Fender deLuxe Hot Rod ,2002

cables ,

it is

How much does the matching transformer heat up with a direct short ?

What happens to the output valves, droppers etc I can't believe they are self limiting ?

Years ago someone asked me to repair a Selmer valve amp that he'd shorted the speaker lead on and the primary side, high impdeance side, of the o/p transformer went o/c , but I never asked how long it was "running" like that.

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

I look for a little crossover distortion when a client wants an early breakup. Have a look at this blokes site;

And view the "How To" biasing demos. This is exactly how I have been doing it for years.

If I were you I would take some time and do some research about how valve amplifiers work, you clearly have little idea. A short circuit is way safer than an open circuit. An open circuit feeds up to twice the HT voltage back into the output valves and can destroy them and lots of other stuff in an instant. A short circuit will take much longer to do any real damage, generally longer than a guitarist will spend playing his guitar with no sound coming out of his speakers.

Get yourself a book on valve amplifiers and read it, you will be amazed about how different the discipline is to solid state. It's never too late to learn.

Gareth.

High Tension - the High voltage supply rail, usually 300 to 500 plus volts. Shove double that back into an output circuit and it is likely to go bang.

Gareth.

This is where my UK / USA Tool Terminology Translator

comes in

Or it would if Google had not decided to de-index it a couple of months back

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

transformer,

the

shorted

o/p

safer

back

to

Left to me I would have been perfectly happy leaving valve stuff back in the fifties and earlier. But these musicians and HiFi nuts will keep buying the stuff and then expect people to repair mid 20th century technology that is unbelievably still being made in the 21st C.

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

screen

On/Off

off

I tried lacquering a test spot but it did not like the metal, would wear away as quickly as the Lettraset type silkscreen printing. I did add a few white Lettraset legends totally worn off A sheet of celluloid over the lettering and under the bush nuts and taped ,hidden, along the sides looks quite presentable.

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

For us 'across the pond', what does 'HT' stand for?

Michael

I'm very glad some of it's still being made, otherwise it would be much harder to find tubes for vintage equipment. I'd never worked on a piece of tube equipment before until a couple years ago when I got an old radio but I was pleasantly surprised how easy it was to deal with, I don't see how it's unreasonable to expect people to repair it, I was able to adapt so any real tech ought to be able to as well. I'm under no illusion of tubes sounding "better" but I do think they're cool, like much other old technology.

B+

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I don't think you realise quite what you're saying there. We had a debate about this on here some time back, and it was questioned as to how relevant valve technology was, when these amplifiers were just old relics of a bygone age, and basically not much sold / made any more. A number of people were surprised at the statistics supplied by the store that I carry out this service work for. The owner said that approximately 70% of his new gear stock, was valve, and that every 'serious' guitar player aspired to owning a valve amp. As far as James' contention that there is not a "better" sound from a valve amp, I think that very much depends on what you're putting through it. There is little if anything better sounding than a Fender Strat played through an (original) Twin Reverb. The smooth response, gentle clipping, and inherent 2nd harmonic distortion, all contribute to a sound that is particularly suited to a guitar.

I think that this needs to be understood, along with a fairly deep understanding of general valve theory, in order to be successful at carrying out this sort of repair. If Mr Cook really dislikes the gear to the extent that he appears to, then I would venture to suggest that this is not a very good platform to be working from, and perhaps he should stick to the semiconductor amps, that he seems happier with.

As far as Gareth saying that valve guitar amps are "designed to distort a lot", I'm not sure that I quite agree with that. I would suggest that they are designed to be fundamentally clean, but with the *ability* to distort a lot, in the right way, in the right hands. In many instances, a musician will require a clean sound from his guitar or other instrument, or even vocals, and it would be a poor design indeed, that couldn't give him that ...

Also, it is perfectly possible to carry out repairs to this equipment, without being able to play a guitar. I've been doing it for the better part of 40 years. All that is needed is a good understanding of how the equipment

*should* perform under test conditions, and this is largely down to experience. If a 'muso' feels that there is something wrong sound-wise with his amplifier, it's usually possible to strip the nuts and bolts meaning out of his muso-speak, and find a technical reason for the problem. Your 'scope and experience will soon tell you when you've resolved that problem.

