Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on December 21, 2013, 08:09:43 am
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Yes it is in pieces but appears to be all there. The head cab is covered with snake skin like material.
The circuit had been severly hacked. The chassis SN is A 17329 puts it at 1974. On the back is says "Fullerton Ca". So from what research I've done so far, think the schematic is AA165. The existing board is pretty pitiful. Was thinking at first I wanted to bring it back, but it's a longer stretch than I had hoped. I will take some pixs soon. Platefire
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:sad2:
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No, but that's good! :l2: cause I got that and a bunch of other stuff for $20.00 which includes, a Fender Excelciar OT, a set of fender cab tilt back legs, a solder sucker, small SS Dukane PA head, Shure Mic pre, Box full of miscl parts/connectors/standoffs/jacks/plugs--plus the fender head came with a brand new unused OT that looks same a Doug's Bassman OT(big). So I'm not crying, but happy!!
Platefire
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WOW, that's an incredible haul of tinkering parts for cheap! Nice job!
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I'm crying cause you guys get all the good junk :sad2:
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$20,impressive.
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Well this all came from a musician friend and fellow tube amp lover/tech I think was under pressure from his wife to clean house, so he called me about the Pro head and piled all the other stuff on after I got there. He was in the process of cleaning out. Realizing this, next time I talk to him I will inform him that he went overboard and I will gladly give anything back he has second thoughts about. Knowing him, it's a done deal. Platefire
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Here are pictures of the snake skin pro.
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Mo pixs: On the OT shown below, I haven't researched those numbers yet to see what it actually is but I believe it was purchased to go with this head but never used.
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This is the reverb Tank. I haven't researched yet to see if it is compatable with the pro circuit.
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Wow! What a mess! :sad2: :dontknow:
IF it were mine, I'd gut it ................ clean it up and put in a Hoffman AB763 board and make it look & sound great. I'd keep the PT & OT and pots if they're good and replace just about everything else including sockets if they need it.
You do GREAT work! Do something really spectacular with this amp, Plate!
Best regards & respect, Tubenit
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Fun! Great haul!
Don't know if you can save that "snakelex" covering such that is looks workmanlike. Appears that the amp chassis was at least stored in the cab so that the chassis is not banged up, meaning bent, deformed. That's most fortunate.
As I said in a previous discussion of some other ratted-out (meaning corroded) Fender head, I wouldn't be in that big a hurry to chuck the parts board & contents. I *would* experiment with some 91% rubbing alcohol on a section of same to see how much crud it might remove. If alcohol submersion can clean it up, you could form your own loooong "bowl" out of some plastic sheeting draped inside a picture frame of 2x4's. Fill it up w/alcohol and drop that board in it, you never know, you might be able to clean it up adequately with $5.50 worth of alcohol along with a slightly stiff brush of some kind you get at the 99 cent store. That's if you determine you have to cut away every wire. Before doing that, I'd just get it back to stock circuit-diagram wise and plug the sucker in. Replace the filter caps, that's certainly a given, but you'd have to do that anyway. Worst that happens it blows up. Second worst, you get it working but cannot overcome the effects of the corrosion, eg; it makes gremlin noises you can't chase down and kill.
Excellent score, Merry amp-fiddling Xmas!
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Thanks, tubenit
I like the idea of restoring the original pro reverb circuit but boy! would that be a job! That's what I leaning to but not sure yet.
I just did a little research and I don't believe Fender ever made the Pro Reverb in a head or I can't find one. Someone must have cut this cab down from a combo or made this head cab.
The output transformer is a New Sensor Corporation NSCO18343 universal/bassman OT 4/8/15 Ohm taps.
The reverb tank is Accutronics 9AB3C1B Long 16 3/4" three spring, Long Delay. Input Imp 10 Ohms and output 2575 Ohms. Made in Korea.
eleventeen---yep! I've pondered on if there is any hope for that old board but the more I think about it, you could put a lot of time into trying to patch it up and still end up with not much. You can't see the terrible hacking someone had done up and down the board. If it was all intact, I might try to fire it up but you would have to redo all the locations where they added components and changed stuff. They have a double pot MV installed, plus another midi pot in the rear through one of the old rca plug holes. It would take a lot of correcting to get close to a test. Platefire
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Te pro was never offered in a head. If a fender head has reverb, it is going to be the larger head. Snake tolex I cannot remember a special issue of silverface 70's Pro head in snakeskin. I do not remember snakeskin in the 70's. That doesn't mean there was not.
The OT is a multitap replacement. Looks like a fine amp to mod to your taste and making it a great players head. Great score.
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Hello Ed
Yeah, I recall the Bandmaster Reverb head I had years ago was a tall head. So that probably means the tank couldn't fit under the massive OT and tubes of this head cab. As a matter of fact I don't have any speaker cab as wide as this head. Any cab I got, the pro head would be hanging off the sides :dontknow: Platefire
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... So from what research I've done so far, think the schematic is AA165. ...
AA165 would have been a blackface amp; however, AA1069 is a silverface Pro Reverb (http://www.el34world.com/charts/Schematics/files/fender/PRO_REVERB_AA1069.pdf).
AB668 would also be a silverface Pro Reverb, in the 1st year they transistioned to those cosmetics, but I haven't seen a schematic floating around for it.
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HBP--If I could sound like this dude on the beat up strat on this vidio--I'd rebuild it back stock in a NY second.
https://www.youtube.com/watch?v=kTzWPrpb-q0&feature=player_detailpage#t=104 (https://www.youtube.com/watch?v=kTzWPrpb-q0&feature=player_detailpage#t=104)
BTW-How do you get the youtube video to appear in post, I've done it before but can't remember what I did?
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You could use a Super Reverb or Bassman OT instead of the New Sensor one you have and get higher power....the Pro Reverb used the same OT as the Vibrolux, and while they do sound good, they are lower power for the same basic circuit as the larger Super Reverb or Bassman. If it was mine I would probably do that, maybe add power scaling and a MV in place of what they put in there, use a Hoffman board, and make channel 2 stock and go with something different for channel 1. I'd leave it in a recovered head too. But thats me...
I'm sure you could turn it into something cool!
What a rats nest on that wiring!
Greg
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HBP--If I could sound like this dude on the beat up strat on this vidio--I'd rebuild it back stock in a NY second.
Sounds like a Strat and a Fender amp... With the right speakers that shouldn't be hard.
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HBP---Well a lot of that combination is in the dudes hands and his ability that I come up short on that end in comparison to him. Can't tweak that into an amp. On the other hand I'm thankful for what ability the Lord gave me and will put it to use!
Still pondering the fate of the pro. Something nice to think about. Platefire
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maybe get an eyelet board and rebuild close to original.
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Before a total rebuild, I would try to put is back to original and see how much work really needs to be done.
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LooseChange, your playing my song! My heart has been telling me to restore and get existing up and running and my heads been saying, "too much trouble", gut it and rebuild. I tend to agree with you and follow my heart.
