Vieta is an audio equipment brand created in Barcelona in 1954 by the Vieta brothers, becoming a major reference for high-fidelity audio in Spain at the time. It still exists today and continues manufacturing audio products.
After some searching, I was able to get this Vieta A-225 amplifier, which I had been interested in for a while.
The A-225 model, in particular, was the last tube amplifier they produced. From subsequent models onward, they made exclusively transistor-based amplifiers. Moreover, it was one of the best Spanish tube amplifiers built in its era.
It is a stereo amplifier delivering 19W RMS per channel, with many inputs: Auxiliary (for example, for ceramic cartridge turntables), high-level reel-to-reel tape, radio tuner, magnetic cartridge turntable, low-level reel-to-reel tape (directly from the heads), and microphone.
It incorporates 10 tubes: Two ECC83 as low-level preamplifiers, another two ECC83 as tone control preamplifiers, two ECC83 as phase inverters, and four EL84 in push-pull pentode configuration as power amplifiers.
It also has a headphone output, switchable via a panel switch, loudness compensation, stereo-mono switching, treble and bass filters, and a monitor output to use as a preamplifier line output, to listen to what’s being recorded on tape, or to connect an external reverb unit.
As soon as I received it, I disassembled it to check the tubes and the inside of the unit:
All the tubes were original Siemens. After testing them with the tube tester, they showed about 73% emission on average, which is pretty good and means they have seen relatively little use.
Inside, it looks practically as if it were new. The only odd thing, which was likely the original fault that put the amplifier out of service, was that one of the filter capacitors had its plastic washer broken—the one that holds it to the chassis—causing it to be loose and therefore not making good contact with the chassis ground. This resulted in a hum due to poor filtering.
The power cable was original, which caught my attention, since usually the Vieta A-225 units I’ve seen had their plugs and/or cables replaced. Moreover, it still has the original Vieta Quality Control department label, indicating the names of the people who personally inspected this specific unit before it was sold. Also, the input voltage it was originally configured for was:
Besides the filter capacitor issue, one of the switches was broken and didn’t work. From what I understand, this is a common problem affecting this model, and there are no modern switches exactly like the originals, so I’ll have to see if it can be repaired somehow.
After the general inspection, it’s time to get to work and give it a thorough overhaul so it works like before.
For the setup, the first essential step is to replace ALL the electrolytic capacitors, since given their age, they are all dry/leaking or close to it. The other capacitors are the famous Bianchi/Philips Mullard “mustard” type, which don’t need replacing because they are polyester capacitors and have worked perfectly for all these years without leaks or issues.
Just in case, I replaced the two EROFOL polyester capacitors with new ones, due to bad experiences I had leaving capacitors of that brand in the past.
A crucial component that must also be replaced is the selenium rectifier used to obtain the negative BIAS voltage and power the preamplifier filaments. Selenium rectifiers gradually fail by increasing their internal resistance until they eventually burn out and explode, leaving a rather unpleasant rotten egg smell.
For this, I decided to dismantle the original selenium rectifier and replace it with modern UF4007 diodes. With a bit of patience, it can be opened and its interior replaced without problems:
Regarding the broken switch, after disassembling it, it’s clear that the mechanism itself is quite simple. Specifically, the broken one had a piece broken off from the side of the plastic part that holds the plunger, which acts as a rocker for the electrical contacts, as you can see:
In order to repair the broken switch, I used a small 3mm steel ball bearing, and to repair the broken plastic, I molded the missing part using a two-component epoxy adhesive. After letting it dry for more than 24 hours, and carefully reassembling all the parts, I tested the repaired switch, and the repair was a total success. Moreover, the new system using the steel ball makes the switch operate more smoothly, preventing the need to apply so much force, which could be the cause of the switches breaking due to stress on the plastic. After that, I reinstalled it on the front panel, and it was ready for use.
After repairing the switch and replacing all the electrolytic capacitors, the next phase was to make a couple of additional modifications to the original power supply circuit to better protect it and improve its performance. The changes were:
- Removing the voltage selector switch and fixing it at 220V: Nowadays, 125V mains no longer exist in Spain, so it makes no sense to keep that functionality, since if that voltage were accidentally selected, the power transformer would burn out instantly.
- Adding a varistor to the transformer primary: This is extra safety in case voltage spikes arrive, protecting the transformer.
- Adding an NTC thermistor in series with the transformer primary: This allows the amplifier to power up more gently, avoiding sudden surges in the valve filaments.
- Adding fuses on the high-voltage secondary: This provides extra protection to the power transformer in case one of the rectifier diodes shorts out.
- Replacing the rectifier diodes with UF4007: UF4007 diodes have much faster switching characteristics than the original diodes, which reduces the so-called “rectification noise” typical of slower silicon diodes like the 1N4007 (noise that valve rectifiers do not suffer from). This achieves a similar effect to using a valve rectifier.
- Adding a snubber network on the transformer secondary: Together with the above, the snubber network reduces this noise even further, almost eliminating it.
Besides these changes, I also added precision 1 Ohm resistors to the cathodes of the four EL84 tubes so that it’s easy to measure the current passing through them with a multimeter (set to millivolts) and thus adjust the BIAS more precisely than the manual suggests.
It should be noted that the Vieta applies a quite high voltage to the plates and screens of the EL84 tubes. Since the transformer is wound for 220V, and nowadays the mains supply is 230V (and in some areas closer to 240V — in my home it’s usually around 236V), this can mean higher voltages than the schematic specifies.
In my case, although this was true, I decided not to modify these voltages for now (perhaps later), since the original EL84s can handle it without problems. Also, the important voltage to reduce would be the screen grid voltage; the plate voltage can be higher as long as the screen voltage remains within permissible limits.
As a future improvement, an active voltage regulator with transistors could be made to lower the voltage supplied to the screens, which would also reduce distortion caused when the screen grid voltage instantaneously dips transiently depending on the audio content (BIAS voltage regulation would also be necessary).
Since changing the selenium rectifier bridge increased the BIAS power supply voltages, I checked the filament DC voltages, which were higher but still within the proper range, so I made no modifications there.
Once these issues were solved, I powered up the amplifier. I had to turn it off because I discovered that a carbon resistor in the anode circuit of the ECC83 RIAA preamplifier was sparking. Upon inspection, I found an internal failure and replaced it.
The chassis finally looked like this after the restoration:
At the second power-on, everything went smoothly. I adjusted the tube’s BIAS so they operated at about 70% of their plate dissipation. Also, using an oscilloscope, I fine-tuned the potentiometers that ensure the phase inverter output is completely symmetrical. After that, both channels worked correctly, delivering their proper power without distortion.
Testing it live with speakers, the result is spectacular. There is absolutely no hum—zero—even when putting your ear close to the speaker, and the sound quality is really good. The restoration has been a success. Now it accompanies me as the main amplifier for my PC/music system, perfectly fulfilling its role.
The amplifier schematic is as follows:
As a side note, the Vieta circuit is actually quite improvable—for example, the tone control is not designed to give a flat response when both potentiometers are set to 50% of their travel, the phase inverter can be improved, and so on… But although these issues could be fixed without problem, I prefer not to, since that would change too much the original character and sound of the amplifier.
Finally, Here it is the amplifier manual in PDF format, courtesy of RetroVieta:
Vieta-A225-instrucciones