What Kinds of Tubes Does Analog Bros. Use For Repairs?
The choice of tube manufacturer and type is determined by the requirements of the specific piece of equipment, our experienced recommendation, and the clients' input and budget. We frequently see the question “what is the best (fill in type) tube”. There is rarely a single ideal tube, but most always a compromise between specific application, availability, reliability and cost. Tube sellers that answer with a particular brand usually have a vested interest in selling that one, rather than taking all of the other factors into account. For one example, due to the high (and constantly increasing) prices of many NOS tubes from the USA and western Europe, we don't feel that it's always in the client's best interest to use the "ultimate" tube set in all situations.
We've set up many musical instrument amps with one set of very high quality NOS tubes for recording sessions and interchangeable (meaning the output tubes have the same bias point) set of much less expensive current production tubes for gigs. This provides the absolute best tone for the recording which will be forever cast in polycarbonate (or vinyl, if you're lucky), and a "touring grade" set. The output tubes are matched to the same characteristics as the "recording" NOS tubes for easy field replacement when it wouldn't be practical to have the amp calibrated. These "touring grade" tubes are for road use, where the tonal subtleties and nuance captured when recording may well be lost in the noise of the venue or inadequacies of the sound reinforcement system (not to mention the physical abuse touring equipment is subject to).
For general purpose "club" and rehearsal amps, we tend to use combinations of NOS and current production tubes that we've found to be very good tonally, reliable and more reasonably priced. We usually make several recommendations allowing the client's requirements and budget to be taken into consideration. Each of the current manufacturers has some types that excel, while other types are merely adequate depending on both the specific amplifier and circuit application. Some of the current production tubes we've found to meet our requirements are manufactured by Svetlana, Sovtek, JJ Tesla and EI. We’ve been very impressed with the quality of the re-issue tubes marketed by Sovtek under the TungSol and Mullard brand names.
If a client requests a specific tube manufacturer and type, we're happy to oblige, particularly if we've had positive experience with that tube in the application in question. We don't use "name branded" tubes, as we purchase in large quantities directly from the importers. We feel that our own system of quality control is superior to, and more cost effective than, a fancy silk screen on a rather generic product. (Some companies [gasp!] put more money into advertising and endorsement campaigns than into proper tube selection.) By the way, Analog Bros. uses NO CHINESE TUBES in ANY repairs. EVER. 'nuff said.
What is bias, what does it do, and can I adjust it myself?
Bias is the term used for the adjustment (or pre set condition) that sets the designed operating point of a tube or other active device in a circuit. Many circuits are designed to be "self biasing" and require no adjustments when the active component is replaced. But in some equipment and specific circuits, when an active device (tube, FET, transistor, etc.) is changed, or its characteristics have degraded over time, an adjustment is required to re-optimize, or calibrate, the performance of the circuit. This also assures that the device is operating within its design parameters.
As for DIY, the answer is maybe. The first issue is that you must be 100% sure of what you are doing inside an energized amplifier. We are NOT in any way suggesting you do this and the following is for educational background only. Of course, if you have a screwdriver you can set the bias on many amplifiers. The question becomes will that setting be optimal for the specific situation? Several factors need to be taken into account to accurately set bias. First, the limits of the particular tube type in regard to the specific circuit need to be adhered to if life expectancy of the tube and amplifier is a concern. Bias is usually a compromise between long tube life and optimum tone. These are not necessarily mutually exclusive, but there is a point of diminishing return in tone and very reduced tube reliability.
So, if you’re interested in biasing the output stage of your amplifier, plan on finding out these factors. What is the class of operation of the output stage, what are the plate and screen dissipation limits of the specific tubes, what are the plate and screen voltages and currents in-circuit and how are you going to measure these parameters while performing the adjustments? If you can answer all of these questions and have some test equipment, chances are you can make a very good bias adjustment.