Fender Amp Bias and Impedance
Jeff answers two questions this month: tips on biasing a Fender Deluxe and matching a Bassman head to cabs with mismatched impedance
I tried to bias a Fender Deluxe AB764 using the transformer shunt method. With the current setting on the bias pot, one tube measures 35mA while the other measures 19mA. What are some possible reasons for the big difference, and how close do the readings need to be? Additionally, the plate voltages are higher than the schematic shows.
Hi Tom, Thanks for your question, but before I answer it I’d like to offer a clarification. The Fender schematic designation AB764, as far as I know, was only used for the Bronco and Vibro-Champ models. The Deluxe Reverb in question here is more than likely an AB763 version.
Since you did not mention installing a new set of matched output tubes prior to your biasing attempt, I’m going to assume that you are trying to bias the existing set of 6V6 tubes in your Fender Deluxe. In order for you to measure the same idle current on both tubes, the tubes need to be matched. If you are measuring the idle current of two unmatched tubes, it is not unusual for one tube to be drawing 16mA more than the other. Installing a new matched pair of 6V6 tubes should alleviate this situation. If in measuring the current of the new pair of matched output tubes you still find a substantial mismatch (more than a couple milliamps), the first troubleshooting procedure would be to note the current at each socket and then reverse the location of the tubes. If the current measurements follow the tubes, then the tubes are not matched. (Hey, it happens!) If the imbalanced measurements remain with the sockets, then you should check the value of the grid and screen grid resistors. If the values of the 1.5K-ohm grid resistors are substantially different (and they really do need to be substantially different in this instance) the amount of grid bias voltage applied to the grid of each output tube would be different, causing an imbalance in bias current between the tubes. The same holds true for the 470-ohm screen grid resistors. If a substantially different voltage appears on the screen grids of both tubes, an imbalance in bias current could occur.
Regarding the plate voltages being higher than the schematic shows, the voltages listed on the schematic are merely an “approximation” of the voltages that will appear in any given production amp. Also, be sure that your amp has the recommended 5AR4 tube rectifier. A replacement Solid-State module, unless it is manufactured to replicate the output of a 5AR4 tube, will produce a markedly higher B+ voltage. I hope that helps you achieve balance in your bias.
Jeff, First off, thanks for writing such a great column, and putting it all into layman’s terms, it helps a lot!
Here is my question: I have a ‘67 Fender Bassman head (blackface) which runs at 50 watts at 4 ohms. The only cabinet I have right now is a Hartke 4x10, which is 8 ohms, but wired in series/parallel. Currently what I am doing is adding a second speaker cabinet to the Hartke that I have loaded with one 12” JBL D41 (I think) rated at 8 ohms. While I believe this is giving me the proper resistance, it is a bit unwieldy. Can I build a dummy speaker load to connect to either of these cabinets to allow me to use just one alone instead of both at once? Thanks again.
The Fat Cat Daddy
Hi Shaun, Thanks for your question and thanks for reading and enjoying the column. I’m glad you find it useful. To answer your question: Yes, but why? You’ll understand once I explain.
The current configuration you are using with your Bassman head seems to be correct. The 8-ohm 4x10 Hartke cabinet and the 8-ohm JBL cabinet, both plugged into the two speaker jacks of the Bassman head, will present the head with the proper 4-ohm load for full power transfer. Since the impedances of the cabinets are the same, the output power of the Bassman will be split equally between both cabinets. If you run only one cabinet however, the output power of the Bassman head is reduced due to the impedance mismatch. Theoretically, it would be reduced by half, but realistically it turns out to be approximately a 40% reduction. This means that any single 8-ohm cabinet used will only receive approximately 30 watts from a typical 50-watt Bassman. Now, let’s examine the potential of using a load box and a single cabinet with your Bassman head. You would more than likely want to build the load box to be an 8-ohm load so that the combination of the load box and a single 8-ohm speaker cabinet would present the correct 4 ohm load. Now, if you refer to the previously stated information, you’re already seeing the potential problem here. Since the output power of the head would be equally split between the two 8-ohm loads, the speaker cabinet would be receiving 25 watts and the load box would be receiving the other 25 watts. Result: there will be 25 watts of power that will be dissipated across the load box and never be heard. What a waste of 25 perfectly good tube watts. So you see, while building a load box would certainly optimize the impedance load on your Bassman head, the net result would be even less desirable than what you currently achieve by running only one 8-ohm cabinet with your head, and I’m sure that’s not quite the result you were looking for.
Moral of the story: we should never waste 25 perfectly good tube watts!
Jeff Bober, Godfather of the low wattage amp revolution, co-founded and was the principal designer for Budda Amplification. He can be reached at firstname.lastname@example.org.