SimpleSE amp: multiple taps with cathode feedback

[mluckow]

I’m building a SimpleSE amp, and I’d like to use it with two different pairs of speakers (not simultaneously, of course). One pair is 8 ohms, and the other pair is 16 ohms (Feastrex drivers).

I’m wondering how to wire it to get two taps, one for 8 ohms and one for 16 ohms. I’m assuming that I’d have separate pairs of positive binding posts for each, but that the negative binding posts would be the same for both the 8 ohm and 16 ohm taps. However, I’m not sure how to wire them to the OPT. I’m using James 6123HS OPTs, so I have 8 ohm and 16 ohm taps on the secondary.

I think that one possibility is to connect the 0 ohm tap to the negative binding post and also to ground, and connect the 8 ohm tap to the 8 ohm positive binding post, and the 16 ohm tap to the 16 ohm positive binding post.

However, in such a case, I don’t know if it would be possible to use cathode feedback. I can't figure out how that would be wired in.

Also, in the SimpleSE schematic, there’s a note next to these wires that says, “The polarity of these two wires depends on the winding direction in the OPT. See table.” The schematic shows the 0 ohm tap connecting to the positive binding post and also to the cathode feedback switch, and the 8 ohm tap connecting to the negative binding post and ground. But I can’t see that there’s a way to use this wiring scheme and get outputs for 8 ohms and 16 ohms.

So, there are two issues here: 1) Is there a way to wire it as shown in the schematic (with the 0 ohm tap connected to a positive binding post) and still get 8 ohm and 16 ohm outputs for my speakers, and 2) Is it possible to get the 8 ohm and 16 ohm outputs and also use cathode feedback?

If anyone knows of any modified SimpleSE schematics out there that show how to wire this, or if someone could explain the wiring to me, I’d be very, very grateful! Hopefully, there's some kind of solution to this.

Thank you very much.

Mike

[tubelab.com]

Some (most of the ones that I tested) OPT's are wound with a secondary having a reversed polarity. Using one of these with CFB results in positive feedback instead of negative feedback. This will increase the distortion instead of reducing it, and in some cases cause the amp to oscillate. The cure is simply to reverse the secondary connections as shown in the diagrams. I have not tested any of the James transformers to determine their absolute polarity.

It is possible to use these transformers in your setup with CFB, but you need to determine how the transformers are wound. Go ahead and build your amp with the two sets of output jacks. I use 3 banana jacks in a row with the center jack connected to the 0 ohm (or common) terminal and the 8 on one side and 16 on the other. This way The speaker can be plugged into the 8 and common, or 16 and common jacks. Place a jumper in the CFB terminals as shown in the non CFB connections. Test the amp and make sure everything is OK.

--------Do not use expensive speakers for this test. -----

Mark a spot on the volume control and play some familiar music so that you can attempt to remember the volume level. Connect the CFB terminals up as shown in the diagram ignoring the 8 ohm tap. (16 ohm tap to the T2 - PRI terminal connected to the wide PC board traces, and 0 ohm tap to the trace going to the capacitor). Turn the amp on, If a loud noise is emitted, turn it off. If not, play the same music to determine if the music is louder or softer. If the music is louder, or the amp oscillated (screams when it warms up) reverse the connections on the CFB terminals. If the test resulted in softer music, the connections are correct.

The fact that you are using a multi tapped OPT allows you to adjust the amount of CFB. Once the absolute polarity of the transformer has been determined, try moving the CFB wire on the 16 ohm secondary tap to the 8 ohm tap. This will reduce the amount of CFB. Choose the tap that provides you with the best sound.

I realize that this may be confusing, so I will post more diagrams on the web site at my earliest opportunity, but I don't have time for at least a week.

[mluckow]

George, thank you very much for your fast and detailed reply! I appreciate it.

I read and reread your reply and am a little confused about the second part. First, when you said to connect the 16 ohm tap to the T2 - PRI terminal connected to the wide trace on the PCB, did you actually mean the T2 - SEC terminal instead?

