Found a amp project today comments welcome.

[jh45gun]

My Plan is to gut this chassis (Zenith 5Z21) except the transformers and tubes and build a fender amp clone from scratch. Amp came out of a old Zenith Phonograph a mono one. Chassis has these tubes 2 6v6's a 6J5 a 12ax7 and a 5y3GT. From what I remember the 6v6's are the power tubes the 5y3 a rectifer and the 12ax7 a preamp tube. Any one know what the 6J5 was used for? Tubes are all Zenith so all orgional. Speakers were two Zenith 12's and two tweeters. It cranked pretty good for a old record player we played a record on it and it sounded clean. Gonna drain the power caps for safety reasons and this will be my winter project. Any one know how to get the tube layout chart off the cabinet with out wrecking it??? Been a while since I messed with any of this stuff, but years ago I took a Electronics repair course ( TV/radio what have you ect back in the 70's when tube stuff was still around so I have had some expirience just have not used it lately. I know my way around a pencil soldering iron still have a digital meter and with a chart handy can still remember how to read a resistor. Still its been a while may have to ask a question now and then. Any suggestions for Fender models with these tubes. I am thinking a Princeton copy maybe. Jim

[jh45gun]

Here is a schematic of the chassis I have:

http://oldradio.ca:83/Radio/HF17R/HF17R.GIF

[hasserl]

I would consider modifying the circuit the amp has first, it looks like it would make a pretty cool guitar amp, rather than gutting it and starting over. I would take out everything on the input side of the 6J5 and give it a single Fender style input, go with a 34K grid resistor right at the tube socket, and use 1M grid to ground. Move the Volume control to the other side of the 6J5. Then I would change the pi to a Fender style cathodyne pi as used in the Princeton and tweed Deluxe. Then clean up that tweeter circuit on the power tubes primary side of the OT, just get rid of all of it and connect the plates directly to the OT primary leads. Add three prong cord if it doesn't have one, and replace the filter caps.

If you don't like the outcome, then think about starting over.

I'll help you redraw a schematic if you want. I enjoy doing these kinds of projects.

[donnyjaguar]

As Hassler mentions there is some definite potential here. The limiting factors are the size of the power and output transformers. In much consumer-grade electronics from the 1950's these were cost-reduced to the point of being pathetic. Hassler's suggestions are all good. I'd try just plugging your instrument in to see what happens first. There's plenty of circuit trickery on the selector switch which will effect the tone. I can explain all this if interested.

[jh45gun]

My amp tech agrees with you Hasserl and DonnyJaguar. He said: It would be best to modify what I have and tweak the tone circuits ect first said I may be supprised with what I have already in place with some simple mods instead of gutting the whole amp. So he agrees with you. I want to do this my self instead if giving it to him to fix for me as that is why I bought this for a project. It sounded good (clean and no hum) playing a record so he suggested make the mods and leave the power caps alone for now until I play it then if it sounds like it needs them to replace them. What do you guys think about that? Hasserl I would appreciate the help with a new schematic and redesigning the circuits as I am a rookie at this though years ago I did take a vocational class in electronics but I never really followed up on it just simple stuff for my self.

For more info looking at the plate I took off tbe phono that the knobs fit through I have Loudness (Volume) and a switch that says record compensator that has 78 RIAA? LP FOR? Presence control Treble and Bass. Volume, Bass and Treble must be Cut as they are - on the left side and normal in the middle and plus on the right side.
Donny any info you have about using the selector switch and other ideas I would like to hear too. I appreciate the help you guys offered. Would it be best to drain the power caps to work on this?

[jh45gun]

Some pictures if that helps any.

http://img.photobucket.com/albums/v454/jh45gun/dcp_0090.jpg



http://img.photobucket.com/albums/v454/jh45gun/dcp_0091.jpg


http://img.photobucket.com/albums/v454/jh45gun/dcp_0092.jpg

http://img.photobucket.com/albums/v454/jh45gun/dcp_0093.jpg

[jh45gun]

I do not know if these numbers on the transformers will help ya any but they are for the power transformer it says: 95-1410 117 Volts 50-60 cycles 138739 Output Transformer: 95-1489 343714 at least I think that is a 3 in the middle if not it is a 8 but it looked more like a 3 on the output tranny.

