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Old 29th October 2006, 12:48 AM   #1
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Default Mesa/Boogie Bass 400 psu

Hi

I'm putting together a parts order for a power supply which is going to be used for a clone of the Mesa/Boogie Bass 400 guitar preamp section only:

Pre and power amp schematic

PSU schematic

DIYaudio user zafira1981 has re-drawn the schematic here.

A few questions:

1. On the original service schematic, there appears to be a bank of caps (marked "220uf 300VDC TEN TOTAL") - can anyone suggest a cheap UK source for a suitable batch of caps, perhaps new old stock? Farnell price would be 20+ for these so it may be possible to save a bit of money if someone can suggest an alternative supplier.

2. What spec does the filter choke need to be? The original schematic seems to say "ORG"?

3. Some of the capacitor sizes seem very difficult to find. Would the following replacements be suitable?

- instead of the 40uF caps after the choke, how about 47uF/400V.

- instead of the 30/500 on the final RC, would 33uF/450V be sufficient?

4. I'm having difficulty sourcing a 200-0-200 transformer (240V primary). Power rating only needs to be enough for 2x 12AX7 since it won't be running the 6L6 as in the original Bass 400. Any suggestions?

Any help is much appreciated.

Greg
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Old 29th October 2006, 12:27 PM   #2
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Hallo

1 - I am from italy..

2 - Filter choke i suggest hammond 155g (7H 40mA)
(only without final section)
3 - Yes rather better 47uF 400V that 40uF 350V
Also for the final rc it is better 33uF but 450V are little.
Above all if the psu works without load the tension overcomes
the 500v
4 - The psu is designed to supply the final tube 6l6
IMHO if You will build only preamplifier section i suggest to
redesigned all PSU. Without the end stage the psu is simplified a lot

I try to draw a new schematics for pre-section only


bye
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Old 29th October 2006, 02:57 PM   #3
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Hi,
1 - In my opinion, minor dificulties in capacitor sizes is rather irrelevant,
just make sure you don`t change then drasticaly:

- as for caps in cathode circuits, the greater the value, the more
bass response you get, the same fits for gain

- the smaller coupling caps in anodes, the smaller bass response

- in earlier voltage filter stages, undoubtly better choice would be
higher cap values

2 - "instead of the 30uF/500V on the final RC, would 33uF/450V be
sufficient?"

- maybe, better coice would be (at least) 47uF/500V (you`ll never
know, what the peak voltage on particular stage will be...)

As for electronic parts dealer, try:
www.Tubetown.de (220uF/350V from 1.8 Eur per pc.)
or try to locate JJ caps, I have the best experiences with `em.
(not because I`m from slovakia...)

You`ll need a reliable tranny, so try to find a dealer, who will make you a custom one...it won`t be that pricy anyway.
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Old 29th October 2006, 03:03 PM   #4
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Quote:
Originally posted by Gold_xyz
1 - I am from italy..
Don't be sad about it. You have the best food.

You make a very good point about re-designing the PSU because it doesn't need to power the 6L6s. I will have a think about it.

I looked at Hammond chokes but the smaller ones are only rated up to 500VDC max. The 155G is only rated to 400V. Perhaps I will have to look at the larger, higher rated Hammond chokes?

Greg
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Old 29th October 2006, 03:04 PM   #5
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Default Re: Mesa/Boogie Bass 400 psu

Quote:
Originally posted by Greg3333

A few questions:

1. On the original service schematic, there appears to be a bank of caps (marked "220uf 300VDC TEN TOTAL") - can anyone suggest a cheap UK source for a suitable batch of caps, perhaps new old stock? Farnell price would be 20+ for these so it may be possible to save a bit of money if someone can suggest an alternative supplier.



That's for feeding the power amp, you don't need anything like that bank of caps for just a preamp.

Quote:

2. What spec does the filter choke need to be? The original schematic seems to say "ORG"?
Again, that's really for the power amp, with the MUCH lower current requirement of the preamp simply replace it with a wirewound resistor.

Quote:

3. Some of the capacitor sizes seem very difficult to find. Would the following replacements be suitable?

- instead of the 40uF caps after the choke, how about 47uF/400V.

- instead of the 30/500 on the final RC, would 33uF/450V be sufficient?
Perfectly fine - you need to bear in mind the pathetic tolerances on electrolytic capacitors - +50%/-100% isn't at all uncommon!. 30 and 40 are not preferred values, 33 and 47 are.

Quote:

4. I'm having difficulty sourcing a 200-0-200 transformer (240V primary). Power rating only needs to be enough for 2x 12AX7 since it won't be running the 6L6 as in the original Bass 400. Any suggestions?
A MUCH smaller current transformer is all that's needed, and there's little point in using a split transformer to generate a single supply - you don't need that much voltage to feed just the preamp.

At a VERY quick glance the valve transformer at
Maplin should be fine (although really far larger than required), use a full wave rectifier rather than a bridge to give about 360V on the capacitors. This transformer also has the heater winding which is useful for you.

But above all, remember the circuit is for feeding a POWER AMP, and not just a preamp, so most of it is irrelevent.
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Old 29th October 2006, 11:44 PM   #6
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a little psu for max ten 12ax7a
Click the image to open in full size.

high voltage
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Old 30th October 2006, 10:16 AM   #7
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Perhaps a bit of overkill on the capacitor sizes?, but that sort of thing is all you need for a preamp.

I would suggest though that you place discharge resistors across the caps to prevent them staying charged up!.
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Old 14th February 2010, 07:44 PM   #8
Per_R is offline Per_R  Sweden
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Hi!
Im a bit curious about what happened to this project, being on the verge of trying something similar myself. The Bass 400 Head has got some good reviews on harmony-central.com and maybe in time i will build the hole thing. At this point I will try to match this preamplifier to either a Gainclone with some kind of paralell/bridged doohicky, or a class-D power amp of some kind...
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Old 20th February 2010, 10:51 AM   #9
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If it's just a preamp you're looking for, look no further than the F2b preamp (there's a thread on DIYAudio as well). The Bass 400 does add some twists to the preamp, which I've incorporated in my own clone.
Also, the working point of the stages in the Bass400 are slightly different from eachother, they should sound different, but that's not something I tried.
But of course, you could also just build the complete preamp of the Bass400, if you want two different channels.
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Old 20th February 2010, 01:54 PM   #10
jbrew73 is offline jbrew73  United States
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hey jarno
i would like to hear more about your clone. like how you use it? studio, live, into a tube or ss power amp? how does it sound?
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