Pentode AB1 or AB2?

[Fuling]

Pentode ABi or AB2?

Merry X-mas!

Ok, here´s the story:

I have a 1,9kohm 120W PP OTP, a 500V B+ PSU and a bunch of 807´s that someday is supposed to be a bass guitar amp.
I want to squeeze as much power as possible out of the tubes.

Do I have to push them into AB2 operation?
I´ve read that AB2 is to no use when it comes to pentodes (they already get close to zeor so to speak), but on the other hand I´ve seen schematics of PP 807 AB2 amps...

Any advices?

BTW: Different datasheets gives different information...

 

[SY]

Haven't tried it with 807s, but in general, the stuff you do to the driver stage to supply the very nonlinear load of the output tube grid in AB2 is stuff that helps make the amp work better even before the onset of grid current (e.g., low source Z with plenty of drive current capability). I like AB2 with the right tubes, others are unhappy with the tradeoffs.

 

[Fuling]

Thanks for the reply, though I don´t know if I got much wiser
I agree with you that beefy driver stages always is a good thing, but the main question remains: Does AB2 operation "work" with pentodes?

I´d be happy if I could get past direct coupled cathode followers or IT coupling.
I´ve messed with grid current tubes before (811A in SE) and it wasn´t that fun...

 

[SY]

What I was clumsily saying is that there are often benefits to be had beyond slightly increased power. That said, the benefits of power accrue the most with tubes that are relatively low perveance.

 

[SHiFTY]

In the STC807 datasheet there is a schematic for a 75W AB2 amp, using direct coupled 6SN7 as drivers. You could ditch the preamp section and use another of your own design.

If you build it replace the 83 rectifier with 2 1N5408 silicon diodes and a TV damper diode in series for slow warm up. This amp needs a very stiff power supply apparently.

I have built an AB1 version using 807s and it sounds really good! I am tempted to try the AB2 but my OPTs are only rated at 35W so there isnt too much point.

Heres a link to the PDF: WARNING! 5.5 MB download!! More data than you can shake a stick at!! STC807.pdf

This guys site retrovox.com.au is pretty awesome, he has quite a few interesing datasheets etc.

Also do a google groups search there are quite a few posts about building AB2 807 amps....

MErry Christmas all BTW

 

[Fuling]

Big thanks fellows!
I´ll have a closer look at that STC datasheet tomorrow!

Merry Christmas!

 

[Sch3mat1c]

What I was clumsily saying is that there are often benefits to be had beyond slightly increased power. That said, the benefits of power accrue the most with tubes that are relatively low perveance.

No, class 2 gains a tremendous amount of power. Triodes benefit the most as evidenced by lazy transmitter tubes with so little current they run class B with no bias voltage. Pentodes can easily double or triple the peak current when overdriven; saturation voltage rises a bit so you can expect around double the power. This requires half the load impedance because the peak current reached by the tube is higher.

You might expect a quad of 807s at a pretty good voltage (like 500V plate, 300V screen) and good grid drive (use cathode followers; high Gm is an advantage for them) to fully satisfy your OPT. Otherwise, a sextet or octet in AB1 would do it.

The nice thing about tetrodes and pentodes is instead of increasing grid voltage positive, you can just increase screen voltage instead. They has the exact same effect on plate curves. Triodes you have to pick and choose and if the power isn't good enough, tough beans.

Tim

 

[audiousername]

This guys site retrovox.com.au is pretty awesome, he has quite a few interesing datasheets etc.

Not sure how on-topic this is, but since Retrovox came up...

I believe the site is run by David Crittle, from Wagga Wagga, and posts on the Joe List (Sound Practices Mailing List) occasionally. He has all sorts of wonderful things in the catalog, mainly unknown valves that no one wants but could be quite useful, like A2293/CV4079 (a 15W high gm triode - hmm, might build an OTL with lots of these one day.....), yet has 212Es and the like... but it would be a great help to everyone if he got it organised in some fashion!

There are also quite a few detailed application reports for valves (mainly from STC Sydney).

No association, just making people aware of a great resource.

Originally posted by Fuling
Big thanks fellows!
I´ll have a closer look at that STC datasheet tomorrow!

Reading valve application reports on Christmas Day... lol... (Well I shouldn't say much, given that I'm posting on DiyAudio late on Christmas Eve....)

I would have had other things to say, but check out that report first. It's quite detailed. 807s and their derivatives/equivalents are a strangely unpopular output valve. Nice to see someone using them!

