12AX7 vs 12AU7 Plate Voltages.

[Structo]

While working on my Hammond Deluxe conversion amp I noticed something.
I was having severe oscillation problems when there was a 12AX7 tube in V2, the phase inverter.

So I stuck a 12AU7 in there and that cured it for the most part.

But something I noticed when checking voltages is that when the 12AX7 was there I measured 195v on the plates.

With the 12AU7 in there it drops to 125v.

Is this because of the increased current draw of the 12AU7?

Pretty strange that it would drop 70 volts.

I haven't used the 12AU7 before so I was wondering if this is a common thing.
I tried several tubes and they all did this.

One was a JJ ECC 832 which was weird because it had uneven plate voltages.
But then I found out it is half 12AX7 and half 12AU7.

[Phil_S]

The 12AU7 is going to draw as much as 10x current compared to the 12AX7. Maybe 10mA vs 1mA in real numbers. This will have an impact on plate supply. Preamp tubes are biased by the combination of the plate load resistor and the cathode resistor, so I am not at all surprised. The other factor at work here is that the plate resistance of the 12AU7 is probably 8-10% of the 12AX7.

Since the tube is self biased, when current draw goes up, you should expect plate voltages to go down. I'm curious, did you compare cathode voltage. I'd guess the 12AU7 has 3x the cathode voltage.

If using a lower gain tube is what you want to do, more appropriate subs for the 12AX7 are 5751 and 12AY7. Both of these have much higher plate resistance than 12AU7 or 12AT7 and are likely to work better in the circuit.

When you put a low gain low plate resistance tube in the PI, you are going to limit the voltage swing when the amp is in operation. This is probably why it cures the oscillation, but it will also suck tone. If you are not able to find the source and need to take this route, get a 5751 and see if it does the job.

[Structo]

Thanks Phil.

Yeah, I just started plugging things in that I had laying around to see if it had any effect.
The 12AU7 was the only lower power tube I had other than a JJ 832 which is half 12ax7 and half 12au7. Weird little tube there.

This chassis is proving to be not very conducive for good lead dress.
The circuit board is pretty much right on top of the sockets.
The preamp tubes may be too close to one another aside from the crappy lead dress.

I may end up just junking this one and start a new Deluxe in a more conventional size.

Here's what I'm working with.

[IMG:800:600]http://i23.photobucket.com/albums/b392/ ... G_0016.jpg[/img]

And the schematic.

[Phil_S]

Build it on one of these:
http://www.apexjr.com/images/tagboard.JPG
These boards measure 2.25" x12". They can be cut with a snips or hack saw. You want to be very careful not to breathe any of the dust, as fibers that enter the lungs never leave, which is why I use snips.

You will have a few layout mods to do, but then it will fit very nicely into what you've already done. When you have some room, you'll be better able to manage the lead dress issues.

I have placed tube close together and it hasn't been an issue. Look at my latest wild hair project: http://home.comcast.net/~psymonds/6SN7amp.htm and you'll see the distance between the power tubes is about 1/2".

[Structo]

I've tried without much luck trying to find the proper resistor values for the long tailed pair when using the 12AU7.

Some of the reading led me to believe the plate resistors should be quite a bit smaller, in the 33K range and that tail should also be smaller.

Can somebody give me some insight on this or perhaps somewhere to look.
I checked out Lord Valve and the Geofex site.

[Phil_S]

If you are having oscillation problems with the PI feeding the power tubes, you can probably raise the grid stoppers (swampers) to 5K without any penalty in tone. Give it a try. It's quick, cheap, and easy.

[Structo]

Thanks I'll try that.

BTW, what did your donor amp start out as?

Is the printed circuit board there just to cover the hole?


Is your tone stack like the one in the schematic?

If it is, how do you like it?

I may try to implement that into my amp.

[Phil_S]

BTW, what did your donor amp start out as?

I have no idea. I bought the chassis stripped on eBay. The PT and OT come from an Arkay FL10, a 2x 6V6 amp. This was the third PT I tried and the second OT. I was afraid to "waste" this pair, but really, they were just right. The OT has a 9K primary. I think, ideally, a pair of 6SN7 wired in parallel PP like I did, would want to see 10K to 11K, so this was pretty close. Triodes will manage a range of primary impedance OK.

Is the printed circuit board there just to cover the hole?

Yes, it is just scrap to cover holes in the chassis.

Is your tone stack like the one in the schematic?

Yes, it is the classic Fender tone stack. I think you'll find it on many Fender designs. I drew the schematic and there are no intentional deviations. I think it is accurate.

If it is, how do you like it?

