Peavey Encore 65 Misc Parts Question

[Enzo]

In this circuit the difference between 20M and 40M is the divider is either 1/45 or 1/90. SO if the difference matters, it has twice the attenuation. In this circuit, the higher the better.

While it won't matter to you or me, when Peavey made these amps, an important consideration aside from electrical specs was availability. They had to have a reliable source of production quantities.

[Dix]

True.... but from looking at Tom's test method & his results, it would appear that the "dual" LDRs are more like a "center tapped single" than a "matched pair in a single package".

His "on" resistance values as drawn are end to end. Which would make the 5C3 (which are plentiful on eBay) a drop in replacement for the 5C3/2 for that circuit if that's the case.

Although, even if we assume "matched pair", I have my doubts that the difference between a 10M off or a 20M off is going to be audible. But if that's the case, then the 5C9 has pretty similar response ratings & is 50M dark. So you can get even more "off". Or parallel a resistor to get you down to 20M.

& if the on resistance is an issue, Peavey's only driving these things with about 10mA. So there's plenty of room to go either way with the limiting resistor to adjust that.

Of course, there's always the possibility that my redneck engineering thought process has short-circuited somewhere.

[Enzo]

I see, in the schematic it is a single.

I might surmise that the double used has the two elements wired in series, so twice the off resistance. SInce it is used as a series element in the voltage divider with R39, maybe it makes for better OFF.

[Dix]

I was talking with regard to a replacement for the one in the reverb circuit. Which is installed with the common center connection not used.

Meaning there was no real need for the use of a dual LDR there. A 4-leg single would have worked just fine rather than the 5-leg dual.

I question whether there was an actual "need" for the use of the duals at all. Other than it was a parts saving move. You want to switch 2 circuits at the same time. Doing that with 1 part rather than 2 would save on parts count, & likely some PCB real-estate to boot. (x2 in the case of the pump circuit switching).

But, as I said, we're not talking about a stereo compressor here. Given the application, does it really matter in this case if say the ground for the "smooth" switch acted a few millisconds faster/slower than the ground for the master volume? (LDR2 in a Rockmaster)

Would not 2 single LDRs with the LEDs wired in parallel work just as well in that spot?

This would all be a moot point if I could get Peavey to get back to me & take my credit card number.

[Enzo]

In the reverb circuit of the Encore a single should definitely work as a straight-up substitute there. The only reason for the use of a dual used that way is likely because it was a part already in inventory, and it would work.

I don't follow. I see a single in the reverb return circuit. Turns the signal off and on into the summation stage V3b.

The doubles both seem to be in the PUMP circuit.

[Dix]

Just a couple more things I forgot to mention above....

In the reverb circuit of the Encore a single should definitely work as a straight-up substitute there. The only reason for the use of a dual used that way is likely because it was a part already in inventory, and it would work. So, no need to source out another unique part.

Also, if you do any searching for info on other LDRs (like I was doing when thinking of using 2 singles as replacements), it can get somewhat confusing. Largely in the case of the 5C4, 5C4/2, & 5C10. Depending on where you look (including Vactrol's own datasheets) you can find those listed as having a dark resistance of 400Meg or 400 ohms. Both of which, as it turns out, are wrong.

The actual dark resistance of the above is 400K.

Also, while most have a forward voltage drop of 2v, for some it's 2.8v.

I've attached the "most accurate" spec sheet I was able to find. It's from an old PerkinElmer catalog. They at least managed to get most of the specs listed correctly.

I say "most" because even they have one error. The last column of the specs chart where it reads "Turn-Off (Decay) to 100 ohms". Should read "to 100K ohms".

But, at least everything else is correct.

[Dix]

Next, the Opto-Isolators (LDRs). Now keep in mind, the ones in the amp were working. And I guess it is rare for these things to blow. But my mission was to determine if a replacement is available should Peavey discontinue carrying the parts.

I ordered one of each from Peavey: the 40101 (21L265) and the 40102 (21L628). Trying to find datasheets for these parts was impossible. But I had a hunch that I knew what the replacement parts are. So I rigged up a circuit to test the theory.

