Is there a way to increase the gain of the Normal channel, without running it through that one half of V2? I tried lowering the value of the 220k resistor between V3 and the 500pf cap of V4, but I do not really notice a difference. Is there something else I can try?
Thank You
http://www.el34world.com/charts/Schemat ... _AA864.pdf
Increasing Gain
Moderators: pompeiisneaks, Colossal
Re: Increasing Gain
For that amp, you could change one or both plate resistors on the NORM channel tubes to 150K for a gain boost. But I would really suggest just using and overdrive pedal set for a clean boost and leaving the amp stock.hired hand wrote:Is there a way to increase the gain of the Normal channel, without running it through that one half of V2? I tried lowering the value of the 220k resistor between V3 and the 500pf cap of V4, but I do not really notice a difference. Is there something else I can try?
Thank You
http://www.el34world.com/charts/Schemat ... _AA864.pdf
If that amp is not a PC board (hate 'em), you could do a lot more modifying the BASS channel..you have the circuit topology there to turn that channel into a screamin'machine.
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Gibsonman63
- Posts: 1033
- Joined: Wed May 27, 2009 1:59 pm
- Location: Texas
Re: Increasing Gain
Running a 12AX7 in the phase inverter will make it hotter because it is a higher gain tube, but you may have to tweak the resistors in the phase inverter to dial it in.
Re: Increasing Gain
HH:
Changing the value of that 220K mix resistor will have hardly noticeable effect on the gain. However, if you want to increase the gain of the Normal channel, you can do some of the following:
1)As PC pointed out, the easiest route is to up the value of the plate resistors from 100K to as high as 220K. Expect the channel to be noisier and brighter, but it will have more gain.
2)Break the connection between the 6.8K resistor and ground on the bass pot and wire a .0047uF cap in series between the resistor and ground (you can solder the cap to the pot back and twist the lead of the cap to the formerly grounded lead of the resistor and leave both on the pot). You can use the bright switch to switch the gain in and out by removing the bright cap and soldering a wire from each of the switch terminals to either end of the .0047uF cap. When you switch the cap in, you will get quite a boost, though your tone control effectiveness will suffer a bit. This is an old Mesa Boogie/ Rivera-era Fender trick.
On the Normal channel of my Bassman, I have the unused triode wired in parallel with the first input stage. That way I have a thicker sound which is variable with the volume pot.
Hope this helps. BTF.
Changing the value of that 220K mix resistor will have hardly noticeable effect on the gain. However, if you want to increase the gain of the Normal channel, you can do some of the following:
1)As PC pointed out, the easiest route is to up the value of the plate resistors from 100K to as high as 220K. Expect the channel to be noisier and brighter, but it will have more gain.
2)Break the connection between the 6.8K resistor and ground on the bass pot and wire a .0047uF cap in series between the resistor and ground (you can solder the cap to the pot back and twist the lead of the cap to the formerly grounded lead of the resistor and leave both on the pot). You can use the bright switch to switch the gain in and out by removing the bright cap and soldering a wire from each of the switch terminals to either end of the .0047uF cap. When you switch the cap in, you will get quite a boost, though your tone control effectiveness will suffer a bit. This is an old Mesa Boogie/ Rivera-era Fender trick.
On the Normal channel of my Bassman, I have the unused triode wired in parallel with the first input stage. That way I have a thicker sound which is variable with the volume pot.
Hope this helps. BTF.
Re: Increasing Gain
Thanks for the ideas/info. I think I will try just flip-flopping the way the channels are set up. I will run the bass channel from V1 to V4, and run the normal channel from V3 to V2 and then V4. Since most guys do not like the bass channel anyway, why should it get the extra gain?
Thanks Again
Thanks Again
Re: Increasing Gain
Another way to increase gain on this channel is to add in the Cesar Diaz Vibroverb RI switchable gain on both cathodes of the normal channel pre-amp tube V3a and V3b.
You need a DPDT switch and mount this on the back panel, then just copy the Vibroverb circuitry. The Diaz mod also switches out the tremelo and the other channel to further increase gain, but this is not essential.
If you dont want the switchable option, you could just hard wire the Diaz mod circuit to the cathodes of V3a and V3b, but the channel will be permanently in the higher gain configuration. Its 2.7k/22uf on V3a, and 2.7k/0.68uf on V3b.
