Kalamazoo to Marshall Mod

[SteveInMN]

Hi All ,

Just kidding with Kalamazoo to Marshall.

I have a Kalamazoo Bass 30 (schematic attached) that sounds pretty jazzy/mushy, which makes sense as it was intended to be a bass amp. It is also highly resistant to breaking up.

I'd like drain off fewer highs/mids, and have a bluesy break up available at noon volume, so the amp is actually useful.

As a start, I'm thinking:
Imitate a Fender input section: replace the 100k's with 68k's on the jacks, and the 470K after them with a 1M.

After that, I am lost and would appreciate you thoughts regarding obvious things that may be holding this amp back.

While I have a truly minimal grasp of circuitry, I have a healthy respect for capacitors, know they can kill me, and know to drain them and verify they are safe.

Thanks much.


Note: the schematic shows 22K R -->500p cap --> ground out of the volume pot. They are opposite that order on the amp: that is,  500p --> 22k R --> ground

[sluckey]

As a start, I'm thinking:
Imitate a Fender input section: replace the 100k's with 68k's on the jacks, and the 470K after them with a 1M.

That won't do anything for you.

Note: the schematic shows 22K R -->500p cap --> ground out of the volume pot. They are opposite that order on the amp: that is,  500p --> 22k R --> ground

When components are connected in series such as this cap and resistor, it makes no difference which comes first.

A different speaker would probably help a lot. Bass speakers were not designed for breakup. If fact, when that amp was new, that would have been considered a fault.

[shooter]

^^^^
2nd speakers


about half the "improvement" I get from a build is speakers
so you're using it for guitar?

[PRR]

It may be short on gain. *IF* that drawing is right, the input JACKS are not wired like a Fender. It's not the resistor values. The unused 2nd input chops the signal in half. To verify, shove a blank plug or dowel in the unused jack, does gain come up?

You can strap the two cathodes of V2 to get a little more gain.

I hope the James tone network is drawn wrong.

[SteveInMN]

As a start, I'm thinking:
Imitate a Fender input section: replace the 100k's with 68k's on the jacks, and the 470K after them with a 1M.

That won't do anything for you.

Note: the schematic shows 22K R -->500p cap --> ground out of the volume pot. They are opposite that order on the amp: that is,  500p --> 22k R --> ground

When components are connected in series such as this cap and resistor, it makes no difference which comes first.

A different speaker would probably help a lot. Bass speakers were not designed for breakup. If fact, when that amp was new, that would have been considered a fault.

OK, I had read that one C/R order was a low pass filter, the opposite high pass, which I guess was wrong.
I was hoping that a few bucks of resistors/capacitors would be transformative, as two new tens would be a couple hundred, but I accept this may be the only path forward, thanks.

[SteveInMN]

It may be short on gain. *IF* that drawing is right, the input JACKS are not wired like a Fender. It's not the resistor values. The unused 2nd input chops the signal in half. To verify, shove a blank plug or dowel in the unused jack, does gain come up?

You can strap the two cathodes of V2 to get a little more gain.

I hope the James tone network is drawn wrong.

Yepper, the inputs drawing does appear to be right. I just tried plugging a separate, unconnected cable into the second jack. The volume did increase a bit, but no appreciable increase in gain.

When you say "strap the two cathodes of V2" - what does that mean?
I have two different schematics of the amp, and the stack drawings are the same. What about the tone stack caught your attention?
All ears, thanks.

[SteveInMN]

^^^^
2nd speakers

about half the "improvement" I get from a build is speakers
so you're using it for guitar?

Yes, guitar, pretty weak for modern bass. Any ideas on speakers? It has two non-original Eminence 10s in it right now, I believe they are pretty low-tier.

[shooter]

Any ideas on speakers

how many friends with amps do you have 


each amp responds different so my speakers in my amp may not fit your amp well, or it might shine, beg, borrow, till you have a idea IF that's a good road.


I run a 2 X12 cab, 90's cream back, paired with a "close-out" Eminence circa 2010


EDIT:

but no appreciable increase in gain

you have pedals, try a gain/tube screamer up front

[danhei]

Somewhat unusual topology. You’ve got a gain stage, followed by a cathodyne phase inverter, and then each output goes through another gain stage (the two sections of the second 6EU7).


Before you swap any other parts out please try bypassing both of the 820 cathode resistors on the second 6EU7 with a pair of electrolytic caps. Value not critical but suggest somewhere in the range of 1uF to 50uF, 10V+. Can use alligator clips just to test (but make sure you don’t short anything else out).