I pulled the chassis out last night and really gave it a thorough examination. It appears to some degree someone was trying to blackface it. The bias supply has been blackfaced. Only thing they failed to put in the coupeling caps between PI output and bias supply--no caps there! Also they attempted to install a post PI MV with a double 1M pot. On the board they installed some odd vaule components but most of the board is original. I think that can be corrected pretty easy. Also I did a PT test with 5U4 rectifier. Got 401VDC output from the rectifier without any connection to filter caps.
This morning I did some major cleanup on board and chassis using Radio Shack Contact lube/cleaner using a lot of Q-tips to get inbetween components. Just takes a lot of elbow grease and TLC. Who ever worked on this amp let their hand rest on the board and components and everything was pressed down. I lifted the board components up off the board, lifted all the attached lead wires up, gave them proper spacing and it's looking a lot better. I now truley believe I can get this existing amp going and will proceed in that direction. I did some measurments on the Head cab, big OT, reverb tank and think I might be able to squeez the reverb tank in there too. The snakeskin tolex is starting to grow on me and matches the front black grill cloth pretty well. I just need to glue some of the tolex loose ends back down. I'm Excited!!! :happy1: Platefirfe
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I enjoyed the restoration of the of my old Fi Sonic. It was a bit of an ugly duckling and I had thoughts of ditching the speaker cab cause it was hacked to bitz but I was glad I spent the time to patch it up. Even having to contend with a dead PT and reverb tank and then to reengineer them to fit into the originals position. Then to my surprise replacing the volume pots put the cream on the pie. Now its no ugly duckling and its great to own a piece of Australian music history. Good luck
Found your matching cab http://www.gumtree.com.au/s-ad/brisbane-north-west/guitars-amps/guitar-speaker-cab-4x12-snakeskin/1033118701 (http://www.gumtree.com.au/s-ad/brisbane-north-west/guitars-amps/guitar-speaker-cab-4x12-snakeskin/1033118701)
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Hi Timbo
Happy new year Down Under. We have the outback Steak house here and we was over there the other day where I had got a gift cirtificate for there. Don't know how close it is to the way it's done Outback or not but I love their 14oz ribeye medium well.
On the Pro, as long as I've been in amp building I've been looking for an old Fender amp that needed help that I could pick up pretty cheap and restore. Believe me I've looked hi and lo and made offers and came up emty handed but this one kind of fell in my lap. It just wouldn't be right at this point to make it into something else without first trying to bring it back to it's original purpose. Someone had the vision of making it into a head, snakeing it and blackfacing it, maybe I can complete the task. That cab you linked would be a perfect fit and match for the pro head but being the old skin flint I am wouldn't put out the bucks for it and wouldn't want to move it without a rodie:>) BTW-have you got a pix of your restoration your were refering to? Platefire
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Thanks Plate, some pic here http://www.el34world.com/Forum/index.php?topic=15917.0 (http://www.el34world.com/Forum/index.php?topic=15917.0)
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Looks like a fun project, but you'll at least have to get matching boots when it's done ;)
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Timbo--looked at your restoration project--cool restoration. I've never seen old style point to point quiet like that. Everything was arranged rather neatly and didn't look like spaghetti. Neatest p to p I recall from a factory job. I kind of scanned over all the post and may have missed something but did you get it sounding like you wanted?
Question: On the pro, I started rewiring the board and got it about 1/3 done. I trying to blackface it but never blackfaced anything before---so I assume you just change the circuit compltetly to blackface pro along with components and values?? The original blackface pro didn't have reverb, so I'm doing everything except the reverb circuit AB763 Pro and the reverb circuit is staying AA1069. Please advise if there is any "got ya's" I need to be aware of in doing this. Platefire
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Question: On the pro, I started rewiring the board and got it about 1/3 done. I trying to blackface it but never blackfaced anything before---so I assume you just change the circuit compltetly to blackface pro along with components and values?? The original blackface pro didn't have reverb, so I'm doing everything except the reverb circuit AB763 Pro and the reverb circuit is staying AA1069. Please advise if there is any "got ya's" I need to be aware of in doing this. Platefire
Blackfacing does not necessarily mean convert to AB763. I would not use that AB763 Pro schematic for this job. Doing so is a gotcha that will almost certainly cause you some grief.
Instead, use the Pro Reverb AA165, which is the correct blackface schematic. There's even a board available over at turretboards.com if you need one.
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I think I speak for most following this thread that "Pro" means "Pro Reverb", meaning, as Sluckey said, just go for an AB165 Pro Reverb circuit. As for blackfacing it, the only diffs I see are the 2000 pf caps between grids and cathodes on the 6L6s (delete them), the "hum balance" type bias control on the later version, and the 47K/47K plate resistors after the 12AT7 PI instead of the 82K/100K plate R's on the earlier version. No real big deal.
That is...unless your amp is the AB668, in which Fender threw lots of add'l parts in an effort to do whatever they felt that they had to do. (Doug has the schem) In that circuit, you have a cathode bias structure (with the 6L6 cathodes sitting on top of resistors and a non-polarized cap connecting the cathodes...) plus the 2000 pfs, plus the 47K plate R's, plus changes in the power supply node resistors.
If the intention is to blackface the thing, then the good news is that the changes are subtractive, eg; you just snip the extra parts out. Look for those 6L6 cathode resistors for your indication.
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Thanks for coming back quickly, by the time I went to bed last night I had 1/2 the board done and it occurred to me that I haven't blackfaced an amp before and might should ask the question.
I just went over what I've done so far and best I can tell the only difference is on the AA165 circuit the network of pre Phase Inverter resistors and caps I have a 47R to ground like the AB763 and the AA165 has a 100R to ground.
The bias supply had already been converted to AA165 using/rewiring the original balance pot when I got it. There is no 2000pf caps to ground on 6L6 grids. It did have a 5/25 cap(like AA1069) on#8 pin cathode of tremolo tube and I changed it to a 25/25 to be AA165. The PI plate resistors are 82 & 100 like AA165.
So I don't know if changing the one 47R to 100R to ground on the PI input is worth the wear and tear or not, you tell me? Looks like overall I'm in good shape and have nothing I just installed to tear out save the 47R. Always Thankful for your help, Plate
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I just went over what I've done so far and best I can tell the only difference is on the AA165 circuit the network of pre Phase Inverter resistors and caps I have a 47R to ground like the AB763 and the AA165 has a 100R to ground.
Fender used a 47Ω resistor on the amps that only have a single 8Ω speaker (like the Deluxe Reverb or Vibroverb). All others, including your Pro Reverb with two speakers, use a 100Ω resistor. So, if you want the NFB level to be right according to Fender, replace that 47Ω resistor with a 100Ω resistor.
