Bass Amp Project

[Shannon Parks]

OK, my suggestion is change the whole volume control stage. The original design is a compromise, in my opinion. First remove the .002uF cap. The 'Lo Boost' isn't really needed now and it can be jumpered (ie the 10M resistor is jumpered out). Replace the .0075uF cap (or .0068uF in your case) with a .1uF coupling cap (450V would probably be fine). Then yank the 500k pot for a 100k audio taper pot. So that stage will just be a .1uF cap with a 100k pot to ground with the wiper to the next stage. You should be able to get these parts at a local radio shack for a few bucks. For even better performance, I would get an even bigger cap, like a .22uF or bigger, though .22uF is plenty. I would also drop that 68k grid stopper at the front of the unit in half - anywhere from 20k to 40k - whatever you have on hand. A 1/4W value would be fine. We want to move the first Miller Effect pole beyond the pole created in the second stage which has now been changed.

The results are you have a little less gain in this stage, but not by much. Basically, we whacked the high frequencies down in line with the bass. The frequency is very flat, totally determined by the cap and resistor for the low frequency and the in series resistance (hence the smaller value volume pot) and Miller effect of the next stage (a -3dB of about 50kHz). We haven't killed the Sunn design at all. The volume control was just a weak point, probably due to the expense of the very large coupling capacitors. Now you have a good flat EQ going into the tone controls. You should be able to evaluate your amp better now.

Shannon

[Scott Anderson]

Thanks Shannon for your help here. I'll try your suggestions as soon as I can. I've already breadboard the preamp per the schematic and besides hum there is no squeal. I must have made a mistake when I wired it in the first place. After I try your suggestions on the breadboard, I'll rewire it for the best sound. I'll keep everyone posted.

Scott

[Shannon Parks]

Thanks Shannon for your help here. I'll try your suggestions as soon as I can. I've already breadboard the preamp per the schematic and besides hum there is no squeal. I must have made a mistake when I wired it in the first place. After I try your suggestions on the breadboard, I'll rewire it for the best sound. I'll keep everyone posted.

Scott

Hi Scott,

Sounds good. To simplify what I said in the previous post, my recommendation is a simple .1uF - 100k audio pot stage. Should be a simple and cheap fix.

Shannon

[Scott Anderson]

Shannon,

I did a little messing with the breadboard last night. I replaced the 68K with a 33k and did the .1uf with a 250K pot (100K on its way). This did lower the bass and made the volume easier to control. I found these old Ampeg bass amp schematics that look interesting especially the B-25. It has a 12AX7 preamp, a 7199 driver which is similar to the 6AN8 and then 2-7027As push-pull. The tone circuit looks interest in that many of the Ampegs used this type of tone circuit. Even the legendary SVT.

Here are some links.

http://users.aol.com/bluemuse/schematics.html

http://members.aol.com/portaflex/schems/b-15-nf.gif

http://members.aol.com/portaflex/schems/b-25.gif

http://members.aol.com/portaflex/schems/b-18-n.gif

Scott

[Scott Anderson]

I sure did. I found an error in the schematic for the pre-amp. There is a .002uf cap from pin 6 to pin 2 of the 12AX7. It should have been connected from pin 6 to the bottom side of the low boost switch. Making this change made all the difference in the world.

My next project is a 120 watt amp. The schematic I’ll be using is from a Sunn 2000S. It's basically the same amp but with 4 – KT88s. It uses the same 6AN8 driver circuit. I have to beef up the power supply (Hammond 278CX) and replace the output transformer (Hammond 1650T). I’ve seen some Ampeg schematics similar as the Sunn but with a more common 7199 driver circuit. I might even try the Poseidon driver board talked about here in diytube.

[jbrew73]

Firstly I like to say, it's what YOU like which is most important, disregard any of the following if it's not right for you...

With bass amps, damping factor is important. Something I did to lower output impedance by half was to double up on the output tubes. You are running ultra-linear (UL) (Local Negative Feedback LNF) which is good but I would say ditch Global Negative Feedback (GNF). From what I can see you have no bypass capacitors on the cathodes of the preamp stages, this is LNF too, which, again, is good. The use of LNF in all the stages reduces or eliminates the need for GNF. Something I will try in my next project, which is a 6V6 push pull amp is 0.005 uF caps between the cathode and anode of the output tubes to limit high frequency response (LNF). I built a few amps and played with GNF, sweeping the signal generator to 100KHz and beyond and found no instabilities, just reduced power with GNF.

Why in heaven's name would you use tube rectification? That is strictly for guitar amps that want power supply sag for increased sustain. For a bass amp you want a tight, stiff (read low inpedance) power supply to deliver high power attack and low distortion sustain. LF4007, fast recovery sand for modern power supplies.

Take out the 10 ohm resistor in the cathodes of the KT88s and replace it with a 1 ohm for each cathode to ground. You are dissipating power in that 10 ohm and it's not really needed, at all. Talk to a guru for recommended bias voltages and take the resistor out alltogether.

Usually, in a guitar amp the tone shaping occurs in the tone controls and the interstage coupling caps. Do you need tone controls? Get rid of them if you don't. They introduce phase distortion which is good for high distortion guitar amps but bad for bass amps. Try increasing coupling caps for more bass.

If you haven't already, read:

http://www.tone-lizard.com/
http://www.normankoren.com/Audio/index.html
http://www.webace.com.au/~electron/tubes/

All great sfuff.

Don

PS:
http://www.diytube.com/forumpix/power_amp.jpg

http://www.diytube.com/forumpix/power_supply.jpg

http://www.diytube.com/forumpix/preamp2.jpg

i'm thinking about building a sunn amp and have a few questions related to this post.


1. when you said doubling up the power tubes did you use the same trannies?
2. the schematics linked are they for a bass amp?
3. in the scematics you show a 274bx/1650n set using 4 6l6's. will the tranny set handle these tubes and which would give more power 4 6l6's or 2 kt88's?

[dhuebert]

1. when you said doubling up the power tubes did you use the same trannies?

I used a transformer that was appropriate for the power level I expected to generate. What I do with the output depends on the primary impedance. For a quartet of output tubes the primary impedance should be half what it is for 2 tubes. You can halve the primary impedance by wiring the output for a certain speaker impedance and using half that ie. wiring the output for a 16 ohm speaker and using an 8 ohm speaker. Doing this increased to output power of the first amp I built from ~50W to ~60W.

2. the schematics linked are they for a bass amp?

Yes.

3. in the scematics you show a 274bx/1650n set using 4 6l6's. will the tranny set handle these tubes and which would give more power 4 6l6's or 2 kt88's?

Its not really a question of which gives more power, its more about ouput impedance and damping factor. The impedance of the speaker (load) is reflected through the transformer to the tubes but this also works the other way, the impedance the speaker sees looking into the transformer is a reflection of the output impedance of the tubes. We want that impedance to be as low as possible for various reasons I won't go into here (unless you want me to), basically better speaker control. The more output tubes you have the better the amp can control the speaker cone which gives a tighter bass. Can the tranny set handle these tubes? It has been in operation for a couple of years now and the owner loves it.

Don

BTW see avatar for current photo of amp discussed here