Powerup thump Valvestate 8200

[Van Esch]

Hello everyone,

It´s official: I´ve met a guy with the same 8200 head and he hooked it up to my speakers: hardly any audible pop.
Mine gives a thump that makes you fear for your speakers every time you turn it on.

I´ve replaced every cap/elco and resoldered everything on the power-pcb. It´s definitely something on the power-pcb because preamp is not hooked up, only powersupply(30volts red/black, 60volts red/red, giving 80 volts after rectifier) and speakers. It does so on left and right speaker, switching does not make any difference.

The rest of the amp works fine. Right now I´m thinking maybe inrush current to the big elco´s and experimenting with surge ntc´s for that.

This is the schematic(also as attachment) from drtube: http://www.drtube.com/schematics/mar...8280-63-02.pdf

I love the amp, but I love my speakers too! Please help!

Greetings, Bram.

[DrGonz78]

Not trying to sound like an A$$ here, but I think you should of at least included the other thread on this...

http://music-electronics-forum.com/t30276/

There are some people here that might not realize you were already working on this amp a while back... So here's is a link...

[Jazz P Bass]

It would be interesting to know if the speaker moves one way all the time. (in or out)
If your power rails are not coming up at the same time, that may be your thump.

[olddawg]

It would be interesting to know if the speaker moves one way all the time. (in or out)
If your power rails are not coming up at the same time, that may be your thump.

Exactly, test for a DC offset on the output. Also the power supply rails should be symmetrical.

[Van Esch]

I started this thread this thread with the same info I gave the last the time plus one new thing: the confirmation that the pop MY amp gives, is not normal to this amp. Nothing since has been done to the amp.

Output dc is 0,057 v left/0,052 v right. Voltage on rails is 41,4 v on C5 side, 41,3 v on C6 side. In the afternoon I can do the speakertest. Any other tests that may find something?

[DrGonz78]

Okay I have been just rapping my thoughts on this one for a while now. I am not tech savvy enough to really help that much here. However, it appears that you have removed the preamp section. Just have to ask if you have removed connections to reverb circuit or effects loop too?

As I recall on the VS100 they are all tied to together in some form.

Edit: yes they are tied together I see it now but only boards now connected are 8280-64-02 and 8280-63-02 correct? This is a fixed bias type amp correct? Those junction points with TR1 & TR2 and TR6 & TR7 may be off now? God this one is beyond me... lol.

[Van Esch]

Correct! 8280-64-02 is really just the outputs on a different little board. The preampsignal passes there as well to the lineout but it´s all disconnected. Effect loops and reverb are onother board tied to the preamp-board. Basically the only things working are all on 8280-63-02: PSU and output. Which doesn´t make a difference: the speaker pops connected or not connected.

The bias seems fixed as there is no adjustment that I can see. I have no idea what tr 1,2,6,7 might do in this case. I´ll do the speaker test lateron, but since popping occurs equally on both left and right, I was leaning toward something wrong in the circuitpart they both have in common.

[DrGonz78]

I am totally guessing here... I think the bias as way of balancing the DC voltage to that final output stage. Lets says some of those transistors are not matched up together... So monitor the output speaker terminals for DC at the start up of the amp. Repeat this a few times at least and see if you see a DC surge upon starting the amp. You could pull those transistor and check the HFE to make sure they are matching. Not saying this is what is going on, but this a very strange problem. Others who know more than I will chime in soon I hope.

Edit: It seems the amp is biased enough... Maybe though at start up of the amp is the one time you hear the bad DC offset take place?

[Van Esch]

At startup the speakers always move inward first, then out. Also something that might be important: once on, then turned off and immediately or until more or less 6 seconds of waiting(elco discharge?After turnoff you can hear the normal hardly audible hiss for about 6 seconds) turned on again there is no thump at all.

[olddawg]

Do you have a scope? Some amplifiers will run all day long with a DC offset. You need monitor DC on the output to see if it stabilizes at 0v after the thump. There may be a momentary mute circuit that is not working. Usually when I see something like this someone has subbed components in a previous repair. Diffamps, coupling, etc.

[Jazz P Bass]

At startup the speakers always move inward first, then out. Also something that might be important: once on, then turned off and immediately or until more or less 6 seconds of waiting(elco discharge?After turnoff you can hear the normal hardly audible hiss for about 6 seconds) turned on again there is no thump at all.

The speaker always goes in first, then out.
When is the thump?
My guess is on the out.
If you hook up your meter to the speaker terminal, to read Vdc, you may narrow down the polarity that causes the thump.

[Enzo]

The amp sits there running with no particular DC offset, right? It only has a thump at turnoff problem? MAny amaps do that, and yes, even within one model, some may do it more than others.

TR1,2,6,7 are the differential pairs at the input of each channel. These are pretty cconventional amp stages.

When you turn an amp off, the power supplies discharge. A couple things MAY happen. If the two supplies do not discharge at the same rate, the relative shift in balance is amplified as a thump. Also, as the voltage drops in the power supply, some amps go unstable. A solid state amp depends on a DC balance between positive and negative rails, and if it cannot maintain that, it will swing its output from side to side. One way to see this is to bring an amp up slowly on a variac. SOme amps will come up smoothly. Others will go through a range where they will hum loudly - due to a momentary large DC offset. This goes away a few volts higher.

SOme advice: it is possible you have a "bad" component, but never assume that problems are always bad parts. That will blind you to other possibilities. Might be a soft filter, sure, but never assume it is only a part.

Those 4700uf main filters? One of them might be less healthy and discharges faster.

Your output devices are different types, opposite polarity types. There may be some variation in the characteristics of one versus the other, and in your amp it may have added up to more than in another amp. And what can happen is that one channel drags one side down faster than the other, but the power rails being in common, it will make the other channel mimic the behavior.

[Van Esch]

No scope unfortunately. Today I´ll try to see the output-dc on startup. It stabilizes immediately to the 0,057v on both channels mentioned earlier. Shouldn´t a mute circuit be in the schematic or do I fail to recognize it? Inrush current not a possibility then?

[Enzo]

There is/are no mutes in the power amp, at least not if your schemaqtic is correct. Preamp may have somethihng for its part, don;t have that handy.

It will be very difficult to characterize the charging/discharging rates of the thing with a meter.

A thermistor in the mains lead as an inrush limiter might help or might make it worse. If this is a moment of instability while the power supplies move, then such a soft start might actually lengthen that noise period. Hard to say.

[Van Esch]

I can confirm that probably: even with a pretty decent Fluke set on max memory it doesn´t register any dc spike on output. A thermistor(s) would have to be pretty light in a ss setup like this to stay conductive with so little current at idle?