I don’t get many ENGL amp repair, so I was a bit annoyed with myself for not photographing this one. I’m grateful to the owner for helping me out by sending me a photo.
Here were the symptoms, in the words of the customer:
Overly noisy (hissing / popping) on both clean and overdrive channels at all times (with guitar plugged in or not)
A couple of noisy (scratchy) pots e.g. Clean and Lead Volumes
Loss of top end frequencies
Clean channel intermittent partial volume reduction coupled with distortion.
The first three symptoms were caused by the 12AX7 (ECC83) valves in positions 1 and 2, which were leaking DC onto the valve grid. This was also causing the scratchy pots.
The fourth symptom didn’t show up obviously during bench tests until I checked the output power. The amp delivered just under 50W into 16 ohms via the 16 ohm transformer tap, but only about 10W into 8 ohms using the 8 ohm tap.
I ordered a new output transformer from ENGL. I fitted it the day that it arrived and this fixed the problem completely. The amp was then soak tested for 90 minutes at gigging power levels.
After some discussion with the customer upon collection, it transpired that he was running two eight ohm cabs in parallel into the 8 ohm speaker tap. This would cause a higher current in the transformer secondary, which may have caused the failure. The correct way to run two eight ohm cabs with this amp is to connect them into the series sockets on the 16 ohm tap.
If you’re in need of an ENGL amp repair, please contact me
This LabGruppen repair was actually for me! As is often the case with people in this trade, I never quite get round to repairing my own gear because I’m so busy with customer’s work. This LAB1200C power amp died with a spark and blown fuse at a New Year’s Eve gig with Newark soul and motown band, the Acetones. Fortunately, a friend bailed me out during that gig and was good enough to lend me an (almost) identical amp to aid with diagnosis.
The amp is a four channel class AB amp, but runs from a switch mode power supply to reduce weight. Cunningly, the Switch mode power supplies can be configured by an ‘MLS’ button to provide ~64V rails for 4 ohm operation, or ~82V rails for 8 ohm operation.
This looked at first like a simple fault – In a SMPS the mains voltage is rectified into DC before the isolation transformer and there were dry joints on the primary side capacitors. This accounts for the symptoms. However after repairing this, the unit powered up but the fans were running at maximum speed. At this point the job was shelved for a significant period of time as customer’s work became a priority.
The construction of the amp is quite complicated, but upon returning to the job, the fix was actually quite simple. A transistor in the heat controlled fan circuit had failed, fooling the circuit into thinking the amp was overheating. After replacing this the amp was fine and passed soak test
All amps are soak tested after completion to ensure sustained high volume performance.
If you need a labgruppen repair, please get in touch.
This is a JHS pedal repair for a good friend and guitarist who now works in London. It’s a two pedal overdrive. Its a Sweet Tea V2 – sounds great!
The pedal had been damaged by the use of the wrong power supply. It was a fairly simple repair to complete. No schematics are available for JHS pedal repairs, but there’s not much new under the sun in distortion pedals. The only funny thing about this one is that they have a top copper pour for V+ and a bottom copper pour for GND. It’s normal to pour GND only so that had me confused for a few minutes.
If you need a JHS pedal repair, please get in touch.
This Blackstar HT100 repair was completed for the co owner of an exciting new Grantham based venture ‘Melody Music Rooms’ in Grantham.
Melody music rooms are a rehearsal space, recording facility and teaching space located on Westgate in Grantham . It’s the brainchild of a group of Lincolnshire and Nottinghamshire music teachers who wanted to provide better music services in the area. Not just a building, they run jam nights and busking sessions featuring local young (and old!) musicians. It’s great to have these guys raising the profile of live music in the area, so do check them out on https://www.facebook.com/melodymusicrooms/.
I’m told that this amp has played Wembley(!) as the owners of Melody Music played some high profile support gigs with their old band. It had been retired when it started blowing fuses, but the owner wanted it brought back to life for the new Melody venture.
There were a couple of valves to replace and the amp had blown the bridge rectifer diodes. Unfortunately Blackstar don’t provide schematics for their amps so it’s always a bit of a tough job repairing Blackstar gear.
Normally at this point I’d put a clip of my repair video, but here’s the repaired amp being used in the studio at Melody Music Rooms.
If you need a Blackstar HT100 repair, please drop me a line.
Because the Hot Rod Deluxe is the world’s most popular valve amp [citation needed], I get a Hot Rod Deluxe Repair arriving quite regularly.
