Phono Input Question

[todd.brust 29]

Okay this might sound like a dumb question, but I still need an answer. On The Heathkit AA-151 there is a magnetic input for the Phono. My question being that there are the right and left channel RCA inputs and the ground pin (no phono ground tie spot on back of amp). Do I tie that to the chassis ground or common ground? I'm assuming the common because almost everything is isolated from the chassis. Just wanted to check so I don't mess up anything.

Here are the schematics if that helps.

http://www.wdox.com/pics/aa151l.jpg

http://www.wdox.com/pics/aa151r.jpg

Thanks

[todd.brust 29]

Anyone? anyone?

[MikeS 0]

Anyone? anyone?

If you look at the schematic, the grounds of the jacks go to cables with a shield wire for ground. Follow that shield ground wire, it shows on the schematic that it goes to chassis ground.

You also asked in the PM from AK about where to ground the turntable on the amplifier. Pick a spot on the chassis somewhere convenient. Try it first and just touch the ground to the chassis from the TT. It'll probably hum until you do ground it to the amp's chassis.

Whether it still hums when grounded to the chassis......that's something you'll have to find out......

[todd.brust 29]

Nice to see you around Mike I asked you on AK about this but also posted here too. Just to get an answer asap.

I looked at the schematic and I am looking at the the input on the Mag Phono and I don't see where you are talking about that it goes to chassis. The only place I see anything going to chassis is in the PS and the output stage. The shielded wire(with the bubble thing around the 3 other inputs) goes to ground. Maybe I'll just look at the actual amp and trace things out as that may be a bit easier.

Otherwise I'll try and just touch the ground wire to the chassis and see if it stops the hum. If not I'll try it off the ground.

But thanks for the help.

I'll give a response and let you know how it works.

[todd.brust 29]

Alright it worked hooking it to the chassis:) Thanks!

Sounds great! Listening to some Zappa LPs as we speak!

[MikeS 0]

Nice to see you around Mike I asked you on AK about this but also posted here too. Just to get an answer asap.

I looked at the schematic and I am looking at the the input on the Mag Phono and I don't see where you are talking about that it goes to chassis. The only place I see anything going to chassis is in the PS and the output stage. The shielded wire(with the bubble thing around the 3 other inputs) goes to ground. Maybe I'll just look at the actual amp and trace things out as that may be a bit easier.

Otherwise I'll try and just touch the ground wire to the chassis and see if it stops the hum. If not I'll try it off the ground.

But thanks for the help.

I'll give a response and let you know how it works.

Yeah, you are right. After looking at the schematic, I notice that. The actual chassis ground symbol is designated like a rake. Plain grounds look like a upside down christmas tree.

You would think all those ground shields in the cables ties in somewhere to chassis ground. The 47K input resistor at the mag phono input is pretty much the grid resistor for that grid of section 6EU7 it feeds into. It also sets the input impedance I think as well....

I almost have to have the amp sitting here to trace out, but your gonna do it.

When I have scratch built amplifiers, the way I've been kinda taught is to like run a ground bus from the PS first cap's ground. You also run the PS xformer's CT to the first cap's ground. From there you run the ground bus to the input jacks, and chassis ground at the input jack's ground pin. Then you try to configure all the HV grounds, (PS, output stage) as close to the PS section as possible, working your way to the lower voltage tube grounds and then signal grounds as you go to the input jacks.

There is star grounding. Which works well, but in this case it takes less wire to ground to a bus than running several ground wires to one spot.

One good rule of thumb is to make sure the PS transformer's CT is tied to the first cap ground of the power supply. I'm sure the Heathkit is....It's obviously has HV chassis grounds separate from the signal grounds.

Old vintaqe tube amplifiers used several ground points on the chassis sometimes.....it can be a PITA. The best thing is just leave things be, and make sure all ground points are clean and well-grounded.

Cool, it worked....the TT ground....

Mike

Edited April 17, 2008 by MikeS

[todd.brust 29]

Thanks Mike.

Well I tried tying it to both chassis and common and they both get rid of the hum. Only difference is that the chassis will make for a much easier grounding spot.

I listened to vinyls on it for a good two hours last night after my previous post and am more than happy with everything. Now I just need some different speakers.

[MikeS 0]

You may as well just run the amp with the cover off, save yourself brain damage. You'll get better ventilation...one thing a tube amp likes is lots of air around it.

Have you tried cleaning the pots yet? Sometimes if you work them back and forth you can work them back in...sometimes.

Whatever you clean the pots out with, go ahead and and buy a can of compressed air as well. Spray the cleaner into the pots, work them real good back and forth, and then blow them out with the compressed air.

Do it a couple times. Then blow them out real good, and let them dry for a few hours, a day, or whatever.

They should do better after cleaning, sometimes they don't. That's kind of bummer with tube vintage amplifiers and integrated amplfiiers. They use strange value pots with various designs, (like dual-ganged) that are hard to replace. AES may have 1 meg replacement pots, but they may not look the same way in design. Or be dual-ganged.

