Bassman AA-165 1965

[bbmade]

Hello all, this is my first post besides a brief introduction. 

I'm working on a Bassman head, 1965 circuit AA-165.  After bringing it up slowly on a variac and seeing voltages stable on the plates of output and pre-amp tubes I noticed V1 and the second half of V2 had low plate voltage.  Schematic shows 220VDC and I had 102VDC. 

Here's the process I went through in troubleshooting:

I drained the power and checked the filter caps, they were the originals and had started to bubble so I replaced them with new Sprauges and checked the resistors in the filtering sections.  The 220K carbon comps had drifted up in value but didn't seem burnt but I still replaced them and kept the originals handy.  The other resisters in the filtering measured just about perfect so I left those.

I brought it back up to power slowly and now had about 120VDC on V1 plate.  I checked the plate resistor for the first half of V1 and it had drifted up to about 140K.  I checked the DC voltage feeding the plate resistor of V1 on the circuit board and it showed 280VDC and the schematic shows 320VDC.

At this point I tried an 82K resistor in place of the 100K that had drifted and the voltage on V1 plate was now 135VDC.

SO THEN....after turning off/draining power....

I took that plate resistor out and put in two pieces of wire to jumper in a 1 MEG linear taper pot to adjust resistance while monitoring plate voltage.  I got it to a even 200VDC and decided to turn it off to see what value resistor might get me close and the pot was at 20K of resistance. 

To me it seems the supply voltage is low but changing from a stock 100K value down to 20K seems extreme yet it gets me higher voltage for the V1/V2 tubes.  I'm wondering what thoughts some of you might have and any insight from more experienced minds might share.  The voltage feeding the V1 plate resistor dropped after getting the actual plate up in voltage which makes sense but still, seems like low supply voltage.

I don't want to change any iron in this amp that's for sure.

Hope to hear back with any and all insights.

Thanks in advance.

-BB

[sluckey]

Set your variac for 120VAC output or just plug the amp into the wall. Measure all tube pin voltages and all 5 power supply node voltages.

What does the amp sound like?

[PRR]

Right: normal wall voltage, and listen.

Getting right-ballpark resistors is more important than exact plate voltage. 100K (+/-20%) _IS_ the "right" value here.

Go down the WHOLE B+ chain from rectifier to preamp power. Where does it differ from factory hints? I suspect a series resistor has drifted up, or a filter cap has gone leaky. Couple other possibilities.

[bbmade]

All the supply voltages are good except that first one which was 280VDC and with a 20K plate value it drops to about 254VDC.  The variac was at 120AC.

The amp sounds good on the normal channel and the bass instrument channel is quiet and sounds anemic.  The deep switch needs cleaning and the bass control in the tone stack doesn't seem to do anything.

[bbmade]

So if I decide to go with a 20K to get good plate voltage does that seem viable?  I understand resistor tolerances but in order to get that plate voltage up to 200VDC it's a 20K resistor instead of 100K.

Thanks all!!!

[bbmade]

The other dropping resistors in series throughout the filter section seemed fine.  I was hesitant to even change the first two 220K resistors, they're probably within spec but I was hoping lowering those and the V1 plate resistor would help.  The bass channel sounds terrible.

Tomorrow I'm getting an order of resistors in stock and will put in a 20K and listen again.

[sluckey]

So if I decide to go with a 20K to get good plate voltage does that seem viable?

NO! Use the correct resistor. There is something else causing that voltage to be low. That's why I asked for all tube pin voltages. There's probably a clue in the other pin voltages.

All the supply voltages are good except that first one which was 280VDC

What resistance does that 27K/1W on the cap board actually measure? Did you replace that 8µF cap?

[bbmade]

Thank you, yes I'm going to put the original plate resistors back in.  I'm finding some more issues with the amp.  All filter caps have been changed by me.  someone previously replaced the filter cap in the bias circuit and a few other electrolytic's on the circuit board.

I've found some microphonics on the board in the coupling cap areas for the normal channel. 

I'm putting the original plate resistors back in now and going to clean all the tube sockets and check a few other things.  Plate voltage has gone up as I've done a some of the coupling capacitors.