[jh45gun]

Found this in a article what do they mean by RIAA? I did see this on my record selector switch? "One thing that won't work too well is to use an old phonograph (record player) amp as is. The problem here is that vinyl phonograph records were recorded with special tone compensation and the playback circuits have to undo this. This "playback curve" (defined by the RIAA) will change an input signal drastically, especially in the treble arena. You might like it, but most likely you would despise it. So if you want to use one of these as a guitar amp, expect to modify at least the RIAA circuit."

[BBQLS1]

I would have no problem with gutting that and building something else. The parts in the pics don't give me a warm and fuzzy. Good luck!

[jh45gun]

BBQLS1 so you talking about the caps?

[jh45gun]

Hasserl when you get the time I would like to see what you come up with for a modded schematic for this project I would really appreciate your help. Been a long while since I messed with anything like this. Jim

[vibroverbus]

I would consider modifying the circuit the amp has first, it looks like it would make a pretty cool guitar amp, rather than gutting it and starting over. I would take out everything on the input side of the 6J5 and give it a single Fender style input, go with a 34K grid resistor right at the tube socket, and use 1M grid to ground. Move the Volume control to the other side of the 6J5. Then I would change the pi to a Fender style cathodyne pi as used in the Princeton and tweed Deluxe. Then clean up that tweeter circuit on the power tubes primary side of the OT, just get rid of all of it and connect the plates directly to the OT primary leads. Add three prong cord if it doesn't have one, and replace the filter caps.

If you don't like the outcome, then think about starting over.

I'll help you redraw a schematic if you want. I enjoy doing these kinds of projects.

+1 to everything

[vibroverbus]

Found this in a article what do they mean by RIAA? I did see this on my record selector switch? "One thing that won't work too well is to use an old phonograph (record player) amp as is. The problem here is that vinyl phonograph records were recorded with special tone compensation and the playback circuits have to undo this. This "playback curve" (defined by the RIAA) will change an input signal drastically, especially in the treble arena. You might like it, but most likely you would despise it. So if you want to use one of these as a guitar amp, expect to modify at least the RIAA circuit."

ahh the history of consumer music distribution...
due to the limitations of record making technology, where high amplitude low frequencies can be a problem, and high frequency response drops off at the top of the audio spectrum, clever engineers figured out early that they could create a work around by establishing equalization standards. when pressing the master, the equipment would attenuate lows and boost highs by very specific amounts. then to get the signal back, all playback equipment was produced with the same equalization in reverse - lows & highs would be adjusted in reverse by the same pre-established formula. this way even though the technology couldn't record high level signals at the ends of the spectrum, it could be 'simulated' for the listener with varying degrees of accuracy. this is why if you ever plug a turntable into a regular line-in, aside from low levels, it will never sound right - it needs the EQ preamp that is built into a 'real phono input' and otherwise will thin and crappy. i always wondered why the industry decided this should be built into the amps and not into the turntables though...

I'm sure somebody else is a bigger expert on this than me, but I believe there were also a lot of different standards for EQ until it was standardized by none other than...

RIAA... Recording Industry Association of America... the same folks who...

fight tooth and nail to keep you from ripping your new copy of Kevin Federline's CD onto your iPod...

[jh45gun]

Thanks for the explanation I did not know what that meant and it was on my selector switch that said 78 / RIAA / LP ect

[jh45gun]

It just dawned on me Hasserl that you were talking about Phase Inverter when you said PI. So never mind explaining.

[hasserl]

Hasserl when you get the time I would like to see what you come up with for a modded schematic for this project I would really appreciate your help. Been a long while since I messed with anything like this. Jim

Okay, just give me a little time. I just checked back in and saw this.

[jh45gun]

Hey No rush I really appreciate the help. When ever you can spare the time like I said I appreciate the help and realize folks are busy in this day and age. Gonna work on a couple of 2x12 guitar cabs tomorrow. Got a friend who has a wood working shop in his garage.

[donnyjaguar]

I like it! I see the RIAA question was addressed so I won't re-visit that as it isn't relevant to MI (musical instrument, not mentally ill :) ) amplification.

There's no retainer rings on those tubes so you're not gonna want to mount it upside down. It certainly is a good starting point for a mini guitar head though.

