PS: More About Dumbleator Settings

[martin manning]

The problem with just reducing the Send pot is that the CF's AC load line will become steeper, and its headroom will be reduced. A 25K pot would be great for maintaining HF, but it's too small to prevent clipping at high input signal levels. To fix that an input divider could be added. Since this is a CF stage, there is no problem with loss of HF at the input due to Miller capacitance, so that seems to be the better place to reduce the signal level. Let's assume that a line level signal is needed when the send pot is at 50% of its value. Working backwards from the send output at 1.7V peak (line level), we have 3.4V peak at the cathode, and dividing by the CF gain (0.95 say) that's 3.6V peak at the input. The signal coming off the last preamp stage could easily be 10x that value, so a 10:1 divider seems appropriate. A 470k/47k could be chosen so as not to load that stage too heavily. A 25k Send pot will now be fine with respect to clipping, and the -3dB point for the Send signal is up around 30kHz with 700pF of cable capacitance. In the end, this is no different from turning the master down, setting it at noon on a 10% audio taper pot.

[ayan]

The problem with just reducing the Send pot is that the CF's AC load line will become steeper, and its headroom will be reduced. A 25K pot would be great for maintaining HF, but it's too small to prevent clipping at high input signal levels. To fix that an input divider could be added. Since this is a CF stage, there is no problem with loss of HF at the input due to Miller capacitance, so that seems to be the better place to reduce the signal level. Let's assume that a line level signal is needed when the send pot is at 50% of its value. Working backwards from the send output at 1.7V peak (line level), we have 3.4V peak at the cathode, and dividing by the CF gain (0.95 say) that's 3.6V peak at the input. The signal coming off the last preamp stage could easily be 10x that value, so a 10:1 divider seems appropriate. A 470k/47k could be chosen so as not to load that stage too heavily. A 25k Send pot will now be fine with respect to clipping, and the -3dB point for the Send signal is up around 30kHz with 700pF of cable capacitance. In the end, this is no different from turning the master down, setting it at noon on a 10% audio taper pot.

Good point, Martin. Not sure what the specific numbers would be for a 12AX7, but here is a good depiction of the headroom limitations when increasing the CF load.

http://www.valvewizard.co.uk/accf.html

Once again, could it really be that HAD came up with a poor design for this thing?

[talbany]

So now in OD mode we have a pot(OD volume) 100k feeding another pot(Master)1M feeding another pot "input" (470K set at 47k) a buffer then another 25K pot.

Why don't we just set up a "Fixed" AC follower for max headroom and pad the signal at the input?
BTW.I thought Part of the charm of the Lator is the added compression and 2nd order you get from driving the CF?

Tony

[martin manning]

Seems like the D'lator send pot is too big, but it does protect against clipping if the CF is driven hard. If that's not something that would ever be needed, then I see no reason it couldn't be reduced, as long as one remembers to turn the master down. A more streamlined approach would be to put the loop jacks in front of the master and pad the send down 90%. Then the amp's master can remain the master with or without the D'lator, which could then lose its recover out pot.

[ayan]

So now in OD mode we have a pot(OD volume) 100k feeding another pot(Master)1M feeding another pot "input" (470K set at 47k) a buffer then another 25K pot.

Why don't we just set up a "Fixed" AC follower for max headroom and pad the signal at the input?
BTW.I thought Part of the charm of the Lator is the added compression and 2nd order you get from driving the CF?

Tony

That would be like diming the Dumbleator's SEND and controlling the send level exclusively withe the amp's master, but the bright cap on that could be a problem. And I agree with your first statement, that would be a lot of pots.

Seems like the D'lator send pot is too big, but it does protect against clipping if the CF is driven hard. If that's not something that would ever be needed, then I see no reason it couldn't be reduced, as long as one remembers to turn the master down. A more streamlined approach would be to put the loop jacks in front of the master and pad the send down 90%. Then the amp's master can remain the master with or without the D'lator, which could then lose its recover out pot.

Yes. Now, if we put a fix padding before the amp's preamp out jack, how could we make up for the signal loss if the Dumbleator were not used? The padding could be made active only by the jack's switch, when used, and that would fix that. But then, if the jack's switch contacts get dirty, we could not use a short patch cable to fix that on the fly-- I've had to do this on gigs -- because it would activate the fixed padding.

I wonder if HAD thought all of the above out!

[talbany]

Another option would be to just drop in a Low Z interstage output transformer (between the send pot and send jack)... 6 bucks
https://www.digikey.com/product-detail/ ... SnEALw_wcB

I wouldn't alter the operating point of the CF?


Tony

[ayan]

Any updates yet, Tony?

Cheers,

G.

