Finally got around to doing some work on my Bassman

Riffraff

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Here's a little background info.
I got a free non-working Bassman 70 a couple of years ago. Someone replaced the power & standby switches and screwed it up then harvested the power tubes and left it in pieces. I fixed it and replaced the tubes early this year and tried it in a little recording and was blown away by how great it sounded. This was the clip.



After that little test I really wanted to use it for recording. I do all of my recording very early in the morning so I record silently with an attenuator and cab IR's and just listen with my monitors. The problem is the Bassman is a little too large a load for my Mass100 at full attenuation. I have a pair of JAN Sylvania 5881's which would drop the power below 60w but needed to bias the amp for those. It was a real slow work day today so I got the job done between calls & emails.

I started with refreshing the power supply. Out with the 42 year old filter caps, in with the new.

1G2pkIA.jpg


The Bassman 70 has balancing control which was actually a really cool CBS change. It let's you dial in unmatched tubes. I added an adjustable bias control to that which makes it really flexible. You do it by replacing the 15k resistor on the balance pot between the wiper and the case with a 10k resistor and a 50k 25 turn pot. It's the little blue thing in the background.

MB0yCmc.jpg


z646NgJ.jpg


I can't wait to try it in a recording tomorrow morning. :hbang:
 
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Riffraff

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Sorry for the delay. It's been tough getting this video & recording done. I'm absolutely swamped at work and have a million honey do projects going at home but I finally got a chance to throw together a video for this necessary mod project. It's nice to be able to play it without worrying about setting my attenuator on fire. :lol:

Knocking a few watts off the top worked out just as I planned it would. The only downside is I won't be be able to fry eggs on my Weber Mass anymore.



500V on the plates. There aren't too many 5881's I will be able to run in this sucker. I'm using a tough old set of Sylvania's right now. I'll pick up a set of JJ's when they are back in stock. Re-issue Tung-Sols would roast in minutes. The tube balance control came in very handy because the tubes weren't well matched. Between that and the adjustable bias pot I installed I was able to dial them in to 33ma on one side and 32.5ma on the other which is damn near perfect in my book. :hbang:
 
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Pete Farrington

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Just to note that power that’s not getting transferred to the load may be getting dissipated by the output valves.
Those late 70s amps may have been designed around the STR387, which was probably the next step in evolution up from the 6L6GC. Perhaps 30W design centre anodes.
Big shoes to fill for the 5881 23W design centre anodes.
Yes they’ll probably current limit and so restrict power output, but in doing so will dissipate more power themselves, due to the saturation voltage across them at peak current being greater.
 

Riffraff

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Just to note that power that’s not getting transferred to the load may be getting dissipated by the output valves.
Those late 70s amps may have been designed around the STR387, which was probably the next step in evolution up from the 6L6GC. Perhaps 30W design centre anodes.
Big shoes to fill for the 5881 23W design centre anodes.
Yes they’ll probably current limit and so restrict power output, but in doing so will dissipate more power themselves, due to the saturation voltage across them at peak current being greater.

Interesting. You gave me something else to worry about.
:)
I was crossing my fingers when I started using it for voltage reasons alone. I really can't open it up to do my recording, I need to work somewhat silently so attenuation is a must. I'm trying not to dump another $250 - $300 on a 4 ohm attenuator. I would love to put the 6L6GC's back in and buy a Powerstation but that's $900 to $1k and I really can't justify spending that much to run a Bassman 70.
:lol:
I'll run these Sylvania's until I can grab a set of JJ 5881's or the Sylvania's blow, whichever comes first.
 
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Pete Farrington

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Maybe consider using 6L6GC, but limiting peak anode current (and hence audio power output) by increasing the value of the screen grid resistors, eg 1k, up to 2k2?
With that type of HT supply, it’s not super hard to wire up a 1/4 power switch, such that the output valves are fed from the HT winding CT, which is an unused supply output at 1/2 the main HT voltage, ie 250V.
Have a look to see how it’s done on the 75
 
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Riffraff

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Maybe consider using 6L6GC, but limiting peak anode current (and hence audio power output) by increasing the value of the screen grid resistors, eg 1k, up to 2k2?
With that type of HT supply, it’s not super hard to wire up a 1/4 power switch, such that the output valves are fed from the HT winding CT, which is an unused supply output at 1/2 the main HT voltage, ie 250V.
Have a look to see how it’s done on the 75
I'm just hacking my way through this stuff as needed and really appreciate the tip. I'll definitely look into this.

Thanks!!!
 

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