The amp has two common faults, which are well documented:
The most common fault is the Low voltage power supply (LT supply) failure. This powers the opamp driven reverb and effects loop. Fender create their 16V low voltage power supply from a 33V and create significant heat in the dropping resistors and zener. This heat then causes the copper to delaminate from the PCB, leading to crackles and bangs and occasionally complete signal dropout. You can see the heat issues on this picture, taken with my thermal camera. The dropper resistors are getting hotter than the power valves! (Update 2020: repair kit available here):
On older hot rod deluxe repairs there’s some PCB retracking work to do to repair the PCB damage. On newer amps I can just take preventative action – replace the 5W dropper resistor with a 5W part raised off the PCB with ceramic spacers and replace the replace the zener also raised off the board.
The second common fault is the grey ‘IC’ (Illinois capacitor) electrolytic filter caps which are prone to failure. Fender use these presumably because they’re the cheapest 450V axial caps around. In fairness to Fender, there are probably hundreds of Hot Rod Deluxe amps using these caps that are still working, but there are also a lot that fail. I use a mix of quality F&T (German) and Nichicon (Japanese) capacitors to replace these parts and I recommend replacement on all Fenders when I’m already removing the board.
I overrate all caps significantly The cost increase is only a few pounds but leads to better performance and improved lifespan. The most important caps are C33 My preferred configuration is:
C36 (overrated by 100V)
C35 (Overrated by 50V)
C33 (overrated by 470V using 2 series caps!)
C31 (overrated by 470V using 2 series caps!)
Update Sept 2020: At the time of this post (2018), great quality Nichicon and F&T axial caps were still available. 2 years later, Axial caps are going the way of the dodo so I’ve switched to using high reliability radial RubyCons with a special adapter board (which I’ve made available to others as a repair kit available here).
The other thermal ‘weak point’ is the footswitch circuit, which gets the same treatment as the other hot resistors in the LT supply.
If you need a Hot Rod Deluxe Repair, please contact me.
This Peavey Classic 30 repair was a bit of a nightmare!
It arrived with a fairly basic problem – the amp wouldn’t turn on. This was down to a loose screw. It held the mains fuse holder internally. When this came loose, the fuse holder was dangling in mid air and shorted mains directly to the metal case of the unit, which caused the fuse to blow – which is a good thing!
I spent an hour on it – tested all the valves and checked the plate and other power supply chain elements – all the preamp valves are good, but the inner power amp valves no longer match so I changed these. There was no means of adjusting the bias so it’s a question of juggling valves until I find a set of EL84s that properly bias.
I then soak tested the amp. This involves running the amp at gigging voltages with a pink noise signal for a couple of hours. I do this to check that there aren’t any problems with the amp that only appear after a longer period.
In this case, the amp started making a crackling noise after the two hour soak. This turned out to have a twofold cause – there was a leaking cap in the tone stack and the HT electrolytics (IC brand) were also worn.
Unfortunately the design of these early peavey Classic 30 amps uses single wire links between the boards. It’s a nightmare to disassemble and these little wire links are a common cause of failure. This slowed down the diagnostic quite severely
If you need a peavey classic 30 repair, please do contact me.
This Polytone Minibrute Repair came in from a Lincoln customer with an obnoxious hum problem. This unit is probably form the 1980s
The issue was solved by adjusting the output stage bias point and improving the connection between the screw mount power supply capacitors and the power amp stage.
The amp is a bit of an oddball in terms of construction, with the preamp contained in the top of the amp, and the power amp in the base of the unit. The power amp unit is held together by the capacitor screws and two large heatsinks.
Another first for Keld Ampworks – the first AMI Jukebox repair. I’ve turned down jukebox repairs in the past because I naively had visions of a 7 foot unit arriving in my rather small workshop! This customer was quick to clarify that the amp unit itself is quite small and transportable from his Lincoln location. Nonetheless he sent me some pics of the full unit (above).
It is a US unit, bearing original battle scars and running on 110V through a transformer.
It was exhibiting a loud hum and excessive breakup. The customer asked me to do a basic valve test and check over and then report back.
It runs a preamp with one 12AU7 and one 12AX7, then a 12AU7 cathodyne phase inverter and a cathode biased 6L6 pair. One 12AU7 tested faulty (replaced with a valve from the customer’s collection), but the main problem was down to the badly worn 6L6 pair (replaced with a brand new matched pair of Russian EHX valves).
US amp manufacturers in the ‘old days’ didn’t tend to Earth their amps. All amps that leave my shop should have correct earth, fusing and safety practice so I fitted an external earth to this unit. Since the unit goes inside another unit this was fitted with a flying lead and solder tag to screw to the main chassis.
This is a favourite toy of the customer’s grandchildren and I’ve since heard that they’ve given it a thorough testing!
If you need an AMI jukebox repair, please get in touch.
Actually, let’s start with a slightly different question…
Are valve amps louder than solid state amps?
