Hi! I want to know if I can replace the EL34 output tube on my eico HF-89 with EL37 tube and if I need to do some modification on resistor value on the circuit.
You may need to reduce the plate voltage significantly, and the grid bias voltage probably needs to be increased into 6L6 territory, -40V or more.
The Mullard EL37 is reputed to be an excellent performer, and I can understand your curiosity, just make sure that you are operating them inside of acceptable plate dissipation, voltage and current values. Since they are yours obviously you can do anything you want with them, and were I you I would try them and if I liked the results would use them. Museum pieces are fine, but...
The Mullard EL37 is reputed to be an excellent performer, and I can understand your curiosity, just make sure that you are operating them inside of acceptable plate dissipation, voltage and current values. Since they are yours obviously you can do anything you want with them, and were I you I would try them and if I liked the results would use them. Museum pieces are fine, but...
With the original configuration of the eico HF-89 (EL34) plate voltage is 487v and bias voltage is -46v
ok, swap the 15k and 10 resistor in bias
change B+ supply to full wave bridge type
Bias at approx -15V @ 275 V B+
-13.5@ 250 VB+
depends where the B+ lies when you change the power.
change B+ supply to full wave bridge type
Bias at approx -15V @ 275 V B+
-13.5@ 250 VB+
depends where the B+ lies when you change the power.
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I did the swap in an HF87 (which had previously been modified for fixed bias). The EL37s performed spectacularly well. 45 watts at midband.
Does EL34 really same as 6L6 in plate supply and the grids cause, i am still thinking to change El34 into 6L6> In case i had read the data sheet from the wed and i saw the class a voltage supply is really the same but the output power is little bit different> Could any one taught me < Thanks A Lot~
They are quite different tubes. Sometimes, the same circuit will work for either, more often not.
While the bias supply can be modified to operate EL37s within their rated dissipation, the one thing that has not been mentioned is that the plate-to-plate load offered by the HF-89 OPTs is not very friendly to UL operation of 6L6 class tubes. While EL37s in an HF-87 modified for fixed bias would indeed return superb performance, the same would not be true for their use in the HF-89 due to the lower impedance the OPTs offer in that amplifier.
If you are determined to use these tubes in your HF-89, then along with the bias considerations to be taken, the best performance will be obtained by also converting the unit to straight pentode operation, with the screens running at virtually the same voltage as the plates -- or better -- at a regulated voltage that is equivalent to what the plate B+ supply drops to under full power conditions in both channels. That will reduce the loading requirements of the tubes, so that they will match the load that the 89 transformers offer, and if the screens are regulated, offer the lowest possible distortion levels with this tube/transformer combination. If the change to pentode operation is done, the original power output rating will be maintained, but distortion and damping will suffer over that of the original design. If left in UL mode, power and distortion will suffer, although good damping will still be available.
It is common to see folks routinely swap out the EL34s in a Dynaco ST-70 for 6L6 class tubes -- but the same problem exists in that scenario as well. While 6L6 class tubes can be made to bias up properly in that amplifier (as well as in the HF-89), and the amplifier will indeed "work" -- and may even produce a sound you enjoy, but there is little doubt that on the bench, both amplifiers will deliver notably inferior performance with such a conversion.
I am hardly saying not to try it. But I am telling you what the typical performance results of such a modification will be. Good luck with it!
Dave
If you are determined to use these tubes in your HF-89, then along with the bias considerations to be taken, the best performance will be obtained by also converting the unit to straight pentode operation, with the screens running at virtually the same voltage as the plates -- or better -- at a regulated voltage that is equivalent to what the plate B+ supply drops to under full power conditions in both channels. That will reduce the loading requirements of the tubes, so that they will match the load that the 89 transformers offer, and if the screens are regulated, offer the lowest possible distortion levels with this tube/transformer combination. If the change to pentode operation is done, the original power output rating will be maintained, but distortion and damping will suffer over that of the original design. If left in UL mode, power and distortion will suffer, although good damping will still be available.
It is common to see folks routinely swap out the EL34s in a Dynaco ST-70 for 6L6 class tubes -- but the same problem exists in that scenario as well. While 6L6 class tubes can be made to bias up properly in that amplifier (as well as in the HF-89), and the amplifier will indeed "work" -- and may even produce a sound you enjoy, but there is little doubt that on the bench, both amplifiers will deliver notably inferior performance with such a conversion.
I am hardly saying not to try it. But I am telling you what the typical performance results of such a modification will be. Good luck with it!
Dave
Tanks for the advice,
I have made this tube amplifier (see link below), do you think is a better idea to modify this one to put EL37!?
THE DIRECT INPUT EL34 AMPLIFIER
I have made this tube amplifier (see link below), do you think is a better idea to modify this one to put EL37!?
THE DIRECT INPUT EL34 AMPLIFIER
Very definitely. The 6K load is still a little low for 6L6 class tubes in triode mode, but it would be a heck of a lot better than trying to use EL37s in an HF89. There will be a slight loss of power using the EL37 , but overall, it should perform very well.
Dave
Dave
From a biasing standpoint, it would be rather easy to modify the amplifier to accept EL37 tubes:
1. Reduce the 10K dropping resistor in the bias filter network to 4.7K.
2. Adjust the quiescent current to 100 ma total for both tubes, by setting the voltage at the TP to 1.00 vdc.
That is it. The pin outs between the two tubes won't require any rewiring of the sockets, so other than the bias resistor change and readjusting the bias controls, that's about it. Listen to it, and let us know what you think!
Dave
1. Reduce the 10K dropping resistor in the bias filter network to 4.7K.
2. Adjust the quiescent current to 100 ma total for both tubes, by setting the voltage at the TP to 1.00 vdc.
That is it. The pin outs between the two tubes won't require any rewiring of the sockets, so other than the bias resistor change and readjusting the bias controls, that's about it. Listen to it, and let us know what you think!
Dave
Sorry, dcgillespie, but for me the EL37 data look to be much more closer to those of the EL34 than to those of the 6L6 family. Maybe that for the intended amplifier EL37 could indeed be a 1:1 replacement for EL34's?
On the other hand I have a suggestion for a helpful modification: Putting a 10R resistor in each cathode lead instead of one for both helps biasing both tubes more properly.
Best regards!
On the other hand I have a suggestion for a helpful modification: Putting a 10R resistor in each cathode lead instead of one for both helps biasing both tubes more properly.
Best regards!
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Kay -- From the data you supplied, the EL37 does look to be closer to an EL34 than that of the 6L6 family. However, my point is more directed at the peak current draw ability between a 37 and 34 when operated in UL mode with 43% taps, which is the scenario at hand. In this regard, an EL37 is much closer to the capabilities of a 6L6GC than an EL34. Indeed, while the EL37 is capable of slightly more power output than a 6L6GC is in this setting, in the real world, the actual performance differences between the two tubes are very small, with both producing significantly less power output than an EL34 will in the same UL setting.
Best regards as well!
Dave
Dave
Best regards as well!
Dave
Dave
Ok tanks dcgillespie, I will try this modification on my amp, when I have done it I will give you some feedback.
William
William
The modification is done! Now I listen my amplifier with the EL37! 🙂 at the first track I slightly notice a VERY little distortion in the high frequency, very present and solid bass and the mid seem to be good. I will listen this amp tonight with a good scotch and tell you more after that.
Tanks for all
William
Tanks for all
William
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