That should wake it up a quite a bit.

[SteveInMN]

Somewhat unusual topology. You’ve got a gain stage, followed by a cathodyne phase inverter, and then each output goes through another gain stage (the two sections of the second 6EU7).


Before you swap any other parts out please try bypassing both of the 820 cathode resistors on the second 6EU7 with a pair of electrolytic caps. Value not critical but suggest somewhere in the range of 1uF to 50uF, 10V+. Can use alligator clips just to test (but make sure you don’t short anything else out).


That should wake it up a quite a bit.

OK danhei, I appreciate your analysis and input, I will try that. Is this what PRR might have meant by 'strapping the cathodes'?  If the amp shows some promise with inexpensive changes like this, I'll feel better about serious money items like speakers.

[shooter]

ERROR 404

[SteveInMN]

ERROR 404

Shooter, I'm thinking you are trying to tell me something, but I'm slow on the receiving end (?)

[SteveInMN]

ERROR 404

Shooter, I'm thinking you are trying to tell me something, but I'm slow on the receiving end (?)

[shooter]

nope, i mistook the post pi gain stage as the pi, so i deleted my comments so i didn't look more foolish than i typically do 


shot-gunning the signal path isn't very productive, once you find some guitar speakers, a scope is real handy, that way you can "see" each stage, where it's lacking or causing "issues", ok there i told ya something 

[SteveInMN]

nope, i mistook the post pi gain stage as the pi, so i deleted my comments so i didn't look more foolish than i typically do 


shot-gunning the signal path isn't very productive, once you find some guitar speakers, a scope is real handy, that way you can "see" each stage, where it's lacking or causing "issues", ok there i told ya something 

No worries,thanks much!

[SteveInMN]

Somewhat unusual topology. You’ve got a gain stage, followed by a cathodyne phase inverter, and then each output goes through another gain stage (the two sections of the second 6EU7).


Before you swap any other parts out please try bypassing both of the 820 cathode resistors on the second 6EU7 with a pair of electrolytic caps. Value not critical but suggest somewhere in the range of 1uF to 50uF, 10V+. Can use alligator clips just to test (but make sure you don’t short anything else out).


That should wake it up a quite a bit.

Oh. YEAH!!! That's the ticket, its got some bark on it now!
While I DID it -- I don't really UNDERSTAND it. Can you tell me why this works?
Thanks danhei.

[danhei]

Oh. YEAH!!! That's the ticket, its got some bark on it now!
While I DID it -- I don't really UNDERSTAND it. Can you tell me why this works?
Thanks danhei.

I highly recommend reading all of Merlin's document on the triode gain stage but at least take a look at section 1.18 (The Cathode Bypass Capacitor).

In a nutshell, an ordinary triode gain stage without a cathode resistor bypass capacitor operates under internal feedback, reducing the gain and distortion, increasing the headroom and bandwidth. These can be good things for a hifi amp or a clean bass amp!

But for your amp as a guitar amp you need more gain: because it has only two gain stages between the input jacks and the output tubes (the cathodyne phase inverter has unity gain) plus you've got a tone stack in there which will have additional signal loss.

Happily, the single easiest and minimally invasive mod for this amp is to bypass those two 820 ohm resistors with caps, giving you a significant gain boost.

And, if you study Merlin's Fig. 1.26 you can see this also gives you the easiest way to change the bass response. Reducing the value of the cathode bypass cap reduces the low frequency gain back to its non-bypassed value, e.g., a 22uF bypass cap will have a -3dB point around 10-20 Hz, well below guitar and even bass guitar frequencies, and at the lower limit of human hearing. We would consider the stage "fully bypassed" with this value. But a 1uF bypass cap has a -3dB point of about 100 Hz, which would mean it rolls off the lowest notes of the guitar, reducing bass or increasing apparent brightness.

So if you've got bypass caps in the range of 22uF or more and you like the sound but think it's too bassy, try decreasing the caps to 1uF (or even less!). Conversely, if you've got smaller value caps in already and you want it to have a bit more low end, try increasing the caps.

You can also give this triode calculator a try. I plugged in some values and ran the 12AX7 model, to illustrate what Merlin also shows. You can see unloaded voltage gain around 66 bypassed v. 42 unbypassed.

Note the 6EU7 is functionally identical to a 12AX7 in most applications BUT IT HAS A DIFFERENT PINOUT so don't go swapping 12AX7s into your amp. You'd need to rewire the sockets, which probably isn't worth doing.

