Fender Bassman 5F6A Repair

[misterc57]

I have had older 1930s amps to work on, but this may be the oldest Fender which are my favorite.

This one had been sitting for decades and I was asked to go through it. I installed a 3 prong AC cord, removed the two .05-600 caps at the ground and standby switch. Cleaned pots and sockets, tested tubes, etc. Brought up slow. Had lots of noise at the preamp tubes , replaced V2 100K plate resistor, and solder a V2 wire that looked like it had never been soldered on. Tested, played for a while and all seemed good. Put back together.

Final check and had some noise again at preamp tubes, chopstick sockets/wiring/connections and getting some noise. Pulled all preamp tubes and tried a different set. Now a new issue, high current draw.

High current starts immediately as I slowly dial up the variac.

Only have high current when standby switch is ON. When OFF no issue and I have negative bias V at the power tubes. Same scenario with all tubes are pulled, or with no speaker load.

Seeing at 5881 tube
plate to standby switch 47 ohms
plate to standby switch 41 ohms

This amp has seen some prior work in the past (filter caps, lots of resistors, etc.). Do not know when. Seems like good work. Not sure why they left a 2 prong AC cord installed.

Also odd, when switching the standby switch there is a momentary pause, then a click sound as it activates on or off.

Has the selenium rectifier in the bias but I have only been asked to check it out and get make it quiet.

Would like to get the high current draw resolved first. Time for help, advice, opinions. Thank you, Mark

ps. I have double checked every wire at every tube pin for good connection and short to ground.

Using the attached schematic

Fender_bassman_5f6a.pdf

[loudthud]

First thing to suspect, Power tubes. Try the tubes in another amp. Try a different set of tubes in this amp.

[Leo_Gnardo]

<and solder a V2 wire that looked like it had never been soldered on.> Had a nice 60's Deluxe Reverb recently, they forgot to solder one tube connection on that too. Good idea to ALWAYS inspect tube socket connections, you never know what you'll find. Wonder how many owners passed the amps along because they had some intermittent problem nobody could solve.

High current draw - no current expected to be drawn for hi voltage while amp's in standby anyway. Excess current in operating mode could of course be defective output tube(s) and/or insufficient bias voltage. Also, even though hi voltage filters were changed at some point I'd still suspect a problem there because "the amp had been sitting for decades." Electrolytic caps really don't like being left uncharged for long periods of time, certainly for decades.

It's wise to replace the selenium rectifier as they can fail, and when they do, emit toxic smoke. Oh boy! Something to look forward to if it isn't done. An ordinary 1N4007 wired across the old selenium rectifier, and the Se's still there for looks but passes no current. There's a bit more forward drop in Se rectifiers compared to silicon so you will wind up with another 2 or 3 volts in your bias supply while the silicon rectifier does all the work and the retired selenium just sits there and looks pretty.

[misterc57]

To clarify... With standby in run mode, high current is present with no tubes installed. So I think that omits the tubes as being a problem. Agree?

Yeah I do not like having the selenium there. I will place a 1N4007 in there. Currently I am seeing around -55 VDC bias with no tubes installed.

I did recharge slow and had the amp running okay for a while, but I have to revisit the filter caps. Also suspicious if the standby switch could be causing problems.

Thank you

[Chuck H]

With standby in run mode, high current is present with no tubes installed. So I think that omits the tubes as being a problem.

I looked in because that's a bitchin' old vintage amp. After reading your opener I wanted to mention the probability of a bad filter cap. Leo already covered that. So I'm just responding to say that your last post makes the probability of a bad filter cap likely.

[The Dude]

If you had ALL of the tubes removed (as you say), including the rectifier, I don't think it's likely caps are causing the issue. Without the rectifier installed there would be no B+. I presume you still had the rectifier tube installed? Otherwise, it's odd that the problem would only present when the standby switch is flipped.

Edit: If you measure resistance from the cap side of the standby switch to ground, what do you get?

[Jazz P Bass]

Please specify what 'high current' means.
Measure it & report it.

[misterc57]

Hey Folks. More clarifications.

When I say no tubes installed = no power tubes and no preamp tubes. The rectifier tube is still installed.

High current = monitoring the variac amp meter. With both power switch and standby switch in run mode, starting at zero volts on the variac, just a slight turn of the volt dial pegs the amp meter. What I would normally see when there is a short.

I will take measurements tomorrow and report back.

Thank you!

[Chuck H]

Hey Folks. More clarifications.

When I say no tubes installed = no power tubes and no preamp tubes. The rectifier tube is still installed.

High current = monitoring the variac amp meter. With both power switch and standby switch in run mode, starting at zero volts on the variac, just a slight turn of the volt dial pegs the amp meter. What I would normally see when there is a short.

I will take measurements tomorrow and report back.

Thank you!

With this report it seems (to me) something is drawing current sans amplifier operation. This is usually a fault in the HV power rail. Most likely a shorting capacitor by default occurance. So that would be the first place to look. And I'll also note that testing capacitors doesn't usually (or even typically) demonstrate such a fault. It is only in circuit with the higher voltage presented by the circuits we observe that the fault occurs. So I'm voting for a failing HV rail filter capacitor as the problem.

[pdf64]

Hey Folks. More clarifications.

When I say no tubes installed = no power tubes and no preamp tubes. The rectifier tube is still installed.

High current = monitoring the variac amp meter. With both power switch and standby switch in run mode, starting at zero volts on the variac, just a slight turn of the volt dial pegs the amp meter. What I would normally see when there is a short.

I will take measurements tomorrow and report back.

Thank you!

That seems bad to me.
I suggest to disconnect and insulate off all the secondary connections of that mains transformer and repeat that test.

[Delta362]

Maybe the rectifier tube took a dump and is shorted? Do you have another rectifier tube that you can try? Does the amp pull a lot of current with the rectifier tube also removed?

[misterc57]

Does the amp pull a lot of current with the rectifier tube also removed?

No

[Delta362]

No

Sounds like you need a new rectifier tube

[misterc57]

If you measure resistance from the cap side of the standby switch to ground, what do you get?

standby switch in run mode about 57 ohms
standby switch in standby mode about 2M ohms

Think I need to make some disconnects down the HV rail and repeat this measure at those disconnect points.

ps. or maybe not. have to give this more thought

[misterc57]

Getting more interesting, in a bad way...

With rectifier tube removed, I am seeing around 58 ohms and 62 ohms at pins 4 and 6 of the rectifier socket. Seeing about 120 ohms across those pins.