The 6550 Experience

[bill bokey]

Thanks Dave !

But that does sound outrageously expensive ! It only cost me $39 for 4 x 6550 tubes the other day.... and it was with Fedex...

[daveweyer]

They must have seen me coming! I had to fill out all these forms as well. Regular shippers must have some kind of deal worked out. Oh well, it's only money. I'll make it up next life.

[shakti]

FInally got my 6550s! Look out for my own experiments if I can sneak in some time in the busy pre-Christmas days...

[Xplorer]

[bill bokey]

No FX, just the Strat and the amp on V7, T6, M5+, B2, P9+
With the guitar volume turned down to around 3 it gets pretty clean...

https://soundcloud.com/castle-made-of-s ... lean-strat

[daveweyer]

Deeply appreciate the experiments and the work that goes into them. Love to see the tone settings closing in on the ones I saw Jimi use. It helps make historical sense.

[bill bokey]

I'm really amazed by the amount of gain with "your" setup. Never had that much gain with a Strat and a Superlead before !
And that's without a bypass cap on V2 !!!

Thanks again Dave !

[chrisom]

Since I have (2) of the 9000-Series Marshall 50w/50w power amps, I was considering coverting one of them to using 6550 tubes, for different sound applications. Looking at the schematic, they are basically the standard Marshall/Bassman style power amp circuit, w/ an additional 1/2 tube at the front end for buffering or whatever. What do you think?

[shakti]

Threw in the 6550s for a veeery quick test. I put in a 15k in place of the 27k in the bias circuit (with a 47k in line with the 25k pot to ground) and still couldn't get them in range for a cold (25-30 mA) bias. I had to parallell in another 15k, so when wiring it up permanent I might try a 10k before the diode, and a 56k in line with the pot.

In any case, with bias at 25-30 mA, I ended up with 510V plate voltage. Adjusting by ear, I liked them more towards 30 mA than lower with that plate voltage, but that only makes sense. I like what I hear, it's more glassy and brittle than EL34s. I hope to do more tests tomorrow, maybe trying shared cathode as well, and also trying with a 12AT7 in V3. I did try it both matched and mismatched (16 or 8 ohm amp setting into 16 ohm cab) and preferred it matched. That means a low primary impedance (1k7) but also less NFB (16 vs 8 ohm with 47k resistor). Mismatched (higher pirmary impedance) sounded to squishy for me, but I'll try it some more, and maybe also try low primary impedance with less NFB (move the feedback wire to 8 ohm tap).
I didn't do anything with the screen resistors, still at 1k.

[Tazin]

I'm really amazed by the amount of gain with "your" setup. Never had that much gain with a Strat and a Superlead before !
And that's without a bypass cap on V2 !!!

Thanks again Dave !

You don't want a bypass cap on V2.

[daveweyer]

Tazin, do you not like the .68 partial bypass on V2? The 1959's were shipped with it installed, just a cheap way to get some more treble. But I know some folks don't like it.

Chrisom, the 9000 series are a bit of a different beast, but they will still work with 6550s. The circuit is something like the "Big Boy" mod that Xplorer posted a few pages back, in that it has an extra tube in the circuit, a driver stage with local feedback to control its gain and lower its impedance among other things. On a purely technical level, the circuit is a "gain staging" circuit, the stage before the power tubes is its own amp--yup, it's a mini re-amp built right into the overall circuit.

We'll talk about that some more...........

[shakti]

I'm really amazed by the amount of gain with "your" setup. Never had that much gain with a Strat and a Superlead before !
And that's without a bypass cap on V2 !!!

Thanks again Dave !

You don't want a bypass cap on V2.

Tazin,

do you mean according to the "Hendrix" amps, or with bill bokey''s particular amp? IIRC it's a '73, which means it has a 100k feedback resistor on the 4 ohm tap, so very little NFB.

IIRC, you said the Hendrix amps you've seen did have a V2 bypass cap, but it may have been changed to an alternate value?

[bill bokey]

My Superlead 73 has a 47k / 8ohm NFB. It's pretty much a '68 SL, the main difference being the missing V2 bypass cap. There are other tweaks as well but they don't add any gain, merely reduce the excessive brightness of the OT

[Tazin]

I find that the 0.68uF bypass cap on V2 works well for humbucker's, but not so great with single coil pups....This is of coarse with a stock Super Lead.

[Tazin]

IIRC, you said the Hendrix amps you've seen did have a V2 bypass cap, but it may have been changed to an alternate value?

shakti, re-read my post on page one of this thread....hopefully that helps clear things up.