CoolWire

This is my last post on this thread. I have found it very useful so thanks to everyone for their input. I've summarised my thoughts on the thread on my [url="http://coolwire.co.uk/tube-amps/northcourt-fifteen-part-i/"]blog[/url] but if you want to skip my rambling and cut straight to the chase, the schematic for my Northcourt Fifteen amplifier can be downloaded [url="http://www.coolwire.co.uk/northcourt/images/northcourt15-pc.gif"]here [/url]and the schematic of v1.0 of my proposed rebuild can be found [url="http://coolwire.co.uk/northcourt/images/northcourt-mk2-1.0.gif"]here[/url]. I doubt that I will build the amp quite as shown here but it will be similar; I am not going to change the phase inverter or power amplifier. Any changes will probably be in the power supply to ensure that there is a large enough reservoir to handle bass transients and sufficient current available to refill the reservoir quickly. I also need to add a power supply leakage resistor to ensure that the capacitors do get discharged when the amp is turned off.

[quote name='spaners' timestamp='1380569799' post='2227317'] Ok, correct me if I'm wrong but I am assuming that Coolwire is planning on home/recording as the sole purpose of building this amp therefore I wont involve myself in the whys and hows as far as stage rigs are concerned. suffice to say that 15w of tube power into a 1x10 bass cab will work just fine. [/quote] Hi Spaners, You're right about home/recording and thanks for all the insights. Really useful stuff. I was thinking that my bass sounded good with a relatively flat EQ and you seem to be saying much the same. You have a point about rebuilding the amp "as is" so I'll follow your advice ... up to a point. I'll create a schematic for the amp as it is before I do anything - it's not quite the same as the schematic I found on the web - but I'm going to lose the volume controls in front of the first valve; they're really not necessary. I'm also going to think pretty hard about the power supply.

[quote name='spaners' timestamp='1380449847' post='2225305'] I have had a bit of a look at the schematic in your link, to my eye this looks like quiet a basic dual purpose amp that should in principle do the job you require. as a first step I would suggest having the transformers tested. If they check out I would start by re-building the circuit on a turret board using new components as resistors and capacitors are very cheap, you should be able to find resistors of the same value. With the capacitors you may have to use close value components but I would suggest that you get the closest higher value where the same value cannot be found. I would also add a stand-by switch before the rectifier vale and probably a solid state rectifier to the heater supply to reduce hum. Update the input supply cable and fuse holder. fit a new set of input/output jack sockets and you should be good to go. If the transformers are not up to scratch then its probably not going to be cost effective though. [/quote] Thanks, Spaners, The transformers check out with a voltmeter (i.e. there don't appear to be any breaks or shorts) so I'm hopeful that the transformers are okay (although someone seems to have wired the ultralinear tap to the anode on one side). I will do pretty much as you suggest although I may use tag strip rather than turret boards and I will change the tone controls a little. I will also pay special attention to the power supply following your comments. I have just bought Merlin Blencowe's book on the subject of power supplies and I'm sure he will have something useful to say on the subject. I'll head off and produce a revised schematic now. I'll be back.

[quote name='JapanAxe' timestamp='1380458362' post='2225498'] I have extensively modded a Harley Benton GA5, which is Thomann's much cheaper badge-engineered version of the Epi Valve Junior. [/quote] This is a real eye-opener for me. What surprises me is that you guys are starting with a straight-forward guitar combo. I thought the speakers would be too small and the bass would roll off far too early to be useful and the open-backed construction would cause problems. You're also foregoing more complex tone controls. I guess Big Muff tone control does at least give you some control over the bass (as I remember it the Big Muff control boosts the bass as you roll of the treble and vice versa). Perhaps I'm worrying too much.

[quote name='spaners' timestamp='1380440871' post='2225148'] I have modded two guitar tube amps to use with my bass. A 5w EVJ and a 15w Fender excelsior, both combos. The EVJ is ok for quiet practice with good tone when I am away in hotels (quiet often with my job) The Excelsior is much louder and I love it for jamming with friends. [/quote] Thanks for that post. Really interesting.. I've got a bunch of questions but I'll limit myself to two for the time being: [list=1] [*]Don't you find the lack of a bass tone control on the E[color=#282828][font=helvetica, arial, sans-serif]xcelsior [/font][/color]something of a problem? [*]Where did you get the replacement output transformer. I am a little concerned that the output transformer on my donor amp might be a little too small. [/list]

[quote name='Mr. Foxen' timestamp='1380321773' post='2223947'] Because bass sounds different. [/quote] That's the bit I'm trying to get my head around.

[quote name='badboy1984' timestamp='1380289893' post='2223345'] My settings on the amp is set flat (12 o'clock position) and EQ on my bass (if is active) set flat as well. I set everything flat to see how to tone goes, and i will adjust if their is a need. I mainly dial in abit of mid and it sound sweet and warm on my LB30. I don't use my valve amp to get that fuzzy/distort sound. I like my valve to sound clean with warmth to it. [/quote] That good to know. Thanks. It suggests that the boost switches are for edge cases and aren't really needed. Do you know what type of tone stack the LB30 uses? I suspect it's FMV but I could easily be wrong. If I'm right, putting TMB at 12 o'clock could mean a 12dB lift for bass and treble.Increasing the mids would be pretty much the same as cutting bass and treble and increasing the gain. In any case it seems that mids tend to get added in rather than cut out. Is that a fair comment? I'm going to see if I can find out more about the LB30 tone conrols.

