Greetings Lads:
Some time ago I wired a Hybrid A mirror image without thinking. I wanted to build it tubes up and all the layouts at the time were fender style. New Avatar to come.
Behold the following pictures of the Amp I just finished for my son. Anyway, it's a Matchless Spitfire clone built in a Hammond AO-35 with original iron. The scheme is basically unmolested with the exception of a 250K two gang Audio pot substituted for the grid load resistors on the finals, to affect a master volume. We will call this the Stoo mod. He suggested it. You will notice in the photo's it's wired backwards as is the tone control. Why can't I keep that straight? Anyway, some may also question the wisdom of locating the PI to output stage coupling caps so far away from the action. I worried that it would result in a towering inferno of instability. Nope. Quiet as a mouse. In fact, when I turned it on after the customary voltage checks and smoke test with the variac I thought I had it miswired because it didn't make any noise. Hmmmm. I'll move the caps over by the PI just for looks I think. They don't belong way out there.
The sound (with a used set of Amperex EL-84s and used 12ax7s) is very very good. I'm driving an Altec 417C so I'll finish my Celestion G12-65 cabinet before getting too excited about mods. The MV works good, though I think a linear pot would be better. If anything I think the amp is less "gainy" than I had anticipated. I may change the 1K2 cathode resistor in the PI to 820 or even 640 to see if I can get more edge. With my Heritage H-535 it of course feeds back those great harmonic sustains effortlessly. Wish I had a strat. Clips in a few days. Head cabinet first to match the speaker cab.
[IMG:800:600]http://i14.tinypic.com/6fzh8ix.jpg[/img]
[IMG:800:600]http://i17.tinypic.com/4tk5j02.jpg[/img]
[IMG:800:600]http://i12.tinypic.com/5zbxa85.jpg[/img]
[IMG:640:480]http://i9.tinypic.com/68c1duv.jpg[/img]
[IMG:800:593]http://i9.tinypic.com/4z2lx1y.jpg[/img]
[IMG:887:615]http://i16.tinypic.com/6blfax3.jpg[/img]
Spitfire build and Dan comes out of the Dunce cap.
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skyboltone
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Spitfire build and Dan comes out of the Dunce cap.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
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Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: Spitfire build and Dan comes out of the Dunce cap.
Congrats on the build, but I gotta say I'm going to miss the avatar though 
Re: Spitfire build and Dan comes out of the Dunce cap.
Mirror images can be a pain if all of your documentation is in it's normal layout. I did a 6v6 plexi in reverse just because it conformed to a steel chassis I already had. You always have that extra step to flip everything over in your mind. Makes your brain hurtskyboltone wrote:Greetings Lads:
Some time ago I wired a Hybrid A mirror image without thinking. I wanted to build it tubes up and all the layouts at the time were fender style.
Your son is a lucky guyskyboltone wrote:Behold the following pictures of the Amp I just finished for my son. Anyway, it's a Matchless Spitfire clone built in a Hammond AO-35 with original iron. The scheme is basically unmolested with the exception of a 250K two gang Audio pot substituted for the grid load resistors on the finals, to affect a master volume. We will call this the Stoo mod. He suggested it. You will notice in the photo's it's wired backwards as is the tone control. Why can't I keep that straight? Anyway, some may also question the wisdom of locating the PI to output stage coupling caps so far away from the action. I worried that it would result in a towering inferno of instability. Nope. Quiet as a mouse. In fact, when I turned it on after the customary voltage checks and smoke test with the variac I thought I had it miswired because it didn't make any noise. Hmmmm. I'll move the caps over by the PI just for looks I think. They don't belong way out there.
..........If it works , don't fix itKeep us posted on the tweeking, can't wait to hear the clips and see the new cab. Congrats!!skyboltone wrote:The sound (with a used set of Amperex EL-84s and used 12ax7s) is very very good. I'm driving an Altec 417C so I'll finish my Celestion G12-65 cabinet before getting too excited about mods. The MV works good, though I think a linear pot would be better. If anything I think the amp is less "gainy" than I had anticipated. I may change the 1K2 cathode resistor in the PI to 820 or even 640 to see if I can get more edge. With my Heritage H-535 it of course feeds back those great harmonic sustains effortlessly. Wish I had a strat. Clips in a few days. Head cabinet first to match the speaker cab.
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RJ Guitars
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Re: Spitfire build and Dan comes out of the Dunce cap.
