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Old 6th January 2008, 05:49 PM   #1
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Default AB2 guitar amp design questions

Hi,

I have posted this question in the thread 'how does class ab2 sound' but it probably wasnt the best place to post it.

I have designed a guitar amp using 2x 807's in AB2 push pull, they produce around 75W in this configuration and require a load of 4k2 - 5kohms.
My first question is are the 2 output transformers i am looking at good choices (price is a factor)?
they are 1650R and 1650N
Here is a link to the specs:
http://www.hammondmfg.com/1608.htm

Also i have 2 x 12ax7 gain stages followed by a 12ax7 phase inverter (LTP type) so in total only 2 preamp valves, i have calculated more than enough gain with them using 120k anode loads and 1k5 cathode bias resistors to produce the required 100v peak swing from 10mV peak input.
(the buffering after the LTP is via IRF820 mosfets to drive the 807's)
But all guitar amp schematics i have seen of this power use another 12ax7 in the gain stage, am i missing somthing? will i be alright using 2 preamp tubes?

Cheers
Craig
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Old 6th January 2008, 07:42 PM   #2
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A 12AX7 isn't up to the task of dealing with grid current in AB2. It'll sound awful.
http://www.retrovox.com.au/STC807.pdf (5.5M) has an AB2 design at the end.
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Old 6th January 2008, 11:58 PM   #3
PRNDL is offline PRNDL  United States
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I'm not an expert at amp design, but here's some of what I've learned.

12AT7 might be a better choice for the LTP, since it has higher current ratings. I believe it is equivalent to the 6SN7 (and 12AX7 = 6SL7), although the octals are usually more linear, which is why they're common in HiFi.

The preamp is designed to present the grid voltage the output tubes want to see (usually a lot more in a guitar amp where output tube distortion is desirable). For 6V6 that is about 25 volts.

The least number of preamp gain stages I've seen in a guitar amp is one (plus the LTP gain) in a Matchless Spitfire or Vox AC15. Modern guitar amps use more gain stages along with master gain in order to get preamp distortion.

I've been experimenting with two gain stages (i.e. one 12AX7 preamp into a LTP, or three preamp gains into a cathodyne), which appears to be rare in guitar amps. Most have three, which I believe is to compensate for gain lost in reverb.

Here's a nice example of a simple 807 amp
http://www.tubes.mynetcologne.de/roe...ematics_e.html
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Old 7th January 2008, 04:28 AM   #4
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Default Re: AB2 guitar amp design questions

Quote:
Originally posted by Craig405
My first question is are the 2 output transformers i am looking at good choices (price is a factor)?
they are 1650R and 1650N
In a case like this, I'd go with the 1650R. The 1650N is rated at 60W, and your design will easily exceed this. From the Hammonds I've used, they won't go much over the rating. Trying to force too many watts through the OPT will cause core saturation, and that's horrible-sounding, even in a guitar amp.

Quote:
Also i have 2 x 12ax7 gain stages followed by a 12ax7 phase inverter (LTP type) so in total only 2 preamp valves, i have calculated more than enough gain with them using 120k anode loads and 1k5 cathode bias resistors to produce the required 100v peak swing from 10mV peak input.
(the buffering after the LTP is via IRF820 mosfets to drive the 807's)
But all guitar amp schematics i have seen of this power use another 12ax7 in the gain stage, am i missing somthing? will i be alright using 2 preamp tubes?
You got a passive tone stack in there? That "extra" 12AX7 in most guitar amps is used for the tone stack.
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Old 7th January 2008, 05:06 AM   #5
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Quote:
A 12AX7 isn't up to the task of dealing with grid current in AB2.
Brett, it doesn't have to. As Craig says, the buffering after the LTP is via IRF820 mosfets to drive the 807's. This arrangement is fine - any tube could precede those MOSFET drivers.

I don't know whether to agree with Miles or not. After all, this is a guitar amp and an OPT designed for 60W might do the job very well (unless the amp is meant for a bass guitar, I suppose). Both OPTs have the same max. primary current rating but the advantage of the 1650N is that you save 4lb in weight. You also get a lower p-p loading with the 1650N, which also needs to be considered.
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Old 7th January 2008, 05:23 AM   #6
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Quote:
Originally posted by ray_moth

Brett, it doesn't have to. As Craig says, the buffering after the LTP is via IRF820 mosfets to drive the 807's. This arrangement is fine - any tube could precede those MOSFET drivers.
Missed that, must'nt have been awake yet.

The IRF's are still going to have a goodly chunk of gate capacitance, even as SF's. Still not sure about the AX7.
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Old 7th January 2008, 07:11 AM   #7
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Hi,

Unless you're dead set on the Hammonds, here's some good bang for your buck:
http://www.edcorusa.com/products/tra...xpp/index.html

(scroll to the 100W ones)


Cheers!
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Old 7th January 2008, 08:01 AM   #8
Colt45 is offline Colt45  Serbia
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I seem to recall Fred putting 200W through a single 1650R with no problems. (6x 807, AB2)

http://www.dogstar.dantimax.dk/tubestuf/dzart-4.htm
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Old 7th January 2008, 11:49 AM   #9
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Firstly thanks for replies all!

I checked out the Edcor page, transformers look fine i would definatly consider if i was in the U.S. but hammond are more convenient and probably cheaper on shipping within the UK.

The 1650R is looking more attractive, the load is slightly higher so i will not be getting max power transfer but is this likely to have a negative effect other than that?
Plus the transformers are the same price, how wierd.. a better watt/pound ratio cant be bad!

I am totally open to better transformer manufacturers in the UK, so please suggest if anyone knows of them.
Sowter are too costly for me though

Miles Prower, thats gotta be it, i dont have a tone stack at moment so i guess im safe until i decide to add one in.
Im hoping the amp will sound so fantastic i wont need to shape tone (HA! i wish).

edit.
That 200w 807 beast is cool, though wouldnt the 6SN7 have too high a Zo for 3 807's in ab2? good to see the hammond is taking a pummeling though.
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Old 7th January 2008, 11:58 AM   #10
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Oh one more thing, if i order OPT from hammond i need filament transformers too (6.3v and 5v).
Im going dc regulated on the 6.3V to see how quite a guitar amp can be.
Am i better off ordering hammonds EI core filament transformers or ordering some standard toroids from RS?
Decisions decisions.
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