In solid state equipment a schematic diagram is needed for quicker fault diagnosis. Design faults are not found very often because unlike valves, transistors fail as soon as their maximum power ratings are exceeded. Also deterioration of materials is not as serious (as in valve equipment) and most of the circuitry operates at lower voltages. Where catastrophic faults have taken place in output stages however, full pcb reconstruction and rewiring might be necessary (see IN PCB’S). Transistor circuits are more electronically complicated and sometimes require lengthy fault diagnosis.

Due to printed circuit board design, in most cases, solid state (and sometimes tube) modern amps must be completely dismantled in order to replace one or two components. Problems are more unpredictable in transistor equipment and in output stages knock on effect, catastrophic faults are often seen. There is more diversity in solid state than tube electronics design. Full restoration may not be necessary in old transistor gear provided the safety issues have been addressed.

There is room for improving reliability though. For instance, in output stages removing and cleaning the heat sink from the old silicon compound and replacing it with new compound, will improve thermal bonding between semi-conductor and heat sink. Also, since potentiometers and switches are screwed on the front panel and soldered on the pcb, it might be a good idea to remove them individually clean them with special switch cleaner. The old solder is removed, the print cleaned and the component resoldered etc.

In hybrid amps for example Music Man or 1960s hifi, anything might be necessary from a simple repair to a full reconstruction if the equipment is very old. Old transistors are very hard to get these days. In modern equipment like Marshall Valvestate the work is a lot more straightforward.

There are factors to be considered when restoring and repairing a valve amp which has been used a lot over the years. Note that pubs, recording and rehearsal studios can be hot and humid places. The amps get hot during use and then cold when they're left in a car overnight. This is thermal cycling. There are also design faults associated with modern tube amps (see NOTE below).

In valve output stages the plate pin/terminal of a power pentode/tetrode such as the EL34/6L6/6550/KT88 is right next to one of the heater terminals (which are always connected to zero volts to avoid hum). Consider now the deterioration of the insulating properties of both pcb material and tube socket due to condensation and thermal cycling. Arcing and burning can take place in this very small physical area between plate and heater.

Internal arcing can cause a valve to fail regardless of external conditions such as dampness and high temperature. Quite often the reason is poor speaker connection when the primary output transformer inductance is the only component connected to the terminal and the magnetic field collapses. If the output becomes open circuited at such high voltages (approx 460v dc) while the amp is driven hard, even for a fraction of a second, then output tube failure is almost certain. Arcing sometimes can also happen externally between the pins on the Bakelite valve base and/or on the pcb/tube socket where it shows as a black carbon coating. Black carbon coating also appears sometimes on the glass of a tube, ie internal arcing and must be replaced. The AC current fed to a 4 ohm cabinet at 100w is 5 amp. If the speaker connection is not good it will reflect back to the primary of the output transformer as a load of a much higher value. This can cause internal/external arcing and excessive screen current. Incidentally its no good replacing a single output tube in push pull stages, they must be replaced as pairs or quads etc.

What has just been mentioned (i.e. intermittent connection of the output load due to condensation and poor speaker connection) can also lead to output transformer failure, which can be of an intermittent nature.

Condensation degrades the insulation between windings. Internal arcing can happen within the transformer creating carbon deposits leading to more leakage and intermittent failure of the output tubes. Sometimes it is impossible to trace this type of output transformer fault unless one dismantles the transformer itself!

If the insulating properties of the PCB material have been deteriorated, a leakage current can be developed between the plate voltage and the control grid. This can cause intermittent reduction on the bias causing thermal runaway, ie red plates or “cherry-ing”.

Now a general problem with fixed-bias circuits in tube amps is the value of the grid resistor. For instance, RCA recommend a maximum of 100k for the 6L6 tube.

High grid resistance value could make the grid become less negative at operating temperatures due to leakage grid and gas currents. A less negative grid voltage results in more plate current, temperature rise and more gas and grid emission current, therefore a knock-on effect that causes tube thermal runaway.

