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Thread: Peavey Encore 65 Misc Parts Question

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    Supporting Member TomCarlos's Avatar
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    Peavey Encore 65 Misc Parts Question

    MEF members:

    I have a Peavey Encore 65 that is in need of some maintenance. I am checking parts and making my shopping list.

    I noticed several caps on the schematic that have electrolytic cap symbols. For example, C7 is shown as a .47uf, 50v. On the board, there is a purple-ish .47 16 volt cap. The schematic shows less than 2 volts at the cathode. (There are other tubes that have a similar setup). The yellow cap in the photo is C5, .047uf, 630 volts. That appears to be a poly film.

    I checked the other "Encore 65" threads here on MEF and other threads. No one asked about that cap. The cap measures fine but I was wondering, is this really an electrolytic?

    Thanks, Tom
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by TomCarlos View Post
    The cap measures fine but I was wondering, is this really an electrolytic?
    It sure is. But if you want to sub in a film cap, no problem. It's not under a lot of stress as a cathode bypass, what's it going to see maybe a volt or two. More than likely it's OK for the next 20 years at least, I'd leave 'er be.

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    The purple is electrolytic. It is marked for polarity. 0.47uF used to be a fairly large and expensive size in non-electro, so they used electrolytics. Now you could just use a non-electro there.

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    Lifetime Member Enzo's Avatar
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    It looks like an electrolytic, is made like one. The schematic shows the polarity next to it. WHy should we think it is not one?

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    Supporting Member TomCarlos's Avatar
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    Mmmm.... my lack of experience with these.... have not seen this before, I cannot find one like this on the net .... take your pick :-) Thanks for the replies.

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    Lifetime Member Enzo's Avatar
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    If it isn't bad, don't replace it. Or if you have some 1uf, close enough.

    Or:
    https://www.mouser.com/ProductDetail...3YSdqOPfP4w%3D

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    Supporting Member TomCarlos's Avatar
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    Next Q - Referring to the schematic pic in #1, any reason why a plate resistor needs to be 1 watt? It looks like all the preamp tubes have a 1/2 carbon installed.

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    Lifetime Member Enzo's Avatar
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    it dissipates about 1/6 watt, but they might have used the larger resistor for noise reasons or some such.

    A lot of film resistors are smaller than what we are used to for a given wattage.

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    Supporting Member TomCarlos's Avatar
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    Thanks Enzo....

    I didn't think the power dissipation would be over 1/2 watt, even when under a full signal.

    On to the next item I am brushing up on - Optical Isolators. I get what they do. Most of the examples and tutorials show a 4 legged sample. Peavey uses a 5 leg bug. #2 and #3 are marked as 40101. The original Peavey part number was/is 21L565. #1 is a 40102. That part number was 21L628. I am trying to find specs sheets on these so I can get a better understanding of how they work.

    Interestingly enough, #3 is connected somewhat differently than #1 and #2. The center leg is not connected to anything. So that is why I am trying to get a spec sheet on this. Oh, one other thing, #3 appears to have some "crud" on the leads. Maybe that isn't a big deal.

    I am recapping this amp, touching up solder joints, etc etc etc. Soon, I will reassemble the board to the chassis. But this is a good time to study up on the circuit design and parts.
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    Lifetime Member Enzo's Avatar
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    Optos are optos. A light source - in this case an LED, compared to the neon bulb in a Fender - and one or two photocells. Photocell is a light sensitive resistor. Dark the resistor has high resistance, Shine a light on it, the resistance goes down. LDR means light dependent resistor.

    Three legs? Just means it has two resistors. One end of each connected together. In the case of #1 and #2, that common end is grounded. The LED has no idea how many resistors are watching. In the case of #3, I think what they did was use the two resistor type, and left the center connection open, so the two resistors are in series now. That just gives twice the resistance when dark, making the reverb more OFF when supposed to be.

    Don't worry about the oxidation on the legs.

