1965 Bassman guts

[mresistor]

A person brought me a BF bassman  -  said it was a 1967, however, looking at the trans codes it appears to be a 1965. With a replacement choke on it.  Has some "newer" mallory paper dual caps that frankly I have never seen before. The dual cap is a 30-30  150v. Also has an older 22-22 on it that has to come out. The HV PS caps are what concern me too. Whoever replaced them went with some blue caps that are huge 20uf . But, they are so large that the tech had to install spacers on the doghouse cover to clear the caps. The tech wrote his name on the chassis in back.  J Bakos.  Anyone know him?
I think this is an AB165 circuit, an early one.

[mresistor]

Also   I am not a fan of Illinois caps anymore. Also, the big blue Sprague Atoms are cardboard outer covering, the likes I have never seen before either. I think they need to be replaced. I will need two new 70Uf , 3 new 20uf and a 16uf   in that dog house.

[mresistor]

This amp very well could be a AA165  that has been modified.

[MoparWade]

Also   I am not a fan of Illinois caps anymore.

Why?

[mresistor]

Also   I am not a fan of Illinois caps anymore.

Why?

I have seen too many fail. There are better caps. My opinion, YMMV.

[mresistor]

Mr. Bakos appears to be based in Atlanta GA..  cool   ;-)   


Question ?   How many of you guys have a "Deep" switch on your amp?       

[sluckey]

The blackface bassman amps were actually made to play a bass through. I played bass through one in the '60s. The deep switch was always on.

[mresistor]

I saw that  on the schematic.  Bass input. Is it a good sounding bass amp?    so what do you think of the wiring?  Maybe replace the brown turds too.   The owner is gigging it.

[2deaf]

Question ?   How many of you guys have a "Deep" switch on your amp?

I have two SF Bassman 100's that have deep switches on them.  I also have a 400ps that has a deep switch on it. 

[mresistor]

Question ?   How many of you guys have a "Deep" switch on your amp?

I have two SF Bassman 100's that have deep switches on them.  I also have a 400ps that has a deep switch on it.

Is it like a "thick"switch   or a "fat" switch?  I haven't fired this one up yet.  I mean   do you have to be a WV or CO coal miner to understand what the effect is?  Just how "deep" is this deep sound?

[sluckey]

The deep switch makes it sound very bassy. I thought the amp was a good reliable small venue bass amp. No one was buying them in the '60s to play guitar. The speakers were ribbed for a bass, not a guitar. Sounded dull with a guitar. Back then screaming treble was popular.

Does the owner have a complaint about that amp?

[mresistor]

Steve - he has some concerns about the pots being scratchy.  I feel that the older paper Sprague HV capacitors should be replaced.  And then we can dispense with the spacers on the doghouse cover. Also  those IL caps are 10uf over spec each  I know not a real biggie, but not correct either. It's in for a refreshing.  I will fire it up tomorrow and see what all the issues are. And how it sounds

[mresistor]

I figured out why the doghouse cover has spacers - there is some really brittle black cushioning material inside that has the imprints of the original capacitors and their shape - so unless you took that stuff out, it would contribute to the lack of space in the cover.. 

[sluckey]

Here are some good pics of the cap board.

     http://ppamps.blogspot.com/2012/12/ab165-bassman-refurbishment-modification.html

[mresistor]

Thanks Sluckey - those are great photos...  wonder why he decided on 2x100uf reservoir caps instead of 2x70uf ? Probably not a big deal as its solid state rect.

[sluckey]

70µF is not a common value today. 100µF@350 is very common.

AES has those dual 25/25µF @ 25V caps for the main board but they are pricey. Most people just replace them with two singles (four for your board). Much cheaper.

Here's a wiring diagram for that cap board in case you can't figure it out from the pics...

[birt]

did you test anything before you start replacing stuff?
i mean you say the complaint was scratchy pots but you didn't turn on the amp yet and you are already planning to replace a lot of caps that might be just fine.


personally i would first fix the problem and test those caps with my ESR meter. if something works as it should there's no need to fix it right?

[HotBluePlates]

... Has some "newer" mallory paper dual caps that frankly I have never seen before. The dual cap is a 30-30  150v. ...

The Mallory with the orange cardboard with blue print (presumably your 30-30 cap) in non-original. But if it's already been replaced, the new replacement is probably fine.

... The HV PS caps are what concern me too. Whoever replaced them went with some blue caps that are huge 20uf . But, they are so large that the tech had to install spacers on the doghouse cover to clear the caps. ...

Again, since they've been replaced, they're probably fine.

