What is this triac doing here? (Guitar Tube Amplifier schematic) - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 18, 2018, 10:10:40 am
content: So my guitar amplifier has recently lost some of it's sound. I suspect the abuse of moving it all the time hast taken its toll and I am going to try and fix it.Findingthe schematicswas quite easy.I took a look at it, and am a bit confused how tubes work but I can generally recognize MOS transistors in the triode tubes, so that is ok. However, I found this triac (T2, found in schematic section D1), and I don't understand what it is supposed to do. C21 has broken one of its legs, but from the function of the triac I can't really see what it is supposed to do. I suspect it is acting as some kind of switch, shorting out C21 at low frequencies? Could C21 being damaged impact significantly the performance of the amplifier?

author: SeanB
author_rank: Super Contributor
timestamp: November 18, 2018, 12:05:12 pm
content: Looks like it is being used as a time delay on the mute line, once it is triggered by an edge from the "crunch" switch it turns on the mute JFET Q2 for around 30ms minimum, probably to mute relay noise during the time the contacts are open, so there are no pop sounds.

author: spec
author_rank: Frequent Contributor
timestamp: November 18, 2018, 01:39:44 pm
content: Hi TheUnnamedNewbieNot sure how much electronics knowledge you have, so I will explain from first principles- just ignore what you already know.To kick off, a TRIAC is a three terminal device: MT1, MT2, Gate. The main current flows between MT1 and MT2 when the TRIAC is triggered. If the TRIAC is not triggered there is a high resistance between MT1 and MT2. Triggering is achieved, either by a permanent signal on the Gate or by a single pulse on the gate. Once triggered, provided that there is current flowing between MT1 and MT2, the TRIAC will stay triggered, even in the absence of a trigger signal.In the Peavey 6505+112 MT2 is connected to the gate of JFET switch Q2. When the TRIAC is not triggered the gate of Q2 will be at 15V and when the TRIAC is triggered the gate of Q2 will be at 0V. Q2 switches something on and off. I haven’t traced the circuit out any further- it is fairly complicated and is drawn in the spaghetti style. Also many component pin numbers are missing.If your objective is to correct the fault with your Peavey 6505+112, I would make two suggestions: TRIACs are as rugged as old boots (the gate is only slightly sensitive) and in the Peavey 6505+112, the TRIAC it having a really easy time, so it is unlikely that the TRIAC would have failed. But while the TRIAC may not have failed, it may have gone slightly leaky (but still be within specification) and with the strangely high value of R60 at 1M Ohm on M1, may be causing a voltage drop sufficient to affect JFET, Q2. If it were my amp I would be thinking of changing R60 to a lower value, maybe 100K Ohms, or even 47k Ohms. But I would need to check the circuit further. Rest assured, though, reducing the value of R60 would cause no damage.[2} Unlike TRIACS, JFETS are odd troublesome things that fail frequently. Either they degrade or suffer a catastrophic failure: open circuit or short circuit. So it would be worth changing Q2 and, maybe, all the JFETS in your Peavey 6505+112 (get the replacement JFETs from one of the top tier suppliers).Of course, the fault may be elsewhere. It is not possible to tell remotely. But I hope the above information has helped a bit. Good luck and don't forget to let us know how it goes.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 18, 2018, 03:25:18 pm
content: Thanks spec for that extended postI haven't used triacs or discussed them in ages, so a refresher is always nice.Now that I think about it more, I think the following is going on. When the channel is switched, T2 triggers. This discharges C21, and in doing so the gate of Q2 brings Q2 in a conducting state, shorting the signal to the ground, acting as a mute, while relays click (as SeanB pointed out). R60 has to be as big as it is so the capacitor takes sufficiently long to charge, giving the relays time to click before the voltage on the gate of Q2 is sufficient for Q2 to go open again. What does confuse me is why they went with 1 M\$\Omega\$ and 33 nF, instead of some smaller resistor and larger capacitor. Perhaps that combo just hit some cost-performance sweetspot.Knowing that, I don't think this was the original problem. With C21 gone, what I should see is that Q2 is always open, and hear pops when switching channels. Of course I will have to measure this to verify my hypothesis.

author: spec
author_rank: Frequent Contributor
timestamp: November 18, 2018, 04:35:38 pm
content: No sweat!Ahh, you know a lot more about electronics than I first thought.Sounds like a good hypothesis..