help grafting a piece of one amp onto another.

[mikepukmel]

Suppose I have a schematic that has, say an overdrive section. The overdrive section has two halves of a 12AX7 cascaded together.

Suppose I want to put a similar section in an existing amp, say a Deluxe AB763, between the preamp and PI. The overdrive section is in the same place in its original amp.

If I know the plate voltage in my amp, and the plate voltage in the amp with the overdrive section, can I just draw the load line for the source amp, using its voltage and cathode resistor combination, then draw the load line for my amp's plate voltage, figure out what cathode resistor I need to make the bias the same in my amp?

Am I on the right track, or should I give up and go clean the basement?

The idea I got was that the bias on the 2 parts of the overdrive section were its 'mojo'. in its existing amp, it has 3.3K and 2.2k bias resistors. Voltages comparable (mid 300's range) I haven't seen cathode resistors that large in any Fender designs. Are these what Ive heard called 'cold clipper'?

[nosaj]

Suppose I have a schematic that has, say an overdrive section. The overdrive section has two halves of a 12AX7 cascaded together.

Suppose I want to put a similar section in an existing amp, say a Deluxe AB763, between the preamp and PI. The overdrive section is in the same place in its original amp.

If I know the plate voltage in my amp, and the plate voltage in the amp with the overdrive section, can I just draw the load line for the source amp, using its voltage and cathode resistor combination, then draw the load line for my amp's plate voltage, figure out what cathode resistor I need to make the bias the same in my amp?

Am I on the right track, or should I give up and go clean the basement?

The idea I got was that the bias on the 2 parts of the overdrive section were its 'mojo'. in its existing amp, it has 3.3K and 2.2k bias resistors. Voltages comparable (mid 300's range) I haven't seen cathode resistors that large in any Fender designs. Are these what Ive heard called 'cold clipper'?

Not sure on a lot of what your asking about, but phase will come into effect depending on each of the circuits and what your add it to.
nosaj

[Chuck H]

I'm with Jason here. That is, a vein started bulging in my head as I tried to figure out just what you propose and what it means. It sort of reads like one half of two questions smooshed together.


Since the plate voltages are similar and you're stuck using standard resistor values anyway why not just use the values indicated in the circuit you wish to emulate? Very finite adjustments to preamp tube bias aren't usually necessary to get the basic effect of the circuit. Sort of like painting a room "Shell white" or "White Sail", if both colors are only different by, say, one drop of some pigment and look generally the same then the "feel" and decor in the room will not be lost by using one instead of the other.

I did have a case where a 6.8k carbon comp preamp cathode resistor drifted up to 7.5k in an amp that had especially nice mojo. Just for look see's I changed it to 6.8k. The amp sounded just as good.

EDIT: (hate to lose Leo's thumb with the edit, but...) The drifted resistor was in one of my own builds (I no longer use cc resistors). I have two other amps built to the same circuit in the hands of players here and abroad. To be fair I did order some 7.5k resistors on the off chance that it might have been a more critical value. So IMO you're not silly for thinking it at all. Just happens that, in this case anyway, it wasn't a critical value. Now, what to do with ten 7.5k resistors? Maybe I'll toss them at cars off an overpass

[g1]

I think what you are asking is how you can get it biased up in the same region of the curve if your supply voltage is different.
I'd suggest trying to change the supply voltage to match, rather than trying to alter the bias and use a different supply voltage.
If you like the sound of the original circuit, the supply voltage is probably an important component of that sound.

[Zozobra]

Can you draw a schematic or at least a conceptual diagram to clarify what it is you're trying to do? If you're just trying to get the supply voltage to each filter node about right then you can change the dropping resistors or maybe consider a parallel feed, rather than series; you see this a lot in rack preamps a la x88r or GP3.

Regarding cold clippers - Your real deal (tm) cold clipper is the 39k found in the SLO and it's many derivatives. This is hilariously asymmetrical at full clip, with one half reaching peak outputs of say 80V but the other half only getting to 3V! This is why the SLO doesn't have an attenuation network between the cold clipper and the next stage as otherwise it'd only overdrive one half of the waveform. I'm sure that it also smashing the last stage+CF into oblivion has nothing to do with it. I wouldn't say 3k3 (for a given voltage) is all that cold, cool maybe, bit not cold. 10k and up is cold and gets pretty asymmetrical. I'll pop a few examples together in LTSpice later.

