The 6550 Experience

[bill bokey]

I can't get more than around 490-500v on the plates at the moment so I set the bias to 28mA, it should roughly equal to 25mA @ 560V on the plates. I also mismatched the OT (4 ohm tap into 2 cabs) and the amp is about 9 dB quiter than it was with EL34s !!!

In a way it's a good thing, the amp is much more manageable now but shouldn't be at least as loud as before ?

[daveweyer]

Man they sure look kick-ass in there! Let's hope they can match the plate sat ratings of the old American Tung Sol.

[bill bokey]

I agree, they look great !


EDIT : after a few minutes the amp is now about 6dB louder ... maybe the 6550s just need a little burn in...

[bill bokey]

I set the voltage selector to 220V and now have around 540V on the plates (and 7V instead of 6.3V for the heater). I set the bias to 25mA but the amp is still a good 2dB quieter than it was with EL34s

Here' are two clips, first with EL34Bs, plate voltage was 470V, bias 37mA :
http://soundcloud.com/castle-made-of-so ... est-1-el34

This one with the 6550s, 540V plate voltage, 25mA bias :
http://soundcloud.com/castle-made-of-so ... est-2-6550

Amp settins are exactly the same, I made sure not to touch them at all.

I know I shoulda played a strat but I started recording with a Les Paul (with the EL34s)

[Xplorer]

sounds beautiful and killer !

i would love to hear it with a strat too ! in cleans, and a bit Wilder tones.

congrats !

[daveweyer]

Yeah, sounds killer. A couple of things on the loudness. The Russian tubes may not have the plate sat ratings of the old American Tung Sol, and if not, you will be restricted in the amount of power you can get out of them.
There is also the issue of class B drive requirements, you may want to try a 12AT7 in the PI socket, alternatively you can lower the feedback a little bit to give more gain in the output stage.
The plate sat rating tells you how much current the tube will draw when the plate voltage is very low, like in the 50 volt range. The more current the tube can draw at 50 plate volts, the higher the power output will be. As the tube swings the output rail down to the 50 volt range, it runs out of current to swing any more, and that's the end of it's ability to deliver any more power.
Russian tubes were terrible at first, but some of the brands have improved that rating quite a bit. I think the Tung Sol name is just one of Mike Matthews' brands now, so basically an Electro Harmonics product.
I'll see if I can find out how good those tubes are.

[bill bokey]

Thanks guys !

I made a few changes : added a pair of 220k bleeder resistors on the creen filter caps as the HT went up above 500V I didn't want to take any risk... but most importantly I replaced the 1K 5W screen resistor with 2k2 10W and they are huge !!!! It wasn't easy finding room for them !



Then I plugeed in a Strat and I turned the bias up a bit, to between 25 and 30mA, B+ went down to around 525V. With the strat I can hear things I couldn't with the Les Paul... and with the wah there seems to be some kind a ... disagreement !
I'll definitely try a 12AT7 in the PI now.

But in the meantime here's how it sounds with a strat (and some fuzz towards the end but it's the Octafuzz with the octave off... more of that later )

http://soundcloud.com/castle-made-of-so ... 6550-strat

[bill bokey]

There's a bit more volume with the 12at7 :

https://soundcloud.com/castle-made-of-s ... 6550-strat


There's room for improvement, I'll work on setting the bias right tomorrow.
The crackling that occurred with the wah probably has nothing to do with amp, it's probably just the jack input & output, I'll check that as well, I need my wah back !

[bill bokey]

The last one for today, with a Fulltone Octafuzz :

http://soundcloud.com/castle-made-of-so ... t-octafuzz

[daveweyer]

Fascinating, I'd love to hear other examples of the 6550 trial. What did everyone else get?
Where are the Beaujolais Hills? Maybe it would be possible to send a set of old American 6550s just for a comparison.

[bill bokey]

Never heard of Beaujolais wine ? It's in France, Dave. And yes, Id' love to try old 6550s !!! That would be a very interesting comparison.
I think the Tung Sol reissue need more burn in, I am just worried the heater is a little too high. Won't that shorten the tubes lifespan ?

[Roe]

you can add resistors to lower the heater voltage to around 6.5v. Try two 0.1R power resistors in parallell

[bill bokey]

Thanks ! I was wondering if that was possible ! So what do you mean exactly : add the two resistors (both in // for a total of 0.05R) in series between PT and first power tube ?
What wattage would be sufficient ?

I was actually thinking of using a Zener diode but don't know if I could ever find the right one ? (0.7V)

[Roe]

Thanks ! I was wondering if that was possible ! So what do you mean exactly : add the two resistors (both in // for a total of 0.05R) in series between PT and first power tube ?
What wattage would be sufficient ?

I was actually thinking of using a Zener diode but don't know if I could ever find the right one ? (0.7V)

yes, two or three resistors in parallel on each 3.15v heater secondary prior to the first (power) tube. Usually 3w metal oxide resistors work well but I always try to measure the voltage drop and to calculate heat diss. I get the 0.1R and 0.22R power resistors of ebay

[bill bokey]

Thanks Roe !