I'll ask Jim to take a look at your instructions--he's our resident EE expert who is guiding us in our construction of the amp. I think he'll be able to understand what to do. Of the three of us who are building amps, I'm the only one who also needs a 16 ohm tap for when I want to use my Feastrex speakers.

Thanks also for posting more diagrams on your website later when you have the time--I'm sure things will be very clear to me once I can see a diagram. I'll check for them there.

Thank you very much!

Mike

[Davec113]

Hi,

I'm building one of the SimpleSE amps with mluckow.

If I don't use CFB, can C12 be eliminated or reduced in value to reduce gain? This is the 1500 uF cap in parallel with the power tube's cathode resistor, or used for the CFB signal if that option is engaged.

Also, I am planning on trying a new JJ ecc99 as a driver tube in place of the 12at7, as I will be using the SimpleSE with a preamp and do not need as much gain. I thought I'd run it by you to see if you've tried this or if you think there would be any issues using this driver tube without any changes.

Thanks,
Dave

[chrish]

I am running my SimpleSE as a straight power amp with no volume control from a Rotel surround sound pre-amp. I am using a JJ ECC81 (12AT7) there is no problem with gain, in fact overall it has a little sess gain than my solid state amps. I would suggest building it as designed. George knows what he is doing

Cheers,

Chris

[Davec113]

chrish,

My preamp has 29 db gain, it can be attenuated down to 12 db by switching in a resistor before the volume pot. I would like to run it at 29 db, but I can't because my speakers are 93 db effecient. I am building the SimpleSE as a power amp with no volume pot. I plan to buid a Pass f4 also, where the high gain preamp will be ideal.

I think the ecc99 will work fine (it might be better than a 12at7) in the amp if you don't need as much gain, but I'd like to hear George's opinion.

Dave

[tubelab.com]

First, when you said to connect the 16 ohm tap to the T2 - PRI terminal connected to the wide trace on the PCB, did you actually mean the T2 - SEC terminal instead?

OOPS, Yes, I meant to say T2-SEC.

I think the ecc99 will work fine (it might be better than a 12at7) in the amp if you don't need as much gain, but I'd like to hear George's opinion.

Sorry, I don't have any hard data yet since I do not have any of these tubes. I put them on my list for my next order, but my time is severely limited right now, so I can't say when it will be.

I am certain that it can be made to work. It is safe to assume that the cathode resistor value would need to be changed. Adjust R10 and R20 for a plate voltage of about 175 volts. The CCS sets the plate current at 10 mA. It can be changed by adjusting the value of R13 and R23 if desired. Since the plate voltage and plate current can be set independently, most any tube with the correct pinout can be made to work.

[tubelab.com]

If I don't use CFB, can C12 be eliminated or reduced in value to reduce gain? This is the 1500 uF cap in parallel with the power tube's cathode resistor, or used for the CFB signal if that option is engaged.

C12 is the cathode bypass capacitor. The CFB terminals should be jumpered for use without CFB. Removing C12 (or leaving the jumper out) will operate the output tubes without a cathode bypass capacitor. There is nothing wrong with doing this, and you will have less gain. I tried it on several different speaker systems and didn't like the sound but it may work OK for some users. Try it with your speaker system to see what happens.

A gain reduction may also be accomplished by placing a resistor in the CFB terminals instesd of a jumper. This allows the amount of cathode bypassing to be adjusted. I would start in the 100 ohm range and adjust to suit.

Reducing the value of C12 slightly is OK but large reductions (to less than 470 uF) may affect the low frequency response since the bypass effect becoms less at low frequencies. I tend to use larger than needed capacitor values to avoid phase shift and ESR issues.

[Davec113]

Hi George,

Thanks for the help. I will let you know what I did and how it works out with the ecc99. I may try it out without changing R10/20 and R13/23 as it should work without changes, then with resistors that set the plate to 175V and 18 mA. Mike is building his SimpleSE as an integrated amp with the 12at7 using otherwise identical parts, so it will be interesting to compare...

Dave

[Davec113]

OK, after searching a bit, it looks like the ecc99 would like to see about 20 mA, so I think I will try running the plate at 175V, 20 mA

Dave