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PostPosted: 12 Aug 2009, 10:14 
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Hi, That SRPP B+ is rather high. The voltages should be quite close between amps regardless of the tubes (as long as they are the same type eg. KT88). The CCS forces the current load on the tubes and in turn the PS to be very close to equal. So here is what I would check. Verify that the power transformers are delivering the same AC to the rectifiers. Verify the CCS current on the two amps. It ought to be the same +/- 1-2%. Check the values of the resistors in the B+ filter string. On may be marked right , but be off value. Look for shorts between the B+ sections. If none of this works then increase the value of the final stage's (in the PS) filter resistor by about 25% and see what happens. Please report back on what you find.

The SRPP type of circuit is quite good, but there is no free lunch. The weakness is the potential for heater cathode breakdown. When ever you go past about 260 B+ the risk goes up quickly. I had one set of tubes last for months at 280 (early project) before dying in a rather dramatic manner. Both channels went within days of each other. With a heater bias of about 1/3 the B+ on the stage and limiting the B+ (I use 225 for most now) everything is great.

The JJ Blue KT88s do not sound like the JJ KT77s. The 88s are more like the Gold Lion KT88s. They are quite good but IMHO the JJ KT77s have an edge in the mids and upper range detail. If you like music with lots of detail (strings etc) then the 77s are the way to go. If you prefer rock, stuff with lots of bass, either brand of 88s are suitable. BTW the JJ 6L6GCs sound a lot like the GL KT88s and while less powerful are a whole lot cheaper. If you don't need the extra power of the KT88s I would opt for the 6L6s. I have a set of Sovtek 6L6WXT tubes that are higher capacity 6L6GCs that I have been listening to for a while. They fall somewhere in between the JJ77s and JJ 6L6s for sound. I personally don't find them as "nice" as the 77s, and prefer the 6L6GCs for rock. They are durable and dependable and may work well in other systems. The only tubes I can not recommend (not their fault most likely) are EH KT88s. They refuse to stay in balance. I have no conclusive reason for this, they just act that way.

Good listening
Bruce

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PostPosted: 12 Aug 2009, 12:22 
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Hi Bruce! I measured the 47k resistors and they're about 46k, and the 100R is about 102R. I think changing tubes and repositioning them in the sockets was influencing the voltages. One of the amp has looser sockets than the other, and the voltages on that one are higher. With a lot of contact cleaner, I think I have brought it down to a stable 265v on both amps, so that may account for the difference between amps. Current is 80.9mA on one and 90.2mA on the other, so no problems there.

As suggested, I think I'll swap the 47k resistor between the two 50uF caps with a 56k (closest value available), and see how that pans out. It could just be unfortunate with everything falling on the wrong side of its tolerance. Will let you know once I get them...

Surprised you say the JJ KT88s sound like GL KT88s. I can assure you mine do not! Although I initially called the JJs harsh (or bright), listening to them earlier today, I actually preferred the sound to the GLs (specific music possibly). Compared to the JJs, my GLs sound like the loudness control has been activated. More bass, less clarity in the treble, certainly not similar at all! I think once i have a full set, I may prefer the JJs.

That'll be a while though. Ordered the new pair from the US. Even with US$30 postage, it is about US$40 cheaper than buying from the local RS. This is an expensive hobby when you don't live in the US!


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PostPosted: 12 Aug 2009, 14:11 
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Hi, Check the resistors in the CCS (LM317). If all is correct the current in one amp should be within 1-2% of the other. 80 vs 90 is not good. All on the ones here (and that have passed through) are within 2 ma per pair of tubes. It would seem that either one of the LM317s is off spec or the resistor setting the levels is. A third possibility is that one is not heat sinked as well as the other, however this is not a usual event as the 317 will shut down if it gets too hot. Right up to spec temp it will perform rather closely to the design.