I add a third position with a 'super gain' cathode circuit (an On-Off-On switch), so that I have three options: Diaz gain, stock, super gain.
I have added this mod to a few BF RI amps with great results. This of course is not a good thing to do if you have a vintage AA864, but if you are after the tone....??
You need a DPDT switch and mount this on the back panel, then just copy the Vibroverb circuitry. The Diaz mod also switches out the tremelo and the other channel to further increase gain, but this is not essential.
If you dont want the switchable option, you could just hard wire the Diaz mod circuit to the cathodes of V3a and V3b, but the channel will be permanently in the higher gain configuration. Its 2.7k/22uf on V3a, and 2.7k/0.68uf on V3b.
I add a third position with a 'super gain' cathode circuit (an On-Off-On switch), so that I have three options: Diaz gain, stock, super gain.
I have added this mod to a few BF RI amps with great results. This of course is not a good thing to do if you have a vintage AA864, but if you are after the tone....??
Its all about the tone!
Re: Increasing Gain
Another simple way is to put a resistor or pot to ground at the low side of the tone stack. You not only get a boost, but the signal is nicely brighter since the tone controls are further removed from the circuit.
I've found 75k works well.
ampdoc1
I've found 75k works well.
ampdoc1
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David Root
- Posts: 3540
- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
Rule of 66
The "golden rule" when changing stage gain in a 12AX7 ONLY is:
100K plate 1k5 cathode
120K plate 1K8 cathode
150K plate 2K2 cathode
180K plate 2K7 cathode
220K plate 3K3 cathode
These values keep the quiescent operating point pretty much in the middle of the voltage swing range, and always sound good.
The calculation is Plate R/66=Cathode R.
Having said that, you can set the quiescent point anywhere you like, eg 100K plate 2K7 cathode, but this is more of a modern setting with more odd harmonic distortion, other things being equal.
In other tubes, eg a 6SL7, these numbers do not hold, but I think if you take for example mu 6SL7=44 vs mu 12AX7=62.5 then about 0.45 multiplier on the 12AX7 numbers seems to work, eg 47K plate, 1K cathode and so on, keeps you in the middle of the voltage swing.
I have found TubeCad very useful for this, much faster than drawing it out on plate curves that's for sure!
100K plate 1k5 cathode
120K plate 1K8 cathode
150K plate 2K2 cathode
180K plate 2K7 cathode
220K plate 3K3 cathode
These values keep the quiescent operating point pretty much in the middle of the voltage swing range, and always sound good.
The calculation is Plate R/66=Cathode R.
Having said that, you can set the quiescent point anywhere you like, eg 100K plate 2K7 cathode, but this is more of a modern setting with more odd harmonic distortion, other things being equal.
In other tubes, eg a 6SL7, these numbers do not hold, but I think if you take for example mu 6SL7=44 vs mu 12AX7=62.5 then about 0.45 multiplier on the 12AX7 numbers seems to work, eg 47K plate, 1K cathode and so on, keeps you in the middle of the voltage swing.
I have found TubeCad very useful for this, much faster than drawing it out on plate curves that's for sure!
Re: Increasing Gain
Thanks for all the additional info. I am just going to make this a stock AB165 with adjustable bias and no funky caps anywhere, and then sell it. I have another Bassman head that is in real bad shape cosmetically. The chassis is good and it has new OEM trannies. I think that is the amp I will try to utilize some of the ideas you guys have given me. The amp I am working on now just needs to be made a Fender again and get in the hands of a gigging musician that needs a good,reliable amp.
David Root -
I checked out Tube CAD. That guy has a HUGE web sight. Which of his software did you buy?
Thank You
David Root -
I checked out Tube CAD. That guy has a HUGE web sight. Which of his software did you buy?
Thank You
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JamesHealey
- Posts: 477
- Joined: Thu Aug 28, 2008 5:34 pm
- Location: Leeds, UK
Re: Rule of 66
Interesting.. one of my amp's I've made hasDavid Root wrote:The "golden rule" when changing stage gain in a 12AX7 ONLY is:
100K plate 1k5 cathode
120K plate 1K8 cathode
150K plate 2K2 cathode
180K plate 2K7 cathode
220K plate 3K3 cathode
These values keep the quiescent operating point pretty much in the middle of the voltage swing range, and always sound good.
The calculation is Plate R/66=Cathode R.
Having said that, you can set the quiescent point anywhere you like, eg 100K plate 2K7 cathode, but this is more of a modern setting with more odd harmonic distortion, other things being equal.
