Bass amp posible mods for guitar use

[chocopower]

Hi there!

lat week i recive an old Spanish amp (1975 maybe) and after testing it with a guitar and a 4x12 V-30 loaded cab i decide mod it for a guitar use.

This is the schematic:

http://www.mundomusicalmeria.com/mun...arc_b2130c.jpg

I think is and strange clone of a Bassman AA864 mixed with a 6G6A blonde Bassman.. but with an EL34 output stage!!!

Has a great sound in the "guitarra baja" chanel (bass guitar), but sound is really mudy in the "contrabajo" channel (contrabass...?ŋ?).
Anywere, i need keep the bass pot all way down to have a reasonable bass frecuency level and some times isnīt enought attenuation...
As you see in the schematic there are no coupling caps at each channel out, just before the P.I.
That could be a good point to start.
Maybe adding a .1 , .047 or a .022 coupling cap there resolve the problem.
What do you think?

Also could be a good idea tweek the NFB loop and...

Any idea??

[Bruce / Mission Amps]

Hi there!

lat week i recive an old Spanish amp (1975 maybe) and after testing it with a guitar and a 4x12 V-30 loaded cab i decide mod it for a guitar use.

This is the schematic:

http://www.mundomusicalmeria.com/mun...arc_b2130c.jpg

I think is and strange clone of a Bassman AA864 mixed with a 6G6A blonde Bassman.. but with an EL34 output stage!!!

Has a great sound in the "guitarra baja" chanel (bass guitar), but sound is really mudy in the "contrabajo" channel (contrabass...?ŋ?).
Anywere, i need keep the bass pot all way down to have a reasonable bass frecuency level and some times isnīt enought attenuation...
As you see in the schematic there are no coupling caps at each channel out, just before the P.I.
That could be a good point to start.
Maybe adding a .1 , .047 or a .022 coupling cap there resolve the problem.
What do you think?

Also could be a good idea tweek the NFB loop and...

Any idea??

Yeah, get rid of all those treble snubbing caps all over the damn thing and use BF Fender values for the rest.

[chocopower]

thanks!!

one more question.. Is an old amp and someone had repair it in the past...
The schematic shown a 4 diodes rectifier bridge, but my amp has only 2..
The 2 grounded ones are lose!

Should i put it again?

regards!!

[Bruce / Mission Amps]

thanks!!

one more question.. Is an old amp and someone had repair it in the past...
The schematic shown a 4 diodes rectifier bridge, but my amp has only 2..
The 2 grounded ones are lose!

Should i put it again?

regards!!

Only if the amp has the original PT, which required the the Full Wave Bridge rectifier.
If it is a replacement PT which has a high voltage center tap that is grounded, then you would only need two diodes for a Full Wave rectifier, not a bridge.
I'd use two 1N4007s in each leg of the PT secondary for some back up though.

Also use a 47pF to 100pF cap across the volume pot like you'd see in a Deluxe Reverb.

[jrfrond]

A question for you Bruce:

After the second stage, you eliminated the shelving network of the .001 cap across the 700K resistor, yet left the .22uF/4.7K snubber. I personally would have went the other way (though I would have lowered the resistor value to a more realistic 220K for more gain and less drastic shelf). I wanted to find out why you chose this route.

It should also be noted that the remaining .22 and .047 caps would require a voltage rating of 400VDC. The schematic shows 220V on the preceding plate. Of course, none of us knows the voltage rating of those caps except the amp's owner, but it needs to be noted. Because the 700K resistor is in there, those caps might be underrated if it is eliminated.

[chocopower]

Only if the amp has the original PT, which required the the Full Wave Bridge rectifier.
If it is a replacement PT which has a high voltage center tap that is grounded, then you would only need two diodes for a Full Wave rectifier, not a bridge.
I'd use two 1N4007s in each leg of the PT secondary for some back up though.

Maybe thats the reason why i have 250 volt DC in the pots!!!

I changed electrolitics and rewire the 3 prog cord.
I found it in a stangre way.
The green-yellow wire gone to a tap in the power transformer. This tap was conected with a poliester cap to another tap in this transformer.
I cut the cap and attached the ground wire to a main trasformer holder...
And now, when i move my volumen pot, BIAS voltage change also....

Whatīs wrong in my acts?




correction...
oh!! shit!! This grounded tap now is ungrounded!! Should i atach it to chasis?

[Bruce / Mission Amps]

A question for you Bruce:

After the second stage, you eliminated the shelving network of the .001 cap across the 700K resistor, yet left the .22uF/4.7K snubber. I personally would have went the other way (though I would have lowered the resistor value to a more realistic 220K for more gain and less drastic shelf). I wanted to find out why you chose this route.

It should also be noted that the remaining .22 and .047 caps would require a voltage rating of 400VDC. The schematic shows 220V on the preceding plate. Of course, none of us knows the voltage rating of those caps except the amp's owner, but it needs to be noted. Because the 700K resistor is in there, those caps might be underrated if it is eliminated.

