Help with early 50s Danalectro Challenger

[Bluefinger]

Hi, I have a "Danelectro Challenger Series S" amp that is not working (like this one: http://kilback.net/pics/Image2.jpg). It seems like there are no schematics available but this thing is different from anything I have ever seen. The volume pot and tone stack are before the first gain stage. Also the filaments of V1 are fed with ~5VDC across pins 4 and 5. The second tube, which seems to be the phase inverter is some kind of octal tube with no visible model name or such. I t seems like the problem is located somewhere around V1 but since I have no experience with such a configuration I don't know where to start. All I have done so far is to replace the old caps and to clean and test the pots. The amp has been tinkered with so I don't know if the wiring is correct at all. Are there similar amps out there with schematics so that I can start somewhere?

ans ideas? ... I'm lost

thanks!

[MWJB]

Hi Bluefinger,

4x6V6?

The amp strikes me as similar to the Commando (GDR8518A & broadly similar to GDR8516A, GDR86517A), with the exception of the tone & volumes being before the first tube (it is possible that the tone circuits in terms of values are identical with the tube driven models, but have a look). Also the Dano JDR8421 accordion amp may have some similarities after V1.

The dc accross the V1 filaments is typical of Red Bank NJ Danos (& subsequently Wards, Silvertone amps made by them). Being a 4x6V6 amp and only V1 has the dc on the filaments (?) I'd look for another resistor between the power tube cathodes and the preamp filaments, as these are used to bias the output tubes, rather than the traditional resistor to ground. Look for ~20vdc on the power tube cathodes?

V2 is probably just a gain stage, twin triode, could be 12SN7 or 6SN7. V1 (12AX7?) is probably a cathodyne PI?

If V1 has failed then the output won't work.

[Bluefinger]

Thanks,

The 12AX7 itself is ok. I tried it in another amp. I never even considered V1 to be PI ... that opens up a completely new point of view. This thing is so cluttered, it makes me dizzy ...

I think the power section is ok too because if I tap on the plates of V1 when reading voltages it produces a pretty loud crackle. Taping on the grids doesn't produce any crackle which seems strange to me. Plate voltages on V1 are around 150 Volts which seems pretty low if this really is the PI ...

I'll check teh power tube cathodes and keep searching ... thanks!

[MWJB]

Any pics?

Is there a big difference between voltages on V1 pin 1 & pin 6?

I haven't seen the exact amp you have, just firing off some thoughts based on the ones I have seen.

Have you tried injecting a signal direct to the 12AX7 grids (be aware it may be wired backwards, pin 7 receiving the signal from the instrument)? Just to eliminate any open coupling caps in the tone circuits.

[Bluefinger]

I can take a pic tonight but it's so cluttered, you will probably not be able to make out anything. But who knows ...

I already replaced all the coupling caps ... that didn't change anything.

[Jazz P Bass]

What would be in the best interest would be to actually sketch out the socket wiring.
Heater pins at the very least.

[Bluefinger]

It took me a while to get back to work on that amp but I have finally taken some readings. filaments read 4.8vac (except V1 which has DC across the filaments). power tube cathodes read 7.8 vdc. These readings seem a bit low, don't they?

Here's teh strange thing: After replacing all the caps in the preamp section I finally do get a sound but the volume is VERY low. If I switch on the tremolo the sound gradually gets quite a bit louder although I don't think that this is the full volume the amp is capable of. If I switch the tremolo off, the volume slowly drops back to the weak signal like it was before. Another thing is that when I switch it off, the volume does not drop slowly until it'S quiet like on any tube amp when the filter caps are drained. It acts like a transistor amp ... if you switch it off, the sound is gone in the same moment. Would that be an indication for a certain problem.



ANother thing: There is an exploded cap that used to go from ground to one phase of the power supply (right at the fuse). The amp is grounded. Do I need that cap ... isn't that the "death cap" that I would need on an amp that isn't grounded?

thanks everybody!!!!!

[MWJB]

7.8vdc is too low for the 6V6 cathodes.

You don't need the death cap on a grounded amp.

Trace the wire(s) from the 6V6 cathodes, does it run straight to the 12AX7 filaments, or does it have a resistor in series and/or another resistor in parallel?

Check the 6##7 plate resistors, Danelectro often put low mu tubes in here and they can eat plate resistors over many years of use.

I still think a pic of the circuit would be useful. Especially the 12AX7 wiring, if someone has already been inside this amp, with no experience of the cathode/heater arrangement, they could have "fixed" something and caused an issue.

Heater sound low-ish...I don't know if I would expect a full 6VAC+ on this amp.

How about some dc readings from the tube sockets?

[Bluefinger]

I just found something different. After replacing the rectifyer I do get decent volume out of the amp and voltages went up a bit as well. Still plate voltages around 200 Volts seem pretty low to me. Is that normal for these amps?

[MWJB]

I'd be expecting over 300vdc, here in the UK I'd reckon on 350-370vdc with 240VAC at the wall. What do you have on the 6V6 cathodes now?

[Bluefinger]

I'll take some more readings. Unfortunately I don't get to work on that amp very often at the moment.

I get about 240VDC on the pins of a known good 5Y3 and roughly the same in AC on the other end of the rectifyer ... these are the only things I can remember off the top of my head. I was so excited about the working amp that I forgot to take any further readings

edit: BTW ... it'S a US model combined with a step down transformer which delivers exactly 115VAC where I have it connected now.

[MWJB]

OK, that doesn't seem unfeasible, some 6V6 models ran around 270vdc at 117VAC from the wall.

[Bluefinger]

ok, thanks. The amp sounds great if you dig late 40s and early 50s chicago blues records. It'S just not as loud as you would expect from a 4x6V6 amp. But at 250V that's not very surprising I guess

[MWJB]

It's low voltage, pretty low gain too I'd guess (if 1x12AX7, half of which is PI with no gain, followed by a low mu stage?).

Maybe see how your harp player likes it? ;-)

I'd still double check that cathode string, sounds like it is still idling hotter than I'd expect.