Which half of Dum-ator contributes most to it's tone/effect?

[Mill Creek Audio]

As a player and a designer I'm curious about the tube FX loop issue... for those of you that have Dumble-ators or clones, do you think the cathode follower or the recovery gain stage contributes more to the recognized attributes of these units? Of course discounting for the moment the effect of any cables used.

[tictac]

My ODS clone has a built in Dumbleator series/parallel loop with a switch that bypasses the loop completely. With the loop engaged I can get a volume boost & maybe a bit more low end; but I find with the loop bypassed I'm still getting a great ODS tone.

I guess it comes down to the way you use the loop. I use my loop to add time based effects and I don't want to add coloration to my tone just the effect. Others may use the loop as an effect to add gain and drive.

For me the ODS sounds great with or without the loop, it's not a critical factor for tone unless you want to add effects after the preamp.

TT

[Mill Creek Audio]

Thanks TT, I was referring to the tone shaping, compression and 3D effect that many talk about with these units. I've even read something like the amp's tone is not complete without it.
I too get great tones from amps of this design without using a Dumble-ator type unit but came up with a novel design for a tube FX loop to solve a particular setup dilemma I had. Just trying to get some opinions to see if it's a worthwhile pursuit (for anyone other than me!)
Also, I've seen some mention of the reversed phase, do most of you using these units flip phase back at the speaker terminals or something like that or does the reverse phase not matter to you when you are using a series loop?

[glasman]

In my screwing around with the loop circuits I have found that the recovery side probably has the most effect (discounting cables). It has a high frequency roll off that starts about 400Hz or so and and drops off gradually. This is influenced to a large degree by the feedback network (or appears to be).

The CF is flat as a pancake from DC to daylight.

I have some graphs somewhere of measured results. I will see if I can dig them up.

Gary

[talbany]

I don't know if anyone else here notices or is it just me... but amps that have cathode follower driven tone stacks seem to yield a much better functioning master volume...Ever tried to put a master volume on a Blackface fender.. Then on a JTM 45..You'll find plenty of info out there about how cathode followers were designed low impedance to drive tonestacks but not much info on how it effects master volumes or Phase inverter loading..(or maybe I lust can't find it) With my loop in circuit I set my master on the amp at 12 o'clock and use the return on the loop as the master and to me get much better tone , sustain and responsiveness, liveliness at whisper volumes I can never get with just the master on the amp. which to me sounds splatty lifeless at low settings 1 or 2.. I would assume it's in the non loading characteristics of low impedance..Since cathode followers offer at best unity gain.. Anyone else notice this or am I swimming alone on this one....


Tony VVT

[Mill Creek Audio]

I don't know if anyone else here notices or is it just me... but amps that have cathode follower driven tone stacks seem to yield a much better functioning master volume...Ever tried to put a master volume on a Blackface fender.. Then on a JTM 45..You'll find plenty of info out there about how cathode followers were designed low impedance to drive tonestacks but not much info on how it effects master volumes or Phase inverter loading..(or maybe I lust can't find it) With my loop in circuit I set my master on the amp at 12 o'clock and use the return on the loop as the master and to me get much better tone , sustain and responsiveness, liveliness at whisper volumes I can never get with just the master on the amp. which to me sounds splatty lifeless at low settings 1 or 2.. I would assume it's in the non loading characteristics of low impedance..Since cathode followers offer at best unity gain.. Anyone else notice this or am I swimming alone on this one....

Would this be with no bypass cap on the master volume?

[odourboy]

I don't know if anyone else here notices or is it just me... but amps that have cathode follower driven tone stacks seem to yield a much better functioning master volume...Ever tried to put a master volume on a Blackface fender.. Then on a JTM 45..You'll find plenty of info out there about how cathode followers were designed low impedance to drive tonestacks but not much info on how it effects master volumes or Phase inverter loading..(or maybe I lust can't find it) With my loop in circuit I set my master on the amp at 12 o'clock and use the return on the loop as the master and to me get much better tone , sustain and responsiveness, liveliness at whisper volumes I can never get with just the master on the amp. which to me sounds splatty lifeless at low settings 1 or 2.. I would assume it's in the non loading characteristics of low impedance..Since cathode followers offer at best unity gain.. Anyone else notice this or am I swimming alone on this one....


Tony VVT

Interesting notion! What I'm not sure about is why you'd get better performance by using the return level as a MV. With a follower in the circuit as with a Dumblator, the MV is now seeing a super-high impedance, so it should not be loaded at all, where as the return level would see no benefit from the cathode follower. So, I can see that there might be value with the follower, but only if you were to set the return level and use the MV as MV - you following that?

[talbany]

Interesting notion! What I'm not sure about is why you'd get better performance by using the return level as a MV. With a follower in the circuit as with a Dumblator, the MV is now seeing a super-high impedance, so it should not be loaded at all, where as the return level would see no benefit from the cathode follower. So, I can see that there might be value with the follower, but only if you were to set the return level and use the MV as MV - you following that?

Not sure I am following your reasoning..Here is how I see it.. Cathode followers have a high input impedance and a low output impedance...

