Keyboard amp repair – Carlsbro Colt 45.

Keyboard amp repair Calrlsbro colt 45-1

This keyboard amp repair was for a Worksop based violinist who uses the full range speaker with his electric violin.

The amplifier was obtained second hand and had developed an intermittent problem in which the amp would drop out almost completely, leaving only a tinny high frequency signal. At first, I suspected that the LF driver or the crossover had become damaged, leaving only the HF driver active in the coaxial speaker. Normally, you would expect an HF driver to be damaged first, but LF drivers are still possible to damage.

However this wasn’t the problem. In the end I fixed the amp by replacing the corroded FX send jack and phones connectors. The shunt contacts on both of these interrupt the audio signal so they must be kept clean. To complete the Keyboard amp repair I then put the amp on soak test for 2 hours to verify that there were no further problems – there were none!

I shot a quick video part way through diagnosis:


If you need a keyboard amp repair, please drop me a message.

Art valve Preamp Repair – Ampworks, Newark

No fault found with this Art tube mic preamp repair, however I did get a chance to do some interesting tests on the unit’s distortion characteristics.

The first picture shows the unit distorting with signals at three different input levels. The Blue trace is basically clean. Part of the valve ‘sound’ is that Valves clip the ‘top’ and ‘bottom’ of the waveform in different ways. Positive excursions are clipped at the input by grid current limiting, while negative excursions are clipped by the output cutoff against the rails, potentially leading to an uneven waveform. This can be seen in the green trace, in which one part of the waveform is rounded off (‘soft clipped’. The Art preamp clipping LED glows orange when valves are clipping. In the pink trace the unit heavily slipped on both top and bottom of the waveform. The clipping is slightly more symmetrical, although it can be seen that one side is’softer’ than the other. The clip light goes red in this condition.


The second graphic shows the changes in the harmonics created by the clipping. The uneven clipping tends to emphasise even order harmonics, with 2nd harmonic at 2K being larger than 3K and so on. Confusingly, waveforms that are clipped evenly create more odd harmonics. YOu can see that 3K harmonic is now greater than the 2K, the 5K greater than the 4K etc.

So there you go. If you have a valve preamp repair job that you think I could help with them please do contact me.

Vintage Vox repair – 1961 AC30 repair

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


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.
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

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


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.


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!

Vintage AC30 repair – 1962 Vox repair

1962 Vintage Ac30 Repair

The guys over at Confetti in Nottingham brought me this vintage AC30 repair. It appears to be a 1962 JMI era Vox model but I’m an amp tech, not an amp historian, so I may be mistaken! It’s definitely an AC30/6 model, only made in smooth black vinyl in 1962. It’s very nice!

The customer brought this to me because they were worried about the amplifier getting very hot. I also noticed that the power cord was damaged and that the amp had a ‘Pin’ style voltage selector common in vintage amp repairs. I always recommend that this is wired out if not already.

The power cord has been replaced and the voltage selector wired to 245 (best match for modern UK), so the switch is now a dummy.

The overheating was caused by leaking reservoir caps which were replaced. Electrolytic caps have a lifetime of about 20 years so leaking caps are common in vintage amps. Faulty capacitors can lead to a much higher current draw through the transformer, overheating it, causing the issues we’re seeing. It also ties in with the internal HT being over 100V too low.
Vintage Ac30 Mains transformer temperature rise
I noticed that the tremelo mode switch had come loose and the resistor wires have sheared at the body of the device. They were replaced using vintage style carbon composition resistors for authenticity’s sake. (Do carbon composition resistors make a difference to tone? According to R.G Keen, The jury is still out!). If you have one of these vintage units, make sure that you keep the switch tight to avoid similar problems.

The amp didn’t run especially hot after the caps had been replaced. The output valves are all matched and measure sensibly, and the amp is cathode biased, so there’s little risk of the output stage being out of spec. I had another vintage AC30 repair in (a cream ’61 model), so I was able to make a proper comparison of the running temperature of the two amps. You can see how close they were in the graph above.

I’m pleased to say that the customer was very happy with the vintage AC30 repair! If you have a Vintage vox, or any guitar amp in need of repair, please get in touch.

Mesa Boogie Bass amp Repair

Mesa Boogie Bass 400+ Repair

This mesa boogie bass amp repair had the highest power valve count I’ve seen. The amp uses no less than twelve 5881 6L6 valves!

Like the recent fender amp repair this valve amp was blowing fuses. Also like the fender repair, the failure was down to a damaged power tube. Two in fact.

Much is made of Mesa Boogie’s insistence upon the use of mesa branded valves on guitar forums and these guys come in for quite a lot of stick. You can read Randall Smith’s defence of the idea here. My opinion is that the principle is reasonable for the majority of non technical users.

In this case, the amp had been fitted with Sovtek valves. Aside from the two failures many of the valves no longer matched well.

By testing mesa boogie valves in my valve tester I know what non mesa valves are within mesa parameters for use in their amps. I could tell that 4 of the sovteks were out of spec, meaning that 6 of the 12 needed replacing. After completing this simple task the amp was fixed.

If you have a mesa boogie bass amp repair please contact me via the contact page.