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Postby ioginy » Fri Jan 28, 2011 3:04 am

sadly no. I tested the plate voltage at both the tube and the OT. the readings are correct.
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Postby dcgillespie » Fri Jan 28, 2011 8:04 am

Pull the 6SL7 driver tube, and then recheck the 6L6 plate voltages. If they return to normal, it will indicate that there is ultra sonic oscillation going on in the power amplifier proper. If they still do not return to normal, then the output stage itself is likely oscillating, indicating the connections to the output transformer are probably still incorrect. I strongly suggest ohming the windings out so you know for a fact which leads are grouped together, and driving the secondary with a low (say 6.3 vac) voltage source so that you can absolutely determine the phase of all windings as well. Only then can you reconnect the transformer with confidence to the output tubes and proceed with your project. This procedure is done with the amplifier off of course, and could produce around 120 vac or so on the primary windings, so use caution when performing it.

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Postby ioginy » Fri Jan 28, 2011 12:55 pm

Hmm, wonderful suggestions. Thank you. I will jump on those as soon as I get home.

I have never ohm tested an OT before. I have read about it before and it seems like a rather simple procedure, being wary of the high voltage of course. A question about that though. The OT has 2,4,8 and 16 ohm windings. Would it be helpfull to test the primaries while feeding the low voltage into all the different secondary taps? Also, how would I determine the phase? Would the AC current be used in this, or could I used a 9V battery?
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Postby dcgillespie » Fri Jan 28, 2011 2:50 pm

Use an ohm meter to verify basic winding groups, i.e. what wires all represent the secondary, and what all wires represent the primaries. While you can do this with the secondary wires still connected into the circuit, you will want to disconnect the two B+ leads and separate them, since there are two separate primary windings. Therefore, you will end up with three sets of wires -- secondary, primary 1, and primary 2.

Each group of primary windings will have three leads -- a B+, UL, and plate lead. Identifying which are the plate and B+ leads in each primary group is easy, as it will be the two wires that have the greatest dc resistance between them. The third wire is the UL tap lead. Mark it as such.

After some thought, instead of using an AC voltage to determine phase, we can determine it this way, still using your ohm meter: Within one primary group, connect one lead of your ohm meter to the UL lead previously identified. Then determine which of the remaining two leads shows the LOWEST dc resistance to the UL lead. Whichever lead produces the lowest reading from the UL lead is the B+ lead. Mark it as such. Obviously then, the third lead is the plate lead within that primary group.

Perform this same procedure on the other set of primary leads to absolutely identify the B+, UL and plate leads for that group. Mark them all appropriately, and then make sure the primary is now wired correctly into the output stage.

On the secondary side, the two wires that have the greatest dc resistance between them are obviously the C and 16 ohm taps. This will be harder to measure correctly, as the correct wires will only read about an ohm or so, with the others reading even less, so it will require careful attention to determine which two actually have the greatest resistance. To determine which wires are which taps within the winding then would require driving the full secondary winding (C & 16) with (for example) a small 6.3 vac filament transformer, and measuring the voltage at the secondary taps. To determine which is the C and which is the 16, use the 70.7% rule. That is, if 6.3 vac is applied to the full secondary, the common lead will be represented by the lead that allows the taps to produce 70.7% of the next highest tap's voltage. Therefore, if 6.3 is applied across the full secondary, the correct lead identified as the common lead will allow the 8 ohm tap to read ~ 4.45 vac between the common and the 8 ohm tap. The four ohm tap will read 70.7% of that voltage, or 3.15 vac, and the 2 ohm tap will read 70.7% of that, or 2.23 vac.

All of these tests are performed with the amplifier turned off!

Hope this helps!

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Postby ioginy » Fri Jan 28, 2011 3:08 pm

David, you are being a fantastic help. I can't describe how much I appreciate it!
I'm itching to do these tests now... wanna go home, wanna go home!