It seems the previous tech was trying to use "original style" parts, but didn't have original values. Those caps are so large they're comical and impractical. But if the amp is working and the doghouse can be secured, they're probably not a problem.

I agree with Birt that if the customer is worried about scratchy pots, you should just fix those. The other things are more about your aesthetic sensibilities, and what you would swap if it were your amp. There's a big cost difference between squirting some contact cleaner and replacing all the filter caps.

[mresistor]

HPB and Birt - it is up to the owner if he wants to replace the HV caps. As for the cathode bypass. Well   I'm sure you guys know when you pick one up and shake it and it rattles like a babies rattle, it's time to replace it. I do those as a matter of course and to put the circuit back to spec. The pots are cleaned and lubed. The amp sounds great. Birt, when I see 51 year old amplifier with an original Mallory double cap in a cardboard tube,  I don't even turn it on until after that cap is out of there. YMMV. 


Sluckey  - what is your preference on bias setting on these bassman amps?

[mresistor]

I replaced both Mallory cardboard tube double caps with single 25uf 50v electrolytics. I know it would be OK to fire it up with only the cathode resistor functioning un-bypassed. But why? I want to hear the bassman like it's supposed to be, not with 51 year old dried up caps.

[sluckey]

Sluckey  - what is your preference on bias setting on these bassman amps?

I don't really have a preference. I haven't plugged into a bassman since '72 and then I was playing bass.

But, since that bias pot is really a bias balance, I'd set it for equal cathode current through each 6L6. Better yet, just install a 1Ω resistor between cathode and ground of each 6L6, then connect one meter probe to pin 8 of a 6L6 and connect the other probe to pin 8 of the other 6L6. Adjust for zero millivolts. If you can't get to zero, set it to minimum millivolts. If this is too much trouble, just set it for -45vdc on the pot wiper IAW the schematic.

This reminds me... That 22K resistor on the bias pot was originally a 15K.

[mresistor]

Steve - I will just use my bias probe but adding 1 ohm resistors is a great idea.  Built from Hoffman parts and instructions in 2007. Good catch on the 22K resistor, I did notice that that one was newer than the rest of the amp.


I will check bias today. Interestingly, it would be pretty cool to convert this amp to cathode bias.

[sluckey]

Interestingly, it would be pretty cool to convert this amp to cathode bias.

How so? Converting to cathode bias would be a step backwards IMO.

[DummyLoad]

Plate V is 456    Pins 5 read -53 volts on the non-adjustable side and -52 volts on the other side -  that also is too high by around by 7-8 volts

no it is not.

as per schematics...
twin & super with 460V is -53V
tremolux with 365V is -45
BF bassman with  420V is -44V

yours is dead on for an amp running B+ that high.

--pete

[mresistor]

Thanks DL  but with .024ma per tube that equates to only 11 watts dissipation per tube, doubled that is 22 watts which seems really low for a 50 watt amp?

[PRR]

> ...22 watts which seems really low for a 50 watt amp?

There's no direct connection between idle power and maximum output in a fix-bias amp.

[mresistor]

at 70% of 6L6GC max dissipation  of 30w  I should be see around .032ma per tube   for am amp output of appx 41 watts right?


aha   thanks PR 



 




 

[DummyLoad]

Thanks DL  but with .024ma per tube that equates to only 11 watts dissipation per tube, doubled that is 22 watts which seems really low for a 50 watt amp?

schematic states -45V at 425V B+. seems odd yours is running at 455V. if that is the case, then the bias supply is tracking B+ properly. e.g. the bias supply is increasing proportionately to the increase in B+. fender did bias cold but as another oddity your pics show a 22K for the bias trim and schematic shows a 15K.


in any case, it seems that your bias is following the fender desired range for the B+ you have.


--pete

[mresistor]

Is it possible the tubes are not performing as they should (old) ?    I can't understand the low power.   I just a couple weeks ago biased a twin and the current and voltage were such that I could compute 70% dissipation per tube. This one isn't any different.

[mresistor]

DL  my wall voltage here right now is 123.6 vac which might explain the Plate voltage being higher.

[sluckey]

Is it possible the tubes are not performing as they should (old) ?    I can't understand the low power.   I just a couple weeks ago biased a twin and the current and voltage were such that I could compute 70% dissipation per tube. This one isn't any different.

How do you know it's not different from the twin? Did you compare the schematics? As I said earlier, the AB165 has a bias balance adjustment. That means you can attempt to balance (match) the tube currents with the bias pot. But you cannot set the bias for 70%, or 60%, or any other specific % because the circuit doesn't work that way.

[sluckey]

DL  my wall voltage here right now is 123.6 vac which might explain the Plate voltage being higher.

