valve amp repair

Vintage Vox repair – 1961 AC30 repair

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Vintage Vox Repair

Shortly after the Confetti 1962 vintage ac30 repair arrived, this vintage Vox repair turned up on my bench too! The 1961 in the picture is on the right, the left amp is a 1970’s vintage vox, which you can read about here.

This is (as far as I can tell) a 1961 model. It was once covered in a cream vinyl, but the customer (who had it from new!) had decided at some point that it would look cooler in black and painted it.(!)

After laying down his axe many years ago, the customer decided to sell the amp and brought it to me to service.

I found that the brilliant channel didn’t work. This turned out to be an out of tolerance resistor in the gain stage and a faulty coupling capacitor. Also the tremelo system wasn’t working. In vintage vox circuits, this consists of 6 triodes worth of analog electronics. 2 are an audio input buffer, 2 create a modulating signal and 2 mix the modulation with the audio signal. The fault was with the modulation circuitry and was again down to a worn out capacitor. This is not an uncommon task in vintage Vox repairs.

As with the Confetti 1962 vintage ac30 repair, I hard wired the mains to 245V for UK operation and replaced the ancient power cord with a modern tri rated mains lead.

I’m pleased to say that the customer was very happy with the vintage vox repair! If you have a Vintage ac30, or a vintage amp in need of repair, please get in touch.

Egnater Repair – Rebel30

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Egnater-repair-rebel30

This amp is a Rebel 30 by Egnater. At first, I had a bit of trouble with the Egnater repair but I’m pleased to say that I found a solution for the customer. The diagnosis itself didn’t prove a problem, the problem was simply the gain potentiometer itself. This is a dual ganged part (2 pots in one). One half of the pot behaved fine, the other half was open circuit in the first part of the turn. This caused a loud clunk at the point where the track re-appears (see the video).

There was no maker’s mark on the pot so I couldn’t be sure what brand it was: probably a far eastern part sourced by the contract manufacturers. It’s quite easy to find dual ganged 500K pots from guitar gear suppliers but they’re designed to fit inside guitars or in ’boutique’ hand wired amps – none of them will fit in to the space in this tiny Egnater repair.

Of course I tried to contact Egnater, but received no reply. Unfortunately I wasn’t been able to find an alternative panel mount part that will fit into the space constraints inside the case. There are switches and capacitors internally that would prevent the available chassis mount parts from being suitable.

However I came up with a fix that sorts some of the issues with the existing pot. There is still a small bit of silence for the first part of the turn, but the very loud bang is mostly gone, with only a whisper remaining, inaudible unless you’re listening for it. The sound returns whilst the channel is still within the ‘clean’, not yet crunchy part of the drive channel turn, so all the useful elements of the gain channel remain.
Egnater-pot-repair
Mathematically, the fix I’ve employed affects the potentiometer ratio only very slightly (as in the graph), I’d argue imperceptibly. The truth is it’s probably even closer than the theory suggests, as real pots don’t curve this smoothly! I’ve done a video comparing the tone before and after the mod, to demonstrate that there’s no real difference, but who knows, you may pick up a nuance. As you can hear, the bang is very evident in the video before the mod, but inaudible after.

The video appears here:

I’m pleased to say that the customer was very happy with the Egnater repair! If you have a Rebel 30, or another Egnater in need of repair, please get in touch.

Blackstar amp Repair – Series 1-45

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Blackstar amp repair

This Blackstar repair was a simple valve amp service before the amplifier was sold. The amp is a series 1-45 2×12 combo. It’s a fantastic 2 channel amp with 4 modes, bright Clean, warm clean, Crunch and Super Crunch.

The amplifier uses Blackstar’s DPR and ISF patents, you can read more about Blackstar’s DPR on my valve amp attenuators page.

The amplifier was performing well, but a test on the valve tester turned up two faulty preamp valves which were replaced. The week before, I’d had another Blackstar amp repair, but as they’re a fairly new brand, I don’t get many Blackstar repairs!

Blackstar amps are one of the many types that I repair, please get in touch if you need my help.

Blackstar Repair – HT40

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blackstar-amp-repair

Recently, a Blackstar amp repair was brought to my workshop in Newark. The amp was an HT40 (the Club 40 combo) in working condition, but the owner wanted to try a new set of EL34 power valves in the amp. He selected a matched pair of Tung Sol with slightly higher gain (according to my valve tester) than then outgoing stock Ruby parts.

However, whilst checking the amp over my valve tester highlighted a fault in one of the Sovtek preamp valves, so I’ve replaced this with a JJ ECC83S.

HT-CLUB-40-Repair

There’s an interesting and fairly unusual element in these Blackstar HT amps – they appear to use a transistor Phase inverter (you can see it in the image at the top), so all the 4 gain stages in the 2 12AX7 preamps are used purely for gain in the preamp circuit. They sound great. This unit doesn’t use the DPR circuit that I mention on that page.

Anyone who knows me as a player, rather than a tech will know I’m a bit of a Blackstar amp fan. I play a Blackstar Series 1 50W head (recently upgraded from a heavy Blackstar Series 1 45 combo). I almost bought one of the these HT40s actually!blackstar-valve-amp-bias

If you have a blackstar repair, please get in touch!

Valve amp blowing fuses – Fender repair – pro reverb

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Fender valve amp blowing fuses

A customer brought this fender repair to me with a common complaint. The valve amp was blowing a fuse every time the standby switch was turned on. A valve amp blowing fuses can be a number of things, but in this case I suspected that the amp probably had a faulty power valve.

This turned out to be true. One of the 6L6 valves had a fault that causes a high current to pass between anode and cathode causing the fuse to blow.

This can occur in all valve amps with all types of valve.  Elsewhere on this blog, there’s a Marshall valve amp blowing fuses that was down to a shorting EL34 valve. There’s also a more recent mesa boogie valve amp blowing fuses.

Once a power valve is replaced, it must be rebiased. I set this fender back to manufacturers spec. Matching power valves should always be used.

If you have a valve amp blowing fuses, please do Contact me for advice.

Post by Daniel Wicks.

Ampeg Repair – SVT pro ii

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Ampeg-repair-svt-ii-bass-amp

What a huge beast this Ampeg repair is! Weighing in at an incredibly heavy 32kg I wasn’t sure that my workbench would take the strain!

It’s an SVT ii powered by 6 KT88 output valves meaning two massive transformers, owner for three mains and one for the output stage. When both must be capable of 300W continuous, that’s a lot of iron!

The owner was finding that the amp would drop in volume after a few minutes of running. It’s a common problem that I was quickly able to trace back to an oxidation issue, meaning a nice low cost repair for the customer.

If you have an Ampeg repair that needs attention, please get in touch , but you’re carrying it in!

Mesa Boogie Repair – Maverick (Dual Rectifier)

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Mesa Boogie Repair - Dual Rectifier Maverick

This Mesa Boogie repair was one of the nicest amps to play, but one of the longest repairs of the year I’m afraid!

In this case, the Maverick had a problem with the channel switching. After a few minutes the rhythm channel would increase in volume, and the lead channel would bleed through an unpleasant fuzzy distortion. The fault was fixed by the opto-isolator replacement in the switching circuit.

It’s never a good thing to discover that your amp is faulty, but a broken amp is particularly bad news for owners of Mesa Boogie amplifiers. This is because Mesa Boogie construct their amplifiers in a way that lengthens the repair time.

The reason is that the amplifier is soldered into position on all four sides. Any changes to the board must be done after completely dismantling the amp (taking 30minutes each time). If several things are replaced in a more complex job, then the costs quickly mount up. Before I add to any myths, it’s NOT because the amplifier is PCB, it’s perfectly possible to make a PCB amp that’s easy to repair! It’s also possible to make a Point to Point amp that’s a nightmare to repair as well – I’ve seen them!

Anyway, what an absolutely great tone this Mesa Boogie has when it’s cranked. It’s completely unlike the rest of the Dual Rectifier series. The Black Widow speakers in this particular combo are brilliant speakers.

If you have a mesa boogie repair, please do get in touch and we’ll talk it through.

Marshall JCM 2000 Repair – TSL 100

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Marshall-jcm2000-repair

Marshall JCM2000 repair

I get quite a lot of the JCM2000 series in for repair – both DSL and TSL. I think it’s because they were so popular, rather than any endemic issues. This particular JCM2000 is a TSL100, the triple super lead model.

I’m writing the blog post quite a while after the repair now, so I’m looking at my notes to remember the issue. IIRC, the amp needed new valves and the owner mistakenly paired valves 1+2, 3+4, instead of the correct 1+4, 2+3. He knew how to bias his own amp and knew about pairing valves, but had always replaced for a matched quad in the past. When he paired the valves wrongly things had become hot and smokey – the valves red-plated For more information on valve amp bias, check my bias FAQ.

The amp had become so hot that a screen grid resistor (R78) – mounted upside down in the amp – had melted its solder connection and fallen out of the amp, melting some cabling where it landed. I replaced this resistor (with a lighter weight, but equally rated component). I also replaced the burned out suppression capacitor that can be seen in the picture.

The lesson to be learned: read my bias FAQ before replacing power valves yourself. If in doubt, ask!

If you have a JCM2000 repair, do get in touch.

Fender Repair – Blues Deville

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Fender-Repair-Blues-DeVille-Repair

Fender repair time!

This is a lovely Blues Deville fender amp that’s seen some serious loving! The owner has been gigging it for 15 years. Unfortunately, first the Low gain input died, then the high gain became intermittent. The owner asked for a full valve amp service and I promised to repair the jacks as part of the service.

Repeated insertion and removal of the jack had moved the jack socket and over time pulled the PCB pads from the PCB. This led to the intermittent crackling that was heard and would have eventually torn the thin copper trace off.

Quality chassis mount switchcraft models were used in the repair to replace the two input jacks. These were wired into the PCB using flexible wires to remove the mechanical coupling to the PCB. Now if the jack socket moves then the wires will move and the PCB will not, and the integrity of the connection will be maintained.

If you have a fender repair, please get in touch.

Post by Keld Ampworks Guitar Amp Repair.

Myth Avoidance: Amp geeks often debate PCB vs turret or tag mounted components. I have to say that unlike many independent amp builders I’m a believer in PCB technology, but for mechanically and thermally stressed parts it is wise separate the component from the PCB to avoid causing stress. This fender repair is a good example of that principle.

Vox Repair – AC30CC2

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Vox Repair, AC30 Repair

Much as its nice to be taxed by a complicated repair, I love a simple job too! Despite a number of problems, this Vox repair was an easy fix for the customer, in fact, thanks to a dodgy screw the hardest part was getting into it! The customer complained of a crackle and intermittent volume dropout – I was able to sort both problems out for just £30!

These amps with the Alnico Blue speakers do sound great. I’d recommend checking them out.

If you have a Vox repair that needs some attention, please get in touch.