Parallel Push Pull 6AS7 or 6080

[Jeb-D.]

the measure 295v on the plates of the 6as7's and something like 110v on the cathode.

That is Vp of 185V, which is a pretty common and good operating point for class-A 6as7 with a load of 2.5k per triode.

build the amp as it is drawn with the only change being cathode bias twin 1k per side and go and it works and sounds great. i changed the opt trans on the test amp to be a much better sounding utc 2.5k and it sounds great.

In that case you have something that can be worked with.

It is good you switched to cathode bias, so now your plate isn't seeing all 355V(as that cathode resistor has to be dropping over 100V). So it may not launch now, but it has to be run in class AB to not exceed Pmax.

The transformer you were using was the other concern. If you switched to a 2.5k plate-to-plate for a paralleled pair that is a much better match than your original transformer. And it probably has a much better DCR as well.

[Jeb-D.]

For your distortion issue.

Make sure your driver can deliver the voltage needed. It will take a lot of grid voltage for your output stage to reach Vgk 0V.

For class A-B you will need to have your cathode resistor(s) bypassed with a capacitor. And adding a little NFB may be benificial as far as distortion goes, but you will need some extra gain to spare.

If you want class-A that plate voltage is going to have to come down to about 185-190V (meaning aprox 290V supply) with an idle current of about 60mA per triode. It will reqire less grid drive voltage and overall you should have quite a bit less distortion.

[poynton]

Originally posted by desperateaudio
............... build the amp as it is drawn with the only change being cathode bias twin 1k per side and go and it works and sounds great. ..........

Hi.

What arrangement do you have on the grid now? Does the TX centre tap go to ground or as you had it in the original diagram? How have you parallelled the sections of the 6080?

Andy

[desperateaudio]

distortion is gone sorry for not adding that. problem was a back test speaker not the amp.

the IT secondary is going to ground and there is now a 1k cathode resistor per tube as opposed to fixed bias.

no grid leak resistors are used though.

Jeb-D when you say 185v you mean the dif between plate and cathode correct?

and at "60ma per triode" you mean 120ma per tube if paralleled correct?

as for parallel on the 6080 i just tied them together with 1k stoppers per grid and a 47 ohm between the plates tied to one side with the power just for a little balance. cathodes tied together with a 1k dropping to ground.

keep in mind guys i have a test version running as stated above with 355v on the plate now for 11.5 hours 5.5 striaght with zero issues

[Jeb-D.]

Jeb-D when you say 185v you mean the dif between plate and cathode correct?

Yes. So that will mean roughly 280V from plate to ground and 285-290V from B+ to ground. A 750ohm cathode resistor per tube will work good at this plate voltage.

and at "60ma per triode" you mean 120ma per tube if paralleled correct?

Yes


On a side note, I've never had problems paralleling these tubes when using cathode bias. Just keep it to around 22W per tube. Figure even the hotter of the two triodes will be below 13W. It will be the balance between between each side of the transformer that is important.

L-pads can be used to balance the sides of a cathode bias push pull, if you find it to be significantly out of balance. E-I core transformers can handle some imbalance.

[desperateaudio]

thanks Jeb-D

on the L-pad issue your are speaking of the l-pad being attached between both pairs of the 6as7's and the wiper to ground via the resistor correct?

thanks again

[desperateaudio]

also no one mentioned the balance scheme at the front of the amp. any remarks?

[Jeb-D.]

on the L-pad issue your are speaking of the l-pad being attached between both pairs of the 6as7's and the wiper to ground via the resistor correct?

Yes, it sounds like you have the right idea. Instead of wiring each pairs cathode resistor to ground, wire each to opposite ends of the L-Pad. Then wiper to ground. Just remember each pairs cathode resistor will have to be bypassed by a cap to ground.

In a class-A push pull you can get away with no bypass cap if both sides share the same cathode resistor because one tube regulates the other for the most part. Leaving an almost constant voltage at the cathode when a signal is applied.

However, if you have to implement the balance circuit, you cannot share the cathode resistor between pairs, so bypass caps are needed.

There is another way to use the L-pad and still share the cathode resistor, but it wouldn't work with the input transformer configuration.

[ja2dhc]

My 6080 amp in single tube PP can output 9W 2%THD with 220V B-Voltage, each 1Kohm bias resister at each cathode.

http://ja1cty.servehttp.com/6080/6080pp.png

[desperateaudio]

very nice single tube version.

I was wondering still if anyone had any comments on my input tube balanced input switch set up?

Thanks again