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New amp project UPDATE - schematics and documentation found
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TresM (Sir)



Joined: 02 Aug 2009
Posts: 472
Location: South Carolina - Humidity Central

PostPosted: Mon Apr 26, 2010 6:56 pm    Post subject: New amp project UPDATE - schematics and documentation found Reply with quote

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=120561895455&Category=43375&_trkparms=algo%3DLVI%26its%3DI%26otn%3D2#ht_2152wt_958


Just bought this
So basically this is a hi-fi wired amp right now. It has one pre-amp stage, and to be an adequate guitar amp, it would need more of them (3-4). Is/are there any particular difficulties with adding gain stages if you plan on re-wiring the amp anyways? I think the B+ voltage has something to do with the output stage, but what are the requirements for preamp stages if there are any?


Last edited by TresM (Sir) on Wed Jul 28, 2010 9:45 am; edited 1 time in total
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loapathy



Joined: 07 Oct 2008
Posts: 760
Location: Maine

PostPosted: Tue Apr 27, 2010 6:30 am    Post subject: Reply with quote

You'll likely end up putting in a SS rectifier and rewiring the 5y3 octal sockets to be preamp sockets. 6SL7s are dual triode with a gain of around 70, so they'd be slightly less hairy than a 12ax7 but a lot bigger in girth. Then it is just a matter of either 1) designing and populating a board to slide in, or 2)doing it point to point.

Any ideas of what you want it to sound like?

You could also take out the octal sockets, put in 9 pin sockets, and use 12ax7s--but that requires use of spacers.
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TresM (Sir)



Joined: 02 Aug 2009
Posts: 472
Location: South Carolina - Humidity Central

PostPosted: Tue Apr 27, 2010 2:40 pm    Post subject: Reply with quote

Ok, that sounds like a good idea, Im sure there are a bunch of guides on how to construct an SS rectifier, and I don't mind the wiring either. I don't actually play, but my brother wants a ridiculously high amount of gain, which I doubt he will be getting. Is there any way to just flat out add sockets to the circuit to increase the gain to at least marshallish levels?
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loapathy



Joined: 07 Oct 2008
Posts: 760
Location: Maine

PostPosted: Tue Apr 27, 2010 4:23 pm    Post subject: Reply with quote

Well... you could take out the 5y3 sockets, get small plate covers for them, and then drill new holes to accept 9 pin sockets (12ax7s, etc). I mentioned spacers in the last post, but I hate that as an option. You end up with 9 pin sockets just hanging out over an open hole with little standoffs--strikes me as dangerous.

An easier option would be to just add a 9 pin socket and have it act as another gain stage before octal preamp tubes. That would give you two wide open gain stages with nothing holding them down, if you placed the tonestack in between, say, the halves of the first octal preamp tube. I'd recommend 6SL7s for that, as well. but if you do this, it'll be a bunch of tweaking of values to get the 12ax7s to behave nicely--also, you need to find out what kind amps you're getting from your PT's filament winding to see if it can handle the current draw.

There are also options like diode clipping if you need insane amounts of gain, but I rarely, if ever, do that type of stuff so I'm not the one to give advice. Marshall Jubilees, Marshall JCM900s, some of the Mesa stuff, all have diode clipping. There's info on this online, I'm sure.
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TresM (Sir)



Joined: 02 Aug 2009
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Location: South Carolina - Humidity Central

PostPosted: Tue Apr 27, 2010 4:33 pm    Post subject: Reply with quote

Thanks a bunch, thats really helpful, and now I can get a list together for stuff I will need before the amp even gets here. I have a spare octal socket, and 2 noval sockets, so that will cut it down a little bit cost-wise. Thanks for your suggestions and anyone else's input is also welcome here.
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loapathy



Joined: 07 Oct 2008
Posts: 760
Location: Maine

PostPosted: Tue Apr 27, 2010 4:55 pm    Post subject: Reply with quote

No problem.

For high gain stuff, you might want to check out the Soldano forum also. Doubt you'll make it into an SLO, but you could steal some of the tricks from a Soldano (i.e., super cold biased preamp tubes which causes clipping, etc).

Also check out Merlin aka Valve Wizard.
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Les



Joined: 06 Jan 2005
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Location: Kansas City

PostPosted: Wed Apr 28, 2010 7:02 am    Post subject: Reply with quote

If you plan on adding tubes, make sure the power transformer can handle the additional current requirement.

It probably can, but there's no guarantee... the math must be done.
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TresM (Sir)



Joined: 02 Aug 2009
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Location: South Carolina - Humidity Central

PostPosted: Sat May 15, 2010 8:22 pm    Post subject: Reply with quote

Arrived a week or so ago. Already got a 3 prong cord picked out for it.


Trying to get this whole thing figured out right.

Top from left to right 6v6 6v6 6v6 6v6 5y3 5y3


This is the underside with all the wiring and the choke
For some reason it wont post right...so you have to go to photobucket to see it.

http://s659.photobucket.com/albums/uu319/alphablok/?action=view&current=102_0945.jpg&newest=1

The little pre amp unit that came with it with a 6av6 tube



I wanted to go ahead and order a quad of 6l6s and use this little transformer I had sitting around as a filament heater.

