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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 8:35 am
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BMW2002Ti wrote:
Not for the real, NOS mil spec TungSol (or any other 1950-60's 5881). TS is notorious for under-rating their tubes. They recommend no more than 350VDC on the plate. Really, they've been tested to 500 VDC and 26 watts per tube. Much tougher than a "regular" 6L6GB, which RCA recommends to set at 19 watts per tube, push-pull, Class AB1 setting.

You maybe thinking the older Bassman 5B6 and 5C6 circuits which ran the 6L6GB or 5881 in cathode-biased, Class "A" mode. Then, you'd have a limit of about 12-15 watts per tube.


19-21 watts (your bias setting)/26 watts = 73% to 81% max plate dissipation. Are you saying the TungSols are 30 watt tubes? 21 watts/30 watts = 70% max plate dissipation.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 10:53 am
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Blue, yep the original TS 5881/6L6WGB can run at 30 watts per tube. In Class AB1, push-pull mode. Just as the 6L6GC CAN run at 35-40 watts in UL mode. I run my 5881's at around 20 watts. No problems, as long as the bias supply, grid stopper and grid-to-ground resistors are tight tolerance. Running tight tolerance resistors on the LTP phase inverter helps, too.

I WOULD NOT run any Russian-made 5881/6L6WGB at this rating. As they are not built to the same standards as the 1950-60's US made 5881 (which are true mil spec'ed tubes). I even run the 1980's Philips/Sylvania mil spec 6L6WGB's at slightly lower idle wattage. Like around 16-18 watts.

Of course you can run the TS cooler. As long as the tone is ok. Leo knew what he was doing using these great tubes in his tweed Bassman (& in other amps, of the era). Placed 400+ VDC onto the plates (when the TS data sheet read 350VDC max). And running them at 36-40 watts output per pair. Setup up correctly, those tube often lasted longer than the speakers.

One note on biasing and power voltages: Using good tubes, I like to run higher plate voltages and lower idle bias current to achieve the same idle wattage. To me, this gives the amp better headroom and transient response. I don't know what your 5F6A DIY runs onto the plates. But, if it's in the 350-360 VDC range, prolly best to be a bit conservative on the idle current. If it's in the 400-425 VDC range, you can try 18-20 watts. See how the amp sounds and how the tubes look like, in action.

Be sure to use a screen resistor that drops the screen voltage about 5-10 VDC below the plate (~500-1000 ohm resistor). This really helps keep the tube from runaway.

Anyhow, good luck with yours!


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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 5:13 pm
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Thanks. That's good info.

Current plate voltage is 432 VDC with the JJ 6L6GCs biased at 38 mA cathode current (about 36 mA plate current).

The amp uses a 470 ohm screen resistor onteh power tubes just like the original 59.

No grid stopper resistor on the power tubes, again, just like the original 59.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 5:14 pm
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Wow. No comments from anybody on my little demo? Was it that bad? Come on, I can take it. :lol: :lol: :lol:

http://www.youtube.com/watch?v=sio3jHx-cKQ

Guess my 15 minutes of fame is over. :cry:

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 6:05 pm
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bluesky636 wrote:
Wow. No comments from anybody on my little demo? Was it that bad? Come on, I can take it. :lol: :lol: :lol:

http://www.youtube.com/watch?v=sio3jHx-cKQ

Guess my 15 minutes of fame is over. :cry:


Ok...I'll go...your playing was fine. Ok so your not the next Duane Allman, that's ok, I ain't the next Clapton either so I guess we ain't forming Derek and Dominos Part II :lol: :lol: :lol: ...And to be honest playing the same thing over for each different setting, makes it a whole lot easier to get a good idea of the amps actual sound...which sounds great.

T2

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 6:14 pm
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Bill, ya did good and the amp sounds great. Your playing isn't bad at all. :wink:

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 6:44 pm
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Blue, your playing is fine. Good recording, too. Heck, playing is a personal thing. If you enjoy it... that is all that matters.

38mA at 432VDC = (0.038) times (432) = 16 watts idle dissipation per tube. Conservative for a 6L6GC, but if the tone is fine... go for it. I've biased 1950's made Tungsol 5881's from 14-20 watts per tube. Depending on Class of amp and whether push-pull or single-ended. No problems. MUST BE real TungSol's.

