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Topic: Vega Lap Steel Amplifier |
Mark MacKenzie
From:
Franklin, Tennessee, USA
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Posted 14 Mar 2008 3:38 pm |
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I am very fortunate to have acquired this amplifier from fellow forumite Roger Peterson. Thank you , Roger!
This is a 1948 Vega High Fidelity Triumphal Model. Roger has owned it since new and it was his first amplifier. I have a same vintage Vega 6 string lap steel so it is a great match.
I didn’t expect the amp to be such high quality. It is very well built! It is push-pull 6L6’s, cathode biased, into a field coil 12” Rola manufactured December 1946 from the date code. The pre-amp tubes are of the loctal type; a 1273 pentode and a 7F7 twin triode. There are two instrument inputs and one mic input. The schematic is very similar to the Gibson BR-1 from the same year. The driver is a 6SN7 GT and the rectifier is a 5U4G.
I have replaced the filter caps and power supply voltages look good. I have replaced the remainder caps with 716 orange drops. I had Weber speakers re-cone the speaker because of a tear.
These amps are interesting in that they separate the pre-amp from the main amp. The pre-amp is at the top of the cabinet with a four prong connecter and a shielded RCA plug carrying the signal all plugged into the bottom chassis at the bottom of the cabinet. I think it is smart to have the heavy transformer and main amp at the bottom for low center of gravity.
The coolness factor is off the scale. I love it. But it is not playing right yet. The instrument channels are pretty low volume and the mic channel distorts quickly and is nice and loud. But… I have a bad distortion if I hit strings hard. That is, if I input too much signal, the speaker quacks and a low frequency oscillation starts. Too low for an audible tone, like maybe 15 hz kinda like a tremelo. I have a signal generator and can produce the bad distortion consistently. I have listened to the pre-amp via the RCA plug and the pre-amp is clean. Its in the main amp. I hooked up an oscilloscope and can see the huge distorted oscillations all the way back to the 5U4G rectifier tube on the plate supply voltage. I don’t have another 5U4G but can’t think of what else it can be. The tubes are original and all tested good on my Heathkit tester.
Any amp doctors out there? Do you think I am on the right track? Can rectifier tubes go bad like this and cause this kind of distortion?
Here’s some pics…..
Thanks for any suggestions.
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Ron Whitfield
From:
Kaaawa, Hawaii, USA
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Posted 14 Mar 2008 8:07 pm Score!
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| Good luck finding everything to ger her rocking. Please let us hear the results! |
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Dave Zirbel
From:
Sebastopol, CA USA
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Posted 15 Mar 2008 4:09 am
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That amp looks like a real gem! You need to send it to my buddy, Skip Simmons. He fixed up an old Oahu from the 1950's for me. He loves antique electronics.
dz |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 17 Mar 2008 6:59 am
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Sounds like Motor Boating.
A problem with deceasing parts in the power supply filtering
interacting with the choke to cause huge voltage swings.
It is causing repeated low frequency voltage sag
that is not being canceled by negative feedback.
And this is leaking into the front end preamps.
So likely there is a yet to be changed electrolytic cap
in the power supply filtering in need of changing.
Not a biggy.
Power supply filters are designed to filter out AC,
but ALSO to decouple the power section that
needs huge voltage swings and current flow,
from the preamp section that wants stability and finese.
Motorboating happens with the cpas stop smoothing
and decoupling the preamps from the power amp supply swings.
A lot of low freq. pass through in the preamps
can also increase the likelyhood of motorboating
even in new circuits.
So if you find too much low freq notes distortion,
also have them out in some caps of higher frequency
pass through filtering.
A little less bass gets through from stage to stage.
And put in the GOOD new versions of old caps.
Not cheap but this baby should sound great.
Kills too birds with one stone;
less bass power consumption so higher relative output
better sound when the caps break in
less risk of motorboating from a susceptible circuit.
Some nice caps are
http://www.mojomusicalsupply.com/images/5162020.jpg
andhttp://us.st11.yimg.com/us.st.yimg.com/I/yhst-8476489043850_1992_3491248
But not limited to these.
I am sure Ken Fox can set you up right.
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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Mark MacKenzie
From:
Franklin, Tennessee, USA
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Posted 17 Mar 2008 8:33 am
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Thanks for the help David and everybody. I used the orange drops for coupling but I used the cheaper blue and black electrolytics. Maybe one of these has failed. I have heard the filter cap of choice is the Sprague Atoms.