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Thanks sluckey
This amp is in a head cab, no speakers. All my cabs I would be hooking this head to are single 8 Ohms speakers save the Boogie that is 4-10 but if I remember correctly, it also is wired for an 8 Ohm load. So I'm thinking that based on that I may be better off leaving it as is. Does that shed any different light on it, or what say you? Platefire
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It's not a show stopper either way.
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This amp is in a head cab, no speakers. All my cabs I would be hooking this head to are single 8 Ohms speakers ...
Understood, but the Pro Reverb had 2x 12" speakers wired for 4Ω.
That matters only in that a 4Ω load will reflect the "correct" primary impedance. Of course, if you're happy with the sound, it's fine to use 8Ω.
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sluckey & HBP
The original OT on this head is missing and was gone when I got it. I did get a new replacement OT with this head that was never installed by the previous owner that is pictured. I asssume this is what they intended to install but never did. It a New Sensor Corperation Fender Type Replacement Output Transformer type #018343 Bassman Universal type OT. Primary is 4K with 2/4/8 Ohm taps. I was planning on installing it and the speaker jack to the 8 Ohm tap. So I understand that the original OT was a 4 Ohm OT/speaker out for a 2-12 cab. So I'm providing the OT background info that you might not have known I was using a new OT with multi taps or am I still not comprehending something because I was thinking this arrangment was kosher for matching my 8 Ohm speaker loads? I do want to achive the "correct" impedance for my existing speakers. Platefire
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Got to thinking I better locate the position that the new OT will need to be to check bolt hole locations in relation to the board. I do not want to peel that old board up to drill holes and insert bolts, the board would be best left alone as much as possible. Well it turns out the bolt holes will fall on either side of the board---ye haw!! :thumbsup: The existing OT wiring holes in chassis will serve as access for the OT primary and secondary leads but will have to remove the existing plastic inserts and drill them out and put in grommits to fit the new holes. The old holes are just too tight to fit the leads through for the new wiring. But I'm thankful its all doable without rocking the boat too much. Platefire
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sluckey & HBP ...
Gotcha. Then you've got it figured right already. :icon_biggrin:
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sluckey and HBP
Confession time. :dontknow: Well for some reason primary impedance and impedance/Ohms has always been hard for me to comprehind and when it is discussed I don't have any visual image or understanding of whats going on to grab hold onto. I know for an amp to work at peak performance it needs to be properly matched with a proper OT and a speaker to match the OT. I have studied it before to try to bring away something that will leave a lasting impression or understanding but so far I still draw a blank when I think about it---that's why I get concerned when somebody indicates that things might not be matched up properly--my pea brain hasn't got nothing to grab onto other than trying to match up proper components. So I always appreciate you guys help because my understanding alone comes up way short! Thanks, Plate
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Tubes operate with high voltages and low current. Their plate resistance is typically high. A typical power tube may have a plate impedance of 5000Ω.
A loudspeaker is a low voltage, high current device. Typical impedance may be 8Ω.
You cannot simply connect a low impedance speaker to a high impedance tube plate. You must use some device that will match the high impedance tube to the low impedance speaker. This device must also isolate the speaker from the high dc voltage present on the tube. And the more accurately the plate high impedance is matched to the speaker low impedance, the more power can be transferred. The OT accomplishes all these tasks very well. In the example above, an OT would need to match a primary impedance of 5000Ω to a secondary impedance of 8Ω to allow maximum power transfer.
Slight to moderate mismatches (say 6000Ω to 8Ω, or 5000Ω to 4Ω) can usually be tolerated, but you will not be able to transfer maximum power from the tube to the speaker. Gross mismatches should be avoided.
The physical size of an OT is largely determined by the amount of power you want to transfer. More power means bigger iron.
If you are auto mechanics savvy, you may want to kinda think of the OT as the transmission sitting between the engine and drive train. Probably a poor analogy, but I ain't into automobiles or mechanics. :icon_biggrin:
To sum this up in one sentence, I'd say, "The OT is an impedance matching device between the high impedance plate and the low impedance speaker."
Now, just how far down this rabbit hole do you want to go Neo?
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Well I surely need to understand it better than I do, so when I run into these issues regarding power amp output being converted to efficent speaker operation, I will have an idea of how to proceed and get the most efficient matching of components
Just to bring it to the simplest terms, I really don't know what impedance means. I do know to imped is to slow down or hold back which sounds a lot like resistance--so I looked it up and found this definition:
Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.
In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude. When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.
So according to that, for DC current there is no differance in impediance and resistance. So I guess my first question would be, why do we call it impedance instead of resistance? Plate
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"So I guess my first question would be, why do we call it impedance instead of resistance?"
Because they are certainly different, and the desire is there to have different terms to accurately describe the situation. A capacitor is not a dead short to AC, it has a frequency-dependent and ufd-dependent resistance to AC. If there were no difference, we could use cheap .01 ufds as filter caps in a power supply. Likewise, we would gain little inserting a choke into a power supply if it did not act to oppose changes both in voltage and current.
We use impedance because inductors and capacitors have a quality differing from pure DC resistance, known as reactance.
Part of what makes this so is the phenomenon of current vs voltage phase angles. In an inductive (L) load, voltage leads current. In a capacitive (C) load, current leads voltage. Were you to combine both L and C in a circuit, you'd have to perform some sort of vector addition to get the equivalent resistance.
(http://i172.photobucket.com/albums/w32/ttm4/reactance_zps2afc8963.gif)
One could study this at great length, it gets wooly. Having some idea of what's going on is beneficial but not required, certainly not to NASA-levels. What's good to know is that maximum power is transferred when sources are impedance-matched and roughly level-matched to loads. I'm not going to pretend my understanding of it is anything great. But you already know your guitar cannot directly drive a speaker; a low-Z microphone usually does not drive the line level of an amplifier very well, and vice versa.
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Thanks, but you got a little too deep right off the bat. You know it would be cool with me if sluckey started another thread regarding impedance as related to guitar amps. I would be right there to learn and ask questions. This is just a suggestion and if there is a interest there by others? If so I would recomend it start simple for us hard heads :laugh: Plate
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Wow! What a mess! :sad2: :dontknow:
x 2
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So according to that, for DC current there is no differance in impediance and resistance. So I guess my first question would be, why do we call it impedance instead of resistance?
Let's get easy.
There are 3 main passive components: resistor, capacitor, inductor.
A resistor presents resistance to the flow of current. An inductor looks like a very low resistance to d.c. (it's a coil of wire, right?), but has a big reactance with a high frequency. A cap looks like an open circuit to d.c. (2 plates separated by an insulator), but a.c. appears to pass through it (as in a coupling cap), so it has a small reactance at high frequency.
Reactance is the a.c. equivalent to resistance, and its value depends on the applied frequency.
Impedance is the sum of resistance and reactance in a circuit, or all the opposition to alternating current.
