Restoring a 1974 Deluxe Reverb - Seeking Advice

[bcmatt]

A friend sold me this mystery 1974 Deluxe Reverb for real cheap. It has no cabinet, doghouse cover, speaker, or footswitch, etc, but it does have the reverb tank in its bag and it works.

20240108_145147.jpg

Once I get this all working well and in a combo cabinet of some kind, I do think I will keep it. That is my intention because it is different enough from all my other amps. I guess it's somewhere related to and in between my 5e3 and SSS builds, but everything I have are heads, aside from my very large and heavy DC30 build. However, on the off-chance that I really don't bond with this style of amp (my liverpool gets played 80% of the time), I do want to make sure that I don't shoot myself i the foot as far as being able to re-sell it. Therefore, I will try to control myself and not mod it to death. With that in mind, I do have questions about how kosher is it is considered to do some changes (aside from recapping and possibly re-tubing once I check the bias):

1) First of all, I do have to put this in a cab, and I do want it to be a 1x12 combo with the traditional dimensions. I happen to have a road-case of appropriate size already. I'm tempted to build a hardwood cabinet or some type... like walnut and maple. Would this actually be shunned compared the classic black tolex over pine with silver grill-cloth? Since I don't have the original cab anyways, there is no need to keep with tradition cab-wise? Or would that make it actually harder to sell off?

2) I'm pretty sure I am this circuit (the A1172)

fender_deluxereverb_a1172.pdf

Is it frowned upon to experiment with a few black-facing mods now? Any thoughts on whether some would be worth trying?

3) Since I will be removing the "death-cap" anyways, has anyone played with some different NFB options on that switch that they really like? I feel like this one would be tempting:

AB763_3-Way_NFB_Mod2.png

Stock, 68 Custom value, and No NFB

4) The Tremolo is the one thing that is currently not working. I've swapped in a few different 12ax7s to see if that is the problem. I do see the little bulb flashing with the set speed. I also can hear a mild ticking (with the bulb flashes) with the intensity cranked, but it does not seem to affect the guitar signal at all. Does this seem like the LDR component is fine? Is this more likely a problem with a nearby cap, or the amp bias affecting the tremolo? If it is the LDR, I would want to order a new one with the rest of the needed components to save on shipping. Thoughts?

[sluckey]

If the neon is flashing the problem is most likely the LDR side of the roach. They are cheap. Replace roach.

[bcmatt]

If the neon is flashing the problem is most likely the LDR side of the roach. They are cheap. Replace roach.

Ok, thanks, I will add it to the order.

[bcmatt]

Well, the parts all arrived from AES. I replaced the filter caps, bias cap and one of the cathode bypass caps in the trem. I haven't done any of the 25/25s that make up the rest of all the preamp cathode bypass caps yet... and am not sure if I will at this point...
I replaced the "Roach" and everything seems to work properly now. Thanks Sluckey!
The original RCA 6V6s just seemed to be too far apart to get a decent bias job. So, I put in the new TAD red base 6V6GTs.

20240202_184444.jpg

This was my first voyage into trying to bias the amp with the Output Transformer Resistance method. It seemed to be more complicated than the old 1 ohm resister method.
I had a heard time trusting my reading of the OT resistance. It seemed to be always moving. The amp is supposed to still be warm when reading it? I have a third cheap multimeter I might bring home to settle the debate between my two multimeters I tried. I settled on the resistance being about 225 ohms between each plate and the center tap. The hard part is that I have to make an adjustment, then take a bunch of reading all over the place. I bounced all over the place.

In the end I was able to get 57.5% plate dissipation:

Centre Tap: 456V
Plate Voltage: 452V

So the voltage drop across half the OT is 4V. 4V/224.5ohms = 17.8mA bias current.

My biggest concerns right now are that the plate current seems so high compared to the schematic (and I already am using a 5u4 rectifier).
Also, I am concerned that the OT resistance is not correct because I did at times read more like 250ohms or something if I recall. My cheapest DMM seemed confident and didn't bounce around aside from going between 224 and 225.

Anyways, if I don't want a really crazy hot bias, the plate voltage ends up above 450V. Is this something I ought to be concerned about or trying to remedy somehow? Is this just due to higher line voltage than 1974?

Anyways, the current plan is that my dad has a bunch of teak hardwood that he will make me a combo cab out of.
In the mean-time I may make a temporary head cab out of pine and tolex.

[bcmatt]

So far I have resisted all temptations to mod... aside from a 3 position NFB switch.

NFB.jpg

[Stevem]

Grab two shot octal tubes ( from someone or somewhere ) , wire , epoxy , shrink tubing, two 1ohm 1% 3 watt resistors and two new octal sockets and make yourself up up 2 of these.
When you make them extend another cathode wire up the wire bunch or better yet half a clip lead for your meter connection.

It’s so easy to just plug them in between your output tubes to read current.

It will be the best spent two hours of time you ever did to build something.

