Bottlehead Kits > Kaiju Stereo 300B amp

My Kaiju Adventure

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Dutch Keith:

Hi there everyone. Let me first briefly introduce myself. I am Keith Marshall, English by birth, and resident of the Netherlands. I have been a professional designer and manufacturer of mainly loudspeakers and latterly solid state phono- and pre-amplifiers for the last 40 years.
Now pretty much retired I have time to spend on indulging in my love for music and tube equipment.
In November last year I built my first Bottlehead, the Kaiju. Before continuing I would like, as so many others have done, to congratulate Bottlehead on the excellence of their construction manuals!

Unfortunately my first impressions were disappointing with, amongst other things, a poor unfocussed sound stage, weak basses and sharp grainy highs. I have a lot of experience in trouble shooting and started, by a process of elimination, to hopefully pinpoint what was going wrong. It was a long process, taking 6 months in all and, just when I was about to give up, the solution came just a week ago.... as a complete surprise!!
 
So last week I happened to notice that my favorite tube supplier were offering a small batch of NOS RCA 5670’s at a very reasonable price. So I ordered a pair, installed them, switched on and started playing one of my favorite albums.

The Kaiju was totally transformed! After all that time I was finally listening to a very sweet sounding amplifier, with an perfectly natural balance and sound stage, really strong but taut bass and smooth extended highs. Just what the Doc(tor) ordered!

Sadly I have to conclude that one or both of the stock GEC tubes were in some way defect or became defect after the initial turn-on. How is this possible?
Any thoughts on this would be welcome!

In a separate post I will describe some of the updates and modifications I have found to be worthwhile.

Cheers! Keith

johnsonad:
There were a surprising amount of variations of the 5670 tube made. I must have at least 15 variations between brands over the years.  They are cheap and easy to roll the tube as long as you adjust the voltage each time.  Try a few more, you may find others that work well to your ears.

Dutch Keith:
Thankyou Aaron, which voltage are you referring to? And how would I adjust it?

Paul Birkeland:
The driver plate voltage adjustment is part of constructing the Kaiju and can be found on page 65.  It you happened to skip this step, the amp may indeed not quite sound its best.

Dutch Keith:
Thankyou Paul, I have found the page in question and I did adjust the voltage as specified (I check off every step).
 Before going further, I made a visual inspection of the suspect tube(s) and found the pins to be very discolored/tarnished black/blue. I cleaned them with a mini buffer wheel and jewellers rouge and plugged them in. Hey Presto! The problems described have now been totally resolved.
Unfortunately I also had an even worse problem with the driver tubes of the Stereomour I built for a friend. These had corroded/bent pins and were totally unusable. They had to be replaced.

It seems like there might be a Quality Control issue here.

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