SSS inspired amp

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Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

Im thinking it is alittle of both.
Im seeing some distortion in the preamp that is probably some crosstalk and perhaps something is being pushed too hard when the standby switch is flipped to "on". It will crackle and squack for a second or two then quickly go silent. im using two 1N5408 (3A) diodes total for rectification. im thinking maybe 2 per string is required. I will keep you posted :-)
Thanks Martin!!! 8)
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

I got the distortion in the front end mostly contained. its much louder now. I will need to re-scope the signal. I removed the master for now and have just one volume control.

I added a second 1N5408 diode in the rectification strings. the standby switching is better but still makes noise. getting there though.

Im not exactly sure what to expect from the sound, as this is a totally weird circuit. BUT when I play it LOUD like half volume it is punchy and clear with a hint of crispy breakup. (humbuckers) at full volume with a pair of 6550's it reminds me of my hiwatt when cranked up. does this sound accurate?
This is all hearsay until I get the signal scoped again.
Synchu
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Re: SSS inspired amp

Post by Synchu »

Did you try to bypass the diodes with 0.01uF ceramic caps (make sure to use proper voltage rating)?
While it doesn't sound like the problem is diode switching noise, it might be just that.

Niki
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

I haven't bypassed the diodes with caps (yet) but I had the reverb hooked up and I was playing around and when I flip the standby the noise is carried through the reverb. you could here reverb added to the noise. so I got a good idea of where to look. My guess is the B+ feed for the reverb transformer is at fault.
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

I think one of the 6550's is faulty. tapping the tube causes crackling. high volumes caused a lot of harsh sounds to come through and I think the faulty 6550 was oscillating. I resoldered the B+ node feeding the reverb transformer and 6550 screen resistors. nothing changed. its gotta be power supply related.
I tightened up the tube socket pins and installed a pair of new 6l6gc's. The standby noise is dramatically reduced but still present. with the 6l6's at max volume the amp has an ECHO of distortion. If I play a crunchy power chord it will echo behind the original notes.

i am going to trace the signal and the standby noise by working from the back to the front with the scope and see where its coming from.
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

finally getting somewhere.
I resoldered all B+ connection and verified my output tube cathode connections. I also measured the heater voltages from the top of the tube sockets. I was thinking maybe it was a poor heater connection.
when the volumes (mv and preamp) are set to zero no sound is produced upon flipping the standby. but when the volumes are turned up alittle the buzz quickly quiets down when flipping the standby.
I then started pulling the tubes and flipping the standby. I determined V3 is part of the problem, which is the reverb recovery and final gain stage before the phase inverter. With v3/v4/v5 (individually) removed no buzz is produced. when I was tube swapping v3 I thought I was going crazy. sometimes I would get a horrible b+ hum from the amp and the next tube it wouldn't hum. so id put the original tube back in and it was quiet. I think we got a cold solder on V3's tube socket. It could also be the reason im getting real nasty distortion with the master nearing max.
That's as far as I got yesterday.
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martin manning
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Re: SSS inspired amp

Post by martin manning »

Can you list the function of all the tubes and the number by which you are calling them out? That will help clarify things since there is no schematic to refer to. Random thought... is there any possibility that you have a bad filter cap?
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67plexi
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Re: SSS inspired amp

Post by 67plexi »

I built a 150 Watt and the standby switch was arcing changed to a 10 amp
switch now no snap crackle pop.
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

Thanks for the reply guys!

I am going to draw up a schematic this weekend.

V1a - tone stack and v1b gain after tone stack
v2a and v2b - Reverb
v3a reverb recovery gain stage
V3b - blending gain stage for preamp and reverb.
v4-LTPI
v5-CF
V6/V7 - 6550 output

I did some scoping and maxed out im getting 90v Peak to peak at the 6550 grids this is with a 60mv 1K signal applied at the input. (6l6's at the moment while the New 6550's in the mail).
BUT at idle the grid bias voltage is -55v, maxed out it drops to -88v.
I am using the 60v Bias tap on a Hammond Twin reverb PT.

I tried playing with the NFB while the amp was maxed out but saw little difference.

With the scope at the master volume input lug and a 60mv 1KHZ at the input , every looked good but as the master volume goes up I saw the signal begin to crimp in the middle and distort. I went back through the preamp and even on V1a's plate coupling cap when the master volume is cranked up the signal still slightly distorts.

To me that's a power supply issue. so I checked the plate voltage idle=447v maxed with signal = 417v I would expect some sagging but it seems like too much.

So im dumping all these observations off so maybe it will rings some bells with you incredibly talented individuals.
I kinda jumped into this project without a lot of planning. my tone stack was entirely too spread out so I needed to wire the stack directly to the pots. I got the preamp itself V1, V2, V3 looking good. It clips when the preamp volume is dimed, but as far as im concerned that's normal.
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

67plexi wrote:I built a 150 Watt and the standby switch was arcing changed to a 10 amp
switch now no snap crackle pop.
I am going to short the switch and see if it makes a difference.
thanks for the feedback 67plexi!

Im using a carling 6A switch (part 110-63).
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

martin manning wrote:Can you list the function of all the tubes and the number by which you are calling them out? That will help clarify things since there is no schematic to refer to. Random thought... is there any possibility that you have a bad filter cap?
I am starting to think it is a bad filter cap.
Node A-feeds the OT 2x 220uf/350v in series with 220k bleeders.
Node B - Feeds Reverb transformer/CF plates(V5) and 6550 Screens 1x 50uf@500v (THIS GUY IS SUSPECT). I was even thinking of maybe splitting this up to give the 6550 screens their own supply.
lovetone
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Re: SSS inspired amp

Post by lovetone »

Ok leave the PI out, pull each output tube out (standby on crackling present) It might point to a faulty tube. Also this could be a screen resistor so if you pull a tube and the crackle stops put a different tube in that position, if the crackle re appeares then try the screen resistor.

Another thing to try, if you end up with all the tubes pulled and the crackling stops so you need one or two tubes to hear the crackling try the tubes in the inner and outer tube base's (sockets) if the crackling is the same what ever then the fault I would think lies in the output transformer or power supply

With regards to a power supply fault it is more than likely to be a resistor.

Hope this helps

Geoff
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

Hi Guys, so as suggested, i swapped the vintage CC 4.7K dropping resistor. Did alittle tube swapping and its functioning perfectly!! (besides the reverb footswitch injecting hum)) A JJ 12at7 was also to blame. I think thats why i was having such a hard time figuring out what the hell was going on when tube swapping. There were 2 issues contributing to the noise. Tube swapping was easy once the PS resistor was changed. I was about to completely rebuild the bias supply haha.

I think im going to try a Local feedback network like drawn in from SSS#02. from the cathode of V1 .any other suggestions that are easy to implement?
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

Reverb Footswitch cable is not shielded. Woops haha.

I got the schematic drawn up. I will get them scanned in tomorrow at work.
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dorrisant
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Re: SSS inspired amp

Post by dorrisant »

Nice job... Glad you got it working.

I recently spent way too much time getting screwed by a PS resistor... Good to see an end to your frustration.
"Education is what you're left with after you have forgotten what you have learned" - Enzo
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