Every now and again, I get a vintage amp repair from the early days of amp manufacture. For what it’s worth, I call vintage amp repairs repairs that involve working on 60s and 70s equipment. Commonly, they’re vintage vox repair, vintage fender repair or vintage marshall repair. This page shows some of the work!
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.
This Fender Brownface repair was something of a labour of love here at Keld Ampworks. It’s a fascinating amp – having started life presumably in America, it’s got a 110V transformer. At some point it made its way to Belgium, where it was ‘converted’ to EU voltages, using a rather scary transformer bolted to the inside of the woodwork. It was later bought by the current owner and brought to the UK.
This was my first Fender Brownface repair. I’ve done blackface Fenders, Tweeds and Silverface fenders but never before a Brownface. Nice to have something new.
My first task was to make the amp safe. The Fender ‘death cap’ is well documented elsewhere so I won’t dwell on it. Suffice to say that it was removed, and a 3 core earthed mains lead with US plug fitted. The fuse and mains power switch were moved to the ‘live’ line. They don’t make ’em like this any more! The scary open frame in-cabinet transformer was also removed from circuit and replaced with a removable US-UK transformer. This makes the amp more ‘original’ and also safer. Double win!
Most of the preamp tubes were still good. The power valves were replaced with a new set of Sovtek 5881s and one preamp was replaced. The valve sockets were all tensioned and cleaned.
Checking the filter caps inside I saw that 2 out of 7 had already been replaced – but with underrated parts (350V instead of 500V!). Of the remaining five, three were leaking electrolyte and so after consulting the customer I replaced all 7. I was able to preserve the original filter cap covers and use them to conceal modern Rubycon parts at 700V.
At this point the amp was much more stable but had a few intermittent crackles and bangs. Many of these were sorted by replacing some coupling capacitors.
The last issues were with the ‘Vibrato’ channel. The vibrato modulation was bleeding through horribly onto the normal audio signal. This turned out to be further cathode bias and coupling issues.
I was quite intruiged when a Nottingham guitarist brought this Marshall Silver Jubilee repair to my Newark workshop. These amps have taken on a bit of a legendary status, due in no small part to their association with Slash and Joe Bonamassa. I love how this amp looks, and it’s got an interesting tone control design, that deviates from the common Fender and Marshall designs.
The amp had blown a power tube, which the owner had replaced without rebiasing, tut tut! It still had very weak output, but occasionally it would blast out at full volume. It was also in need of a good pot clean.
First thing to do was to disassemble the amp. Marshall amps from this era are quite nice to work on and easy to take apart.
The described problem was caused by oxidation in the FX loop circuit. This is a common problem with many amps and was easily fixed.
I observed some crackling on the EQ potentiometers which turned out to be slightly leaking coupling capacitors in the tonestack.
I then attempted to rebias the amp but discovered another problem. The plate voltage of 490V requires a bias current of to give 70% bias. I couldn’t get higher than 18mA with the valves the customer has fitted. Viewing the output waveforms in the scope, the amp showed significant crossover distortion. I checked the bias supply and found it to be working correctly. Although they tested within spec in my valve tester, it just wasn’t possible to bias these JJ EL34s in this particular amp. I fitted a new set of EHX valves and was able to bias the amp properly to 34mA.
This amp sounds great, even when I play it!
If you have a Marshall silver jubilee in need of repair, please contact me
Now, is it good things or bad things that come in threes? It must be good things. This was the third of three vintage vox ac30 repairs that came to me over a few weeks in summer 2015. Vintage amps have a particular smell and when there are 3 vintage vox amps in the workshop for repair you soon become very used to it!
This amp is probably the toughest repair job that I’ve worked on over the last 6 years. Another repair company in Lincolnshire, experienced with vintage amp repairs, had looked at the amp but had ultimately had to give up. I don’t really blame them.
This vintage Vox AC30 (there’s a picture of it on the 1961 vox page too) is I think a 1970s model. I’ve estimated it at 1973, with substantial modifications through the years. However, I’m an electronics geek, not an amp history geek, so I may be wrong. Someone who’s opened up the amp in the past has written “1978 ish ? 1986 as well, mostly ?” inside it. I judge it to be 1973 because the vox logo is a traditional VOX logo as used in the 60’s, but this could have been swapped during it’s lifetime. Early 1970s vintage Vox AC30s used PCBs too. STragely, this doesn’t look like any of the 70’s PCBs that I could find. So, the jury’s out on this one!
