Restoring a 1974 Deluxe Reverb - Seeking Advice

[bcmatt]

In regard to the mv, I did a quick installation of the type 3 ppimv, just because it is the least invasive. I just need a little help taking the edge off the volume so that we can be a little more free with the channel volume. I'll try this out for a bit and see how it serves.

20240209_125002.jpg

We still have live drummers on stage with no cage and they mostly play what feels good to them. If they are extra hard hitting, we ask them to back off about 10% in how hard they hit so that we can push it more through the line arrays.
All that is to say, I just want to keep it that my amps aren't the main problem for the sound guy... let's keep it so the drummer is always the main offender. That said I, also put the amps towards the rear of the stage with small plexi shields to not beam anyone directly.

[didit]

Thanks! I'll definitely try that. So, you want a total of 120R per side including what the resistance the secondaries already measure? I'll check out your thread

Yep. Another way to say it is a static resistance at the rectifier socket, plate to plate, of roughly 240R or more including both resistors and the entire secondary.

Best ..

[bcmatt]

Yep. Another way to say it is a static resistance at the rectifier socket, plate to plate, of roughly 240R or more including both resistors and the entire secondary.

Best ..

Cool, I'll try measuring them tomorrow when I bring it home.

20240208_180107.jpg

[bcmatt]

I haven't even pulled the chassis out yet, but I pulled the rectifier out and measured from the outside of the socket (I think between pins 4 and 6) to be 115 ohms.
So I guess I want to add 2x 68 ohm resistors as well...

[didit]

Hello --

I haven't even pulled the chassis out yet, but I pulled the rectifier out and measured from the outside of the socket (I think between pins 4 and 6) to be 115 ohms.
So I guess I want to add 2x 68 ohm resistors as well...

Yes. Best recall, what I'd used were a pair of 2W 68R metal film resistors. If you want a larger drop overall, increased resistance would be OK but care and attention to the dissipation increase please.

I should have taken a bit more time and sketched some rationale. Benefits are: lower voltage and more so reduction of pulse current on rectifier plates; and moving heat from the drop, distributing the thermal density. Ideas around more nuanced benefits from the overall change to dynamics of power supply impedance have been suggested, however I haven't done the research or first-hand analysis and less comfortable asserting them. Perhaps others will weight in.

Best .. Ian

[bcmatt]

Hello --

I haven't even pulled the chassis out yet, but I pulled the rectifier out and measured from the outside of the socket (I think between pins 4 and 6) to be 115 ohms.
So I guess I want to add 2x 68 ohm resistors as well...

Yes. Best recall, what I'd used were a pair of 2W 68R metal film resistors. If you want a larger drop overall, increased resistance would be OK but care and attention to the dissipation increase please.

I should have taken a bit more time and sketched some rationale. Benefits are: lower voltage and more so reduction of pulse current on rectifier plates; and moving heat from the drop, distributing the thermal density. Ideas around more nuanced benefits from the overall change to dynamics of power supply impedance have been suggested, however I haven't done the research or first-hand analysis and less comfortable asserting them. Perhaps others will weight in.

Best .. Ian

I ordered some 3W Metal Oxide off Amazon. Not sure how well I trust them, but I don't have much in the way of resisters below 470ohm already and ordering from my typical electronics supply places require a lot more patience and shipping expense. The reviews seem positive...

I definitely would be curious to hear more explanation of adding resistance to the PT secondaries... in addition to the drop in voltage that I need.

Would being before the rectifier reduce the possible sag compared to dropping resisters after the rectifier?
I assume that this would be generally be safer for the rectifier since the voltage is reduced before it gets to its anodes. I'm just curious what else this ends up doing electronically.

I read through your other thread a couple times, but it doesn't talk much about this part...

You mentioned totaling "at least 120R" on each side... So that is a minimum to reach certain PT characteristics? But maximum is more determined by hitting the desired voltage?

[didit]

Hello Matt --

I ordered some 3W Metal Oxide off Amazon.

I'm sure those'll be fine.

I definitely would be curious to hear more explanation of adding resistance to the PT secondaries... in addition to the drop in voltage that I need.

Would being before the rectifier reduce the possible sag compared to dropping resisters after the rectifier?
I assume that this would be generally be safer for the rectifier since the voltage is reduced before it gets to its anodes. I'm just curious what else this ends up doing electronically.

