The 6550 Experience

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

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Re: The 6550 Experience

Post by revolver1 » Thu Jul 11, 2019 7:58 pm

More experiments today...

I swapped out the 100pf disk bright cap and put the 4700pf back in and also swapped the 470pf dog bone in the tone stack for a 500pf silver mica.

Quite interesting, the bright cap definitely gave it alot more presence in the room. When I kicked the wah on with the guitar volume full up it would go into feed back, I think on a big stage this would be completely useable. I'm quite convinced it was a big bright cap, that said for my needs I'm going to try a dog bone I have that reads 2600pf. I really enjoyed the 100pf but I did feel it needed a bit of a lift and I also felt the 4700pf was a bit much so i ll see how the 2600pf works out. I think in the days before sound reinforcement you'd definitely want the 4700/5000pf in there, especially on the bigger stages it would have space to breath.

I generally prefer the bigger caps but for this amp I think its maybe a bit much for where I'll be using it.

The silicon fuzz is doing what it should, I'm getting fat cleans up to 7. Then rock n roll 7 to 8, it starts hotting up 8 to 9 and then over the top rawk from 9 to 10. With the germanium fuzz I prefer it with more fuzz and less volume but the silicon is just right the other way round. I'm amazed at how fat and sweet the clean tone gets with the guitar rolled back compared to the amp on it's own.
At the moment I can see the fuzz being on all the time with this amp.

So far i think the 100pf bright cap and dog bone in the tone stack sounded best with the fuzz and the wah. The 500pf mica was better when it was just the amp but it lacked some of the really tasty overtones or magic I was getting with the fuzz but the draw back with the bone was it could sound a bit harsh with just the amp so i had to really work the guitar volume and tone to get the sweet spots but when I hit them it sounded great.

I don't know exactly what the dog bone cap does whether it blocks more bass or allows more mids through but it definitely imparts some magic it was alot of fun to play and I was getting a familiar vibe from it although it can be a bit harsh at times.

So at this point I'm thinking I'm facing a balancing act between bright cap and the tone stack cap. I'm going to swap out the 500pf for a 560pf mica and see how it goes. I'm hoping the 2600pf bright cap will be somewhere in between the extremes of the 100pf cap and the 4700pf. Also hoping the 560pf in the tone stack will give a slight mid lift like I was getting from the bone but with out the harshness.

I'm up for trying all the combos I can at the moment with what I have to hand. I've never tried a bone on the mixer or even the 560pf on the mixer. I don't really know what to expect so I should just give it a go and find out.

I have been looking on amp archives to try get a feel for what type of caps generally went into the 69's for the mixer and stack but they all seem to be every possible combination under the sun.
I'd be interested to know if Dave can remember what type or combos of cap types turned up in the "Jim Morrisson" shipment.

I am concerned about the electrolytics like I said the other day I was getting 478vdc on the plates, I need to up the voltage handling. I'm going to remove the pre amp cap and mount a small board on top so I can take the wires back through the hole. So I'll have the screens P.I and pre amp on top. Theres not really enough room inside. I took a long look at it but I think the caps will get too hot from the P.T and the P.T needs all the air circulation it can get.

I'm going to stick to essentially the same values a dual 50uf for pre, a 16/32uf for 48uf on th P.I 2x dual 50uf for 50uf on the screens and 2x dual 100uf for 100uf on the mains. All the other caps are ARS but I could only get F&T for the dual 100's. So far I really like the ARS, I have no idea how or why but I think they some how sound better at least as far as current production goes. Anyway hope it doesn't muck things up because it's really getting there.

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Re: The 6550 Experience

Post by revolver1 » Sat Jul 13, 2019 8:02 am

Managed to fashion a little perf board on top. I removed the preamp can and used the holes to mount the stand offs for the board so I didn't have to drill any more holes.

Decided to try a 32+32uf on the pre amp to see if that takes the edge off. I prefer the feel of 32,32 anyway so well see.

Looking forward to seeing what the 560pf in the tone stack brings with the bright cap in between the values I've already tried.