I wonder what the failure rate of these new cheap caps is. They are all 450v Maybe I should have used 600v ones. They are also small in this circuit. 10microfarad.
I will double check them.
Motorboating is a good description of the sound. I was thinking maybe the tube was setting up this oscillation.
Thanks for the help. This is a great forum! |
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Don Davis Jr
From:
Georgia, USA
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Posted 17 Mar 2008 8:55 am
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| Dave Zirbel wrote: |
That amp looks like a real gem! You need to send it to my buddy, Skip Simmons. He fixed up an old Oahu from the 1950's for me. He loves antique electronics.
dz |
Hey Dave,
I sure want to thank you for making this post. I talked to Skip Simmons a long time ago and have been unable until now to retrieve his name and contact info.
I have an old, old Sansui 2xEL84 Hi-Fi amp I wnated to ask him about making into a guitar amp.
Thanks again,
Don |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 17 Mar 2008 3:25 pm
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Yeah the atoms are good.
Maybe go up to 30's.
20 seems quite low.
Does the schematic say 10's !!!
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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Mark MacKenzie
From:
Franklin, Tennessee, USA
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Posted 17 Mar 2008 4:47 pm
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Actually David, I have no schematic but the original filter caps were 10's.
I had to trace and draw my own schematic. It looks almost exactly like a Gibson BR-1.
I think I will go up to 20 or 30 microfarads though. |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 18 Mar 2008 9:26 pm
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Mark I was showing you amp to Burmese electronics guys on my border run yesterday.
I said I was looking for old amps or Heathkit stuff etc Had a few pic with me including your Vega.
It got several knowing smiles, but no bites.
They said sure in Yangoon there is stuff like I want.
So maybe there next time.
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 21 Mar 2008 3:14 pm
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| Quote: |
| I wonder what the failure rate of these new cheap caps is. They are all 450v Maybe I should have used 600v ones. |
No, 450v is plenty! I doubt there's even 350v in that old amp. Most amps made back in the '40s weren't designed for modern 6L6GC's, GB's, or GT's but for the plain ol' metal-envelope 6L6 tubes, or the old 6L6G "Coke-Bottle" shaped glass tubes. Voltages commonly ran from 250v-350v max., and outputs were considerable lower than today's 6L6GC based amps. Usually, two 6L6's meant about 18-25 watts. The tubes were capable of more, but speakers back then weren't!
As to your "motorboating" amp, check for old (small) electrolytic caps that are used as bypass caps. They usually aren't bigger than your thumb and they're notorious for causing the problem you describe. |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 21 Mar 2008 5:40 pm
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Donny good points.
Here is a BR1 schematic
http://www.el34world.com/charts/Schematics/gibson/BR-1.pdf
Look at the 20mfd across V1 cath restistor 1.5k
and 20mfd across V2A & V2B cath R 1.8k
There is a 0.05 on a 47k to ground after V2b also.
That seems to be the tone stack,
(too little coffee so far)But if it opened...
At this amp's age ALL caps are suspect.
Good luck.
DD
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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Mark MacKenzie
From:
Franklin, Tennessee, USA
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Posted 22 Mar 2008 5:07 am
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| Thanks for the help. I tried a new 5U4G but same symptoms. I will look at those caps if I get a chance today. What I don't understand is that changed every cap. Maybe I could have missed one but I don't think so. Seems I always get the head scratchers..... or maybe its just my head... |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 22 Mar 2008 7:30 am
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It could be that a new cap is working BETTER than the old one...
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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John Billings
From:
Ohio, USA
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Posted 22 Mar 2008 1:17 pm
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| Well,,,, if it's like a BR-1, you're gonna be very happy when you get it workin'! I've got one, and it's great! When I went to Gibson/CMI 35 years ago, and asked for a schematic, the guys in service just started laughin' at me. |
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Mark MacKenzie
From:
Franklin, Tennessee, USA
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Posted 24 Mar 2008 2:31 pm
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Cool John, it is very close to the BR-1. Right now I have very low volume on the instrument side and the Mic input has great sounding distortion and compression but nice volume. If I back off the guitar volume it cleans up. I wonder if your amp matches the BR-1 schematic from the above link. The only noticeable difference from my Vega amp is the use of loctal tubes and my volume pot on the Mic input is before the tube not after. On the instrument side it is also before the tube just like the BR-1. You know the BR-1 was the amp used on the solo on Rock Around the Clock?