A transformer blocks d.c. on the primary from appearing at the secondary, but passes a.c. signal, like a cap. Unlike a cap, it can step up or down voltage. Power in equals power out, so if voltage is stepped up, curent is stepped down by the same ratio.
If ohm's law (in one form) is Resistance = Voltage / Current, then for a.c. it is Impedance = Voltage / Current. Stepping up voltage and stepping down current is also stepping up Impedance. The OT takes a high voltage, low current at the primary and steps voltage down and current up at the secondary; so it has stepped impedance down, and matched a, say, 4kΩ primary to an 8Ω speaker.
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Thanks HBP
I have been studying the info you provided. Forgive my dumb statements or questions:
is Impedance related to ac only as in the grid the guitar signal travels on?
Also is an inducter the hookup wire we use in an amp? You said "coil of wire"--did you mean wire wound on a coil like a transformer or was you just talking about a length of hookup wire? The longer the wire, the more resistance?
So I really didn't know the transformer blocked dc and passed ac like a capacitor but that should be obvious because the OT has high voltage dc on the primary and ac only on the secondary.
Ok I'm thinking out loud---in the OT primary you have high voltage dc and ac grid signal. So the high voltage dc is blocked at the OT primary and the ac grid voltage is stepped down and the current is increased by the same equal ratio the ac is steppped down on the secondary? so I'm thinking by tapping this step down process at different points is where we get the multi Ohm taps?
Also I'm assuming from what you said lowering voltage and increasing current lowers impedance and vice versa higher voltage and lower current raises impedance???
Also I'm wondering at what level or threshold does a ac signal go from being high impedance to low impedance or vice versa?
OK-you can straighen me out now :dontknow: Thanks, Plate
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is Impedance related to ac only ...
Resistance is the opposition to the flow of current. Impedance is the opposition to the flow of alternating current, and includes resistance as well as the frequency-dependent element of reactance.
Said another way: d.c. is like a frequency of 0. Reactance has a formula with frequency of the a.c. applied, so if the frequency is 0, the reactance is also 0. If impedance is resistance and reactance, but reactance is zero because we have d.c., then the only thing left is resistance.
See how resistance matters to both a.c. and d.c., but impedance includes an extra element that only applies to a.c.?
Also is an inducter ... You said "coil of wire"--did you mean wire wound on a coil like a transformer ...
Yes.
Inductor = choke = transformer winding. It is very literally a wire wound into a coil, because the field set up around the wire strengthens and increases the inductance of the resulting coil. Below radio frequency, inductors/chokes/transformers will be wound in coils, and will generally have a metal core of some kind to increase inductance.
You know already that transformer windings have some resistance, but the 4kΩ primary of your OT doesn't measure as 4000Ω when you attach your meter to measure resistance. Also, a choke may look like thousands of ohms at 120Hz, but have a d.c. resistance of only tens of ohms. You should also notice that "d.c. resistance" is redundant, but has become common because there are other places where we might wrongly say "resistance" when talking about something that only exists with an a.c. signal.
So I really didn't know the transformer blocked dc and passed ac like a capacitor but that should be obvious because the OT has high voltage dc on the primary and ac only on the secondary.
Yes sir!
It's one of the important functions of a transformer that we often don't think much about: keeping d.c. levels separated while coupling a.c.
You could say a cap and a transformer (or more broadly, a choke) do the same types of things, but come at the problem from opposite directions. Which is a better choice in a circuit pretty much comes down to the frequency being handled and cost considerations.
Ok I'm thinking out loud---in the OT primary you have high voltage dc and ac [plate] signal. So the high voltage dc is blocked at the OT primary and the ac [plate] voltage is stepped down and the current is increased by the same equal ratio the ac is steppped down on the secondary? so I'm thinking by tapping this step down process at different points is where we get the multi Ohm taps?
Yes sir!
Power is the same at all the taps. The 16Ω tap has twice the voltage and half the current (for the same power) as the 4Ω tap.
Example:
- Say there is 16vac at the 16Ω tap. 16v / 16Ω = 1A. 16v * 1A = 16w
- The 4Ω tap has half the voltage as the 16Ω tap, or 8vac. 8v / 4Ω = 2A. 8v * 2A = 16w
- See how the voltage and current changed for each of the taps, but power stayed the same?
Also I'm assuming from what you said lowering voltage and increasing current lowers impedance and vice versa higher voltage and lower current raises impedance???
Yes sir!
If you firmly know ohm's law and the equation for power, then pick a number for power (say, 10w). Pick a number to be your voltage (let's try 500v), and calculate your current for that power (10w / 500v = 20mA). Use ohm's law to see what resistance allows the current to flow when the voltage is applied (500v / 0.02A = 25kΩ). Now keep the same power and pick a radically different voltage, and repeat (25v; 10w / 25v = 400mA; 25v / 0.4A = 62.5Ω).
You can see whichever way voltage goes, so does impedance. You might say if the impedance gets bigger, it takes more voltage to push a given amount of power through the impedance.
Also I'm wondering at what level or threshold does a ac signal go from being high impedance to low impedance or vice versa?
You know Einstein? It's all relative...
For a preamp tube plate, 600Ω looks like a low impedance. For an 8Ω speaker, 600Ω looks like a high impedance.
"Low" or "high" only makes sense when you talk about it in reference to some other impedance or circuit point/element.
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Your last statments was a little more complex, so I might have to take one at a time so I won't get to scattered :w2:
Ok--Impedance includes resistance, reactance and frequency. So you are saying dc always has a freqency of 0 and with 0 freqency there is 0 reactance. So in a tube amp we have locations where it is strickly dc, other locations strickly ac and then others with ac & dc combined. So reactance only applies where dc and ac are in the same location? So in a dc only world impedance would only include the resistance element but when ac is included the freqency & reaction would be the reaction of ac to the dc element in the neighborhood.
I keep thinking frequency of what. If frequency is something repeditive at a repeating rate---are we talking voltage, current or something else?
I'm only repeating what I think you said so you can tell me where my comprehinsion is wrong. Thanks, Plate
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So in a tube amp we have locations where it is strickly dc, other locations strickly ac and then others with ac & dc combined...
There is AC (which is where current 'flows' back and forth around a reference point), then there is DC (where current flows in one direction only), then there is varying DC (where more or less current flows in one direction only)
Resistance inhibits the flow of current. Impedance inhibits the amplitude of AC
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You got some nice info from HBP, as usual!
"Ok--Impedance includes resistance, reactance and frequency."
Nit: No, just the first two, resistance & reactance. Reactance is a frequency-dependent form of [resistance] the opposition to both voltage and current, which occurs in capacitors and inductors in AC circuits. (C's and L's) Since I use the word "frequency" in that definition, the implication is that we are talking about an AC circuit. "Frequency" will be a mathematical factor in whatever equation we use to figure reactance, so, in a DC circuit, where frequency is zero, those terms will algebraically drop out and reactance will be zero. Thus we'll be left with just DC resistance, which in a DC circuit, is identical to DC impedance. This goes back to your first question: We use the different terms (impedance and resistance) to imply whether we're talking about an AC or a DC circuit. We rarely use the term "DC impedance". Once we have figured out the reactance of a capacitor, for example, we can use that reactance (measured in ohms) exactly as we would in Ohm's law...though not for power calcs. THAT is the value.
