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DIY Valve/Tube Practise Amp

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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.

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Having had an amp with both James (passive Baxandall) and Fender style tone stacks on otherwise identical channels, I much prefer the James stack. Also, it'll help you make the most of your 15 watts as it retains more of the midrange, where a lot of your perceived loudness lives. If you want a switchable bass cut, I think the most elegant way to do it would be to switch the cathode bypass caps on one or more stages of the preamp. I found the calculator at [url="http://www.ampbooks.com/home/amplifier-calculators/cathode-capacitor/"]http://www.ampbooks.com/home/amplifier-calculators/cathode-capacitor/[/url] really helpful for this.

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[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?

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[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?

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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.

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[quote name='CoolWire' timestamp='1380274335' post='2223037']
'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?
[/quote]

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.

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[quote name='CoolWire' timestamp='1380289616' post='2223340']
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.
[/quote]

PCB does stop circuit tweaking, moving centres is just changing component values. PCB means keeping the layout right is easier.

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[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.

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[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.

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[quote name='CoolWire' timestamp='1380318971' post='2223904']
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]

Because bass sounds different. But since barely any cabs can handle more than about 300w, why bother with 500w indeed. Its all willy waving. 200w has been excessive for my purposes, and my purposes are being retarded loud.

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[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.

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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. In my opinion 15w is a good size for home and will get loud enough to upset the most tolerant of neighbours, it is also a very good base for modding due to the fact that the PCBs are fairly large and the tube sockets are not board mounted like most of the modern amps. The excelsior can also be picked up quiet cheaply too. The biggest difference between a bass amp and a guitar amp is the output transformer, and what you are looking for is bigger with a better low range response . I have used Hammond OTs in both of mine. I used a 1650F in the excelsior ( [url="http://www.hammondmfg.com/1608.htm"]http://www.hammondmfg.com/1608.htm[/url] )The second thing to consider is the valves you chose. Valves are graded and that grading number tells you how clean or dirty the valve is going to be (the higher the number the cleaner the sound) so you should look for matched output valves with a grading figure of 7 or higher to get max clean head room. Increasing the size of the power supply capacitors will also help as this provides a larger reserve of power for the tubes (low frequency's require more power)typical values for these would be 22uf in a guitar amp and increasing to 47uf will generally be big enough to do the job.
I have also experimented with coupling capacitor values and cathode capacitor values but in my humble opinion these sort of changes don't make a great difference to the sound or volume (but it doesn't hurt to fit better quality components as you experiment) hope this is in some way helpful
[size=5] Warning Valve amps should only be worked on by people with the relevant experience and understanding of the dangers involved in working with high voltage electrical components [/size]

here a couple of pictures of the excelsior

[url="http://imageshack.us/photo/my-images/580/q7fl.jpg/"][/url][url="http://imageshack.us/photo/my-images/69/vkrq.jpg/"][/url][url="http://imageshack.us/photo/my-images/689/pebf.jpg/"][/url][url="http://imageshack.us/photo/my-images/9/es9o.jpg/"][/url][url="http://imageshack.us/photo/my-images/818/p904.jpg/"][/url][url="http://imageshack.us/photo/my-images/405/a4jp.jpg/"][/url][url="http://imageshack.us/photo/my-images/855/6zp8.jpg/"][/url][url="http://imageshack.us/photo/my-images/35/cj33.jpg/"][/url]

Edited by spaners

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Coolwire
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.

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[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) ...

I have used Hammond OTs in both of mine...

[size=5]Warning Valve amps should only be worked on by people with the relevant experience and understanding of the dangers involved in working with high voltage electrical components [/size]

[/quote]

I have extensively modded a Harley Benton GA5, which is Thomann's much cheaper badge-engineered version of the Epi Valve Junior. Mods included:[list]
[*]Sorting out the DC voltage
[*]Fitting a screen grid resistor
[*]New Hammond OPT
[*]Jensen speaker (I have the combo)
[*]Tidying up the preamp circuit
[*]Tone stack removed, replaced with tone circuit from Big Muff
[*]Gain and master volume
[/list]
I got my information from sewatt.com, an excellent valve amp modding forum. I am sure there is scope for optimising the preamp to suit a bass, but definitlely replace the under-specced OPT if you go down this route - it also give you all the output impedance options you are likely to need.

