ALs tube amp thread.

[TechnoBeaver]

Well I think I may have finally found some parts to tackle a tube amp project.

The donor is a 50's? Rogers-Majestic console. I can't find specific info on it.

It has the following tubes.
7Q7 Pentagrid
7A7 Vacuum Pentode RF/IF-Stage
7B6 Double Diode-Triode Audio
6v6GT beam power tetrode
5Y3GT Full Wave Power Rectifier

So I'm thinking I need the last 3. I also have a 6AU6 and a 6J5 if needed.

I guess it will be more of a scavenge then a conversion, that's fine.

I know I have hours of research ahead of me, but any input/advice/links are appreciated.

[TechnoBeaver]

Now were talking. If you noticed in the multi way forum I posted speakers and such that I got from a flea market.

Any how check this out, it's got :

12AX7
12AT7
6AV6

2 x 6V6GT

5Y3GT Rect.- I also have one of these in the other amp.

I am think I might have something doable here.

I wonder would this thread be better in the TUBE/VALVE section???
Feel free to move it.

It also came with a nice vintage Jensen alnico speaker. I think this may sound nice with a guitar as most woofers do. I don't know if you would call it a driver or woofer? It had a basscoil in front of it for LPF, but in reality could just be a driver. I'm confused. lol

[indianajo]

I think you are thinking about making a guitar amp??? People use the word "amp" to mean different things, on craigslist electonics list the only kind of amp is the kind that uses 12 VDC and drives 1 ohm speakers at an insane and unlistenable watt levels. When I make an amp, it is for recorded music, or maybe my organ or apple macintosh driven keyboard.
Actually, I like your donor cabinet. A new turntable in the top, some freshening up of the electrolytic caps and the speaker rubber, you'd have a nice monophonic hi-fi to match the collection of mono organ records I was just donated. (Nobody wanted them, not even the charity resale shop). Or, you could hide a speaker in the bottom and a tweeter in the lid and go for new fangled "stereo".
My current system, that I am playing a lot of organ and high church music on this week, looks like I need to install a neon "BEER" sign (chose your brand, it's mortons up there isn't it?) and a wire mesh grill to keep the beer bottles off the faces of the musicians.

[Cassiel]

12AX7
12AT7


2 x 6V6GT

http://www.blueguitar.org/new/schem/...6-77_schem.gif

[TechnoBeaver]

Thanks for the link Stalker, it will come in handy. I am checking a few videos on youtube with that amp.

I wouldn't mind a little more gain, although I beleive the fender users 12AT7s, so I guess I could mod it a bit.


I spent a couple hours tonight removing most of the things I won't need, 12v lighting pigtails, feedback loop, tone stack, etc.

[TechnoBeaver]

Ok making some progress I suppose.

First I wired up a 3 prong power cord, 2.5A SloBlo fuse and a regular power switch.

I also installed the speaker and input jacks.

I removed the tone controls and lights etc, trying not to disturb too much.

When I first got the amp I plugged it in, it hummed. However I should have plugged in an audio source first to see if it actually works.

I wired up some gain stages, with no luck. I plug in my guitar and it just faintly hums a bit like before. Oh boy.

The preamp is loosely based on Mesa/soldano. I also used the layout posted by stalker above as a bit of a guideline.

I was hoping to make some progress on my own with out asking for too much help, but now I'm stumped.

I understand its probably a bad cap(s) maybe a tube. Probably my wiring as well.

I was hoping to at least get some noise. Wah. I don't mind spending the money on some new caps etc, I'm just not sure on values yet. I do however have more old parts I could use.

Any tips or tests would be appreciated. In the meantime I'll keep on reading. I can post better/zoomed in pics of anything.

[indianajo]

Test on electrolytic caps. Date inscribed is yyww where yy is last two digits of year and ww is week 1 -53. 2010-19yy>20 is a bad cap or likely to go bad if it ever warms up. Tall can caps for tubes come from FP factory, sold in US by triodeelectronics.com or tubesandmore.com. Or you can put 4 axial caps on two phenolic headers under the chassis, often cheaper. tubeandmore has the phenolic headers, also glass reinforced boards with lots of turrets on 2"x1/2" spacing favored by guitar amp builders. Both have 600v solid core hookup wire, lacking the ROPS certification for sale in europe. Once you get the cap to a non-explosive state (old electrolytics sometimes explode when the water boils) turn on side, clip negative of 600V scale to chassis, touch with one hand plus of meter to output tube grid. Speaker should make big pop. Hope you have read tube safety for newbies at top of tube amp thread. Tube repair website www.geofex.com/ampdbug/ampdebug.htm is often useful. Best of luck in the future.

[TechnoBeaver]

Yes I have been following safety rules.

I'm going to try that trick tonight when I get home. Thanks! Every little bit would help me to narrow it down.

Is there a safe way to hook the DMM up to the preamp stages to veryify signal going through that, ie voltage swing when I play the strings?

[indianajo]

my Sears $29 dmm is useless at frequencies other than 50-60 hz, the lowest AC scale it has is 200V which is also pretty useless for audio. I proved it with a scope later, the meter misses stuff in the audio frequencies (8vpp) it should see. I bought a 20 mhz scope off craigslist for $40, and a scope probe for $50 (B&K scope, scope probes from mouser.com) If you can't find a used scope, find a used low wattage output transformer like from a bogen PA amp or something- I picked up one at Salvation Army resale for $15. wire inputs of transformer from capacitor off plate (600v .1 capacitor) to ground, secondary of transformer to a little speaker. Again, turret board or phenolic header is usefull to wire this up safely. Ensure wire to cap rated 600v. In a pinch, wire the transformer from this chassis to the preamp tube output caps, instead of the output tube output cap. You should hear something.

[Nigel Goodwin]

Originally Posted by indianajo

my Sears $29 dmm is useless at frequencies other than 50-60 hz, the lowest AC scale it has is 200V which is also pretty useless for audio. I proved it with a scope later, the meter misses stuff in the audio frequencies (8vpp) it should see.

Almost all multi-meters are only designed to read 50/60Hz, they are useless for audio - either a scope, or an AC miliivoltmeter, should be used.