Hey David, I think you are right on suspecting the cathode bypass caps. I removed the 6L6 cathode bypass caps and the motorboating went away. Then I tried using the original 6L6 GA's which tested good but the motorboating came back. Back with the modern 6L6 GC's and tone and headroom improved with no motorboating. I think I will try to replace the pre-amp cathode bypass caps with better quality and see what happens. All were replaced and I used electrolytics. Do these caps have to be electrolytics?
This amp is getting close. I do really like it. |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 24 Mar 2008 5:46 pm
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No need for electrolytics I believe.
Just keep them 35-50volt rated.
Nice oil filled caps would be nice.
These days many great choices to be had.
Bumble Bees or Black beautys too.
Messing with the caps is a great way to open up tones.
Get an old Mullard tube or 2 in the front end.
This can be a fine little amp.
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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Jim Newberry
From:
Seattle, Upper Left America
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Posted 16 Apr 2008 3:28 pm
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Guys,
I just "won" the little brother to this amp on eBay. It's a Vega Commander with an unusual single-ended 6L6 setup. Tubes are:
V1: 6SL7GT
V2: 6C5
V3: 6L6G (cool coke bottle 6L6)
V4: 5Y3
The date stamped in the back of the cabinet is Dec of '47. The speaker is a 10" Jensen PM10C with the OT mounted on the speaker frame.
I got it today and have only been able to power it up; I have to get it home to plug something in to it.
I haven't been able to find a schematic for it on line. Has anybody got any more information (like a schematic???) about this little guy?
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Ron Whitfield
From:
Kaaawa, Hawaii, USA
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Posted 16 Apr 2008 5:51 pm
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| No info, just more drool. |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 17 Apr 2008 3:21 am
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That's not the OT, but a Field Coil for the speaker.
Mainly used prior to the permanent magnets we now use.
It generates the feild that the sound coil interacts within.
Yes drool worthy for sure.
Becareful turning it on especially if
it has not run in a long time.
Put a 100w light bulb in series with
the hot side of the wall current.
If it lights appreciably
when 1st turned on SHUT IT DOWN FAST.
And take to a tech for cap replacement.
Bad caps can take out other components.
Treated nice and this baby should sound
WAY COOL!!!
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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Jim Newberry
From:
Seattle, Upper Left America
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Posted 17 Apr 2008 9:05 am
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DLD, thanks for that. It obviously had been used at least a little recently, so I went ahead and fired it up. I'll open up the chassis this weekend and check on the caps and make myself a parts list for restoring if necessary.
I believe that Jensen is actually a permanent-magnet speaker (thus the PM10-C model number)and that is the OT. I know the big brother amp that started this thread has a Rola field-coil in it.
Anyway, it's clean as can be, fairly quiet, stays clean with single coil pickups at full volume on the Instrument channels, but shows lots more gain on the Microphone channel. Cool amp.
I suspect drool isn't very good for the electronics, however. |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 18 Apr 2008 8:39 am
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Well one of these came in for a work up.
Not as pretty as that Vega, but interesting.
Sounds OK, a warm bluesy sound, and a gritty bluesy sound depending on channel.
except the speaker is fading away.
So maybe caps and a speaker and she is a happy camper.
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many!
Last edited by David L. Donald on 18 Apr 2008 9:20 am; edited 2 times in total |
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Jim Newberry
From:
Seattle, Upper Left America
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Posted 18 Apr 2008 9:07 am
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| David, did that Harmony have the same tube compliment or something? I see it also has advertised the "PM" speaker... |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 18 Apr 2008 9:52 am
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I believe it has;
Two 6J5 preamp tubes,
Looking those up
they are a medium gain single triode tube,
like half a 12AT7 I think.
So they use two.
One 6V6 power tube
And one 5Y3 recto
like my '61 Champ,
Speakers seems to be Pretty Minimal
But likely this means Permanent Magnet.
WOW, that IS modern!!!
Who knows maybe we'll get stereo someday? 
Well I also see it has a:
"Large, Handsome, Pig-Grain covered cabinet."
Not at all sure how you get the grain off a pig???[/i]
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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Jim Newberry
From:
Seattle, Upper Left America
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Posted 18 Apr 2008 11:41 am
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| I know the putt veers to the left if you putt against the grain of the pig. |
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David L. Donald
From:
Koh Samui Island, Thailand
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Posted 18 Apr 2008 6:28 pm
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To my ever lasting surprise, under the dust bunnies,
I found a schematic and parts list!!!
I was nonplussed to say the least...
_________________
DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many! |
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