From Aiken

"The cathode follower *is* an amplifier stage, but not a voltage amplifier in the typical sense. It is used as a "buffer" amplifier, which means it has a high input impedance and a low output impedance. This means it does not appreciably load the previous stage it is connected to, and the very low output impedance allows it to drive low impedance loads without much signal voltage loss".

With a series loop configuration like the D-lator The return pot essentially becomes the MV since it is the last load/volume control before the PI..The return level is after the follower ---Low impedance
Thats all..Did I miss something you were asking..

Tony

Would this be with no bypass cap on the master volume?

Ahh! Yeah!!...Loops have bright caps as well..

[Smitty]

Tony,

Remember my Dualdrive from the Nashville show. It had a regular old Dunlop High Gain volume pedal in the D-loop. I thought it worked pretty much as you described. Since then I've modded the pedal so it has a cap across the pot. Sounds better.

Smitty

[talbany]

Tony,

Remember my Dualdrive from the Nashville show. It had a regular old Dunlop High Gain volume pedal in the D-loop. I thought it worked pretty much as you described. Since then I've modded the pedal so it has a cap across the pot. Sounds better.

Smitty

Hey Smitty I hope you are well and yes I do remember that amp.. It did have those qualities... I enjoyed playing all your amps even if I woke up the whole floor...You have a great line going and should be proud...

Here is a cool experiment..This seems to work a bit better with those HRM or Bluesmasters..than the classics... Set it up with a loop inserted and whatever effects you desire..Set it how you normally would but run the master volume at around 12 o'clock...kick in the OD set the OD volume nice and high 2-3 oclock as well as the drive get it nice and juicy... now turn the return way down just cracked open whisper volume..I do this with all my HRM amps and see if you don't get wall collapsing distortion that is clear articulate not mushy and sustain out the ying yang!! at whisper volumes.. Now try and get the same thing without the loop.. This is what I am talking about!!..I wish I could have the time to post some clips of this.. But I never have any frickin time...Please let me know your results..


Tony VVT

[glasman]

Found the graphs.

Analyzer is an Ono Sokki CF-5220

Cathode follower response. Gozinta to Gozouta.


Recovery amp

[talbany]

As it relates to Master volumes or send buffers


The cathode follower *is* an amplifier stage, but not a voltage amplifier in the typical sense. It is used as a "buffer" amplifier, which means it has a high input impedance and a low output impedance. This means it does not appreciably load the previous stage it is connected to, and the very low output impedance allows it to drive low impedance loads without much signal voltage loss. For example, if the previous stage had an output impedance of 100K, and you tried to connect a stage with a 10K input impedance to it, you would only get 9% of your original signal when you connected the second stage, because of the voltage divider formed by the 100K output impedance and the 10K input impedance. If you insert a cathode follower with a 1K output impedance and a 1Meg input impedance, the 10K stage can be driven with 90% of the original signal, because there is now effectively a 1K:10K voltage divider instead of a 100K:10K voltage divider.

As it relates to tone stacks..

The cathode follower is basically an active "impedance transformer", in this sense. The reason the cathode follower is used in driving a tone stack is not only because the tone control network impedance is relatively low in comparison to the output impedance of the previous stage, so it would cause a loss of gain, but more importantly, the tone stack is a filter network that is designed to ideally be driven from a zero source impedance to achieve it's proper frequency response. The cathode follower provides a very low source impedance that allows the tone stack to work as designed. If the tone stack is driven from too large a source impedance, not only will there be a loss of gain, but there will be a different frequency response to the network, typically quite a few dB loss of the highs. The cathode follower prevents this loss, allowing the tone stack to retain more of it's theoretical frequency response. If you go to tone stack calculator and change input impedance see what happends..

Gary's flat frequency responce of the CF confirms the theory..This is cool stuff!!

Info source Aiken...

Tony VVT

[Mill Creek Audio]

OK, so it seems the consensus is that the recovery stage is where the magic happens. I'm not talking about the benefits of unloading the send side of things, I mean the tone shaping and perhaps compression when driven a bit that the recovery amp provides. I noticed that no one chimed in about the phase issue, maybe I did not go far back enough and missed where it might have done to death. In hi-fi the cathode follower is not the favorite choice, many don't like the sound compared to the gain stages so similar to what we see in guitar amps. My response to those issues in the early 80's was to use a stage like a SRPP with a low or medium mu tube for the send amp. Same hi Z in and VERY low z out (some input padding and a ton of signal on the output to deal with) but it gives you phase reversal and a VERY sweet sounding send amp (also flat response but a very different sound than a cathode follower). Now your series or parallel loop is in phase with an even nicer amp tone and feel.
Your take?

[talbany]

The SRPP really performs best matched to a fixed load impedance,..Like first preamp stage in most typical Hi Fi ....However Tone stacks or PI/w NFB are not fixed load so we don't know how it will perform.. AX7 wired for pentode?..Ahhhh!!..Phase issue intrests me more.. Just me!!!

Smokin Joe 2...https://taweber.powweb.com/store/smokeII_schem.jpg

Input stage Tony VVT

[Structo]

Here is the clickable link.

https://taweber.powweb.com/store/smokeII_schem.jpg