The 6550 Experience

[bill bokey]

Although I never posted there, I read the topic about Jimi's gear at West Coast Organ and Amp and thank Dave so much for all the info he shared with us.
I have always played my Marshall amps with EL34 and thought I'd give those 6550 a go. I know I'm not the only one so maybe we could share or experiences here.
I have a '73 Superlead that doesn't sound very good (well, not as good as I'd like it to) although I reverted it to 68 specs. It'll be a perfect base for tweaking.

I read the 29 pages of the West Coast topic, and wrote down as much info as I could. For now I'll stick to the 3 tube amp, so no 5687 until further notice. Here's what seems clear enough :

- 6550 Tung Sol output tubes, biased at 25-30 mA (cathode current), Class B
- 560V plate voltage, about 530V on screens
- 2k2 / 10W screen resistors
- star grounding
- 100µF mains filter caps
- 12at7 V3 (PI)
- 470 + 22k PI cathode
- shared cathode V1
- settings : Volume 7, Treble 4, Mids 6, Bass 4, Presence 9

What is yet unclear to me :

- what was the plate dissipation of those 6550 ? 35W or 42W ? I'm only asking cause I doubt I'll get 560V with the Dagnall T4074 so 25-30mA won't mean much... If it's 42W and 560V then 25mA means 33% plate dissipation. Is 35W then 40% (even less than that as it's actually cathode current that Dave measured, not plate current)
- bright cap ? was there one or not ? I've never played a 6550 amp but on my Superleads I have to have the bass dimed or it's just too thin. So if he had the bass pot on 4 I'd guess there was no BC. Any thoughts ?
- PI plate resistors... the usual 82k/100k or a more Fenderish dual 47k or even lower ?
- same with grid bias resistors : stock 220k or lower then 50k ?
- how to mismatch the OT to be closer to 4-5k primary impedance ?
- Dave said something about bleeder resistors on the screens filter caps, but what value ?

I ordered a quad of TungSol 6550 and some 2k2 10W resistors, for the screens and a pair of 220µF ARS caps. Until they arrive, I'm hoping some of you will answer some of my questions and ask some more.

[shakti]

Nice topic! I have a '69 clone and am waiting for a quad of Tung-Sol RI 6550s to put in there.

I think you got those mods right, but it did appear that they may not have all been used in all amps, and maybe not all of them in one go. Tazin confirmed that the purported Hendrix amps he had seen (but could not post photos of) did not bear witness to such extensive modding.

Dave did say he did not lower the bias grid resistors from the stock 220k values.

What I will do with mine:

- change bias drop resistor from 27k to...15k? to try to get the right range
- bias to about 25-30 mA and see where my plate voltage is at. I'm guessing 500-510, so if I feel like it I will try using the 230V tap on the power transformer to bump the plate voltage
- leave the screen resistors at 1k if I go with the lower plate voltage
- leave the PI tail resistor at 10k - this was stock in the amps that Tazin saw
- experiment with negative feedback; I have it at 47k@speaker jack. This would have been stock in Hendrix' amps, but since he used a full stack it would have been 8 ohm effectively. With my setup it will be 16 ohm, so I need to rewire it, or...
- mismatch by using a 16 ohm cab with amp set at 8 ohm. This bumps the primary impedance from 1k7 to 3k4. But this one baffles me - if they were going for more power, why use a higher plate load?
- try a 12AT7 in V3
- maybe try shared cathode on V1

I am going to leave the stock filter caps, and I already have a star/Larry ground system on my amp.

[bill bokey]

Dave did say he did not lower the bias grid resistors from the stock 220k values.

Yep, that's why I'm a bit confused... He intially talked about making them smaller than 50k and lowering the plate resistors as well, with a 12at7. And later on on the thread he indeed said they should remain stock. Dave, if you read us, can you clarify this ?

- bias to about 25-30 mA and see where my plate voltage is at. I'm guessing 500-510, so if I feel like it I will try using the 230V tap on the power transformer to bump the plate voltage

Good point : I should have thought of that, I usually do it the other way around, to lower plate voltage...

My amp also already has star/larry grounding but I'm not happy with the filter caps (well, the bass isn't tight enough so I blame them) and will try higher capacitance.

