Bassman 70 maintainance

[Bernardduur]

Hello all!

I wanted to ask about my Bassman 70. It is still in the original Silverface specs so I thought to upgrade it a bit as I have done to my Fender Silverface Twin. My idea is to alter the phase inverter to blackface specs and to change the bias to a blackface bias next to normal ELCO and tube replacement.

Now I was wondering; the unit is rated at 70W (hence the 70 in the name)...... does this require a special bias / bias setting? As a 6L6GC is rated at 30W max you could'nt reach the 70W specs, right?

Schematic

[pdf64]

You are mixing up the amplifier output spec (70W) with the output tube's max plate dissipation spec (30W). Have a read through the Aiken articles to see if they clarify the distinction for you.
http://www.aikenamps.com/Advanced.gif
Re blackfacing, what are you hoping to achieve? It's got an ultralinear output stage and so is fundemantally different to a BF type amp, it's never going to respond the same no matter how many resistors/caps you tweak.
Best to make sure the tubes and caps are good, and enjoy it for what it is - a powerful clean amp.
Keep the output tube balance as it's useful, but add a bias pot. Peter.

[Bernardduur]

Thanks for the advice! As I am just browsing around the world of amps and amp tweaks I still need to learn a lot....... the link is pretty usefull (well, the link is to a GIF file but you know)

As for the phase inverter, I noticed a great difference when I changed it in my Silverface Twin. The amp was a lot more direct and open sounding, something it lacked before. I hoped this change in this amp would do the same.

As for the bias, would a bias pot + balance pot be a big advantage over a simple blackface bias set?

[jbltwin1]

bias supply

Using the balance PLUS bias adjust allows you to more evenly balance unevenl matched tubes. Not a big deal in todays world of matching everything but if you have REALLY unmatched tubes(more than 10mA apart) it gets you there. Tone won't change, just versatility. A good feature if you're trying to use up singles that aren't matched. Mike.

[Bernardduur]

Ah, I though that..........

But it can be omitted if one should use matched tubes, right? As I know I would never use unmatched tubes

[jbltwin1]

bias adjustment

No NEED for it at all with matched tubes. HOWEVER, if those tubes drift at different rates, it allows you to adjust for it, if you wish.

[Bernardduur]

Thanks!

I'll check into this then!

How about the phase inverter; I know from my Twin that the altering of this little circuit resulted in a brighter, more biting tone...... more openness. Would this also happen with the UL output?

[pdf64]

Sorry about the link, I should have checked it - doh.
Altering the component values in the phase splitter to typical BF won't cause a problem, so if you've done it before and like the results then go for it. I was just hoping to get across to you that it's a different amp than a BF bassman, more powerful, and with lower output impedance and distortion due to UL configuration. Also due to UL it will need those first power supply filter / reservoir caps working well, so check the B+ ripple at high signal levels. Peter.