Some 70s vintage Vox AC30s came with reverb circuits, but this amp uses a solid state reverb circuit that I can’t find any record of in 70s amps. You can tell that the amp didn’t ever have a valve reverb because it’s a 10 valve unit. The ’78 reverb unit used 11 valves with the extra triode used to buffer the signal into the spring reverb and to make up gain after it. But the amp had a reverb market on the front panel, I assume this must have been fitted later.
I’ve spoken before of the peculiarities of doing amp repairs on guitar amps and other electronics is that the actual process of repairing can take just 5 minutes, but the fault replication and diagnostic period is often much much longer. This problem is made worse in situations when the problem disappears intermittently. This one would disappear as the amp warmed up, after 20min or so.
The fault turned out to be a loose turret connection between the normal preamp volume control and the phase inverter. The connection appeared to be physically intact with a good solder joint. Grounding the point through the pot appeared to have no effect. However when removing this turret it broke in two. There must have been solder holding the piece together, but not making a reliable connection.
After that was fixed, we decided to wire out the solid state reverb unit and return the unit to vintage AC30 spec by adding back in the LoGain input on the vintage channel. While doing this I noticed that someone who’d worked on the amp, probably whoever fitted the reverb had wired up the input jack wrongly, leaving the brilliant channel jack socket with a high resistance to ground at all times. This turned the unit into a pickup point and the amp was feeding back on its own at high treble settings, with no guitar plugged in. Nightmare! Anyway this was soon fixed.
When finished, this is easily one of the nicest sounding amps I’ve ever played. I love the way the Vintage Vox AC30 circuit distorts. It’s easy to see why it’s become a popular circuit for the likes of Dr Z, Trainwreck, Matchless, Bruno etc. to copy. You can see a bit of it in the quick video I did to celebrate it’s finish!
This vintage vox AC30 repair took a good deal of effort, and I was the second repairer to work on it, bit I’m glad to say that it is now fixed. If b you have a vintage vox AC30 and it’s in need of a bit of TLC, please so contact me via the contact page.
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.
This Vintage Fender tweed repair is the oldest amp I’ve worked on to date. I’m informed it was made in 1953, its a 5C3 design.
The 5C3 deluxe tube complement is 2x 6SC7 for preamp and phase inverter and 2x 6V6 running a push pull output stage to about 12W. The amp uses a valve rectifier – the stock is 5Y3. The customer was running the whole amp run from a nice meaty 240V to 110V transformer from maplins to provide the US mains voltage.
The customer brought the amp in as not working and requested an HT capacitor refit.
I noticed that the HT voltage gets to 500V at inrush before dropping to below 450V (the rating of the existing caps). Since the amp doesn’t have a standby switch this means that the caps are subject to significant stress at turn on. I experimented with a 5V4 rectifier valve to reduce the inrush current, however the 5V4 results in a higher HT with not much headroom before the 450V rating. I recommended 600V caps for the repair to allow for a good safety margin.
600V 15u/16u caps aren’t that common in these days of low voltage electronics – unfortunate for those attempting a vintage fender tweed repair! So we used 350V electro caps in pairs. This didn’t look as pretty, but functionally provides the same performance. I never recommend NOS electrolytic capacitors as electrolytic capacitors degrade even when out of circuit.
I removed the low value bleed capacitor on the primary side of the mains transformer as this is considered unsafe by today’s standards. If the cap becomes faulty and passes DC then the amplifier chassis can become live. Unlikely, but not nice! I always insist upon following mains safety procedures – even on vintage amplifiers.
The fault with the amplifier was actually a loose ground connection underneath the eyelet board – simply fixed.
One last problem – these 6SC7 valves in this particular period of vintage fender amps have a horrible tendency to be microphonic. The speaker vibrates the cabinet, the cabinet shakes the rather old valve base, which shakes the valve and the whole thing takes off in LF feedback. There’s a probably a reason Fender dropped them after this model! I first assumed that the fault was with a worn out valve but on ordering a replacement I now believe that this is likely to be a feature of all 6SC7s. The solution I found was to replace the valve base with a more mechanically rigid Belton one, to stop the valve moving in its base. Interested to hear from anyone else who’s experienced this!