bassman working like dream now

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nealski
Junior tube assistant
Posts: 1
(3/25/04 7:14 pm)
 bassman working like dream now
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 have to admit i was the victim of Hoffman's law again, during all the changes that i tried, i did hook up one of coupling caps wrong, which resulted in loss of volume.

now that it's fixed it is difficult to turn it all the way up, which is the way it's supposed to be!!!!!!!!!!

thanks for the input guys

laters neal





(here's the old post)

i have been reading this forum for a while now and i think it's about time i stuck my head out--especially because i could do with some help.
I'm a beginner with a basic knowledge in circuits and electronics, i also have a band play guitar and sing.......

A couple of months ago i bought a broken bassman head on ebay. it's a AA371 circuit.
It didn't make any sound at all, as the PT and the filter caps were fried.
After getting a new PT and filter caps i was able to get a tiny bit of sound out of it (about 20% of the volume i'm expecting)


I decided to replace parts to eliminate things that could be broken, so I replaced all the electrolytics on the board and
most of the resistors including the ones in the doghouse.

I also decided to try and do some 'blackfacing' to it as well.
I changed the PI section to blackface specs, changed the tone stack on the bass channel to Marshall values, used a 500pf cap into the PI, changed the mixing resistor from the normal channel to a 220K one.

Things then started to come together when i changed the feedback over to the 22k resistor and the .1 cap after installing a 100R resistor to ground, i did swap the secondaries from the OT as well.

I had sound, but also quite a bit of hum, i checked the heater wiring and did it using Doug's guide on it, every other preamp tube was reversed to the one before.

Anyway, i still haven't been able to get rid of the hum (i think it's about 100 Hertz-i'm in London) it's a middle G slightly sharp on guitar.
And the funny thing is when i swap the feedback loop back over to the way it's in the AA371 circuit i lose all the power again. (i have just changed the last blue cap, the .1 cap in the long tail of the PI and now i am getting about a quarter of the expected volume)
I did some measuring of voltages tonight: here is with the blackface feedback loop.

V1 p1 212
p3 2.1
p6 200
p8 1.77
V2 p6 258
p8 1.86
V3 p1 193
p3 1.61
p6 245
p8 2.02
V4 p1 281
3+8 66
p6 270
p7 23

with the AA371 feedback loop
the voltages stay the pretty much the same, i am using a 25k feedback resistor and a .1 cap (i lose the cap for the blackface hookup)
the power tubes voltages are both at -48 bias voltage,
and pins 3 and 4 are around 437 on both.

okay, sorry about the length of this my first post, i'm sure there is some other structural problem there ???
Or i'm another victim of Hoffman's law--ironic as that's my name too.

anyway, any pointers will be much appreciated (hope it all makes sense)



Edited by: nealski at: 4/2/04 12:43 pm
 
nealski
Junior tube assistant
Posts: 2
(3/26/04 10:06 am)
 Re: bassman problems
--------------------------------------------------------------------------------
 i have installed a ground buss behind the pots, cleaned the inputs and changed the bias system the way Doug describes it
in his pages.

cheers neal
 
Tiny Daddy
I will work on all amps
Posts: 569
(3/26/04 11:05 am)
 Re: bassman problems
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 "used a 500pf cap into the PI" If this is the coupling cap from the 7025 plate to the PI grid then 500p is too small. Use .01uF or even .022uF.

Edited by: Tiny Daddy at: 3/26/04 12:53 pm
 
ganzonimx
Hey get your own solder
Posts: 439
(3/26/04 12:19 pm)
 Re: bassman problems
--------------------------------------------------------------------------------
 Do you have low volume on both channels?

The voltage on V4 pin 7 = 23 V is a little low (but might be OK after the blackfacing), what about pin 2 of V4? Double check the values of the resistor you changed around the PI.

Did you change the pot values when you changed the TS? The AA371 and the AA864 does not have the same values of the pot, and it makes quite a difference.

Cris

Edited by: ganzonimx at: 3/26/04 12:28 pm
 
nealski
Junior tube assistant
Posts: 3
(3/26/04 12:54 pm)
 Re: bassman problems
--------------------------------------------------------------------------------
 thanks for those ideas guys, the low volume is on the non blackface feedback loop version.
On the blackface feedback loop version the power is almost there but there is this hum. (the two channels behave in the same way in the two different versions)
I will check those other voltages (pin 2) tomorrow, and check the pot values, i haven't changed any of the pots yet.

will post an update then

thanks a lot already

neal
 
nealski
Junior tube assistant
Posts: 4
(3/26/04 3:38 pm)
 Re: bassman problems
--------------------------------------------------------------------------------
 hi guys

have decided to stay in tonight as opposed to meeting some
attractive friends in a pub, okay i feel better now.

the voltage on pin 2 on v4 is 22 volts, have double checked
the resistor values on the PI and they are okay apart from a slight discrepancy between the 2 1meg ones.
One is 990k and the other is 1010k, i also get a thumping type sound when i switch off (on) standby.

I had a think about the pot values and compared them to the AA864 circuit but realised that at this point I didn't really want
to change the preamp section that much, so the only thing
i have done was to get rid of the caps that bleed treble to ground apart from that it's still stock AA371

at the juncion of the 1meg and 22k and 470R resistors i also
get about 70volts which i thought should be higher??

the sound that i am getting is already quite impressive though

thanks for listening

laters neal
 
Tiny Daddy
I will work on all amps
Posts: 572
(3/26/04 4:03 pm)
 Re: bassman problems
--------------------------------------------------------------------------------
 "i also get a thumping type sound when i switch off (on) standby"

You need to reconnect the first pair of filter caps to the other side (load side) of the standby switch.
Also see my previous edit regarding the coupling cap.

 
nealski
Junior tube assistant
Posts: 5
(3/26/04 4:10 pm)
 Re: bassman problems
--------------------------------------------------------------------------------
 thanks man,

god i got so blinkered i hadn't even seen that post from before
yes that's the coupling cap i meant, will try a bigger one,
but used it because i had seen it in the AA864 and AA165
diagrams.

thanks again

laters neal
 
nealski
Junior tube assistant
Posts: 6
(3/26/04 4:51 pm)
  Re: bassman problems
--------------------------------------------------------------------------------
 you the maaaaaaaannnn tiny (big) daddy

it's fixed and it sounds amazing

yes for some reason the yellow and red wires have to be connected the opposite way to what it says in the diagram.
I just checked now and you were right the yellow one goes to the big filter caps and the red one to the preamp filter caps

i never bothered to double check that, well i am a beginner

thanks a lot again--love the forum

cheers neal
 
 
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