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Bassman 70 Line Out Issue
Bassman 70 Line Out Issue
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Old 16th August 2019, 12:59 PM   #1
gbowling is offline gbowling  United States
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Default Bassman 70 Line Out Issue

I have a Bassman 70 that has a line out.

When the line out is plugged in without a speaker connected, it works fine.

When the speaker is connected without the line out connected, it works fine.

But when you plug in the speaker and the line out, I get the following issues.

- The line out signal is many times louder, so loud it's unusable.
- I also get a very loud hum from the line out, even if the volume is turned all the way down.

None of these issues are there if the speaker is unplugged.

Here is the schematic. I'm unsure how to read the "jacks" for the speaker outputs. RE, the connections with the speaker plugged in vs unplugged.

To me it looks like plugging in the speaker switches in the output of the tranny into the connection from the line out (left side of 2.2K resistor). Which would pull in signal from the output tranny that isn't there when the speaker isn't plugged in.

But surely that's not correct as why would plugging in the speaker affect the line out in such a dramatic way? So I'm probably not reading the circuit correctly.

Any help appreciated.

Thanks, gabo
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Old 16th August 2019, 02:37 PM   #2
turk 182 is offline turk 182  Canada
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??? the signal level is changing....what exactly are you connecting the line out to?
and be aware that running a tube amp without a load is bad....(but with no load on either jack the secondary will be grounded (if all is correct) )


can you specify which speaker output your connecting to?



and not sure about this but it seems the line/recording jack is part of the feedback loop or derived from the same circuit so whats attached could vary the feedback signal affecting gain, hopefully others will weigh in.


in my tenure as a soundman with older Bassman's if had no luck what so ever with direct connection to the jack in question and i'm always more concerned with grounding of those old amps.

Last edited by turk 182; 16th August 2019 at 02:50 PM.
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Old 16th August 2019, 05:05 PM   #3
gbowling is offline gbowling  United States
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Connecting the line out to the line in of a mixing board. The speaker connection is the main speaker connection, which is the bottom jack in the diagram.

Nothing is plugged into the ext-speaker jack.

Yea, personally I wouldn't use this jack, there are other ways to do it. However, the musician that owns this wants to use the speaker on stage and use the line out to take to the house board.

Even though I wouldn't do it this way, it should work, and I'm pretty sure the original design didn't function like I'm seeing this one. With the very loud noise on the line out and the huge change in volume depending on whether a speaker is connected or not.

So I'm just trying to get my head around how it's suppose to work. I'm going to do a re-cap on this amp regardless as all the caps appear to be original. There are also a few resistors that show signs of overheating that I will probably also replace. All this may resolve the issues, but I would like to know what's causing the issues instead of just fixing with a shotgun approach.

gabo
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Old 16th August 2019, 06:19 PM   #4
turk 182 is offline turk 182  Canada
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with no speaker connected, the secondary of the output transformer should be shorted that means the feedback loop level is disturbed.


try running a high wattage dummy load on the output/speaker jack.(reduce input gain to the console, if necessary with a resistive pad)



i would also look at a repeat transformer to isolate the line/recording jack could be the noise is from a loop created between the amp chassis and the console ground returns.


avoid random recapping until the issues are resolved.



in the day this was designed recording console inputs where 600 ohm balanced and line was 1k but that's less relevant in this case...or is it...

Last edited by turk 182; 16th August 2019 at 06:23 PM.
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Old 16th August 2019, 06:24 PM   #5
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by gbowling View Post
Even though I wouldn't do it this way, it should work, and I'm pretty sure the original design didn't function like I'm seeing this one. With the very loud noise on the line out and the huge change in volume depending on whether a speaker is connected or not.
Circuit works fine, you are using it wrong.

1) that Line out is LOUD by modern standards, puts out 2V RMS or more, and is designed to drive another tube power amp, same or similar as the one built-in , whose sensitivity is also around 2V RMS.
Remember: 70's stage levels.

You are plugging it into a way more sensitive Mixer Line IN, sensitivity probably 100mV RMS or so; Basman Line out is easily 20X to 30X too loud.

You are attenuating full amplifier output (its Line Out is just a sample of Speaker Out) by shorting speaker out by unplugging speaker, a mess.
You will KILL your power tubes quickly.

Solution: tame that hairy Macho Man 70īs Line Out to modern weenie levels: add a 22 ohm resistor in parallel with the current 270 ohm one and try again.

Of course, with a proper cabinet plugged in.

Quote:
I'm going to do a re-cap on this amp regardless as all the caps appear to be original. There are also a few resistors that show signs of overheating that I will probably also replace.
NO, donīt messwith that fine old amp unless really needed.
You are not describing any symptom leading to a specific problem.
No, "operator error" does not qualify

Quote:
All this may resolve the issues, but I would like to know what's causing the issues instead of just fixing with a shotgun approach.
Worst is that you will fix nothing amd possibly botch something.

Ok, try my suggestion and post results.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.

Last edited by JMFahey; 16th August 2019 at 06:26 PM.
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Old 16th August 2019, 06:37 PM   #6
turk 182 is offline turk 182  Canada
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i was just musing about the attenuation of the "stock" divider/pad and saw JM's suggestion of parralleling the 270 with a 22 ohm that would change it from an approximately 10:1 to a 100:1 pad....much better me thinks...

but i would still like to see transformer isolation used.
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Old 16th August 2019, 07:09 PM   #7
gbowling is offline gbowling  United States
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"you are using it wrong." hahaha, story of my life!!

Thanks for that JM. I guess what's most confusing is the thing is labeled "Line/Recording" which would lead the un-informed to believe you could plug it into a recording input!

Also, thanks Turk for the transformer suggestion.

I think what I will do is essentially both. Install a 22ohm in parallel, then recommend the musician take that into something like a radial JDI or ProDI. Giving him the option to do that or go direct in without it.

Still need to sort out the very loud hum that comes out of that interface. But padding it down will bring that down enough to see whether it's also the same as what's present in the normal speaker output.

Thanks, gabo
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Old 16th August 2019, 07:18 PM   #8
turk 182 is offline turk 182  Canada
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Quote:
Originally Posted by gbowling View Post
Still need to sort out the very loud hum that comes out of that interface.
ground loops are the bane of my existance....
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Old 17th August 2019, 02:41 AM   #9
Rick PA Stadel is offline Rick PA Stadel  United States
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Yep, bet two nickels it's a ground loop issue.

Solve it all with a floating direct box on the Input, IMHO. Why fuss with trying to make an inferior solution work? Much better to use the far cleaner signal from before the power stage adds its dirt.

Better still, the musician can make adjustments to what he hears, without you having to watch closely and grab knobs on the board to 'fix' (read 'undo') every little change made at the amp.

Good luck persuading the player, though.

Regards,
Rick
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Old 17th August 2019, 12:09 PM   #10
gbowling is offline gbowling  United States
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hmmm, "floating direct box on the input"

I guess you're referring to something like a radial JDI, like you would use for taking the instrument direct into a console? But instead take that into the amp? And NOT the "thru" jack into the amp, but rather the output of the JDI into the amp?

gabo
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