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Wiring a 6AS7G to be Push Pull within its own envelope

Hi All,
I am trying to get some info for a friend, on how to wire a Twin Triode in
a SE 6AS7G Tube Power amplifier. We want to try and wire the tube as Push
Pull in which the two Plates inside will push pull to gain more power and cancel
out the distortion levels! Could that be like Trying to wire a twin triode tube
in Ultra-linear mode? Is this possible to get the Single twin triode power tube to
Behave as if two tubes were used in Push pull output stage? Has anyone ever
tried this? If so, would not the Voltage have to be higher in order to get the current
up to a higher level? Any thoughts would be appreciated! Lydia​
 
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It should not matter

I can see that being a problem with SE amps where channel separation would matter, but if both triodes in one tube are used in push-pull, how would this matter?

My thought is that Distortion, would decrease and the signal is sent thru one
side to the other and It should be exactly the same as if one were to use this
as a Push Pull Pair for each channel. I would imagine the distortion to be around
2.8% in full 6-8 watts output pere channel as most of the 2nd Harmonic distortion will be gone. The 6AS7G should be stabilized. It could not hurt to have an Adj. Fixed Bias regulator for biasing these type of tubes in Output stages. Hopefully, We would end up with a 6AS7G SE PP Class A tube amplifier.
I think Stability is Key here. If Noise or crosstalk is involved it should be canceled out in this Push Pull wiring, True? Any thoughts............. Lydia
 
Cross talk between the two triodes in these is a non-issue. Very low internal resistance, only a few pF of capacitance, and extremely low mu, all means cross talk will be next to nill.

These make great little PP amps, the low rp means the OT can be low impedance, which means relatively few turns and much smaller parasitic inductance and capacitance in the OT.

The only difficult thing with them is the low mu, aim for +-100V drive for the grids. (Actually not that difficult, the low gain gives low miller capacitance which makes them easy to drive)

b+ around 200V, grids at negative 80-90volts is a good starting point.
 

45

Member
2008-12-18 2:29 am
UK
The only difficult thing with them is the low mu, aim for +-100V drive for the grids. (Actually not that difficult, the low gain gives low miller capacitance which makes them easy to drive)

b+ around 200V, grids at negative 80-90volts is a good starting point.

You don't need +/-100V drive.
150V/60 mA at about -65V bias is a very good compromise for this tube. Primary load 2K-to-2.5K plate-to-plate for PP or 1.4-1.5K for SE. You only need a driver capable of +/- 70V swing. Positive grid drive is not recommended. In my experience it is better to stay below 10W plate dissipation (for each section) in order to achieve some useful life.
 

Shoog

Member
2002-08-15 10:16 pm
Eire
I run mine at 100V 100mA which has 30V (B+ of 130V) across the cathode resistor. This then only requires 30V of drive for full output and the grids are relatively easy to drive at this bias point. Very reliable and a much fuller sound than run at lower currents and higher voltages.
The triodes are very variable between sides so simply tying the cathodes together is a recipe for disaster.

As for bias arrangement, what is shown below works, though LM317 CCS at 100mA work better in place of the 330R;
 

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Shoog

Member
2002-08-15 10:16 pm
Eire
Various.
I have used 15VA toroidal's, but only with a step down of 6:1+1. I think any step down of 3:1+1 or above would work.
I have also used the OEP input transformers and the Z3003E is a good candidate for small money. I have also used a microphone line driving transformer from OEP with a 9:1+1 ratio - worked fine.

I have also had success with the encapsulated Talema toroidals as input phase splitting chokes. They work fine in this role, but the interwinding capacitance is just to high if configured as an input transformer, even with a step down ratio. My guess is that just about any toroidal transformer will make an excellent high performance input phase splitting choke.

Of course the secret of success is having a low source impedence, and for that I have used a toroidal step down transformer at the output of my preamp - that thing could just about drive the speakers directly.

PS, that circuit was never built as shown so is only there for reference to the output bias arrangement. However the circuit design approach has been used in my current amp of two years standing.

Shoog
 
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You don't need +/-100V drive.
150V/60 mA at about -65V bias is a very good compromise for this tube. Primary load 2K-to-2.5K plate-to-plate for PP or 1.4-1.5K for SE. You only need a driver capable of +/- 70V swing. Positive grid drive is not recommended. In my experience it is better to stay below 10W plate dissipation (for each section) in order to achieve some useful life.

Yes agree, these tubes work great on lower voltages as well. I've used them with a 75V rail and high current and they really sound great. So with lower plate voltages naturally the drive voltages can be lower. My current amp has 200V on plates and about -80V on grids. I've had 210V and -95, but prefer the lower values. 40ohm cathode resistors and panel meters for monitoring ensures safe operation. I use one triode section in each tube, two of those in parallel, each triode running at 15watts. As Shoog mentioned the two triodes are poorly matched, easier to find two triodes in seperate envelopes that are similar. It seems triodes on pins 123 are more similar to triodes on same pins on other tubes than they are with the triode on pins 456. Have been running the same tubes like that for many years, no sign of over loading. As long as the total temp is within limits, a lonely triode section can handle at least 15watts.
 
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Shoog

Member
2002-08-15 10:16 pm
Eire
I think its best to use CCS biasing to ensure that everything stays happy. This makes the inevitable triode mismatch in a single envelope immaterial. I have run mine this way for years without incident and everythings stays tickety boo until a point when they go into oscillations.

Shoog
 

45

Member
2008-12-18 2:29 am
UK
Mismatch can be overcome just using a transfomer with some gap to allow DC inbalance. Otherwise one could use a fixed bias.
Actually it could be back bias which doesn't require a dedicated supply. One just moves the cathode resistor in another position and decouples the filtering for each output tube. Using a resistance value a bit higher than needed leaves some room for adjustment. Note that the back bias resistor will experience the total current drawn from that PSU secondary. It can be regarded as a midway solution between a true fixed bias and self bias.

An historical example is the Brook 12A:

http://www.diyaudio.com/forums/tubes-valves/164434-brook-12a-clone.html