More general tube guitar amp questions (new pic)

[Mbira]

I am working on two different old tube amps. One is a perty old Vega. It runs one 6L6 and a couple of preamp tubes. This is one of those that has the output transformer connected to the speaker, so I can't really test it with a power resistor. When I first powered it up it gave this aweful humming sound (bad caps-I think). So I replaced the power caps. The only caps I didn't replace were a couple of the 600v .01uF caps. Now the amp doesn't have the hum, but the output seems very low to me. It was using an old on-off volume knob combo that went right from the input to the knob to the grid. It said 500M (?!?) on it. It was unable to turn, so I replaced it. I measured it and came up with 500K linear for a value. I have never seen a 500M put before (would this be common?)
Now I have very low output. This amp has three input jacks and only one of them are connected to the pot. The other two go in parallel straight into the grid.

I guess my question is
1) How much output "should" I expect from one 6L6? (18ish watts?)
2) Would not replacing those 600v .01uF affect the output?

This is already long enough, so I'll post the other question in another thread.

 

[Baltimore]

A single 6L6 is good for probably 5-10 watts out....how quiet is "very low output?"

 

[PRR]

Post voltages.

 

[ebartlet]

I have a SE 6L6 amp with 425V on the plate. The manufacturer rated it at 10 watts. I can't stand to be in the same room with it wide open, without some form of hearing protection. No matter the B+ (within reason), it sould get pretty loud. If those .01's are coupling caps, they could cause low output, but I'm guessing with the way things fail that they are probably not the problem. Have you measured voltages around the tubes?

 

[Gus]

Joel

When you power up an old amp and it hums let it run for a hour or two(if the hum starts to drop after 10 or so mins and nothing is smoking or glowing I do this outside on cement and watch the amp) with a single coil guitar plugged in and the volume turned up a little and listen to the hum if it starts to drop the caps might reform.

Now this is not for "don't change the vintage caps" crazyness but sometime old caps reform and measure fine on a meter like a sencore LC102. Sometimes you have to change caps.

Two things with this
1st some new cheap electros are not that good IMO
2nd if the old caps work it one less thing you might break when you start to change things.

Many years ago when I first started playing with electronics I would often change more than I had to, sometimes adding a problem.

Like PRR posted voltages will help.

Another thing with 2 wire old amps check with a meter in AC volts from an outlet ground to the metal face chassis to check if you will get shocked.

Have you looked at R.G.'s site geofex.com in the tube amp troubleshooting section?

 

[NewYorkDave]

Are you sure the speaker is good?

And are you saying that the "output transformer" appears to be mounted on the speaker? You also mention that the amp is really old. I'm wondering if you have a field coil speaker there. If what you mean is just that the leads from the output xfmr are soldered to the speaker... well, don't be lazy, desolder 'em! Just be careful not to overheat the terminals and screw up the leads to the voice coil.

It said 500M (?!?)

500K. Before, say, the mid '50s, "M" was frequently used where we would use "K" today. This comes from the Latin "Mille" (thousand). We still refer to a period of a thousand years as a millenium. The convention now is that M means million (Mega, from the Greek megas, which means "great") and K is Kilo, which means thousand, also from Greek.

 

[Mbira]

I'm not sure that the speaker is good. The nasty grumbling hum thing is gone now that I have changed the power caps. The OT is actually Connected to the speaker and then goes to what looks like another OT behind where the speaker maginet is. It is a "Rola" speaker.
The other tubes are a 6N7 and a 6C5. I'll post voltages this evening.

Thanks guys.
Joel
Here's some pics if interested:
http://rattletree.com/DSC00137.JPG front
http://rattletree.com/DSC00139.JPG back
http://rattletree.com/DSC00168.JPG insides[/quote]

 

[NewYorkDave]

Well, since your pictures don't show details of these transformers, we're still guessing at this point--although that spec of "2500 ohms" printed on the speaker gives an interesting clue.

