Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: davidwpack on May 27, 2019, 06:59:20 pm
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I've had this little fella in the closet for a while and think I might try to do something with it. Anyone recommend something I can build in a 10.5"x3"x3" chassis? Pentode preamp, se, and tube rectifier. I really dig the box and want to build something in there. I've got 2 vibro champs so I'd like to deviate from that direction. Here's pictures.
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2 more.
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plexi pre with a SE 6V6 very abused spec's :icon_biggrin:
head only, or with speaker?
head only; same pre, SE EL34 sweating
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I'm thinking I'd like to use a speaker in there. Probably not that speaker I'm thinking.
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I've had this little fella in the closet for a while and think I might try to do something with it. Anyone recommend something I can build in a 10.5"x3"x3" chassis? Pentode preamp, se, and tube rectifier. I really dig the box and want to build something in there. I've got 2 vibro champs so I'd like to deviate from that direction. Here's pictures.
What's wrong with it the way it is? Seriously, with only one preamp socket, most practical circuits are going to be similar to either a Champ (two triodes), or what you have now (a single pentode.) I suppose you could use a 7199 or similar, but that just brings me back to the question: what's wrong with it the way it is?
One thing that is wrong with it is that it runs straight off the mains so has no PT. But, anything you do with this cab and chassis will require the addition of a PT no matter whether keeping the rest of the circuit untouched, modifying, or ripping it all up re-doing.
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Yeah that why. I intend on adding a PT. I don't have a schematic and have no idea what the tubes are and I'm pretty sure it's probably an electrocution hazard. I've debated restoring it as original for years but...
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I don't have a schematic
so, you're qualified now, create one, should take you a nice relaxed evening V1pin1 to .....pin2 to...... :icon_biggrin:
I would do it even if I decided to gut n build
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What about using a 6BM8 tube which has a triode and pentode in one tube. The 6BM8 pentode sounds different then a 6V6, IMO. This schematic is just an idea and is NOT a proven design. I have no idea what your PT is, so take this as a draft concept where you'll have to work out the PT and B+ rail.
You said you wanted something that was not VibroChamp-ish. :dontknow:
with respect, Tubenit
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Yeah, I'm going to draw something up when I figure out what to build. As far as the existing amp goes, I wasn't able to figure out what the tube compliment was so I just never drew one up.
That looks really interesting Tubenit. I'll definitely consider that. Also, I'm wondering why 75k is printed on the speaker?
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That may be part of the Magnatone family. Could be worth some money. :dontknow:
If you want something simple that's not just another champ. look at this. Simple schematic at bottom of page...
http://sluckeyamps.com/smoky/smoky.htm
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Yeah, I really like that Sluckey. I may use a tube rectifier since I have a hole for one...or I may just bend a new piece of metal for it and Just pull this old one out and keep it as it is. It's just a two-sided L-shaped piece of metal. I should be able to bend a new one pretty easily. Thanks! I might build that one.
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It's got a 50L6. That's a fine power amp. Small for stage but ample for bedroom or recording.
Tuck an isolation transformer high in the cabinet. Restore what's there, it isn't much. I bet 95%+ of the parts are still good. I'd even keep the 59-cent OT and 89-cent speaker for now.
The general plan will be the early pentode-preamp Champ (and many others like my Kent) except working on 100VDC.
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Only one thing, in SE amp you really don't need a tube rectifier, if you want you can do without
Franco
BTW you say nothing about the original schematic
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That may be part of the Magnatone family. Could be worth some money. :dontknow:
Wondering IF it might be the Magnatone 107? Or similar to it?
With respect, Tubenit
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Thanks PRR. I think that's pretty close.
Kagliostro, there is no original schematic. The only writing on the amp reads Don Noble.
All the tubes in there are the exact same size including the preamp tube. (About the size of a 6V6.)
If I can restore a safe version of the original I would love to do that beginning with PRRs suggestion of an isolation transformer. Thanks guys!
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I believe the Magnatone connection is right. Don Noble amps were contracted to Magna and maybe Danelectro, but that chassis sure looks like Magna. They became part of the Strum and Drum distribution empire of lower priced instruments. You have all Octal sockets, so if it were me, I'd keep it that way, using a preamp design from that period, of course with a PT or an isolation T. 6SL7; 6SC7; 6SN7 or Pentode 6SJ7 are all possible choices for a preamp.
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Yeah, that pretty much exactly what I was thinking with the 6SN7. There's hardly any info on Don Noble amps but I think you guys are right about the Magnatone connection. One guy was saying that they were made by Magnatone, Danelectro, and Ampeg.