In other tubes, eg a 6SL7, these numbers do not hold, but I think if you take for example mu 6SL7=44 vs mu 12AX7=62.5 then about 0.45 multiplier on the 12AX7 numbers seems to work, eg 47K plate, 1K cathode and so on, keeps you in the middle of the voltage swing.
I have found TubeCad very useful for this, much faster than drawing it out on plate curves that's for sure!
Stage 1 220k 2k2 0.68uF
Stage 2 100k 3k3 22uF
Can you tell from looking at this what kind of harmonics are being made etc? I just dialled in these resistor values by using my ears.
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David Root
- Posts: 3540
- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
Re: Increasing Gain
Difficult without more info. I ran tubecad assuming 300V B+ on your two stages and stage 1 is biasing at -1.37V so the quiescent point is closer to the zero bias line. So you would clip that side of the signal with a large input voltage.
Stage 2 is biased to the right of center point at -2.1V, so you would then be clipping the other side of the signal.
This is conventional practice in high gain preamp design, only usually these two stages are sandwiched between a clean first stage, which could be any of the "66 rule" combinations I showed, and a fourth stage that could be clean or totally funky for OD tone, like the Solano 100K/39K/no cap trick, for example.
However, to answer your question, you should get more odd order distortion than even order with both your stages, because they are at off-center of swing. More on your stage 2 because your operating point is further off center.
I cannot remember the formula for calculation off hand, it is based on how far off center your point is voltage wise, but I'm sure someone here does. I'd have to go to RDH4 to dig it out.
Stage 2 is biased to the right of center point at -2.1V, so you would then be clipping the other side of the signal.
This is conventional practice in high gain preamp design, only usually these two stages are sandwiched between a clean first stage, which could be any of the "66 rule" combinations I showed, and a fourth stage that could be clean or totally funky for OD tone, like the Solano 100K/39K/no cap trick, for example.
However, to answer your question, you should get more odd order distortion than even order with both your stages, because they are at off-center of swing. More on your stage 2 because your operating point is further off center.
I cannot remember the formula for calculation off hand, it is based on how far off center your point is voltage wise, but I'm sure someone here does. I'd have to go to RDH4 to dig it out.
Re: Rule of 66
Jim Marshall and Ken Brand likely did so, as well. Here a tweak, there a tweak, everywhere a tweak-tweak....JamesHealey wrote:
Interesting.. one of my amp's I've made has
Stage 1 220k 2k2 0.68uF
Stage 2 100k 3k3 22uF
Can you tell from looking at this what kind of harmonics are being made etc? I just dialled in these resistor values by using my ears.
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David Root
- Posts: 3540
- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
TubeCad
Hiredhand, I bought TubeCad and SE CAD, but Tubecad is all you need. He has another one that is push pull, but because he's a dyed in the wool HifI guy it's all set up for pentodes wired as triodes, which ain't much use to me.
TubeCad is $40.00 and worth every penny. I use it all the time. The only negative in it is the long tailed pair is not what we use, but variation C is a bit closer. (You get thirteen different stages each with four variations).
If you don't want to shell out the $40, Martin Manning has a piece of software he has put on this forum which is also very good.
TubeCad is $40.00 and worth every penny. I use it all the time. The only negative in it is the long tailed pair is not what we use, but variation C is a bit closer. (You get thirteen different stages each with four variations).
If you don't want to shell out the $40, Martin Manning has a piece of software he has put on this forum which is also very good.
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: Increasing Gain
Take look at the resistance coupled amp tables in the back of a RCA manual.
There are three ways to get more gain; more voltage, larger plate resistor,
or a larger grid resistor for the following stage.
by increasing the resistances to get more gain the impedance goes up too
so.... the value of the coupling cap can be smaller....
Your going to be stuck matching the impedance of the gain stage to the tone stack.
move the tones or choose a different tone control
There are three ways to get more gain; more voltage, larger plate resistor,
or a larger grid resistor for the following stage.
by increasing the resistances to get more gain the impedance goes up too
so.... the value of the coupling cap can be smaller....
Your going to be stuck matching the impedance of the gain stage to the tone stack.
move the tones or choose a different tone control
lazymaryamps
Re: Increasing Gain
Andy, et al....
Thanks for all the info, I appreciate it.
Thanks for all the info, I appreciate it.