While waiting for Peavey parts, I ordered a VTL53C/2 from eBay to see if it came close to the 40101. I dropped that into my test circuit. I changed the voltages and R1 of the test circuit to force 40ma through the LED (as per the specs- see photo). I determined the resistance across the outer legs of the LDR to be ~ 1500 ohms. Is that close enough to the 2000 ohms as shown in the datasheet? I found my first substitute. A few days later, Peavey sent me the replacement part for the 30240101. Lo and behold, it was one in the same, a VTL53C/2. So my theory and testing proved it. Mystery part #1 solved!

As for the 40102 (the Red Dot), I found specs for the VTL5C2/2. That is what I thought the part could be. The Peavey part arrived and it had the original 21L628 number on it. So I changed up my test circuit (as shown) to push 40ma through the LED (as per the spec for the VTL5C2/2. Per the voltage measurements, I calculated the resistance across the outer legs of the LDR to be ~ 500 ohm. Is that close enough to the VTL5C2/2 spec of 700 ohms? Unfortunately, this part is getting harder to find.

Unless anyone can weigh in to the contrary, I think the LDR mystery might be solved.

Look for these parts just in case Peavey ever runs out.

70240102, 21L628 -> VTL5C2/2 (Edited)
30240101, 21L265 -> VTL5C3/2

Holy coincidence Batman!!!!

A couple years ago I became the lucky inheritor of a much used, abused, rode hard & put away wet, Rockmaster amp. Which is basically the 120w head version of the Encore (& w/o reverb). I managed to get it up & running again. Other than the case being about completely shot & butt-ugly, it's only real issues were a blown fuse, a couple bad solder points, & in need of fresh power supply caps. & since I'm one of those "path of least resistance" types, I just fitted the chassis with fabbed-up rack mount ears.

There was one item that I didn't fix. The pump circuit switching. & like you I set about researching the LDRs to source 'em, & came up with about the same exact info you did. Or should I say lack thereof.

I had run across a few old posts (in fact, by Enzo I believe if memory serves) about calling Peavey. At the time even those posts were over half a decade old, & since we're talking about an amp with a born-on date in the early 80s, I just figured no way they're still available. So I never bothered.

So, as I was sitting having my morning coffee, for whatever reason the mood struck to see if I could find out anything on replacements for 'em that was new or that I missed the first half dozen times around. Imagine my surprise when Google sent me here.

Guess I actually DO need to call Peavey. I can't believe they still have those things available.... in 2020.

As a way of saying "thanks for the info" & maybe help you out a bit if possible.... here's what I know about 'em. (some of which may or may not be accurate... I'm by no means an LDR expert)

From what I've been able to find, I'd say you're spot-on with your assessment. It seems in the world of LDRs "in the ballpark" would be considered a "tight tolerance". "In the same zip code" is considered close enough. So yea, 1.5k would be close enough to 2k. Ditto 700 & 500. Especially since, as far as Vactrol is concerned, there's only one other option, the 5C4/2, & that one's min on resistance is only 150 ohms. So, I think you've called it right.

What I've heard, the problem of getting 2 single LDRs to "match up" with anything that vaguely resembled precision was the reason behind the creation of the duals. Peavey using them in this circuit may have been more of a convenience thing than a necessity. My own idea, push come to shove & if i could figure out/guess the value of the dead one, was simply to use 2 singles in place of the dual. It's not like this is a stereo compressor where each channel absolutely has to exactly mirror the action of the other to the microsecond.

As for measuring the "dark" resistance. If you refer to the footnotes of the description sheet as referenced back from the data sheet, you'll find that dark resistance listed is as measured 10 seconds after turn off. Which is why you can't simply just measure that cold. Apparently it keeps climbing past that after it's off.

(footnote 3 on the last page of description sheet) "... ultimate resistance is many times greater than the value at 10 seconds."

One last tip I've got... If you do ever have to end up replacing one, make sure you heatsink 'em as you're soldering 'em in. Apparently, these things really don't take to being heated up much very well.

Anyway, thanks again for the posts on it. I'll have to give Peavey a call this week.

[TomCarlos]

Thank you G1 - I went back into that post and fixed my errors.

[g1]

...

[TomCarlos]

MEF Members (and future visitors):

I want to close the loop on a couple items discussed in this thread.