Nice work... would love to hear those clips when you get them done.... Your son should be ecstatic!
rj
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Re: Spitfire build and Dan comes out of the Dunce cap.
> I'm driving an Altec 417C
Is that no good for guitar?
An Altec 12" speaker has followed me for decades. Something like 417 but I always get Altec numbers mixed up. I figured I could get rid of it by hiding it in an overgrown Champ. It's true vintage and well broken-in. I do fear it could be too mellow, so I let the Champ run a bit raw (trace of NFB). But if it isn't going to sing I better go shopping.
> wired backwards as is the tone control. Why can't I keep that straight?
Bust-open a pot. Look how it works, where the moving wiper touches the end-lugs. Hold that vision. Stand on the player-side of the panel, turn the knob to "10", imagine which end-lug the wiper is touching. Keep your finger on it. Now go around to the back of the panel and put your "more" wire on that lug.
This works good for volume controls, where it is usually pretty clear where "more" is. The signal from the previous stage. "Less" is ground, which is "0" on most volume pots.
Most passive treble knobs are "volume controls" which don't touch bass, so they work nearly the same. "Less" may be a tap-down on the path the main signal flows through, but it is usually clear that it is more-groundy than the small cap bleeding highs onto the treble pot.
Fender Bass controls always make me cross-eye. They are really 2-terminal rheostats, not 3-terminal pots. We use 3-lug pots because that's all we can buy. We can ignore the 3rd lug or strap it.... in some circuits, strapping will limit glitches when (not if) the wiper loses contact, but in a Fender Bass knob the glitch is hardly worth fretting over.
Recall that, in typical chassis layout, it makes a difference if pots are mounted lugs-up or lugs-down relative to the guy with the soldering iron. What is "left lug" one way becomes "right lug" the other way. Rotate, not flip.
Put 1Meg or 2Meg grid to ground on your finals. As is, when (not if) the wiper loses contact, the grid voltage is undefined. Leakage may cause it to drift negative (cool, OK) or positive (meltdown!). While the spec says 500K for 6BQ5 self-bias, that's a 2% worst-case spec, 98% of production 6BQ5/EL84s will be perfectly stable with a couple megs on the grid. And you probably won't leave it no-contact for long: the power and tone will be wrong, and in early stages of wiper failure a good knob-tap will "fix" it.
The 0.0022uFd into the 1Meg volume pot implies about 85Hz bass rolloff. That would be wise for a wee amp with a six-inch speaker. It seems bass-shy for a large speaker until you have tried it and found it boomy.
Your 0.01 Tone knob cap is bigger than the 0.0022 coupling cap before it. If not for the volume pot between (or if Volume must be worked at "10") then it implies a treble-droop lower than your bass cutoff. If Tone is turned to "0", treble rolloff is 230Hz to 59Hz(!) depending on volume control setting. It isn't a treble control, it is everything except your bottom octave more-or-less. If you want "more gain" and "less bright", I'd be aiming at 1KHz, or around 8 times higher, or another zero on that cap: 0.001uFd. If you want less interaction with the Volume setting, put it before the Vol pot where impedance is near 70K all the time, think 250K-A pot and 0.002-0.003uFd.
In all my years, I've never really liked this type treble control. But anything better needs more gain to throw-away. (Hmmmm. with that 2-input longtail, there's a trick.... one I may never have seen used in this form.....)
I do think that two gain controls in a low-gain amp is too many: one or the other will normally have to be full-up for decent responsiveness.
> PI to output stage coupling caps so far away from the action
Because if you put them "where they belong", they are practically rubbing the input jacks. You have gain over 1,000 from jack to cap, and your choice of polarity.... it would beg for feedback, and Murphy's Law ensures a positive feedback condition will happen. (Unless you wanted an oscillator... then Murphy would ensure that negative feedback swamped the desired PFB.)
I think that whole chain is cozy for a line-level input (the original plan) and risky for higher gain like a guitar amp needs. That it hasn't given trouble is testament to good detailing (or incredible luck). Don't change a thing.
Is that no good for guitar?
An Altec 12" speaker has followed me for decades. Something like 417 but I always get Altec numbers mixed up. I figured I could get rid of it by hiding it in an overgrown Champ. It's true vintage and well broken-in. I do fear it could be too mellow, so I let the Champ run a bit raw (trace of NFB). But if it isn't going to sing I better go shopping.
> wired backwards as is the tone control. Why can't I keep that straight?