I wonder if my employer would allow me to bring my project here.... hmmm. O:)
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Postby ioginy » Fri Jan 28, 2011 11:30 pm

Well, I checked the 6SL7 first and sure enough, something was a-miss. I checked everything and found I had used a 220K resistor instead of a 220 ohm between the cathodes. I changed it out and WHAM! There is the volume I was missing. I checked the plate voltage and it is back up at 460v.

The amp sounds amazing, however it sounds amazing for guitar. The distortion is smooth and rich, truly a pleasure to play. Sadly there is too much distortion for bass. I swapped the 12AT7 with a 12AU7 and it really cleaned it up, but that is a band-aid fix. I would like to keep the 12AT7, but reduce the distortion. I didn't change any of the caps or resistors when I swapped the 6SL7 preamp tube for the 12AT7. Would bringing up the plate voltage increase clean headroom? I figure 12A?7's generally have 100K plate resistors and I currently have a 470K in the first stage and a 220K in the second. I don't have much experience with gain adjustment so any help would be appreciated.

None the less! Up to this point, thank you so much for your help. it has made this project so much easier :$
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Postby dcgillespie » Sat Jan 29, 2011 6:13 am

Again, this gets into a question we were dealing with earlier. With regards to bass guitar, is the issue one of too much gain over driving the power amplifier section, or too much distortion being generated in the power amp section when a given level of power is reached? Swapping the preamp tube for a 12AU7 provides lower gain, meaning that for a given level of power, the gain control has to be turned up higher. That may give you the impression it has "cleaned things up", but the real question is, is the amp now playing louder with less distortion because of the tube change? I would doubt that it is.

Since the intention of the amplifier is to play bass through it, that places tremendous demands on the output stage. Looking back over your effort throughout this thread, one of the things you have done versus the original design is to use a non-standard OPT. While I have no doubt the transformer you used is a good transformer for it's designed purpose, it's purpose was not for what you are intending to use it for.

No doubt (this is meant in a positive way), you looked at the schematic of the Scott, thought hey, it uses PP 6L6s with fixed bias, so its OPT will make a great OPT for the Ampeg circuit. True, it will "work", but very likely allow for a limited power output and power bandwidth of the original Ampeg design. Since bass guitar requires much more power capability from the output stage, that is why the distortion issue is showing up most with its use.

The Scott amplifier uses push-pull 6L6s true, but they are still actually operated in cathode bias mode, at reduced B+ voltages versus those of the Ampeg design, and more specifically, at a greatly reduced screen voltage relative to the plate voltage, versus the Ampeg design. All of this works to raise the required load impedance the tubes need to operate into, and reduce the power output as well -- all as versus the Ampeg design. The Scott amplifier's OPT is likely about a 6000-6500 ohm impedance primary, where as the Ampeg requires a load closer to 3800-4000 ohms. The Scott was likely a 25 watt amplifier, where as the Ampeg is likely a 50 watt amplifier, all based on the schematics you have provided. Therefore, using the Scott transformer in the Ampeg design will very likely reduce power output, and increase distortion significantly over that of the original transformer specifications. This can be confirmed by measuring the turns ratio and then calculating the intended primary impedance of the Scott transformer. The procedure can easily be found on the web.

If the concerns above are (likely) found to be true, one thing you can try is to change the reflected load back to the output tubes. This is easy enough to do, by simply operating the rated load of your speaker on an output tap that represents double the speaker impedance. That is, if you have an 8 ohm cabinet, connect it to the 16 ohm leads on the OPT. This will have the effect of limiting the effective power bandwidth of the transformer, but also halving the reflected impedance back to the output tubes, and therefore (likely) producing an operating impedance that is more appropriate for the Ampeg operating conditions. Since the Scott was a high-fidelity design to begin with, but you are now using its transformer in a guitar based application, some loss of bandwidth in the transformer can be tolerated, such that the transformer might operate much better in this application, when connected as suggested.

None of this may be what you wanted to hear, but again, I hope it helps!

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