Amp Tech Thread / Ask a tech Q

[Instant Dan]

I didn't do that though, I changed the tubes while the amp was off. It was only after I powered it up and took it off standby did I notice a horrible screeching noise.

[Hemisaurus]

Thank goodness

No I was suggesting rather than turn you amp on all the way, which if you have a problem, can cause further damage, just turn it on to standby, and see which of your tubes was glowing, and swap around the one that wasn't.

If the problem follows the tube, bad tube, if it stays with the socket, bad amp.

[Instant Dan]

Can't do that now

Well I guess that is what I get for messing with the tubes again late at night and tired. I was trying the switch around method that hemi recommended and I somehow (even with being ever so gentle) managed to snap off the main male plug of the EL34 tube into the tube socket (pic for reference)
http://www.newoldsounds.com/images/large/T-RUBY-EL34-Q_LRG.JPG

[Hemisaurus]

So I gathered

Don't worry, it's not unheard of, the plastic base is usually just soldered on, the tube is formed around the electrodes, which are left sticking out, then the plastic base is slipped over and the pins soldered to the electrodes. Hopefully it can be fixed with some tweezers, and you won't even need to replace the socket.

[Instant Dan]

If you blew the B+ fuse, it wouldn't turn on? Right?

[Hemisaurus]

The heater supply is separate, so the tubes would still glow, the bias supply is separate, so there may be some wackiness, but there shouldn't be any sound.

[Instant Dan]

I took her to Jake at http://www.steamboatampworks.com since I had heard from others in Houston he did great repairs and at fair price. Some of the other (older cats) actually refused my business, not going name names. Needless to say, I have little respect for them.

Check out his 'detuned' cabs and amp heads. Got me gassing for one of his 2x12's and Steamboat Classic 18.

[Hemisaurus]

refused why? seems an odd way to run a business.

every so often I feel I should just work on simple, single channel tube amps, no Mesa crap, no solid state Crate, but I never do

[blackkrosses]

I took her to Jake at http://www.steamboatampworks.com since I had heard from others in Houston he did great repairs and at fair price. Some of the other (older cats) actually refused my business, not going name names. Needless to say, I have little respect for them.

Check out his 'detuned' cabs and amp heads. Got me gassing for one of his 2x12's and Steamboat Classic 18.

What an adorable logo these amps have...

[SpaceTrucker]

So I got one of these http://www.freepatentsonline.com/1922415.html

Hemi, Its a fucking public address system from the 30s, (my model is from 36) It has no values on the schematic. But it looks like it has been repaired in the 50s or 60s Due to the telephone wire in it(my brother said that, and he would know) I mean, it still has some of the cloth braided wire on it. and sprauge caps. its running a single 6l6 a 6j7 rectifier and a 5y3gt All Westinghouse.... The transformer looks like the old tweed fenders. I'll try and get a picture of it. it is sweet looking.

Would you be able to point me in the right direction to resurrect this REAL point to point amplifier(as in no boards)? It's so cool, I mean, I payed 15 dollars for it So I would be willing to build a whole new preamp just to play a little guitar n bass through it(or just guitar if it can't take the bass) I google image searched it but none of them date that early, mine is made of wood with only one row of 1/8th inch mono jacks.

[Hemisaurus]

Well it might be tricky to do it all online, certainly if you were local, I'd give it a shot.

That link really isn't showing me anything, but 6J7 should be the input tube, not the rectifier, the 5Y3 is the rectifier.

You should be able to get an amp going from the tubes you already have. Sharp cutoof pentode and whatever tone stack (if any) you want. It's like Sunn's suicide amp, without the suicide part. It's a 3 tube special, have a look at the Airline Wards schematic, different tubes but same type, 1 sharp cutoff pentode for a pre-amp, 1 beam pentode for the power amp, 1 rectifier.



You might want to move the tone stack to past the preamp tube though, so where C4 is, put the tone stack (and volume) and lose the volume and tone stuff on the input. Adjust to suit 

[Hemisaurus]

Also found this link
http://music-electronics-forum.com/t8219/
the guys scribbled a rough schematic of something similar, but you say you have a schematic pics


Does it work atall? I'm thinking from the thirties, it probably will need a cap job, NB 1936 is the first year for the 6L6, I looked it up, because I thought it was in the forties. I'd be tempted to leave it as much 'as is' redo the caps, try putting a signal into the grid on the 6J7 and see what you get.

Do some research on the xfmr or the unit as a whole if you can, PA drivers were sometimes designed for odd output impedances.

Post as much detail as you can, pictures part numbers, etc.

[SpaceTrucker]

I don't know if it works, I was too afraid too try it out. also 2 wires disconnected when I opened it up. (well I had to clip them) It did have like a 3 inch speaker in it to talk into and a flapper to press down and talk, but I somehow misplaced it. I did notice on the old(and decrepit) power cable it was burned between the two prongs. It is sitting on my workbench open right now.

Here is the patent number 1,922,415 us patent. That is all the info I can find about it. the "schematic" doesen't have any values so it isn't much help to me. 
Working on pics right now.

[Hemisaurus]

I'm not an expert on the US patent system, but I believe it's more to cover the method of an implementation than the implementation itself.

ie. a system using vacuum tubes that allows a microphone, or microphones to drive multiple loudspeakers.

You may find that same patent number in a lot of PA amps, anyone got one can confirm?

