UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

YC-156 · 15th Sep 2006, 6:57 am

Hmm, in no particular order:

Older transformers should not be pushed. The wire insulation etc. is not up to the abuse of the modern counterparts.
The current rating of your transformer is valid when it is powering a resistive load, like ... a resistor. For a capacitor input filter, which I suspect you will be using, the average apparent transformer load is 2 to 5(!) times higher than you would expect from the output load current and voltage. <--- This is the reason why you often have people complaining about overheating mains transformers for their nice DIY class A audio amp designs. Few DIY websites and books explains about this issue, which is based in math and physics, not opinion. Bottom line: Transformers powering class A amps must have the power rating of their HT winding derated(!) by a factor of 2 to 5. The exact figure depends on the components used.

Source: Radiotron Designers Handbook by RCA, Fourth Edition (1952). Page 1175, chapter 30, ' Rectification', section 2 (III).
As previously mentioned in another thread some time ago, then the 83 *will* *simply* *die* if you run it at high current in a common capacitor input filter in a PSU. Additionally, if you did use one 'properly' with a choke filter coil, the output voltage would be much less, reducing output and a suitable choke may be difficult to obtain. Mercury vapour rectifiers, while they can take high average current, can not survive the high peak currents found in a capacitor input filter.
Agree on the 807 and their ilk like 6V6, 6L6 etc. They don't really look that nice when powered. I have an 'open frame' 807 without the internal shielding and it glows pretty nicely, but that appears to be an uncommon type of construction.

My choice for a push-pull stereo amp would be EL34s, even if the shape isn't that fantastic. At least they are large and available. They glow reasonably well too, but eats 1,5A of heater current a piece.
I wonder what the current rating of the 26V heater winding is? How much does the transformer weigh?
The problem with using triodes compared to tetrodes/pentodes is that - everything else being equal - you will draw less current through the triode. Read: Less power for equal anode voltage. The voltage gain of triodes may also be significantly lower than pentodes, so you may need an extra gain stage to really break down the walls with volume. Finally there is the Miller capacitance issue and the possible problems with having to run grid current. You may need a real power driver to push a pair of triodes.
Two things with 813s and similar higher voltage power valves: You won't easily find an output transformer, since they are worth their weight in gold (more or less) to the radio amateurs building classic style AM transmitters.

Secondly, I at least won't help you use them. The voltages involved are simply too dangerous to be used in a transportable stage amp, and the mechanical construction required to make it 'road-and-audience' safe wouldn't be trivial. No open style chassis here would do IMHO.

Stepping off my

Frank N.

15th Sep 2006, 6:57 am	#2
YC-156 Retired Dormant Member Join Date: May 2005 Location: Aarhus, Denmark Posts: 281	Re: output valve choice Hmm, in no particular order: Older transformers should not be pushed. The wire insulation etc. is not up to the abuse of the modern counterparts. The current rating of your transformer is valid when it is powering a resistive load, like ... a resistor. For a capacitor input filter, which I suspect you will be using, the average apparent transformer load is 2 to 5(!) times higher than you would expect from the output load current and voltage. <--- This is the reason why you often have people complaining about overheating mains transformers for their nice DIY class A audio amp designs. Few DIY websites and books explains about this issue, which is based in math and physics, not opinion. Bottom line: Transformers powering class A amps must have the power rating of their HT winding derated(!) by a factor of 2 to 5. The exact figure depends on the components used. Source: Radiotron Designers Handbook by RCA, Fourth Edition (1952). Page 1175, chapter 30, ' Rectification', section 2 (III). As previously mentioned in another thread some time ago, then the 83 will simply die if you run it at high current in a common capacitor input filter in a PSU. Additionally, if you did use one 'properly' with a choke filter coil, the output voltage would be much less, reducing output and a suitable choke may be difficult to obtain. Mercury vapour rectifiers, while they can take high average current, can not survive the high peak currents found in a capacitor input filter. Agree on the 807 and their ilk like 6V6, 6L6 etc. They don't really look that nice when powered. I have an 'open frame' 807 without the internal shielding and it glows pretty nicely, but that appears to be an uncommon type of construction. My choice for a push-pull stereo amp would be EL34s, even if the shape isn't that fantastic. At least they are large and available. They glow reasonably well too, but eats 1,5A of heater current a piece. I wonder what the current rating of the 26V heater winding is? How much does the transformer weigh? The problem with using triodes compared to tetrodes/pentodes is that - everything else being equal - you will draw less current through the triode. Read: Less power for equal anode voltage. The voltage gain of triodes may also be significantly lower than pentodes, so you may need an extra gain stage to really break down the walls with volume. Finally there is the Miller capacitance issue and the possible problems with having to run grid current. You may need a real power driver to push a pair of triodes. Two things with 813s and similar higher voltage power valves: You won't easily find an output transformer, since they are worth their weight in gold (more or less) to the radio amateurs building classic style AM transmitters. Secondly, I at least won't help you use them. The voltages involved are simply too dangerous to be used in a transportable stage amp, and the mechanical construction required to make it 'road-and-audience' safe wouldn't be trivial. No open style chassis here would do IMHO. Stepping off my Frank N. Last edited by YC-156; 15th Sep 2006 at 7:05 am.