    Look at #3. Normally the LED is lit by current through R37. This shines on the photocell lowering its resistance. So the reverb passes through it from the control to V3b. Reverb is ON. Ground the tip of the footswitch jack, and it shorts across the LED, making it dark. That allows the photocells to go high resistance, effectively turning off the reverb.

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    Supporting Member TomCarlos's Avatar
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    Thanks Enzo...

    After reading your explanation, I found a white paper on Vactrol Optical Isolators. Vactec introduced the compact Resistive Opto-Isolator branded as Vactrol

    I'll see if I can find the Vactrol (or other brand) replacement part numbers (if I ever need them) and distinguish the different between the 40101 and 40102. I am guessing the difference is in the dark resistance spec and temperature coefficient. More to come on this.

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    I finished recapping all the electrolytic caps and replaced a few resistors that were slightly out of tolerance. So the amp is passing a signal - with a couple issues. I will deal with Issue 2 another time.

    Issue 1 is a non working reverb circuit. I will deal with the LDRs later. But for now, I have a burned out Reverb Transformer. I discovered this when measuring 0 Volts at Pin 6 of tube V3A. The Blue Wire of the Transformer has 455 volts. But there is nothing at the pin for the Red lead. I removed the wires and there is an open on the primary windings.

    I'll give Peavey a shout tomorrow and hoping this transformer (705-00152) is available. If not, does anyone have a lead on a replacement? I cannot find specs on the Peavey Xformer.

    Thanks, Tom

    UPDATE: Photo of the transformer and the reverb circuit.
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    Last edited by TomCarlos; 03-24-2020 at 06:13 AM.

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    Lifetime Member Enzo's Avatar
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    Specs/schmecs, it is a 12AT7 tube driving a reverb transformer. I'd drop a Fender in there in a heartbeat.

    meanwhile, pull the transformer, it needs to come out anyway, right? Open it up and see if possibly it is just a failed connection between the winding wire and the insulated wire. Of a broken winding wire. Might be fixable.

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    Supporting Member TomCarlos's Avatar
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    Thanks Enzo.... I found the Fender replacement, wasn't sure if it would be a match. But yes, I will yank it out and look at the leads coming out of the paper to see if they are cut or damaged.

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    Ok... new reverb driver / transformer is in and the reverb circuit is working. I got the Fender Replacement Transformer (SKU P-TF22921) from Antique Radio Supplies. Perfect fit. Unfortunately, even though this is a "Fender Replacement", don't expect the deep lush sound that you'd get from a real Fender Amp!! But for a Peavey, hey, can't complain.

    In a day or so, I will come back to this thread. Enzo got me thinking about the LDRs. Mine are working. But I want to learn more and see if I can figure out the differences in the 40101 and 40102. I bought myself one of each from Peavey and they should arrive next week. In the meantime, I setup a test circuit to evaluate an Xvive VTL5C3/2. No, you cannot measure the "Off Resistance" of 10 Meg. Your meter will show an "Open." But I was able to push 40ma through the LED and was able to determine that the resistance across the series resistors was close to 1.5K (close enough to the 2K stated value). And sure enough, if you decrease the current through the LED, the resistance on the LDR outputs increases. However, the LED current to Resistance Out is NOT linear. If you look at the datasheet's "Output Resistance vs Input Current" curve, that shows how the LDR behaves. Anyway, when the Peavey LDRs arrive, I will report back.
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    Lifetime Member Enzo's Avatar
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    Doesn't need to be linear. It is used as a switch. LED is ON or OFF, not slowly dimmed down.

    I doubt the reverb tone is from the transformer, rather the circuit around it.

    70240102 is the Peavey part number. The part itself is a 21L628. The 21L628 is the data sheet to look for.

    I don't offhand know the difference, but one thing that comes up is speed. Light them up the resistance goes down. Turn the light off, and some revert to high resistance faster than others. it may be difficult for you to test for that. 200 milliseconds versus say 20 milliseconds.

