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I am currently playing bass through a Musicman HD130 (watts) which is set up as a guitar amp, running 4 EL34, ss preamps and a single 12AT7 PI. Although i am very satisfied with the rock and roll sound, it doesn't appear to have the lows that bass amps normally have. It's rather light below 90 Hz. What i would like to know is, are there certain resistors/ caps to change here? I had meant to change the value on the 'bass' cap, but i do want the bass to stay tight enough. I know enough about guitar amps, but before i start messing i'd like to ask first.By the way, do Fender Bassman players (those who play bass through it) ever try and change things in their amps?
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Heck of an amp to play bass through.
The first opamp has a diode clipping circuit!
Short of modding the amp you could try a bass boost pedal.
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I'm looking at the 2100-130 schematic. Most LF rolloffs are set at 16Hz. With several of them cascaded through the various stages the overall rolloff might be in the 40Hz range. The biggest LF bottleneck is 160Hz at the Deep switch. So just play with the Deep switch closed or increase the 10K (R47?) at the 12AX7 pin 7.
If one or more of the LF determining caps has lost capacitance due to age, that would reduce bass but they are all film caps IIRC, usually pretty reliable.WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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There is an ugly distortion coming from the gain channel when i play guitar through it. Would that be the diode clipping circuit you're talking about? Funny that it is not at all prominent when i play bass. I'm just not comfortable messing with transistor preamp circuits. I don't know what values should do exactly what..Originally posted by Jazz P Bass View Post
Is there any way i can get rid of that diode clipping circuit, if it produces that distorted sound? I would like it as clean as possible.Last edited by ballynally; 03-24-2011, 08:32 PM.Comment
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Thanks. will take a look. Only one tube here. That's the PI. Is that what you're referring to?Originally posted by loudthud View PostComment
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Yes, it's a gain stage followed by a Concertina Phase Inverter with negative feedback returned to the cathode of the gain stage.Originally posted by ballynally View Post
The diodes in the preamp really aren't there for clipping, they are there to protect the input op-amp when large voltages are present. The distortion you hear is probably slew rate limiting. Those op-amps could really use an upgrade.WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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What speaker(s) are you using? Guitar speakers don't take too kindly to bass guitar.Comment
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thank you very much.Was looking at the 10k (r47). Apart from that, should i be looking at the cap (c33) as well? It seems very specific for the deep swich. Any important upgrades you could recommend?Originally posted by loudthud View PostLast edited by ballynally; 03-25-2011, 12:11 AM.Comment
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I've been looking through old threads (search musicman) and there was one report of caps going low in value (0.1uFs) and another of an amp that seemed to be losing bass so there could be a problem with a batch of caps that were used. To get an accurate reading you'll most likely have to unsolder one end and measure the capacitance. C33 is out of the circuit when the Deep switch is closed because there is a short across it. You could install a 100K trim pot in place of R47 and make sort of a variable LF control.
There are adapter boards available that will adapt the metal can to a DIP package, and a suggestion to use a wire-wrap socket and just squeeze the pins into a circle. One person installed NE5534s and they drew too much current, the Tremolo stopped working in low power mode because the 16V power supplys dropped too low.WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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I would just use it as a power amp. It has a main amp in jack doesn't it? Use a small bass amp with a line out or a preamp into the main amp in. I have a cheap little Gorilla Bass combo amp with a 12" JBL in it. (It even has a built in compression circuit.) It's fine for low volume a stuff and recording. Surprizes the hell out of people. If I want a big loud rig I just use the line out into a power amp and a 2x15 cab. I got that little amp for nothing and fixed it.Comment
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My friend uses one of these as the bass head in his studio, and I've used it a handful of times at live gigs. My impression is that it is a very clean amp and has no problem handling lows on a normal 4-string. I usually play w/ the master volume on ten and the channel volume up around a half. Just my 2 cents, but I think if you're getting distortion at less than those settings than something's off.Comment
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Thanks. It is very clean... with the bass. I think there is a higher frequency buzz that becomes to the fore when playing electric guitar through it.I've tried the Master on full, but still get the buzz when i open the gain above 3, so, yes, i think something is wrong. Given the many gain stages here, and my uncertainty about what does what exactly, i don't know where to start. The amp can handle the bass al right, and there is no sign of bass frequency slippings. I just would like some more below 80 Hz frequencies.Originally posted by mrm0to View PostComment
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setup is with 1 15" 300 watt bass speaker, so no worries there..Originally posted by nashvillebill View PostComment
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