Where did you ever mention plate voltage or grid voltage? D/L is even quoting voltages but I can't find them in this thread. Did you delete a post?

[mresistor]

Sluckey   the plate voltage is  456v and each 6L6GC is idling at .024.   I figured each tube was at around 11 watts plate dissipation.  (yes  I deleted a post  but was unable to keep DL from catching it, I made a mistake and was trying to correct it, sorry)

[mresistor]

In this Bassman, isn't that (what is supposed to be) 15K resistor on the bias pot called the bias range resistor? So I understand that you can balance the two tubes to eliminate 120cycle hum. Why can you not set the power tube current to produce a desirable power out level, or plate dissipation? Is a twin not fixed bias. Can you explain why a person cannot set output tube dissipation on this Bassman?

[mresistor]

It appears to me one could select a different resistor to raise or lower the negative voltage on the non-adjustable tube to get a higher cathode current figure, and then use the balance pot to adjust the other one to be equal to it or for reduced hum. Is this not correct?

[sluckey]

Bass amps are normally biased cold in order to produce a clean sound. Different goal from a guitar amp. I understand that bassman amps are now popular with guitarists but they need some tweaks to be optimized for guitar. Seems like you are getting in to this amp a lot more than what the owner wanted. I thought all that was needed to do was spray the pots?

15K resistor on the bias pot called the bias range resistor?

Calling that the bias range resistor is a recent phrase. It was not called bias range when it came out of the factory. Fender never intended for you to change that resistor to set the bias.

Can you explain why a person cannot set output tube dissipation on this Bassman?

Of course you can change it if you modify it.

It appears to me one could select a different resistor to raise or lower the negative voltage on the non-adjustable tube to get a higher cathode current figure, and then use the balance pot to adjust the other one to be equal to it or for reduced hum. Is this not correct?

Of course. That's called a modification. Most people would convert that bias balance circuit to the AB763 adjustable bias circuit.

[mresistor]

Sluckey   it is already modified by a man named J. Bakos out of Atlanta GA.  Plate voltage x cathode current has a direct correlation to power dissipation of a 6L6GC and these 6L6GC's are running to low on dissipation. This is what I am trying to address. The power out of an amp that is low on it needs to be looked at. That is what I am doing.  DL says the bias voltage is tracking the higher plate voltage, to a point but not exaclty. 456 is 107% of 425. And 53v is about 118% of 45v. There is a difference.  So how do I go about getting the power up on this Bassman? 


And I read about converting to a AB763 type bias circuit and it sounds like an excellent idea I would have to go over with the owner.

[mresistor]

For right now, I'm thinking that replacing that 22K resistor with the original value of 15K would bring this bias voltage back down and bring the current up on the 6L6's. Maybe this is a wrong thought.   

[mresistor]

It occurs to me that the reason that 15K in the bias circuit was changed is because a previous owner was running different output tubes that required more bias voltage to keep the plate dissipation down.

[mresistor]

Bass amps are normally biased cold in order to produce a clean sound. Different goal from a guitar amp. I understand that bassman amps are now popular with guitarists but they need some tweaks to be optimized for guitar. Seems like you are getting in to this amp a lot more than what the owner wanted. I thought all that was needed to do was spray the pots?

Steve  he said he would like to have it sound better, and to replace what I thought needed to be replaced, and I am being conservative in my approach. The owner is a performing artist and gigs with this amp, using a 6 sting guitar. So anything that can conservatively be done to improve the performance for guitar is going to be welcomed. He is also into "tone" as I am. Thanks for the info on cold biasing from the factory, I was not aware of that. But why not just use a KT88 or some HIFI tube to do clean bass?  6550?  Any way   I am going to replace the 22K with a stock 15K  and check bias again. I'm using a spare set of 6L6's not the customers.

[sluckey]

I am going to replace the 22K with a stock 15K  and check bias again. I'm using a spare set of 6L6's not the customers.

I wouldn't waste my time biasing a spare set of tubes that are not the customers.

[mresistor]

Changed the resistor to a 15K  tested it on my crap tubes, then put the customers tubes back in -  the 6L6's are now running 34ma per tube,.. very nice. At 5 on the vol dial there is a bit of hum though,  but not too bad. Definitely better power out and sound.

[DummyLoad]

what's the B+ with 34mA?


--pete

[mresistor]

Pete - it's 457v   indicating around 15 watts per tube

[mresistor]

This Bassman now sounds glorious!  Really nice and powerful.  The hum was from the lights in my other room and that I was using a strat in single coil mode.  The owner is going to be very happy. Thanks you guys for sticking with me on this amp.   I appreciate the help.   :-)

[mresistor]

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