So I have been looking at a bunch of different 6AS7G output stages (single ended NOT OTL). I have come across some different operating points. One I found at 280v, 90v cathode bias 90mA per section... which is a little over max plate wattage. Another is 245v, 77v cathode and 77mA per section.
I am looking to do 1 6AS7 per channel, so paralleled sections. 8-10W output.
Any guidance as far as a good operating point for them? Of course I will get matched 6AS7... in fact I have a box of 6080's that I was going to use for another project but never did.
Input stage... no idea. NFB... no idea.
Richard
I am looking to do 1 6AS7 per channel, so paralleled sections. 8-10W output.
Any guidance as far as a good operating point for them? Of course I will get matched 6AS7... in fact I have a box of 6080's that I was going to use for another project but never did.
Input stage... no idea. NFB... no idea.
Richard
That should be asked from the manufacturer: https://sklep.toroidy.pl/en_US/p/TTG-EL84PSE-Tube-output-UL-transformer-2,5kOhm-EL84-6V6-PSE/588
8-10W in SE is a stretch. But OK for PP.
The first time I tried PP 6080s in PP the result was 25W with a 400V PS.
About 125V of that was in the cathode bias resisters. That was 1998. In a boot strapped arrangement.
Recently running it again with Mullard 6080s. But this time 20W, otherwise same conditions.
The 1998 verse was tested with JAN 6080s.
The first time I tried PP 6080s in PP the result was 25W with a 400V PS.
About 125V of that was in the cathode bias resisters. That was 1998. In a boot strapped arrangement.
Recently running it again with Mullard 6080s. But this time 20W, otherwise same conditions.
The 1998 verse was tested with JAN 6080s.
Attachments
Those OPTs look interesting. But if they are designed for 200 mA why do they list 6V6 & 6BQ5 in parallel?
But they would be OK for 6AS7G/6080 triodes in parallel. I thought Poland was in the Euro monetary union. Maybe not.
What is the approximate conversion from Zlotkis to USD? My son is married to a Polish girl, her parents came here after WW2
And my sister is married to a Polish guy in New Jersey. So I have a connexion to Poland. Many here.
Tomorrow is another day on the tractor. Snow has started again, 8-10 cm tonight.
Instead of 8-10 Watts!
I guess the reason is the 2.5k impedance, which is optimum for parallel connected EL84 and 6V6. The same 200 mA specs they have with parallel connected EL34 SE OPT (1.65 kΩ).
https://sklep.toroidy.pl/en_US/p/TTG-EL34PSE-Tube-output-UL-transformer-1,65kOhm-EL34-6L6-PSE/577
Googling I found the Zloty is about 0.25 USD. So at that the prices for those OPTs look very good. But getting them to North America is probably expensive.
The 6V6 & 6BQ5/EL84 families in triode do not test well as reported by many. A while back I bench tested a Decware clone, it was not able to develop One Watt. In general, the higher triode mu requires large plate voltages. Then plate dissipation is a problem.
There are many common TV vertical & horizontal tubes with mu ~5. They do work very well at 250- 300V on the plate.
Over the last few years I've tried experimental amplifiers as Class A 6BX7, 6BL7, 5998, 6SN7, 6EA7, 6LU8 & so on.
Both SE & PP. So the 6SN7 was about One Watt while the 5998 managed a clean 10W. And all within specified dissipation rating.
The PP 6LU8 in Class AB1 clipped at 35 watts.
Thank you all for your replies! so 100ma, 250v, and -120v bias into 1.5K for both sections of the 6AS7 in parallel for SE? This won't over voltage the tube in anyway? I am for sure going the SE route and not PP. Thanks again!
I've ordered toroidy from TME. Shipping is very cheap, only $9 flat rate to the US, actually cheaper shipping than any other transformer I've bought in the US.
https://www.tme.com/us/en-us/katalo...y=100028&search=toroidy&mapped_params=2:1081;
You don't have to worry about over voltage. However, this operating point will run the plates way too hot. The spec sheet (Frank's) specifies a PD= 14W/plate. 250V at 100mA is 25W. You might get a few mnore watts, but the heavy load will lead to more distortion. The loadline I posted will give slightly less than 10W into 3K (paralleled sections) out with very little distortion, and stays well within spec.
The other bit of weirdness is that the spec sheet recommends at least 30 secs of prewarm before turning on the HV. The 6AS7 was intended for series pass duty, and so has a VHK rating of 300V. That makes for a thicker than normal insulation between the filament and cathode, and a greatly increased time for the cathode to come up to working temp.
^
|
It's a Class A SE loadline. Since the Q-point is 40mA, the max current is limited to 80mAmax, and a Vpk= 22.96V. When the final just hits cutoff, the voltage will be 485.29Vmax. That gives a current swing of 80mAp-p -- the outer limit of Class A. A Class B, PP loadline would start at 250V since the PP pair conduct on alternate cycles, and the PP OPT operates on differential current only.
Class AB PP would require a load of 24K/P-2-P, and Class A PP 12K/P-2-P as it would be a balanced, two-phase source. As given, the OPT needs to match 6000 : 8 for one section or 3000 : 8 for paralleled sections (or whatever your speeks are).
|
It's a Class A SE loadline. Since the Q-point is 40mA, the max current is limited to 80mAmax, and a Vpk= 22.96V. When the final just hits cutoff, the voltage will be 485.29Vmax. That gives a current swing of 80mAp-p -- the outer limit of Class A. A Class B, PP loadline would start at 250V since the PP pair conduct on alternate cycles, and the PP OPT operates on differential current only.
Class AB PP would require a load of 24K/P-2-P, and Class A PP 12K/P-2-P as it would be a balanced, two-phase source. As given, the OPT needs to match 6000 : 8 for one section or 3000 : 8 for paralleled sections (or whatever your speeks are).
I looked at buying through TME, but they seemed to have a very limited range of these transformers that they stock. Let us know how they work out if you get them!
I’m curious at what output level one would get such low second harmonic distortion from a 6as7/6080 tube.
If you don’t mind sharing your spice file and the models for the tubes you are using, I am very interested, I’m closing in slowly on a pp 6as7/6080 amp, and curious to perhaps eventually build one single ended also to hear the difference between the two topologies.