Sound City 50 PA Plus Bias issue **fixed**

[Bubbastain]

So I recently picked up a Sound City 50 pa Plus. The Sovtek EL34s that we’re in there were severely mismatched. The lowest I could get them was 30ma and 47ma. I swapped tubes with each other just to see if the hot tube was hot in the other socket and it was. So I ordered a pair of JJ 6CA7 tubes and asked for a “cool” pair.
They came in and I installed them. Still, the bias trim pots would only bring them down to 40ma on each side with both trim pots all the way down. I measured a plate voltage of 507, so that has the tubes running at about 80%.
So I’m thinking that changing the bias resistor would give me more range. I’d like to be able to go at least 20 ma colder at the lowest setting. I’d also like to replace the bias cap(s) at the same time since it seems to be recommended. I can’t find a schematic for this amp though. I’d appreciate any help. I will be taking it to a tech for some other things, but I’d like to do what I can myself first.

https://i.imgur.com/N2hLHMh.jpg

[shooter]

This one is in Doug's library, probably close enough

http://el34world.com/charts/Schematics/files/soundcity/soundcity50plus.pdf

[sluckey]

Here's the schematic...

     http://el34world.com/charts/Schematics/files/soundcity/soundcity50plus.pdf

There's only one bias cap, C25, 100µF @ 64V. Replace with 100µF @ 100v. Click on the capacitor link at the bottom of this page.

You may not be able to set the bias colder on this amp. When you set the bias pots at their coolest setting, you already have the max negative voltage this circuit can produce. You may get a slightly more negative voltage (cooler setting) by increasing the value of R43. Ain't gonna be much cooler though. Hopefully replacing the bias cap will increase the negative voltage some.

[Bubbastain]

Thank you so much. I’ll replace that bias cap as soon as I can. I appreciate you making it simple for me by marking the cap on the picture. The amp sounds pretty darn good. Tubes are running hotter than I would like but I’m not playing it very loud at all yet.

[octal]

If you still can't get the current down low enough after changing that cap, you can increase the value of the cathode resistors for the output tubes. It appears to be a combination bias scheme. For some reason, one of them on the schematic is labeled 68 ohms and the other is 22 ohms... think that's most likely a mistake unless they were deliberately trying to introduce some asymmetry for tone reasons. It'll be easy enough to inspect your amp and see what's in there, I'm guessing that the values will match.


EDIT: The schematic shows the resistors, but that chassis gut shot doesn't. So, it's a bit of a guess what you'll find inside.

[PRR]

I don't have a big problem with 80%.

If you can identify and disconnect one end of the bias winding, you can connect it to one side of the 6.3VCT winding, get another 4V or about 10% which might put you closer to a 70% zone. Do it with tubes out: one way gives 4V more and the other way makes 4V LESS. When you got it making more negative bias, then try with tubes.

[sluckey]

A voltage doubler circuit is an easy mod if you want to pursue. Only a few more components and you have plenty or room and extra turrets near the existing bias cap.

[kagliostro]

A voltage doubler circuit is an easy mod if you want to pursue.

Something like this ?



Franco

[Bubbastain]

A voltage doubler circuit is an easy mod if you want to pursue. Only a few more components and you have plenty or room and extra turrets near the existing bias cap.

That sounds interesting. I have a new bias cap coming Friday. I’ll pop that in and see how much of a difference that makes. I was told by the tube supplier that I was sent a “very cool” pair of tubes. Any recommendations for swapping out the R43 resistor?

[sluckey]

Let's hope a fresh cap will bump the voltage up just enough for you. Honestly, it doesn't bother me that your amp is idling at 80%. But it would bother me that both bias pots are set to minimum. I'd have to do something about that.  The voltage doubler would be my first option. A small 12VAC transformer would be my second option. You have plenty of room for either. The transformer would be the simplest to implement.

You have 4.7K now. Don't get crazy with it. Try 5.1K, 5.6K, 6.2K, 6.8K, 7.5K, 8.2K, 10K. Don't go above 10K. I'd start with 10K to begin with. If it doesn't make much difference no need to try the other values.

[Bubbastain]

Well I looked in my stash of resistors but it appears that the ones in there are 1 watt versions and most of mine are 1/2 watt. So I’ll have to order some more stuff since I think RadioShack is gone.

[Bubbastain]

I just installed the new 100uf 100v Bias cap in place of the original 100uf 64v Bias cap. Brought the number down only slightly 39.3 and 39.7. I’ll try the resistor next, but it seems like the voltage doubler mod might be the best bet. I wonder how did these things run off of regular tubes it’s whole life without destroying them on a regular basis?

[sluckey]

How much ac voltage do you have feeding the cathode (banded end) of the bias diode?

[silverfox]

A voltage doubler circuit is an easy mod if you want to pursue.

Something like this ?



Franco

I'm thinking the 1N4007 is a ripple blocking resistor. If not, what is it for? Hmmm. On second thought, it won't block "Ripple". Sorry, I couldn't resist the averment. But it won't provide any additional protection from hum.

silverfox.

