The 6550 Experience

His guitar slung across his back, his dusty boots is his cadillac.

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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.

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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?

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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.

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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.

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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?

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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.

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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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Re: The 6550 Experience

Post by frenchie » Fri Nov 30, 2018 2:13 am

Xplorer wrote:
Wed Oct 31, 2018 10:28 am
You know, peoples can also be nice by nature and enthusiast because they like it.
I think axeman is right , you really should have kids , you're bloody good at fairy tales man :lol:
so cute ! :lol:
Try to avoid attempting to analyse them psychologically first tho .... Because that pseudo science is a total horseSHIT pile floating away on murky waters , and it says a lot about the integrity of people advocating its use , sure it generate billions yearly in "misguided introspections" of brainwashed masses so they come back later and spend again (*) , but so does cocain and no one's yet chanting it's potential merits enough to recommend adding it to the world's toddlers diet


(*: not even counting the " moist finger in the wind " mental desease branding of political opposition that end up in mental wards in dictatorial regimes , soon genuinely looking balls out nutters cuz of the chemicals flood in their veins done by "nice by nature" white coats , looking all respectable and shit ... )

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Re: The 6550 Experience

Post by axeman » Fri Nov 30, 2018 2:45 am

Wow frenchi, you’ll do well in talk radio.

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Xplorer
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Re: The 6550 Experience

Post by Xplorer » Fri Nov 30, 2018 11:24 am

Frenchie .... You know everything on earth, even with some pretty ignorant views. Good for you.
I propose that we leave these out of topic considerations where they are and let the 6550 thread going.

revolver1
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Re: The 6550 Experience

Post by revolver1 » Wed Jul 10, 2019 5:56 am

Hi all, it's been a while... My neighbours have gone on holiday so for me that means a chance to do some amp test driving. I have a couple of issues and also some questions.

So first off (disclaimer) at the moment I don't have a Strat so I'm using an R8, I'm using 9's and have Bare Knuckle mules with a light vintage stagger on the pole peices.
The whole rig is new including the guitar so I'm having to dial it all in as I go but its coming together nicely.

So I put it through my StoneAge 2x12 with Scumback H55 and H75 and used a BMF Wah with an Analogman Silicon Fuzz BC183 and MJM UniVibe. The Fuzz was kinda noisy but as the volume is turned up to about 9 to 9, 1/2 it quietens down. I also tried my analogman red dot germanium fuzz which sounded great but for the kind of sound I'm looking for with this it's the silicon fuzz, vol 9.5 fuzz 2.

I was running through a bit of killing floor just grinning from ear to ear, the throb, drive and hair. This is definitely the closest I've ever gotten to what I hear in my head. Some times because of the LP I can hear tinges of Cream in there and it also cleans up great on the guitar vol.

I think in my attempts of the past with a super lead and a germanium (also then had rosewood and a maple necked R.I strats) I could get a close approximation but everything would have to be dialled in exactly but I couldn't get clean to filth off the guitar volume. The clean sound had no body or size and was quite hard to play, I just knew something wasn't right but this setup is a whole different ball game.

Even though it's a LP, hanging out single notes with a bit of vibrato just has all the right overtones and certain Jimmi chords the harmonics come out in the right way. Its what you want, like playing the "sound" of the equipment as well as the guitar.

So on to problems and questions....

I had the same prob with some of my other amps, so using just a wah (no fuzz) I would get a sort of resonant howl. With my Castledine wah it was toe down and with the BMF it was heel down. It's kind of like feedback but not, guess it sounds like whale song!

It seems go improve if I back off the presence to 6 or less. Do you think this is just down to the routing of the purple presence wire or do you think I have an ossilation problem?

Next issue. So the first time I tried the amp I felt the drive wasn't quite right. To try put it into words the drive was very up front and in your face where I'd like a bit more dynamics to it. I can hear this quality in all your sound clips where the guitar vol is cleaned up and it reminded me of the tone I hear on Vilanova Junction from that remastered album that came out a few years back with the unheard material on it. Alot of the tracks had "that sound" to them and you guys are definitely getting it. For reference I had the normal channel on 8 and channel one on 7.

So I changed the preamp droppers from 10k + 8.2k to 8.2 + 8.2k for a little more pre amp head room. I also changed the 4700pf Volume pot bright cap for a 100pf.
As I said before this was sounding great and with every thing thrown in it was very close but still missing that sound you guys are coping.

So I've given it some thought and even went back to re-read some of the west coast thread and now I'm reasonably convinced I've been looking in the wrong part of the circuit.
I think you guys all stuck with the 220k bias splitters where as I thought I was doing the 6550's a favour by installing 150k there.

So I'm using a Mullard 12at7 in the P.I but having re read the other thread I now understand the 150k will change the P.I drive characteristics and I think this is where my solution is, it needs to go back to 220k.

There was talk of changing the 10k P.I tail to 22k or 15k. I know it said to try it so if I'm using the 12at7 and the 220k splitters and first try it with the 10k tail what difference would I hear by swapping the tail resistor for 22k or 15k? What would it do to the sound? Has any one else tried this?

I will probably want the 4700pf bright cap back in there with this change, might even try a 2000pf dog bone for kicks.

Lastly I'm wondering what your opinions are on ceramic or silver mica in the tone stack? It was a while ago but I had experimented in the past on a superlead with this and have a general feeling about it but I wanted to ask what you all think? I have a mica on the mixers and a dog bone on the tone stack, wondering if I should change the dog bone for a mica?

So to summarize my long winded set of questions;

. Wah howling (presence wire routing or ossilation?)

. 12at7, 220k bias splitters, P.I tail 10k, 15k or 22k? Effect on sound?

. Silver mica or ceramic?

Thanks guys, look forward to your response.

:rockon:

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Re: The 6550 Experience

Post by revolver1 » Wed Jul 10, 2019 2:05 pm

Bit of an update, I re-routed the purple presence wire and the howling is sorted. No more were wolfes. I had the presence up at 9 with the fuzz and wah on with no problems. I'm glad that was an easy fix. :thumbsup:

I switched the buffer on in my BMF wah, really thought I'd not like it but it's great. I've got it after the germanium fuzz and before the silicon and it sounds great with both, its defiantly a bad mo fo of a wah certainly sorts the pre fuzz issue.

I swapped out the 2x 150k bias splitters with 2x 220k and its sorted out the drive sounding too up front and in your face. I think before it kind of emphasised what the dog bone brought to party.

Still not sure what difference I'd hear with upping the P.I tail but I'd like to hear what Dave has to say about it or any one else with experience.

I think I may try swapping out the dog bone and put the 4700pf bright cap back in. I'm theorising a silver mica in the tone stack will smooth things out a bit and the bright cap will bring back a bit of edge. Let's see.

I checked the bias while I was there and the wall voltage was a bit on the high side, I was getting 478vdc on the plates which has got me a little concerned about the electrolytics.
What do you think?

I might have to get a bit creative if I need to squeeze another 3 caps in?

I think it's getting there but I'd still like to hear peoples opinions. :thumbsup:

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