Blonde Bassman Power Sag.

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eanderso22193
Junior tube assistant
Posts: 88
(2/7/04 5:00 pm)
 Blonde Bassman Power Sag.
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 I've got a '61 or '62 Blonge Bassman 6G6 that I've almost resurrected completely. But now, when you have the volume up past about 5 or 6, and strike a chord really loudly, after about a second the volume drops drastically and then comes back up. It does this on both the Bass and Normal channels. I hooked up a bias meter to the 6L6's and hit the chord and watched the bias current go from about 37mA to all over the place.

Any ideas on what the problem might be?

Eric
 
6G6
Junior tube assistant
Posts: 73
(2/7/04 6:58 pm)
 Re: Blonde Bassman Power Sag.
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 Have you replaced the filter caps in the bias circuit?
Those get overlooked sometimes.
Also, the diode they used back in the day wasn't real rugged.
Simple, cheap fixes that might help and sure won't hurt.
 
eanderso22193
Junior tube assistant
Posts: 89
(2/8/04 11:10 am)
 Re: Blonde Bassman Power Sag.
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 I had to rebuild the bias circuit. The amp was in such a terrible state, having been converted to 100 Watts and having a solid-state rectifier added. I'll check the bias circuit again to make sure I got it right, and maybe use a different diode. I used one I found laying around the shop that looked like some I've seen in older amps. It seemed to test ok, but I'll try something newer. Thanks.

Eric
 
Tiny Daddy
I will work on all amps
Posts: 482
(2/8/04 12:02 pm)
 Re: Blonde Bassman Power Sag.
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 Check the coupling caps to the grids of the output tubes. They should be no larger than .1uF. Or maybe they are old and need replacing?
Also the output tube grid resistors that connect to the bias supply should be no larger than 220K, maybe even change to 150K.
If these parts are too large the caps can get charged up when the amp overloads, which affects the bias momentarily.

 
eanderso22193
Junior tube assistant
Posts: 90
(2/9/04 9:24 am)
 Re: Blonde Bassman Power Sag.
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 Cool, I'll check that. I know I just replaced the caps with .047 orange drops, but it still did it. I'll check the resistors and try changing them perhaps.

Eric
 
GordonSims
Junior tube assistant
Posts: 36
(2/23/04 11:30 pm)
 Re: Blonde Bassman Power Sag.
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 I had a similar experience with a 6G6A I scratch built on a Bassman 10 chassis. It would kind of stutter then kick in when you hit a hard chord. Really noticable if you muted the strings with the heel of you hand and made a nice pop. Like you say the V2 voltages etc. went all over the place when you made the amp take a transient. I put in a master volume ( ganged 500K pot across the output of the phase inverter) which made it worse. I think the only reason it was worse is because you could turn down the master and still survive in the same room with the volume over 6. The normal channel is pretty much a straigt ahead brown face preamp and it didn't have the problem. Only the base channel did. I also scratch built a 6G6A on a silver face bassman chassis and used the bassman 10 power transformer which had taps for a tube rectifier. That way you'd get some cool sag when you played past 6. Again same problem with the bass channel but without the master volume you are playing very loud on 6. If you compare the second preamp tube in the bass channel of the 6G6 and 6G6A you will see a 1 meg resistor across pins 1 & 6 on the 6G6 and not the 6G6A. I tried adding the resistor thinking it was like a feedback across the the B side amplification stage. So feedback should decrease gain and make it tighter and more stable. I tried it and it does help a bit. At the end of the day I decided I just don't like the way the bass channel sounds or behaves. So I sold the more authentic version I built on the silver face chassis and I Marshallized the bass channel on the Bassman 10 chassis. It now looks like the JTM-45 preamp and sounds and behaves much better. You can turn the volume up to 10 with the master down (to keep the cops away) and no weird hesitation when you slam a chord. Has anyone else been trying to figure out why people on ebay are willing to pay so much for 6G6 bassman which as near as I can tell is just a brown face normal channel and a evil bass channel? Why not build a brown Pro, Concert, Bandmaster or Super. They all have the brown tone and the harmonic vibrato as a bonus.
 
GroundhogKen
Forum Moderator
Posts: 2296
(2/24/04 8:20 am)
 Re: Blonde Bassman Power Sag.
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 You have blocking distortion. You can read about it here:

www.aikenamps.com/Blockin...rtion.html

Tiny Daddy has already suggested part of the cure.
Reduce the power tube coupling caps to .047uF and make sure the bias feed resistors are 220K or less.

Also, add grid stopper resistors (1.5K) on the grids ( pin 5 ) of the power tubes.

Reduce the value of the coupling caps leading into the phase inverter. They have different values in the vaious 6G6 circuits. There may be one from each channel.

In this circuit for example:
www.ampwares.com/ffg/sche..._schem.gif
I'm talking about the .1uF before the treble pot. and the .05uF before the 470K resistor on the normal channel.


Ken.
 
GordonSims
Junior tube assistant
Posts: 37
(2/25/04 1:00 am)
 Re: Blonde Bassman Power Sag.
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 Ken,

Thanks for the info. My bassman problems sound like the preamp version (V2?) since the normal channel worked fine under the same slammed chord and high volume setting. I think "gridlock" sounds more sinister than blocking distortion. Only gridlock could make me want to dust off my oscilloscope to see what's going on.
 
 
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