UltraPhonix with a few twists

[fred.violleau]

My question is : do the resistor need to be balanced on both sides as they are also acting as a voltage divider right?

Fred.

if the resistors are the same value then you get a 50% drop in gain with a voltage divider I seem to remember.

M

Indeed Marcus!

[talbany]

I though about using another relay in order to ground the signal of the channel that is not being used instead of using mixer resistors.
It would be synchronized with the input relay.
I know @Sluckey is using a switch on his dual marshall amp to ground the unused signal but only after the first stage of amplification.
Would this avenue allow me to tweek the two channels output independently?

It's hard to say? I've found it's generally a good idea to isolate them there to help any bleed-back as well as the added loading.
It would be easy to do! you can run it off the same relay switching supply as the input. A simple test would be to just ground it out for now with an alligator clip before the mixers and see if that helps first. As far as the difference in gain the tweed style tone control doesn't have the losses as the TMB/Bluesmaster does. You can also try a 12AY7 on the tweed spot? cut some gain there and see if that helps? just an idea.
BTW. Sending your signal through a rather large resistor (470k)before the D-Lator will dull your tone

Good Luck

Tony

[martin manning]

Fred, do you have an updated schematic for this amp? Skimming through the thread I don't see one that shows the Tweed section and the mixing.

[fred.violleau]

I've found it's generally a good idea to isolate them there to help any bleed-back as well as the added loading.
It would be easy to do! you can run it off the same relay switching supply as the input. A simple test would be to just ground it out for now with an alligator clip before the mixers and see if that helps first. As far as the difference in gain the tweed style tone control doesn't have the losses as the TMB/Bluesmaster does. You can also try a 12AY7 on the tweed spot? cut some gain there and see if that helps? just an idea.
BTW. Sending your signal through a rather large resistor (470k)before the D-Lator will dull your tone

Good Luck

Tony

Thanks Tony, I had not thought about using a tube with less mu. I might have one at hand, and if not, I need to order a new set of preamp tubes, so I will get one as well!
I will test this morning how grounding one signal affects the output.

Glad that you mentioned that 470k would dull the tone, because, it was sounding great when I tested the channel totally isolated with no mixer resistor, so something must have changed with the load?

I will report my findings in a few hours.

Thanks!

Fred.

[talbany]

Fred
Just to be clear.
There are a number of ways you can do it. If you decide to go with the relays you could wire it one of 2 ways. Either ground out the channel your not using or 2 the no-load option which is to lose the mixing resistors and switch them through the relays.
Either one should still work and becomes up to your ears as to which one you prefer. If you just ground it out you still have a load there (220k to ground) Although judging by your last post it sounds like you prefer the sound of the no-load option. Again let your ears decide.
Looking forward to what you find.

Tony

[fred.violleau]

Thanks again for all the suggestions Tony!

I did try to ground the unused signal and it works fine for the bluesmaster.
I will have to tame the Tweed channel, because it is still distorting.

So far I will add another relay, ground the unused the signal.
Bluesmaster, will not have any load. Tweed channel I am still debating with the best way to tame the output signal.

I will try a heavy load for now and see how it sounds.

In the meantime, need to order another relay and redoo my relay board.

Fred.

[martin manning]

I’m trying to understand the Tweed side schematic. Recall that a cathodyne PI (as used in the Tweed) has essentially unity gain, and now you are running it into a LTP, which has significant gain.

[norburybrook]

I’m trying to understand the Tweed side schematic. Recall that a cathodyne PI (as used in the Tweed) has essentially unity gain, and now you are running it into a LTP, which has significant gain.

I think that's the crux of the matter Martin.

can an amp have switchable phase inverters? i.e a Cathodyne for the tweed and a LTP for the BM feeding in to the output section.


M

[martin manning]

You could, but it would be easier to attenuate the Tweed side output before mixing at the LTP input. Keeping all the Tweed gain stages in, to maintain its character, is probably a good idea; just has to be balanced with respect to the BM side.

[norburybrook]

You could, but it would be easier to attenuate the Tweed side output before mixing at the LTP input. Keeping all the Tweed gain stages in, to maintain its character, is probably a good idea; just has to be balanced with respect to the BM side.

but isn't the cathodyne PI a big part of the ' tweed' sound ?



M

[martin manning]

My understanding is they are clean and well balanced unless driven into clipping, so I don’t think it’s a significant part. Notice the mods people make to the cathodyne PI are aimed at balancing it (Dumble) and keeping its operating point stable (Ampeg style fixed bias).

[didit]

You could, but it would be easier to attenuate the Tweed side output before mixing at the LTP input. Keeping all the Tweed gain stages in, to maintain its character, is probably a good idea; just has to be balanced with respect to the BM side.

but isn't the cathodyne PI a big part of the ' tweed' sound ?

To a first order, yes. Entire output stage distortion has major contribution & particularly in those cathode-biased power tubes.

Best .. Ian

[erwin_ve]

My understanding is they are clean and well balanced unless driven into clipping, so I don’t think it’s a significant part. Notice the mods people make to the cathodyne PI are aimed at balancing it (Dumble) and keeping its operating point stable (Ampeg style fixed bias).

My understanding was that the LNFB at the Tweedle Dee cathodyne PI was to make the transistion between clean and overdrive a bit smoother.
Do you mean that by the mods people make?

[martin manning]

I forgot about that LNFB, yes that will help keep it under control. The “Ampeg mod” is something else done to 5E3’s outside of the Tweedle Dee context. Good point about the cathode biased power stage, Ian.

[talbany]

Thanks again for all the suggestions Tony!

I did try to ground the unused signal and it works fine for the bluesmaster.
I will have to tame the Tweed channel, because it is still distorting.

So far I will add another relay, ground the unused the signal.
Bluesmaster, will not have any load. Tweed channel I am still debating with the best way to tame the output signal.

I will try a heavy load for now and see how it sounds.

In the meantime, need to order another relay and redoo my relay board.

Fred.

Fred
Sounds good!. After you get the relays set up you can adjust the load resistor to set the amount of gain. However, there will be a point where if you go to heavy the amp will clean up but your tone might suffer so it's a balancing act. The 12 Ay should also help as well. I doubt you will get it totally clean. Remember the tone stack

Tony