Weird Tube Preamp "Gain Recharge" Problem

[Greenback25]

It's a Langner DCP-1 preamp clone with added Langner Elite and Hyper mods, the schematic is here:

langner DCP-1_ thrash preamp Shematic.pdf

The problem that I'm encountering is a bit weird: I play a constant DI guitar signal through it and, for the most part, the preamp reacts normally to the DI with the expected gain level. However, as soon as the DI loop restarts, there is a little bit of blank space where the preamp doesn't get a constant stream of audio signal fed through it and then the preamp gets into a "recharge" type of mode, where the gain is noticeably below the level and muddier than it should be and it needs a good 10 seconds or so to get back to the gain level it's supposed to have.

You can hear the recharge phase in the beginning of the clip and after 10 seconds or so, you'll hear how the preamp should in fact sound:

https://vocaroo.com/1hLROmdXI4bz

I already checked the following:

All 5 preamp tubes have been giving me a "good" reading on my Orange Valvetester MK II, nevertheless, I replaced V1 with a brand new tube and the problem still persisted.

I checked the functionality of the lead channel potentiometers and they all are doing their job correctly, from the gain all the way to the limit (channel volume) control.

This problem only really occurs if there is no signal present for however short a period of time it seems and then it falls back into that recharge phase, otherwise, it works quite well.

What could this be? Anything that I should check in particular?

[xtian]

Measure resistance from each gain stage's grid to ground (except V4b). None should exceed 1M+62K.

[R.G.]

+1 to xtian.

Most likely it's a bias problem. The event discharges a cap in the signal stream and it takes seconds to ramp back into normal operation.

One possibility for debug is to watch the plate voltages while flipping the switch. If it's a bias problem, the plate will have a big voltage change.

[Greenback25]

Measure resistance from each gain stage's grid to ground (except V4b). None should exceed 1M+62K.

Thanks for the response! Since I'm not a technician, I only know the basics, should I approach this task like this:

1. Discharge the Preamp by checking Pin 1 on any of the preamp tubes so they're below 1V

2. Remove the preamp tubes

3. Use the Multimeter in Resistane mode and put the black probe to ground (mains chassis grounding spot) and the red probe into pin 2 and pin 7 per preamp socket

Or would I need to leave the tubes in there and check them via the terminals which are exposed on the lower part of the socket?

[xtian]

No need to remove tubes, but yes, discharge first.

[Greenback25]

No need to remove tubes, but yes, discharge first.

I measured Pin 2 & 7 versus Chassis Ground resistance and here are the results:

V1 Pin 2 & 7 = 2.6 k

V2 Pin 2 = 0.86 k
V2 Pin 7 = 2.6 k

V3 Pin 2 & 7 = 3.84 k

V4 Pin 2 = 100 k
V4 Pin 7 = 2.64 k

V5 Pin 2 & 7 = 98 k

These measurements were made with a discharged preamp and removed preamp tubes, so that I can get better access to pin 2 & 7. None of them are above 1 Megaohm. Does that mean that everything's correct when it comes to grid resistances?

Is pin 2 with 0.86 Kiloohm on the first triode side of V2 faulty? You already singled out V4b so I kinda expected it to be different than V4a, but you didn't mention V2a and b, so maybe the fault lies here?

Here are some pics of the circuit around V2, maybe some mistakes are evident there, I don't know... Is there anything else that I should measure?

20260714_011723.jpg

20260714_011721.jpg

[sluckey]

All those resistance readings are crazy wrong. Looks like you measured from the tube side of the socket but counted pins in a clockwise direction. That's backwards. You should start at the key (wide space) and count pins in a counterclockwise direction when measuring from the tube side of the socket.

[Greenback25]

All those resistance readings are crazy wrong. Looks like you measured from the tube side of the socket but counted pins in a clockwise direction. That's backwards. You should start at the key (wide space) and count pins in a counterclockwise direction when measuring from the tube side of the socket.

Damn, I totally forget that, viewed from the top, things are a bit different. Thanks for pointing it out!

Now that I've hopefully used the correct 2 and 7 pins, here are the results:

V1 P2 = OL / Over Limit
V1 P7 = 60 k

V2 P2 = 60 k
V2 P7 = 84 k

V3 P2 = OL
V3 P7 = 0.13 Ohm

V4 P2 = 62 k
V4 P7 = 140 k start, steadily rising

V5 P2 = 0.79 M
V5 P7 = 0.79 M

I assume V1 pin 2 is has a problematic reading, as well as V3 P2? Is this indicative of a resistor failure? What else should I test next?

