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Post subject: 1955 Bassman bias
Posted: Thu Sep 15, 2011 9:52 am
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Can I adjust the bias on a 1955 Bassman?


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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 10:47 am
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Yes, you can. But, you'd need to change fixed resistor values. You'll need to place a pot into the bias power supply circuit, in order to be able to adjust the bias via a pot.

This requires some juggling of fixed resistor values in the circuit. And somehow placing a pot onto the chassis. What version (circuit type) of Bassman do you have? (eg: 5G7, etc )


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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 10:56 am
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BMW2002Ti wrote:
What version (circuit type) of Bassman do you have? (eg: 5G7, etc )


If it's a 1955 edition, that year coincides with the introduction of the narrow-panel Bassman and the amp would likely be the 5E6 or 5E6-A circuit revision.

HTH

Arjay

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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 11:17 am
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Arjay, Thanks! Okee-dokee. To put in a bias pot--- I'd remove the 56 k-ohm resistor to ground (paralleling the filter cap in the bias power supply circuit). Get a 10-k-ohm, bias pot from AES (item #R-V10KL-BP-FND). And place it inline where the 56-k-ohm resistor used to go and tie a fixed 40-50 k-ohm resistor IN SERIES with the pot to ground. The reason for this is if the pot goes bad or someone cranks the pot so the bias is too hot, you still have some negative voltage.

Probably best to glue the pot onto the chassis somewhere near the bias supply diode, as it'll be easier to adjust it. Kinda good thing about tweeds. Don't need to drill holes.

Hopefully, that should do it.


http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 11:23 am
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BMW2002Ti wrote:
To put in a bias pot--- I'd remove the 56 k-ohm resistor to ground (paralleling the filter cap in the bias power supply circuit). Get a 10-k-ohm, bias pot from AES (item #R-V10KL-BP-FND). And place it inline where the 56-k-ohm resistor used to go and tie a fixed 40-50 k-ohm resistor IN SERIES with the pot to ground.


That's the ticket.

The added pot could also be installed by fabricating an "L" bracket from aluminum or brass sheet-stock then anchoring it to the chassis by using an existing fastener (say, one of the screws holding the PT or choke in place). I really wouldn't trust an adhesive, even one intended for high-temp applications.

HTH

Arjay

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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 1:03 pm
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BMW2002,

Your link is for a Bandmaster 5E7 circuit. My '55 Bassman is a 5E6 or 5E6-A.
Would your description of adding a bias pot still be accurate?

Thanks


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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 2:19 pm
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Here's the 5E6-A schematic, Kurt......

http://173.83.74.46/schematics/bassman_5e6a.pdf

HTH

Arjay

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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 3:15 pm
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I will use 20 k pot in serie with a 47k resistor or close with the wiper go to both 220k.
If you keep the 56 k you won't be able to have a "hotter" bias. Just more cold than you have now.

Look a the layout and shematic of this Deluxe Reverb this will show you how this should look.


http://www.el34world.com/charts/Schemat ... _AB763.pdf


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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 4:01 pm
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stratele52 wrote:
I will use 20 k pot in serie with a 47k resistor or close with the wiper go to both 220k.
If you keep the 56 k you won't be able to have a "hotter" bias. Just more cold than you have now.


Now that I look more closely at the plate voltage (405VDC), that's true. And doubly so since this voltage is likely even greater due to the higher primary voltage that the PT is receiving from the wall socket.

Good call, Stratele!

8)

Arjay

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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 5:54 pm
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Stratele, You are correct. Should be around 10k-ohm pot + 50k-ohm fixed, 1/2 watt resistor to ground. So that the midrange of the pot will equal a total resistance of around 55k-ohms. Since this amp runs 6L6GA and not 6L6GC's, the negative voltage needs to be around -40VDC and not the usual -45VDC in later Bassmans.

I use 10k-ohm because it easier to find OEM 10k-ohm bias pots, than 20k-ohm. Ones with blade adjustment and not shafts.

Thanks!


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Post subject: Re: 1955 Bassman bias
Posted: Thu Sep 15, 2011 8:16 pm
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Some very interesting suggestions guys!


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