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Maybe this will help
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YES! Thank you! I was about to ask how to hook up an isolation transformer. I'll try to go through the amp this week and verify component values. Thanks again!
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The guy on this link
https://www.worthpoint.com/worthopedia/mots-vintage-noble-tube-amplifier-176981826 (https://www.worthpoint.com/worthopedia/mots-vintage-noble-tube-amplifier-176981826)
say he is not sure if the tubes are as per the original schematic
but on his amp there are 35Z3 - 12SL7 - 50L6
so seems that preamp use a triode
EDIT: Compared, the two circuit are very close, also if in the photo of the Noble of the other guy I see a cap between preamp tube and power tube that I can't see in your amp
Your's
(https://i.imgur.com/CASoaNz.jpg)
Other's
(https://i.imgur.com/IzaZQ2c.jpg)
Franco
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Which cap? I'll look tomorrow when I get home from work tomorrow and compare. I'll try to get a better lighted picture too. If there were any mods done they are very old mods. Everything looks original to me. Thanks for the info!
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Which cap?
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I think your amp is stock, the other may be "revised"
If you take close photo for the whole circuit and for each tube we can draw a schematic
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No prints on tubes ?
Franco
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No all the writing has faded off the glass. If I remember correctly I believe one may have had a serial or some kind of batch number on the bottom but nothing indicating the tube types.
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all the writing has faded off the glass
a good spot light and "fog" the tube, sometimes revels hints of what it mighta been
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My guess is that it's close to the Magnatone 107 as suggested by Tubenit. That drawing is missing a grid leak resistor on the input tube. It also looks like the input tube is grid leak biased, unlike the schematic. I've made those changes, and added pin numbers to the drawing just to get you started. Now it's up to you to do the leg work and verify and correct the schematic to match your amp.
You can find a table of resistor color codes on the web. Tube datasheets, if you want them, are at https://frank.pocnet.net/ (https://frank.pocnet.net/).
We could try to draw a schematic from photos, but I think you should do it yourself. It'll be a good learning experience for you. :icon_biggrin:
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I tried to draw what I can see (assuming the tubes are 12SL7-50L6-35Z3), but is very difficult with the actual photos
(https://i.imgur.com/f4dc5AC.jpg)
May be Tony Bones got it
Franco
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Yeah I intend on drawing one up. I just never did because I didn't think it would matter not knowing what the tube types were. Now that I have some idea I'll get one drawn up. Thanks!
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Drawing the circuit may very well show you the tube types. It's a simple amp. Shouldn't be too challenging with the amp in front of you. I would start by assuming the 'other' amp tube types are correct, except for the rectifier and that's no big deal.
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If you draw a schematic, please share it
Thanks
Franco
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I will. I'll try to get a start on it when I get home this morning. Thanks for the help and picture Kagliostro!
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I followed Shooters advice and fogged them up and found writing on the tubes. They are: 12SL7, 50L6, and 35Z5. I'll try to get a schematic done this week. Thanks everybody.
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:thumbsup:
Franco
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Note: the filaments of the 3 tubes are probably wired in series: 12 + 35 + 50 = 97 VAC. There are a number of small vintage amps like this with similar circuits overall. Check where your amp's filament supply originates, because this varies. E.g., in the Alamo Capri there is a 98V filament supply on the PT secondary.
But on the Lectrolab schematic I posted, the filament supply is sourced from the AC wall supply, probably standard 110 VAC at the time. So a series resistor of proper W rating was often used to drop 110 VAC to 97 VAC, under load (i.e., with tubes IN). Today's wall supply is typically about 123 VAC, so you may need a larger value series resistor for a larger voltage drop from 123 to 97.
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I think that large resistor Is connected to the rectifier socket in your amp
You can see It on the pictures
Franco
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Yes it is. Also that cap that you couldn't see is there. It was just bent over. I'm getting a little of it hand-drawn. I'll get it transferred over with the J Schem when I get it completed. So far it's checking out to be pretty much what Toney Bones posted. I tested one resistor that read 56k but should be 4.7M according to the color code but I am testing them in-circuit so it's probably off. The printing on the caps has long since worn off, if I have to I'll disconnect them and use a capacitor checker. Might be a few days before I get it completed because of my work schedule but I will get it finished and will post it here when I get it sorted out.