    Here:
    http://denethor.wlu.ca/pc300/optoiso...troduction.pdf

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    Supporting Member TomCarlos's Avatar
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    Unfortunately, finding a datasheet for the 21L628 or the 21L565 has been next to impossible. I will see what kind of parts Peavey ships.

    Per the measurements from the actual circuit, there is 9ma flowing through the LED (in the LDR). The center point of the internal resistors is not connected to the circuit. So the circuit is using the outer pins only (both resistors in series).

    In the meantime, I have a question on tube V3A (12AT7) of the reverb circuit. As you can see in the photo, V2B (12AX7) and V3B (other side of the 12AT7) appear to be biasing as expected. The issue is with V3A. It seems to have twice the current flowing through the tube as expected.

    Blue Lead voltage at top of Reverb Transformer, 434v
    Plate Voltage = 427v
    Grid Voltage = .05v
    Cathode Voltage = 4.62v

    I changed tubes (3 different 12AT7 tubes), same results.
    I checked R31 and it measures 475 ohms.
    I replaced C20 with a new 1uf electrolytic cap, same results.
    In fact, changing the cap from the .47 caused a slight increase in the voltage.

    So it's not clear to me why I am getting close to 10ma when I would be expecting to see around 5ma.

    Any ideas on this?
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    Lifetime Member Enzo's Avatar
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    Difference in DC resistance of winding.

    Read the thing I posted about the Vactrols. They are equivalent. I don't know which numbers are which, but the document describes the diffrences in types. Should be able to reason the applications.

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    So it's not clear to me why I am getting close to 10ma when I would be expecting to see around 5ma.
    Tube current is mainly determined by the bias voltage (grid voltage minus cathode voltage). The small difference in plate voltage cannot explain the much higher current (actually it would take a higher plate voltage to increase current).
    You may have a leaky C19 and the the actual gid voltage might be higher than .05V because your meter resistance loads down the voltage.

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    Supporting Member TomCarlos's Avatar
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    Thank you Helmholtz... I will take a look at C19 as well.

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    Quote Originally Posted by TomCarlos View Post
    Thank you Helmholtz... I will take a look at C19 as well.
    Just took a second look at the schematic and I meanwhile doubt that a leaky C19 could be the culprit as the voltage at the junction of R29 and R40 should be zero.
    Does R31 have a good ground connection?

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    Supporting Member TomCarlos's Avatar
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    The junction of R29 and R40 have .001v.

    Pin 7 has .024 volts.

    I removed R31 to check it. When I reinstalled the resistor, I scraped away at the trace to make sure the resistor lead had a good solder connection to the trace.

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    Don't forget the joker g1's Avatar
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    Do you get that same 475ohm reading for R31 when it's connected in circuit?

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    Another thing Tom, your voltages written on the schematic indicate a 700ohm resistance for the transformer. Try disconnecting C21.
    Fender numbers are usually around 1K7 here I think. The peavey voltages indicate the peavey transformer primary resistance would be around 4K.

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    Supporting Member TomCarlos's Avatar
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    G1 -

    R31 measures 475 ohm in circuit.

    I removed C21 and no change on Pin 8, still at 4.6v.

    I think you figured it out. I was pondering the notion of swapping out one transformer for another, that it would (perhaps) behave differently. Your analysis makes sense. I see how you came up with the 700 ohms DC Resistance for the Fender Transformer. And following the same logic, the Peavey transformer would be approx 3700 ohms.

    Per the schematic....

    Current Calculation: 2.5v / 470 ohms = 5.32ma

    Transformer DC Resistance = (465 - 445) / 5.32ma = 3753 ohms

    That is a big difference.

    So let me see if this is correct. The Fender's lower primary resistance allows for more current to flow through the tube. As a result, without changing the Cathode resistor, a higher voltage will appear. No matter what the Cathode resistor, we will have 10ma flowing through the tube. So it wouldn't make a difference if R31 were lowered to ~250 ohms. We would still have 10ma through the tube. And this would be ok because the max current rating through a 12AT7 is 15ma.