Good listening
Bruce

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PostPosted: 12 Aug 2009, 14:18 
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Hi, On the tubes and sound. I personally prefer all the JJ ones over all the others. I suspect the differences are related to the speakers. Some are very linear in impedance and some not. This in combination with the tubes impedance can introduce a host of differences in the final sound. My system generally has a rising impedance with frequency. Not huge, but there. This in turn would effective alter the loading on the tubes. Perhaps (just a guess here) the GLs work better into a higher Z with the increasing frequency. If someone out there has some info on this, I would like to hear about it. This is one of the areas that affects tube amps where SS ones are rather immune (very low Z out). This could make a nice series of posts that link amps and speakers.

Good listening
Bruce

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PostPosted: 12 Aug 2009, 19:34 
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Sorry, made a typo there. I meant 89.9 not 80.9 :)

I won;t have a chance to test it today, but one thing that just occured to me, is that my speaker load is 6 ohms on the 4 ohm tap. Could this light load be to blame? If so, is an adjustment in the PS the only way? I want to retain the ability to use 8 ohm speakers in future, but I assume adding resistance in the output is a no-no...

Cheers,


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PostPosted: 12 Aug 2009, 21:10 
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Well, just for the fun of it I built a prototype with a pair of 6AK5s.

I hit about 3.3V p-p on a 4ohm speaker when distortion sets in. Probably 0.34 Watts total.

Hopefully the attachment worked. First attempt.


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PostPosted: 13 Aug 2009, 02:07 
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Right, I had just enough time to do a quick test on one amp, and no luck :(

Wiring the 6 ohm speaker to the 8 ohm tap directly, and via 2 series resitors yeilded no difference, other than more volume.

This time the SRPP anode was at 271v and the mains measured 248VAC, with 368VAC out of the PT.

Making a change to the second 47k resistor should be quite easy, as it is not on the PCB, so that's going to be my next move. I may add some extra space between the cap pins and the resistors in case the heat is upsetting things.


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PostPosted: 13 Aug 2009, 10:54 
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Hi. 89 and 90 are close enough. The amps are fairly insensitive to the load so I would try the 8 ohm outputs and see if it works better. It sure seems as if there is something amiss in the PS. I would make the adjustement if you can't find a source for the difference.

I love the 6AK5 version. It is similar to one I have in the works as a headphone amp. I plan on using 6J6 dual triodes (they share a common cathode BTW). Power output would be in the same range as yours.

Good listening
Bruce

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PostPosted: 13 Aug 2009, 11:02 
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Hi, If the 368 is under load it probably is a bit on the high side. Under load mine drops to about 350. At 1.4 times that it will put you over (in theory) 500 volts. Since all but the 6L6s can handle that without any problems, I would make the increase in the PS resistors to bring the anode voltage on the SRPP down to around 225. It will give you a bit of reserve if your AC mains fluxuate. Our local ones here stay right on 120 so this is not a concern. Elsewhere a 10 volt change can cause serious problems if you are running close to the tubes ratings.

Good listening
Bruce

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PostPosted: 13 Aug 2009, 22:26 
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I tried another test tonight. I sub'd 6AQ5s in the output and adjusted the bias current to give a peak with before distortion set in. I also switched to a 8 Ohm resistive load.

With one volt p-p in (Ri = 47K) I was getting 2.84V into a 8 ohm restive load. This with a 150V B+, and 0.52V ripple. The plate output was near 150V p-p for both tubes so they were driving as hard as they could. I adjusted the current source for maximum undistorted output.

120Hz is at 10mV so I calculated 49dBv signal to noise (60hZ). I need to get it to at least 69dBv so I've got some work to go.

I suspect the transformer is not what it was advertised to be (like that ever happens on e{vil}bay). It is supposed to be a Lafayette 6V6 pp output transformer. However it has two sets of secondaries. One is lowZ and looks like multiple speaker taps. The second may be a 70V line tap, but it shows 88V across the output and taps at 62v and 26V.

I'm going to take the transformer to work and do some measurements in the lab tomorrow and see if I can get a better idea of what it really is.

Steven


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