This amp is exactly what I had imagined. Volume is not overpowering. It has almost 100% clean headroom. For overdrive and ear splitting volume, I've got other amps. I started out with a different PI, but realized it was just too much gain/voltage swing for the triode finals.

[Phil_S]

I've tried without much luck trying to find the proper resistor values for the long tailed pair when using the 12AU7.

Some of the reading led me to believe the plate resistors should be quite a bit smaller, in the 33K range and that tail should also be smaller.

Can somebody give me some insight on this or perhaps somewhere to look.
I checked out Lord Valve and the Geofex site.

I'm out of my depth here. I think you'd want to cut them in half, roughly, and double the cathode, but I'm not sure. See if you can troll through some of the Fender schematics. I think you may find one that uses a 12AT7 for the PI. That would be a reasonable guide. The AT7 and the AU7 share many characteristics. It will change the character of the amp significantly.

What is it you want from this amp? Knowing the goal would help. Right now this is all in a vacuum.

[Structo]

Well I started out wanting a Tweed Deluxe but I really didn't want the dual inputs.
I stumbled onto this schematic when I was researching Hammond organ amp conversions.

Right now I would just like to get it stable.

It is for the most part.
I changed the swampers to 2.2K.
Amp seems more compressed and saggy at large volume.
It also wanted to motorboat at a certain point on the volume pot, at around 4 o clock.

I still can't figure out why the volume control operates the way it is.
There is less than 1/8 travel before it goes from pretty clean to totally distorted. Then from there it really doesn't get much louder, just more wooly sounding.

So far the best tube layout is a 12ax7 in V1 and a 12au7 in V2.
I wish I had a 12at7 but I don't.
If I put a 12ax7 in V2 then it squeals if the tone control is dimed.
I've shortened the grid wires as much as I can and placed the leads where they are not interfering but it is so cramped everything may be just too close.

I put a 47pf bright cap on the volume control and that seemed to help the tone a little bit.

[Markusv]

[quote="Phil_S"]Build it on one of these:
http://www.apexjr.com/images/tagboard.JPG
quote]

I love these boards

Have buillt 3 amps on them now
Not as pretty, no, and thinner etc but the bendable tabs will do a lot for you and components are easily inserted. They are plenty strong just not as rigid as Garolite

My 2 cents

Markus V

[Phil_S]

Tom,
Please take what I say with a grain of salt, as I am an amateur with no engineering background. I wasn't paying close attention before because I didn't have any idea what you were trying to accomplish. One thing that amp is doing is hitting the finals pretty hard. That's why the 12AU7 calms it down. You've to to very 2 pushy gain stages going into a LTP PI, which acts like a third gain stage.

V3 is a spot where I think you can make great use of that tube you've got that's half AU7 and half AX7. It will take a bit of rework. Build the 12AU7 side as the third gain stage [1]. This will add a little something, but not a killer amount. Build the 12AX7 side into a split load (cathodyne, concertina) PI. Look at the 5E3 Deluxe for an example. The concertina has zero gain and simply splits the signal.

[1]12AU7 gain stage, just guessing here....01 or 02 coupling cap to grid, plate load R between 47K and 68K, and cathode R between aobut 820 and 1500 ohms. Use lower Rk with lower Ra and higher Rk with higher Ra.

People will argue the limited voltage swing from the concertina won't pump your power tubes real hard, but you might experiment with it to see if the three stages in the preamp give the signal enough gain, you might just be OK. You know, folks like the 5E3 and don't seem to complain about its lack of tone, so that concertina PI must be good for something.

Just a thought. Good luck.

[Structo]

Hi Phil,

Thanks for looking into that for me. Yeah I understand the way the front end is on that schematic is pushing the tubes pretty hard.

Not sure you understand though, there are only two preamp tubes.

The schematic shows V1a and V1b, the the PI (V2), then the 6V6's (V3 & V4).

I probably should check into the concertina PI though because although the amps sounds pretty good, it sounds more like a plexi than a Deluxe at this point, which isn't necessarily bad but I want more range on the volume control.
As it is now, from zero there is barely an 1/8 turn from clean to mean.
As you go higher on the volume pot it gets pretty muddy, not louder.

Right now I have the ECC832 in the PI spot and that keeps the squeal away unless I dime the tone pot.

It is currently wired like a Deluxe tone control.

I wish I could figure out the squeal when the tone control is dimed.
If I back it off just a touch, it doesn't. I don't get that.

[Wayne]

<dumb question> It's not a linear volume pot, is it? </dumb question>

W

[Structo]

Not dumb at all, one of the first things I checked, it's audio taper.