#183 w/Switchable HRM, Almost Finished

[Structo]

A double wide footswitch, I like it!

David, I put those current limiting resistors in my footswitch, on each LED.

[David Root]

I guess you can put them just about anywhere Tom. I just followed HADs practice of putting them close the PS +ve output. I use 150R resistors in the FSW itself in parallel with the LED to regulate its current draw. So I have to rewire all six FSW buttons because the AES wiring is screwy.

bepone, I have wondered about changing the recovery stage in the loop to better accomodate the 12AT7, but since it sounded OK to me as is I haven't messed with it so far. FWIW Normster may have "adjusted" the response as he used only one .1uF, one .22uF and one .47uF cap in his design, and changed a couple of other components a little too. I will review that compared to the "original" version I looked at, which is the "Claus-Umbleator" schematic in the Files. I will check out the difference in output impedance with the 47K Ra, I use Tube CAD for that kind of analysis, so much faster than graphing it by hand from the plate curve. You might want to look at Kevin O'Connor's Tube Buffered Loop in TUT 1, I did that once, it is probably overkill but is very neutral in either parallel or series operation, and it uses a 12AT7.

Incidentally I just ordered a NOS pinched waist '59 Amperex 7062 to try in the loop position, wondering if the long plate design will have more transparency than the conventional short plate. I also have some '53 blackplate GE 12AT7s that I might try too. What have you used?

[Structo]

Gottcha.

Can anybody tell me the difference in using different value resistor for the artificial center tap for the heater winding?

I see 100R used a lot but even on Dumble amps they can vary from 100R, 120R and 150R.

Does the larger 150R actually drop the voltage any?

[David Root]

Theoretically it should I guess but maybe not a significant amount. 1.2 or 1.5 X squat is still squat!

[bepone]

hello david,
i build many fx loops in the past, mainly for soldano type of amps (all DIY) with 12AX7, 12AT7, 12AU7, 5693, mosfets, subminiature tubes, and for every tube i picked working point from the graph.. i have no experience with "dumble type " fx loops..

from my experience bigger current loops, like 12AU7 and 12AT7 working 100% transparent with any effect..

but 12AX7 buffered loop is not 100% transparent (repeat, in soldano SLO type of amp) and have very pleasent smoothing effect in overdrive.. so 12AX7 loop works "for the tone".. i never built dumbleator, nor fx buffered loop in dumbles (i have 3 #183 units) so i'm very curious.

E180CC (7062) is excellent tube for fx loop, note that plate handles 275Vmax , regards,

[bepone]

Gottcha

Can anybody tell me the difference in using different value resistor for the artificial center tap for the heater winding??

no, it doesn't matter, while resistors value are the same, i put 470R when dont have 100R near

[ToneMerc]

Gottcha.

Can anybody tell me the difference in using different value resistor for the artificial center tap for the heater winding?

I see 100R used a lot but even on Dumble amps they can vary from 100R, 120R and 150R.

Does the larger 150R actually drop the voltage any?

Tom, I've seen as high as 180R and I believe he used whatever he had on hand. Unless ohms law fails me it's opposite relationship, as the smaller the resistor the greater the load on the filament winding. In the greater filament loading scheme of amps vs milliamps, I think the value is irrelevent on filament voltage drop.

3.16V/100 ohms= 32mA, whereas 3.16V/180 ohms= 18mA

TM

[David Root]

Thank you bepone, that is very interesting! I'll definitely compare 12AX7 and 12AT7 in the loop and post what I find here. Once I get the footswitch done!

Is that a function of plate resistance limiting bandwidth?

[ToneMerc]

David, good job taking the circuit to another level.

TM

[dcribbs1412]

David, good job taking the circuit to another level.

TM

Agreed..Sir you have raised the bar

Congrats

Darin

[David Root]

Thank you Gents, I just hope everything works properly when I crank it up! I should be able to get it finished and tried out over the coming long weekend.

I have three NOS GE 12AX7WA and a quad of NOS Mullard Xf4 just bursting to sing their little hearts out in it and I don't want to disappoint them!

[bepone]

Thank you bepone, that is very interesting! I'll definitely compare 12AX7 and 12AT7 in the loop and post what I find here. Once I get the footswitch done!

Is that a function of plate resistance limiting bandwidth?

yes please, let us know diferences
i dont know for sure, but ECC83-12AX7 has bigger internal resistance than other tubes, so its driving capabilities are weaker.. this means certain loss of highs when certain effect is connected with long cables..in which percent will be manifested, i don't now, depends of connected effect..
from my experience and soldano amps (12AX7 tube in loop) there is not 100% transpanency, but with 12AT7 (and resistor adjustment for it) there is. dumble uses different cathode follower arrangement, so i'm very curious about it.
also i'm very interested if will internal fx loop like in your case, will smooth some highs when amp play in OD mode, regards

[bepone]

I have three NOS GE 12AX7WA and a quad of NOS Mullard Xf4 just bursting to sing their little hearts out in it and I don't want to disappoint them!

xf4 tubes are so god, that you can put them in cheap and anemic amp to make alive quickly. when you put them in excellent amp, they have 3D effect, excellent choice

[David Root]

I just finished the "DoubleWide" TM footswitch. Here's a couple of pix in case someone else wants to try it. Making sure you have the wires soldered in the right place takes half the time.

I have the amp on my Variac with cheap tubes in it just in case, heaters and bias look OK. Once I am satisfied that all is kosher, I'll put in the real tubes and check it out.

Forgot to put in the 220K ground reference on the loop return output, so Major Humm was on the job. Also have an issue in the relay PS, will look at that tomorrow.