Peavey Classic 50/50 – EL84 screamer

The classic Classic Peavey 50/50

Thanks to Paul in Kenilworth, CT,  I picked up this real Classic at a great price. They realistically go for even up to R5 000 in good nick and then one still has the overseas shipping charges to contend with and possible re-tubing. I do believe there are many of these power amplifiers floating about in South Africa so keep your eyes open.

What is so marvellous about this amplifier is that it can be easily modded, can be set up for home use and best of all these are stereo units or more in pro circles, two channel 50W RMS. And yes, of course – 50W per channel for tube amplifiers at a near giveaway price is the main attraction. Bear in mind that a 35W per channel EL34 aplifier is going to empty your wallet of some R20 000.00.  But keep this a secret please!

Power output of the Peavey Classic 50/50

Sceptics are often quoted as being surprised at the output performance of these amplifiers as they use the baby brother of the EL34, the EL84.  Well not really. The EL84 is a high gain pentode which requires very little drive to bring it into saturation. Aha!  This explains the popularity amongst the overdrivers. The 6BQ5 is the USA equivalent. The 6V6 is less sensitive. The EL84 used in guitar amplifiers brings out a distinctive and prominent treble tone. Do not confuse this with lacking bass – EL84s when in the right configuration make exceptional quality amplifiers.

The manufacturers of guitar amplifiers regularly configured the output stages of these EL84s to run at a plate (anode) voltage of between 400V and 425V – over 100V that of the manufacturer spec. Before jumping to conclusions remember that tube specs are given for max plate current and voltage. Reducing the plate current and supplying high voltages is not uncommon. One just cannot do both – the EL84 can dissipate up to 14W in a controlled environment – controlled meaning reducing screen grid voltage as well. These amplifiers are known to run sometimes for up to 15 years+ without a tube change. Coming from a radio telecommunications background it was not uncommon for SSB transmitters to never have their output tubes replaced. In class AB1 an SSB transmitter is also running at about 50% efficient, 3kW in for 1.5kW out in marine use.  The bigger issue was power supplies – often over 100kg to power these beasts. Tubes are survivors and are many times more resilient than the older RF transistors (even in audio use).  So tube amplifiers designed around the manufacturer spec is often downplayed for many years of reliability. Even at 400V applied to the plates of an EL84 does not mean it will fall over in 3 months to a year as many doom and gloom harbingers decided.

The one thing to be careful of though is driving any tube amplifier without a load.  Especially when pushing a tube plate voltage to the max such as the Classic series. This will cause breakdown in the primary windings of the output transformers. Note the two diodes across the EL84s. Wonder what they are for?

One channel Classic 4040 Power Amplifier
One channel Classic 5050 Power Amplifier

Looking at the schematic it’s no wonder that these were so popular. Besides the design being super simple tube rollers had fun because the tubes are very easy to change.

Classic 5050 EL84 Output Octet
Classic 5050 EL84 Output Octet

These amplifiers rely heavily on the added fan cooling often not seen in low power tube audio. As the tubes are pushing their maximum ratings within a very compressed space however it’s not just a good idea, it’s essential.  These Peavey amplifiers are also known for their compact size and weight, a 2U chassis makes it easily transportable, in rack or out.

Troubleshooting these Peaveys

Trawling through the net one often is surprised not to see skilled technical folk not warning wannabe repairmen to ensure that whatever voltmeter, DMM etc they will be using is indeed high impedance. Years back one would have a VTVM (Vacuum Tube Voltmeter) which was designed for this and measurement of ultra low voltages.  The cheap low Ohmic meters of the day, chiefly analog of course, would play havoc on tube bias.  Unsuspecting DIYers were often surprised at the rosy glow coming from the anode when measuring control grid to ground.  So, please be careful.

There are four main checks when it comes to tube performance: Plate or anode voltage, screen grid,  control grid bias and heater voltage.   If you are not sure, have not worked on tube gear before then now’s the time to call in skilled help.  Tube voltages kill!

All voltages and currents should be in the service manual, but not always.  The general rule of thumb is to remove the tubes with power off and capacitors discharged. Clean the socket properly – tube heaters draw a fair whack and if not seating properly there will be a very large degrade in performance. I mention this as well because an amplifier I repaired years back lost bias through poor socket connections with the tubes going into saturation and showing a healthy red glow from the anode. So just be extra cautious, socket pins can cause problems, especially in road gear.

Heater filaments very seldom burn out. I have found that because of the current draw on bigger amplifiers (and some smaller) if there is a fuse-holder make sure that the fuse is seating/connecting properly.  This was a big problem with portable TV sets where the last thing the tech looks at is the voltage drop across the fuse. Even older fuses cause a drop but don’t now make this a fetish – just make sure the heaters sit at spec value.

Control grid voltages: always in audio negatively biased to Cathode. Below is a brief description and also gives transformer secondary voltages.

Bias supply
Bias Supply – control grid bias and 12V relay coil for stereo / mono use.
V-Relay
V-Relay – the relay coil is grounded in “Mono” mode.
Tube Bias
Tube Bias

Mains transformer 110V or 220V primary

Secondaries

  • sec 1       300VAC @ .318 amps  =  425V feed to anode/plate  /  100 Ohm 5W dropper to screen grids
  • sec 2      41.5VAC CT @ .318 amps =  – 58V to control grids
  • sec 3      6.19VAC @ 5.16 amps = heater supplies
ART Pro MPA2 Digital with Peavey Classic 50/50
ART Pro MPA2 Digital with Peavey Classic 50/50

Next up will be the test, tube pre-amp to tube power amplifier.

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4 comments

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1

The classic 5050 has a fixed bias rail. What happens when tubes start losing emission? Will there be current hogging. My technician said that as one tube goes the rest will soon go as well so best to replace them all. I was told this is not true.

Author2

Hi Joe - there are a few ways to bias the tubes and currently the most popular is fixed bias where you can adjust the voltage, some use cathode bias (which is more common than one thinks) and then we have auto bias. All tubes can be set to a fixed bias or with mod, an auto bias of sorts. The gurus of sound like the fixed bias where each tube can be adjusted to draw optimal cathode current. Any output amplifier using parallel devices will have gain differences unless they have been matched - there will be current hogging. Often we read about audio amplifiers through the audio forums but if one wants to learn about tubes I can promise you that the best reading matter anyone can get their hands on is going to be especially the older (1970s and prior) ARRL handbooks. As high powered linear transmitters were part of my training I think many of the older telecom support engineers could spin some very good yarns on the repair and modifications of tube gear. Biasing played a very important role of course. I hope this assist you with an answer - there are many ways to skin a cat.

3

For those interested in tube amplifiers and strings remember that paper cones are often the best way to hear them. Looking forward to the next article - I have the MPA2 but not the 50/50. Be interested to hear some views here. I can get this amp for about R4 000.00 in pristine condition with Russian military tubes, the incredible 6P14P-EV

Author4

For reader's information the NOS (new old stock) 6P14P-EV is rated as follows:
6P14P = 6BQ5, EL84 and 7189 tubes.
E = Extended or long life (>5,000 hours)
V = Mechanically ruggedized for low microphonics and reliability. Limited 65mA Cathode current, 300-500V anode voltage. These tubes cost around R500.00 to R750.00 per pop landed RSA. Ouch!