"So you are saying dc always has a frequency of 0 and with 0 frequency there is 0 reactance." Right!
"So in a tube amp we have locations where it is strickly dc, other locations strickly ac and then others with ac & dc combined. So reactance only applies where dc and ac are in the same location? So in a dc only world impedance would only include the resistance element [right!] but when ac is included the freqency & [reaction] (think you mean "reactance") would be the reaction of ac to the dc element in the neighborhood.
Uhh, no. The capacitors and inductors in an AC circuit have reactance. It is a quality or characteristic that is dependent upon (among other things) freq and (as one would assume) the number of ufds or henrys in the cap or coil.
It's not whether AC and DC are combined (as they often are) We almost always have to separate the AC from the DC part to analyze what's going on. The AC in a circuit "sees" caps and coils and finds that those components have this reactance effect upon both the AC current and the AC voltage. If said AC is combined with DC, those components do not affect the DC part.
"I keep thinking frequency of what. If frequency is something repeditive at a repeating rate---are we talking voltage, current or something else? "
Voltage and current.
What are some clues that DC resistance and (AC) impedance are different things?
You measure the DC resistance of a 6.3 volt filament winding on a power transformer and come up with .2 ohms. You take 6.3 volts and divide by .2 and per Ohm's law come to the conclusion that if 6.3 volts were flowing in that secondary winding across .2 ohms, you would expect 31.5 amps to flow. Nope.
You go to a tube spec sheet, say a 6AU6, for a high frequency design you are working on. You find a section called "interelectrode capacitance" which talks about the teensy capacitors that in effect exist between the grid and the cathode and the plate and the grid. Because capacitors are what, 2 separated pieces of metal, right? So the electrodes inside a tube have itty-bitty caps of 1-2-3-4 uuf between them, because they are separated pieces of metal. We don't care much with audio, but at high freqs, these are short circuits to AC...so these tubes can't be used above certain frequencies.
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We know what resistance is. (If you don't, learn.)
"Impedance" means something extra.
As said, it can be reactance (capacitor, inductor). Now we need to think about Frequency.
It *can* also mean there is a voltage dependent aspect of this "resistance". Consider a Silicon diode. Infinite resistance one way, the other way is 30 ohms at 1mA but 3 Ohms at 10mA. It's a very "bent" resistor. It's bentness depends on voltage or current, but not (much) on Frequency.
But Audio usually means wide-band, in both frequency and voltage. Usually stuff acts "the same" for 100Hz to 10KHz and 0.0001V to 100V. When it doesn't, usually we "want it that way". Tone-controls bite highs more than lows. Speaker impedance is all over the place but we usually pretend they are a "resistor" because the deviations have been allowed-for by the speaker designer.
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On the pro reverb I drilled holes for the OT and also drilled out the wiring access holes larger and installed new grommits. I didn't install the OT though, the thing is so heavy I'll wait and put it on last thing to make handling the chassis easier while doing the final wiring. Got a parts order in with Doug. As soon as I get those in I can do the final wiring, install OT and fire it up and see where I'm at with this amp.
On the impedance study, Thanks! My brain is reelen from the influx of info, may take me some time to digest. Plate
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Archive this please!! Very nice insight gents!
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Got parts in for this one today from Doug and couldn't wait to put them in. I installed all AA165. I fired it up about 3:00 PM on my light bulb limiter and no smoke so I went whole hog 120V. It started squealing like a pig right off the bat so I had to switch 6L6 plate leads--that did it. Both channels working pretty lively. No reverb or trem. Don't know if this is one that has to have trem switch plugged in to activate? Also don't know it the reverb tank that came with this one is good or not--it may be the tank is bad. I haven't installed any 1 ohm resistors on the 6L6's cathodes yet---just set the bias -51 like schematic and power tubes seemed happy. I got a 5U4 rectifier in there, don't have a GZ34. The thing was running pretty quiet. Some voltage readings are:
V1=239,230(normal chan)
V2=244, 268(rev chan)
V3=405(rev driver)
V4=273, 262 (rev recovery)
V5=215, 435 (trem)
V6=234,233 (PI)
V7/8=Plates-436/439, Screens=433/435
Forgot to check heater voltage
Platefire :icon_biggrin:
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I guess the good news is that you are running with that skungy old parts board, yes? Congrats!
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Yep! At first I thought the hacker hadn't messed with the board very much but I was wrong. They had disconnected stuff, changed values, terrible soldering, caps added in strange places and extra pots. Apparently they had been reading about a lot of mods that could be done to this amp but didn't have the where-for-all to carry them all out. The only mod I can see that was done successfully was converting the bias from balance control to bias control--like the AA165. Fortunatly I had most of the stuff I needed in stock. It's all AA165 except for one thing they did I kind of like and will wait on changing out---the ground wire for the reverb control pot terminal runs to the gound terminal of the rev chan vol ground terminal--when you turn up the reverb in adds gain(like crazy) that really makes a nice feel/tone. Of course it also adds reverb but it sound so good you hardly don't mind. Anyway I'll play with it until I decide if it's useful or not:>/
BTW-fixed the reverb and Vibrato---reverb tank had a ground wire from the output transformer that had fell off the rca jack and just soldered that back. The vib had one leg of the opto eye disconnected that may have been one of their gain mods, that I resoldered back. Plugged in my vib pedal from my Silvertone 1482 and it switches it very well.
I added the 1 ohm resistors to grounds on the 6L6 cathodes. Got an old set of tubes in there kind of unbalanced running one at 33ma, the other at 24ma and bias voltage about -49.
Gosh!! The thing is sounding good to me. The tone stack is kinda weird but I'm learning to work it.
The responsive feel is impressive. It's fender sound but a little different. Platefire
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At the risk of stating the obvious, having a parts board with the terminals laid out in a way that suggests the component layout is powerful juju. Compared to building on a generic turret board where all the terminals are equally spaced; and only at the edges... with no terminals part-way across the board ..it's a major difference. Even if you are changing a silverface amp to a blackface one. (I don't know offhand if there are differences in the black/silver parts boards, but they are certainly very minor) The proprietary layout lets you see mods and changes very easily.
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Well I guess I should have clarified all the pre-existing mods wasn't all on the board--some on the chassis and some on the tube sockets. A duel one meg MV pot in the vib intensity pot hole, a 250K
midi pot in the rear vib rca jack footswitch hole, several caps on preamp tubes sockets between pins--I snipped off. On the board it was mostly cathode resistors and bypass caps of different values than stock. Also removed a terminal strip where they apparently used to wire the duel MV up. Most of the wiring to hook up these mods was either never installed or removed, so you could only guess to what they was doing.