Yes, valve amps often are louder than solid state amps. As an example, we could take a Marshall MG30 (30W) and a Vox AC30 (30W) and compare the two. There’s no question at all that the Vox will sound significantly louder to any objective listener. Or take a Fender Blues Junior (15W) and a Fender Frontman 15 (15W). The perceived volume difference is huge.
But hang on, given the price differences in those examples we’re not really comparing apples with apples are we?
How much louder are valve amps than solid state amps? Let’s talk about perceived loudness
It’s often said that a valve amp sounds “twice as loud” as a solid state amp. There’s a whole field of psychoacoustic study to delve into here. For the purposes of this discussion, let’s focus on one psycho acoustic rule of thumb (source).
a power increase of 10dB results in double the perceived volume.
So if we look at the electronic (and electro acoustic) design of valve amps and solid state amps and we can account for 10dB of difference then we can begin to understand why some valve amps are louder than some solid state amps.
Why are valve amps louder than solid state amps?
1. Player demographic.
This is the biggie.
Transistor technology is more affordable than the thermionic valve. There’s no doubting that.
So the cheaper amps are often solid state, the more expensive are often valve.
Cheaper amps are more likely to be sold to ‘bedroom’ players and children. Someone who’s gigging is more likely to spend more and be able to afford a valve amp.
The amp development engineers know this, so they design ‘bedroom’ amps to give sensible volume control in a bedroom range. The user can turn the volume dial to about 12 o’clock before the parents complain. They design ‘gigging’ amps to give sensible rehearsal volumes from about 9 o’clock and full on ‘fight the drummer’ volumes at 12 o’clock. This is just sensible design.
If we were to give this concept an ‘electronicsy’ name, we’d call it the amplifier sensitivity. It could be measured by turning all controls to a certain position (eg. 12 o clock) and determining how much signal we need to put into the input jack in order to get 1W output.
How much difference does this make? As very few manufacturers list AC measurements on their schematics it’s very hard to put a dB figure on this without experimentation, so I’m not going to make one up! I’ll be measuring amps as they come it to try and put a figure on it!
See point 1, solid state amps are often used at the price conscious end of the market
Speakers have a sensitivity, measured in dB/Watt
Let’s look at some real speakers, chosen at random from Celestion’s range:
Celestion Vintage 30 – used in mid and high end gear. 100dB/W
Celestion 70/80 – used in low mid range amps. £500ish. 98dB/W
Celestion Rocket 50 – used in amps around the £200-350 mark. 95dB/W
You can’t get manufacturer efficiency data for the guitar speakers used in even cheaper amps, but you can be sure that most will be poorer. Some specs are available for cheap 12in PA woofers such as the MCM 55-2973 at 91dB, roughly 8-10dB lower than comparable high end PA woofers by Fane, Eminence etc.
Already we can see that an amp with a high end speaker has the potential to be 10dB louder than a cheap amp with a low end speaker.
Cheap speakers have low sensitivity
So what happens if we use the same speaker? Read on…
3. Output load rating and power amplifier current/voltage control
Valve amps use an output transformer. Although its primary purpose is just to match the high impedance valve output with the low impedance speakers, there’s a side benefit – many transformers have tappings to allow a valve amp to match a 4 ohm, 8 ohm or 16 ohm speaker. This means that the amp delivers roughly the same power into any load.
Very very few solid state amps have an output transformer because they’re not needed – it’s easy to create a low impedance output with transistors.
So this leads to a scenario in which a solid state amp outputting 15.5Vrms will deliver roughly 60W into 4 ohms, 30W into 8 ohms and 15W into 16 ohms.
When the engineering department tell the marketing department this, guess which figure the marketing team put on the front of the amp?
A real world example:
a Marshall MG100 solid state amp:
20Vrms = 100W into a 4 ohm speaker,
20Vrms = 50W into an 8 ohm speaker,
20Vrms = 25W into a 16 ohm speaker.
Meanwhile a Marshall DSL100:
20Vrms on 4 ohm transformer tap = 100W into a 4 ohm speaker
28Vrms on 8 ohm socket = 100W into an 8 ohm speaker,
40Vrms on 16 ohm socket = 100W into a 16 ohm speaker.
It is possible to build a voltage drive (with a switch) OR current driven solid state amp that will deliver the same power into different loads. But guess what? It’s more expensive due to the higher voltage rails required, see point 1.
Depending on the scenario, this can result in between 3dB and 6dB difference in power output between two amps when they’re plugged into the same speaker UNLESS the speaker is 4 ohm
4. Output impedance
Most solid state amps are voltage controlled (certain era Roland JC, Marshall 80XX series, Fender ultimate chorus and some Trace gear excepted). voltage control results in a low output impedance
Valve amps with no negative feedback around the transformer have a large amount of current control
Rod Elliot of ESP (https://sound-au.com/valves/valve-trans.html#s21 ) does a worked example that shows that these differences
As for how much difference this makes, it’s hard to say for certain. Time to experiment, methinks!