Even without values, a schematic can be a handy thing. A schematic without values, kinda like a road map without street names, you can get from start to finish, but if you get stuck in the middle you have to look around for a 'landmark' (like a tube) to find your way. I await the pictures 

[SpaceTrucker]

http://s1129.photobucket.com/albums/m508/SpaceTrucker89/?action=view&current=DSCN1676-1.jpg

http://s1129.photobucket.com/albums/m508/SpaceTrucker89/?action=view&current=DSCN1679.jpg

http://s1129.photobucket.com/albums/m508/SpaceTrucker89/?action=view&current=DSCN1680.jpg

http://s1129.photobucket.com/albums/m508/SpaceTrucker89/?action=view&current=DSCN1684.jpg

http://s1129.photobucket.com/albums/m508/SpaceTrucker89/?action=view&current=DSCN1689.jpg

Here you are.

As you can tell its like a P.A in a school or office. And it is very heavy for its size. but the wood is so pretty, its not a laminate its actual wood and a very light wood at that. Like cherrywood or something.

Info on a piece of paper attacted to the inside of the amp;
Teletalk model 224 m

pilot bulb use no. 44 westinghouse or equivalent 6.5volt at 250 mills bayonet socket.

ac 50/60 cycle 115 volts 30 Watts
Notice uses about 15 watts in idle position

Licensed by Reyarch products inc. under US patents of American telephone and telegraph company and western electric company inc

Made by webster electric company Racine Wisconsin U.S of America.

[Hemisaurus]

Websters are very common, lots of people convert them.

It does look like that model has been bastardised a bit, those aren't all the original caps, it does look a bit of a rat's nest, and obviously be careful of the uninsulated cap on top of the 6J7 as that carries the plate voltage I imagine.

No pic of the schematic? In the 9th and 10th pics, is that the 6J7 on the right?

Mainly you need to make sure the amps in good enough health you can turn it on. You need to have an appropriate load on the output. You need to make a suitable input network, might just need a resistor on the control grid of the 6J7.

[Hemisaurus]

OK that transformer hooked to the 6J7 was freaking me out 'til I checked the datasheet.

The top hookup is for the grid, so that's the input.

WARNING: What I'm going to say next could very well kill you, or worse, damage the amp!

If you have a big set of brass balls (well insulated of course), you could just say fuck it, hook a speaker up to the back (any mention of impedance?), get a few clip leads and jump wire a cable to the top of that tube, maybe via say a 22-68K resistor and chassis ground (removing the connection to that weird transformer)

I would do this, but of course I have a variac on my bench, and would check for dangerous voltages before doing it.

The safe way would be to first check for any suicide caps, that connect the mains to the chassis, and remove them.

Then fit a 3 wire cable with a chassis ground.

Then try very hard to find information on the load impedance, there may be marked taps somewhere?

Then, hook the amp up to a GFI before turning it on.

All the rest depends on what kind of test gear you have.

You can power it up without the 6L6, and thus save the transformer from damage, whilst you inject a signal into the 6J7, you can use a voltmeter on the cap of the 6J7 to ground check for any DC voltage. You could try running a signal through the mic transformer.

Basically, if you can't get info on that output transformer you can go from a potential amp to a pile of toast in a few seconds flat

[Instant Dan]

Well I got a response back from the guy today. He said there was some grounding issues but easily fixed and just needed new tubes. I lucked out.

[Hemisaurus]

Cool

You realize there's only so much we can do via the internet, right 

[Instant Dan]

Yep, hence why I need to step up my gear repair knowledge.

[Hemisaurus]

Yep, hence why I need to step up my gear repair knowledge.

Best place to start (for free)

http://www.diyguitarist.com/Misc/J_Darr.htm

[SpaceTrucker]

The 6j7 is opposite the big transformer, Where all the big caps are.(left hand side)

So, what if I don't have any test gear. Except like, alligator clips, a volt meter, solder gun, and Balls to spare. So what your suggesting is put a gator clip on the top of the 6j7 as a signal-input?(don't you need two wires for that?) and put a beater speaker on one of the six outputs(Not shown)? and hope for the best?

Along with doing the safe thing and hooking up a 3 prong(grounded to chassis, like grinding/sanding a shiny part or just strait on it?).

P.S 15$s ain't too bad is it?

Oh yeah and I noticed another Hole in the chassis, that I could fit a preamp tube socket into.

I have not found any info on the output transformer yet. Only info I could find was 1800 ohms on a single (I want to say field coil) speaker from a webster electric amp from 1935, that was like a organ amp or steel guitar amp. I hope that isn't the case.

[hayseed]

Yep, hence why I need to step up my gear repair knowledge.

Best place to start (for free)

http://www.diyguitarist.com/Misc/J_Darr.htm

Great link!
This may have been asked already but; what is a good mod i can do to my RAT2 to make it more bass friendly? Where does one get the parts to do so and do you have any suggestions?

[Hemisaurus]

Great link!
This may have been asked already but; what is a good mod i can do to my RAT2 to make it more bass friendly? Where does one get the parts to do so and do you have any suggestions?

Bass friendly usually means upping the values of the capacitors in the signal path.

I don't have a RAT 2 schematic, this guy has a mod'd one.

http://illuminist.tk

Looking at an original RAT schematic



I'd want to up those two .022uF to .22uF and see how that sounds.

Then maybe change the .0033 as it's part of the filter circuit.

[Hemisaurus]

Answering myself, I found a better RAT schematic.



according to the blurb, RAT 2 is the same as the Turbo Rat but with 1N4148 diodes for clipping.

With that, I'll stick to my original opinion on changing those 2, maybe 3 cap values.

or you could try this guys mod, sounds like he plays jazz guitar

http://www.diyguitaramp.com/rat.html