12dw7 to 12ax7 conversion?

[m6tt]

Because a transformer was changed out, it seems reasonable to convert a 12dw7 socket to the more easily available 12ax7. The grid resistor on the au7-like half is 1MOhm. The resistor on the cathode is hard to read, but appears to be 650 ohm. The plate is directly coupled to a the transformer, and receives about 250v. It would be especially good to have a plate resistance around 20Kohm or so. What exactly do I need to do here...I've seen the resistance coupled tables, but I am not sure if I am reading them right.
Also, there is a stage nearby which has a capacitor coupling plate and cathode. What is going on there? is it an oscillator of some kind? On that stage (a 12ax7) the grid is grounded through 1Mohm, the plate is connected via a long wire to a resistor to which the cathode is also connected. The cathode is also connected to ground by a 1.2k gold tolerance resistor, in addition to connection to the cathode by the resistor and capacitor. The resistor which connects the two is also connected to a large cap which through a variety of capacitors and resistors connects back to the grid. It seems like a big loop to me and doesn't make much sense.
Thanks for the help if you can, I can't make anything close to head or tail of it.
Is it a vibrato?

[Dave Cigna]

There's no way anyone here can make sense of what you have from your description alone. You need a schematic of the amp. There are a lot of schemes at http://www.freeinfosociety.com/elect...page.php?cat=1. If it's not there, then try googling. If you still can't find it, then draw up your own. Yes, really! Get some paper and a pencil and some good light and start drawing. Redraw it in a way that makes sense.

Seeing the schematic might answer all of your questions. If not, you can present it here and someone will be better able to help.


Then you can start thinking about converting to a different tube type.

-- Dave

[m6tt]

Unfortunately there are no schematics that I can find for this amplifier, a hammond A0-48 power & reverb amp (not the transformer company, the organ company). The device was designed to do quite a bit, and near to the end of the tube era as well. It is not cleanly laid out _at all_ inside. I have been trying for a while to sketch the whole thing out, but I have only had limited success...it is point to point wired which makes for much rewriting and confusion indeed. I have had some success with individual stages, but there are a lot of fancy tricks going on here that are quite a bit more than what might go on in a simpler amplifier. Additionally, unlike say a fender amp or something, quite a lot of it is outboard...a _lot_ of plastic and RCA connectors that make it more confusing as well. I know what those do, but it does make it much harder to draw out, because everything is bent head over tail to get to the connectors instead of just stage to stage. I can tell what is going on in most of it, but there are some parts, like the second part of my original post, which seem to make no sense at all.

[gingertube]

The 12DW7 is 1/2 of a 12AX7 and 1/2 of a 12AU7 in one envelope.

Ampeg used them a lot in their guitar amps. The 12AX7 'ish triode was used as a gain stage and the 12AU7 'ish triode was used as a buffer or driver. If a 12AX7 could have done the job it would have been the preferred device in the original design.

12DW7 are in current production and should not bee hard to find. It is unlikely that you will be able to change the circuit to suit a 12AX7 without accepting some compromises. I'd leave it the way it is.

Cheers,
Ian

[m6tt]

I suppose that may be the case. The au7 half was being used as a SE triode output stage that drove (through a transformer) a 8 ohm Gibbs reverb tank. The amp works and sounds OK with a 12ax7, but it definitely doesn't sound the same. The 12dw7 which was in there originally is still good, but it makes terrible swishy sounds on start up. Additionally, the transformer that it drove was changed. It is probably a largely irreplaceable part. As such I am sure impedence issues will arise, so I figured switching it to a 12ax7 with its higher plate resistance would be wise. The transformer I replaced it with was a SE output transformer for a 50c5 radio. I believe it was 10k ohms --> 4 ohms, i.e., 20k ohms into 8. That is an easier to acheive plate resistance with a 12ax7 than an au7.