Flamin' Marshall AVT 20!

Anything to do with the things that make the music we listen to.
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Dave the bass
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Post by Dave the bass » Tue Mar 13, 2012 5:06 pm

Paul Barker wrote:In all seriousness if it is only 35 watts is it not just crying out for an adapted output section to push pull KT88 or EL34?

You could sell kits.
Yeah, probably, but I'd be up against some heavy hitters. Fender, Marshall, Mesa Boogie, Woolworths Wall of Sound, Zang Nung Po industrial Amp Manufacturing company of China.

I'll stick with DIY for now! :-)

DTB
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Post by pre65 » Tue Mar 13, 2012 5:15 pm

TDA2050 will drive a valve in A2.

I know, cos I dun it. :wink: :lol:
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Dave the bass
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Post by Dave the bass » Tue Mar 13, 2012 5:16 pm

I ride up the A2.

I know, cos I do it :lol:

DTB
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Mike H
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Post by Mike H » Tue Mar 13, 2012 5:16 pm

Nick wrote:
Paul Barker wrote: A single ended 833a 100 watt amp wouldn't need any protection.
Not, but others in the room might :-)
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Post by Mike H » Tue Mar 13, 2012 5:37 pm

Dave the bass wrote:Looking at the TDA2050 datasheet <pushes nerdy glasses up sweaty nose and blinks awkwardly...> shows lower values of R and higher values of C than Marshall have used. 2R2 and 0.47uF typically, it specifically states in the literature higher values of R can lead to oscillation..... Ooooooooo.

DTB
He's right, he's not wrong, that'd be my next mod....

Input bias & feedback resistors are 22k as well, dunno what Marshall made 'em as :?:

Lookit at table on page 5, 'recommended value: 22k', I see also C3, 4, 5, 6, 7: 'smaller than recommended value: danger of oscillations'
 
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marshal avt 20

Post by johnyk » Tue Jun 12, 2012 12:49 pm

i Have also had the same r53 melt causing the c71, to also melt, after reading this thread plus some others i replaced the two mentioned components and r53 instantly melted again...... i am pretty new to messing with the inner workings of amps ,,,,, so reading further on and asking some advice i came to the conclusion, to change the ic chip,,, the post's on this thread, refer to the IC chip as being a tda2050, but mine is a lm1875t, i am going to put a lm1875t in as that's what ive taken out, but is mine different to yours? as lm1875t and tda2050 have different voltages plus many other differences...... mainly how come in the pdf on this thread it says tda2050?
..... am i missing some crucial detail being new to this? :shock:

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Dave the bass
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Post by Dave the bass » Tue Jun 12, 2012 1:46 pm

There's probably more than 1 variant of AVT20 MrK. The cct listed on this thread doesn't exactly tie up with what have in my amp either. HT for the ECC83 is generated differently than the cct diagram suggests IIRC.

If your cap and resistor ignited as soon as you replaced the IC I'd be looking elsewhere for a/the fault. My own AVT20 only goes into oscillation when all gains and the treble control is turned up fully. The amp must have a load BTW, don't power up without a speaker being connected. FWIW, My amp had two dry joints on those PCB speaker terminals which probably wasn't helping things.

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Post by Mike H » Tue Jun 12, 2012 3:02 pm

From Maplin TDA2050 30W amp kit blurb:

R6 (2R2 resistor) gets hot or burns out

In certain configurations the output of the amplifier can couple to the input causing large feedback oscillations around 100kHz. At this frequency, the feedback is inaudible, but if the power supply has enough current capacity, it can cause R6 to burn out. The feedback can only occur if the input is left floating. Also, the circuit is more susceptible if loose wires are connected to VIN and OUT as these act as antennae.

If the input is hard wired to a previous stage, for example a preamp, you probably will not experience a problem. However, if the the input will be open circuit some of the time (for example, if it is wired to an audio jack socket), you can take the following steps to remove the feedback loop:

• If the input, output or power are connected with loose pairs of wires, twist the pairs together to reduce their effectiveness as an antenna.

• Alternatively, use shielded cable for the input and output.

• Load the input with a 10k resistor. Connecting a 10k resistor from VIN to GND will reduce the sensitivity of the input to interference.

• If the input of the amplifier is wired to a jack socket, you may find an extra terminal that connects to the signal terminal when there is no plug inserted (see the datasheet for your connector). By connecting this to ground you can ensure the input will not be floating when nothing is plugged in.


From TDA2050 datasheet (R4 = R6 above):

Component: R4; Recommended value: 2.2R; Purpose: Frequency Stability; Larger Than Recommended Value: Danger of Oscillations

Datasheet doesn't give any info for components except the values, in the kit list R6 is 0.25W film type, this is nowhere near high enough power rating IMO. What's the point of it burning out precisely at the moment when the circuit needs it most?

HTH
 
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Post by johnyk » Mon Jun 18, 2012 3:31 pm

Dave the bass, yes the fault is somewhere else, my first fix was just replacing the r53 and c71, but not the ic..... R53 just melted,

since my first post I replaced R53 again also replaced the ic 'lm1875t'
R53 smoked and the amplification was very noisy, i quickly turned off the amp, so you are right the fault somewhere else,

i then, just to see what would happen, unplugged the speaker and plugged in external speaker cabinet and turned up,
R53 did not melt or smoke any more although like in your post and what other people have been getting the amp was very noisy and whistling,
also i tried the cd input connected to the external speaker, the whistling wasn't there but there was excess noise......

i will read further on tinterweb and ask more knowledgeable friends of mine and if it gets mended i will post what i did on here, at the moment however i'm a little stumped, i hope the things i have tried may shed some light on what the problem with our avt's could be.

oh and the very first thing i did before any thing was fix all the dry joints, because among others the speaker pins where dry.

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Post by Mike H » Mon Jun 18, 2012 3:59 pm

Try it with the input shorted to ground maybe. :?:

If it still does it there may be a broken PCB track or bad connection or something like that.

It is increasingly suggesting (to me) something of that sort. It shouldn't be this difficult to make it behave.
 
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Post by johnyk » Wed Jun 20, 2012 2:52 pm

Yes thank you that worked, thank you.

the original problem must of been the all the dry solders in the amp causing snap crackles and pops, and whining.

After I removed the amp from the casing and fixed all these bits I failed to realise that the casing connected to the input jack was earthing it just like that extra wire Mike H mentioned, so this was the problem when I tested it not fully assembled I was missing this all important earth wire, silly me!!

and so that R53 just kept burning out and wild noises blasted from my amp.

So i have a new ic chip, new r53, new c71, and i have strengthened the existing earthing from the input jacks by cleaning the contacts and putting a tiny blob of solder just to make sure, between the earthing chassis and the jack contact earth. and put a new valve in it.

my amp now works thank you for your help and to all who put info on this forum........ however i now have a new problem with it

there is a noise like a second saturated guitar in the backround that just rings on a fraction of a second behind what i have just played, it increases when the reverb is turned up, the amp didnt do this before i had any problems with it .....

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Post by Mike H » Wed Jun 20, 2012 5:48 pm

A-ha!! Mystery solved. Super Image

Edit: as regards the original problem, not sure about the second :?
 
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Post by chris661 » Wed Jun 20, 2012 10:42 pm

Try the old valve again - I'm wondering if it hasn't gone microphonic.

Chris

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Post by johnyk » Thu Jun 21, 2012 10:30 am

well thank you again mike H, and chris661 cheers, tried both valves i have, the one that was in is only about 6 months old and have tried a brand new one in it too, sounds same unfortunatly :?

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Post by Mike H » Thu Jun 21, 2012 3:47 pm

"it increases when the reverb is turned up"

Isn't that what 'reverb' does?

Could be just the spring-line vibrating? Is it a spring-line? If so, at which end is the reverb level control, input to the reverb line, or output? If input, then rattle the springs, it'll come through...
 
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