I can whip up a schematic with some ideas too, but when working with some unknowns it won't be a simple matter of connecting the dots and turning it on (at least I'm not that good!) but we can get it pretty close and to the point where minor tweaking will bring it in-line. I'll re-use as many parts as possible but you'll have a bunch left over you won't need. My OrCAD computer is kaput right now so I'll have to do something free-hand and scan in. Let me know if you want warm and fuzzy or optimized.

DJ

[donnyjaguar]

Just a quick update. I started putting my thoughts down on paper but not sure when I'll have time to finish it off. Here's the approach I've taken.

Remove the vacuum tube rectifier and use 1N4007 diodes.
Add mains & standby switches
Bypass cathode resistor of outtput tubes with 47uF/35V
Use 6J7 as the first voltage ampliier
Make DC voltage doubler out of 5V rectifier filament winding and use dropping resistor to power 6J7 (this will provide DC for the first filament where its most critical)
Add 10k pot to balance hum on 6.3V filament winding, remove ground connection.
Add 3rd prong for chassis ground
Put 6SN7 in socket where 5Y3 was, 1st half of which used immediately after 6J7.
Add treb/mid/bass Fender style tone stack driving 2nd half of 6SN7.
(Use 1M shunted with 330k for Bass;500k shunted with 500k for Treble; 50k shunted with 15k for Mid and you can re-use all your controls :) )
Volme (2M) either before or after this 6SN7 (experiment)
12AX7 configured as conventional paraphase splitter driving output tubes
NFB resistor value to suit.
Reconfigured secondary of output transformer as per more conventional designs grounding Yellow lead.

I'm guessing this amplifier could be re-worked for less than 20$ in new parts - especially if you're like me and have lots of spares laying around.

I'd pull the selector out of the unit and put the Standby switch in its place. The power/mains switch can go on the back.

The power supply needs to be engineered to serve up the right voltages on the amplifier and this is going to chew up the most time for me. You can tell by the original design that the amplifier is barely able to fully drive the output section. Now the amplifier will be more powerful and it *should* have enough grunt to overdrive the output section.

DJ

[jh45gun]

I do not have any spare parts laying around so I will have to buy them which is no big deal. Just a few questions why diodes over the tube is it more stable or is it because you want to use that tube socket for the other tube and this way you do not need to add another socket? So you take out the tube and make a bridge rectifier right. One more thing to add for convenience would be a pilot light and I would not mind a pre amp out though I suppose we could accomplish that with a capacitor and a resistor and a jack across the speaker too if need be.

[hasserl]

Well, I'm a firm believe in the KISS principle. It has served me well over the years and I've learned to trust it. With that in mind I reworked teh schematic along the lines I mentioned in my first post.

Note I added a coupling cap afterthe first gain stage to isolate the volume control, that I moved there from the input, from DC voltage. I put it a .047 to match the .047 used in the Bass control circuit. This value could be played with to tune the low frequency roll off at this stage.

The Volume pot itself I left unmarked. You could reuse the existing volume pot, but at 2M it is fairly high value. You might want to instead try 1M pot there. It's up to you. It wouldn't hurt to try the 2M.

From there to the PI I left everything as is. I don't know how that Presence circuit is going to sound. But I would just try it and find out. It may be cool.

Then I gave it a Princeton Reverb style cathodyne PI circuit. I went with the .1uf input caps, but these could also be tweaked to fine tune the frequency response of the amp.

And finally, I did not add a cathode bypass cap at the power tube cathode resistor. You could add a 25uf electrolytic there to boost gain. Again, I'd try it as it is and see how it works, then start making changes to fine tune it to your preference.

Hope it helps:

http://img201.imageshack.us/img201/1905/hassezenithampschemvv6.jpg

[teleamp]

http://cgi.ebay.com/Hammond-Organ-AO-35-Tube-Reverb-Amp-DIY-Guitar-EL84_W0QQitemZ180072535544QQihZ008QQcategoryZ73371 QQrdZ1QQcmdZViewItem


Make your on Doctor fast VW or SuperAmp 17 (correct chassis for early S17).