Merry Christmas everyone

 

[EC8010]

AB2: Tha anode of a transformer coupled output valve sits at HT voltage with no signal. For linear operation, it can swing to double the HT voltage or zero volts. Making a triode's anode swing to 0V is difficult; the Vgk = 0V anode curve is usually a long way away from 0V. Forcing the anode to swing to 0V requires that the control grid be driven positive - AB2. Because the anode now swings further, AB2 extracts significantly increased power from a triode. Conversely, a pentode can already swing its anode close to 0V, so AB2 doesn't help much.

Driving the control grid positive imposes a heavy load on the driver circuitry, so the driver has to be beefed up. A beefed up driver is more easily capable of driving Miller capacitance etc linearly. Of course, it could be argued that you don't want linearity in a guitar amplifier...

 

[audiousername]

Conversely, a pentode can already swing its anode close to 0V, so AB2 doesn't help much.

It doesn't help it swing closer to 0V much, but *2 operation does allow greater current swing. Of course, with pentodes you could just increase screen voltage to do that, so it probably isn't as helpful as it sounds

 

[Fuling]

Ah, it´s amazing to see that all of you guys spend time in front of the computers debating tube operations instead of celebrating Christmas Big thanks!

Well, I´ve had a look at the schematic in the STC datasheets and I must say that I have some doubts about the cathode follower arrangement. 1/2 6SN7 per output tube seems a bit weak, I´d rather have something a little beefier.

What about a transformer coupled driver stage?
I have some old output transformers intended for EL81 PP amps with 10-25-50-100V output taps, maybe one of them could be used as an IT? Since the secondary winding is specified as voltages and not impedances I assume that the 50V tap is a center tap?
I guess I´ll have to run some measurements on one of them to verify this.

 

[SY]

Fuling: Nothing says "Christmas" more than a bank of cheerfully glowing tubes! Transformer coupling is a very common way of doing AB2; the great Fisher 50A is a perfect example.

No, class 2 gains a tremendous amount of power.

Only if the tube has low perveance. As EC8010 points out, a high perveance tube will swing the plate close to zero volts anyway, so the power pickup is not large.

 

[EC8010]

Ah, its amazing to see that all of you guys spend time in front of the computers debating tube operations instead of celebrating Christmas

I'll have you know I've just been out doing my Christmas shopping! Tomorrow, I will doubtless be wielding a screwdriver to assemble a "batteries not included" - and it's unlikely to be a piece of real electronics with battery grid bias (either AB1 or AB2)!

Merry Christmas.

 

[Fuling]

Nothing says "Christmas" more than a bank of cheerfully glowing tubes!

True!

I´ll do a search for Fisher 50A out of curiousity.

Thanks!

 

[EC8010]

Nothing says "Christmas" more than a bank of cheerfully glowing tubes!

Now there's an idea. Christmas tree lights using a series string of ECC82 somewhat overrun to make them glow brighter. Or maybe a load of 85A2?

 

[Fuling]

Or maybe a load of 85A2?

Hehe, why not a bunch of 811A or 813 or something that really glows??

 

[Brett]

Transformer coupling is a very common way of doing AB2; the great Fisher 50A is a perfect example.

Anyone have a schematic of this amp they'd like to share?

 

[Fuling]

Anyone have a schematic of this amp they'd like to share?

I searched for it but all I found was a place that used to sell schematics but does no longer... Sad.

 

[SY]

Now there's an idea. Christmas tree lights using a series string of ECC82 somewhat overrun to make them glow brighter.

That's the best use for these tubes.

 

[nanana]

i'm not sure where you heard that "pentodes are no good for AB2", but i think this is just ****... the 807 (which is a beam tetrode anyway) was designed for it. look at the data sheet. the previous comment about the driver stage is the main key to a successful amp design in AB2. a class AB2 amp is mostly all about the driver stage... it does help if the output tube you are working with is optimized for positive grid current. but the 807 is, most definitely! beam tubes can be particularly optimized for this kind of use because the cathode can be made for short fast current dumping (this means a coated cathode and not metal), and the screen grid can be moved closer to the control grid in order to get the velocity up (the dynamic impedance down), the control grid will be heavy and plated with gold or orther work force retarding metals (platinum, iron, etc,). tubes that are made this way typically have a big difference between plate and screen voltage ratings... like the 807 (600V Ep/400V Es), 829 (which is optimized for AB1, AB2, B, and C), and most transmitting tetrodes.
jc