stromberg carlson

[eruannu]

got an old Sc signet 33 i am working on im new at this so try to bear with my ignorance but i need to know how to calculate the proper value and how to install a bias pot for this amp
as well this amp has a tube taht perplexes me it has a 6U8 which i am familiar with as an occilator tube but not as a preamp tube it handles the phase inversian task as well as a gain stage for the phono channell
will evil things happen if say i swap this tube with a 12ax7

[Enzo]

Evil things? Well it won't work for one thing. The two tubes have very different pin assignments.

Bias supplies are all calculated the same. STart with the voltage at the first filter. This would be the "raw" bias voltage. Then decide the target voltage - the bias voltage we want at the tubes themselves. I know you want a ranfe, but pick the voltage at the center of that range.

Now it is a matter of creating a voltage divider. From that raw supply, there is probably a resistor to the second filter, and from there a resistor to ground. The bias for the tubes comes off the second filter. The voltage at the second filter will be a percentage of the raw voltage, and that determined by the ratio of the first resistor and the resistor to ground. Specifically, the bias will be the percentage determined by the ground resistor divided by both resistors added together.

For example of the raw negative supply was -80 volts, and the first resistor was 10k, and the resistor to ground was also 10k. Then the point where the resistors meet - the point where the second filter connects - would be -40 volts. 10k/(10k+10k) x 80 volts = 40

Let's say we want to make the 40 volts adjustable. Then we vary the ration of the two resistances. Typically we make the lower resistor a variable one. So if we replace the lower 10k resistor with a 20k pot, then with that pot set midway, 10k itself, we would get the original 40 volts again. But if we dial it all the way up to 20k, then the bias voltage will be:

20k/(20k+10k) x 80v = 20k/30k x 80v = 2/3 x 80v = 53 volts more or less.

And of we dialed the pot all the way over to zero, then the bias voltage would be... uh oh...zero. That because the pot would now be grounding the bias voltage.

So instead of a 20k pot, I might replace the 20k pot with a 5k resistor and a 15k pot in series. Or whatever. Point being we don't want to let bias ever be zero, so instead of a simple pot, we use a pot AND resistor so there will always be a minimum.

[eruannu]

thank you for the bias info
what would be a sutable sub
the two i was given were
the 6ea8 and the ecf80
are there any others or are these even good subs

[Enzo]

Sub? I'd just get another 6U8. Looking at Antique Electronics, they sub a 6KD8 for it, and they ask $5.85 for the tube.

[eruannu]

ok if we are dealing with 470 ohm resitors what pot and resistor are we talking about and where cani get replacement feet or are the stick on computer kind ok because i have a lot of those in my parts bin

[teleamp]

I've got a SC model 33 with tubes from the early fifties, the amp is my holy grail for tone standards. I love the way it sounds.

MikeY

[eruannu]

yeah it was given to me for a consulting job and when i craked it open the power section was almost identical to a 59 bassman so i threw a small party and ave since then been modding it into my ultamate tone machine which so far sounds awsome with a tele plugged into it it dosent like my prs at all so i am modding it into a both a more usefull rocking machine over all and dialing in adjustments such as tone capacitors to bring it into perfection i am thinging on a couple switches on the tone caps to add a little extra or less capacitance mid show to adjust tone

[eruannu]

ok im an idiot and the first to admit it the power amp pof the unit is vaugly similar to a bassman in the same way that a ac30 is similar and it is cathode biased bye yall