Edited by JapanAxe

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[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]
I think what Coolwire is getting at is that guitar needs less power to be effective in the mix at whatever volume. Especially when a distorted guitar sound is used, a 15 watt all valve amp with a good speaker will suffice on stage, a 60watt all valve bass amp will have difficulty balancing to that amp when on full in a mix. 10 times is maybe a little drastic in most cases, but definitely a factor bigger in real terms.

Most 2 way speakers do not have the same power rating for the tweeter as for the woofer for similar reasons. Music requires much more power at the lower frequencies than the higher ones as a rule (why pink noise was developed for testing systems as opposed to just using unweighted white noise).

Edited by Karnage

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[quote name='Mr. Foxen' timestamp='1380490923' post='2226202']
The fact that the standard guitar cab is a 4x12, whilst standard bass cab is a 4x10 is part of why bass needs more power.
[/quote]
Almost as many bass players use 2 cabs as guitarists use one 4x12. The biggest cab I've used for guitar is 4x10 myself, and switched to smaller as it was quite simply overkill. There is a power reason on average for a reason, and it's not the speaker area, but the balance of power required.

A PA system will have more powerful subs than mid-tops, despite the mid-tops often covering more octaves of frequency response in real terms. It's how music is balanced in power terms.

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[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]

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[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.

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[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.

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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.

Onto Coolewires questions.
[b]Thanks for that post. Really interesting.. I've got a bunch of questions but I'll limit myself to two for the time being:
1.Don't you find the lack of a bass tone control on the Excelsior something of a problem?

2.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[/b]

1. I have made a few small changes to the amps tone control, these involved adding a tone control pot to replace the two position tone switch and changing the value of the high and low pass filter caps within this circuit. This was done to add a little more control rather than to change the way that the amp was voiced and to improve the quality of the components. Main reasoning behind this are that firstly I like to keep things simple and secondly that it already sounded good. why fix it if it aint broke ?

2. I got my output transformer direct from Hammond manufacturing in Baisingstoke. [url="https://www.hammfg.com/contact"]https://www.hammfg.com/contact[/url]
They will sell direct to the public so just phone them. (they carry a lot of stock)

Quote.
[b]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.[/b]

I started with the EVJ after picking it up at a boot sale for a tenner, just for something to fiddle with.
The speaker in this little combo, an 8 inch, was never going to do well with bass. I fitted an old car sub I had kicking about. worked just fine and sounded good. You have to remember in days gone by all amps where basically the same so you will find early bass amps quiet often would be open backed. The reason for this is that an open back speaker cab is always louder but has less control/definition ie it adds speaker distortion (desirable for guitar/not desirable for hifi) Modern bass technique's and sounds demand a more hifi sound hence the closed back and ported cabs of the modern day and the high power output that is needed to control the speaker, that powers not just there to make it loud folks !!! something you all fail to realise.

I got the excelsior after walking into a local shop and finding the shops owner playing a bass into it. We chatted for a bit about tube amps, things bass and the weather (as you do) Then after a coffee n a fag I gave the amp a go with a mustang. I just loved the way it looked and sounded, He offered me M8s rates on the amp and it was mine. This amp has a 15 inch speaker that worked well with bass and is still working well and just getting better. The only reason for modding the amp really was to satisfy my curiosity and see how much difference making the changes would make. Bear in mind that it was only ever going to be used at home.
Its also worth saying that most of the mods I tried had little or no effect on the sound as far as I could tell and most were done with a view to improving power supply elements or to fit higher quality components. Again I repeat, if it aint broke why try to fix it ?
Your comment about bass rolling off to quickly is where the bigger output transformer comes in. By rolling off I assume you mean drops in volume in comparison to the higher register. This is due to the small output transformers in a guitar amp where the roll off would typically be -3db @ 100hz in comparison to - 1db @ 30hz of the Transformer I have used.
As I said in an earlier post I would recommend that you rebuild your amp to the original spec and try it out before you make any changes that may or may not improve the amp. That way you will have a subjective bench mark as to the effect your changes make to the sound good or bad. Make changes one at a time, that way you will know what to undo when the result is not what you expected and remember the guy that designed the amp probably knew a lot more about tube amp topology than we will ever know collectively. Therefore don't expect your mods to make it sound dramatically different.
Hope you find this in some way useful

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[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.

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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.

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