[jh45gun]

Thanks for the Schematic Hasserl you did a excellent job on it. I will have to compare both schematics (the origional and yours) and make a parts list and go from there. About all I have here now is a power cord. LOL Got to have my friend make me a copy of the schematic my printer does not do it justice even after I cleaned the heads it is a bit fuzzy and hard to read the values. I have to go to the library tomorrow maybe I will pull it up there and have them print it. Again Thanks. When I get the parts and get things changed around will let you know how things are going.

DonnyJ I am looking for more of a clean sound than a over driven sound as my Silvertone 1472 will overdrive if I crank it. I am hopeing to get something that sounds a bit brighter than my Silvertone for a more Fenderish sound. Now I use my digitech PR80 for reverb and other effects on the Silvertone and use the Black face modeling part of the pedal to brighten up my Silvertone. The pedal makes the Silvertone sound good.

[vibroverbus]

Remove the vacuum tube rectifier and use 1N4007 diodes.


why not either put a switch in, or use a plug-in SS rectifier? I make my own from old dead octal tubes and 1N4007's... then again I'm a big fan of tube rectifier sag too...

of course that takes away the steal-the-socket option...

[vibroverbus]

Well, I'm a firm believe in the KISS principle. It has served me well over the years and I've learned to trust it. With that in mind I reworked teh schematic along the lines I mentioned in my first post.


I like it. I like the idea of keeping most of the existing stuff in there and tryin' it. The heck with just-another-clone-Fender/Marshall! The tone controls will be isolated so you won't get the classic FMV interactivity, which is a good thing. Leo actually did some of this in Brownfaces/Blonde amps were he'd distribute part of the tone stack to each gain stage.

Keeping the feedback loop run that far back is also more of a hifi oddity, my guess is that will end up being a bit dry for a guitar application but who knows? Run back to a pre-amp stage like that I'd expect the presence knob to be much more tone-impactful than a conventional PI feedback setup. Again I agree with the 'try it for now' approach.

[donnyjaguar]

Nice schematic. I'm just in the process of scanning mine in and will share also. Its more complicated than yours but the reason being is that I'm concerned that there won't be enough overall gain in the amplifier unless you add more amplifying stages. I've designed it so that it will give the option to pound the output stage pretty good and get that transformer saturated. :) More to come...

[jh45gun]

Donny J, Sounds Good since this project is an expirement the more options I have the better I appreciate your input as I do Hasserl's and Vibroverbus's I did have problems reading Hasserl's schematic when I copied it and I thought it was my printer but an other printer did the same so can you scan it in high resolution if that is not a problem?

[unklmickey]

Well, I'm a firm believe in the KISS principle. It has served me well over the years and I've learned to trust it.

always a good plan!

Hasserl,

i'd like to make a couple of comments, and i hope you'll see them in the spirit they are intended.....constructively.

the first one is minor. avoid using jpg for drawings whenever you can. it speckles them. instead, use gif.

the second one comment is about the PI.

that's probably a good choice for this application, but notice that the upper 6V6 was previously fed from the plate of the first half of the 12AX7.

the lower 6V6 was fed by the plate of the second half of the 12AX7.

with the change you drew, you have inverted the signals to the output.

not a huge issue, except that the NFB will now be "PFB".

i'll guess the simplest way to fix that, is to just switch the connections at the .1 uF caps.


other than that, i think it looks like a great plan.

cheers,

unk

[unklmickey]

why not either put a switch in, or use a plug-in SS rectifier?...

my vote (whatever THAT'S worth) would definitely be for switchable SS rectifier.

having the option of tube or SS, without yanking a tube in and out would be a huge benefit IMHO.

[jh45gun]

my vote (whatever THAT'S worth) would definitely be for switchable SS rectifier.

having the option of tube or SS, without yanking a tube in and out would be a huge benefit IMHO.

Ok since I have not messed with this kind of stuff for a while what would that benefit me?

[jh45gun]

Interesting observation about the Jpeg making it speckled so if I changed the file myself to a gif would it clear it up for me to print it or would Hasserl have to do that?

[jh45gun]

When I tried to save it again to recopy it, it only gave me the Jpeg or Bitmap options not gif.

