6AN8 PI

[TIMBO]

Hi guys, Rather than side track billcreller's Sunn post I thought I'd better start another. Looking for a build for my 18w iron (something different) and see that Sunn have a 6AN8 in the PI spot, well I just happened to have one.

What do you guy's think of using this PI with EL84s....... maybe 6V6s. Thanks

[kagliostro]

Why not ?

RCF used that tube in PA amp driving el84 tubes (and also el503)

about 6v6 they require a bit more drive than el84, but I think is feasible

Ciao

K

[sluckey]

Looks good to me. Looks like you found a guitar preamp. I'd probably prefer 6V6s since this is a clean sounding amp. Have you considered a UL OT or are you mainly looking to use your 18W OT?

You won't need series caps at the first B+ node with that PT.

[silverfox]

I too have had questions regarding the 6AN8 used in this fashion. I've got that type circuit- Perhaps? in a Univox 1221.

What are the benefits of using a Pentode in the phase inverter section?

For reference: http://www.el34world.com/charts/Schematics/files/univox/univox1221.pdf

Silverfox.

[sluckey]

What are the benefits of using a Pentode in the phase inverter section?

Technically speaking, the pentode is not the PI. The triode is the actual split load (cathodyne) PI, very similar to that found in the Princeton Reverb. But since the cathodyne PI only has a gain of slightly less than one, it's common to consider the preceding voltage amp as part of the PI.

With that in mind, the pentode is capable of more gain than a triode. The pentode may have a different sound than a triode but I'm guessing that the extra gain was the main reason for choosing the pentode.

[HotBluePlates]

What are the benefits of using a Pentode in the phase inverter section?

... I'm guessing that the extra gain was the main reason for choosing the pentode.

I notice in both schematics presented that feedback is applied from the OT secondary to the bottom-end of the 6AN8 pentode cathode resistor.

If you have more open-loop gain (i.e., use a pentode instead of a triode), you can apply more feedback and not make the input to the power amp so insensitive that it needs super-huge driving voltage.

Said another way, Sluckey's right: pentode is easier to drive while also providing extra gain that can be taken by the feedback loop to reduce distortion and output impedance.

[silverfox]

Thanks for the information. I have noticed the Univox seems to have a Richer tone;  I'm guessing higher harmonic content from the Pentode; not to say, higher distortion content. hard to define the difference the Pentode seems to provide, (higher wave count?).

The use of a voltage doubler also may benefit from a high gain Pentode driving the power amp. Just a guess on that.

Silverfox.

[DummyLoad]

looks cool. build it! 

EL84 over-driven can sound fizzy. 6V6 usually sound smoother over-driven...

--pete

[TIMBO]

Thanks guys, All good info. Sluckey, the plan is to use the 18w OT and i'll still try the ELs first. The caps in series is that way cause I already had a board made up with the rectifier diodes and caps on it , just recycling  

"CLEAN" you say, might need a bit guidance to dirty it up a bit, the NFB I'm guessing can be adjusted to suit???

When setting out the chassis for the tube placement I marked three holes, two for the preamp and PI but I was not going to drill the second preamp till I got this up and running, but in the frenzy of drilling I drilled the second hole   what an idiot, so two spare TRIODES. DO I hear OD circuit or extra gain stage/tremolo. thanks

[HotBluePlates]

"CLEAN" you say, might need a bit guidance to dirty it up a bit ...

Every Marshall amp I can think of has feedback around the output stage. Are Marshall amps clean only?

[TIMBO]

Thanks HBP, I'm getting better at putting these things together but I fail when it comes to how it all works. This build is like most of the others I do, a bit from here and a bit from there and a S**T load of luck. Thanks for your input, always appreciated. 

[TIMBO]

Hi guys, This one has fired up well, with nothing went bang.
At low volume (a bit late at night) sounds good.

With a standard 18w type PT is putting out 300v on the HT leads, this then after rect is 387v at node A. So at the plates of the EL84s (JJ) 379v. Again I have power tubes running way over there max limit, this is after adjusting the cathode resistor from 135r(2x270r in parallel) to 250r (100r and 150r in series) this giving 11w per tube.So i'll keep an eye on it. PT seems to be OK as well.

[TIMBO]

Hi guys, Had a bit of time to tinker. Ed had posted that Sunn amps are loud and clean. Well he was right, with only a volume control, I was only able to get it to 3 on the dial and with the ELs now bias at 10w each this thing sounded like 50w and my old speaker box was vibrating across the room and with only a hint of breakup.

I tried a PPIMV but it didn't seem to work  When it was on 1 it was already very loud and when turned up just sounded crappy.

Just a thought.... would adding another gain stage (CF) to V1a add more volume as well as extra gain????

AND adding a volume pot between the TS and V3b control the volume????


Any other thoughts on controlling the volume. Thanks

[kagliostro]

I've just find this in my archive

may be there is a solution for your problem

---

About the use of a CF, the CF has a gain of 1 max but acts as an impedance adapter

so if you insert it between V1b and the TS the response will be better

---

I'll give a try to the Plate and Cathode split resistor may be it is what you need

Ciao

K

[sluckey]

One of the simplest things to do would be put a 470K resistor between the top side of your volume control and that .01µF coupling cap. This creates a voltage divider and knocks down the signal reaching the volume control by about 50%.

I don't understand why the PPIMV didn't work.

[Ed_Chambley]

Hi guys, Had a bit of time to tinker. Ed had posted that Sunn amps are loud and clean. Well he was right, with only a volume control, I was only able to get it to 3 on the dial and with the ELs now bias at 10w each this thing sounded like 50w and my old speaker box was vibrating across the room and with only a hint of breakup.

I tried a PPIMV but it didn't seem to work  When it was on 1 it was already very loud and when turned up just sounded crappy.

Timbo, what value dual pot did you use for the PPMIV?  I have a similar setup (not exactly the same) using dual 250k that works well.  Mine has EL34's and v1 is bypassed a little differently, but the circuit is very similar.  Did you remove the 220k resistors when you installed the PPMIV?

[TIMBO]

Thanks ED, PPIMV is working well, it's just a little fuzzy at low level,1-2 on the dial.

Got a mate to give it a good thrashing and its a loud mean machine. Its at its best with dials at 9 giving a great crunch and overdrive. At 4-5 does clean very well at high volume.
BUT there is no best of both worlds when you lighten the attack on the strings the touch response is just not there.

The same guy also gave my TOS a workout with great success, in clean channel and kicking in the overdrive he was able to lighten the attack and have a sparkly clean and up the volume on the guitar and BAM full on crunch without touching the amp.

It was nice to finally get a treat when you put a REAL musio in front of your amp 

SO, what to do with this amp???

I have rebiased the ELs to 10w each but it sounds way louder than 20w but that could be because it was plugged into a 4x12 cab and even in a 2x12 it was the same.

Is this perceived loudness coming from the PENTODE as the preamp circuit doesn't appear to be much different to any other circuits.
Is my thinking right that the signal from the preamp is being amplified but not pushed into distortion by the pentode to make it sound louder and the only way to get it to distort the signal is to push the preamp with the volume pot at max this then loosing any touch response 

I like the different tonal qualities of the pentode and it does work well with the ELs, so to use it with a pair of 6L6s or 34s would be huge.

Is there a best of both worlds with this amp, touch sensitive and awesome crunch.Thanks

Just a few changes to the curcuit