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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon Jun 01, 2015 6:12 pm
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That is the coupling cap to the phase inverter, ya, they get noisy and microphonic, but seldom lose signal. I think you found it.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon Jun 01, 2015 7:15 pm
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Loss of signal is gone so far :roll:
however still not near loud enough for a 50 watt tube amp and a lot of ugly sounding distortion at the tail of notes when cranked up.
very possible that is from the speakers, I get 6.9 ohms at the plug so were hitting close to the 8 ohm range
two of the speakers have "repairs" done to them with silicone

thinking about bypassing the master to see if it is failing, i replaced the volume pot 1 meg at the studio channel and it made a little improvement.

If I kick up the input with clean boost it gets loud, on its own its not much louder than a blues jr

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon Jun 01, 2015 11:46 pm
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Are the 6L6GCs biasing into nominal range? Ugly distortions on top of notes usually is a filter cap issue. I know that you changed them out, so unless the wiring was messed up (feeding the tubes anode plates) --- it prolly isn't the caps.

Maybe, the phase inverter section. You have tried another 12AT7? Could be those 0.1mfd coupling caps. Or the resistors on the tail (cathode) of that 12AT7. Voltage measurements around this area may help find the problem.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 5:55 am
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Beemer I changed the two .1 with mallory 150's I had and made some progress

The only real issue now is ugly bees in a can distortion that is now at the onset of low notes with the volume past 3 .
I swapped out all tubes again just in case , I have 6L6WXT that are idling at 32ma with the bias pot maxed out
I have new 12AX7 and 12AT7 modern made tubes in the preamp (its not the tubes) !

gonna do a short term test with my 2x12 bassman cab its only 4 ohms so I'll keep it short
The 1x12 cab at 8 ohms is only a 25 watt speaker so....... and the bassman 10 original speakers are rough, I only replaced the one so far.

But we do have progress :mrgreen:

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1968 Bandmaster with 2X12 cab C-rex speakers
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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 7:43 am
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" I have 6L6WXT that are idling at 32ma with the bias pot maxed out. "

This doesn't seem right. Are those tubes new? The voltage in the bias supply should be around -48VDC after the 3.8K-ohm resistor. Between the two 50mfd/70VDC caps. What anode plate voltage is on the 6L6GCs?

How did you arrange the grid-to-ground resistors on the 6L6GCs, after you changed the balance pot to a bias pot setup? The two OEM 100k-ohm resistors may need to be changed to 220k-ohms, to get the bias point back up around 40mA (mid turn of pot).

This:

Image




To this:

Image


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 8:00 am
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-50.4 between the two 220K (I changed it to a true bias )

Pin 3 off far left 6l6 (from the back of the chassis) 511 volts
pin 3 Far right 6L6 508 volts

Ah that may indeed be a problem, When I removed the .002 caps from pin 1 to pin 8 on the 6l6's I left pin 8 to direct ground without a resistor

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 8:14 am
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No, I think pins 8 to chassis ground is fine.

-50.8VDC with the pot in the middle of turn? 500+ VDC on the plates seems high. Is this measured with the 6L6WXT is the sockets? Maybe, better around 465-475VDC. What are the voltages on the anodes of the 12AT7 and 12AX7? Maybe one is too low, for whatever reason, causing the 6L6GCs to be too high. Or one of them is idling too low, due to problems with the cathode resistor and/or bypass cap.

Anyhow, 32mA would seem fine for 500VDC on the plates. But, you may need to go to 7581 or 7581A, if you want those tubes to last.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 8:41 am
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V-1 pin #1 262 volts
V-2 Pin #1 247 volts
V-3 Pin # 1 315 volts

Going to rebuild the PI and work on the studio tone stack
to follow this mod advice
http://www.premierguitar.com/articles/S ... Bassman_10

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 9:20 am
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The low gain may be normal for this amp. I wouldn't be a bit surprised if the Blues Jr is louder.
Bass amps are designed with low internal gain to keep them in the background dynamically.
If the volume is OK with pedals, it's probably normal.
If you want to pump it up, change the 820ohm feedback resistor to @2.2 to 4.7Kohm


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 10:25 am
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TimsAudio, I have a 1968 Bassman. It is not a low gain or attenuated amp. A guitar through a 2 x 12 inch cab is louder with the Bassman head than my blonde Tremolux.

Plus, the odd distortion riding on top of bass notes indicates something is wrong. I'm thinking a problem similar to blocking or farting distortion.

blues bondsman wrote:
V-1 pin #1 262 volts
V-2 Pin #1 247 volts
V-3 Pin # 1 315 volts

Going to rebuild the PI and work on the studio tone stack
to follow this mod advice

http://www.premierguitar.com/articles/S ... Bassman_10


Well, the plate voltages on the gain and PI stages seem ok. BTW... does this amp have a master volume control? How do you bypass it? Or is it still in the circuit?


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 10:48 am
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Yes the master volume is still there
Best way to bypass ?

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 1:22 pm
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How does the amp sound with the MV at full throttle? Half throttle?


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 1:54 pm
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well full throttle and the break up is earlier on the volume dial
half and ya gotta crank it to get the buzz

Funny thing is, it isn't a vibration thing, the cab is five feet away from the chassis
Just got back from my secret electronics house and I got several bags of caps, resistors and a five lb roll of Solder at a stupid low price !

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 4:58 pm
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Yep did all this this afternoon
Replaced the .002 on the 6L6's
Image

Rebuilt the Phase inverter circuit (yes I used Metal Oxide)
I built it based on the mods link I posted earlier from Premiere Guitar Mag
Image
notice the Mallory 150's in place of the blue turds ?

Then I reconfigured the Studio Channel also according to the Mod link
Image

Why Metal Oxide resistors ??? Stability man ! this baby wants to go spastic and so I tightened up the specs to keep it in line !
and it worked !!!
The only thing I didn't do was change out the bright cap, cause man that dude is BRIGHT !!!
maybe later, i seldom use a bright cap as I'm a Strat and Tele Guy !

The amp now sounds so good ! a bit woofier than its neighbor the 68 Bassman Head and Cab, however I hear that replacing all those CTS dog speakers opens it right up.

No after taste to the notes , twice as loud !, no cutting out, Strong low end !! and you dont hear the caps drain when you shut off the standby any more, I think I forgot to tell you guys, when you shut off the standby before the repairs today you would hear the notes even after the switch was thrown and it sounded like bees in a can.
All better now !!!

Thanks to those who helped !!! You guys here are the best and talking it through sure helps when your stumped

_________________
Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Tue Jun 02, 2015 5:02 pm
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More Victory laps :mrgreen:

Now my shop is trashed !
Image

I did buy something NOS at an excellent price
A full five LB reel of solder 40/60 Resin core for 35 bucks at an old electronics shop.
Image

What do you guys think about that .1uf I left in the tone stack ??? I'll bet a .047 would help get rid of a little more woofiness ???
feel free to comment

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


Top
Profile
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