I had read some where that the invertors (Phillips???) of the el34 ran them up to 800 volts to prove you could get 100 watts from a pair.
If you check the data sheet you will find they are running in class "B". great for PA but bad for hifi.
Don't be disappointed if the transformers have limited freq response.
If you check the data sheet you will find they are running in class "B". great for PA but bad for hifi.
Don't be disappointed if the transformers have limited freq response.
Menno van der Veen used 740V B+ on EL34s in one of his 'project' designs. Advising to use beefier tubes.
https://www.mennovanderveen.nl/inde...push-pull-pentode-with-twice-the-output-power
https://www.mennovanderveen.nl/inde...push-pull-pentode-with-twice-the-output-power
As has been mentioned, the EL34 datasheet gives a design example with Vp = 800V, Vg2 = 400V, Ip = 25mA, Pout = 90W- 100W.
They call this operation "Class B"
But the tubes actually run very hot at PD = 20W or 80% PDmax!
Note that this operation requires a very large OT primary impedance of 11k to avoid severe overdissipation with signal.
Now if you have an OT with an Raa of 11k, power output with Vp = 420 will be poor (maybe 25W) and screen dissipation is likely to be very high.
They call this operation "Class B"
But the tubes actually run very hot at PD = 20W or 80% PDmax!
Note that this operation requires a very large OT primary impedance of 11k to avoid severe overdissipation with signal.
Now if you have an OT with an Raa of 11k, power output with Vp = 420 will be poor (maybe 25W) and screen dissipation is likely to be very high.
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It is a line output transformer, but with 3 secondaries, and 18 windings on one of the secondaries with accessible taps, so I am hopeful I will find a suitable turns ratio.
It ought to be possible to break one tap, then end up with 4 windings with the same number of turns in parallel, and a reflected impedance of 2.5K for an 8R loudspeaker.
S1 is the primary (1470 turns)
S2A + S2B + S2C + S2D (plus the parallel windings) are 88 turns
S2E and S2F also 88, so A + B + C + D in parallel with E + F is a turns ratio of 16.7.
16.7 sqrd * 8 is around 2.23k impedance.
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I don't understand the table. How 735W??
What do you mean with line output transformer?
Is that for a 70V or 100V PA line system?
I have a 50s Siemens PA amp built exactly to the EL34 datasheet example (Vp = 800V) with a 100V line output.
It delivers close to 100W with 2*EL34 into a 100R(!) load.
Regarding the OT:
A real world OT is a compromise between I²R losses and turns number (responsible for inductance and bass response).
When you use an OT designed for Raa = 11k with much lower impedance, winding currents (limited by wire gauge) and losses (causing power loss and more importantly heat) will significantly increase.
Result would be OT overheating and eventual failure.
If you compare the winding information (W = turns) with the schema earlier on, there are multiple taps for different loads. There is an option for 9R load.
The turns (Windings) correspond to the diagram below.
The plan is to remove the voltage doubler and hopefully reconfigure the OPT (the taps are soldered to tags) to try and create a meaningful anode to anode load for typical loudspeakers (8R).
I realise some fettling will be required 🙂
The turns (Windings) correspond to the diagram below.
The plan is to remove the voltage doubler and hopefully reconfigure the OPT (the taps are soldered to tags) to try and create a meaningful anode to anode load for typical loudspeakers (8R).
I realise some fettling will be required 🙂
Looks like 1470/(18+26) would make a 33,4:1 ratio, or 8,9k/8R. Should be a decent load for a pair of triode wired EL34s operating at a fairly high plate voltage if you are willing to trade some output power for low distortion and output impedance.
Please correct me if I am wrong, but say I was to aim for 6k anode load and 8R load, then I would be angling for a turns ratio of
SQRT(6000/8) which is 27. There are a total of 1470 primary turns (2 * 735) so that means the number of turns for the secondary is 1470/27, so 59, which isn't so far away from 53, S2F on its own or (S2F || (S2B + S2D)). It looks feasible on paper.
Thanks, there would seem to be options with the OPT, at least to justify adjusting the PSU and operating points.
If the winding ends are accessible from the outside you should have plenty of interesting options, but be warned that the overall quality of the OPTs is probably not too impressive. My old EL6400 OPTs (that look similar to yours) don't make it far past 20kHz and suffer from low Lprim, resulting in a rather weak bass response even with triode wired 807s as output tubes and a bit of global feedback.
I have a transformer with the exactliy same windings, it is on a EI 108 core, stack is 62mm, it is from a philips 120W pa amp with 4 el34, Ua =740
The 176wdg output is 100V into 83ohm, so, neglecting Rdc, 1470:88wdg would be 5500ohm into 19,7ohm
Philips used anode to anode "sparkgaps" for transient protection. The inductance of this transformer is good down to approx. 40Hz
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