My comments were only suggestions to make that channel sound better.
The natural -3dB point of that 700K and 1nF cap is around 230Hz...so I am assuming anything over 500Hz to 1000Hz will sound boosted by the time it hits that extra gain stage. That could be good or bad if this is really a bass amp.
Eliminating it will get more mids and lows into the amp so it might not be terrible useful to remove it but I've never seen a Fender clone with anything like that in it and I think that is where he is going.
Bypassing it will allow more signal to the next triode, which all ready has has it's gain cut by that weird 220nF, 47nF 1M sitting on the 4k7 filter.
But you are right, he could go either way.... leave the 1nf||700K filter and dump that shunting/snubbing 220nf,47nf/4k7 filter on the grid of the final 12AX7 triode and replace the grid resistor with a 470K to 1m (keeping all the extra gain of the final preamp before the phase inverter)... I can pretty much promise that doing it that way will have too much gain... or, get rid of the 1nF||700K high pass filter and keep the full bandwidth snubbing/shunting filter and use the little extra gain that will be there. I guess it depends on what the amp is intended to sound like.
That same extra stage in a Fender reverb amp for mixing reverb and dry signal (remember it has that 3m3 to 4m7 with the 10pF high pass cap) and the output is limited by the 50K load the vibrato intensity pot is imposing on it, his has no "after the fact" attenuation.

Why did I choose that method? Only because he stated liked the way the other channel sounded... and the other stage bypasses that weird filter with the extra gain stage and is isolated by the 220K resistors before the coupling cap to the PI.
The parallel 220nF cap and the 1M resistor gives the following triode a more correct DC grid bias point (1M + 4k7) while at the same time the .220nF cap shunts about 95% of "all frequencies" of signal audio going to the following grid, right to the grounded 4K7 resistor. That's quite an AC voltage divider.
I'm sure the idea was to make that stage have a little more of something after the high pass filter but not enough to sound like a big gain boost and preamp distortion in the PI.

I'll be willing to bet they are at least 400v caps but yes, be sure the caps actually are in the 400v to 630v range.

[jrfrond]

I'll be willing to bet they are at least 400v caps but yes, be sure the caps actually are in the 400v to 630v range.

One would HOPE so, but I can remember Peavey using low-voltage rated shaping caps after larger, higher-voltage coupling caps in the not-too-distant past in the Classic series. The loss and phase shift kept these amps from really opening up, so I would mod them by removing the coupling cap, but then the shaping cap's voltage needed to be raised so that it could also block DC. Somehow, I don't think Peavey had a corner on using underrated components wherever possible. I'd rather be safe than sorry, that's all.

Looking back at the Spanish amp design, that area we were discussing would be a perfect place for a master volume. It would solve the network problem and also make the amp a bit more versatile, since you could get it to breath a little fire with the gain way up.

[chocopower]

Looking back at the Spanish amp design, that area we were discussing would be a perfect place for a master volume. It would solve the network problem and also make the amp a bit more versatile, since you could get it to breath a little fire with the gain way up.

I think that could be a good way.

A .047uf cap (600 volts) and a 1M pot mounted as a voltage divider will do de work!

Isnīt?

[chocopower]

Hi there again!

After re-testing all the layout i found a strange mod (it try to bridge the channels...). I remove this mod and suddenly... 250volts in the pots!!

Whatīs wrong?... i finf it... The person who made the mod grounded all the main circuit board throw the shield of the wire used in the mod...

I add a new ground wire for the entery circuit and every thing seems be ok.

One more question. As said in another post, the sound is great but i need keep the bass pot all way down to cut the excesive bass.
I know 2 ways to fit this problem.

1. Add a coupling cap just before the Phase Inverse... (.1 or .047)
2. Change the .1uf cap in the tone stack for a .047uf

Of course, i can A/B one method or other... but what the hell!!
Whatīs your opinion?

[chocopower]

Hi again...

Finally i reply my self... I put a .022uf cap working as coupling cap. Problem solved..

After this i tried to tweek the NFB loop resistor. Original is a 82K with a paralleled 120pf.
I change them for a 820 ohm one, as in Blackface Fenders.
Suddenly the amp begin to make strange noises.....as an old radio!!
To much feedback? Maybe a 27K, (as in the 5F6A) work better?

[Bruce / Mission Amps]

Hi again...

Finally i reply my self... I put a .022uf cap working as coupling cap. Problem solved..

After this i tried to tweek the NFB loop resistor. Original is a 82K with a paralleled 120pf.
I change them for a 820 ohm one, as in Blackface Fenders.
Suddenly the amp begin to make strange noises.....as an old radio!!
To much feedback? Maybe a 27K, (as in the 5F6A) work better?

Jeeze.
You built it into a generator.
The NFB circuit is a voltage divider.
You can't just change the 82K resistor to what ever you want, especially not by using one that is 1/100th the size, without change the bottom one too!!
Look at it like this,
Your amp,
82K sitting on a 4k7 resistor...
82,000/4700 = 17.44:1
Fender,
820ohms/47ohms= 17.44:1
You don't for a second think that was a coincidence do you?
If you want less NFB change your 82K resistor to 100K.
Because your 82K resistor has a cap across for tuning... I'd mess with that too. Right now it looks like 120pF is fairly small so the NFB at above 15KHz would be higher, maybe for better high frequency stability?

[chocopower]

hi!

upssss.. Now i remenber this from an old topic.........

In my other amp (Pro-reverb clon) the NFB is 820/100ohm...

What about with the .1 cap in series with the 22K resistor to ground?
I supose it work as a presence fixed pot. isnīt?

And the 120pf paralled cap... i just removed it....