How NOT to fix a noisy Vibrato switch and damage your DRRI build. Help Please

[Guitardropper]

A year ago somebody brought a brand new DRRI pcb (probably replaced from a new amp for a turret board) to my hands and i decided to make an amp out of it (and play around with the components a bit). So i made a beautiful sounding heavily modified DRRI that was fine tuned (with a lot of time and research) to my taste. After that this became my favorite amp.

And now the description of the problem:

After reading on a hurry some advice on the internet ''I did something stupid''. I tried to eliminate the popping sound that the vibrato foot switch was making when turning on and off by wrongly connecting a 220pf cap to the white and the grey wire of the vibrato/reverb foot-switch.

After that turned the amp on and quickly realized that:

• The vibrato effect stayed always on, no matter what (even when i removed the cap and disconnected the switch)
• The Power tubes started to glow bright red hot (something is going really bad)

I immediately turned off the amp to prevent additional damage.The current state of the amp is:

• Power tubes get red hot when opening the standby switch
• Sounds comes from the amp normally (with the exception that Vibrato is always on)
• Reverb is working
• Controls are working
• No components shows signs of damage or burning (visual inspection)

The amp was biased properly at 27ma, components (caps, transformers, pots, tubes, reverb tank, switches and sockets etch) are all new. The pcb and most of the resistors and some caps on it are the only stock components and these have only a few hours of operation. The amp was stable and working great up until now.

I have modified some amps and built a couple in the past and i know that you should never do something you are not sure or does not understand completely (especially in a hurry) but it seemed like a simple fix with not much risk at the time and i feel really really bad that i damaged my favorite amp with my stupidity.

Thanks in advance for you help.

P.S. If you need i can post more info, photos voltages etch.The closest amp tech is located 400km away so i think i have to do it by my self. My soldering skills are up to the task but i don't have a deep understanding of the amp circuit (i can read schematics).

[sluckey]

A year ago somebody brought a brand new DRRI pcb (probably replaced from a new amp for a turret board) to my hands and i decided to make an amp out of it (and play around with the components a bit). So i made a beautiful sounding heavily modified DRRI that was fine tuned (with a lot of time and research) to my taste. After that this became my favorite amp.

That was a considerable challenge. Did you have all three boards or just the main board?

connecting a 220pf cap to the white and the grey wire of the vibrato/reverb foot-switch.

Assuming the amp is wired IAW the schematic, that cap cannot cause the symptoms you describe. Something else must have inadvertently happened.

[Guitardropper]

I had all 3 boards and did a lot of reasearch (on how not to create ground loops, how to wire everything etch) before i start (that was my second built). I took my time with checking everything and taking precautions.

As for the cap , considering its position in the circuit , i thought that could not make a lot of damage so i tried it. But when i switched the vibrato on to see if it cured the popping, i heard what it seemed to be a louder pop(not dramatically louder) then i switched it off and it was off. Then i wanted to make sure before i removed it i switched it one more time and it stayed on. By that time (my face was relatively close to the amp) i smelled a hint of something that started burning  i saw the power valves red hot and turned off everything.

I thought that the cap was charged and released the charge as i was switching on and off and that it cooked some component in the vibrato section that had as a resault to have it always on. But the power tubes is a mystery for me

[vampwizzard]

you said the amp WAS biased to correct values. What are your voltages now? Id be hunting down voltages and properly functioning parts from V4b on through the power section.

[Guitardropper]

you said the amp WAS biased to correct values. What are your voltages now? Id be hunting down voltages and properly functioning parts from V4b on through the power section.

I ll post a gutshot showing the switch and everything (sometimes  a picture describes it better) as well as the current voltage and amperage on the power tubes, in the picture you can see only v8 (v7 have similar values)

[Guitardropper]

Here is the schematic with the readings on the testing points i could measure with a multimeter (a couple of readings are really off)

[sluckey]

WHOA! Your number one priority is to get TP5 happy. It must have approx. -50VDC. Pull the output tubes and don't put them back in until TP5 is happy. This is the single cause for your tubes redplating and the tremolo being on always. Check the components in the blue circle.

Also with the amp turned off, measure resistance from TP5 to chassis. What have you?

[Guitardropper]

WHOA! Your number one priority is to get TP5 happy. It must have approx. -50VDC. Pull the output tubes and don't put them back in until TP5 is happy. This is the single cause for your tubes redplating and the tremolo being on always. Check the components in the blue circle.

Also with the amp turned off, measure resistance from TP5 to chassis. What have you?

The resistance reads 19.7K from tp5 to chassis without changing anything (output tubes stilll installed)

[sluckey]

(output tubes stilll installed)

PULL THEM OUT. YOU'RE KILLING THEM.

[Guitardropper]

(output tubes stilll installed)

PULL THEM OUT. YOU'RE KILLING THEM.

But the amp its turned off. Also in order to take any readings up to this point as to minimise the possible damage i had everything hooked up and opened the amp for a couple seconds, took the reading and close it  immediately before the tubes start to red plate or reach high temperatures. I ll have them out immediately before the next power on.

Thanks for your help. I am searching right now for the bad component

[sluckey]

But the amp its turned off.

You're right. But you don't need the power tubes installed to fix this problem. And they will be in danger every time you turn the amp on. Better to put them aside until you have the bias voltage problem fixed. Probably won't take long.

If one of those three components in the blue circle don't fix it, then measure the AC voltage from the transformer that feeds the bias diode.

[Guitardropper]

Probably won't take long.

If one of those three components in the blue circle don't fix it, then measure the AC voltage from the transformer that feeds the bias diode.

 r69 is the first i desoldered and its dead has no resistance reading. I ll check the diode and the cap just in case. Also i dont have any fp (fireproof) resistors is it ok to put a metal film 1/4w just to check the voltage on tp5 for now and buy one later tonight or should i go and buy one  before doing anything further?

[sluckey]

I think that 1/4 watt resistor will be ok just for testing. Heck, I would just put a gator clip lead jumper across the resistor for a test. But for sure get the proper resistor before you call it done.

[vampwizzard]

Sluckey, per the usual, is going to give the A+ solid advice in troubleshooting. I was probably wrong to not clarify to take the tubes out or do it on a variac.

My mentality in figuring out where the problem would've occurred (for future forum readers): you shorted two voltage carrying lines together and to ground. In my experience, the ground loop and its non protected devices are the most likely to go bad.. in the lowest impedance path under ground fault. Not sure if that helps or is 100% accurate but its helped me debug a bunch of problems in the past.

[Guitardropper]

I was away for a couple hours. I went to a local store but couldnt find the proper replacement for the resistor. I hooked up a metal film 22ohm 1/4wat and everyhting came back to normal. I then installed the tubes put the bias probe and confirmed the right voltage and amperage. Now all i have to do is to find online a FPresistor and  order it.

It may seem like a simple thing for most of you but i was really upset in case i had fried my amp (althought the clean guitar sound that was coming through was giving me an indication that things didnt had the time to go terribly wrong and burn the tube or a transformer)

Sluckey thank you very much for your help you made my amp and my day!

[sluckey]

My pleasure. It was fun.