Been listening to a bit of BOG to keep the reference in my mind. Its such a solid sound he had around that time I'm starting to feel like my last attempt was very, very close. I think there's a lot to be said for the sweet spot of the fuzz dial and the guitar settings. With the 4700pf bright cap it leaves a lot of power up your sleeve.

I'm also thinking when I checked the bias last I gave it a nudge trying to keep it around the 25ma mark but the wall voltage was jumping about a bit. I think it sounded better before further into class B, slightly more interesting with the fuzz.

Found out ARS do a 100×100uf electrolytic but can't get any in the UK so I've ordered 2 from the states. I'll just have to wait for that to show, untill then I've got the F&t comming that will do for testing. So just waiting on the caps now to give it a blast.

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Re: The 6550 Experience

Post by revolver1 » Tue Jul 16, 2019 3:48 am


I was just re reading some and came across a point I may have missed for the mods.

You said that if you raised the plate voltage you may well have changed out the 820r /330uf V1 cathode to adjust the clipping with a 1k resistor but it didn't specifie the electrolitic value?

Bearing in mind mine is a 50w, I have 478vdc on the plates with 2x 8.2k droppers. Should I look to do this mod?

I'm so close to it now its ridiculous, just fine adjusting to get the right level of brightness so it doesn't over bear the interesting harmonic content. I guess it kinda comes down to the drive characteristic which is why I ask.


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Re: The 6550 Experience

Post by revolver1 » Tue Jul 16, 2019 7:16 pm

Today I did alot of swapping components in and out and I'm really quite happy with the results.

The 2600pf brightcap sucked badly and so did the 560pf in the tone stack so out they came and eventually I put back the 4700pf bright cap and the 470pf dog bone in the tone stack, with the other changes I think I'm settled on it. The 500pf mica in the tone stack just doesn't sound as interesting as the dog bone and I'm convinced about the bright cap now as well.

I swapped the dog bone in the phase inverter for a mica that came in at 48pf, I think this has sort of made a slight change to the drive characteristic in a good way so that's staying in.

Upping the voltage handling of the filtering has worked a treat, I feel a bit better now it's not on the limit of meltdown and it still feels much the same apart from the 32/32uf on the preamp that I think is a change for the better.

50/50uf is kinda good because it feels powerful and I'm maybe still a bit on the fence about it because I can see it working with a Strat running 10 to 38's for a bit more whallop on the low strings. It certainly packs a punch but i think it makes it a bit harsh at the same time, for the moment I think 32+ 32uf is doing a better job.

I feel like I play better with 32 32uf because of the touch sensitivity, 50 50uf is great when it's all running at full tilt but it's a bit of a one trick pony and I'm not entirely sure it sounds right. The uni vibe didn't really gel with it as well either and rolling the guitar volume back sounds better with the 32uf. I think I really need to sort a Strat out before I commit to 32uf but for now I'm pretty happy with it.

So 1x 32/16uf for 48uf on the P.I, 2x dual 50uf for 50uf on the screens and 2x dual 100uf for 100uf on the mains.

Now for what I think has made the biggest change for the best. I still would appreciate some advice from some of the more technically adept guys about this.
So I've ordered up some 1k 1/2w that I'll be waiting on for a bit with the intention of pulling the 820r on the cathode of V1 and subbing in the 1k.

While I'm waiting I went through a bunch of 820r carbon comps looking for one that measures high and I found one that came in at 885r. So I've put it in and its sounding much better. I think between this and the 32uf pre filter its sorted the harsh edge it had. The iskra I had in there measured 831r, I measured the CC after I gave it a damn good thrashing earlier today and in circuit it now measures 908r.

I've got 478vdc on the plates with 2x 8.2k droppers I'm not sure if I've got enough voltage to try the 1k?
In the West Coast mods thread I think Dave said he did this to the 560vdc amps to adjust the clipping, I'm thinking the 908r is maybe a middle ground compromise for my 50w?