Once I've got that done I'm going to remeasure all the voltages of the pins.

[92Volts]

Voltage is low because that tube is drawing too much current through the plate resistor. Low voltage feeding the plate resistor is also consistent with the tube drawing too much current.

Textbook cause for too much current is wrong cathode resistor-- too low in value. Or wrong tube type, 12AT7 or 12AU7 will draw too much if used instead of 12AX7/7025.

Failing coupling caps will definitely cause the tube to draw too much current! DC voltage at the grid should be 0 for biasing with these simple gain stages.

[bbmade]

Thanks!  Interesting you mention the coupling capacitors.  I've replaced the coupling capacitors for the tone stack in the normal channel and the plate voltage has gone up.  It may be time to replace the same ones in the bass channel as well.  I unsoldered the control side of those coupling capacitors to see i they were leaking any DC and only saw about .002 volts on a couple of them and one showed .1 volts but I think the amp deserves some brand new ones.  I'm going to swap those and see if the plate voltage goes up some more.  Now it's up to 184VDC.

The grid wire of V3 is like a frickin' microphone right now.  The bass instrument channel just barely makes any noise tapping on it with a wood probe but on V3 it's like tapping on a microphone.  I've re-soldered the terminal on the socket after wrapping the wire tight on the terminal (wishful thinking) and it didn't help.

So, on the bright side plate voltages are going up without messing with the stock plate resistors.

[92Volts]

Your voltmeter has some input impedance/resistance, if it's a cheap one it might be less than the 1Mohm resistance between most grids and ground. In short, measuring there may affect the reading, and you may read less than what's normally present in the circuit.

Anyways, 0.1v DC would be plenty to influence the bias on something like a 12AX7... replacing coupling caps is not a bad idea.

Microphonics can be capacitors, or tubes. A little bit will happen in all tubes, so it depends how hard you're tapping the board, but a cap may be at fault.

[bbmade]

Yeah I've tried a few different tubes for V3 and it all sounds the same.  I have a tube tester too for basic emissions tests and shorts, nothing fancy.  I'm using a Fluke 289 meter.

If I touch the wire or even lightly tap on it with my wood probe it makes a lot of noise.  When I tap on the board in the area of the wire it makes some noise but it's far less than the 250pf ceramic cap that was there and another ceramic cap was coming apart so those got swapped out and also the .1uf and .047uf caps.  So to wrap up tonights work I've cleaned all the tube sockets and will let this rest until I can find some more time to work on it.

At first I thought someone was in there converting this to a AB 165 circuit but there were some original coupling caps in this amp which match the AB 165 schematic and not the AA 165 schematic.  This amp seems like it's caught somewhere between the AA 165 and AB 165.

Thank you to everyone who chimed in and if anyone else has input I'm all ears.  Love learning more about tube amps!

[tubeswell]

The plate load for a triode (e.g. 12AX7) determines the stage's gain and the tube current. Plate voltage is affected by the tube current and the plate load value. The more tube current there is, the lower the plate-to-cathode voltage will tend to become, and vice versa - all other things being equal.  The grid-to-cathode voltage determines micro-adjustments to the tube current and also provides an effective input signal voltage operating region. There is a practical limit to how much you can decrease a plate load resistor before you overheat the tube.  Conversely, there is a practical limit to how large you can make a plate load resistor before the tube runs so cold that gain becomes unpredictable. You need to stay within a 'normal' range of operating characteristics to ensure reliable tube operation. Having said that, there is some flexibility in load resistance and bias voltage, within which you can experiment, which will not harm the tube. However, you shouldn't cause the tube to run so hot that components (or the tube itself) gets destroyed by excessive heat.  There is usually also an absolute maximum operating voltage that you should never exceed, lest inter-electrode arcing occurs (which can also destroy the tube). All this information is on tube datasheets.


Generally for a 12AX7, typical gain stage supply voltage (i.e. 'B+' in the US or 'HT' in the UK) for the stage can range between about 60V to 380 or so volts. Typical plate loads would normally be between 47k to about 470k, with 100k being quite common. Typical bias resistor values range between a minimum of about 820R up to 39k, with 1k to 3k9 being quite common. For a typical Fender gain stage, a plate load of 100k and a bias resistor of 1k5 is quite common. This delivers a plate voltage that is typically about 2/3 of the supply voltage for the gain stage in question, at a bias voltage of between 1-2VDC.