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I don't know man. I'm slowly drawing stuff out but I'm having to draw pin numbers on the schematic that aren't normally included. For instance, a 4.7k resistor connects pin 6 to pin 8 on the 12SL7 (cathode to heater) and I'm not sure how to do that on a schematic. On the 50L6 pin 1 is connected to pin 8 via a __ resistor. Pin 8 of the 12SL7 is also connected to the on switch. Here's what I drew so far. I'll get back on it my next day off.
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...in SE amp you really don't need a tube rectifier...
I think you already knew this: *Here* the several tube heaters are in series across the 110V wall voltage. A 5 tube radio (https://en.wikipedia.org/wiki/All_American_Five) heater string adds-up near 110V. This amplifier omits two tubes so will have a power resistor for the ~~13V excess. (Actually 5-tube radios aimed at 121V, two more 12V heaters.) Omitting the rectifier makes an awkward 12V+50V heater string.
It would be reasonable to re-wire to parallel heaters and get a 12L6. They are not so common, but tubedepot has 'em for $4.
However a 2-tube amplifier just looks bare inside. Keep the rectifier for "cheer".
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Oh ok. I wouldn't have thought of that ...but yeah I plan on keeping it in there.
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> resistor connects pin 6 to pin 8 on the 12SL7 (cathode to heater) and I'm not sure how to do that on a schematic.
It may go to heater pin, but sure *both* go to "ground". Actually "switched power return".
Yes, the super-simple construction is confusing, but there are few variations possible. The plans posted by Tony and others WILL be 98% same as yours. Print one of those, then trace your amp against it, noting different values (occasionally missing/added parts).
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Will do. Thanks for the tips PRR!
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Ciao PRR
...in SE amp you really don't need a tube rectifier...
I was saying that only because I was thinking he want to gut the original circuit and build a different circuit
in that eventuality the choice for a PT will not be obliged to something that can supply also a Tube Rectifier
I like much more the idea of a refurbishment of the original
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Those old resistor can easily have drifted, measure them one by one and replace if necessary
Franco
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better pics? more light, in focus, & closer shots. :icon_biggrin:
--pete
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I'll get some new pics tomorrow and try to get more drawn up when I get home.
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Ey! Here's pics for those that wanted them. I'm trying to do this one by meself so hold off on posting yer versions for a bit if you would...although I think it's going to be remarkably similar to the one Tubenit and Toney Bones posted. I have more off-time next week so I should have something by then.
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Check out the power supply upgrade!
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Here's what I've done. Don't know if it's right but I believe all the parts are there. :icon_biggrin: Like I said I put extra pin numbers that aren't normally included on schematics so I guess it's kinda like a layout-schematic. A schematical layout? Anyways, if no one sees anything terribly wrong I'll draw it up on the computer my next day off.
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You're definitely getting there!
I didn't go over the whole drawing with a red pen or anything, but a few things jump out at me. One, I can't tell B+ from GND. Lines are crossing each other, but I can't tell which are supposed to be connected electrically and which aren't. You might have gotten yourself mixed up too. :icon_biggrin:
I think that the wire between 12SL7 p8 and 50L6 p1 should be GND.
There's a grid leak resistor missing on the 50L6 (p5 to GND.)
Maybe other errors too, but get that straightened out and we'll go from there. :icon_biggrin:
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Ok. Thanks for looking! Probably be my next day off but I'll get it. Thanks!
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I think this may be a Dickerson, made by Magnatone circa 1948, per this site: http://magnatoneamps.com/dickerson.html (http://magnatoneamps.com/dickerson.html)
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Just took a quick look...pin 5 isn't connected to ground. I did miss a resistor on the 50L6 socket that connects pin 1 to pin 5. So the only 2 things connected to pin 5 are the .02? cap and the 470k resistor from pin1 to pin 5.
Edit: pin 8 of the 12SL7 is directly connected to pin 1 of the 50L6 via a black wire.
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Added some humps and added the resistor to the socket.
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This is what I see...
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I think Sluckey's right. I was trying to figure out to express it when he posted a great diagram.
Note that pin 1 on the 50L6 is not connected to anything in the tube, but it is used as a tie point for the signal common a.k.a. floating ground. Pin 8 on the 12SL7 is the last heater pin in the chain and is connected to the floating ground.
I can see in the first photo you posted that there is a 4.7Meg grid leak resistor on the input stage. Is there really a 4.7K cathode resistor too? Seems like both would provide too much bias...
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Yeah. Those numbers are right. Looks like yellow, violet, red unless the colors have faded. Sluckey, the pin numbers need rearranged on the 12sl7 on yours. Also there's a cap to ground on the power switch. I found a mistake in mine. I put a jump where there should have been a connection but I think it still works if I fix that. Or am I way off? This amp/tubes are weird.
