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Old 10th April 2005, 01:21 PM   #1
Trout is offline Trout  United States
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Question extremely low output

Hi Guys, I just picked up an older instrument amp chassis at an auction yesterday. Nice well made and wired unit But, Its got a couple issues.
#1 Terribly low output level
#2 tremelo nonfunctional ( possibly a footswitch jack issue)

I Checked ALL of the tubes, All appear to be new or better.
I then replaced ALL the tubes with NOS RCA's from my collection.
Same results extremely low volume output. Not distorted or rough sounding, Just Low level.

Unit has Pair of 6V6GT's ,5Y3, 3-12AX7 bugleboys, And a 12AU7 .

But, Right now, Im trying to get the output level up. Im thinking I should be getting at least 12-18 watts out of this thing.

This unit is not exactly complicated but has had some tampering internally.

A resistor has been changed in the power supply section, (appears to be correct value) And defective standby switch has been jumpered.

One thing that I feel may be the problem?? The OPT.

The guy I got it from had plugged it in to see if it powered up, And see if the tubes lite up, BUT, both secondary OPT leads were grounded to the chassis.

Any Ideas would be of great help.
Gene

PS, I can have pictures tonite, I'll post back then
Thanks again Guys!!
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Old 10th April 2005, 01:58 PM   #2
Yvesm is offline Yvesm  France
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Hi !
Your acquisition looks very similar to a Fender amp !

Quote:
One thing that I feel may be the problem?? The OPT.

The guy I got it from had plugged it in to see if it powered up, And see if the tubes lite up, BUT, both secondary OPT leads were grounded to the chassis.
Double check the loudspeaker connections, specially if a jack is used.

Yves.
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Old 10th April 2005, 02:43 PM   #3
Trout is offline Trout  United States
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Quote:
Originally posted by Yvesm
Hi !


Double check the loudspeaker connections, specially if a jack is used.

Yves.
Hey,
I never mentioned, There was no output jack, The Leads were twisted together and grounded. ( So to speak,) 1 side of opt comes out of tranny and is grounded, Then a lead from another ground location on the chassis was twisted together and taped to the second tranny out lead. So in reality, Both leads had seen a power up grounded.

Im going to have to post a picture I think, I found a cap not attached to anything coming off one of the 6V6's. And as of yet cant find a place within its reach it belongs, Dadly, I have to call it a day already (8:30AM) Time to hit another auction.

Gene
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Old 10th April 2005, 02:50 PM   #4
SY is offline SY  United States
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First things first. Do check all the input and output connections.

But before running after defective output transformers (about number 50 on the list of likely problems), do some basic checks. Are the power supply voltages correct? Is signal getting from one stage to the next?

edit: you might want to check and see if the amp is made for some funny load like a 70V line.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 10th April 2005, 04:31 PM   #5
Trout is offline Trout  United States
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Quote:
Originally posted by SY
First things first. Do check all the input and output connections.

But before running after defective output transformers (about number 50 on the list of likely problems), do some basic checks. Are the power supply voltages correct? Is signal getting from one stage to the next?

edit: you might want to check and see if the amp is made for some funny load like a 70V line.

I did some further studying of the basic circuit, 2 12AX7's are being used as 4 input/instrument stages each of which have there own jacks.

Ive still not found even with a magnifying glass where the loose on 1 end cap attaches. Its an old standard brand ceramic .022 tubular. Connected to the 6V6gt on pins 1,5,6 which are all tied together. The other end reaches to the 12ax7 prestage but where it actually attaches is a mystery.

Mind you, sound does come out of the speaker, Clean, Hum Free and wide freq response, Just no power, I compare it to a 9Vtransister radio from the 60's in volume.
gene
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Old 10th April 2005, 04:41 PM   #6
SY is offline SY  United States
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That might be your problem. Two of those pins are basically no-connection, one is the control grid. So the cap probably came from the plate of the driver (or the cathode if the driver is a split-load phase inverter).

Replace the cap anyway. If it's bad, it will cause damage, and a new one is like fifty cents.
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And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 10th April 2005, 06:17 PM   #7
Trout is offline Trout  United States
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Quote:
Originally posted by SY
That might be your problem. Two of those pins are basically no-connection, one is the control grid. So the cap probably came from the plate of the driver (or the cathode if the driver is a split-load phase inverter).

Replace the cap anyway. If it's bad, it will cause damage, and a new one is like fifty cents.
Bingo, Just after I posted, I figured out which point on the 12AX7 the cap connected to, BANG,, It works!!

The Tremelo is still a mystery though, I get no effect out of it, But then again, I've played 30yrs without one, So I guess its not a huge issue other than I have issues letting anything beat me LOL

BTW: Quick Side Bar, I love the New avatar!!
Gene
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Old 11th April 2005, 02:42 AM   #8
SY is offline SY  United States
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Quote:
Originally posted by Trout

BTW: Quick Side Bar, I love the New avatar!!
Gene
Woo-woo-woo! Which is probably what I would have said in that tube treasure chest you mentioned on the other thread.

Glad that puppy is working again. I'm curious about the tremolo thing- does this amp use a light bulb/LDR combo in that stage? And a side bar, what kind of guitar are you playing?
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 11th April 2005, 04:24 AM   #9
Enzo is offline Enzo  United States
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Oh, PLEASE tell us what the amp is next time. Then we can just look up the schematic and talk about it knowledgably.

How old is the amp? Look for date codes on the caps, controls, and transformers.

The trem only works when the footswitch is on on most amps. The reverb on the other hand is usually ON until the footswitch turns it off. There are usually feedback caps - typically three in series - to make the LFO. These caps get leaky with age and the thing won't oscillate anymore. That is usually what is wrong with oldre trem circuits.
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Old 11th April 2005, 04:27 AM   #10
Trout is offline Trout  United States
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Quote:
Originally posted by SY


Woo-woo-woo! Which is probably what I would have said in that tube treasure chest you mentioned on the other thread.

Glad that puppy is working again. I'm curious about the tremolo thing- does this amp use a light bulb/LDR combo in that stage? And a side bar, what kind of guitar are you playing?
Im not exactly sure how this tremelo is hooked up,, It has a strength and speed pots, Connected to a 12AX7 and has a foot switch wired in normally open position. I assume its only function when switch is tripped. I jumpered the Jack, No effect. Tubes good, But Im thinking its possible one of the pots are dead, (next to the broken standby switch) No optical devices anyplace.
My Guitar

I play/own a american strat.. one actually built in the USA lol
Lace sensors and possibly one of a single year that came stock with the dreaded wilkinson roller nut ( defective direct from fender)

Imagine this, Brand new in the early 80s strat with a defective nut, Mis aligned bridge, 1 nearly striped neck screw, And a truss adjuster thats messed up. All for the great low prive of 1479.00 New !! And I was there when they unpacked it at the music Store.

I had always been a "ahem", gibson man Someday soon I may actually finish restoration on my original Axe. 55 melody maker I got in 1960 used for 45 bucks.
Gene
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