I'm not a big fan of carbon comp. I had a few in another build, just the cathode, slope and nfb, although they sounded good at first when I swapped them out after a year or so I realised how much punch I'd been missing.

With the exception of the last bit of advice I could do with I feel I've taken it as far as I can for now without a Strat.

I'd like to get a way of recording it, for starters I'd like to hear how it records and secondly I'd like to put a clip up if it sounds any good. I wont do it until I get a Strat though but for now all I can say is in the room it sounds very, very;


Big thanks to Dave and to every one else for the inspiring contributions. :clap: :thumbsup:

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Re: The 6550 Experience

Post by daveweyer » Thu Jul 18, 2019 12:21 am

If you sub a 1K for the 820 on V1, you will raise the plate voltage. This may be good if the stage starts clipping on the negative plate swing at full input volume from your guitar. I mainly tried to get clipping to be reasonably symmetrical on both extremes of plate swing. It will never be totally symmetrical because the plate resistor is not a great constant current source, especially when the tube is drawing very little current.
I always say give it a try; the rules are, well, just something somebody said!

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Re: The 6550 Experience

Post by revolver1 » Sat Jul 20, 2019 7:07 am

Hi Dave, thanks so much for the reply. So glad your around, it's so quiet at the moment.

I've been trying to get some audio clips together but I'm not the best when it comes to tech, some how I've uploaded a clip that's floating around in cyber space somewhere and I've got absolutely no idea where it is. I've also managed to to gain a small group of followers that have liked some rap gibberish bilge that I'd never even heard before let alone posted, no idea how that happened but it seems I'm hip and groovy with the youth of today although im not sure im happy about that.... I think I've sussed it out now but I have to re-record which means the neighbours are gonna love me.... I'm expecting dog shit in a paper bag for Christmas. :lol:

It's really coming together now, first off it was it was too harsh and the drive seemed too focused maybe brittel is the word. Ive corrected this by swapping the 47pf dog bone for a mica also swapped the 50/50uf pre amp can for a 32/32uf but the biggest improvement was the V1 cathode resistor. So I'm waiting on some 1k to turn up but for the moment I have a carbon comp that reads 885r and it's doing a good job. I'm going to try the 1k but it may be good as it is.

Going over the old posts and swapping out the v1 cathode resistor has lead me to the conclusion that my problem is all about the clipping. The 32/32uf can has helped take some of the hash edge off and makes it sweeter. It sounds right but it loses some of the size, power and punch that I'm convinced had to be a part of the mix.

Given that the goal was to put together ballsy punchy amps, a power house back line for the main sound generator rather than relying on a P.A system as is the norm today. I'm convinced it needs to be 50/50uf on the pre amp, I'm sure Jimmi would have appreciated the extra power and punch.

The 4700pf bright cap sounded bad with the 50/50 cap but works great with the 32/32uf, I'm also convinced 4700pf is the right way to go so something else needs to change to get the 50/50uf to work. Even though it sounds about right at the moment with the 32uf my thinking is to attempt the mods you mentioned in the phase inverter. It think there maybe some uneven clipping going on there that if I can correct I could then get the 50/50uf can back in and it would sound and feel right.

What I'm wanting to know is are the value of resistors mentioned going to work for my 50w application? As I've said before I'm getting 478vdc on the 6550 plates. The values mentioned were to replace the 82k, 470r and 100k with 56k, 1k2 and 68k and to replace the 10k tail with a 15k or 22k.

Do you think this will work for me or do I need different values?
I unfortunately i don't have a scope so I have to assess it by ear but I feel I'm looking in the right area.

I have to go through my box of tricks, I think I have most of the resistors but I will probably need to order one or two. So if you think I may need different values I would be best to order it all together because of postage and taxes.

On another note and obviously just my personal feeling. I've been following the W.C fuzz debate and although part of me thinks this could well be it and is certainly woth perseverance. It sounds like an awesome fuzz circuit anyway but I'm finding I'm getting from thick sparkly cleans to full earth shaking fuzz with my silicon fuzz and just using the tone and volume on my guitar.

