Fender Bassman 50 AB165 Rebuild Questions

[misterc57]

I have a basket case head I am attempting to return to life. I have replaced all filter caps and the 1500 R and 470 R at the 6L6 sockets.

I am watching V and current as I bring it up.

I am trying to measure V at each tube pin. Every time I try to read the V5 6L6 V at the 470 R (either side of this R pins 4 and 6) I get a disturbing spark and snap which does not happen on V6 6L6.

This happens with or without a tube inserted. The R is not loose.

Any suggestions on what would cause this?

Schematic attached.

Thank you. MarkO





bassman_ab165.pdf

[misterc57]

Stupid mistake on my part. I had left a lead on the 470R that I could not see and it was touching the chassis.

[TomCarlos]

Update: As I was typing my reply, you found the problem. Wonderful. But still, replace that power cord !!!

For starters, replace the power cord !! You should not have a braided ground acting as the chassis ground.

If you removed the tube on V5 and you are still getting the arching, try and disconnect the leads going to the tube and measure at the end of the wire. Be careful when doing this. If you still have a spark on V5, trace that wire and see what is going on at the other end. Make sure there is no solder or small wire strands on V5 (between the pins).

[misterc57]

Thank you. In the last hour or so I did put in a proper 3 prong cord and replaced the (2) .022 coupling caps going to the 6L6s.

Right now I am trying to get the 6L6 bias set. I can get V5 to 35mA but not V6. The problem is at the socket as I have swapped the 6L6s with the same result. Only getting about 17mA at V6 with V5 set correct, and the max I can get on V6 is 21mA.

[g1]

Only getting about 17mA at V6 with V5 set correct, and the max I can get on V6 is 21mA.

Does pin4 voltage measure same for both V5 and V6 ?

[misterc57]

I replaced a bunch of resistors in the bias circuit. I still cannot get V6 to bias any higher than 20mA. I am looking to be at 35mA. Any suggestions?

Thank you

[misterc57]

does pin4 voltage measure same for both v5 and v6 ?

v6 444.6
v5 444

[mikeydee77]

What is the grid voltage on the 1500R resistor? I don't see it mentioned in the thread

[misterc57]

What is the grid voltage on the 1500R resistor? I don't see it mentioned in the thread

-50

[g1]

-50

You show different grid voltages for V5 and V6 so the idle currents will be different. You replaced both 1500R grid resistors? Do they both still measure 1500 ohms?
Try checking the idle currents again with the phase splitter tube removed.

[Helmholtz]

The grid stoppers don't drop voltage.

A difference of 2% in grid voltage could be due to mains voltage fluctuation when measured at different times.

[misterc57]

You show different grid voltages for V5 and V6 so the idle currents will be different. You replaced both 1500R grid resistors? Do they both still measure 1500 ohms?
Try checking the idle currents again with the phase splitter tube removed.

I did replace both 1500R, and did check R again. I will triple check again tomorrow.

The grid voltage changes on each tube when I adjust the bias pot. I try to set the bias pot to where grid V are both equal, in this case within .5 V for each 6L6. From there I try to adjust the bias.

Tomorrow I will try reading and adjusting again with V4 removed.

Thank you

[misterc57]

The grid stoppers don't drop voltage.

A difference of 2% in grid voltage could be due to mains voltage fluctuation when measured at different times.

The readings I recorded in the picture for each valve were taken at the same time.

I am running off a variac and usually dial it up at each test, so I do get slightly different readings each time I do a test.

[misterc57]

Also I wonder if the bias V is too high coming from PT. Hard to read the schematic but I think it shows -45V compared to -50 that I am reading.

[Leo_Gnardo]

Skimming thru the responses and your photos, something I see is that your "bias" pot is actually a balance pot. No wonder "one side goes up & the other side down." Bias in this case is set by the fixed 15K resistor between pot body ground and pot soldering lug. If you vary the value of that R, then bias overall will increase or decrease. Looks like you need more bias current, therefore lower bias voltage. One solution - you can clip parallel resistors onto the existing 15K. Here's where a resistor sub box is handy if you have one. If not, try say a 220K resistor for starters & see if that tube with 20 mA doesn't start moving more current. Might have to go yet lower resistance to find a comfortable spot say around 30 mA, then tweak balance pot to get them both the same current.

Another solution, one I use a lot, is a fixed resistor in series with trimpot in place of that bias setting resistor, then you can dial it in 'zackly where you want. Say, a 5K pot in series with 10K instead of a 15K fixed R.

FWIW I've encountered some 6L6's that need surprisingly low bias voltages to work, in the 42 - 44V region. Tubes that are "hard to wake up" apparently - but once proper bias current is established they work fine.