Fender deLuxe Hot Rod ,2002

Do you mean R97? 1.6k 2 watt?

Ron(UK)

Yes, i was reading it upside down , goes to tip of footswitch plug, but I would call it a 1W, when I replace it, it will be bigger. Not the main problem which is drop in gain and distortion intermittently

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

There is a repository of guitar amp schematics on the web, including most of the vintage Fender models, including their many revisions. Google would probably pull it up, or I might have bookmarked it. I'll check....

jak

This is a good site for diags

Ron(UK)

Indeed it is. I don't think it was the one to which I referred...possibly even better!

jak

The gain on both channels is too high in my reckoning. Feeding only a 1mV RMS, 400 Hz, (600 ohm source) into a guitar amp input I would not expect the output to be on the uncomfortable side of listening to continuous sine at vol setting 3.

0.12V ac rms over 8 ohm speaker at "3" and 0.39V ac at "6". At "3" setting 0.19V ac (measured as if it was sine RMS) on both splitter driver outputs No distortion at that sort of low level input. What and where in the preamp to look for somethiong amiss?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Have you tried it with a guitar?

Ron

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splitter

Nope, I've always intended picking up a magnetic pick up from somewhere but never have, let alone a guitar. But all other amps i've fed this sig gen into then around 100mV is the norm for just speaker output signal monitoring purposes, rather than dummy load checking

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

How do you repair guitar amps if you dont have a guitar? It`s not all to do with specs and test gear you know, some guitar amplifiers have huge amount of gain and are intended to produce a quite specific colour of distortion.

Do you have a typical stage microphone to test PA amps?

Ron

input

listening

but

norm

load

Balanced or unbalanced Sure mics for those tests and sig gen for guitar amps. I repair amps, not disrepair them. So a good sine in and a good sine out, with any luck a clean channel, or at least minimisable distortion. If its a "repair" job concerning the wrong sort of distortion then I turn them away. I find muso terminology for such stuff impenetrable as do they, not understanding temminoloy as cross-modulation, clipping etc.

I have quite enough clutter here without a guitar as a test instrument and I still would not understand what they're interpretation was for the wrong distortion/s anyway.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Don't you understand that striving for a clean distortion free sound isnt right with a lot of guitar amps? A perfect waveform doesn't give the sound a player might want. It`s the distortion that gives them their characteristic sound. Setting up the bias for example - the technically correct bias isnt necessarily the right bias (if you get my drift) They aren't hifi amps, they are designed to colour the sound the way a guitarist likes.

Ron

Your measurements mean nothing in a guitar valve amp. They are designed to distort. A Lot.

1. Put a sine wave at around 400 Hz into the power amp in jack, use a dummy load, set the bias with a scope, measure the power output and observe the waveform to make sure its not far off what it should be. Thats your power amp working then.
2. Plug in a guitar and play it. If you can't hear anything amiss or don't know what you should be listening for then don't bother trying to fix valve guitar amps, or you'll be going round in circles measuring things, wondering why there is so much distortion and the frequency response isn't very flat at all.
3. If you are going to be fixing valve amps you need a stock of valves. Best bet is to change the whole lot to a set you know works, then go through the amp fixing any faults. Then go back and fit the originals, changing any that are bad.

Gareth.

As you say Ron, part of a valve amp's sound is the fact that it can be made to go smoothly dirty, and often even has a channel designed to do just that by deliberately overloading the second or third stage with pre and post gain controls. I always think of the old Canned Heat number 'Let's Work Together' as a premium example of how a guitar sound can be horrendously distorted, whilst still being sufficiently musical to find its place in the song. I've no idea as to whether that guitar was pre-processed with a fuzz pedal, but I have heard stage musicians get a similar sound from just making use of the overdrive facilities on their amp. 100mV does sound like a lot of input compared to a guitar pickup, though. About the maximum that I normally drive these things with, is 20mV.

Arfa

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Besides, testing with a guitar is the fun part.

1 hour repair =3D 10 mins to replace bad capacitor + 50 mins testing with your favorite guitar...

And she`s buyyiying a staiairway to hev ennnnnn

but

Schema for this one , or near enough , was on that site, ac voltages agree reasonably well throughout from input to output. I assume that with the test

4mV in and gain controls at 50 percent they mean using a dummy load or serious ear defenders required.

After all that it looks as though it is the 8 ohm / 4 ohm (auto) diverter switch in the unused Switchcraft 1/4 inch external speaker socket. It looks like a silicone rubber loop around the tip contact is the way around that, after cleaning. The internal speaker tip contact could do with one also. Its only 5 years old

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

What happens if in process of owner stowing away or pulling out the cables , foot switch etc from that convenient stowage box next the speaker, he unknowingly pulls out the internal speaker plug. Turns the amp on and tries turning up the wick to get some output. With no speaker plug inserted the 8 ohm line from the o/p transformer, it is grounded via the built in switch contact in the socket like an input socket..

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

In that case very little will happen, the switched socket shorts out the output. However if the socket or speaker cable were open circuit then there`s a good chance that the output tranny, valves or valve bases would flash over.

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We were forbidden to (try to) play that where I worked...