[d95err]

It sounds like you’re describing the topology of a Dumble Overdrive Special. I would start from there and experiment. If you want to add a Dumble-style overdrive section to an AB768 style amp, I would out it before the reverb section.

[eschertron]

I agree with the other posters that are saying the operating point for the shoehorned stage(s) should be close enough for a 'comparable' supply voltage. Tweaking can come later. What will matter more to the sound is the signal getting to the new stage. Make sure it's comparable to the signal in the original design. Also, you will need to consider adding a PSU node. In this case, you can make the voltages in the preamp just about any that you want. Note that it's not a bad thing to have lower voltages in the first couple stages of a dirty amp.

[mikepukmel]

It sounds like you’re describing the topology of a Dumble Overdrive Special. I would start from there and experiment. If you want to add a Dumble-style overdrive section to an AB768 style amp, I would out it before the reverb section.

Yep, that's exactly what I wanted to do. (And, sorry, Chuck, another one of my rambling multi-questions). Added another node off the power supply. I have another node there already off the choke, added one more off that. Tried two dropping resistors, first 5k, fiddled with it a bit, then 10k. More controlled with current voltages with the 10k. That's not saying much, since its now so far from the AB763.

Yes, I have a schematic, finally (finally! Wahoo), getting somewhere with KiCad. I have all of the components and wires in the schematic, except the power transformer (they don't have an old tube transformar that I can find). I do not yet have all of the component values typed in, so its not quite ready to post. Hope to get that done tomorrow, will post.

Basically: the tube socket that would be Tremolo in the AB763, I replaced with a piece of the dumble overdrive special, the schematic 183.
grafted on to the Normal channel, right after the preamp, and before the 220k mixing resistors that lead to the PI.



*** Fast forward a few days:

Got the first trial grafted in, sounds pretty good, for what it is. Your suggestions were spot on. I tried two versions: one with a 10k dropping resistor and one with a 5k ( 2 x 10k's in parallel), The higher voltage 5k version, when the overdrive was turned up, created genuinely nasty sounding, raspy, ugly distortion. The kind that makes you want to hurl. with lower voltages, sounds pretty good.

Beefs up the Normal channel. I don't know the right words to use, brighter, but not in a bad way. More 'lively', sparkly? Oh sorry don't want to get started on the amp tone adjective thing again.

I started with the trim pot on the input side about 1/2. Will fiddle with that, drag the scope out and see what a sine wave looks like.

Also, would like to try a few higher supply voltages. Maybe 1/2 way between what it is now, and whatever I got with the 5k resistor. I didn't measure the voltages with th 5k dropping resistor, but currently,

2.8 and 2.0 cathodes.
173 and 238 plates.

For grounding, I did what overall scheme I got from the many great grounding threads: cathodes grounded first to the (-) lead coming back from the ps cap that feeds that tube, then ran to the chassis from there, point is between preamp and PI.

[mikepukmel]

Related question: I have a trim pot mounted on a little piece of fiberglass board I had left over from another project. Its not a turret or eyelet board. The only trim pots I could find in 100k were teeny things, with pins that bend really easily. Is there an off the shelf thing I can use to mount the trim pot. If this whole thing works out, Id like to make it more permanent, i.e. drill and install a few more turrets, clean up the wiring a little better and permanently mount the trim pot someplace.

[mikepukmel]

I agree with the other posters that are saying the operating point for the shoehorned stage(s) should be close enough for a 'comparable' supply voltage. Tweaking can come later. What will matter more to the sound is the signal getting to the new stage. Make sure it's comparable to the signal in the original design. Also, you will need to consider adding a PSU node. In this case, you can make the voltages in the preamp just about any that you want. Note that it's not a bad thing to have lower voltages in the first couple stages of a dirty amp.