I just wanting to bring it back to a stock fender circuit, and to my understanding AA165 was the new circuit revised from the non-reverb Pro AB763 to add reverb. I think this amp might have originally had the aa1069 circuit but not sure? It's kind of nice to have an old kinda vinage(73) fender of my own. Ever since I started working on tube amps I've had my eye out for an old fender to fix up and well here it is. Platerfire
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My bias supply has a pot as in the AA1069 schematic which I think may be a bias balance control.
The previous owner converted that same pot to a general bias supply like on the AA165/Ab763. It works pretty good but in turning the setting clockwise I run out of movement before I reach my desired bias setting. Topped out I can only get 31Ma where I would like to get 35 to 40 on the 6L6's. As I remember this can be adjusted by changing the resistor value on the pot to ground that is presently 27K-----do I need to go with a lower value resistor? I've adjusted these years ago but forgot what I did.
The Vibrato seems kind of weak. On the speed it adjust the speed pretty well to about 50% on the pot then seems just to loose it's effect into nothingness. Also on the intensity, it don't seem to provide the depth I'm use to on my other amp vibratos. What can be done to make it stronger?
Thanks, Platefire
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do I need to go with a lower value resistor?
yes
What can be done to make it stronger?
replace the roach
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OK, Thanks sluckey
Yeah they got two 50K paralled for 27K. Guess I'll start with a 22K then 15K or 10K as needed.
On the existing roach, it went south on last amp fire up. When you hit a note with any kind of authority
the vib photo light goes out and you just get static. Light stroke and it blinks off and on but weak. Platefire
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On the existing roach, it went south on last amp fire up. When you hit a note with any kind of authority
the vib photo light goes out and you just get static. Light stroke and it blinks off and on but weak. Platefire
Say what? The blinky neon light in the roach is totally independent of playing notes. Unless something is wired wrong or the board is conductive.
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Nice job so far
I'm certainly envious..in a nice way!
I want one too!
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Sluckey, thanks for coming back an indicating that appeared to be a ground out problem on the roach. I went back and did some more checking--no continuity between board and eyelets at roach, checked continuity between roach leads and connecting components---good, and then I checked the vib footswich connection---ah ha! On the Silvertone 1482 footswitch I am using I had to use a 1/4 to RCA adapter and the adapter was grounding out. So after correcting that it is working normally again but still weak. I like to get a "Crimson and Clover" type of wave even though I may never use it but can't get close with this--very undefined. So I'm probably still up for a new roach. Platefire
Toxophilite
Thanks. Got some new pictures I haven't posted yet since I reworked it. I will soon! Think it looks a little healthier how--but regarding looking for these things is very hard in my area. Of course you can buy them all day long off e-bay if your willing to pay the price! I've been checking flea markets, pawn shops, antique shops, garage sales and even tried to get folks I know of that has old fenders in their closets doing nothing to sell with no success until now--14 years. I'm amazed that all the deals some folks on this forum turn up contiually. Hang in there and be patent and sooner or latter you'll snag one. I'm living proof :laugh: Platefire
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Somehow it's hard for me to think of this 1973 amp as vintage because to me 1973 don't seem that long ago but gosh!!! Thats been 40 Years :dontknow: I really think of vintage as 50's or 60's.
Here are some shots of my test and tweak set up. On an amp like this I just live with it a while--test and tweak until I'm satified. This chassis still looks pretty rough especially on the face plate--I'm just going to call it "Road Worn" since that is popular now days. Platefire
BTW-In case someone wonders about those large red resistors on the board that looks like caps, those are NOS resistors I bought at a flea market that are probably vintage 50's at least but they are spot on accurate. If they say 100K they usually measure 100K and I use them when I haven't got a more modern one in stock.
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It won't win any awards for prettiness, but I am glad you were able to get the old parts board crankin'.
The money you saved on a new board---if you can buy a set of blackface faceplates I predict you will make yourself very happy. It'll make you smile every time you look at it. At least the front, the back is passable.
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eleventeen
I read your post early this morning and really did give it some serious consideration. I finally decided I really like the personality of that old beat up face plate. First thing people think when they see that is,
"What in the world happened to that"? I like the mystery of what it went through to get where its at. On the other hand the blackface faceplate is still something I could do in the future.
Last night I changed the resistor on the bias pot from 27K to 22K and got some more ajustment leverage with my bias. Probably 20K would even be getter but with the 22 I was able to get the bias right in the middle like I like it--not to cold, not to hot, just right! That sure made a difference. It's sounding like a true USA Fender now. Tried several sets of used 6L6's in it. Tried a set of Svetlana's that came with the amp the are pretty well matched but I just didn't like the sound. I've was running a set of Sovtek 6L6GB's that sound pretty good but are way off from each other. I ended up leaving a set of Sovtek 5881/6L6WGC that sound real good to my ears and are more closely matched.
I worked on the headcab today to glued some of the loose snake tolex back down. Also had to remove the existing plywood & cardboard bottom plate off the reverb tank and install a thinner piece of mounting cardboard on the bottom because the clearance between the bottom of the OT and bottom of cab is so tight. Since the amp is working well, I decided to go ahead and mount the chassis and put it all back together. I ended up with only about a 1/4" clearance between the bottom of OT and top of reverb tank, but I got it in there. A Pro Reverb shall have a working Reverb :evil5:
After I got it all back together everythng was working sounding great and then my Vibroto started ticking--should have known not to put it back together to quickly :BangHead: This head is reminding me of my first early 70's Silverface Fender Bandmaster head. My first great sounding amp. Even has the same teal stipe around the faceplate. Bet it even had close to the same circuit before I blackfaced it. I'm just real happy with the way it's sounding and responding especially since I got it biased up. Platefire
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Ticking vibrato??? Here are a couple tips that may help...
Click on this link and look at the second photo...
http://timeelect.com/jbl-twin.htm (http://timeelect.com/jbl-twin.htm)
Attached is a Fender Service Bulletin that deals with ticking vibrato...
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For your vibrato, replacing the roach is likely the best way to go. Sometimes the trem can be weak because too much outside light gets into the sensing part of the roach and taping it up can help. They are very delicate and easily broken though and many times if you try to take it off the board so you can tape it up you'll break something! Also, if you want a slower speed, change one of the 0.01uF caps in the feedback part of the trem circuit to a 0.02uF...ceramic is just fine. A lot of Silvertones like a 1484 or 1485 have roach trem that has more intensity than the Fenders, and this is because Silvertone brought it in after one gain stage whereas Fender mixes it in after two.
