GemTune GS-02 Circuit SE KT88

[Thinker]

Power supply:



Circuit:



Some questions:

isn't the 150uF hard on the 5Z3PA? Also for the primaries of the output transformers on the right channel the red is connected to pin 3 of the KT88 and the yellow goes to the B+ but on the Left channel it was done in reverse with yellow to pin 3 and red to B+, Also the Left channel if putting out 2x the power of the right channel. I tried switching the output and the preamp tubes and the output power difference is consistent, the left channel is always 2x the power of the right. Thoughts?

 

[Thinker]

How could I figure out which way is the proper way to wire the output transformer? Also I noticed that pin 7 of the KT88 heater is connected directly to ground is that suppose to reduce hum? (on the left channel it's pin 2 that connected to ground)

 

[gadget73]

150uf is extremely hard on any tube rectifier. I don't think any of them are rated for that kind of first cap. You can get by with a bigger cap if the other limits aren't pushed, but 150uf is a bit of a stretch even if you're nowhere near limits for voltage or transformer resistance. I think I'd be considering making that a 47uf. As far as I can find, a 5Z3PA is equivalent to a 5U4GB, which is rated to deal with a 40uf first cap.

Having an unbalanced heater circuit actually makes things more inclined to hum. The usual better way of doing it is running pins 2 and 7 to the transformer, and either grounding the center tap of the heater winding or using a pair of 100-470 ohm resistors to go from each leg of the heater circuit to ground in order to balance it relative to ground. The heater wires are typically twisted to cancel any hum field they generate as well. A single wire won't cancel. For some extra trick stuff, and depending on circuit needs, you can connect the real or fake center tap of the heater circuit to some point to elevate the heaters above ground. A typical spot is the cathode of the output tube if its cathode biased, but resistor dividers off B+ also work. This is sometimes more critical than others. In some circuits, there is a pretty fair amount of voltage on a tube cathode, and elevating the heater supply is needed to maintain heater-cathode voltage limits. I'll take a guess and say this one probably isn't one of those cases. Usually you see that with direct coupled tubes or with cathodyne inverters that have the cathode at a hundred volts or more in some cases.


Reversing the leads on the output transformer will turn your negative feedback into positive, assuming its got any. I don't see any tie from the speaker output terminals back to the driver tube so there may not be any. Also, positive feedback tends to make the amplifier squeal and do other really stupid things. The other thing that reversing them will do on a UL output stage like this is change where the screen tap is relative to the plate if the tap point didn't happen to be 50%. Generally speaking red goes to B+, but its hard to say for sure. I guess if you were so inclined you could do some transformer testing to figure out the ratio and determine where that tap point is, and then figure out what wiring makes sense based on that. The screen is typically "closer" to B+ than the plate.

 

[babblefish]

What are the total connections/leads available on the the primary side of the OPT? Reason I ask is because for a typical OPT with UL windings, there should be 5. Is there any chance that you've got the grid from one of the KT88 connected to the wrong winding? Such as the center tap instead of one of the UL windings? Maybe to the incorrect UL windings? Or, if the channel with less output has a problem with either a bad UL winding or bad connection to said winding, that channel would be essentially be running in triode mode, which would explain the reduced output.

 

[Thinker]

Yeah I thought that the 150 uF was too big, this amp doesn't have diodes before the rectifier either, so the tube is doing all the heavy lifting. (side note the MP 428 amp with the 300 ohm resistor after the 5U4G does great, that resistor really helps reduce inrush (if I'm using that term correctly))

I did the 100 ohm resistors artificial center tap on another amp and elevated it, and it really helped that amp out. That's why I was scratching my head when I saw this, I was wondering if there was another method that I wasn't aware of or if it was just poorly wired.

I took the end bell's off one of the transformers to see if there was any labels, there weren't There's only 3 wires that come from the primary side of the transformer, If I remember right I thought the UL tap was 43% or something close to that, I don't understand why having it flipped (plate and B+ switched) would cause such a change in output power. I'm wondering if one of the outputs is just crumby.

Other kinda weird thing I noticed when I was testing it with the analyzer, with 0.75 volt 1 KHz signal even though there's the power difference between the channels they're both reading about 1% distortion, and this I really don't get, when I turn the volume pot to full with the 0.75v 1 KHz signal the analyzer says that the Left Channel is putting out 1461 watts and the right is putting out 842 watts. So I have no clue how it's coming up with those numbers.

I've been rather disappointed with these Chinese amps lately.

 

[gadget73]

The UL tap position varies depending on particulars. Some tubes want it more to one end than the other, some designs change it too. The common push-pull spot is around 40%, but I know extremely little about SE UL designs. I do know that if the screen were open, you'd get zero output. Having the screen taps be at different places will change the power output level though. Closer to the plate makes it act more like a triode which reduces power output. Closer to the B+ makes it more pentode-like and makes more power.


Those power numbers are just not logical. No way you're getting 1400 watts out of this The other question is whether the difference is one of sensitivity or actual power output. What I might suggest is driving the left channel to full power just before clipping and noting your output voltage at the speaker terminals as well as input voltage at the jacks. Do the same with the right channel. If the input voltage is considerably different but the output voltage is similar, then one side is more sensitive than the other for some reason, probably some issue within the driver stage. If the output voltage is much different, then its a problem in the power output stage. Usually big differences in sensitivity are related to feedback connections. If this has no global feedback loop, then I'd be looking at the 100uf cap on the 6N8. An open cap would increase local feedback on that stage, making it less sensitive.

If you want to turn the output voltage numbers into wattage numbers, its volts squared divided by the value of the dummy load.

 

[Mklevitt]

New to the site but found this post. I have a Gemtune GS-02 from a few years back whose output transformer has crapped out. I would really like to salvage the amp but can't reach Gemtune directly (for obvious reasons) to get a replacement. Was hoping the specs would help me find an OEM replacement. Any chance you could dig up these diagrams, or any other leads so I don't have to throw away an otherwise useful piece of equipment?

Thanks in advance...

Power supply:



Circuit: