EL34 driver for PT15

[The Shadow]

Question:
I've had a bit of a read up about tube regulators, do they need some sort of RC network around them to drop the
noise they generate or will the ultracap cancel the noise anyway?

Steve

[NickG]

Yes, you can cut down the noise with a small cap, but use too much and it will oscillate. A zobel as you say is better. 100R and 0.1u would be a good place to start.

[The Shadow]

Thanks for that info Nick

[Mike H]

6AC7 as driver valve (original thread, 'EL34 driver for PT15')

"I don't know if the 100k on g3 will work for 6AC7 but I'm curious if it does."

Greetings, first time on this forum, but must reply to this subject as I've recently experimented with the 6AC7 with some significant discoveries.

Firstly though, you should never connect ANY pentode's suppressor grid to anything except cathode. There is NO manufacturer's application data I've seen that says otherwise in any circumstances. Even if wired as a triode (g2 to anode), g3 is still never tied to the anode as well, probably because g3 is not intended to conduct current, unlke g2, and so does not have any power dissipation handling. (Unlike g2...) However, one exception is the 6AC7 whose g3 can be connected to ground instead, but ONLY if you're using it to amplify video or RF.

Anyhow I had also long wondered whether the 6AC7 would make a good driver valve (even before seeing this discussion thread), while providing, as it would, a low impedance drive to the o/p valve grid(s). I already had two old ones but one of them went kaput while I was trying to set it up, so I bought two 'new' unused RCA ones.

RCA's data sheet says it should not be used as an input valve for audio because it is too noisy, meaning mostly hum pick-up from the heater, hence a DC heater is recommended for low-level audio signals. But also there is a fair amount of radio pick-up type noise as well, especially since the 6AC7 is a high-frequency valve. More serious though is unacceptable distortion, characterized by a rounding-off of the negative half-cycles (as seen at anode), which I'm guessing might have something to do with the sharp cut-off characteristic. In other words the signal grid is loading the input on the positive half-cycles, despite the input peaks remaining well below the cathode bias of +5V. This behaviour increases in proportion to signal amplitude (i.e. is progressively worse with increasing level), which is why I thinks it's related to the sharp cut-off behaviour...

HOWEVER, if instead g1 is fed from a LOW IMPEDANCE SOURCE then both these problems are cured at a stroke, it kills the noise AND restores correct o/p waveform shape. I suggest a 6J5 or half 6SN7 cathode-follower, where the cathode chain = 2.2k + 22k, with 1M grid bias resistor tapped into the chain at the bottom of the 2.2k and o/p taken from the top. DC at this point about +50V. HT = 300 - 350V.

The 6AC7 itself is set up as follows: HT = 400V, anode resistor = 22k (Vishay 9W vitreous enamelled wirewound), screen resistor = 68k 2W, g2 is then decoupled to cathode thru 220nF polypropylene. g3 connected to cathode. Cathode resistor = 470R bypassed with 100uF. The gain for one of these RCA examples was X130, the other X135, which is practically as good as a high-gain EF37 (EF86) with a 200k anode load, BUT with the added bonus of low impedance output. Anode and screen DC can vary widely between 150 - >200V, making Ia between 9 - 11mA, but could be set more precisely by altering the cathode bias resistor as appropriate, although the data sheet recommends biasing for Ia = 10mA in any case. Input grid bias resistor seems best as 100k, requiring say 470nF cap from the preceding 6J5 cathode.

Incidentally, wired as triode (g2 to anode, but always leaving g3 on cathode!) works well too, same anode load of 22k, gives a gain of X35.

[The Shadow]

Hi Mike and welcome to the forum.

When I had my PT15 amp running It was driven by 6AC7s in the end and sounded great. The preamp driving the whole shebang was just as you suggest.

I had a preamp using four 6J5s two per channel, constant current grounded cathode sections direct-coupled to cathode followers, which gave the 6AC7s just the sort of low impedance source you mention.

Using a simple common cathode input stage using a 6F8G twin triode into the 6AC7 drivers sounded horribly distorted, but I couldn't work out why at the time. Thanks for that valuable insight.

Steve

[vitalstates]

Originally Posted by Mike H

6AC7 as driver valve (original thread, 'EL34 driver for PT15')

"I don't know if the 100k on g3 will work for 6AC7 but I'm curious if it does."

Greetings, first time on this forum, but must reply to this subject as I've recently experimented with the 6AC7 with some significant discoveries.

Firstly though, you should never connect ANY pentode's suppressor grid to anything except cathode. There is NO manufacturer's application data I've seen that says otherwise in any circumstances. Even if wired as a triode (g2 to anode), g3 is still never tied to the anode as well, probably because g3 is not intended to conduct current, unlke g2, and so does not have any power dissipation handling. (Unlike g2...) However, one exception is the 6AC7 whose g3 can be connected to ground instead, but ONLY if you're using it to amplify video or RF.

Hi Mike

welcome..

I am curious about the above statement as the words 'should' and 'never' have me a bit worried. I have a variation of the ubiquitous SE EL34
amp which I've modded in the past.. On the suggestion of another member on this forum I did indeed connect suppressor grid to ground, with a profound improvement in all areas. It has been running like this for about 18 months.

Any chance you could explain what problems I'm likely to run into by doing this?

Regards

Ed

[Corne]

Firstly though, you should never connect ANY pentode's suppressor grid to anything except cathode.

To get true triode characteristics g2 and g3 should be connected to anode. If g3 is connected to ground any electrons hitting the g3 wire will flow to ground but in triode mode they should have gone to the anode.

g3 is not intended to conduct current, unlke g2, and so does not have any power dissipation handling

The suppressor grid was introduced to battle secondary emissions from the anode to g2. In his function the power dissipation is virtually zero so this specification omitted from the datasheet. But g3 can handle some power and if it's connected to anode the current that hits the widely spaced g3 wire isn't large enough to heat g3 to destruction, it can't even heat it enough so that it start glowing red. It's perfectly okay to connect g3 to anode.

No worries Ed.

There is NO manufacturer's application data I've seen that says otherwise in any circumstances

True almost all pentode datasheets only have data for g3 connected to ground. Look at the datasheet of the EF14 you'll find it has applications with g3 tied to anode.


Corne

[Richard]

Hi Mike and welcome!

Never say never as they say! You raise an interesting point on g3 connection. I've seen circuits with EL34 g3 connected to anode. One of the supposed attractions of this valve is that it could be wired as more-nearly a true triode than most other tet/pents. I accept there seems to be no data for this connection with it.

I always assumed (maybe wrongly) that why it wasn't suggested (for EL34) was a practical/historical reason of compatability. It was launched to compete with tetrodes such as 6L6 and if the triode connection had been suggested with g3 connected to anode there would have been trouble when using tetrodes in triode EL34 amps. Thus g3 was always connected to ground (for pentode and triode) to replicate the fixed tetrode pin 1-8 internal connection.

Sound quality is interesting, as Ed has found with his (Ed, I can't see a problem with g3 to k even if your circ says otherwise), so maybe this also is a reason (i.e. the valve still benefits from suppressing reflected electrons when wired triode).

For an example of g3 connected to anode see the 6AU6 sheet attached (an RF valve I know but used in audio quite a bit too).