1959 Fender Bassman 5F6-A Power output??'s

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7/6/2004 6:53 AM

Paul Bamberg

1959 Fender Bassman 5F6-A Power output??'s Gentlemen;   I have the above amp on my workbench and it is putting out only 25 watts of undistorted output into a 2 ohm resistive load. What is the typical undistorted output you all see???   1. The amp is all orginal, except for 1 screen resistor and the .02 cap connected between the treble and bass controls in the control stack (someone changed it to .1).   2. The amp at idle has 465 volts on the 5881 plates, under power sags to 429. The idle ripple at the Output xformer center tap is 3VAC at idle, increases to 7VAC at full power.     The amp has a microphonic 12AY7 tube and the amp makes random one shot noises when idling (sounds like psssh pssh).     This thing could stand new tubes, as well as new CAPS this thing is 45 yrs old. The owner has asked that I look it over and clean the controls. As far as Im concerned a re-cap and new tubes are in order, but the owner must make that call. Again my ??s are what power output would you all expect and what would be a good idle current for the power tubes.   Sincerely   Paul Bamberg

 

7/6/2004 3:56 PM

SpeedRacer

35mA on the outputs is where I usually bias them.   Power output sounds bad.. but then again the lack of headroom and easy distortion might be something the player loves about the amp.. Should really be closer to 40W, I usually test at 400Hz. You'll have to tweak the eq to get the max clean signal through.   Sounds like the rectifier tube is a beater.. is it a 5AR4/GZ34? The caps really ought to get done if they've never been done if the amp gigs, but again if the guy loves it as is, just explain these things and be prepared to let it go. Last thing you want to do is give him back a 'tight new amp'.. IMHO.     I'd also doublecheck the OT part# and make sure it s the right one.     jm2c

 

7/6/2004 5:44 PM

Wild Bill

What Speed said...     As well, it's cheap insurance to replace all the bypass caps across the cathode resistors in the preamp stages. These often get forgotten but they are electrolytics after all and will dry up over the years just like their bigger cousins in the filter section.     When they dry up the sneaky thing is that they don't short, they just sit there and do nothing! What they are supposed to do of course is provide an AC signal bypass. With no bypass action the gain of the stage is much reduced, which might account for the low power you measure. Everything looks good but there's insufficient drive coming from the preamp.     .02     ---Wild Bill

 

7/6/2004 7:18 PM

Paul Bamberg

Thanks guys for the timely response!!! I tested with 200hz sine wave, with tone stack optimized, looking for the sine wave just start to flatten. I checked all xformers and choke, they are all original and have the correct P/N's. The rect tube is a small Mesa Boogie GZ34. The 12AY7 was real microphonic whilst the chassis was inside the amp, once removed and on the bench I was able to turn it WO and play it. Wow these things really do sound wonderful. I had to put some old RCA 6L6's in it, because the old tubes rattled so bad it drove me nuts. I did get the go ahead from the owner to recap it (all caps), clean pots, clean sockets, install all new tubes, set to correct bias between 35 to 40 ma. Im concerned about making the amp sound tighter, due to new components!! All the stuff that comes across my work bench is modern tube stuff and xistor junk (yuk), so I don't have any experience with vintage customers. So any help from you all is terrific. I did check the preamp output and phase inverter output stages and they were still clean when the output stage was starting to flatten out. The original output tubes (Sovtek) were biased at 24ma one tube and the other 50ma (not exactly balanced). I use the output pri resistance vs voltage drop method to check the bias. Any other ideas and or comments are greatly appriecated.

 

7/8/2004 8:03 PM

Wild Bill

So what kind of power output are you measuring now, Paul?     You should also keep in mind that guitar amps were rarely measured at absolutely clean power levels. A certain amount of distortion is taken as a given. If you look at typical operation stats on a data sheet you'll see that 40-50 watts from a pair of 6L6's will have a few percent distortion.     You've probably already read the articles explaining how tube distortion is more pleasing to the ear than transistor distortion - warm and thick as opposed to harsh and fuzzy. Also, most electric guitar is SUPPOSED to be distorted! If it were perfectly clean it would sound like an acoustic guitar. Of course, it's all the variables of that distortion that determine what's pleasing and what's not.     My point is simply that you should not treat a guitar amp as you would a hifi amp - apples and oranges.     .02     ---Wild Bill

 

7/9/2004 1:50 PM

Paul Bamberg

Thanks to everyone's advice I discussed a new game plan with the owner (he's not a guitarist, but plays his harmonica thru the amp). I will replace the output tubes and 12AY7 tube only. The owner was concerned too many changes might effect his tone too much. I went ahead and checked all the coupling caps, all tube voltages etc and everything seems OK. In fact when I orginally checked the output wattage I had my dead load set to 4 ohms not 2. When I retested at 2 ohms with my old RCA output tubes the wattage was 37 watts, which is real close to the mark. The owner also gave me a 5E1 Champ to check out for him as well. The champ needs a new 6V6 and Im replacing the 6V6 cathode bi-pass cap and the coupling cap. The champ's output waveform didn't look good, prior to flattening out (the preamp waveform looked great). There was a slight amount of DC leakage (50-75mv) on the coupling cap going to the grid of the 6V6, and Im not so sure about the cathode cap neither. The champs dates to 1957, remarkable how one guy gets this much vintage amps. The good thing is Ill get to look after these gems for a long time.   Sincerely Paul Bamberg

 

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