So I have an old Harmony 430 (Valco built, same as the Gretsh 6162, Supro Royal Reverb) that uses a pair of 6973's for its power tubes. Since these tubes are becoming much harder to find (I've read that the new production EHX version sound nothing like real 6973's), I've been toying with the idea of converting it to run EL84's.
The problem is that I LOVE the sound of 6973's, and I don't want my Harmony to change in sound THAT much.
Does anyone have any insight to potential tweaks that can be made that would keep the overall sound similar should it run EL84's?
I've attached a schematic for reference.
The problem is that I LOVE the sound of 6973's, and I don't want my Harmony to change in sound THAT much.
Does anyone have any insight to potential tweaks that can be made that would keep the overall sound similar should it run EL84's?
I've attached a schematic for reference.
The 6973 is a beam power tube and the EL84/6BQ5 is a true pentode. They are going to sound different regardless of what you do.
Rewiring for a 6AQ5 might get you closer to the sound you want if you match bias points.
Rewiring for a 6AQ5 might get you closer to the sound you want if you match bias points.
Because of the higher voltage ratings the 6973 can not unconditionally be replaced by an EL84. EL 84 Va max= 300v , while Va max 6973=400v.
You might use the Russian 6P14 EV which has a Va max > 400v. But watch out: the pin out of these tubes differ.
6973 : 1= g2 2=nc 3= g1 4,5 h 6= g1 7= cath. 8= g2 9 = a
6P14 EV, EL84 : 1= nc 2 = g1 3= cath. 4,5 H 6=nc 7= a 8 = nc 9= g2
The 6P14 EV is a very good sounding tube and has a tube life almost double that of an EL 84.
You might use the Russian 6P14 EV which has a Va max > 400v. But watch out: the pin out of these tubes differ.
6973 : 1= g2 2=nc 3= g1 4,5 h 6= g1 7= cath. 8= g2 9 = a
6P14 EV, EL84 : 1= nc 2 = g1 3= cath. 4,5 H 6=nc 7= a 8 = nc 9= g2
The 6P14 EV is a very good sounding tube and has a tube life almost double that of an EL 84.
6CZ5 should work in this circuit - same pinout except pin 8 is marked "internal connection - do not use" - (might be connected to the screen like 6973...) disconnect it to be safe. $9 for NOS at findatube.com. (Also used, tested 6973 for $10 each, $21 for NOS).
At one point in time (long ago):
The 6BQ5 was a Beam Power Tube
The EL84 was a Pentode Tube
The specifications of the 6BQ5 and EL84 tubes (electrical specs, not necessarily the sound - that is arguable), were almost identical.
At a later point in time:
Guess what? Manufacturers started dual-marking the tubes as 6BQ5/EL84 or EL84/6BQ5.
And, you can find some that are true pentodes with suppressor grids, and some that are true beam power tubes with beam formers.
As was previously mentioned, the 6BQ5 and EL84 will be too hot and short lived at the voltages present in your amplifier.
Modifications will be necessary. It is not just the 350V B+, it also is the 23V self bias of your tubes. The difference between those is the plate to cathode voltage. You can not be using 23V self bias for 6BQ5/EL34, the self bias voltage will be perhaps less than 1/2 of the 23V, maybe even 10V.
That will make the plate to cathode voltage be about 350V - 10V = 340V.
A low capacitance B+ first filter capacitor, followed by a choke that meets critical inductance rules, will lower the B+ voltage.
The power transformer will run cooler. Critical inductance can be calculated for 50Hz mains, and 60Hz mains, if you know the total current draw on the B+.
There is not a good way to know what the amplifier will sound like unless you perform the mods that are needed for 6BQ5/EL84.
The 6BQ5 was a Beam Power Tube
The EL84 was a Pentode Tube
The specifications of the 6BQ5 and EL84 tubes (electrical specs, not necessarily the sound - that is arguable), were almost identical.
At a later point in time:
Guess what? Manufacturers started dual-marking the tubes as 6BQ5/EL84 or EL84/6BQ5.
And, you can find some that are true pentodes with suppressor grids, and some that are true beam power tubes with beam formers.
As was previously mentioned, the 6BQ5 and EL84 will be too hot and short lived at the voltages present in your amplifier.
Modifications will be necessary. It is not just the 350V B+, it also is the 23V self bias of your tubes. The difference between those is the plate to cathode voltage. You can not be using 23V self bias for 6BQ5/EL34, the self bias voltage will be perhaps less than 1/2 of the 23V, maybe even 10V.
That will make the plate to cathode voltage be about 350V - 10V = 340V.
A low capacitance B+ first filter capacitor, followed by a choke that meets critical inductance rules, will lower the B+ voltage.
The power transformer will run cooler. Critical inductance can be calculated for 50Hz mains, and 60Hz mains, if you know the total current draw on the B+.
There is not a good way to know what the amplifier will sound like unless you perform the mods that are needed for 6BQ5/EL84.
