The 6550 Experience

[shakti]

Well, reading your post on page 1 didn't really clear that up, as you don't say anything particular about it except alluding to a "hybrid Super Bass" circuit.

But with a stock '69 Super Lead as a starting point, a .68 uF on V2 should be in there at least from the beginning, then we can tweak by ear and compare with the visual evidence we have.

I did discover today that the volume 1 bright cap is essential for this kind of setup! I had already put in the 6550s, biased cold, and a 12AT7 in V3. With a '69 CS left hand Strat played right hand, into a Castledine Supra-Vibe with preamp engaged, I am trying to get those clean/fuzzless BOG tones, like on Stepping Stone or Them Changes. But whichever way I tweaked it, I could not get the right balance between overdrive and clean, cutting treble or overall warmth. Too muddy no matter what I did, and with some kind of harshness coming in if I tried to compensate with treble.

Enter the 5000 pF bright cap. I had abandoned that kind of cap many, many years ago as I didn't like how it made the amp piercingly bright at lower levels. But in this setup, with non true bypass wah, coiled cords, and (this is another small piece of the puzzle) daisy-chaining into another amp, it made all the difference in the world! I love it now - it brings back in the punch and cut in all the right places, while those coiled cords, Uni-Vibe preamp and daisy-chaining brings in the fullness and warmth just like it should! Wish I had time to make a clip but had to leave for work, but this is another big step towards BOG tone! Folks, leave that 5k bright cap in there!!

I had rewired V1 to shared cathode, but may go back to split, as I tend to leave the bass very low (2 or below) even with this setup.

Another thing I really like about the bright cap is how it works with the fuzz. Rather than add brightness it seems to add some kind of searing, cutting sizzle that *really* works with a fuzzface. I was using my Sunface NKT275, which shouldn't be the right fuzz for BOG at all, but with this setup it actually works quite well!

[bill bokey]

I also really like it with the 5nF bright cap (although I must confess I didn't try without...) and with the bass very low (around 2).
What were your settings ? And speakers ?

[shakti]

Settings were:
Presence 8-9
Bass 2
Mids 7
Treble 4-6
Volume 7

As mentioned I had a coiled cord from guitar to a non true bypass wah, then into the fuzz, Supra-Vibe and pedalboard switcher, to another coily cord into the amp. Then I added another cord from the low input of channel 1, to a "dummy" receiving amp that was not switched on. This last bit must not be ignored, as it both cuts the overall level of the signal coing into the amp (making it cleaner), as well as toning down the treble ever so slightly.

Cab is a 1969 Marshall B cab with T1281s.

I am going to have some fun with bright cap in my other amps now...with shared/bass style preamps I've been using 100 pF a lot of the time, but with this split cathode preamp the 5000pF is actually quite cool, as I tend to always turn it up to at least 7 anyway. I know this is really old news to a lot of you guys, but for me it was a bit of a shock as I had written it off long ago. But again, with all this capacitance in the chain, something really cool happens...and watch out with a fuzz! That searing "take-off" sound was a lot easier to get, as it adds gain in the right places (mids and some treble, but not bass).

[bill bokey]

ok so we're using the same settings, cab and coily cables ! The main differences seem to be the V2 bypasse cap, daisy chaining and filtering (I guess your amp has 50µF all around has it ? )

The first clips I recorded were with an old Vox wah (non true bypass) but I had to replace it with a newer one and I'm not sure it has or has not a true bypass.

So far I only tried with a "mismatched" setup (amp on 4ohm into two 16ohm cabs).

Can you record some clips ? Does your setup sound like mine ?

[shakti]

I'll record a clip eventually, but don't know when I'll have the time. Compared to yours I think yours is perhaps a touch gainier, or maybe it is more squishy? I would try it without the mismatch. And you have less NFB than I do, which also adds gain. And you don't have a Univibe preamp, right?

Yes, mine has 50 uF all around (well, 100 for the PI).

[shakti]

Here's a photo before I changed the bias resistors, V1 cathode and added a bright cap.

[bill bokey]

What's your NFB ?

[Xplorer]

Then I added another cord from the low input of channel 1, to a "dummy" receiving amp that was not switched on. This last bit must not be ignored, as it both cuts the overall level of the signal coing into the amp (making it cleaner), as well as toning down the treble ever so slightly.

thanks for this advice, i didn't imagine it would have an effect to put a cable between the low bright channel of the amp to another amp even switched off. definitely got to try that.

the 5000 pf , ok. i tried once and it brought some interesting trebly saturation but i removed it, not for me for the main use i have. but i'll try again now !

what do you think ? regarding a west coast mod in a 45/100 instead of a superlead : a 45/100 preamp + a 5000 pf bright cap + some 2k2 + some 6550 + a 12at7 + 5 x 100uf power caps instead of the 32uf + a dynaco A431S output transformer + a 45/100 power transformer 560v ?

Dave told me which resistors to change on the preamp but regarding the SL mustards values 0.68 and variations compared to a 45/100 ... will it matter so much ?

yes .... the fuzz volume into a univibe preamp , it does something ... into this kind of amp.

a huge step toward BOG ? mmmh ! coool, very interesting, can't wait to hear your clips when you'll have some time.

[shakti]

OK, you asked for it (or did you?), so please excuse the total suckage of my playing here. Just messing about trying to capture some sounds. Same caveats as always; the room is small, it's attenuated (at -6 dB IIRC), the recording equipment is crappy and adds its own kind of distortion, so don't expect this to sound like the Fillmore East. When summer comes around I may be able to capture something at full volume with better acoustics.