Bust-open a pot. Look how it works, where the moving wiper touches the end-lugs. Hold that vision. Stand on the player-side of the panel, turn the knob to "10", imagine which end-lug the wiper is touching. Keep your finger on it. Now go around to the back of the panel and put your "more" wire on that lug.
This works good for volume controls, where it is usually pretty clear where "more" is. The signal from the previous stage. "Less" is ground, which is "0" on most volume pots.
Most passive treble knobs are "volume controls" which don't touch bass, so they work nearly the same. "Less" may be a tap-down on the path the main signal flows through, but it is usually clear that it is more-groundy than the small cap bleeding highs onto the treble pot.
Fender Bass controls always make me cross-eye. They are really 2-terminal rheostats, not 3-terminal pots. We use 3-lug pots because that's all we can buy. We can ignore the 3rd lug or strap it.... in some circuits, strapping will limit glitches when (not if) the wiper loses contact, but in a Fender Bass knob the glitch is hardly worth fretting over.
Recall that, in typical chassis layout, it makes a difference if pots are mounted lugs-up or lugs-down relative to the guy with the soldering iron. What is "left lug" one way becomes "right lug" the other way. Rotate, not flip.
Put 1Meg or 2Meg grid to ground on your finals. As is, when (not if) the wiper loses contact, the grid voltage is undefined. Leakage may cause it to drift negative (cool, OK) or positive (meltdown!). While the spec says 500K for 6BQ5 self-bias, that's a 2% worst-case spec, 98% of production 6BQ5/EL84s will be perfectly stable with a couple megs on the grid. And you probably won't leave it no-contact for long: the power and tone will be wrong, and in early stages of wiper failure a good knob-tap will "fix" it.
The 0.0022uFd into the 1Meg volume pot implies about 85Hz bass rolloff. That would be wise for a wee amp with a six-inch speaker. It seems bass-shy for a large speaker until you have tried it and found it boomy.
Your 0.01 Tone knob cap is bigger than the 0.0022 coupling cap before it. If not for the volume pot between (or if Volume must be worked at "10") then it implies a treble-droop lower than your bass cutoff. If Tone is turned to "0", treble rolloff is 230Hz to 59Hz(!) depending on volume control setting. It isn't a treble control, it is everything except your bottom octave more-or-less. If you want "more gain" and "less bright", I'd be aiming at 1KHz, or around 8 times higher, or another zero on that cap: 0.001uFd. If you want less interaction with the Volume setting, put it before the Vol pot where impedance is near 70K all the time, think 250K-A pot and 0.002-0.003uFd.
In all my years, I've never really liked this type treble control. But anything better needs more gain to throw-away. (Hmmmm. with that 2-input longtail, there's a trick.... one I may never have seen used in this form.....)
I do think that two gain controls in a low-gain amp is too many: one or the other will normally have to be full-up for decent responsiveness.
> PI to output stage coupling caps so far away from the action
Because if you put them "where they belong", they are practically rubbing the input jacks. You have gain over 1,000 from jack to cap, and your choice of polarity.... it would beg for feedback, and Murphy's Law ensures a positive feedback condition will happen. (Unless you wanted an oscillator... then Murphy would ensure that negative feedback swamped the desired PFB.)
I think that whole chain is cozy for a line-level input (the original plan) and risky for higher gain like a guitar amp needs. That it hasn't given trouble is testament to good detailing (or incredible luck). Don't change a thing.
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: Spitfire build and Dan comes out of the Dunce cap.
PRR wrote:> I'm driving an Altec 417C
Is that no good for guitar?
An Altec 12" speaker has followed me for decades. Something like 417 but I always get Altec numbers mixed up. I figured I could get rid of it by hiding it in an overgrown Champ. It's true vintage and well broken-in. I do fear it could be too mellow, so I let the Champ run a bit raw (trace of NFB). But if it isn't going to sing I better go shopping.
When I was a wee lad, Mr H Alexander Dumble let me hang around his shop and bug him half crazy. I ran errands like to Quement electronics in San Jose for parts. He ultimately built me an amp out of a fender Rhodes piano chassis he had laying around. This would have been probably about 1970. The Rhodes had (IIRC) 4 6L6s and a 2 ohm ouput. Alexander (Howard at the time) used a pair of 6L6s and I could only afford one speaker. Anyway, I built the head cab, face plate, and speaker box and he just about "gave" me the chassis but insisted that the only speaker worth owning was an Altec 417C. He had a stack of 8 ohm there in the shop. He said the impedance mismatch would make it a bit bright. It did.