[unklmickey]

hi jh45gun,

once jpg speckles a drawing, changing formats won't help. the damage is already done.



switching in a pair of diodes to bypass the tube rectifier (as Vibroverbus suggested), will make the power supply "stiffer" (reduce the sag).
the voltage will also be a bit higher.

given the choice of only one, i'll choose the tube rectifer, and the sag.

but if it's switchable, you can use whichever suits you, for the sound you are going for at the time.


cheers,

unk

[jh45gun]

Yea I tried to recopy it and I still cannot read it well. I can on the computer just cannot on a hard copy. I have heard others refer to the term sag and I do not know exactly what they mean and is it desireable in a tube amp?

[unklmickey]

i think it's extremely desireable, but there are some cases where it can muddy things up a bit.

you'll do well to read up about this. but in a nutshell, sag is a drop in the power supply voltage, at high volume output. it causes a compression effect. then as the signal weakens, the power supply recovers.


unk

[hasserl]

Okay, here it is in gif format. See if that works any better.

http://img215.imageshack.us/img215/8345/hassezenithampschemyu8.gif

[unklmickey]

PM sent.

[hasserl]

Okay, I went ahead and just emailed it to you. Also, unklmikey is probably correct. We should probably flip the output's from the pi to the power tubes.

[jh45gun]

Thanks Hasserl it still did not print well on my printer I will have my friend try it if not we will just have to read it off the computer with out making a hard copy. So what your saying is where the two caps that are .1 coming off the second half of the 12ax7 to crisscross them so after the the top .1cap the wire goes to the bottom 6v6 and the bottom wire from the .1 cap goes to the top 6v6. I apprecate the help and I did not want to ask you to redo the schematic as I did not want to be a pain. I appreciate the extra work. Jim

[jh45gun]

Just curious considering what I have here for tubes and other components what do you think this will wind up as wattage wise for the output? I am hopeing around 15 anyway.

[donnyjaguar]

I apologize for my rather rough drawing. Busy at work these days! :)

http://img69.imageshack.us/img69/3686/reworkedscanpa7.th.jpg (http://img69.imageshack.us/my.php?image=reworkedscanpa7.jpg)

You can see that I've used DC on the first VA stage. There could be enough voltage/current here to power the filament of the 6J5 tube also, but it shouldn't hum anyway. I've bypassed all the Kathode resistors to increase gain but it may not be necessary to have them all in there. I tried to use lots of the same value components and there's room for substitution on many. the 47k in the NFB loop is a guess as I don't know the ratio of the output transformer. The amplifier will be stable with no feedback at all so its a matter of choosing the appropriate value to bring it down to a reasonable gain. I resisted the temptation to run fixed bias, but that option is available with additional complexity in the circuit.

Have fun! Even if you don't use my circuit I think there are some good ideas in there that someone may benefit from.

Cheers!
D-J

[jh45gun]

Thanks Donny though the tube is a 6J5 so would that make a difference? For starters I am going to start with Hasserl's schematic as for a first time project I want to keep it simple. Still this is good to have posted as you never know, I may find an other one of these or may some time want to change this one. For now I think I better go the simple route since I have not messed with any thing like this since the 70's. I do sure appreciate all the help I have gotten here. Your a good group I appreciate it. I may have a question here and there after I get into this. Might be a week or two before I get started. Gonna have my friend here working on it with me I figure two heads are better than one to eliminate mistakes.

[jh45gun]

I am going to have to go to the library tomorrow if they are open and use their printer and see if it works I think my printer is getting low on ink. I just tried to print a copy and it looked pretty faded.

[jh45gun]

I am hopeing the mods make this sound more like a amp just plugging it in as is was pretty disapointing as you can read about in my other post. I expected a little more than what it was. Or will changing the circuits bring it to life?

[vibroverbus]

I am hopeing the mods make this sound more like a amp just plugging it in as is was pretty disapointing as you can read about in my other post. I expected a little more than what it was. Or will changing the circuits bring it to life?

as you've been told a few times, if you are trying to play through the stock phono EQ circuits, it will sound like crap. not going to get any better until you heat up the soldering iron... stop posting and start working!
:D

[jh45gun]

Well I did not figure it would sound that great just plugging it in but some one did suggest to try that so I did. Now I know and your right it sounded like crap. :D

[vibroverbus]

yeah... if it was an old PA amp it might not sound too bad 'bone stock' but phono EQ is bound to be terrible for guitar unless you have really weird taste in tone...

[jh45gun]

Just bumped it up since I made a updated post on the project in case some one wants to check out the schematics