[martin manning]

I'm not clear on how a 1:1 transformer like the one linked above would help here. For a pre-master loop, the arrangement below would preserve the original functionality at the jacks when the loop is not in use (the ability to externally jump the loop jacks if the return jack's switch contact fails). New features are the front panel MV function is maintained when the “power amp in” jack is used (more sensible IMO), and the option to reduce the preamp output to line level when the loop is in use. If a Dumbleator is used, its send pot can be reduced in value (down to 25-50k, to make the CF stage a better low output impedance buffer), and it will no longer need a recover out pot since that function is now provided by the amp’s MV (the D'lator recover out can be turned full CW or eliminated). S1 could be a DPDT mini toggle mounted above the jacks, which would be easy to add and provide a convenient place to hang the divider resistors. Rearranging the order of the loop and master volume would require a bit of work. Admittedly, the switch is a potential failure point, but not a likely one. Something similar could be used for an on-board D'lator, where S1 could become a loop bypass.

[talbany]

Gil
Sorry for the delay as I got a last minute sub job in DC and the site went out last night.

I didn't have a 15K pot on hand so it was a 50K and 25K pots. Both seemed to work OK. The only thing I noticed was there was a bit more compression when going to the lower value pots. It was pretty subtle so I don't know if you will notice this or not. The really interesting thing was the lower value pots increased the lators ability to sustain (especially the 25K) which was pretty cool!. Of course, the send pots became really sensitive but we already knew this. As much as I tried I couldn't get the CF to distort so it never ran completely out of headroom. (I've heard followers distort)
So far I prefer the 25K for now and am leaving it in there to try on the next few gigs and see how it feels
Of course YMMV depending on your setup tubes/cables and effects.
Master on 7
Lexicon LXP-15
Stock D-Lator
Amps were
High Plate Skyline
Low Plate Skyline Music Man
Let me know if you have any questions.

Tony

[ayan]

Gil
Sorry for the delay as I got a last minute sub job in DC and the site went out last night.

So I didn't have a 15K pot on hand so I tried both 50K and 25K pots. Both seemed to work OK. The only thing I noticed was there was a bit more compression when going to the lower value pots. It was pretty subtle so I don't know if you will notice this or not. The really interesting thing I noticed was the lower value pots seemed to help increase the lators ability to sustain longer (especially the 25K). Of course, the send pots became really sensitive but we already knew this. As much as I tried I couldn't get the CF to distort so it never ran completely out of headroom. (I've heard followers distort)
So far I prefer the 25K and am leaving it in there to try on the next few gigs.
Of course YMMV depending on your setup tubes/cables and effects.

Let me know if you have any questions.

Tony

Thanks very much for the tests and report! I think I will order some pots when I get back home from Italy and try for myself as well. I guess one mandatory question would be whether you noticed a decreased sensitivity of HF response to the send pot setting....?


Thanks again!

G.

[talbany]

Gil
Yes (naturally). It wasn't drastic but noticeable. Unfortunately, I had to use some rather high capacitance cables since my good ones were still on the equipment truck and were stuck using my cheap shop cables so you are just going to have to hear for yourself!

Safe Travels!

Will send you a clip later.

Tony

[martin manning]

I'd recommend scoping the D'lator send signal to see when it starts to clip. With a 25k send pot, and the nominal 260V on the CF plate, I think that will occur when the input (preamp out) voltage hits around 15V.

[ayan]

I'd recommend scoping the D'lator send signal to see when it starts to clip. With a 25k send pot, and the nominal 260V on the CF plate, I think that will occur when the input (preamp out) voltage hits around 15V.

I can try that when I’m back at home, unless someone beats me to it. *hint*.

[norburybrook]

Gil
Sorry for the delay as I got a last minute sub job in DC and the site went out last night.

I didn't have a 15K pot on hand so it was a 50K and 25K pots. Both seemed to work OK. The only thing I noticed was there was a bit more compression when going to the lower value pots. It was pretty subtle so I don't know if you will notice this or not. The really interesting thing was the lower value pots increased the lators ability to sustain (especially the 25K) which was pretty cool!. Of course, the send pots became really sensitive but we already knew this. As much as I tried I couldn't get the CF to distort so it never ran completely out of headroom. (I've heard followers distort)
So far I prefer the 25K for now and am leaving it in there to try on the next few gigs and see how it feels
Of course YMMV depending on your setup tubes/cables and effects.
Master on 7
Lexicon LXP-15
Stock D-Lator
Amps were
High Plate Skyline
Low Plate Skyline Music Man
Let me know if you have any questions.

Tony

Tony,

I have a Dumbleator in a box waiting to be built here so i'm interested in your findings. Are your pots log or Lin? I'm going to give the 25K a shot as I like how you described the tone


M

[martin manning]

It’ll be fine if you keep the master volume down low enough. I’ve just never really warmed to the idea of the knob on the front panel becoming essentially useless when the D’lator is in the loop.