[kagliostro]

Not a solution against humm

the diode permit to the 10uF cap to charge quikly, when the amp is shut down the 100K resistor perform a low velocity discharge of the capacitor

this way your tubes will recive bias voltage for a longer time at shutdown with protection purpose

http://www.valvewizard.co.uk/bias.html

Franco

[silverfox]

Not a solution against humm

the diode permit to the 10uF cap to charge quikly, when the amp is shut down the 100K resistor perform a low velocity discharge of the capacitor

this way your tubes will recive bias voltage for a longer time at shutdown with protection purpose

http://www.valvewizard.co.uk/bias.html

Franco

Okay, I have personal experience with the lack of protection circuit feature. i.e. No diode or slow discharge feature. I've considered modifying my bias circuit to get this feature because: If you quickly cycle the power switch on my build, the power supply diodes blow. I believe this is due to the power tubes lacking a bias, due to the sudden draining of the cap, so when the power is turned back on the power tubes go full conduction. I also always employ the Standby switch and that likely has prevented the short from occurring again. Need to pull the chassis and change the circuit.

Thanks for the explanation,

silverfox.

[Bubbastain]

So I had some success today lowering the bias. I removed the 4.7k resistor from R43 and put in a 10k in its place. The bias dropped by 2. Went from almost 40ma for each tube to 37 and 37.7. Is there any danger or possible negative effects if I were to go much higher? 34ma would be 70%. I will most likely have a tech do the voltage doubler mod that was mentioned before I need new tubes again, but if I can lower it a bit more for the time being with a 15 or 20k resistor that would be easier, quicker and cheaper. However, I don’t want to harm anything by going that route.

[sluckey]

Try it and see. No harm.

I would add a 12.6v transformer. Easiest, no brainer way to do it. Here's the filament transformer I would use...

https://www.ebay.com/itm/Hammond-166F12-Filament-Transformer-12-6VCT-300mA-New/122789774903?hash=item1c96d74637:g:iqAAAOSwzQFZ-~~c

[Bubbastain]

Try it and see. No harm.

I would add a 12.6v transformer. Easiest, no brainer way to do it. Here's the filament transformer I would use...

https://www.ebay.com/itm/Hammond-166F12-Filament-Transformer-12-6VCT-300mA-New/122789774903?hash=item1c96d74637:g:iqAAAOSwzQFZ-~~c

Thanks for your help with all this. I’d order that transformer right now but honestly I wouldn’t know what to do with it. I can swap out a part for a part and make shure I’m not going to kill myself with a shock, but that’s about it. Anything more seems like I should have a tech do it.  Unless however it is just a swap this for that type thing.

[sluckey]

Mount the transformer near the bias cap. Look at the bias diode. Cut that dark brown wire connected to the banded end of the diode and tape the cut end of the wire. Connect a transformer black lead to the banded end of the diode. Connect the other black lead to chassis ground at the positive end of the bias cap. Tape off the green yellow wire. Connect the two green wires to the existing filament string (pin 2 and 7) of the nearest power tube.

[Bubbastain]

Just purchased the transformer.

[Bubbastain]

Mount the transformer near the bias cap. Look at the bias diode. Cut that dark brown wire connected to the banded end of the diode and tape the cut end of the wire. Connect a transformer black lead to the banded end of the diode. Connect the other black lead to chassis ground at the positive end of the bias cap. Tape off the green yellow wire. Connect the two green wires to the existing filament string (pin 2 and 7) of the nearest power tube.

Transformer arrived yesterday. I’ll probably install it today or tomorrow. I have one question though. The two green wires, one green wire to pin 2 and the other green wire to pin 7 right?

[sluckey]

right

[Bubbastain]

I just installed the transformer. Boy did it drop the number, or should I say increase the negative bias? Without touching the bias pots one tube dropped from 37ma to 1.8! I bought it up to 30ma, raised the othe bias trim pot which dropped the first one, and then brought the first one back up to 30ma. I’m waiting for the tubes to cool so I can check the other tube. It sure would be nice to have two bias probes right now.  Haha! Here is a picture. I’ll post final results tomorrow. Thank you so much Sluckey.

https://i.imgur.com/vr5sjlE.jpg

[sluckey]

Are you able to adjust the bias below and above your desired bias point? You may want to change R43 back to the original 4.7K value.

[Bubbastain]

Are you able to adjust the bias below and above your desired bias point? You may want to change R43 back to the original 4.7K value.

Yes. Even with the “very cool” pair of tubes they sent me I can still go above my desired range. I’m still fine tuning the final numbers since each bias trim pot affects the other and I only have one bias probe. When I first opened up the amp I was delighted to see two separate bias trim pots. Of course I had thought they were completely independent. Unfortunately not. I’m going to order another bias probe tomorrow.

I’m also going to replace the capacitors in the amp too. If there are any other small capacitors or resistors that you think I should replace I’d appreciate any suggestions.