The 6550 Experience

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shakti
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Re: The 6550 Experience

Post by shakti » Thu Nov 01, 2018 5:13 pm

LOL, don't even bring the speaker issue up...talk about rabbit hole!

I do have multiple cabs, but in the room I am currently using them I can't get away with playing them unattenuated through multipe cabs. Maybe sometime in the summertime - I live in the countryside on a small farm, so I can do it in the barn when no one else is around.

I put a post phase inverter master volume in mine, and much to my surprise I actually prefer the sound of the amp with the master turned down a little bit. This type of master volume reduces the negative feedback progressively as you turn it down. Maybe that was one reason I like the sound better - essentially turning down the master volume introduces something into the equation which is similar to using multiple cabs, at least in how the amp is interacting with the speakers.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

revolver1
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Re: The 6550 Experience

Post by revolver1 » Tue Nov 06, 2018 5:21 pm

Today I decided to put the NOS Tungsols in instead of the reissue Tungsols. I have to correct my sel as I previously said they were re branded as RCA but they are actually Sylvanias.

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I had to go with an 82k for the bias range resistor with the reissue tubes to get to 25ma but with the NOS tubes I had to change back to a 56k to get in range. I got sick of pulling the bias circ to bits, by chance back when I made the board by mistake I drilled two holes for wires in the bias circuit so now I've put two turrets in the holes I drilled and connected them with a thick bus wire to hold in place so now I can chainge out the resistor as required without upsetting the bias circuit.

I fired it up to recheck the bias and when I was done I wanted to have a listen for hum with the tone and volume up so I removed the aligator clip I had for my meter monitoring the plate voltage and just as i set the clip down on the side BAM! fizz, a little fireworks and smoke... Brown trousers...

I immediately switched it off at the wall and took a look. first thing I saw was soot and scorch marks by the heater leads on the PT and imeadiatly thought well crap tha'ts the PT shot but on closer inspection it was the snubber cap that crapped out. It was NOS and in hind sight a bad choice, I will subsequently removing these from all my amps and replace with new caps. Lesson learnt...

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So then I've gone back in and replaced the caps and because I had the bus wires on the bottom of the board that was a massive pain in the butt that I got fed up with, looks nicer but when you have to go back in its a massive pain so I decided to put the bus wires on top.

Apart from the fuse in the plug which I replaced every thing seems O.K except that when I flip the stand by off there is an audiable loud pop, I thought the snubbers were supposed to eliminate this?

I've done a 100w before and this was my first shot at a 50w, deviating from the instructions I had to implement the snubbers so now I'm questioning If I did it right? Would you mind taking a look? Any ideas of what could be causing the "pop"?

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axeman
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Re: The 6550 Experience

Post by axeman » Tue Nov 06, 2018 5:28 pm

Beautiful build, but to let you know Reissue Tungsol are actually 6L6 and Silvania did not make 6550. The tubes in your amp are actually Real Tungsol 6550.

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Wed Nov 07, 2018 12:19 am

Did it pop when you had the NOS caps in there?

revolver1
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Re: The 6550 Experience

Post by revolver1 » Wed Nov 07, 2018 4:52 am

Thanks for the kind words axeman. Interesting about the reissue Tungsol, I didn't realise they were 6L6.

Dave, yes it was doing it with the NOS caps in.

revolver1
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Re: The 6550 Experience

Post by revolver1 » Wed Nov 07, 2018 8:27 am

Sorry fellas, false alarm. It must have been the caps settling in. I've had it on this morning and operated the standby twice and there was no poping.

I've done a star grounding scheme and it's not too bad. There is the regular hiss that seems a bit louder than my 100w but it does seem to amplifie the PT hum a little more than I'm used to. It may improve once it's screened inside the head shell. I've cranked all the volumes and presence with no squeeling so that's all good.

Did I read either in this thread or the first one that there Is extra noise due to class B operation or was it the high voltage?

At ground No.4 down by the presence knob by the PT I've grounded the bias circuit, the secondary from the PT, and the shared PI and screens cap. Could that be the problem?

That was the way the 100w was done but I have seen other JTM45 lay outs with the PT secondary grounded at the mains filter cap. Also I guess there is some kind of compromise that has to be made with the shared filter can ground point.

revolver1
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Re: The 6550 Experience

Post by revolver1 » Wed Nov 07, 2018 2:48 pm

Just a thought, don't know if any one else has experienced this but the 6550's kinda rest on the brass bolt heads of the socket rather than seating flush on the valve socket itself due to the fact the base is so much wider than an EL34. Could that maybe be conducting vibration from the PT through the chassis and into the tube to be amplified or is that something that would only happen with a microphonic pre amp tube?

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Thu Nov 08, 2018 2:21 pm

One would hope the 6550s were not that microphonic. In the best cases, I have used flathead countersunk screws to anchor the tube sockets.

Keep in mind on the grounding that there is noise on the ground wire between the filter caps and the PT because of the heavy ripple current there--120HZ stuff. It develops because there is some resistance in the wire. You do not want sensitive circuits grounded between those points. Let the point at which the common is grounded to the chassis be at the ground of the most sensitive stage, usually the input jack or even the cathode return of the first stage. Run the ground from the bias caps directly to the PT common, and not to the negative wire of the main filter caps; technically, the PT CT or in this case the point at which the bridge common is connected to the PT is the place where the impedance is so low that no ripple voltage can be formed, at least very little. In a star, all grounding leads would converge here, and would be run separately out to the returns from the circuits. But the chassis would be grounded to the circuit commons at the input stage. All this is done so the sensitive stages cannot listen to the ripple on the filter caps.

revolver1
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Re: The 6550 Experience

Post by revolver1 » Thu Nov 08, 2018 3:27 pm

Thanks Dave, sounds like I've done it right. The heater CT is grounded at the farthest point with the input jacks, the bias is with the PT secondary CT and the shared screen/PI cap. The mains filter cap is grounded at the filter cap position pretty much the last point before the main ground to the IEC conector.

It's not so bad just a little noisier than I expected its certainly not as bad as standard grounding. It could even be down to the PT hum being picked up through the chassis by the pre amp tubes.

daveweyer
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Re: The 6550 Experience

Post by daveweyer » Thu Nov 08, 2018 6:58 pm

You may want to relocate your heater CT ground to a different place on the chassis, or as an alternative, put a 50 ohm 5 watt pot across the heaters and ground its middle pole (the wiper), after you disconnect the heater CT wire. The adjust the pot for minimum hum; hint, it usually will not be in the middle.

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