Condensation and thermal cycling is a perfect recipe for hidden dry joints, as any old TV repair engineer will tell you. Dry joints in the control grid circuitry can cause intermittent tube failure. Also at full output power if one or two or three of the output tubes become disconnected occasionally (because of poor connections i.e. worn tube sockets or dry joints) then the remaining other tubes will have to do all the work and will eventually fail. 

IN PCB’S
The remedy for the deterioration of the pcb material in output stages is to remove all the damaged area (due to burning for instance) and fill it with high-strength/high-temperature epoxy (sometimes mixed with colloidal silica).  The solder has to be removed from the suspected dry joints (sometimes most of it) and one has to access how strong the adhesion of the copper track is onto the pcb. Then new solder has to be applied and where the adhesion is poor, the copper track must be removed, and the connections must be hard wired using copper wire of appropriate gauge..

A tube amp over 25 years old needs to be thoroughly looked at and treated as a restoration because the switches, sockets and terminals are made of Bakelite and other plastics. Would you have old used plastic electrical fittings in your home? These amps become hot, work on very high voltages, in today's electronic world no other equipment works under these conditions. The insulation in ceramic tube sockets does not deteriorate and carbon deposits due to arcing can be cleaned. However, the metal part of the socket weakens and it doesn't hold the valves as well so its a good idea to replace them too. Fender valve sockets do not hold the valves well even if they are relatively new for instance. In some amplifiers the diameters of the holes must be enlarged in order for the new better quality replacement sockets to be fitted and this can be a lengthy procedure.

A visual inspection under a magnifying glass is necessary to check for corrosion and material wear. Because of the high voltages electrical safety is paramount. Old fuse holders and electrical switches must be changed. The mains switch makes the valves glow but the second one is a high DC voltage switch responsible for about 480v in many tube amps. A big part of a job is to remove solder from old deteriorated joints, grind and clean the terminals until clean metal can be seen; then resolder using a little bit of solder paste and clean again. Cracks, blisters or other structural problems must be investigated. In older amps, (especially VOX AC30s) high voltage terminals suffer from horrendous structural problems. These terminals must be replaced using new high quality tag/turret boards. Sometimes new terminal boards must be constructed using materials such as Nylon 6/66 and individual tags.

In restoration jobs the output stage including the biasing circuit must be rebuilt. The characteristics of various components drift with time and use so they need replacing. The output valves are replaced, its fired up to see how it performs. I do a series of bench tests using a dummy load, overdrive and so on, to check how each valve performs after the amp has been correctly biased. If the maximum ratings of a tube given by the manufacturer are exceeded tubes fail on a statistical basis, unlike semiconductors which fail on an absolutely certain basis. NOTE : This is why some amplifiers which exceed those maximum ratings may work with certain valves for years and then one puts in a brand new good set and biases the amp properly but then after a while you end up with one or two blown ones. THIS IS A COMMON DESIGN FAULT ON MONDERN VALVE AMPS AND A PROBLEM WHEN IT COMES TO REPAIRING THEM!

The position of certain components may have to be changed, away from components that are heat sensitive. Some resistors are replaced for ones with a higher power rating. I check the condition of high voltage electrolytic capacitors, and may change some of them if they have blisters. Switches and sockets are checked and the whole amplifier has to be cleaned with special electrochemical's. Some potentiometers and signal sockets and switches may need replacing too.

I then do a sound test which leads us on to the small valves, and of course when the amp starts being used. one may discover other little problems... noisy operation, low gain and repairs on circuits like vibrato, reverb etc might be necessary too.

"When I got the amp a couple of years ago, I had it fully serviced by my good friend Theo Argiriadis as it was producing some noise from its speaker outputs. Theo knows these amps inside out, and carefully checked out the amplifier, replacing some tired capacitors and resistors in the process. He also recommended fitting a standby switch to the rear of the amp to allow the amp to warm up before switching on the HT circuit and hence prolong the life of the valves (the new switch on the rear engages mains power, the original front switch turns on HT). After testing, he commented that this was one of the best Cadets he'd heard. We decided to leave the original Mullard output valves and Brimar preamp valves for the time being as they were still within spec and sounded good. Since this service, the amp has had occasional use and has performed faultlessly."

 

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