Basically I want lots of versatility from this amp, more pre-amp tubes, lots of knobs and switches, and one of those cool graphic equalizer thingys Smile.
I also saw these solid state rectifier plugs that you just plug into the socket and re-bias the amp, which means i need to find:


Some super versatile and complicated pre-amp circuit from an awesome amp, with the corresponding tone stacks.

A bias circuit

Parts lists for said circuits.

Some way to measure the ratings of the trannies, without killing myself.

some understanding of what the hell I just got into.


Last edited by TresM (Sir) on Sat May 15, 2010 8:38 pm; edited 6 times in total
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loapathy



Joined: 07 Oct 2008
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Location: Maine

PostPosted: Sat May 15, 2010 8:26 pm    Post subject: Reply with quote

I'd do separate bias controls for each power tube. Makes it much easier to retube something if you don't need to screw around with "matched" pairs and quads.

I'd need a schematic to figure out what's really going on there, as well.
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TresM (Sir)



Joined: 02 Aug 2009
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Location: South Carolina - Humidity Central

PostPosted: Sat May 15, 2010 8:37 pm    Post subject: Reply with quote

Quote:
I'd do separate bias controls for each power tube. Makes it much easier to retube something if you don't need to screw around with "matched" pairs and quads.

I'd need a schematic to figure out what's really going on there, as well.


Separate controls for each tube sounds a bit complicated, and the ones that i was ordering were already matched in a quad.


Couldn't find a schematic anywhere online, or by calling Magnavox, Phillips, or Magnasonic.


So maybe a gutting and rewiring of some form is in order?
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loapathy



Joined: 07 Oct 2008
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Location: Maine

PostPosted: Sun May 16, 2010 4:37 am    Post subject: Reply with quote

Separate bias controls are not that difficult. I've started summer work so I won't be around that much but I am sure someone like Tom (structo) or the tangelolemon dude can help.

As far as the other stuff you'd like to do, it is all possible dependent on the current draw of the tubes.

It is a matter of finding the circuits you want to use, and finding the proper layout to make them work without oscillation.
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TresM (Sir)



Joined: 02 Aug 2009
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Location: South Carolina - Humidity Central

PostPosted: Sun May 16, 2010 5:03 am    Post subject: Reply with quote

Thank you for your suggestions and time.


Ill go ahead and look into the separate bias controls.
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TresM (Sir)



Joined: 02 Aug 2009
Posts: 472
Location: South Carolina - Humidity Central

PostPosted: Wed Jul 28, 2010 9:45 am    Post subject: Reply with quote

I FINALLY found schematics for the amp. I got really lucky, and found pretty much complete documentation on the entire amp, and the radio tuner assembly, which i don't have.

A chart of resistance ratings for all the tubes
Full Schematic of the amp (not the preamp though)
Full BOM with part No. and full specs.
Transformer specs
Photograph layout with components labeled.

Its actually for AMP 132, but my AMP 128D is in the same series and layout.

I know I want to replace the entire preamp with a new one, the old one was connected to the power amp with an octal socket and cap, which was attached to 8 wires, that went to the preamp.

transformer (Power) specs
PRI- 117VAC 1.26A
Sec1- 574VCT .190ADC
SEC2- 5VAC 4A
SEC3- 3 6.3VAC 4.64A

Transformer (Output) specs
PRI 3.9k ohms CT
SEC 3 ohms, tap@1.5 ohms

Filter Choke specs
Total Direct current .190ADC
DC resistance 51ohms
Inductance (0 Current 1000~) 1Hy

So given that a single 6l6GC (6.3vAC) has a 900ma heater current, would i be able to replace the 6v6s with the 6l6GCs, and do an independent bias of each?
If im right on this, the current draw of all 4 is 3.6A (6.3v), and the tranny can supply 4.64A on the 6.3v secondary, so that leaves me with 1.04A spare, and each 12ax7 tube draws 0.3A on the 6.3v secondary, so i could have 3 12ax7s, and then have .14A spare?

the 6v6GT, on the other hand, has a draw of 0.45A on the 6.3v secondary, so what should I do with regards to planning? If i went with the 6v6GT, I am assuming that I would have enough spare amperage for about 9 and 1/2 12ax7s.

Maybe I'm completely wrong with all the 'math', and anyone who could help me out or give suggestions is more than welcome to do so.

Links to the Schematics-
Most of the components listed belong to the radio receiver, so view accordingly.

http://i659.photobucket.com/albums/uu319/alphablok/componentlistcont.jpg

http://i659.photobucket.com/albums/uu319/alphablok/Schematic.jpg

http://i659.photobucket.com/albums/uu319/alphablok/Resistanceratings.jpg


http://i659.photobucket.com/albums/uu319/alphablok/Photographiclayout.jpg


http://i659.photobucket.com/albums/uu319/alphablok/Partslist.jpg


If there is anything else you would need to give advice please let me know and i'll try and find it for you. Thanks in advance
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Bootsypratt



Joined: 01 Dec 2006
Posts: 1236
Location: Columbia, SC

PostPosted: Wed Jul 28, 2010 10:06 am    Post subject: Reply with quote

Where in South Carolina are you from?
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TresM (Sir)



Joined: 02 Aug 2009
Posts: 472
Location: South Carolina - Humidity Central

PostPosted: Wed Jul 28, 2010 10:09 am    Post subject: Reply with quote

Im technically located in Elgin, but really Columbia, near Sandhills.
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