The 'Doctor Valve' Bias Boy

Hi all. Whilst the idea of some kind of unit for bias testing/checking/setting-up on valve/tube amplifiers is far from an original idea, there are a few units available on the market that do an adequate, reliable job. Most simply do one tube at-a-time, whereas some can do two tubes. However, very few of these actually give a reading of their own… requiring an additional multi-meter to display the details from the said bias unit(s).

My Australian friend Mervin (Doctor Valve) came up with this idea of making a more accurate and practical way of doing the bias set-ups on tube amps. When I say more accurate, it’s not that any of the available units out there would be inaccurate… but it’s more to the point that Mervin’s idea will do all the tubes at the same time! Thus making it more accurate in the sense that instant readings are available, instead of having to turn the amp off, and then moving a single tube unit to the next tube… then the next tube and so-on. Let-alone having to wait for each tube to reach working temperature every time too! So in this respect it is also very practical!

The one shown in this article is my own… kindly built for me by Mervin, as a gift whilst here in the UK on a five week visit. It was my idea to add the "X10" switch which moves the decimal point on the meter for a reading of 2000m/a; instead of the normal 200m/a full-scale reading. For myself this was quite a nice little touch as it allows readings of beyond the normal 200m/a range, and is ideal for seeing both individual tubes’ currents and the total (all two or four tubes’) current when the amp is driven to full RMS power output. This switch is however optional and is not required for the general person wanting to set the tubes’ bias under non-load conditions. This also requires the use of suitable test equipment which… once again the average person wouldn’t have. Bottom line here… the "X10" switch is optional.

The unit is really simple to use. Replacing two or four tubes in an amp with the in-line octal plug/socket and then inserting the tubes into these sockets. Once the amp is fully warmed-up, each tube can be read by each individual corresponding "V?" switches. The accuracy of checking all the tubes like this is superb! Even tubes that are bought "ready matched" from various companies providing this service, and although these are usually fine, they are generally not matched on the same voltages and "real world conditions" that your amp(s) would put them under! This being the case and as good as matched tubes can be, the situation can actually change when used in an amp… in the real world! Mervin’s Bias Boy eliminates this/these issues completely!

As most tube amps only have one (or no) bias adjustment, then with the Bias Boy you can swap the tubes around until you get an even balance on the two push-pull halves. Furthermore, if you have two bias pots, you can get things even closer and finally, if you are lucky enough to have a bias pot on each of your output tubes (the ultimate way to do it really!) then you can get things absolutely spot-on!

And just as a last word here. It can also be used for six or eight tube output stages too. I have just set-up a Sound City 120 with it… doing four tubes first and then when I had all the matching correct, I inserted these four tubes directly into the amp and then used the Bias Boy for the remaining two tubes… knowing what readings I already had from the first four… cool stuff!

Cheers, John.

A very useful & practical piece of equipment.

And the innards. No rocket-science here or ultra-neatness…just a good working job! The blue multi-turn pot in the top/left corner is for adjustment of the X10 switch but… as I said… Mervin provides this as optional.

Showing the complete unit with the octal in-line plug/sockets too.

… And the same here too. The cable used for the loom is Silicone covered… ideal for not getting scorched or burnt when lying against hot tubes! It is also very flexible.

The octal plug on the unit itself - with the loom filled-in quite simply with a hot melt glue-gun! Originally this loom could have been hard wired to the unit but, Mervin is considering a second loom which features B9A plug/sockets as an addition. This would allow the unit to do EL84’s too!

(Please excuse the scruffy looking power transformer on this Status/Impact 50 watt head… though it did work fine!) Here is the unit in operation on a 50 watt Impact… showing tube number 2 a little on the high side at 55m/a!

Please note that I have no intentions of making these units myself. If you require more information, availability, prices etc then please contact Mervin (Doctor Valve) on the following:

Home Tel: 0061 2 4329 7454
Mobile Tel: 0061 4023 40030

Mervin ships this item worldwide and welcomes any enquiries.

Cheers, John.