I read through your other thread a couple times, but it doesn't talk much about this part...

Yea. It's safer in terms of overloading the rectifier, and it's pulse current as the 60Hz voltage ebbs/flows primarily versus anode potential. There is a max voltage spec (arcing) but up above 1kV. It will also make less strain on the transformer.

My model of sag are the dynamics from sudden impulse-driven discharge of the filter capacitors when the power stage is pushed versus the charging rate from the upstream supply. This impact is the sum of all the inductance and resistance before the first filter capacitor. I would imagine only subtle differences whether it's the rectifier plate resistance, or resistors before or after it. An actual choke (inductor) changes everything. In that amp there is one between the plate and screen nodes in the power string, which helps stabilize transients for preamp and screen voltages, but not impacting (so much) the power tube plates. Somewhat mysterious but quite obviously so.

A brief sketch. Again, maybe others want to step in with more details/nuances, etc.

You mentioned totaling "at least 120R" on each side... So that is a minimum to reach certain PT characteristics? But maximum is more determined by hitting the desired voltage?

Yea. 120R is a minimum to approximately match the older higher internal resistance model of Fender OEM transformer. That change did make a clear audible difference. And one that similar tweaks producing equivalent plate voltages did not.

If you have the time and patience perhaps playing with the Duncan Amps power supply modelling application can help illustrate how changes in various spots in the chain move the voltages and currents around.

Best ..

[bcmatt]

OK, thanks Didit, that's what I was curious about. I appreciate that.

So, the resistors arrived this morning and I installed them. They seemed to work pretty well. I haven't removed my other dropping resistor(s) yet, but just wanted to check where things are at now. It sounds good. I think improved, but who can know for sure without real blind tests.

20240212_123651.jpg

Ok, so now my voltage hitting my other dropping resister is 426VDC (which is now only 6 volts above what the schematic states. You could consider this to be the difference in mains voltage. Actually, since I am reading 124.2VAC right now, I would actually expect it to be significantly higher.
So with my post-rectifier dropping resistor, I am reading 411VDC at the OT CT. and 406VDC at the 6V6 plates (when biasing to about 62% plate dissipation - (21.2mA)).

I'm a huge VVR fanatic, so I'm all for lower plate voltages, not that running 9VDC lower than the schematic makes much difference. I think I'll run it like this for a bit, with all the resistors in. But, I'm thinking the end goal will be to remove my post-rectifier dropping resister, and just keep your PT Secondary Dropping resistors instead. Then I will try to find some CL-80 or CL-90 Thermisters to drop a couple volts on the PT primaries as well. (I'd like to get the heater voltage closer to spec as well.) Once I have those thermisters (maybe just 1), I will decide whether I ought to tweak the values of the PT Secondary Resisters a little bit...

Anyone have any thoughts on this plan?

[martin manning]

Where's the heater voltage now?

[bcmatt]

Where's the heater voltage now?

6.6VAC

[martin manning]

Not bad, but about 5V of line voltage reduction would take it down to 6.3.

[bcmatt]

Not bad, but about 5V of line voltage reduction would take it down to 6.3.

That would be sweet. Apparently this can make a huge impact on tube life... which is a big deal these days with tube prices.

[bcmatt]

My thermisters arrived today and I installed 1 CL-90 and 1 CL-80. I tested them at my office with a line Voltage of 122VAC. I'm getting about 3.4VAC drop across the CL-90 and about 2.3VAC drop across the CL-80.

So, I also removed the extra 250 ohm dropping resistor because it's no longer needed.

My heaters were at 6.17VAC. I biased up the 6V6s at 62% dissipation with the plates at 407VDC. 4.81V drop across OT primary (224 Ohms) to CT. So that's 21.5mA dissipation.

I think everything is about where I want it now, because at home I often get even higher line voltage closer to 125VAC.
I'll try playing it for awhile and re-assess things.

I also added a few tube retainers because they seemed a little precarious hanging there. They weren't going to fall out, but I would nervous in a combo...

20240223_180035.jpg

[martin manning]

Nice. The voltage seems to be under control and, bonus, you have inrush current limiting.

[didit]

Seems like you've found that nearly finished sweet spot ..