To wire up the old socket brake the tube in a paper bag then brake the rest of the inner glass such the you can heat the tip of each pin ( a real hot iron is needed for this) and remove the internal wires, then you can solder in your new wires by stripping them long enough such that the stick out the tip of the pin.

Try to use old tubes that have the tapered nose pins since they do no harm to a socket as opposed to some modern flat nose pins.

In terms of idling the average 6V6 in these Fender amps I have found that more then .018 amp buys you nothing but output tubes that get used up faster.

[martin manning]

I had a heard time trusting my reading of the OT resistance. It seemed to be always moving. The amp is supposed to still be warm when reading it? I have a third cheap multimeter I might bring home to settle the debate between my two multimeters I tried. I settled on the resistance being about 225 ohms between each plate and the center tap. The hard part is that I have to make an adjustment, then take a bunch of reading all over the place. I bounced all over the place.

Strange behavior on the meter. Often that happens if there are stray voltages around, but the plate should be open circuit when the power is off. Maybe poor contact of the probes to the measuring points? Reflow those and see if that helps. If you are within ten ohms, the error in the calculated current is going to be less than 5%, less than 1 mA. Once you figure out which tube is highest, you can just focus on that one. It's pretty quick really.

If you want to make up a bias test socket, Hoffman has a kit: https://el34world.com/charts/BiasChecker3.htm

...if I don't want a really crazy hot bias, the plate voltage ends up above 450V. Is this something I ought to be concerned about or trying to remedy somehow? Is this just due to higher line voltage than 1974?

Probably. What rectifier are you using? A 5U4 will knock the voltage down vs. a 5AR4.

Anyways, the current plan is that my dad has a bunch of teak hardwood that he will make me a combo cab out of.

Yacht Rock machine!

[bcmatt]

Grab two shot octal tubes ( from someone or somewhere ) , wire , epoxy , shrink tubing, two 1ohm 1% 3 watt resistors and two new octal sockets and make yourself up up 2 of these.
When you make them extend another cathode wire up the wire bunch or better yet half a clip lead for your meter connection.

It’s so easy to just plug them in between your output tubes to read current.

It will be the best spent two hours of time you ever did to build something.

To wire up the old socket brake the tube in a paper bag then brake the rest of the inner glass such the you can heat the tip of each pin ( a real hot iron is needed for this) and remove the internal wires, then you can solder in your new wires by stripping them long enough such that the stick out the tip of the pin.

Try to use old tubes that have the tapered nose pins since they do no harm to a socket as opposed to some modern flat nose pins.

Thanks, I think I will finally get around to making some probes one of these days. Good advice on the paper bag... I'll definitely do that.

In terms of idling the average 6V6 in these Fender amps I have found that more then .018 amp buys you nothing but output tubes that get used up faster.

I read somewhere that Fender often released these from the factory at just over 50% dissipation.

Do you have somewhere you usually find yourself at? I am all about preserving tube life as a pretty high priority... especially these days. This pair of 6v6GTs cost me $70USD. That really hurt. It also freaked me out when I got some sparks measuring plate voltage one time. the probe must have got a bit too close to pin 2 as well. I unplugged the amp immediately, but I was so scared I had just wrecked the tube. As careful as I try to be, remeasuring for the 6th time after a bias adjustment, I guess my focus must have faltered. The tubes still seem fine, but I felt pretty stupid. Some socket probes could save me some real drama.

[bcmatt]

Strange behavior on the meter. Often that happens if there are stray voltages around, but the plate should be open circuit when the power is off. Maybe poor contact of the probes to the measuring points? Reflow those and see if that helps. If you are within ten ohms, the error in the calculated current is going to be less than 5%, less than 1 mA. Once you figure out which tube is highest, you can just focus on that one. It's pretty quick really.

If I read it now with the amp being off all night, both sides read pretty much 222ohms and it only moves around within about 1ohm difference. But what about the whole thing about it being easier to read accurately when the OT is still warm? Is that really a thing? I just find it more stressful and rushed.

Probably. What rectifier are you using? A 5U4 will knock the voltage down vs. a 5AR4.

It is already using a 5U4. I watched a video last night with Uncle Doug looking at the the different Silverface circuits. (I confirmed mine is the 1172 because of the unfortunate placement of the standby switch in addition to the 5U4 and other components. I'll move the standby switch to after the first filter caps. He also seems to feel strongly that because these models also appear to use the same transformers as the AB763, you should be able to switch to the 5AR4/GZ34.
I feel like something is fishy there. Why would all the schematics read 415V plate voltages despite their different rectifiers? I'm wondering if they went through several PT specs while using the same code for the PT.

20240203_093632.jpg

Doug also seemed confused about why some would have a heater center tap and others wouldn't. He didn't have all the models in front of him, just those different early 70s schematics. (AB763, A1172, AB1270, AB868).
It would be nice if there was another type of rectifier I could drop in to knock down voltages and volume a little. I suspect it might be extra hard on most 5Y3s I'm resisting the urge to install VVR in this thing... but if it looks like I'm going to be hanging on to this long term...

Yacht Rock machine!