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Like this. I know it's a weird approach but is it remotely right or just a mess? :icon_biggrin: Just trying to figure out if I grasped anything with this amp.
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You have two B+ and two "grounds", mixed-up. (I know how it is.)
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Yeah. Those numbers are right. Looks like yellow, violet, red unless the colors have faded. Sluckey, the pin numbers need rearranged on the 12sl7 on yours. Also there's a cap to ground on the power switch.
Ha. I thought I fixed those pin numbers before doing anything. They're fixed now. The cap has always been there.
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Thanks guys!!! I guess drawing up a schematic for tubes that I'm not familiar with must require drawing a schematic I'm not familiar with. Shoulda went with what I already knew. Makes much more sense now. Thank all you guys for your help!!!
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i see same as slucky except i see 2x 47K grid stoppers for V1a.
see attached.
--pete
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Ciao Pete
I see someting strange
R3 - R11 connection
or other way, R3 is also connected to ground :w2:
(https://i.imgur.com/kZFQiaO.jpg)
Franco
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Ciao Pete
I see someting strange
R3 - R11 connection
or other way, R3 is also connected to ground :w2:
Franco
Franco, i believe that it is drawn correctly. :icon_biggrin:
--pete
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I like your drawing better Pete.
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I like your drawing better Pete.
thank you.
--pete
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I got 47k too. Looks like yellow violet orange.
Edit...Sluckey you have 50sl7 for the power tube.
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If the current draw is .15A per tube (=.5A), I should add probly a 1A fuse...and a 110VA or better isolation transformer? (I'm going to build a bucking transformer). Is that about right? Thanks!
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If the current draw is .15A per tube (=.5A), I should add probly a 1A fuse...and a 110VA or better isolation transformer? (I'm going to build a bucking transformer). Is that about right? Thanks!
The filament current is .15A for each tube, but since the tubes are wired in series the total filament is still only .15A. Isolation transformer is a good idea. If you want a fuse put it on the primary of the Iso tranny. You don't need a bucking transformer. If you are concerned about the very slight increase in filament current due to todays higher wall voltage, just increase the size of that big series resistor. If that resistor is really 150Ω it's already the correct size for 120VAC. I wouldn't bother with it.
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Oh ok. So the 35VA transformer from tubes and more should be fine then it looks like. Thanks again.
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... I belive that It Is drawn correctly
Oh, I too, that kind of PS Is really confusing
Franco
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> someting strange ... R3 - R11 connection ... that kind of PS Is really confusing
Draw it out "conventional". DC power flows top down.
As an amplifier: instead of ~~110V of DC supply, this stage feels more like 60V total supply. But it is the 1st stage, signal level is low, this is OK (for a *cheap* amp). The 0.1uFd cap keeps the V1A cathode "grounded" for audio frequencies. The stage will work as an audio amplifier.
Why would we do such a terrible thing to an audio amplifier stage?? Well, this transformerless power supply can be plugged-in either way. If the 120V wall outlet is 120VAC and "zero", with pre-1960s plugs you had a 50:50 chance that B- would be 120VAC. This might hum, and you might flip the plug for less hum. However the "Neutral" wire is never dead-zero. And there are some conditions where it has a substantial voltage to dirt-ground.
Look at this user. The bunny is barefoot, on concrete, leaning on a steam radiator. These are all good grounds, or at least better conductors than a living body. And this body is holding bare-metal connected to the amplifier and its uncertain connection to lethal voltages.
This particular amplifier seems to use a trick that was legal for a few years. If you work it through, the "only" path from user to wall-outlet is through R11 C3. As drawn, this is not a lethal shock. 4.3mA is less than 5mA and 5mA is still the standard for anti-shock GFI cut-outs.
But parts fail, and the cheap capacitor often fails SHORT. (Likewise, heater-cathode insulation can short, and a later standard apparently required this first heater be isolated.)
And the amplifier is less good than it could be. And the 0.1uFd is not a great ground for hum.
1920s-1940s automobiles had no seatbelts or dual tail-lights. Most of the old cars I see fixed-up and run on the road have seatbelts and full tail-lights now. This is basic save-your-ass thinking. Likewise these old shock-hazard amplifiers *MUST* be re-rigged with isolation transformers... this isn't 1959 anymore, and you are not broke young parents trying to encourage your child with music while also feeding the kid.
