Ampeg SB12 Bass Amp Build

[dawsonaudio]

Hi, I'm going to be building a similar amp as the Ampeg SB12. Below is a schematic I found off the web. I have a power transformer from another project I was working on that was powering 2 6v6's. I don't think it will work with the 6l6's...it might be too small.

What I'm not sure about is which of the components would need to be altered to make the 6v6's work.

I was also wondering if the tube rectifier should be changed. The tube that was installed with the 6v6's was a 5u4, I believe.

Thanks for any help here.

Nate

[img:800:600]http://www.drtube.com/schematics/ampeg/sb12-jp.gif[/img]

[Stevem]

Your Pt will work but it will only have enough current to make may be 12 watts of clean power from 6L6s unlike the 22 or so that the right PT could make.

Rectifier wise the 5U4 will work also but the 5 volt heater supply needs to be rated for a full 3 amps, not 1.9 like a 5AR4 needs.
A 5U4 has a higher internal impedance than a 5AR4, so for a givin amount of AC voltage input from the PT it will put out less dc voltage for the amp to run on.
Also a 5u4 unlike a 5AR4 is directly heated tube so it does not have a Amp happy , power supply filter happy slow turn on like the latter does.

A power transformers B+ secondary winding will need 100 to 120ma just to power 2 6V6 tubes ( no less the preamp tubes )and 145ma to rev up two 6l6 tubes and have good punch when played wide open!

[martin manning]

Do you have any specs for this PT you are considering? If not, at least measure no-load voltages all around (including the primary voltage you had when measuring the secondaries) so we can see what you are working with.

A 5U4 has a directly heated cathode, but it must get hot before it can begin supplying current so it will provide a soft start. If your PT was previously feeding a 5U4 then presumably it has enough current capacity in the 5V winding. The biggest difference will be the higher voltage drop (maybe an additional 25-30V) over a 5AR4.

As far as other changes you will need an OT that has a primary impedance compatible with 6V6's at the plate voltage you will be running. I would add screen grid resistors (470 ohms would do), and increase the 1k control grid resistors to ~5k.

Two 6V6's will be pretty limiting for a bass amp. I assume this is for practice or recording use?

[dawsonaudio]

Yeah, this amp will be used for practice/recording purposes only.

I have a few used power transformers at my disposal that were included in a organ parts box from a friend some years ago. One is a H-AO-22111-1 power transformer 355-0-355V @ 150MA, 5V @ 3A, 6.3V @4A. The other is a H-AO-30241-1 power transformer...not sure the specs on this one at the moment...and the other is a AO-24110-1...also not sure the specs right at the moment.

I do know that the 22111 came from an M3 organ/field coil set-up...I google searched the 30241 and it must have come from a Hammond H series. Trek II sells a replacement for it called the HRT-1 here: http://www.trekii.com/HRT1.html. The last transformer is a 24110-1 and looks the same as this one on ebay: http://www.ebay.com/itm/like/1510717139 ... rid=229466.

On a side note, I was looking through my box of tubes and found a pair of 7591A's which I believe are similar to 7868's which could also work in place of the 6v6's. I think the 7868's were used in the Ampeg Bass Amps of that time.

So my main concern at the moment is to find out what kind of ratings all of the transformers have and hopefully pick one that will work. I'm going to try to give some Hammond people a call today and maybe get some spec's on these other transformers.

[Stevem]

A 5U4 is one tenth of a second faster at coming up to peak voltage and the peak voltage will be some 54 volts less than a 5AR4.

[M Fowler]

I think that first transformer you noted 150mA will work out great I have built three bass amps running 6L6GC using only 150mA and it has plenty of loudness especially if your only using it for home or recording.

Mark

[dawsonaudio]

So using that first transformer, do you think it would work with a pair of 7591a's/7868 tubes? Now that I've found these 7591's in my stash, I'm going to go off of this schematic:

[img762]https://buildanamp.files.wordpress.com/ ... -sb121.jpg[/img]

Thanks for the help here.
Nate

[Stevem]

Both tubes sound great and I have a 66 reverbrocket that uses the 7591's and I have the 7868's in a Fury amp so I can vouch for them both, but the 7591's are far more affordable both new aftermarket, or NOS !

[dawsonaudio]

I have some questions about the schematic...there is a 100 ma 200piv diode...is that just your traditional diode? Also, I'm not sure what type of Bias and Balance Pot I should use...it is calling for a 15k and 100ohm.

Thanks for the help.

Here is a clearer version of the schematic I'm using from Dr. Tube:
[img:800:600]http://www.drtube.com/schematics/ampeg/sb127868-jp.gif[/img]

[Leo_Gnardo]

I'd not worry too much about the bias rectifier diode. Almost any competent rectifier will do. I use UF4007 or 1N4007 because I got bags of 'em. Also your bias is derived thru a resistor from hi voltage windings in this amp, so having a 1000 PIV rated rectifier keeps some safety 'headroom' between actual circuit voltage and the rectifier's PIV rating.

10K pot ought to do, good luck finding a 15K pot these days. You'll need to adjust the value on the ground/wiper connection to keep the sweep of the bias adjustment in the appropriate zone for whichever output tubes you select.

I'd stick with 6V6 but no objection to experimenting with 7591, not as if it's brain surgery swapping a couple leads.

Also there's 2 advantages to using 5AR4/GZ34 rectifier: slow warmup (it really is a lot slower than 5U4 to warm up) so you can skip a standby switch, plus a bit more power and who'd object to that in a bass amp.

[dawsonaudio]

What value should I use at that ground/wiper connection if I can't find a 15k pot and go with a 10k? How about a 24k pot? And the resistor to change is the 39k in the schematic, correct? I will be going with the 7591 output tubes that I've got.

Thanks for the help here.

[Leo_Gnardo]

What value should I use at that ground/wiper connection if I can't find a 15k pot and go with a 10k? How about a 24k pot? And the resistor to change is the 39k in the schematic, correct? I will be going with the 7591 output tubes that I've got.

Thanks for the help here.

Making a couple assumptions here, 7868 and 7591 like to see a bias voltage around -20 to -26V, and the original R values in the bias resistor string, looks like they planned for about half a milliamp current thru that string. So if you use a 24K pot, reduce that 39K resistor to 30K and you'll have a wider adjustment zone for the bias voltage, no complaint about that. 27K or 33K resistor may be more easily found, use either, no worries. If you go with a 10K bias pot, I'd just leave all else as on the schematic until you fire up the amp and see what those 7591's are doing, then change the range setting resistors (39K and 10K) only if you can't find a comfortable bias point.

[Structo]

I believe the 100 ohm pot is the "humdinger" used to balance the heater voltage, adjust for least hum.

Or you can use two 100 ohm resistors, run from each leg of the heater to ground to form an artificial center tap.
If you do that, don't use the hum pot

With that said, what is with the cap on the heater center tap?

[martin manning]

Not a humdinger pot Tom, this is used to balance the power tube idle cathode currents by divvying up the 100 ohms between them and ground.

[Firestorm]

The earliest version of the SB-12 (the one with 7868s) has a 100 ohm wirewound pot on the cathodes of the output tubes, wiper to ground. It's purpose was to balance the 7868s' idle current. Apparently, 7868 pairs don't always match very well. It may not be necessary and there are better ways to do it.