Hello - I've been playing around with different tubes in my Boyuu Equis A10 SET. This originally came with 6N2 pre-amp, EL34B output and 5ZP rectifier. This was pre-built and I must say it has been a brilliant performer from day 1, very pleased with it. The first test was to try 6N2P-EV. There was no perceived difference to my ears, however these valves were bought for longevity. I've also swapped to 6CA7 'fat tube' and after about 50 hours of burn-in cannot really tell any difference...
My final trial has been with a pair of 6P3P J grade 'coke bottle' valves and these appear noticeably more quiet, i.e. volume is a lot lower. Given that this amplifier is 'self biasing' can anyone help with anything which might explain why the 6P3P vales are a lot quieter? I have a bias meter so I can measure the bias current for each type of output valve for each channel.
I also tried a Sovtek 5AR4 in place of the 5ZP to see if a slighlty lower B+ voltage drop (17v instead of 20v) might have an effect... nothing that I could tell. During testing, all sources and speakers have remained the same. Many thanks in advance.
My final trial has been with a pair of 6P3P J grade 'coke bottle' valves and these appear noticeably more quiet, i.e. volume is a lot lower. Given that this amplifier is 'self biasing' can anyone help with anything which might explain why the 6P3P vales are a lot quieter? I have a bias meter so I can measure the bias current for each type of output valve for each channel.
I also tried a Sovtek 5AR4 in place of the 5ZP to see if a slighlty lower B+ voltage drop (17v instead of 20v) might have an effect... nothing that I could tell. During testing, all sources and speakers have remained the same. Many thanks in advance.
EL34 is a pentode and 6P3p is a beam tetrode. They have slightly less gain, due to the type of valve and totally different bias requirements.
Hi Jon - I've seen EL34-B descirbed as a 'beam tetrode' as well. I haven't looked at the constrcution compared with EL34. Is this correct?
Maybe a daft question - with self-biasing circuits I presume there has to be some component change then for 6P3P instead of EL34-B whilst 6CA7 has such close characteristics it's plug-in compatible...?
Maybe a daft question - with self-biasing circuits I presume there has to be some component change then for 6P3P instead of EL34-B whilst 6CA7 has such close characteristics it's plug-in compatible...?
Looking at both EL34 and 6P3P data sheet and assume a few things. Let say the Amp is running at 25W plate, 280Vp and 250Vg2, the Cathode resistor (Rk)can be set at 150 ohms with a plate current (Ip) of 90mA. Move that Rk of 150 ohms to the 6P3P. Looks to me it could end up at an Ip of 83ma. If I further assume the amp uses a 5K SE OPT, looks to me, the 6P3P can live there happily after. Providing the assumptions are close to the real schematic.
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Beam
Tetrode
EL34-B
KT66
KT88-98(KT88), 6550 and CV5220
5881A(5881) 6L6G, 6L6GA, 6L6GB and 6L6GC
6L6GCR(6L6G) 6L6G and 5881
6P3P 6L6G
Pentode EL34A(EL34) 6CA7
EL156 KT series and 6550
Hello - I've uploaded the A10 schematic. The cathode resistor is 500 Ohms (being 2 x 1k in parallel). I haven't measured any of the voltages, so maybe that's something I should do (I'm experienced working with 3-phase and other lethal voltages...). I have an LCR meter so could measure the nominal impedence of the output transformer.
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The LCR meter is not going to tell you much about the OPT, you need to apply DC current and large enough AC voltage to get a good idea about its primary inductance. The winding ratio could be done with just a low voltage AC supply. All of the above should be done with the OPT out of the circuit, not connected to the tube or the load.
Ok - thanks for the tip... I will drop a quick email to the supplier as they have been very helpful in the past. Easiest route and hopefully the most accurate information! I would presume it will be suited to EL34B SE operation initially.
The Cathode resistor difference (150 vs 500 ohms) was mainly from difference between assumptions (guess) and the actual circuit. I guessed Pentode and a 250V screen. The schematic you posted seems to say Ultra Linear and at around 350V screen voltage. Sorry if I confused you. My new guess is that this 500 ohms Rk should put the bias voltage to about 21V (from an ultra linear curve on the web). Properly where it should be if the amp designer were to make 12W output per their management spec and at the same time meet the EL34 plate spec. Higher power and also meet tube spec means trade off. 2nd Harmonic might have to give a little.
You might want to know the OPT impedance and measure the V plate, V screen and Vk if you want to reduce the Grid Bias/Cathode resistor (higher idle current).
You might want to know the OPT impedance and measure the V plate, V screen and Vk if you want to reduce the Grid Bias/Cathode resistor (higher idle current).
Hello this is the diagram of the A9, (6n9 preamp) , the a10 has the 6n2 preamp and has hybrid rectification.