Arfa

What is the US term for EHT (Extra High Tension) , say more than 1KV ?

B++ ?

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

relevant

bygone

a

Strat

Sentences like "There is little if anything better sounding than a Fender Strat played through an (original) Twin Reverb" are absolutely useles on an electronics repair forum.

Now if you could point to a site that has electrograms and spectrum alalysis plots, and sound files, of differnet guitars/pickuups/amps/cabs being played in different modes along with , (consensus if possible) muso "technical" language in their terms, their audio description, then it would indeed be a very useful addition. With an extra file giving sound files of common fault conditions simulated, ie fault induced clipping rather than musical effect, pops , hiss, crackle, hum, buzz, half complementary pair signal, etc that techies could point owners to, for such noises appearing intermittently and rarely on the work bench.

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

B+ is the plate supply, no matter how high the voltage. It is from the early days of radio:

The 'A battery' powered the filaments.

The 'B battery' powered the plates.

The 'C battery' provided the grid bias.

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What do you call the seriously high PDA (post deflection acceleration voltage) in an oscilloscope or TV, surely something other than B+ ?

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

I don't understand, in general valve gear is far far easier to repair than modern surface mount solid state stuff, Once you get a grip on the basic theory, it`s all so logical.

Ron(UK)

It doesn`t really work like that with valve music gear. anyway, most of the stock faults on the more common amps are well known among the guys (and gals) who repair the stuff for a living, and well documented on the web.

Ron(UK)

Well, not if you work on this stuff a lot, and have at least a basic understanding of the 'musicality' of particular guitar / amplifier combinations. If you hear a cheapo 'starter' guitar, played through a fully solid state amplifier more suited to vocals, or a keyboard, then listen to say a Fender / Fender combination, you'd know what I mean. I suspect that most on here who do this work seriously, know exactly what I'm saying.

Despite what some on here would sometimes have you believe, occasionally, it

*is* just subjective. I would actually think that it would be pretty difficult to analyse the difference to the point of being able to demonstrate it, technically. By suggesting that this was 'not your thing', I intended no offence to your abilities. I know many engineers that are very good in many fields, but not in the repair of valve amps. I am sure that there are others out there who, unlike me, are properly musical, that have a far better ear for problems than I do.

As an example, a few months back, I had a valve band amp in that just 'didn't sound right'. The owner said that it was quiet, and sounded 'thin'. It wasn't actually all that quiet, but maybe a little compared to what he was used to. 'Thin' was a good description of the sound. You couldn't really say that it lacked in bass, or any other register really. It just sounded, well thin ! With a sine wave and a 'scope and a power load and meter, there was barely anything amiss with the output waveform or power level. The one thing that did show on the 'scope, was a slight asymmetry to the wave, but it was slight, and could have easily been missed. The problem turned out to be the screen feed resistor on one of the output valves (just a 2 rather than a 4 valve lineup). It was completely open, so there was no screen voltage on that valve at all, which would have meant that it was barely working, so you might have expected to have seen a much more distorted waveform at the output, as you would have with a semiconductor amp.

When the resistor was replaced, and that valve's contribution to the output stage was restored, there was little difference in either the overall sine output power, or the waveshape, but the sound was now much better than before. It now sounded 'right'. Now I know that doesn't explain the mechanism of why or just how it sounded 'thin' before, and 'round' after, but as an engineer doing a lot of this stuff, I knew that it was now repaired, and the owner agreed. He made a point of calling the shop, and telling them how 'good' (there we go again ...!) it sounded compared to how it had for some time. So I make no excuse for using phrases on here like that I used with the Fender ...

Arfa

B+ is a class of supply, B is high voltage, and + indicates it is negative ground, so all readings will be positive. Individual supplies may be labeled: 'Second anode', 'High voltage', 'Focus'. It depends on the item. You see transmitters with thousands of volts, at high current. Before the metering it is still B+, and after may be called the plate supply, or plate voltage.

You need to download the scan of Radiotron Designers Handbook (4th edition)

is one source. This is a

25 MB download, but has more about Vacuum tubes than any other book you will likely find in a single book than any other.

has other books on tubes, as well.

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