Then they has the infamous ceramic coated "PEC". Printed electronic circuit I suppose it means. There are ceramic square looking things with several leads going to them. They have internal capacitors and resistors in them, and are tied into the tone pots.

Do the tone controls sound ok?

Mike

[todd.brust 29]

I've been running the cover off for a while now. No more smells!

I have not yet gotten the pots cleaned. I tried to just work them a bit and see if it got better, but with no luck. All the ton controls work fine and don't make any noise when adjusting. The volume pots are the only ones with trouble. I'll have to make a trip to radio shack sometime later this week/weekend and pick up supplies.

I hooked up my PSONE last night to listen to CDs for the first time. Everything seems to be working great!

I was wondering about the output tubes but referenced google and found my answer. They seem to glow blue inside the tube, not the filaments. Just blue lines on the inside of the tube. Kind of tripped me out at first, but from what i found, it said that something(I forgot what) glows blue internally.

Hows the Altec coming Mike?

[MikeS 0]

That blue glow you see is hovering right outside of the cathode, and between the cathode and plate, correct?

That's the only blue glow I've noticed inside EL-84/6BQ5 in my experiences. Some larger tubes like 6V6/6L6 and my sovtek 2A3's for example, get a blue glow on the inside of the glass envelope. I can't remember off hand what the cause is, stray electrons striking the glass?

You start seeing pinkish/purple clouds or spots, that's usually a sign of cathode poisoning. With cathode arc/death nearing.

But you'll hear static from the tube with this condition. Had a NOS 80 dollar 2A3W/5930 do this after a month.....That'll chap a guy's ass, especially with a 30 day guarantee.

You may want to hit a place like Guitar Center or a pro sound joint that carries Caig products that JimJ mentions.

I'm cheap, and I've had good luck with the RS contact/control cleaner and lubricant. It's worked with many a carbon pot.

I used this stuff with that Pioneer SX780 I found for my friend. Worked well with the VC pot, which was really scratchy.

"Hows the Altec coming?"

I've cleaned measured and tested the PS transformer, you know.....the one with the hole drilled into it. I repaired the burnt insulation on the heater windings. Checked for DCR, and then ran the thing unloaded for a 7 hours, to check for overheating.

It passed......it ran rather cool unloaded.

I found a posted Sams Photofacts schematic of the Altec 345A posted online. I also have the original operator manual's schematic, but the Sam's schem shows DCR for the PS xformer secondary leads. My measured DCR #'s matched right up.

I'll see how the PS xformer holds up with a load when it's in the amp, it should be ok.

I emailed and had my retired EE audio buddie explain how to measure a output transformer out of circuit using a signal generator and a scope, using a simulated plate load for the primary and a dummy load for the secondary.

Using square waves, I could monitor varied frequency response between each output transformer and check for similiar characteristics, or un-similiar. Basically, I did it to check for possible shorted windings, which would effect the square wave response.

They passed, and seem pretty balanced between the two.......not perfect, but close enough.

So now that I know the iron is ok, I can continue.

I need three 40/40 uF 500 volt capacitor cans. The ol' 49 old ones ain't worth cocking with. Bummer that, those cans are gonna run a good 100 bucks for what I want.

I scrounged up six 0.1 uf K40Y 1000 volt Russian coupling capacitors. The ones like in my little Magnavox amp. They will fit with just a wee bit of room. I lucked out.

I'll need some power resistors, and some new 50 uF 50 volt capacitors for the negative supply for bias. Also some 50 uF 3 volt capacitors for the 6AU6 bypass cap....maybe I can find a Nichicon axial cap at radio shack in 16 volt with some luck.

A bunch of under chassis clean up, and wire cleaning mostly......that's about it....I also need a matched quad of EL-34's. The original tubes are all mis-matched brands. They all test somewhat close, but I'd rather just start over with a fresh quad.

I'll save the only two Mullard EL-34's for a SE amp or sumthin'.

I also need to find some 6C4's. I need around 200 bucks to do all this. But I got bills at the moment, so I've just been "prepping" more or less. I'm currently sanding all the mouse markings off the chassis. It's a major chore I hate it, but the metal comes out pretty much like chrome.

Hopefully, this amp sounds clean and powerful enough to make my LaScala's boogie when I got a few beers in me.

I'm a no life......yikes

[todd.brust 29]

Yeah the blue low is in strips close to the plate. No other colors besides the filament glow. I'll be picking up some pot cleaner this weekend after I get paid friday! I'm broke too .

Nice steal on that amp now that you know the transformers are good. I had the same problem with the Heathkit finding the PS cap. I wanted to keep similar values without having to hide caps on the underside. I ended up going with a Dynaco ST-70 PS cap. The 30/20/20/20 and another 32/32 from PE. The dynaco cap cost me about 45. Pretty close to OG values.

Speaking of caps, I got a bunch of high voltage caps from school they were going to throw out. A bunch of different capacitance values with voltage ratings anywhere between 500 and 1000.

Good luck with that Altec. You'll have to let me know how it goes and get me some pictures of the progress. Well that is when you get the cash!