Repair job bounce

He was running line-level signal from a mixer into the input, had the line out from the amp connected to the same mixer, and accidentally routed the output directly to the input

** Very devious thinking there.

The actual scenario is quite simple and what many musos do all the time.

Guitarists that is ...

..... Phil

Related to some kind of distortion or effect box before the input, like maybe one of those tube compressor/limiters that have a transformer coupled output and put out a hot signal. Or running the guitar into a mic preamp first and using similar output stage.

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** Normal use, no unusual devices and something he had done many times.

..... Phil

Smashed his guitar on the amp on stage of course trying to be Jimi Hendrix

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** Might have felt like hitting me with his guitar though.

When I explained it was a very clear case of input overload damage that I had done a mod to prevent in future, he asked rudely:

" Why didn't you do the mod the first time ? "

As if.

..... Phil

..... Phil

Bit strange - you say the circuit is all solid state except for the

6L6GC output tubes. But the schematic shows a 12AX7 is used too (V1). What is odd is that the pin-outs at the bottom right of the diagram show only the 6L6GC base connections. The 12AX7 base connections aren't mentioned. Probably just a simple omission.

--

Jeff

I hope you tell us the answers later...

Some remarks, not necessarily what you were looking for:

Pulling both speaker outputs leaves no line out signal.

Running with open output may cause 6L6 internal flash over. If that reaches g1, Q3/Q4 will suffer.

The 75K/680E dividers before Q3/A4 may not bias them properly into class AB. It depends on the 6L6 characteristics.

The D16,D17 diodes will lower the max reverse Vbe but hardly to a safe value. And they overload the TL072, shifting it's operation point causing distortion. Maybe a selling point for guitar amps...

Arie

------------------------ Phil Allis>

** Of course, it's not safe to run a tube amp with no load.

** Can be fatal for any tube amp.

** Q3 & Q4 are special selected devices from Motorola.

SJE = special type.

Bias is within limits with them installed.

D16 and D17 temp compensate the bias.

** No, Q3 & Q4 have low input impedances due to NFB.

The TL072 cannot produce more than sufficient drive.

** Not before the 6L6 tubes are square waving.

You are miles off.

..... Phil

Played 'loud' with the amp turned off.

Cheers

--
Clive

----------------- Phil Allis> >

** Not the cause

You have to what made the TL072 fail.

.... Phil

No, it isn't The 'S' prefix is how Motorola marked house numbered parts for customers. Motorola quit making transistors, two decades ago. They may or may not have been screened at the factory. There are several reasons for Ho use Numbering. One is to reduce employee theft, and another is to prevent w arranty fraud.

Their Semiconductor products were spun off into ON Semiconductor in 1999.

In another thread you made a comment about ';an old HP Curve Tracer. They n ever built any for sale. If you do some research they built one and only on e for use in house by one engineer. Only one unit. It wasn't until they bec ame Agilent that they started producing equipment to test semiconductors on the production line.

The guitarist follows this ritual first, before the smashup:

1. power on the amp
2. turn the amp gain all the way up
3. turn the guitar gain all the way up
4. stand in front of the speaker and face it
5. plug the guitar into the live amp
6. turn the gain down a smidge to stop the shrieking feedback
7. if U1's still alive tune the guitar otherwise exit the ritual
8. strum the A chord
9. strum the D chord
10. strum the E chord
11. play an ascending arpeggio
12. play a descending arpeggio
13. pick through the first few bars of "Stairway to Heaven" a few times
14. Go back to standing in front of the amp and face it to create the shrieking feedback necessary for a Hendrix state of mind.
15. play a rough rendition of the "Star Spangled Banner" once

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
She set out one day In a relative way And returned on the previous night.

never built any for sale. If you do some research they built one and only one for use in house by one engineer. Only one unit. It wasn't until they b ecame Agilent that they started producing equipment to test semiconductors on the production line.

I used to have a Marconi badged sig gen that Marconi never made. It turned out to be some other mfr they had rebadged, then their evaluation rejected. It was afaik a 1 off.

NT

The feedback around the output transformer back to IC4 is DC coupled, the line out jack is DC coupled too, and if the line out jack is connected and there's a DC offset/ground potential difference on whatever you plug it into you get a big DC offset at the output of both the IC-4 drivers

DC offset at IC4 outputs won't couple through the capacitors to do harm.

How are you going to get DC offset across the transformer exactly?

I think a problem is the shorted output transformer when no speaker plugs are in will also short out the feedback so if there is any input the amplifier will try its very hardest to amplify it.

piglet

Are D16 and D17 shown right way in that drawing?

piglet

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** ROTFL !!

..... Phil