So I was thinking of trying a single ended EL84 in triode mode with a constant current source using a LM317 with 275V at the plate, but I had a couple questions:
Does the size of the bypass cap matter?
How much current is appropriate to set the LM317 at? Is 25mA fine?
Thanks for any help 🙂
Does the size of the bypass cap matter?
How much current is appropriate to set the LM317 at? Is 25mA fine?
Thanks for any help 🙂
Hi I am using the LM317 on SEP el84 amp. the size of the bypass cap matters as it sets the low frequency. I use 100uf X 25v Elna Cerafine with mine. I am also using a 27ohm 1/2 watt resistor to set the voltage/current. If you go with a slightly larger resistor you will have less dissipation. with the 27ohm resistor that works out to about 34ma idle current. cheers, Dak
The appropriate current is determined by the HT volage and the output transformer primary impedance, and should be set to give maximum power output for that HT voltage and primary impedance.
To get the maximum power an EL84/6BQ5 is capable of, use HT = 257 V, transformer primary 5200 ohms, cathode current 53.5 mA. You will then get just under 5 watts of audio into the transformer, and the lowest harmonic distortion at 1 watt compared to other combinations of HT, load, etc.
Note that CCS biasing will give a lower power output than cathode resistor biasing, which in turn gives less power than fixed voltage bias, which will give 6 watts.
To get the maximum power an EL84/6BQ5 is capable of, use HT = 257 V, transformer primary 5200 ohms, cathode current 53.5 mA. You will then get just under 5 watts of audio into the transformer, and the lowest harmonic distortion at 1 watt compared to other combinations of HT, load, etc.
Note that CCS biasing will give a lower power output than cathode resistor biasing, which in turn gives less power than fixed voltage bias, which will give 6 watts.
5 Watts seems quite high for triode mode, doesn't it? That sounds more like pentode or ultra-linear power output to me. It's also biased quite hot (a tad over specs unless I'm mistaken)
I want my tubes to get some longevity out of this, so I don't want to tax them too hard (and my speakers are 97dB efficient on 1W at 3m), so I'm perfectly happy with 1-2 watts of power.
When I checked a LM317 current calculator it said a 27 ohm resistor would set the current at ~46mA of current. Or am I mistaken (I'm actually not sure)?
Errr... Yes, I didn't read your question sufficiently carefully, and missed teh word "triode". Many sorries. I took the biasing straight from Philips data for SE pentode operation though.
Philips data for triode mode:-
Ht 259 V; Cathode current 34 mA; Transformer secondary 3500 ohm.
Power output into transformer for const current biasing will be about 1.7 watts
Because if you don't, the CCS will not allow any signal current out of teh cathode, and there will be no output.
For LM317 (or other voltage regs like it) as a CCS, the current regulation is defined by the internal reference voltage (1.25V) divided by the resistor you add from the output pin to ground or negative potential.
LM317 CCS Calculator
current (I) = 1.25/R
1.25/27 = 0.046296 or about 46mA
33 ohms would give you about 38mA (1.25/33 = 0.03788)
and so on...
If you use one CCS, like a tail, to make a LTP, there will be very good quality output. No need to bypass.
Why have two CCSs and bypass them?
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