Why does every schematic I have ever seen of these particular amplifiers show a dual triode input stage, But leave the one grid not connected? There should be an external wiring to both grids. Grid connections of one grid to the other, are not to be assumed just because the grids are side by side in the same circle (in the envelope).
And by the way, I would never tie the cathodes to a common self bias network. That is not a good way to get balanced current in the two triodes. Use Separate self bias resistors. Nice, these amps do not use bypass caps on the input cathodes, so in this case, no separate bypass caps needed.
Also, tying the grids directly together, and the cathodes directly together, and the plates directly together can be a good way to tempt the circuit to be an RF oscillator (these tubes Do work at RF). Wires are parasitic inductors (L), and there is capacitance (C) in the circuit. LC is a resonator, even if it is at hundreds of MHz. Tube rollers beware, some tube types may oscillate, others may not. Use Separate grid stopper resistors, and tie the other ends of the grid stopper resistors to the correct point at the input.
Many of these amplifiers use printed circuit boards, and that makes it hard to implement the above suggestions.
And by the way, I would never tie the cathodes to a common self bias network. That is not a good way to get balanced current in the two triodes. Use Separate self bias resistors. Nice, these amps do not use bypass caps on the input cathodes, so in this case, no separate bypass caps needed.
Also, tying the grids directly together, and the cathodes directly together, and the plates directly together can be a good way to tempt the circuit to be an RF oscillator (these tubes Do work at RF). Wires are parasitic inductors (L), and there is capacitance (C) in the circuit. LC is a resonator, even if it is at hundreds of MHz. Tube rollers beware, some tube types may oscillate, others may not. Use Separate grid stopper resistors, and tie the other ends of the grid stopper resistors to the correct point at the input.
Many of these amplifiers use printed circuit boards, and that makes it hard to implement the above suggestions.
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@Longroad... interesting, your A10 power supply B+ rectification has an additional two SS Diodes, 1N4007, so wonder what the difference in B+ voltage would be as my circuit relies on 2 valve ("toob") diode rectification....
Mission720,
The two 1N4007 diodes in the bridge will only drop the B+ by about 1Volt. Of course adding two more tube rectifiers and filament supply will drop Lots of volts (many 10s of Volts) versus using the 1N4007 diodes.
The two 1N4007 diodes in the bridge will only drop the B+ by about 1Volt. Of course adding two more tube rectifiers and filament supply will drop Lots of volts (many 10s of Volts) versus using the 1N4007 diodes.
I modified the amplifier and sounds realy good now, in triode mode, with the ul tap connected it is a terrible sound. But my only problem is, the bass is not good. So i think it are the output transformers. Lowered the kathode resistors to 430R in place of original 500R. And each cathode of the preamp has his own resistor and capacitor. For me the 6ca7 is better than the el34 ( has more dynamics and lesser distortion) and i got a nasty hum while starting the amplifier ( used the 5u4g or 5y3g) and replaced it for the 274B and the hum was gone. I can share the diagram if someone is interested. But im not impressed with the bass. That is the only complain..
Hi, you can add bass and dynamics by increasing the capacitance of the capacitors C103, C203 = 470 microfarads. I also replaced the cathode resistors with a Px24-25w 500 Ohm. We had to change the power transformer (interturn short-circuit winding). Installed Raphaelite 280W
Raphaelite 280W power transformer PW280AB-230 EL34 6L6 KT66 push-pull tube | eBay
It is interesting to see the diagram of your amplifier after the changes, my looks like this.
Raphaelite 280W power transformer PW280AB-230 EL34 6L6 KT66 push-pull tube | eBay
It is interesting to see the diagram of your amplifier after the changes, my looks like this.
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kronzilla,
I am glad your amp sounds good in triode mode. I am not surprised.
What manufacturer and part number of the output transformers are you using? Are they part of the kit? How much is the cathode current in the 6CA7? (cathode Volts/430 Ohms)?
What model are your loudspeakers?
Happy Listening!
I am glad your amp sounds good in triode mode. I am not surprised.
What manufacturer and part number of the output transformers are you using? Are they part of the kit? How much is the cathode current in the 6CA7? (cathode Volts/430 Ohms)?
What model are your loudspeakers?
Happy Listening!
Sorry i was very busy lately. It are just the stock transformers.
For tubes im using nos ecc81 as preamp, 6ca7 ( el34has lesser headroom) and 274b for rectification. Post the diagram when i have time
For tubes im using nos ecc81 as preamp, 6ca7 ( el34has lesser headroom) and 274b for rectification. Post the diagram when i have time
Today i tweaked the amplifier to my best sounding. I remoded the triode connection and set the tube in penthode. And each kathode of the ecc81 has a 82k feedback resistor. I replaced the 274B for the Electro Harmonix 5u4GB, the el34's getting little too hotcto my taste and the sound seems too compressed. I tested with kt88 and that sounding the best. Bass is also very deep. I regretted that i bougjt such 'bad' amplifier but after tweaking im very happy.
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