Long story even longer. If you look in the files section here, you'll find Altec 417Cs in any amp he built from the early 70's.
Around here, they get no respect. Some say they are awful. Here's my take on it. They don't break up. If the distortion isn't in the amp you ain't gonna get any. If something sounds shitty in the amp, the speaker won't mask it. This in my opinion is exactly why Alexander used them. He wanted to hear what his amp was doing, not what the speaker was doing. I've got an 8 ohm stock "C" with the aluminum dust cover (currently in that cabinet) and a 16 ohm 417 no suffix with a paper dust cover. I've never had it hooked up to anything. I suspect it'll be warmer sounding but still not likely to color the amps output.
Since this amp is such a low gain critter, I'm gonna see what it'll do with a speaker that has break up designed in. Anyway, I'll noodle through your suggestions on the tone of this amp and see where that leads.
Thanks again
Dan H
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: Spitfire build and Dan comes out of the Dunce cap.
>Now go around to the back of the panel and put your "more" wire on that lug.
This works good for volume controls, where it is usually pretty clear where "more" is. The signal from the previous stage. "Less" is ground, which is "0" on most volume pots.
Most passive treble knobs are "volume controls" which don't touch bass, so they work nearly the same. "Less" may be a tap-down on the path the main signal flows through, but it is usually clear that it is more-groundy than the small cap bleeding highs onto the treble pot. <
It was this business of the "more" and "less" that I wasn't getting. This makes perfect sense.
>Put 1Meg or 2Meg grid to ground on your finals.<
Done with one megs that I had on hand.
>The 0.0022uFd into the 1Meg volume pot implies about 85Hz bass rolloff. That would be wise for a wee amp with a six-inch speaker. It seems bass-shy for a large speaker until you have tried it and found it boomy. <
Changed to .005 today. Wow! Kinda like a really different animal
>Your 0.01 Tone knob cap is bigger than the 0.0022 coupling cap before it. If not for the volume pot between (or if Volume must be worked at "10") then it implies a treble-droop lower than your bass cutoff. If Tone is turned to "0", treble rolloff is 230Hz to 59Hz(!) depending on volume control setting. It isn't a treble control, it is everything except your bottom octave more-or-less. If you want "more gain" and "less bright", I'd be aiming at 1KHz, or around 8 times higher, or another zero on that cap: 0.001uFd.<
Didn't get to this one yet, will tomorrow.
>If you want less interaction with the Volume setting, put it before the Vol pot where impedance is near 70K all the time,<
Did this one too. Really kind of changed the character of the control from a treble boost to a bass boost. Sounds best dimed unless running the Gain control dimed and backed off on the Master. Then it adds definition to chords. Question: I just swung the wiper wire over. This effectively strands the 180pf cap in the coupling string to the PI. Does it have a better home?
> think 250K-A pot and 0.002-0.003uFd. <
A little at a time. Down the road yet.
>I do think that two gain controls in a low-gain amp is too many: one or the other will normally have to be full-up for decent responsiveness. <
This works surprisingly well if you want to add grind at lower volumes. Still gonna dime the gain control though. I'm trying various combos and haven't decided much yet.
> PI to output stage coupling caps so far away from the action
I moved them off the rectifier socket anyway. I have picked up a little hum but not from that. It goes away when I turn off my bench lite. hehe. More changes though.
This PT will not put out 348 with a tube rectifier. More like 297. I pulled all the tubes and hooked up an estimated load of 90ma with a resistor bank and that's what you get. So, as a result, instead of 250 on the PI plates I had about 221 and 200. I changed the plate load resistors to 100K and 82K and the PS series resistor from 22K to 18K and now have 240 and 225. Closer. I also changed the PI cathode resistor from 120 ohm to 134 ohm. Cathode voltage now about 52 volts.
Instead of 150 volts on the parallel input plates I had 123. I changed the PS series resistor from 22K to 4.2K and dropped the plate load resistor from 220K to 193.2K (I got about 20 of those. Big precision metal films I picked up at a swap meet someplace) I now have 150 on the nose on the input plates. I didn't measure the cathode voltage. I'll do that tomorrow.
So what did all this do? It's now a more gainy pushed shape tone if you want it. I think you could change the MV control to a couple of different resistor values and control them with a couple of relays and a foot switch. Anyway if MV is dimed it's just a sweet little Miss Clean Thing. This all made the Post PI Master more effective. I need to play it more to really decide what it''s worth. The "gain" control now doesn't change the treble/bass ratio. The amp is fuller and bigger sounding. I was tube rolling and put a microphonic 12AX7 in the first hole and of course the amp took off, I wonder if it's close to being pushed as hard as it can up front? I wanna take a closer look at the input Rk to see what that ratio is. Anyway this is turning out to be a nice little 15 watter. I'm gonna make me one too I think but maybe with a real chassis, some tone controls and a little recovery triode with no bypass cap. Degeneration. Gotta smarten up on tone controls though.
Thanks PRR
Dan H[/b]
This works good for volume controls, where it is usually pretty clear where "more" is. The signal from the previous stage. "Less" is ground, which is "0" on most volume pots.
Most passive treble knobs are "volume controls" which don't touch bass, so they work nearly the same. "Less" may be a tap-down on the path the main signal flows through, but it is usually clear that it is more-groundy than the small cap bleeding highs onto the treble pot. <
It was this business of the "more" and "less" that I wasn't getting. This makes perfect sense.
>Put 1Meg or 2Meg grid to ground on your finals.<
Done with one megs that I had on hand.
>The 0.0022uFd into the 1Meg volume pot implies about 85Hz bass rolloff. That would be wise for a wee amp with a six-inch speaker. It seems bass-shy for a large speaker until you have tried it and found it boomy. <
Changed to .005 today. Wow! Kinda like a really different animal
>Your 0.01 Tone knob cap is bigger than the 0.0022 coupling cap before it. If not for the volume pot between (or if Volume must be worked at "10") then it implies a treble-droop lower than your bass cutoff. If Tone is turned to "0", treble rolloff is 230Hz to 59Hz(!) depending on volume control setting. It isn't a treble control, it is everything except your bottom octave more-or-less. If you want "more gain" and "less bright", I'd be aiming at 1KHz, or around 8 times higher, or another zero on that cap: 0.001uFd.<
Didn't get to this one yet, will tomorrow.
>If you want less interaction with the Volume setting, put it before the Vol pot where impedance is near 70K all the time,<
Did this one too. Really kind of changed the character of the control from a treble boost to a bass boost. Sounds best dimed unless running the Gain control dimed and backed off on the Master. Then it adds definition to chords. Question: I just swung the wiper wire over. This effectively strands the 180pf cap in the coupling string to the PI. Does it have a better home?
> think 250K-A pot and 0.002-0.003uFd. <
A little at a time. Down the road yet.
>I do think that two gain controls in a low-gain amp is too many: one or the other will normally have to be full-up for decent responsiveness. <
This works surprisingly well if you want to add grind at lower volumes. Still gonna dime the gain control though. I'm trying various combos and haven't decided much yet.
> PI to output stage coupling caps so far away from the action
I moved them off the rectifier socket anyway. I have picked up a little hum but not from that. It goes away when I turn off my bench lite. hehe. More changes though.
This PT will not put out 348 with a tube rectifier. More like 297. I pulled all the tubes and hooked up an estimated load of 90ma with a resistor bank and that's what you get. So, as a result, instead of 250 on the PI plates I had about 221 and 200. I changed the plate load resistors to 100K and 82K and the PS series resistor from 22K to 18K and now have 240 and 225. Closer. I also changed the PI cathode resistor from 120 ohm to 134 ohm. Cathode voltage now about 52 volts.
Instead of 150 volts on the parallel input plates I had 123. I changed the PS series resistor from 22K to 4.2K and dropped the plate load resistor from 220K to 193.2K (I got about 20 of those. Big precision metal films I picked up at a swap meet someplace) I now have 150 on the nose on the input plates. I didn't measure the cathode voltage. I'll do that tomorrow.
So what did all this do? It's now a more gainy pushed shape tone if you want it. I think you could change the MV control to a couple of different resistor values and control them with a couple of relays and a foot switch. Anyway if MV is dimed it's just a sweet little Miss Clean Thing. This all made the Post PI Master more effective. I need to play it more to really decide what it''s worth. The "gain" control now doesn't change the treble/bass ratio. The amp is fuller and bigger sounding. I was tube rolling and put a microphonic 12AX7 in the first hole and of course the amp took off, I wonder if it's close to being pushed as hard as it can up front? I wanna take a closer look at the input Rk to see what that ratio is. Anyway this is turning out to be a nice little 15 watter. I'm gonna make me one too I think but maybe with a real chassis, some tone controls and a little recovery triode with no bypass cap. Degeneration. Gotta smarten up on tone controls though.
Thanks PRR
Dan H[/b]
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.