Peavey Encore 65 Misc Parts Question

[TomCarlos]

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.

[Enzo]

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

[TomCarlos]

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?

[Enzo]

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.

[Helmholtz]

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.

[TomCarlos]

Thank you Helmholtz... I will take a look at C19 as well.

[Helmholtz]

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?

[TomCarlos]

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.

[g1]

Do you get that same 475ohm reading for R31 when it's connected in circuit?

[g1]

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.

[TomCarlos]

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.

[Helmholtz]

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?

[TomCarlos]

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.

[Helmholtz]

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.

[TomCarlos]

Will do!! Thanks!!!