First, the bias of the 6L6s. I rigged up a temp circuit as suggested by Enzo. It worked, yeah! In the end, at the moderate volume level that I play at, I really couldn't hear any difference between a 50% or 70% bias point. But there certainly was a difference when I yanked the Scorpion in favor of a Jensen C12K 100W (4 ohm) speaker. So for now, the original bias circuit will remain, the Jensen too!

Next, the Opto-Isolators (LDRs). Now keep in mind, the ones in the amp were working. And I guess it is rare for these things to blow. But my mission was to determine if a replacement is available should Peavey discontinue carrying the parts.

I ordered one of each from Peavey: the 40101 (21L265) and the 40102 (21L628). Trying to find datasheets for these parts was impossible. But I had a hunch that I knew what the replacement parts are. So I rigged up a circuit to test the theory.

While waiting for Peavey parts, I ordered a VTL53C/2 from eBay to see if it came close to the 40101. I dropped that into my test circuit. I changed the voltages and R1 of the test circuit to force 40ma through the LED (as per the specs- see photo). I determined the resistance across the outer legs of the LDR to be ~ 1500 ohms. Is that close enough to the 2000 ohms as shown in the datasheet? I found my first substitute. A few days later, Peavey sent me the replacement part for the 30240101. Lo and behold, it was one in the same, a VTL53C/2. So my theory and testing proved it. Mystery part #1 solved!

As for the 40102 (the Red Dot), I found specs for the VTL5C2/2. That is what I thought the part could be. The Peavey part arrived and it had the original 21L628 number on it. So I changed up my test circuit (as shown) to push 40ma through the LED (as per the spec for the VTL5C2/2. Per the voltage measurements, I calculated the resistance across the outer legs of the LDR to be ~ 500 ohm. Is that close enough to the VTL5C2/2 spec of 700 ohms? Unfortunately, this part is getting harder to find.

Unless anyone can weigh in to the contrary, I think the LDR mystery might be solved.

Look for these parts just in case Peavey ever runs out.

70240102, 21L628 -> VTL5C2/2 (Edited)
30240101, 21L265 -> VTL5C3/2

[TomCarlos]

Thanks Enzo...

Earlier, I found an old thread from 2010 titled "Not Enough Bias Voltage?" In that thread, you offered advice on the Bias circuit for the Peavey Butcher amp. The bias circuit in that amp is identical to that of the Encore 65; even though the schematic looks backwards!! So I will try out your suggestion.

[Enzo]

You increase R67 because it works. Decreasing R58 won't. R67 is only 470 ohms. The bias voltage is the result of the voltage divider the two resistors form. R58 is 47k. If I knock R58 all the way down to 4.7k - 1/10 its present value - it only drops the bias 5 volts. In other words with 470 ohms you have no leverage for the divider. Plus the divider draws more current from the supply. I can raise R67 by a factor of 10 for the same 5 volts, but it is easier on the system and if you make that resistor variable you get a better range control.

[Helmholtz]

So there are a couple ways to do this. As Helmholtz suggested, I could increase R67. The other option is to lower R58. Perhaps if time and energy allow, this would be a good candidate to add a variable bias pot into the circuit. I have seen examples of how to do that. R67 stays the same. R58 is replaced with a fixed resistor and pot. But for now, a good experiment would be to tack a resistor in parallel with R58 to see if I hear a difference.

I recommend to increase R67 because this actually improves bias supply filtering while lowering R58 has the opposite effect.

[TomCarlos]

I apparently wrote my notes backwards! So it should go like this...

The more negative the Control Grid voltage, the lower the plate current.
If you make the CG voltage less negative, that will increase the plate current.

So there are a couple ways to do this. As Helmholtz suggested, I could increase R67. The other option is to lower R58. Perhaps if time and energy allow, this would be a good candidate to add a variable bias pot into the circuit. I have seen examples of how to do that. R67 stays the same. R58 is replaced with a fixed resistor and pot. But for now, a good experiment would be to tack a resistor in parallel with R58 to see if I hear a difference.

Enzo.... What is wrong with running cool? I guess nothing. That is why I asked the question wondering if the change in current (operating point) would have change overall sound of the amp. So this would be an experiment so to speak. And since I do not rely on an amplifier operating like a Shredding Machine, running cool is probably OK !!

[Enzo]

As was said, -57 on the grids will be even cooler. And so what is wrong with being a bit cool anyway?