There arenít actually many valve (tube) instrument amplifiers manufactured above 200 watts out there. Two of the obvious exceptions would be Ampegís SVT model and Marshallís VBA400. Both these amplifiers are seriously powerful bass guitar amps.
The SVT originally used 6 x 6146 tubes for the
output; mainly used in transmitting circles, though for audio they can produce about 140 watts from a pair with around 750 volts on the plate (anode). This valve has a top cap (connection to the top of the valve). It is this connection which carries
that lethal high voltage! During later production of the SVT, the 6146's were changed to 6550
output tubes. (the American
answer/equivalent to our good old British KT88)
The Marshall VBA 400 (first introduced around 1998) boasts a bit more power than the SVT, and uses no less than 8 x 6550ís at the output making this probably one of the biggest of all tube guitar amplifiers ever manufactured.
Both the 6550 and KT88 tubes are supposed to be interchangeable (equivalents) with maybe a small bias tweak(?). I personally donít quite agree with that statement. Yes they are interchangeable with the said bias tweak, but Iíve always found that the 6550 just does not quite keep up with the original GEC KT88. Nevertheless, it is still a good beefy bottle.
You well may be wondering where all this is leading to! Well, although the SVT has been around for quite a long time and the VBA 400 has been produced since about 1998, I myself was designing and building bass amplifiers much bigger than both of these from the early seventies at the age of 21! This was a long time before any of the big companies started making such beasts, and yet mine were even bigger and better!
One of my first attempts was back in 1971, when I designed and made a 500 watt bass amplifier running 8 x KT88ís on the output. These were run with 750 volts on the plates (anodes) and 400 volts on the screens (grid 2ís,); fixed bias of course. Both the mains and output transformers were completely designed and built by me too. They sport 5Ē stacks of the highest grade 248 type laminations and weighed a ton!
To drive the 8 x KT88ís up to 500 watt without clipping I used an ECC81 (12AT7) for the phase splitter and 2 x ECC82ís (12AU7ís) as two cathode followers: one driving each quad of KT88ís. This amplifier would deliver its full 500 watts right down to 25cls and up to around 16kcls. It sounded amazing on bass guitar driving a single 18Ē cabinet and a 4 x 12Ē cabinet!!
There were many features packed into my designs, such as:
(1). There were 8 x separate bias adjustment potentiometers (one per each output tube). This facilitates the perfect equal setting of each output tube current.
(2). There was a rear mounted meter and an 11 way rotary switch. This switch provided position 1 - plate voltage; position 2 - screen voltage; position 3 - bias voltage; and finally positions 4 thru 11 displayed each individual output tubeís current. Perfect for ease of matching the 8 tubes up with the individual pre-set bias controls.
(3). Another design I built in was a protection relay. This was energised right on the end of the negative bias supply. Its main contacts were then on the negative part of both the plates and screens HT. The scenario envisaged was that if the bias should fail for any reason, the main HT would simply be shut down, protecting all the output tubes. Can you imagine the cost of replacing 8 blown up KT88ís in those days, let alone what they would cost now?
All three of the above additions I have used many times on my own designed/built amplifiers, and yet even today with much greater availability of components and cheaper costs, I still havenít seen a production amplifier with these facilities/additions!?
This first 500 watt amp was fitted in a wooded case and had a black vinyl covering (like a Marshall, Laney etc). It was also fitted with an integral cooling fan - something else that has just started to appear in large tube amps from the major manufacturers!
Not long after making this amplifier, I had the privilege to be chosen as the bass player for a three month tour of the Middle East backing Martha Reeves & The Vandellas (Jimmy Mack fame and more). I took this amplifier along with my single 18Ē & 4 x 12Ē cabinets and used it for nearly the whole tour, which started off in Egypt, then went on to Bahrain, Dubai, Abu-Dhabi, Kuwait and a few other places of which I canít remember the names of now.
About 3 weeks before the end of the tour, whist performing at the Intercontinental Hotel in Dubai, I was approached at the end of the evening by some other English musicians who had popped in to see the show. They introduced themselves and began to explain that they were the resident band on a floating hotel moored just across the street in Dubai harbour. It turns out that the converted boat they were performing on was built in Newcastle, UK, and that they themselves all came from Manchester, UK. The bass player was well impressed with my sound and particularly the 500watt amplifier. It was like nothing heíd ever seen or heard before!
For the next few days he pestered and pestered me to sell it to him. Eventually, I agreed to do so but the problem was that I needed something to finish the tour. We ended up agreeing on £450 (a lot of money back then!) plus his Fender Bassman 100 for me to use on the rest of the gigs. I sent the money back to the UK via electronic mail and brought the Fender back too, which I eventually sold after making myself another 500 watt monster in 1973! Not a bad deal eh!?
I would love to find/hear from this guy again. I often wonder what happened to that amplifier. I canít even remember his name or the name of the group either.
Iím sorry the photoís I have included with this article are not too clear. Unfortunately photography has never been one of my fortes, but although the photos were originally ok, they have faded with time. Nevertheless, all I have discussed above, like the 8 x KT88ís, the meter/rotary switch, bias potentiometers, both transformers etc, can still be clearly seen.
I made, all-told, four of this style of 500 watt amplifier and sold them all.
During the late 80ís, I made a 4u rack mounted version. I only have the one photo of this, and it is not to clear Iím sorry to say.
And lastly in the 90ís came the biggest tube amp I have madeÖ.Öto date!
This absolute monster was 8u (rack mounted) high, had three enormous transformers, 16 x KT88ís (ÖÖ..yes 16!), 500 watts per channel (stereo) or 1000 watts mono, and four cooling fans (two sucking and two blowing). It took two people to lift this beast. The only mistake I made on this enormous baby was not to take any photos of the internals? Only the front and back.........what an idiot!
Cheers for your interest, John.