BTW: I used the blue circled pin as pin 2 in the measurements above and the red circled pin as pin 7. Should be correct now, but just want to clear up any potential misunderstandings...

20260714_011721.jpg

[xtian]

V1 P2 = OL / Over Limit
V3 P2 = OL

These are obvious failures (assuming your measurements are accurate. Look at the schematic for V1.2: the grid is connected to ground via R2 and R1, meaning you should read 1M+62K to ground.

V3.2 is connected to ground via R34 (62K), or directly to ground if that "Rel 1" switch is closed.

Start with V1.2 and figure that out. Should be easy.

[TUBEDUDE]

Looking at the board I'd recommend touching up several of those joints. A handful look dry a few of the plate resistors and a 4K7 look like they need some solder.

[Greenback25]

V1 P2 = OL / Over Limit
V3 P2 = OL

These are obvious failures (assuming your measurements are accurate. Look at the schematic for V1.2: the grid is connected to ground via R2 and R1, meaning you should read 1M+62K to ground.

V3.2 is connected to ground via R34 (62K), or directly to ground if that "Rel 1" switch is closed.

Start with V1.2 and figure that out. Should be easy.

Aren't the capacitors in the vicinity interfering with correct resistance measurements? For example, I re-measured R1, the input resistance which should me 1M, but instead i'm reading a very low Ohm rating of maybe 0.15 or something like that.

The R2 resistor, which should be 62k, meausures about 60k, so I assume that one is correct, even though I measured it without removing it from the circuit.

Is the R1 resistor in this case the culprit behind the OL reading on pin 2? It's still a bit weird to me, as without the 1M input impedance, I assume the tone must be extremely dark, but the preamp sounds ok IMO except for that recharging behavior of the gain structure.

Regarding the V3.2 pin: The preamp tube constellation starts with V1 on the right (when viewed from behind the preamp), then followed by V3, then V2, then V4 and V5 on the far left.

So, inactuality, it was V2 with with the OL reading on pin 2, not V3. In any case, the corresponding grid resistor for V2 is R5 and I measured that one slightly above 60k and it should be 62k.

Seeing how both V2 (which is on the lower row on the schematic) and V1 are both directly connected to the input and share teh C1 (58nF) cap, could this be the definitive indidcation that R1 (1M) is definitely at fault here, or is its reading messed up due to the C1 capacitor right next to it?

Here are some additional pictures:

r1.jpg

v1.jpg

v1r2.jpg

[pdf64]

Aren't the capacitors in the vicinity interfering with correct resistance measurements?

No.

For example, I re-measured R1, the input resistance which should me 1M, but instead i'm reading a very low Ohm rating of maybe 0.15 or something like that.

The shorting contacts on the input socket are doing their job. Try repeating the measurement with a plug in.

The R2 resistor, which should be 62k, meausures about 60k, so I assume that one is correct, even though I measured it without removing it from the circuit.

We're looking for gross anomalies with these tests.

[sluckey]

Measure dc voltages for pins 1, 2, 3, 6, 7, 8 for all little tubes. Measure ac voltage for pins 4, 5, 9 for all little tubes. Some pins will read zero volts but that is important too. Post results.

Measure dc voltages for pins 3, 4, 5, 8 for all big tubes. Pin 8 will be in the millivolt range but it's important. Measure ac voltage for pins 2,7 for all big tubes. Post results.

Tell us about your meter. Make, model, and cost.

[R.G.]

I read through this again.

Do you know for sure the DI loop is not creating the fade recharge? Is the loop have no issues at all when its signal goes to another amp? It's probably fine, but on weird problems it's good to check everything. Techs with lots of scar... er, experience check their meters habitually to make sure they're functioning right.

Do you have an oscilloscope? If so, you could just probe the plate signals when it's acting up, and know which tube/section is having a problem. The commentary so far about resistors and bias and cold solder joints is a good first guess, but there are other possible issues. Same for watching plate voltages in normal versus bad behavior. If it's a bias shift it will definitely show up on the plate, and you will know which set of part to look into more deeply.

Another dodge to find the acting-up stage is to use an audio probe into another amplifier. This will let your ears discover the stage that's acting up.

Finally, it might not be bias or resistor drift. A dark horse candidate might be a squark of ultrasonic or RF from the Di or the preamp's response to something the DI does that isn't audible. In this case, the symptoms should show up on multiple tube sections.

[Stevem]

I did not read through this all, but it sounds like a issue covered on the Duncan amp site in there technical section under preamps .