I think back in the day riding the guitar volume was the done thing, although it's not inconceivable i think pedal dancing over complicates things especially when dealing with singing at the same time. There's definitely bootlegs I've heard that certain songs are just the straight amp but again he's still riding the Volume, I feel Jimmi just went with what felt right at the time and almost played the amps like an instrument.

I think it's also woth keeping in perspective, I've seen alot of footage that gives me a strong impression of what Dave., you had previously mentioned. Jimmi could get really down when his sound wasn't happening as in the gear is not reacting in the way he wants. One example the 69 Sweeden performance you can sense the tension like Jimmi's not getting the sound he wants and is frustrated trying to sort it on the fly while Noel's like oh man not again just get on with it and play your damn guitar. Or other occasions when the crowd is asked to be cool while they get it together. Jimmi must of talked about it with Buddy and Billy because they seemed more understanding rather than the tension I could see with Noel. Just my interpretation obviously.

I know years ago my band mates could have bludgeoned me to death over the way I'd get if my sound wasn't happening. :lol: :lol: :lol:
But for Jimmi there were so many variables going on especially if playing in different countries with what ever was going on with the wall voltage and variables of thermal effects in the amps them selves then throw in a sensitive germanium transistor and your in the lap of the gods for it all to come together. Im guessing Jimmi would have appreciated the stability of the silicon fuzz, it would at least be a little more consistent night to night.

I think it's worth repeating because it keeps being said fuzz volume on 10 and fuzz on 0. But Dave, you have definitely said the fuzz should be somewhere from 2 to 4. I've been setting the fuzz backed off just enough to let the wah breath volume on 9.5 to 10. So if you look at a clock face it's a nudge past 9, I have one of those battery saving switches pots so this equates to fuzz around 3 or 3 and a bit and it sounds right to me. The fuzz is an analogman silicon BC183.

I've got germanium and silicon both correct stock circuits and for my money germanium is early Hendrix and silicon is BOG. I agree the germanium is more fun with a bit more fuzz in the mix i prefer cranking it up but it doesn't cut it for BOG, the silicon on the other hand with Dave's suggested settings is spot on but if you crank the fuzz up forget it, absolutely no way I can see that being it in any way. Actually it's impossible to play.

Also cant emphasize enough it's a completely different beast with a modded amp in all the right ways. But we cant lose sight of the fact that you can cop Hendrix tones with stock gear because he managed it before all the mods. So for all those that argue the no mods point I think you do have a point but for me you cant get the latter tones with out them.

Amp settings are exactly as Dave said, I just tweak it a touch to compensate because every amps going to be slightly different. I have a nudge less presence and an equal nudge more bass and treble. Volume one on 7 and I've tried Volume two on 4 as suggested but I also like it cranked to 8 for the way it thickens up. But setting wise it's pretty much as Dave said and its scary close.

For what it's worth i also think Shakti is on the money with the idea of daisy chaining to a second EL34 amp. I think it's in the blend of 6550 and EL34. I find I can play some things on an EL34 amp and think yeah that sounds like it and then other bits it kinda doesn't. Both amps would fill in the gaps like mixing speakers. Looks like the speaker mixing was almost certainly going on more from replacing blown speakers but still a happy accident. That's been my personal theory for a long time and the Woodstock photo evidence seems to back up the 6550/EL34 bit and then theres Dave's speaker replacement testamony. I think that would all add up to a massive broad spectrum sound scape.

The sound I've managed to recorded so far on the modded amp set up really isnt a million miles away from the sound of a stock 1959. The big difference is on the stock 59 I can either get a close clean tone or a close dirty tone but no way of getting from one to the other and definitely no way of doing it off the guitar volume and the other effects don't interact correctly.

The West Coast mods for me is the only way I can get there where all of it works and all off the guitar volume.
:clap: :thumbsup:

Wow, that was a bit of a ramble... :whistle: :roll:

So Dave, any thoughts on those phase inverter resistors?

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