[bill bokey]

I think you got those mods right, but it did appear that they may not have all been used in all amps, and maybe not all of them in one go. Tazin confirmed that the purported Hendrix amps he had seen (but could not post photos of) did not bear witness to such extensive modding.

That's antoher good point : I most certainly got carried away writing down all the mods thinking they all belonged to the same amp !

I do remember reading about a purported Hendrix amp which had shared cathode, a small (ceramic disc ?) V1b coupling cap... was it 1nF ? can't remember. I'm quite sure it also had a 470nF tropical fish capacitor on V2. Does that ring any bells ?

[Roe]

6550 users often decrease NFB. I've tried real nos 6550As and they sound too stiff for me. kt88s are much nicer

[shakti]

I tried KT88s, but wihtout any other changes (just lowered the 220ks to 150k) and while it certainly had more headroom, it didn't knock me out. But will have another go with 6550s.

[daveweyer]

Wow, good to hear some experimenting with 6550s going on.
I suspect that stiff will be a pretty good word for the 6550. To get the circuit going, just keep to the minimum mods. Bias at 25 ma. This will keep idle tube dissipation under 15 watts, which gives you 20 to 25 watts to play with above idle for each tube. Class B circuits of this type can approach 60% efficiency, which means only 40% of the available input will have to be dissipated as heat. If the tubes draw 500 ma at full power then the power input will be 280 watts, 40% of that will be 112 watts of dissipation between the four tubes, a cinch for 6550s.
The grid resistors at 220K will work fine until the tubes get really worn.
Adjust your power supply filter caps for the amount of stiffness you want, it's okay to have more on the screen supply.
You can use 1.5K resistors on the screens, but 2.2K will offer more screen current limiting, and just a touch softer response at louder volumes. You'll hear it when you play through the amp.
As I have said more times than you wanted to hear, the screen dissipation is the critical factor when the plate voltage and the screen voltage of the tube are about the same at idle, because every cycle when the tube plate swings down to its plate sat value, about 50 volts, the screen is still at 500 volts or whatever the HV holds at during high current draw. So for most of the cycle, the screen is higher than the plate, and when that happens the screen does all the dissipating because the electrons want to flow to the screen--after all, it has the higher potential and is therefore the attractor--electrons bounce off the plate and head back to the screen, called secondary emission.
This can cause screen dissipation to exceed 20 watts, and burn out the tube. Screen dissipation should be held to about 7 watts on these tubes, same for an El34.

The input cathodes can be run from one resistor and capacitor, but try it to see what it does for your application.
Same goes for bypassing adjustable controls with a small film cap. And try different types of caps too. Especially try tantalum capacitors on the cathode bypass of the input stages. Ceramic disk caps often sound best in tone and bypass circuits, often (Like in old Fenders) the sole coupling cap to the output stage is a ceramic disk.
Each amp as you know has a personality, so there is a sweet spot for each one depending on your playing styles. For Jimi, it was largely at full volume in complete square wave clipping. I'm sure it isn't that for most of you.
Of course later in his career he crept away from the maelstrom sonic landscapes into much more subtle shades and tones, and the 6550 offered that hard edged blues tone, whether he actually preferred that for blues is anyone's guess though, seems like most articles about it has players liking the soft edged tones of EL34s and 6V6s.
It always depends on the application.
Knowing Jimi, he probably played whatever characteristics the machine had to his advantage, and learned them so he could control them better.

[daveweyer]

Answering a few more questions about using 6550s. It was suggested that lowering the NFB would be desirable in amps using 6550s. Yes definitely, give it a try. It changes the distortion characteristics, and can actually make the amp more stable from an HF oscillation standpoint. You get more hum and noise, but you can deal with those in other ways if you get a sound you like. It's also not quite so stiff, and lets the speakers play a greater role in the sound of the amp.
Even try no negative feedback if the amp is inherently quiet enough.
There are other ways to use negative feedback besides around the loop of the OT. We can talk about that too.

[Xplorer]

Coool ! good luck with your experimentations !
will Watch this exciting thread
i think that my superlead might change quite a bit someday ...