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    There seems to be a misconception. A DCR difference at the plate of a few k won't change tube current. As the internal plate resistance of the tube is much higher than the plate load DCR, it's the tube that dictates the current (acting as a constant current source).

    Does grounding of V3A, pin 7 change cathode voltage?

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    Supporting Member TomCarlos's Avatar
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    Thank you Helmholtz. In fact, if I used different 12AT7 tubes, I will get a different voltage at the Cathode.

    I just thought it was odd that the other tube voltages came close to the schematic but this one was off.

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    Just did a graphical construction of the operating point with Rk = 470R. My results perfectly match your measurements (4.7V/10mA) There is some extrapolation involved, so the error might be up to +/- 20%.

    I tend to think that the values in the schematic are wrong. As a cathode resistor of 470R will result in a plate current of around 10mA, the max plate dissipation (2.5W) of the 12AT7 is exceeded by around 66%!

    For safe operation the cathode resistor would need to be increased to 1k or more.

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    Will do!! Thanks!!!

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    And a shout out to Enzo who did mention "DC Resistance of the winding" in #19. I didn't connect the dots on that. So thank you.

    Last topic on this amp, I wanted to post some notes on the Bias Control for this amp. I guess we refer to this as the non-adjustable fixed bias circuit? Note: I am using JJ 6L6GC 30 Watt tubes.

    I made some notes in the picture below. As you can see, the 6L6 tubes are running "cool." I don't see that altering R67 or R58 buy me much. The problem must be that the transformer is loaded such that we can't get the desired -57 vdc at the junction of CR12 and R67. So as we lose voltage across the divider and 220K Control Grid resistor(s), I can only get -52.2v at the Grids. I don't know if the difference of 3 volts would make a substantial difference, in raising the plate current, dissipation, or overall sound of the amp.

    I have seen other threads where it was suggested a redesign of this circuitry (some type of full wave negative rectifier) could get you there. After reading your comments on my other thread (Pre Planning For Modifying A Fender Super Reverb Amp), it is clear that "current" in the secondary winding must also be considered. So I don't think this is an option.

    Unless someone has a rebuttle, I think this is as good as it gets.

    Reference - Rob Robinett Tube Bias Calculator
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    As you can see, the 6L6 tubes are running "cool." I don't see that altering R67 or R58 buy me much. The problem must be that the transformer is loaded such that we can't get the desired -57 vdc at the junction of CR12 and R67. So as we lose voltage across the divider and 220K Control Grid resistor(s), I can only get -52.2v at the Grids. I don't know if the difference of 3 volts would make a substantial difference, in raising the plate current, dissipation, or overall sound of the amp.
    If you want hotter bias (higher plate current) you need less (negative) bias volts. That's easy to achieve by adjusting resistors. Just increase R67 value.

    BTW, the voltage drop across R56/R57 is most probably caused by your meter (indicating 10M meter resistance) and not real as grids don't draw current.

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    Quote Originally Posted by Helmholtz View Post
    There seems to be a misconception. A DCR difference at the plate of a few k won't change tube current. As the internal plate resistance of the tube is much higher than the plate load DCR, it's the tube that dictates the current (acting as a constant current source).
    I was thinking more about the possibility of a partial short in the transformer, or a leaky parallel cap.

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    I was thinking more about the possibility of a partial short in the transformer, or a leaky parallel cap.
    Both effects would reduce or even kill reverb drive but neither of them could noticeably change tube current as the tube acts as a constant current source with an equivalent internal plate source DCR above 40k.

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    Last edited by Helmholtz; 03-29-2020 at 10:22 PM.
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  35. #35
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    As was said, -57 on the grids will be even cooler. And so what is wrong with being a bit cool anyway?

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