I see a big problem with the AC cord, besides the fact that it's two-wire. Can you spot it?

(PS: the pics really don't need to be that huge).

 

[alk509]

[quote author="NewYorkDave"]I see a big problem with the AC cord, besides the fact that it's two-wire. Can you spot it?[/quote]

The Mickey Mouse strain relief? The old as farts insulation? It's actually three wires, though - look at the pics of the inside.

Peace,
Al.

 

[Mbira]

[quote author="NewYorkDave"]Well, since your pictures don't show details of these transformers, we're still guessing at this point--although that spec of "2500 ohms" printed on the speaker gives an interesting clue.

I see a big problem with the AC cord, besides the fact that it's two-wire. Can you spot it?

(PS: the pics really don't need to be that huge).[/quote]

The AC cord isn't 2-wire. I replaced it with a three-wire. Looking closely, I'm guessing the problem is that I put the hot side on the fuse end that can be grabbed, yes?

The pics were mainly taken for my own benefit. Sorry, no disclaimer on the size. Here's one of the transformers (a little smaller)
:
http://mhumhirecords.org/DIYpages/tn_DSC00169.JPG
http://mhumhirecords.org/DIYpages/tn_DSC00170.JPG

 

[Mbira]

PS, guys I'm going to tidy things up, I'm just trying to get the bugger working right now.

 

[alk509]

[quote author="Mbira"]I'm going to tidy things up[/quote]

Are you kidding me? That looks like the gold standard of neatness compared to the shit I build! :thumb:

Peace,
Al.

 

[PRR]

There's no output transformer inside the case. Just Power and a choke.

The speaker IS a field-coil speaker. It uses an electromagnet instead of a permanent magnet. The electromagnet coil is used as the first filter choke.

The plate of the 6L6 goes to the output transformer on the speaker. The other end of that 2,500 ohm winding goes to the main filtered B+, which happens to be the low end of the field coil.

So that cable from chassis to speaker needs 3 wires: raw B+ from rectifier heater, 6L6 plate, and a filtered B+ back to the chassis for the low level stages. The 4th pin really should be ground! That is not necessary until something shorts.... when (not if) it shorts, the speaker metal would rise to +400V; if the speaker is grounded, a short will pop a fuse or something.

6N7. Argh. I hate them. twin triode with common cathode. Makes perfect sense in a push-pull amp, which this isn't. Makes sense if both sides are used parallel, but you don't really get much gain that way. So we have a cascode amp with common cathode, a very nasty affair.

Get the 6N7 and 6C5 pinouts from Frank's, start tracing. Along the way, get voltages. Be very suspicious of a triode Plate that isn't 60V-250V, a cathode that isn't 1V-6V, or a grid that isn't ZERO volts +/-1V.

I give 50:50 odds the 6L6 is not as healthy as it once was. For testing, I'd put in a 1960s-1970 vintage 6L6GC that was known to work correctly (ie, rob a happy Fender Twin or something). When all is well with the too-new tube, put the old tube back and check for significant bias shift. More than a Volt of DC on the grid at idle is a sign of gas. Watch closely for an hour: it may wander around 2V and finally settle to less than a volt if the gas getters-out. But keep a close eye on it over several days and be sure it stays settled. I had a 6L6G that would not settle down, so I put it on the display shelf.

Oh: a 500 Meg pot is preposterous. Use 250K, 500K, 1Meg, it isn't critical.

> How much output "should" I expect from one 6L6? (18ish watts?)

Heck no. 6L6 plate dissipation is 19 or 21 watts max. SE efficiency is 50% max. The most you can get is 11.5 Watts. (Yes, that's more than 50%: they include the 14.6% distortion.) Later ratings only quoted 10.5 Watts. The 2K5 load leads us to the 250V 80mA suggested operating point, 6.5 Watts. That's no joke: thousands of Fender Champs flog a 6V6 to 5 Watts and make mighty noises. Today there is little reason to drag a Marshall 100 Watt around: 5 Watts is plenty for feed the inevitable PA system, and more than you really want in the studio.