[quote name='Mr. Foxen' timestamp='1380289992' post='2223348'] Totally wrong. Same power is same output. It might be different output that you want, probably more lows, that comes from power supply and output transformer size. [/quote] Interesting. I figured it was down to the efficiency of the bass speakers or something. If a 50W bass atack is as loud as a 50W guitar stack why bother with 500W bass amps? Perhaps its the psycho-acoustic effect of guitar amp distortion that means bass amps have to be more powerful because they usually remain clean? A heaviy distorted guitar signal will always sound louder than a clean signal with tehe same amplitude. All other things being equal, 500W only sounds twice as loud as 50W.

[quote name='Beer of the Bass' timestamp='1380281856' post='2223179'] Having had an amp with both James (passive Baxandall) and Fender style tone stacks on otherwise identical channels, I much prefer the James stack. [/quote] Thanks for that. I've read this view before; that the James tone controls suite the bass better. Which frequency would you recommend for the mid-range notch? 500Hz?

[quote name='badboy1984' timestamp='1380274795' post='2223050'] Probably just buy a Ashdown CTM15 and you will be all set. Is a 15watt all valve bass amp. It is small as well. [/quote] I did think about that but I'd rather take the DIY route as I'll be able to tweak it to get it to sound as required. This is impossible with commercial offerings that are built on a PCB. The issue for me is that - being a gigging guitarist - I haven't had the need to use bass amps in anger so don't know which features are the most useful. I will find out as I go through this exercise but your experiences will help me get their faster. For example, what tone settings do you use on your LB30 and why/when?

I should come clean and let you know that I'm really a guitar player but I have always loved playing the bass and I thought it was about time that I built a small bass amp to complement the 1W guitar amp that I built some years ago. So I've bought an Ashdown Mi10 cabinet because it is physically the right size for the home environment and because it seems to cope okay with the frequency range of a four-string bass (they don't publish the frequency response). So now I need to build the amp. I've bought an old 15 Watt valve amp to act as a donor so now all I have to do is work out what preamp circuit I should build. You'll find some pictures and more details on [url="http://www.coolwire.co.uk/tube-amps/a-small-bass-amp/"]www.coolwire.co.uk[/url]. Now I'm working on the rule of thumb that a bass amp needs 10 times the power of a guitar amp to achieve the same output. That means I am looking to build a 10W bass amp to get the same feel as my 1W guitar amp. I have no idea where I picked up this idea and I may be totally wrong. Can anyone confirm this? The next thing I have to worry about is tone controls. Old valve bass amps seem to use a standard FMV tone stack but, in a domestic environment, I think it is important to allow bass cut to allow for the proximity effect but this is not possible sing the FMV topology. Does anyone have any thoughts on this? There appears to be a new breed of small bass heads like the Ashdown LB30. I haven't seen a schematic for them but I note that they provide switches to change the range of the bass, mid and treble controls. Does anyone have one of these amps? Do you use these additional controls? I have never found a mid control of an FMV tone-stack to be particularly useful; it just seems to complicate things and really only seems to exist to ensure that no gain is lost. If I'm aiming for a clean sounding amp then losing gain is not a big issue. How much dirt would you want to get from a valve amp? I know there are a lot of questions here and I apologise but if I can pick your brains I won't make as many mistakes myself. I return I'll publish the schematics for anyone else that wants to build there own amp. I don't see many (any) schematics for small valve bass amps. Thanks.

Thanks to you all. I will head on over to amps-and-cabs shortly. Not sure that I'm autarkic but I do enjoy playing with technology to see what it can do.

Hello to everyone. My first bass was a '64 Hofner Violin Bass which I played in a few bands before deciding I was a better guitar player. The years role by and the Violin Bass is now in the hands of my son but I have decided it is time that I learned to play the bass properly (whatever that means). I have purchased an SGC Nanyo bass - I cannot remember the model number but it is lovely with a walnut (?) cap on a maple body and a maple neck. It has EMG active pickups. The only thing wrong with it that I can see is that someone filed the brass nut too deep for the A string; a small fragment of beer can is acting as a shim for the time being but I need to get i fixed properly. I guess I'll need a new nut - any recommendations? Anyway, I now need an amp and being a guitarist it has to be a valve amp and being a bit of a DIY enthusiast I'm going to have to build it myself. Now I know about guitar amps but not about bass amps so I need help. Seems that low-wattage valve, bass amps are becoming quite popular so I'm sure some of you out there will have some opinions on the features that a bass amp should have and how how powerful it should be etc. So what do you think my next step should be? I have recorded some of my thoughts so far on my blog: [url="http://coolwire.co.uk/category/tube-amps/bass-amps/"]http://coolwire.co.u...amps/bass-amps/[/url] Anyone else interested in this kind of thing?