SSE to PP, FAIL, why?
I tested converting SSE to PP. Just for fun, because I had parts, but failed.
As can be seen from the figure,
1. I used Sowter 8920F (10K:10K+10K) input trans to split the phase of single ended input.
2. I fed the opposite phase into the input of SSE.
3. Instead of using 2 SE OPT for two EL34s, I used one PP OPT. B of SSE board goes to B of PP OPT. P1 of OPT goes to the plate of one EL34, P2 goes to the other EL34.
4. B+ was about 400 V (300V rectified by diodes).
I thought this was going to be a class-A PP. Aside from AC noise, the sound was very hazy, and started to distorted when I increase the volume half-way.
Can anyone suggest me what was wrong?
I have done this and it works well. There is a thread in this forum where another builder did it too.
The connections you mention are all required, but there are a few more that you didn't mention that are also needed, maybe you missed one.
The screens of the output tubes need to be connected. They can be wired in the same manner as the SE connections depending on your desires and OPT. As in the SE hookup you can run a jumper from the screen terminal to the plate terminal for triode operation. Connect the screens to the UL taps if your OPT's have the tap and you want UL. You can also jumper the screens to the B+ terminal for pentode operation.
The cathode caps need to be hooked up. CFB can be used in a P-P amp but it requires a very good OPT with excellent balance. I would connect the jumper that grounds the caps for initial testing.
If the amp is already working it will operate in class A up to about twice the power that it made in SE. Increasing the drive further will cause a transition to AB but should not cause clipping. Some experimenting with the bias (cathode resistor) can be needed to optimize the transition point VS power output. I have used a vacuum tube PI and preamp with the SSE board to make a guitar amp that cranks about 60 watts out of a pair of EL34's from the Allied 6K7VG power transformer.
An OPT in the 3500 to 4500 ohm range is best for maximum power, I used 6600 ohms for best HiFi sound. A true class A P-P triode amp would probably want an even higher load impedance.
I forgot to mention those connections.
Output stage is in triode mode, and CFB capacitor is connected to the ground. I am going to experiment with UL and CFB once the amp runs fine.
Output tranny is Hashimoto 40W 5K PP, which I suppose to be very good one ($$$).
Each board and channel worked very well in SE mode, when I tested with 3K SE OPT.
One quick question arose in my mind. Do I have to use a resistor for the input of input transformer?
Call me stupid, I forgot to connect ground from the boards. It worked fine in SE mode, but with the floating ground, phase was not correctly split.
Now, two SSE makes fantastic sound in PP mode. No hum so to speak. Clean highs, warm mids, punchy lows. I used top-grade parts including famous Sowter 8920F. Hashimoto OPT is only second to Tamura, as known among Asian DIY community.
SSE is great, of course.
I have TSE with 300B tubes, but TSE might be even better for PP conversion. Fixed bias of TSE may work better for balancing out the DC in PP OPT.
Here is the pic of 2SSE_PP amp.
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