Greg
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sluckey
Thanks a bunch! That something I'll have to address when I go back in. Should have known better to button it up this soon. I was just really anxious about putting it back together to see how it all fits because I got it in parts all torn down. Thankfully all the parts were there and wasn't lost. Once I got the tolex loose ends glued back down, it went together really well. I use vibroto quiet a bit, so I want to get it up to snuff. I went to the site you had liked and copied the pix showing the new cap .01 Mylar and the lifted roach leads and put it in my Pro computer file.
Greg
Thanks for the tips. I did notice that the light seems to be almost poking out one end of the roach covering. I tried very lightly to get the boot covering to slide down a little to better cover the light up but it didn't budge and I was afraid to use too much force, because like you say, it does look delicate. The roaches are not that expensive, so I may just go ahead a pick one up for next maintainance cycle:>)
Platefire
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> I really like the personality of that old beat up face plate.
The original James Bond car had flippable license plates to confuse people on his tail. (As if the DB-5 was so common there would be two on the same road; might work on my Honda sedan today).
(http://cdn.screenrant.com/wp-content/uploads/James-Bond-Spinning-License-Plates.jpg)
Pivot on each side, spanking-new faceplate to barn-find faceplate at the push of a secret button. (Or just flip it with your finger.) Since the pots/knobs get in the way, duplicate them; now you can have two different sounds pre-set at a flip.
Yeah, I can't even keep my gate-latch working so I shouldn't be suggesting secret-agent mechanisms.
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:laugh:
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PRR
Well I've heard of a blackface, brownface and silverface but never heard of a two faced amp :laugh:
Really from a distance you can't really tell the face plate is so marred, just on close inspection.
Played the Pro in my church band today. The thing is loud and clean. I started off on 2 and eventually got up to 3. It was nice and quiet with very minimuma operating noise. It took my pedals very well. The amp can do the surf tones very well the long delay 3 spring reverb. The snake skin covering did raise some eyebrows but the drummer liked it. You know how them drummers are :dontknow:
Platefire
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Got a general question about preamp tube shield/covers. I put the preamp tube tube sheilds back on when I was putting the amp back together and could swear it effected the tone in a negative way where I ended up pulling them back off.
I was just wondering if anyone had any similar experiances. Is this just a figment of my imagination or is there something to it? Platefire
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I'm thinking it's just a figment.
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Yeah, could be. I do know when your tweaking and testing an amp your super sensitive about every little thing. I did do a search and found this discussion. Plate
http://www.thegearpage.net/board/archive/index.php/t-859318.html (http://www.thegearpage.net/board/archive/index.php/t-859318.html)
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The Snake Head Pro had been doing so good and last night I lost volume in the reverb channel. You can still hear the guitar at a very low volume. I checked the normal channel and it's fully up to speed--so it's not the power amp. I did try another good 12AX7 today in V2 but with no change. Anything else I need to try before pulling the chassis? Platefire
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V4
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sluckey
Man! I'm so glad for your "V4" post. That was it :happy1:
Thanks so much for calling that to my attention--I had missed that gain stage. Glad to have the Snake Head Pro up and running again. Platefire
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When I got this amp up and running I took voltage measurements and biased it and everything was in reason except for V5 Vibrato tube =V5 plate #6 that directly connects to the vib roach. The schematic shows 360VDC there but my reading was 435. Also on V5 Plate #1, schematic voltage says 280 but I got a low reading of 215.
I did measure the Ohms of the 100K resistor to roach in place and it read right at a 100K and also read the 10M R in place to pin #6 plate that read close to 10M. So I was kind of puzzled at what was causing the voltage to be 75 volts above what it should be? As I recall I checked for leaking caps also but didn't detect any. The voltage from B location power rail measured 436 and schematic reads 440. The 435 reading at roach is almost identical to the 436 B location power rail reading which makes me think it would have to be the 10M or 100K has drifted even though it reads correct in place. Never done much trouble shooting to a vib--so my solution would be to change out the roach along with those resistors and see where it stands. Platefire
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You will get different voltage readings depending on the VIB footswitch. If switched off or unplugged the voltage at pin 6 would be about the same as the B+ feeding the circuit.
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Well Thanks sluckey
Next time I open it up and check it, I will make sure I do my reading with it engaged. Planning a future maintenance run where I hope to tie up all the loose ends--new roach, new reverb chan #1 input jack, new set of 6L6GC's & bias job--the list is growing. Platefire
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Here are the finish pictures of my Snake Head Pro. I may install some hooks to wrap the power cord on and maybe build a cover for it. It could use a back-plate also, so maybe this isn't the final chapter. Platefire
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Got a question? The Pro head makes a ton of spring clash/rattle noise driving down the road unless your on a perfectly smooth road---and those don't hardly exist anymore. So could you stick a piece of foam rubber
under the springs in the tank to help this without effecting the reverb operation. Or what is the best way to address this?? Thanks Platefire
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New shocks. :icon_biggrin:
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Har! The car is just one year old :w2: Should I trade it in and try another? I could take the head along for the test ride to see how it does. That way I could be sure not to buy another reverb rattling car. Surely there is a better solution than this :dontknow:
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If you have room try mounting it on the front panel. Might make a difference. Or put the tank in a bag. Or turn the head over on it's back or face down. Or haul it in the trunk. Or get a tank with a locking device. There may not be a cure.
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The height of this head is to low to mount it on the front panel, turning it over is worth a try and I have a SUV so the truck is the same as the drivers compartment. A bag may be a possibility but the clearance under PT is less than 1/4" which may would put it in contact with PT which may not be good. Not important enough to me to buy a new tank. I hauled plenty of reverb equipped amps before but this one in more noisy than normal. I'm not on the road any more and most of my hauling will be between the house and the church ten miles away. So I will probably grin and bare it with no more hauling that I will be doing.
I still wonder about the foam rubber. Most tanks come shipped with foam rubber around the springs that is to be removed upon installation but the reverb tank is an electronic device. Why does the springs have to be free moving to still perform their duty. The foam rubber is non conductive---so how would that effect the sound leaving some in there to restrict movement? Don't the electronic signal traveling through the spring create the reverb and why would impeding spring movement restrict this. I'm sure there is a good reason but I'm not getting it---unless the foam held moisture and became conductive--that would be a problem. Platefire
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Why does the springs have to be free moving to still perform their duty. The foam rubber is non conductive---so how would that effect the sound leaving some in there to restrict movement? Don't the electronic signal traveling through the spring create the reverb and why would impeding spring movement restrict this. I'm sure there is a good reason but I'm not getting it
The 'signal' moving down the actual springs is not an electrical signal. The input transducer converts the electrical signal from the reverb transformer into a 'MECHANICAL' signal that causes the spring to vibrate as it travels to the other end. At the other end, another transducer acts as a pickup to convert the mechanical signal back to an electrical signal to be passed on to the recovery tube circuit. If you inhibit the movement of the springs you will severely impact the reverb sound, maybe even kill it completely. Give it a try.
The foam rubber pad was put in the tank to prevent the springs from vibrating excessively due to shock during shipping. Excessive vibration could damage the fragile transducers. If you really want a good solution replace the tank with one that has a locking device (like the one I showed you). That locking device was designed to prevent excessive vibration during transportation. It has a red warning label to remind the user to 'unlock' it for normal operation.
Tolerance is the cheap solution.
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OK, I didn't realize the significance of the picture---I thought you was just showing me top mounted reverb, I didn't realize it was locking. I've always heard about a locking unit but never seen one. I was trying to see the lever/switch that you flip to lock it but didn't see anything that looked like a lever or switch. Guess they more expensive than a regular tank? Platefire
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Well it's time to go back in. The #1 input jack for reverb channel broke. I ordered another vibrato roach to install while I'm in there. Trying to decide rather to install it normal first and see how it works first or install the mod sluckey pointed out on an earlier post. Platefire
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And I thought installing a silly input jack would be a simple task--WRONG! I've never wired a pair of input jacks like on this AA165 schematic and trying to read the layout and schematic correctly but apparently I'm not following it right. I've done spent several hours on it. The way I had it wired up the first time I had run away gain and after cutting the connection wire from input jack #1 hot term to input jack #2 switch term, input jack #1 started working right with normal gain.
So am I reading it right input jack #1 has the normal 1 meg ground to hot term but what is unusual and is not working for me is the connection from #1 jack hot term to #2 jack switch term. Is that right? or am I reading it wrong?
Right now I have input jack #2 disconnected from connections to input jack #2 as it shows on schematic until I can figure it out. Might be easier just to put a separate 1 meg R on input jack #2. Sure could use some help on this because I did want to wire it up like original. Schematic/layout attached. Thanks, Platefire
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Look at the input jacks on the other channel. Wire it the same. Or look in the Library. Doug has a very clear picture showing how to wire input jacks.
http://el34world.com/charts/CommonHookups.htm
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Thanks sluckey
I finally got it hooked up right where everything sounds right---I think? Guess I had never actually wired up an old fender double input channel before. Never knew it was hooked up like that. kind of mind boggeling at first. Well actually still got it hooked up wrong with the schematic in that I got the 1 meg ground to grid going to jack #2 from the #1 Jack switch term(instead of the hot term). Don't know exactly what that will do but when I plug into each jack separately--seems to be working fine that way. Platefire
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Look at page 6 of this pdf...
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf
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Thanks Sluckey, I will study it. I did go back and try to re-wire it correctly but it only got worst and started the high gain thing again. Best I can tell I had it wired up correct this last time and it still wasn't right. I am going to go ahead and replace the other jack also so I can be sure it's not a faulty jack. Another thing I can't understand is checking grid Ohms through the hi jack it's not dividing the Ohms down to 33K to the tube like it should be, still reading 68K. How can something so simple turn in such a monster? Got to let it go for now. Platefire
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Has anybody got any ideas in the hookup of inputs jacks would cause your gain to be out of control to where by the time you turn up to 2 or 3 volume the amp completely distorted. I never encountered this problem before and it's kind of strange to me as related to input jacks. Will losing ground connection cause this or what is happening here????
I keep wondering if this is related to my input jack installation or something else, but it didn't start until I installed this new #1 jack on the reverb/vibroto channel. I would like to understand, if possible, what's causing this gain problem before I install a new #2 jack because if I install that and it's hooked up like the schematic shows--and its still doing that---I don't know. Very frustrating. Platefire
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I suspect the jacks are not wired correctly.
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Hope it's simple as that. I will rewire in a new #2 jack and see what happens. What it appears to me #1 jack is wired like any standard fender jack, 1 meg hot to ground and ground to switch. Jack #2 had jack #1 hot wired to 2's switch. Both jack #1 & #2 hot is wired to grid through their own 68K resistors to tube. If I got it wrong please let me know. Platefire
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Installed a new #2 jack hooked up like the schematic. Still run away gain! I went over and over my hookup of both jacks and could find no problem. I began to go over the lead dress several time separating wires because I had decided the problem wasn't in the jacks. After going over the lead dress re-arranging wiring for about the third time---bingo--run away gain gone! I still don't know what was causing it but for right now it's gone and amp is performing right.
Next thing is to install new roach, but not tonight. I've had enough for now. Platefire
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Installed the new vib roach this afternoon. The vib seems to be working strong without any ticking. Jammed through it the better part of an hour and it seems to be running well/sounding good. Guess the next thing I'll do for it is build a back-plate. Platefire
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OK, This is pretty old, a little over a year. This is the 1974 Fender Pro Head I picked up for $20.00 in pieces. The cab is not stock/normal. From what I can tell someone cut down a combo or another head and made it into this head you see but the chassis and parts dates to 1974. I can tell the cab was cut down from something else because of the uneven carpentry done in the re-assembly. I still haven't identified what model cab the front black grill and badge came from???
If anyone can ID it, please let me know. I would like to know what cab they made this from.
The reason I'm posting is I finally got around to building a back-plate for it. Didn't have enough black tolex to cover it, so I used imitation leather/vinyl material from wal mart and glued it down with contact cement. I call it my Snake Head Pro because of the snake tolex. To reiterate previous info of this thread, it was modified to 1965 Blackface specs and sounds great. So these are the finish pictures including the new back-plate I made for it. So as Paul Harvey(remember him) use to say "This is the rest of the Story" Platefire
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I love it when a job comes together, well done Plate. :icon_biggrin:
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Thanks! Ever since I got into tube amps 15 years ago I have been looking for a vintage Fender amp without having to get it off E-bay. Well 1974 is not very old vintage compared to 50s & 60s and it's a long ways from being original stock but represents some sort of break through in my original vintage Fender quest. Platefire
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Cool amp and 74 is vintage, the problem is we are too so 74 does not sound vintage. :laugh:
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Yeah, the 70's just seems like yesterday to me. What year would be latest vintage now, 90's??
BTW: can you tell that one of the openings in the back-plate is 1/2" longer than the other one? It was a mistake in my layout and I only discovered it after I had already cut it out. It went in anyway!
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Don't know if I have any bragging rights around here, so I won't go for that. How about happy accident!?!
Anyway this Snake Head Pro Reverb I've been playing over lately and it was sounding ok but the reverb
was getting a little noisy cause when I turned the reverb knob down the noise would disappear.
So I replaced the reverb driver and recovery tubes---problem solved, a lot quieter!
But wait a minuet whoa---the thing sounds suddenly awesome and the response is very flexable even at low volumes---currently playing at 1 1/2. Even the tremolo seems to even stepped up a notch--which it needed to because I needed a little more intensity.
So I'm a little flabbergasted that just the reverb tubes replacement had given it the added oompth to take it from OK to Wow! I'll sure pay closer attention to the health of those reverb tubes from now on as they seem to effect more than I knew. :dontknow: Platefire