That amp is in fabulous shape and very Collectable. Don't muck it up.

 

[adamasd]

Thats a great looking amp. I got to play with one fo those once awhile back. The owner said it was a Model 120, he was the original owner. They are fantastic sounding amps. I sure hope you trace up a schematic for that amp. I tried to get the owner of the one I played to let me in it, but he would have nothing to do with that. Even with my offer of giving it a check up.

adam

 

[NewYorkDave]

I was referring to the lack of a strain relief. But now that you mention it--yes, the hot line coming in should go to the rear terminal of the fuseholder.

 

[solder_city]

please dont take this personally mbira, because its not meant to be personal, but you really need to build up your tech knowledge and chops before you offer your services for pay. cut your teeth on your own stuff, not other peoples (unless they understand what theyre getting in to). assisting a more experienced tech is also a great way to learn if you can get into a shop thats willing to take on an eager young apprentice.

ive just noticed that you seem to post a lot of requests for help with your amp repairs and its mostly pretty basic stuff your asking about

not trying to be a dick, definitely dont want to dampen your enthusiasm for learning the trade, i just dont want to see you get in trouble with a customer

 

[Mbira]

I was referring to the lack of a strain relief. But now that you mention it--yes, the hot line coming in should go to the rear terminal of the fuseholder.

The cable actually wraps around inside behind that back panel and tucks into this cloth bag where there is a strain relief.

I give 50:50 odds the 6L6 is not as healthy as it once was.

Yes, it rattled like a blown lightbulb. It was the first thing to be replaced.

I will check voltages on those tubes. I already got the pinouts on those buggers.

soldier_city:
Since this is such a basic problem, why don't you tell me what's wrong? . But after taking a deep breath, I realize that you don't know me or my work, so I won't take this as an attack. For a little clarification, I DO work on my own stuff, and in fact that is how I came into this business. You might think otherwise-from our extensive internet relationship-but I'm one of the best techs in town. I have over the last 2 years fixed-lets look at my invoices...I have fixed 231 amps over the last 2 years. The five music stores that I work for are all happy, and I have more work than I can handle.

When I get stuck on an issue, I ask my friends for help. Most of the folks I trust :shock: are in here.

I have found that even when I start to know something, the next amp I get seems to throw me a loop. This is the learning process for me. If you don't like it, don't bring your amp to me.

:guinness:
Joel

 

[Mbira]

OK, after the weekend delay, I think I have seen my most recent f*&^up
Check out this cap in this pic (Dave, the pic is much smaller):
http://mhumhirecords.org/DIYpages/DSC00168q.jpg
I wired the + going towards ground. The - is connected to wires going to the plates of the preamp tubes and the grid on the 6L6. Is this backwards?
Voltages are over 300v going into the OT-thru that four connecter plug thing. Then is about 90v coming out and going into the choke. Then on the other side of the choke (where that suspect cap is, voltages are dropped to 40v.

Is that my problem?

:guinness:
Joel

PS: why is there a .05uF/600v cap going right after the pot and into the grid of the 6N7? I understand the filtering, but why such a high voltage?

 

[PRR]

> Is that my problem?

One of them.

Post voltages.

Remember: we are not there looking over your shoulder, to see, feel, and smell what's happening. The pictures are dark (get a desk lamp). Voltages are useful clues.


> why such a high voltage?

Because the original wax paper was 200V, and failed (of old age), and the repair dude had 600V in stock. You could use 10V here, but nobody bothers to make 0.05uFd film at low voltages.

 

[Mbira]

PRR,
Sorry, I thought I did post voltages in terms of seeing that there is something obviously wrong (IE, after the choke , all voltages are below 40v).

I will change that cap and see if voltages change, then I will post voltages on every pin.
Thanks for your help.
Joel