Q & A – Replacing / Biasing Amp Tubes

Here are a few questions we’ve received about replacing and biasing amp tubes (see iMojo link below). Andy Turner, Mojo Co-Owner, provides a few answers:

Q:  How bright should the filaments light up with new tubes? Preamps, power tubes el84, 6l6, 5y3 rectifier tubes…shouldn’t they all light up very bright signifying they’re new and full of life??

A:  Not only do different tubes have different filaments and have them in different places – but also different manufacturers of the same tube have different placement and intensity. I am afraid each tube type and manufacture is its own study in filament science. To make matters even more confusing, even if you could see the filament directly in every tube, it would not tell you anything about the life of the tube. All it does is warm up the tube so that electrons will flow.  The plates and cathodes which do not glow unless the tube operated incorrectly are where the tube “wears out”.

Q: I have a Fender Twin amp an every time I switch it on – after maybe 15 minutes - a high pitch ring starts coming through. Is that a sign that it isn’t biased? Or is it a easy fix?

A: Probably one of the input gain stages is micro phonic – or the reverb return. If your amp has had output tubes replaced without proper biasing, no doubt it should be checked. However, it sounds likely that you problem could be solved with preamp tubes.

Q:  What happens if I bias it, the tube voltages or whatever don’t match up? What is the best way to drain the amp of all the excess power before I start work on the amp?

A: Opinions vary on what is too far unbalanced with regard to good tone. Some people intentionally unbalance an amp. With regard to a properly functioning output section, you would want each side to be within a 3mA of each other. You can usually drain an amp with a 10K 5w cement resistor and a set of test clips. Amp off strap the resistor across the first filter.

Do you have questions ? Email us at info@mojotone.com.

Philips JAN 6080WC OTL Amp Project

Tired of listening to output transformers?  Make your next project an OTL amp using the military-grade Philips JAN 6080WC output tube.  This NOS U.S.A. made tube was subjected to the most rigorous vibration tests for military use, and as a result, should prove to be less microphonic than the Russian versions, and last longer too.

For the low-budget builder, a simple cathode-follower or a common cathode resistor bias will push both sides of the tube in the output section.  This is due to a low plate resistance (280-300Ω), well-matched triodes, and the tube’s ability to pass a lot of current.  These are also great for low impedance headphone drivers in the studio. Compared to other dual-triode configurations, the 6080 is more rugged than the 6AS7G, and much more affordable than the 2A3.
Check out some examples of the 6080/6AS7 used in OTL audio amplifiers:
Then, if you find yourself reminiscing about your days with output transformers, keep our 15W SE transformer in mind.

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