Quickly on the specs; it's the 69 spec clone you see in the photos above, but with V1 cathode rewired to shared (330uF/820R). 47k NFB resistor on the speaker jack. 6550s biased to just under 30 mA at 510V plate voltage. V1 is a Siemens & Halske 12AX7, V2 is a Tesla E83CC (usually not a good choice for V2 as it's a fairly stiff sounding tube, but I just threw it in there), and a Mullard 12AT7 in V3.

The guitar is running through a (non true bypass) Vox wah with Castledine guts (the battery was kind of weak, BTW), an Analog Man NKT 275 Sunface with volume full up and fuzz halfway up, a Castledine Supra-Vibe with the preamp set for just a slight boost, then into the amp. Cab is a '69 B cab with T1281s. In one of the videos you can see where I unplug and plug in the daisy-chain to another (unused) amp, but I don't think such subtle details show up well in the recording, it's easier to hear (and feel) for the player, in the room.

First video is with a Fender CS '69 Strat, left hand strung right hand, maple fretboard. Everything is stock except the saddles which are Highwood notched saddles. This guitar definitely needs some upgrades - it's just a bit harsh and sterile, kind of one-dimensional. A better steel block, better springs and maybe even some different saddles should do a lot, but it also needs better pickups as those CS69s are just kind of harsh sounding with very little depth to them. I tried a different guitar in the next clip, see below.
In this clip it was running matched into a 16 ohm load, so in other words the feedback is 47k at 16 ohm. Settings were P 8, B 3, M 7, T 4, V 7.5.



Second video is with a partsocaster with a vintage 1969 neck (rosewood board), all top notch vintage correct hardward, and Don Mare Josie/VaughnC pickups. It's more of a mid-60s style guitar, better for AYE sounds, but it's a much more responsive instrument than the one above. In this clip I added another 16 ohm load in parallell, but still matched, so it's effectively running 47k @ 8 ohm NFB. Settings were almost the same as above, but I backed off slightly on the presence to about 7, and I think I slightly adjusted the volume as well.

[bill bokey]

Is the fuzz always on in the first clip ?
And are you using the same setting as before ?

[Tazin]

Here's a photo before I changed the bias resistors, V1 cathode and added a bright cap.

Nice build....And the WIMA caps give the amp that 'authentic' look. Are you using 50uF+50uF cans for all the B+ filtering? I don't think it matters all that much, but the line share of the plexi era '69 amps ran the two 1K5 swamp resistors on the outer two output sockets only.

Do the lastest videos you posted have the 5000pF brite cap on the bright channel volume pot?

[shakti]

Is the fuzz always on in the first clip ?
And are you using the same setting as before ?

The fuzz was switched on at 2:21 and then remained on for the rest of the clip IIRC. It's not always easy to tell when it is on or off, but I think that's a plus, because that has always puzzled me about the BOG recordings...really hard to tell which clean and semi-clean sounds are with fuzz but guitar volume backed off, and which ones are straight into amp.

I also don't know how much a real Uni-Vibe preamp boosts. The Castledine has a variable output volume, and I don't know which setting corresponds best with a vintage Uni-Vibe. But it definitely adds a little bit of character, and also smooths out the fuzz a little bit.

I hear more distortion in the clip than what I hear on the BOG recordings, yet not as much gain with the fuzz full on. I think that's due to several factors; the room and recording (and mixing) probably accounts for a very large percentage of that difference, then there's the guitar in the first clip which adds a touch too much harshness and not enough silky, woody depth ("clean distortion") that a good vintage Strat does. I am also considering removing the V2 bypass cap...what say you, Tazin?

[shakti]

Nice build....And the WIMA caps give the amp that 'authentic' look. Are you using 50uF+50uF cans for all the B+ filtering? I don't think it matters all that much, but the line share of the plexi era '69 amps ran the two 1K5 swamp resistors on the outer two output sockets only.

Do the lastest videos you posted have the 5000pF brite cap on the bright channel volume pot?

Yes, all ARS 50+50 uF caps in this one. I know the original Plexis only had two 1k5 swamp resistors, but it's safer with a resistor for each. IIRC I chose 5k6 or thereabouts. I never really compared though - is there a discernable difference betwen 2 vs 4?

Yes, 5000pF bright cap for both of those videos.

[daveweyer]

There was a method to the madness in only using two screen current limiting resistors. I mentioned this before, but parallel tubes tend to have "lumped constants" a place in the frequency spectrum where all the little inductances and capacitances add up together at the same place and cause the amp to turn into an oscillator. Varying the stopping resistors on the active elements of the tubes can cause the various inductances and capacitances to be spread out so they don't all "lump" together at one point.
Marshalls had a terrible problem with oscillating at high frequencies, especially when operated into square wave clipping. Staggering the stopping resistors helped solve that problem.
Along these lines, you may want to consider some 2K resistors on two of the tubes, and 2.5K on the other two.
In an amp with high plate voltage, you should always limit the screen current. The plexis had much lower plate voltage so they could get away with it, at least for a week or so until the EL34s shorted out.
Still the idea of only two stopping resistors had real merit besides saving a few pennies.

[shakti]

Actually we're talking about the swamp resistors on the grids (pin 5), and not the screen grid resistors. Or am I missing something?