Orange Amp Repair – Rockerverb 100 MKII


This Orange amp repair is a particularly shocking example of an amp which someone has attempted to repair without really having an understanding of what they’re doing. The customer brought it to me with two issues: a broken reverb (victim of the botched repair) and an intermittently faulty gain channel.

orange-amp-repair2I started out by performing a basic valve amp service upon the amplifier. This involves testing all the valves with my valve tester. 3 preamp valves were found to be faulty: this proved to be the cause of the gain channel dropout. This Orange amp repair didn’t require checking of the bias of the amp, because the Rockerverb MK II has Orange’s proprietary ‘DIVO’ power valve management, meaning that the amp biases itself. It’s the only system that I’m aware of that doesn’t require matched valves in the output stage, so when one of the EL34s was found to have a snapped key on its base, I installed a 6L6GC in its place! I also fixed the snapped key so it could be used.spring-reverb-repair

Turning my attention to the spring reverb repair, I noted that one of the phono cables had a broken connection. The connectors had been made up without any strain relief and this had allowed damage to happen during normal use. This is an easy fix and after fixing the broken connection I resoldered the other reverb intact connector so that both had adequate strain relief.

Upon testing the faulty spring reverb I discovered that it still wasn’t fixed so I turned my attention to the unit itself. Worryingly, one of the sockets was surrounded with glue on the outside. When I opened it up I realised that some fool(!) had tried to fix the loose socket by flooding it with superglue. Worse, as you can see in the picture the glue had dripped inside the unit and fouled the springs quite badly.

I was able to remove the superglue, but they remained quite distended. The unit still works as a reverb so my customer decided not to have me replace the tank.

If you’ve got an Orange Amp repair or a spring reverb repair, please don’t bodge it, just get in touch!

Marshall JCM900 repair – Ampworks Newark

Marshall-JCM900-repair-2 copy

This JCM900 repair arrived after the owner found that it was lacking in volume.

It’s a 2009 unit, two channel, with a 2203 type circuit on channel A and a higher gain circuit on channel B.

The problem appears to have been with the amplifier bias. Although the valves were matched, the bias on this JCM900 repair was set very high. The bias set the dissipation at about 95% – Marshall suggest about 70%. This causes the output to compress much earlier than would otherwise occur. Fortunately this is easy to fix.

I then offered to soak test the amp which is a means of testing the amp in the conditions it would be in rehearsal or gig. This is great for revealing those hard to find problems that would otherwise missed. It revealed a minor fault with the preamp – a crackle appeared behind the note transients. Since this was a relatively minor problem, the owner chose not to have it looked into.

I sometimes have been known to complain that some amp manufacturers give little thought to ease of repair – mesa boogie have had some stick from me in other reports. Some marshall repairs – thinking the DSL range in particular – are a bit difficult because of their complexity but this JCM900 repair is a delight to work on. They’ve really thought things through both in terms of reliability and ease of repair. It’s probably the easiest channel switching amp to repair that I’ve seen.

Although the channel A had a great plexi type tone, the owner isn’t a big fan of the Drive sound in ChannelB – he’s thinking of modding the amp to acheive the fabled Silver Jubilee tone so stay tuned – this one might turn up again.

If you have a JCM900 repair, please do get in touch.

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!

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.

Valve amp blowing fuses – Fender repair – pro reverb

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.

Marshall JCM 2000 Repair – TSL 100


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.

Cornford Repair – MK50 ii

Cornford Repair

This Cornford repair came in for a service. Conford amps are often beautifully constructed amplifiers that sounds as good as they look! The owner uses the MK50 ii bias switch to use 6L6 and EL34 valves. He was using E34Ls (different to EL34s!!) and wished to have the valve amp biased to move the EL34s again. For further details of my service procedure, please see this page

There is some question over the future of Cornford after the departure of designer Martin Kidd (‘He’s so proud of it, he put his name on it!’ – see pic 😉 ). I don’t know how true all the gossip is. I don’t know how it may affect Cornford owners if they need their amps repairing, but I do know that if you have a Cornford repair or a valve amp bias job, you can get in touch. Your baby will be in safe hands!

Hughes & Kettner Repair – Tube 50

Hughes & Kettner Repair

This Hughes & Kettner repair is a tube 50 brought in for a valve amp bias. The Tube 50 is a fixed bias amp, meaning you need to bring the amp to a tech to have it adjusted (confusingly!!) by a technician. The opposite of fixed valve bias is cathode valve bias. Confusingly cathode bias is not usually adjustable. To make sense of this, check out this article

The Tube 50 is a 2 channel valve amp with three ECC83 in the preamp and two EL34s in the power amp. My apologies – the image is a stock image – not the actual Hughes & Kettner Repair.

If you have a Hughes & Kettner repair or a valve amp bias job, please get in touch.

Laney Repair – GH100L

Laney Repair

What is it with laney amps that makes me forget to photo them?

This Laney amp bias GH100L just needed its bias adjusting to accommodate a new set of EL34s. The GH100L is a fixed bias amp, which means the bias is adjusted (and quiescent current is fixed) by a technician. The opposite of fixed valve bias is cathode valve bias. Confusingly cathode bias is not usually adjustable. For more info, check out this article

If you have a laney repair or a valve amp bias job, please get in touch.

Marshall Repair – JCM2000 DSL100

Marshall Repair

This Marshall Repair was brought to me with one of those sneaky ‘hard to find’ problems. The amp was blowing fuses when the customer was playing it. He’d taken the amp to another repairer who hadn’t found any problems. Unfortunately the amp continued to blow fuses.

Guitar amplifier repairs of this kind often have to be treated in a different way. After an initial examination, opening up the amp and checking for any burnt components or loose joints I placed the amplifier on soak test. This involves playing a representative signal through the amp into a dummy load in the conditions that the amp was in when it was blowing fuses. During this soak test I observed that under certain conditions one of the power valves was sparking internally. I swapped the set of valves, re-biased and then placed the amp back on soak test. Since the amp wasn’t blowing fuses or sparking any longer, I decided that it was ready to go.

If you have a Marshall repair job that needs doing, please get in touch via the contact page.