Haha! Yeah, that was the first thing I thought of... so this will be like a boat-deck? But he said he had more than enough 3.75" teak boards to make this whole thing. I think we'll use a skunk stripe of maple(light) or walnut(dark) to go down the middle between the teak boards.

417480505_1104108150744173_6690026632760972324_n.jpg

Since it might be several months, I think I'll build a pine/tolex head cab for it in the meantime.

[martin manning]

If I read it now with the amp being off all night, both sides read pretty much 222ohms and it only moves around within about 1ohm difference. But what about the whole thing about it being easier to read accurately when the OT is still warm? Is that really a thing? I just find it more stressful and rushed.

So probably stray voltage is screwing up the resistance measurement. Maybe you can get a stable warm resistance measurement if you warm the amp up and pull the power tubes before measuring? Otherwise you could make a socket adapter, for which I'd go with Hoffman's kit.

The resistance coefficient of copper is about 0.39%/deg C, so if the OT is warm, say 20C above a 20C ambient (putting it at 40C, or 104F), the resistance would be 7.9% higher. If you measure primary resistance at room temperature and then set bias when the OT coil is hotter, the error will be going in a conservative direction. You'd be ~7.3% low in actual current using the 225 ohms cold measurement and targeting 4.5V to get 20 mA when it's at +20C.

Why would all the schematics read 415V plate voltages despite their different rectifiers? I'm wondering if they went through several PT specs while using the same code for the PT.

Yea, something strange there. 5AR4 should put out 20-30V more than 5U4.

It would be nice if there was another type of rectifier I could drop in to knock down voltages and volume a little. I suspect it might be extra hard on most 5Y3s I'm resisting the urge to install VVR in this thing... but if it looks like I'm going to be hanging on to this long term...

I think lowering the voltage is a good idea. Maybe MOSFET B+ reducer in the PT CT? That would mean disturbing the PT CT chassis ground point, though.

Yacht Rock machine!

... so this will be like a boat-deck? But he said he had more than enough 3.75" teak boards to make this whole thing. I think we'll use a skunk stripe of maple(light) or walnut(dark) to go down the middle between the teak boards.

Even better! Teak planking with narrow strips of Holly in between is classic decking and cabin floor treatment.

[bcmatt]

Ya, so the PT is feeding the 5U4 374VAC on the rectifier plates.
I'm thinking they must have changed the specs of the "025130" transformer over the years but kept the name.

So... it looks like I ought to do some MOSFET fixin' anyways...

Perhaps I should do some sort of VVR solution that limits the maximum voltage that the pot will allow to be at like 90%.
I'm trying to remember if Dana Hall's VVR2 board had a specific resister for that because I do have one of those in my toolbox that I pulled from my 2204:

20240203_133834.jpg

I have a few solid state components in my toolbox:
I have these MOSFETs:

20240203_133900.jpg

And these guys:

20240203_133957.jpg

Then I have several Zeners: the 1N4740A
They are 1W 10V Zeners
Then I also have some 1N4007G diodes as well.

The main problem is where to fit this idea. I'm thinking maybe I could remove the current bias pot and replace with a resister since the VVR board needs to have a little bias pot on it anyways (which I may not have a 1m trimpot anymore...)

[martin manning]

I would be inclined to go with the MOSFET B+ reducer in the CT for this. The nice thing about that is the FET tab (drain) isn't at high voltage like it is in the VVR. The drop in voltage will be approximately equal to the zener voltage, and you will need ~50V. Is the PT CT soldered direct to the chassis or to a solder lug? Finding a place to bolt the FET to the chassis is a problem. A more expensive but possibly better solution would be to get a Hammond 290BX PT and drop it in there.

[bcmatt]

I would be inclined to go with the MOSFET B+ reducer in the CT for this. The nice thing about that is the FET tab (drain) isn't at high voltage like it is in the VVR. The drop in voltage will be approximately equal to the zener voltage, and you will need ~50V. Is the PT CT soldered direct to the chassis or to a solder lug? Finding a place to bolt the FET to the chassis is a problem. A more expensive but possibly better solution would be to get a Hammond 290BX PT and drop it in there.

I think you are right that I am jumping to a too-involved of a solution right away. Yes, the CT is soldered right to the chassis, I think through a tab on a transformer bolt maybe but the solder connects right to the chassis.

20240203_152331.jpg

I'll try to get up to speed what that this specific MOSFET B+ reducer would look like... but I think I would lean that way for now.
I'm confused what everyone else with a 5U4 deluxe model does, because I would assume they all would be on too high of voltages now with nowhere else to go rectifier-wise. You'd think this would be a common problem.
I would also think there would be a reluctance to give up the original PT because of the vintage value these days. Wouldn't replacing with a modern PT that doesn't have those old date-codes reduce their resale value?

I myself haven't really had a chance to play this thing yet, so I don't know where it fits in my heart... but I'm only about $600 CDN into it so far.

[martin manning]

My thought re swapping in a new PT is that you can save the original and it is completely reversible.

[bcmatt]

My thought re swapping in a new PT is that you can save the original and it is completely reversible.

That's true.