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Many Thanks PRR
Now all is way clearer
So this circuit tried to supply to the intrinsec defect of such kind of Power Supply
The improvement due to the use of a standard Power Transormer will diminuish the electrocution hazards, and, I suppose, also the noise added to the signal
In this case, will an isolation transformer solve both problems or it will be only an insurance against electrocution without effects on the noise ?
Thanks
Franco
BTW: Steam radiator is common in your country ? We have and had hot water heated radiators, I spent 24 years working at wholesale of hydraulic, sanitary and heating equipment and never hear about steam radiators in civil houses
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I was reading this site and this guy mentions adding a diode for full-wave. I wasn't sure if it was advisable.
https://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/ (https://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/)
Edit: not diode. SS bridge?
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Hogwash! His whole premise for adding the FWB is based on the fact that the rectifier tube is only 1/2 wave and he says" a transformer is designed for full wave operation, blah, blah, blah. He totally missed the fact that the series connected filament string is still operating at full wave and draws .15A @ 120V and that is an 18 watt load on the transformer. This is a much greater load than the puny halfwave rectifier tube has to supply.
IMO, it's a total waste.
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Ok. I figured you guys would have mentioned that if it were true. Thanks!
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I agree with Steve talking about the consumption related problems
but ....... looking the whole thing from one other side
Will the use of a FWB (also to feed heaters) give as result less noise and hum ??
Franco
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less noise and hum ??
the series connected filament string is still operating at full wave
Without a "re-design" it is what it is
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Oi! I've got my parts and got it together finally. Sounds really good but not nearly loud as it should be. By comparison, cranked, it's as loud as my vibro champ on 4. The on/volume is scratchy and spotty as hell though. I'll get it cleaned up and see what happens. Here are some pictures for you. I used a pedal enclosure as a junction/switch/fuse box. Mounted them with Velcro and a couple screws to the back of the rear panel. Thought it'd hum but not very much at all. Thanks for everyone's help!
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:thumbsup:
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Nice, way to stick with it!
not nearly loud as it should be
I'm thinking based on the schematics above, 4 on a vibro-champ is about all you're gonna get without some Mods
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Oh ok. I was thinking it would be at least as loud. I mixed up some tube data. I see now that it's about 4 watts for the 50L6.
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so you have a 5F1-2 fenderish, put volume at 10, Marshall 4X12 cab and jam :icon_biggrin:
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> about 4 watts for the 50L6.
Not with 110V power. More like 2W fully optimized. Less with lowest-price parts like a cheap amplifier would use.
Fender never made anything this small.
It can still annoy the neighbors.
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> about 4 watts for the 50L6.
Not with 110V power. More like 2W fully optimized. Less with lowest-price parts like a cheap amplifier would use.
Fender never made anything this small.
It can still annoy the neighbors.
aha! amp build competition: who can build an amp with the most power using cheap tubes for lowest cost. :icon_biggrin: tubes must be available on fleabay or from the NOS houses; e.g., tubes and more, etc.. you must use an output transformer; not a power transformer for the output transformer. surplus Ok. $200 limit.
--pete
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DIYaudio.com $100 amp competition
https://www.diyaudio.com/forums/instruments-and-amps/190738-hundred-buck-amp-challenge.html
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:l2:\
now you start, I already built at least 5, they all got re-purposed to something useful :icon_biggrin:
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DIYaudio.com $100 amp competition
https://www.diyaudio.com/forums/instruments-and-amps/190738-hundred-buck-amp-challenge.html (https://www.diyaudio.com/forums/instruments-and-amps/190738-hundred-buck-amp-challenge.html)
IMO, 200bux is more realistic and would likely increase probability of results that would be worth keeping and actually sound worth a crap.
you'd be ahead if you fab your own chassis or use the cheap hammond project boxes. scrap toaster ovens work. :icon_biggrin: old radio chassis' too.
--pete
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:l2: \
now you start, I already built at least 5, they all got re-purposed to something useful :icon_biggrin:
wuss! you've got one more shit-build in ya! probably already got the parts pulled. :icon_biggrin:
--pete
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wuss!
:laugh:
I'm playing in the sand(amp)box quit picking on me :new1:
I do have a nice preamp TUBE board all wired n waiting, but I spent 1/2 my allotted funds, or all if I limit to $100, does it count if they've already been expensed off the books?
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wuss!
:laugh:
I'm playing in the sand(amp)box quit picking on me :new1:
I do have a nice preamp TUBE board all wired n waiting, but I spent 1/2 my allotted funds, or all if I limit to $100, does it count if they've already been expensed off the books?
cook the books however you like... uncle sammy may no likey tho. :m5
--pete