Dumbleator tube?

[drz400]

I'm assuming by that 10K load resistor the Dumbleator used a 12AT7 and not an AX7 or maybe a 12AU7, anyone know???

[Bob-I]

I'm assuming by that 10K load resistor the Dumbleator used a 12AT7 and not an AX7 or maybe a 12AU7, anyone know???

I tried a 12AX7 and 12AT7. The 12AT7 sounded much better. I don't know what Dumble used.

[drz400]

I'm assuming by that 10K load resistor the Dumbleator used a 12AT7 and not an AX7 or maybe a 12AU7, anyone know???

I tried a 12AX7 and 12AT7. The 12AT7 sounded much better. I don't know what Dumble used.

Judging by the load resistor I would think it has to be a 12AU7 or 12AT7 since otherwise it would have been more like a 120K load resistor if it is around 300V supply. Plus the fact Alexander would have picked a tube with better drive than an AX7, but thanks! I guess there are not too many of them floating around

[Fischerman]

Are you refering to the 10K resistor at the cathode of the cathode follower?

I had someone build an amp for me (not a Dumble clone) and he originally put a Dumbleator loop in it. I had major headroom problems and it just sounded crappy with a 12AX7 (I didn't try any other tube types). I ended up connecting the 1M grid resistor to ground and connecting a 2M2 resistor to B+ (fixed-bias)...and then using a split-load on the cathode (signal taken at the junction of the voltage divider). IIRC, I used an 82K/22K divider and used a 100K Send Level pot. I left the Return just like a Dumbleator. This worked really well in that amp. I found out later that it's actually very close to the Send circuit in a Mesa Dual Rectifier.

Also, I tried a 12DW7 in the FX loop socket and it adds a little bit of clarity (I'll admit...it might be me hearing what I want to hear ). With a 12DW7...the Return gain stage is still a 12AX7 type triode but now the cathode follower Send triode is a 12AU7 type. Since the CF doesn't add any gain you don't lose any gain by going to the 12AU7 but the lower output impedance (I think?) seems to help. But I don't expect everyone to try it...food for thought. I also like to wire V2 in my homebrew Marshalls 'backwards' so you can use a 12DW7 there too...again...a little different...a little better to me.

[drz400]

Are you refering to the 10K resistor at the cathode of the cathode follower?
.

Exactly, in a cathode follower that is the load resitor and 10k is way to small for a 12AX7 should be at least 47k or 120k is ideal, a 12AX7 also doesnt have the drive that a 12AU7 has or 12AT7, also have to be careful not to excede 300v across the tube. I'm quite sure Alexander did not use an AX7, it just wouldnt be like him.

[Fischerman]

Since the Return stage is the same tube...would a 12DW7 be a good choice for a 'stock' Dumbleator circuit? I don't know how they were wired...in order to have the CF on the 12AU7 half you would have to wire the CF to the first triode (pins 1-2-3).

In any case, it doesn't seem that the Return gain stage circuit would be ideal for either a 12AT7 or a 12AU7...it seems more suited to a 12AX7 but I'm stretching a bit by saying that.

[drz400]

Since the Return stage is the same tube...would a 12DW7 be a good choice for a 'stock' Dumbleator circuit? I don't know how they were wired...in order to have the CF on the 12AU7 half you would have to wire the CF to the first triode (pins 1-2-3).

In any case, it doesn't seem that the Return gain stage circuit would be ideal for either a 12AT7 or a 12AU7...it seems more suited to a 12AX7 but I'm stretching a bit by saying that.

You can just do a standard return stage and you would have plenty of gain from any tube but the 12AT7 and AU7 will give you a much better drive

[pamaz67]

The best sound I got from the dlator has been with a 6819 tube. If you have one, give it a try.
Great headroom and harmonics complexity, especially on the clean side. Of course not a great gain as a 12ax7, but being a loop it's not supposed to amplify in amplitude so much......
Ciao
Paolo

[heisthl]

I have used both the 12AX7 and 12AT7 and they both work ok as long as you go for 300V on the first stage and end up with 200V on the plate of the second stage. You will have to adjust the dropping string for which tube you decide to go with, the 12AT7 draws more current. My guess (and it's only a guess) is, that at the time of this design, HAD was inspired by the Fender reverb driver circuit which uses a 12AT7. Of course I don't even know if this is really the actual schematic of a Dumbleator. Maybe someone out there has actually seen what tube was used.

[glasman]

Here are my rule of thumbs for the loop.

I shoot for about 275 volts on the cathode follower plate and 200 to 220 on the plate of the recovery amp. I have found this gives me the best tone with the 12AX7. Lower than this and you run into the headroom problems.

The cathode voltage I find normally around 15 to 16.5 volts. This gives roughly a plate current of 1.4ma and a plate dissipation of ((275-16.5)* 1.4ma) 0.36 watts. All within the range of the 12AX7.

Granted that a 12AU7 with its lower gain, might be a better choice as I want the gain to be established from the preamp and not the loop.

FWIW, my measured gain of the recovery amp is about 33db.

Regards,
Gary