If you decrease the plate load to 20k, you not only risk running a 12AX7 triode too warmly* (too much tube current), but you decrease the effective gain of the tube because output impedance will be increased and plate output signal swing will be more limited.


* A 12AX7 triode is rated for 1W maximum plate dissipation, and you usually want to say well under 1/2W.


Whereas a tube like a 12AU7 prefers to be run with a 10k to 20k plate load, and does not perform at all well with a 100k plate load.

[guitardude57]

Check your PS Caps...that is an old circuit, and they likely are leaking.
I just rebuilt this same circuit.  2 of the caps were leaking out the ends, so I replaced all of them.
Including the bias caps and the V1 supply filter.
All the Carbon comps to the plates had drifted up 20+ %, so those were changed.


This is a pretty darn gainy circuit.


Any time your voltages are low, look at the Power Supply filters...


And leave the plate resistors at the correct values, per schematic.

[bbmade]

Yep, I've replaced all the filter caps, swapped the dropping resistors in the filtering section to metal oxide and swapped some of the coupling capacitors.  None of the blue Astrons are leaking but there were some ceramic discs that were bad.  Voltages are now where they should be.  The amp sounds pretty good.  I was researching some mods to these AB165's today and tried a few things to make it more pre-CBS Blackface. 

I still have a ways to go but now the bass instrument channel sounds much better.  I don't want to do the Marshall mods that some people like.  I feel like if you want a Marshall tone...get a Marshall but I understand the appeal.

I think I might make the bass instrument channel a little more like the tweed era Bassman in the preamp/tone stack.  We'll see.

I plan to check the bias, get rid of the death cap and polarity switch and re-do the AC plug.  Someone crimped the new cable on to existing wires rather than just solder it in place.  I might do a quick demo video when it's done and upload to YouTube as a private video to then embed here...New to the forum so not sure what is or isn't possible in that regard...I'll do some homework here on the forum!!

[guitardude57]

I'm glad it is functioning better now, and voltages line up as they should.


Also, consider swapping the bias arrangement to a more standard adjustable, like found in the ab763.
I did that, on the last one, and got a more stable adjustment.
The balancing bias deal there...don't care for it.


It is easy, and only a couple changes.  Replace the 50V bias cap with a 100uF 100V. Watch polarity
on that one...

[bbmade]

I've done a little work to the bias circuit.  The electrolytic has been changed but I will double check its value.  I did a huge order for parts and have a sneak suspicion I forgot to add any caps that would work there...We'll see!

Hoping to finish up this amp within another week.

Really appreciate all the replies here, what a great resource.

[mresistor]

Also, consider swapping the bias arrangement to a more standard adjustable, like found in the ab763.

Guitardude57 I think you're thinking of an AB165 which was messed with by CBS ( I assume) and has local feedback on the power tubes and that funky bias circuit. This is a pre CBS AA165.
 I've had a couple of the AB165's come through that were already modified to be AA165's and I personally like them much better that way.


http://el34world.com/charts/Schematics/files/fender/Fender_bassman_aa165_schem.pdf


bbmade does your amp have the local feedback. Maybe post up a picture of the innards for us.   

[mresistor]

I suppose the AB165 could be considered pre-CBS and it was Fender's engineers that changed it from the AA864 - AA165 style circuit and I like the previous versions PI configuration better too.

[guitardude57]

The one I worked on was an AB165...1967.


Had the funky bias set up, I changed for what had been a better bias layout
used in several amp series... And found on the AA165.

Did not change any other part of the circuit...other than drifted plate resistors,
and PS caps.


The amp sounded mighty fine after...and horrible before.

[lego4040]

   I never should of let mine go, I loved my 1965 Bassman. Treat her kindly, this amp gets mean when turned up. I had mine repaired by John Lebrio years ago when I lived in Staten Island and was blown away literally when it was played threw a 4x12 Marshall stack