Wich B+ do you recommend?
Do you change something in the schematic (resistors or capacitors values)?
That's 18.5 dBs, not bad at all, how much volts swing?
Wich topology is better SRPP or cathode follower?
My friend is using actually Erno Borbely unbalanced preamp with a gain of 20dB & he's not happy.
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My guess at the gain of the original 6DJ8 circuit in post number 2 is about 24. My guess at the series circuit I proposed with the output moved to pin number 6 of a 6DJ8 is about 15 to 17. My guess at the gain for the 12AU7 in the series circuit I proposed is ~8.5 (also as stated by DF96).
You may want gain and buffer.
DT
You may want gain and buffer.
DT
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Possibly I need more tubes, I'm not sure because I'm newbie, I suggested more tubes involved for example an input with a twin cascode or SRPP 12AU7 with a cathode follower 6DJ8 to lower the output impedance, total 5 tubes for a stereo preamp.
Hi Sy
Perhaps you mean this
http://valvewizard2.webs.com/SRPP_Blencowe.pdf
How are the equations of SRPP with LED?
Best regards
Johann
Try an ECC82 instead of ECC88, change 150 ohm resistors by 1.5Kohm resistors, 0.1uF cap by 1uF cap, 150uF bypass cap may be increased to 470uF without problems, +B=350V (well filtered), 100Kohm pot is OK.
With an output impedance in the order of 2.5Kohm and a gain in the order of 10 you don't need anymore.
Float the filaments at say +80V and be happy.
Best regards
Johann
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Joined 2009
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Hi Merlin,
Let's start with the amplifier.
I see it is a balanced input amplifier, meaning it wants two out-of-phase signals not just a single input signal. If you are going to build a dedicated pre-amplifier to drive it, you could consider a pre-amp with a balanced output. It would be a shame not to take the opportunity to do it as the amplifier is designed to work this way. Running it with a single ended input should also be possible. You might want to confirm with your friend if a balanced input is desired before choosing a design.
Here's a source of information: Aleph-X 100w Amplifier Construction Notes
There's a thread here with a simple schematic but it does require a transformer and good ones are not that cheap:
http://www.diyaudio.com/forums/tubes-valves/92081-balanced-tube-preamp-class-d-power-amps.html
Tubes take time to warm up so you need to consider if you need a 'mute' relay or something between the pre-amp and the amplifier to avoid a turn-on 'thump' from the speakers or just remember to turn on the pre-amp first.
For a single ended preamplifier, like the one you started with, this link may be useful:
DIY 12AU7 Tube preamp
it shows you the design and the results, it also explains how it works which is always nice. The gain looks about right and the performance good. I feel like making one myself
Let's start with the amplifier.
I see it is a balanced input amplifier, meaning it wants two out-of-phase signals not just a single input signal. If you are going to build a dedicated pre-amplifier to drive it, you could consider a pre-amp with a balanced output. It would be a shame not to take the opportunity to do it as the amplifier is designed to work this way. Running it with a single ended input should also be possible. You might want to confirm with your friend if a balanced input is desired before choosing a design.
Here's a source of information: Aleph-X 100w Amplifier Construction Notes
There's a thread here with a simple schematic but it does require a transformer and good ones are not that cheap:
http://www.diyaudio.com/forums/tubes-valves/92081-balanced-tube-preamp-class-d-power-amps.html
Tubes take time to warm up so you need to consider if you need a 'mute' relay or something between the pre-amp and the amplifier to avoid a turn-on 'thump' from the speakers or just remember to turn on the pre-amp first.
For a single ended preamplifier, like the one you started with, this link may be useful:
DIY 12AU7 Tube preamp
it shows you the design and the results, it also explains how it works which is always nice. The gain looks about right and the performance good. I feel like making one myself
OK this schematic came from here:
DIY 6922 / E88CC Tube preamp
On that page you'll see his loadline for the 6922:
If you plot the Watt Dissipation line over this, you'll see the problem.
The solution is to alter the loadline (by putting a db-pad or volume pot on the output) and also the quiescent-point (idle-current) by adjusting the bias via the cathode resistor, and possibly the voltage.
The 6922 is a great tube, but it is a low-noise fragile signal tube, not meant for this kind of manhandling.
The Dot which represents the idle-spot should be well below the max-dissipation curve.
The voltage swing, which will meander over the line anyway, should never spend more than half its time outside the boundaries.
Although Philips allows 2 watts dissipation / triode,
The Telefunken sets 1.5 watts as the real limit. Obviously we go with the lower rating.
Also, it is generally recommended that your design stay at 70% of max wattage rating for safety and to allow for variation between tubes and brands.
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Here's a better loadline and bias0-point for the 6922.
It sits at 70% of the conservative max dissipation (1 watt).
With this bias-point you stay in 70% zone, and tube stays out of trouble most of its life.
The load resistor becomes about 12k5, the cathode resistor is 830 ohms, the bias voltage is -5.
The cathode floats about 150 volts above ground.
The tube coasts in its comfy spot, with very little loss of headroom and great gains in tube longevity.
It sits at 70% of the conservative max dissipation (1 watt).
With this bias-point you stay in 70% zone, and tube stays out of trouble most of its life.
The load resistor becomes about 12k5, the cathode resistor is 830 ohms, the bias voltage is -5.
The cathode floats about 150 volts above ground.
The tube coasts in its comfy spot, with very little loss of headroom and great gains in tube longevity.
Attachments
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Here's a better loadline and bias0-point for the 6922.
It sits at 70% of the conservative max dissipation (1 watt).
With this bias-point you stay in 70% zone, and tube stays out of trouble most of its life.
The load resistor becomes about 12k5, the cathode resistor is 830 ohms, the bias voltage is -5.
The cathode floats about 150 volts above ground.
The tube coasts in its comfy spot, with very little loss of headroom and great gains in tube longevity.
That's your proposed schematic?
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