5. Valve power amps are designed to distort
A distorted signal has a higher RMS (up to 1.414 times higher) than a clean waveform. Valve circuits are often more distorted than solid state, even on the ‘clean channel’. However FETs and diodes can be used to distort solid state amplifiers.
But solid state power amps are often not designed to distort. Why, is it the tone? Well it is true that some solid state amps distort in an unpleasant manner, but many distort quite pleasantly!
In truth, the primary reason goes back to point one again. Distortion increases the RMS of the voltage signal, which increases the power that the solid state devices must be designed to dissipate, so more money must be spent on heatsinking and cooling them and more money must be spent on providing a power supply that can supply extra the power because…
Simplifying the maths here, if a clean signal passing into a power amp is fully distorted into a square wave, it will increase the RMS voltage (or current) by about 1.414x.
So cheap solid state amps (and some cheap valve amps) will normally allow the power to sag significantly when distorting the output stages.
Best case scenario: The described increase in voltage equates to 3dB more power. However, you’ll only actually hear this level difference when the amps are dimed and the power amps are distorting and when comparing a fully distorted valve amp to a totally clean solid state amp. So it’s not a reasonable comparison.
6. Distortion adds harmonics
This is the first one that’s actually related to technological differences between valves and solid state.
A distorted signal has a higher RMS than a clean waveform. Valve circuits are often more distorted than solid state, even on the ‘clean channel’. However FETs and diodes can be used to distort solid state amplifiers.
It’s also true that the distortion adds extra frequency content in the most sensitive part of the human ear.
Conclusion
At the start of this article we discussed the claim that valve amps sound twice as loud as solid state amps. We equated this to a 10dB difference in sound level.
Volume control sensitivity: To be Confirmed by experiment
Speaker sensitivity: up to 10dB difference
Output load rating: 0-6dB difference, depending upon the cabinet impedance
Output impedance: unknown
Distortion: up to 3dB difference, but only when the amp is dimed.
Harmonics: unknown.
With various combinations of the factors above, we can easily account for 10dB difference in power and thus a perceived doubling of volume, but what we’ve really seen is that thing that results in valve amps sounding louder than solid state is the target cost price of the unit as it’s designed. If the engineer designing a solid state amp for a pro market were given the same target cost price as the engineer designing a valve amp for a pro market, then a solid state could easily be designed with:
a sensitive volume control,
expensive 100dB/W or greater speaker,
4x voltage headroom to accommodate different speaker loads
Current drive to cancel output impedance effects
Multiple power devices and sufficient heatsinking and a large power supply to allow substantial power amp distortion
And we’d end up with a solid state amp that was as loud as a valve amp. Let’s do this!
Sadly I’m sharing the story of a line 6 DT25 repair nightmare.
The amp was blowing fuses. This was a relatively easy diagnosis despite a not very repair friendly construction. The power transformer was damaged, by a short, probably on the primary side.
After a bit of research it appears that the line 6 DT25 is prone to these failures.
I contacted Line 6 (owned by Yamaha) technical support for repair. It took a bit of persuasion to get a response, but I was finally directed to a third party company in Norwich called sontec, Yamaha’s spare parts dealer in the UK.
I ordered over the phone in December and was forewarned of a 6 week manufacturing lead time, which is reasonable. The part cost £90 something
After 8 weeks the customer services representatives on the phone still could give me no indication of delivery date.
In February they informed me that the part had been manufactured and it would be sent out within a couple of weeks.
After repeated phone calls I was finally informed on 2nd March the item had shipped from Europe. 5 1/2 weeks after the shipping date.
I was then informed on the 9th March that the item had arrived in the UK with Sontec and would be shipped out directly.
It was actually shipped a week later on the 16th, according the Sontec packing note.
It then arrived 4 months after ordering in a damaged condition, with the PCB connectors smashed.
Sontec were unable to provide replacement plugs or even a part number for me to buy my own, saying I’d have to wait for another transformer, subject to similar lead times. They did however refund £30 of the original cost
3 months I’m still waiting for the replacement transformer but I’ve fashioned a fix for the damaged connectors and got the amp returned. I’m not confident that the replacement will ever arrive.
My customer was a retired pro musician from the golden era of British blues music. These days he records extensively and releases on reverb nation. You can check his current material here. He was understandably annoyed with the terrible support on the amp, but was kind enough to leave positive comments about my work.
Review. Rowan did a fantastic repair on my Line 6 DT 25 amp !! He quickly diagnosed the fault and expertly repaired it. I can highly recommend him !! He’s also a very nice man and that helps a lot !!
I’m a bit wary of taking on another DT25 repair. But if you’re in the same position do get in touch anyway.
