone stack (asterisked on the schematic). This shouldn't really affect the tone, since the knee is at 68Hz? I wanted to see if gain increased, but it's not that noticeable. Still lower than it should be.
- Changing speakers. I have two cabs available, a cheap Marshall speaker (from an MG50DFX, which is what I've been converting to make this build) - which is a bad speaker and may have some treble roll off. And an Egnater Rebel 112x cab. It COULD be that both speakers are bass heavy, but it's quite an extreme effect to be caused by speakers.
- Output Transformer. I'm using the 18W Push Pull offering from AmpMaker. I haven't swapped it, but I have another one of these in another build (the AX84 Lead preamp into 6V6 poweramp), and this amp has treble on tap when I need it. Admittedly the NFB with Presence control helps with that. I don't think it's the OT but it's possible.
- Lowering the coupling cap between V1 and the PI down to 5n (was 22n). I couldn't hear any difference.
- Removing the cathode bypass cap on the power tubes. Again, I couldn't hear any difference.
- Swapping preamp tubes. No effect.
- Checking voltages. They all look normal enough to me, but I'd appreciate another eye cast over these. This is my second amp build so I only know what I've read on the forum - particularly I don't really know what to expect from the PI. My B+ is a touch low because I've got a 200 ohm sag resistor in place for now (I might take this out - I kinda like how it feels, but it drops too much B+ from what I've read). Here are the voltages:
B+ (off the rectifier, after the sag resistor): 322 (without the sag resistor this is 355)
Screen supply: 306
PI plates supply: 270
Preamp tubes supply: 263
V1 (input stage of Normal + Wreck channels)
Pin 1 (Plate): 175
Pin 3 (Cathode): 1.30
Pin 6 (Plate): 169
Pin 8 (Cathode): 1.39
V2 (Wreck channel stages 2 and 3 - probably not relevant)
Pin 1 (Plate): 236 (this is a cold clipper stage)
Pin 3 (Cathode): 2.46
Pin 6 (Plate): 172
Pin 8 (Cathode): 1.35
V3 (PI)
Pin 1: 198
Pin 2: 50ish (forgot to take an accurate reading)
Pin 3: 71.2
Pin 6: 207 (note unbalanced PI so this is higher than pin 1)
Pin 7: 50ish
Pin 8: 71.2
V4 (power tube)
Pin 3 (Cathode): 10.5
Pin 7 (Plate): 319
Pin 9 (Screen): 299
V5 (power tube)
Pin 3 (Cathode): 10.5
Pin 7 (Plate): 320
Pin 9 (Screen): 298
Could this kind of thing be caused by a bad solder joint? I've reflowed most of them, and have a better soldering iron coming tomorrow.
If I can't find a problem, my current solution would be to increase the plate resistor to ~220k for more gain, then add a high pass filter / lower some coupling caps. But I'd rather work out why this is happening and do a proper job
Obligatory gutshot: https://drive.google.com/file/d/1hOF72_ ... sp=sharing
Any help is appreciated!
Regards,
Simon