Its funny, I thought it would generate distortion sound like a fuzz stomp box, but the way it is, its a little more gritty, tone is nice, but nothing like what Id call dirty. I think I like it. COuld not crank the amp the few nights I had to work on it it was too late, will try tomorrow or Friday when the house is empty.

[mikepukmel]

It sounds like you’re describing the topology of a Dumble Overdrive Special. I would start from there and experiment. If you want to add a Dumble-style overdrive section to an AB768 style amp, I would out it before the reverb section.

Its actually on the normal channel, since as is, it doesn't sound very good, I never use it.

[mikepukmel]

Can you draw a schematic or at least a conceptual diagram to clarify what it is you're trying to do? If you're just trying to get the supply voltage to each filter node about right then you can change the dropping resistors or maybe consider a parallel feed, rather than series; you see this a lot in rack preamps a la x88r or GP3.

Regarding cold clippers - Your real deal (tm) cold clipper is the 39k found in the SLO and it's many derivatives. This is hilariously asymmetrical at full clip, with one half reaching peak outputs of say 80V but the other half only getting to 3V! This is why the SLO doesn't have an attenuation network between the cold clipper and the next stage as otherwise it'd only overdrive one half of the waveform. I'm sure that it also smashing the last stage+CF into oblivion has nothing to do with it. I wouldn't say 3k3 (for a given voltage) is all that cold, cool maybe, bit not cold. 10k and up is cold and gets pretty asymmetrical. I'll pop a few examples together in LTSpice later.

80v up and 3v down! Wow, I never thought anything was that asymmetrical. Ive seen some posts here and there on this blog, that just show a little dimple on one side, and clean on the other. Thanks, would be great to see teh spice sim's.

[mikepukmel]

I'm with Jason here. That is, a vein started bulging in my head as I tried to figure out just what you propose and what it means. It sort of reads like one half of two questions smooshed together.


Since the plate voltages are similar and you're stuck using standard resistor values anyway why not just use the values indicated in the circuit you wish to emulate? Very finite adjustments to preamp tube bias aren't usually necessary to get the basic effect of the circuit. Sort of like painting a room "Shell white" or "White Sail", if both colors are only different by, say, one drop of some pigment and look generally the same then the "feel" and decor in the room will not be lost by using one instead of the other.

I did have a case where a 6.8k carbon comp preamp cathode resistor drifted up to 7.5k in an amp that had especially nice mojo. Just for look see's I changed it to 6.8k. The amp sounded just as good.

EDIT: (hate to lose Leo's thumb with the edit, but...) The drifted resistor was in one of my own builds (I no longer use cc resistors). I have two other amps built to the same circuit in the hands of players here and abroad. To be fair I did order some 7.5k resistors on the off chance that it might have been a more critical value. So IMO you're not silly for thinking it at all. Just happens that, in this case anyway, it wasn't a critical value. Now, what to do with ten 7.5k resistors? Maybe I'll toss them at cars off an overpass

The topology is so different in the original amp: it has a SS rectifier and much higher voltages overall, and a standard "chained" power supply, dropping resistor, and cap, followed by dropping resistor and cap etc. What I have is fairly bastardized, more or less to avoid rewiring the whole thing.

Oh man, don't toss those 7.5k resistors, are they 1 or 2 watt?

[nosaj]

Its funny, I thought it would generate distortion sound like a fuzz stomp box, but the way it is, its a little more gritty, tone is nice, but nothing like what Id call dirty. I think I like it. COuld not crank the amp the few nights I had to work on it it was too late, will try tomorrow or Friday when the house is empty.

Well if you crank it wouldn't that empty the house?
nosaj

[mikepukmel]

Well if you crank it wouldn't that empty the house?
nosaj

Well, sometimes. But most often, the people cleared would be me.

Got the schematic mostly typed in. It might be missing a couple of grid stops. And I haven't yet figured out how to put a signal label e.g. "A" that refers to some other point labelled "A". Seeing as its Thanksgiving, and about 14 degrees outside, Im not betting that I can get the house to get some more smoke test playing in today.

Would like to fiddle with the plate voltage on the OD tube.