[daveweyer]

The point was also raised about mismatching the OT secondary impedance to change the load on the output tubes.
Yes, a lower P-P primary impedance gives you more power, and, more distortion.
But I found a very sweet spot for the output tubes if you've got about 560 volts to play with, and that is in the 4K range.
You will have a total of 250 watts to play with even at that impedance, so not much point in trying for more power.
Say the tubes can swing 500 volts out of the 560 available on each phase, square that sum and divide by the P-P primary impedance, then subtract about 20% for losses. Even at 4K you'll have too much power, at 2K you'll strip the cathodes.
If the PT has enough current, you'll get 160 watts out of 6550s, plenty enough to blow the speakers.

By the way, I heard the clip of the 1987 50 watter, really SWEET! Love it.

[Tazin]

I think you got those mods right, but it did appear that they may not have all been used in all amps, and maybe not all of them in one go. Tazin confirmed that the purported Hendrix amps he had seen (but could not post photos of) did not bear witness to such extensive modding.

That's antoher good point : I most certainly got carried away writing down all the mods thinking they all belonged to the same amp !

I do remember reading about a purported Hendrix amp which had shared cathode, a small (ceramic disc ?) V1b coupling cap... was it 1nF ? can't remember. I'm quite sure it also had a 470nF tropical fish capacitor on V2. Does that ring any bells ?

And let me add that I would not concern myself with things like ceramic disc and tropical fish capacitors. Currently there is no proof that componets such as these were present in an amp when Jimi was using them. Besides, just because one amp might have had things like these does not mean all of them did. Personally, I would stick to a hybrid Super Bass type circuit.

[daveweyer]

I think Tazin's right on this, for the trial amp, just stick to what's there. That way you'll have a better comparison of what the 6550s are doing.
Once you get that dialed in you can try other things to see if it gets you anywhere.
Gotta remember that every amp was tested for what it sounded like, based on a comparison with a memorized sound quality, usually three of us standing around listening to a player putting it through its paces.
So for Jimi mods there was no "one thing" that we did to every single amp, except maybe replace the tubes.
Tweaks worked on some amps, not on others.
You have a bag of tricks, you try some of them, all of them, or none of them. Results are supposed to lead.

[Tazin]

I think Tazin's right on this, for the trial amp, just stick to what's there. That way you'll have a better comparison of what the 6550s are doing.
Once you get that dialed in you can try other things to see if it gets you anywhere.
Gotta remember that every amp was tested for what it sounded like, based on a comparison with a memorized sound quality, usually three of us standing around listening to a player putting it through its paces.
So for Jimi mods there was no "one thing" that we did to every single amp, except maybe replace the tubes.
Tweaks worked on some amps, not on others.
You have a bag of tricks, you try some of them, all of them, or none of them. Results are supposed to lead.

Dave, I think you summed it up nicely.

[bill bokey]

Thanks guys, it all makes perfect sense. As I've never used 6550 nor experimented with class B it's good to have the info Dave gave us as a guideline.
Tazin, what do you mean by hybrid Superbass circuit ? Shared cathode and lead tone stack ?

[daveweyer]

Bill, by all means share your feelings about the 6550s. You know you can run EL34s in class B too. The distortion is gnarlier in class B; there is a little grit that comes in at the zero crossing of the waveform because the tubes are completely cutting off of conduction right there, so a little notch appears on the waveform.
My guess is that you will need both types of amps to complete your "arsenal" of sounds as my friend Gary Rowles calls it. I checked out your Facebook page to see and listen to the mods you've done, and they looked really nice, sounded great too.
Keep in mind that class B was used primarily to save the tubes from meltdown, I didn't know if Jimi would like the sound. Apparently it was in keeping with where he was heading during 69 and 70 because he used those amps the way I set them up, returning them for service from time to time without instructions to "change them back the way they were". So we kept going with the 6550 mods and the hot transformers.

Now I hope I didn't mislead anybody here with all the suggestions I made about mods when I first started posting about them. I just sort of summed them all up in a few posts, and there were a lot of mods, some of them radical.
They didn't all go in one amp.
For the experimenters in the group I can share some of the radical mods if you want to try them. It looks like this will have to be an empirical experiment, since so far not much visual evidence has been shown to verify the actual physical existence of the mods. I know one guy who could share some of this, but he won't.
But if you built one, and the sound told you that this is what you heard on this or that performance, then you could prove the mods by hearing. I guess that's the only option left at this point until someone unknown grants us an enlightening view into the past.