[sluckey]

Also put a blank plug in the unused input jack as PRR suggested. Gain should come up considerably. If you like it, you may want to rewire like the Fender Hi/Lo input jacks. The low jack would give you what you have now and the high jack would give you more gain. You could reuse the same 100Ks and 470K.

It may be short on gain. *IF* that drawing is right, the input JACKS are not wired like a Fender. It's not the resistor values. The unused 2nd input chops the signal in half. To verify, shove a blank plug or dowel in the unused jack, does gain come up?

[SteveInMN]

Oh. YEAH!!! That's the ticket, its got some bark on it now!
While I DID it -- I don't really UNDERSTAND it. Can you tell me why this works?
Thanks danhei.

I highly recommend reading all of Merlin's document on the triode gain stage but at least take a look at section 1.18 (The Cathode Bypass Capacitor).

In a nutshell, an ordinary triode gain stage without a cathode resistor bypass capacitor operates under internal feedback, reducing the gain and distortion, increasing the headroom and bandwidth. These can be good things for a hifi amp or a clean bass amp!

But for your amp as a guitar amp you need more gain: because it has only two gain stages between the input jacks and the output tubes (the cathodyne phase inverter has unity gain) plus you've got a tone stack in there which will have additional signal loss.

Happily, the single easiest and minimally invasive mod for this amp is to bypass those two 820 ohm resistors with caps, giving you a significant gain boost.

And, if you study Merlin's Fig. 1.26 you can see this also gives you the easiest way to change the bass response. Reducing the value of the cathode bypass cap reduces the low frequency gain back to its non-bypassed value, e.g., a 22uF bypass cap will have a -3dB point around 10-20 Hz, well below guitar and even bass guitar frequencies, and at the lower limit of human hearing. We would consider the stage "fully bypassed" with this value. But a 1uF bypass cap has a -3dB point of about 100 Hz, which would mean it rolls off the lowest notes of the guitar, reducing bass or increasing apparent brightness.

So if you've got bypass caps in the range of 22uF or more and you like the sound but think it's too bassy, try decreasing the caps to 1uF (or even less!). Conversely, if you've got smaller value caps in already and you want it to have a bit more low end, try increasing the caps.

You can also give this triode calculator a try. I plugged in some values and ran the 12AX7 model, to illustrate what Merlin also shows. You can see unloaded voltage gain around 66 bypassed v. 42 unbypassed.

Note the 6EU7 is functionally identical to a 12AX7 in most applications BUT IT HAS A DIFFERENT PINOUT so don't go swapping 12AX7s into your amp. You'd need to rewire the sockets, which probably isn't worth doing.

Super, thanks danhei, this gives me a LOT to go on! I appreciate the education.

[SteveInMN]

Also put a blank plug in the unused input jack as PRR suggested. Gain should come up considerably. If you like it, you may want to rewire like the Fender Hi/Lo input jacks. The low jack would give you what you have now and the high jack would give you more gain. You could reuse the same 100Ks and 470K.

It may be short on gain. *IF* that drawing is right, the input JACKS are not wired like a Fender. It's not the resistor values. The unused 2nd input chops the signal in half. To verify, shove a blank plug or dowel in the unused jack, does gain come up?

Thanks sluckey I will try this!

[PRR]

....all of Merlin's document on the triode gain stage but at least take a look at section 1.18 (The Cathode Bypass Capacitor)....

Good stuff, but in this case, save the two bucks and just strap one cathode to the other.

It is push-pull. They work in anti-phase. Two antiphase cathodes "bypass" each other very well. In the end you even save one resistor (use one of half-value). In production that turns a slim loss into a slim profit. On the bench the strap is just a clip-cord.

[SteveInMN]

....all of Merlin's document on the triode gain stage but at least take a look at section 1.18 (The Cathode Bypass Capacitor)....

Good stuff, but in this case, save the two bucks and just strap one cathode to the other.

It is push-pull. They work in anti-phase. Two antiphase cathodes "bypass" each other very well. In the end you even save one resistor (use one of half-value). In production that turns a slim loss into a slim profit. On the bench the strap is just a clip-cord.

OK PRR! - jumper the cathodes directly  (no caps, no resistors), and run a 410ohm (or there-abouts) resistor from the center of the jumper wire to ground. Do I have that straight?

[PRR]

I was suggesting to just jump it to try. But you have gone past that point, which is fine too.