Parafeeding a Tetrode

[dsavitsk]

I am working on a little 12V guitar amp with a 12k5 as the output tube. An example of one of these can be seen at http://www.sophtamps.ca/schematics/sopht_12k5-v6.jpg

The problem is that the only output transformer I have around for this cannot have any DC on it. So, I was thinking about adding a plate load resistor and a cap to couple the transformer. The questions is, will this work, and is it a reasonable solution?

On the attached picture, the left side is the way I've seen this tube used, and the right is what I am thinking about doing.

[oshifis]

I don't think is would work at such low voltage. You should provide sufficient bias current that mandates a relatively low value of R, but then it shunts the transformer in AC point of view.

I would insert an air gap in the transformer. The lower cutoff frequency will rise then, but guitars don't go down to 20 Hz anyway, are they?

[Jeb-D.]

dsavitsk,
If you want to run parafeed off the 12V you should choke-load the plate. If you do go the resistor route you will have to raise the B+ so that the Plate operates at 12V+, not B+ 12V. Remember a resistive load can't swing above B+ as an inductive load can. Also, it has a much higher DCR than a choke or transformer winding. You will loose much of the available output power too, unless you make B+ large enough to use a plate resistor value that is 10x the value of the transformers primary impedance.

I'd advise against a CCS in case that thought was brewing. The Tetrode acts as a CCS too, so they will fight with each other. You can add a shunt resistor from plate to ground to allow a CCS, but that approch would be janky IMO.

You could hit up edcor for a custom transformer. They say their engineering charge is only $20. And I can't imagine them charging any more than $17 for the transformer itself for that application.

[dsavitsk]

I'm not really worried about output power -- I need about 3 to 4mW here. Also, the transformer in question is a $3 1K:8 Radioshack thing that I was hoping to get away with. I did try to add the gap, but they seem to have epoxied the thing shut.

This really is a cheap and goofy project. But, what it sounds like is that while I am going to lose lots of power and while a larger B+ would be ideal, that this will "work" in a sense. That is, this is not an incorrect way to hook up the tube and the transformer, just a fudging of proper operating points. Does that seem right?

[tubelab.com]

Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.

[Cycline3]

Originally posted by tubelab.com
Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.

I dunno.. that transformer he's got is small - coin size small. I've seen it. On the other hand, if it dies... it's only a few bucks.

Unless size is a real issue, Parts Express has a $4 transformer that handles 10W and I've built a couple SE guitar amps with 6L6s and it works great for that. Maybe it would work here...

70V Line Transformer for tube output

From the reviews of it:
Ron Banks from LaPorte IN
I keep several of these on hand for old tube radio restoration. The .62 watt tap is approximately 8000 ohms, the 1.25 watt tap is about 4000 ohms and so on. These are well made and have more iron than most of the transformers in the old sets. They don't look out of place on the chassis and allow you to use most any speaker. Using the 1.25 watt as the center tap they work in push pull tube amps as well.

[dsavitsk]

Originally posted by tubelab.com
Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.

I'll give it a try and see what happens. Worst possibility is I am out $3.

Originally posted by Cycline3


I dunno.. that transformer he's got is small - coin size small. I've seen it. On the other hand, if it dies... it's only a few bucks.

Unless size is a real issue, Parts Express has a $4 transformer that handles 10W and I've built a couple SE guitar amps with 6L6s and it works great for that. Maybe it would work here...

I've got a couple of 70V line transformers around, so I'll try one of those too. Good suggestion.

[dsavitsk]

Originally posted by tubelab.com
Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.

Well, it seems to work fine. Well, even. In fact, it sounds better than the much larger 70V line matchers I have. Who'd a thunk? It is a little *splatty* in the bass, but overall it makes a nice late night guitar practice amp.

[Jeb-D.]

Well, it seems to work fine. Well, even.

That's cool. Do you have a driver stage or are you driving the 12k5 straight from the guitar?

[dsavitsk]

I used a 12U7. Schematic is basically the same as http://www.thomasdog.com/amps/sopht/ except I used 1M pot as grid leaks between each stage so I had a pre and post control, and I added a brightness switch to switch between a 4.7u and a .01u cathode bypass on the first stage. Also, the wall wart I used is unregulated, so I built a CRCRC filter (1000u, 4.7R, 2200u, 4.7r, 2200u) to get rid of the hum.