Enjoy! Then Play On

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 8:58 pm
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T2Stratman wrote:
bluesky636 wrote:
Wow. No comments from anybody on my little demo? Was it that bad? Come on, I can take it. :lol: :lol: :lol:

http://www.youtube.com/watch?v=sio3jHx-cKQ

Guess my 15 minutes of fame is over. :cry:


Ok...I'll go...your playing was fine. Ok so your not the next Duane Allman, that's ok, I ain't the next Clapton either so I guess we ain't forming Derek and Dominos Part II :lol: :lol: :lol: ...And to be honest playing the same thing over for each different setting, makes it a whole lot easier to get a good idea of the amps actual sound...which sounds great.

T2


Thanks T2.

Yeah, I sound more like Duane and Gregg's lesser known brother, Walter. He was the one that tripped over the cables at the Fillmore and fell into the audience and was never seen again. :lol: :lol: :lol:

I really wanted to keep the demo simple to show off the amp and not my skills as a player. I think I succeeded. :D

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 8:59 pm
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63supro wrote:
Bill, ya did good and the amp sounds great. Your playing isn't bad at all. :wink:


Thanks. I'm really pleased with the sound of the amp. It was a lot of work but well worth it. :D

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Tue Feb 07, 2012 9:04 pm
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BMW2002Ti wrote:
Blue, your playing is fine. Good recording, too. Heck, playing is a personal thing. If you enjoy it... that is all that matters.

38mA at 432VDC = (0.038) times (432) = 16 watts idle dissipation per tube. Conservative for a 6L6GC, but if the tone is fine... go for it. I've biased 1950's made Tungsol 5881's from 14-20 watts per tube. Depending on Class of amp and whether push-pull or single-ended. No problems. MUST BE real TungSol's.

Enjoy! Then Play On


Thanks. To be honest, I was really nervous about recording something and posting it on YouTube for all the world to hear.

The parts for the Presence mod came today and after I install it, I'm going to play with the power tube bias a bit. I have found that the JJ 6L6GCs really seem to like 38 mA cathode current in a Fender amp, but I'm goign to experiment a bit. I'll post the results.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Wed Feb 08, 2012 6:11 am
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Bill, the natural distortion on that amp is seriously sweet. Anyone who thinks the drive channels on the HRD series amps sounds good, really needs to listen to your sound sample. :mrgreen:
That's what it's all about.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Wed Feb 08, 2012 6:00 pm
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63supro wrote:
Bill, the natural distortion on that amp is seriously sweet. Anyone who thinks the drive channels on the HRD series amps sounds good, really needs to listen to your sound sample. :mrgreen:
That's what it's all about.


Thanks. :D

That's the difference between power tube distortion (5F6A) vs preamp tube distortion (HRD).

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Fri Feb 10, 2012 10:50 pm
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Made a few tweaks to the amp tonight.

First, I replaced the 59 vintage Presence circuit with the modern Presence circuit used in the 59 Bassman Reissue.

59 vintage Bassman Presence circuit:

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59 Bassman Reissue Presence circuit:

Image

The Reissue circuit blocks the DC that is present on the vintage circuit, making the operation of the Presence control very quiet by eliminating the scratchy sound that the vintage circuit exhibits as the knob is turned.

I also reset the power tube bias. I was able to adjust the bias to give me 50 mA cathode current (about 48 mA plate current) at a plate voltage of 425 volts. This works out to be an idle power of 20 watts, or 70% of the maximum power dissipation (30 watts for the JJ 6L6GC tubes). The tone has noticably changed, becoming thicker and just begining to break up at a slightly lower volume level. I would characterize the sound by saying that my original bias setting of 36 mA plate current at 432 VDC plate voltage (50% max power dissipation) is clean and crisp, perfect for Country music. The new setting of 48 mA plate current at 425 VDC plate voltage is creamier sounding at the same volume setting and perfect for the Blues. I haven't cranked the amp wide open yet as it is after midnight, but I will this weekend. 8)

Interestingly enough, the snap/crackle/pop noise I had riding on the normal background hiss is completely gone. I'm not sure if it was due to the new Presence circuit or finding the bias sweet spot for the power tubes. After tweaking the tone and presence controls slightly, the amp sounds better then before.

I have ordered some new Orange Drop and silver mica caps to replace the stock caps in the tone circuit. I'll see if that makes a noticable difference.

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Last edited by bluesky636 on Sat Nov 03, 2012 11:34 am, edited 1 time in total.

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Feb 11, 2012 12:37 am
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I think FMIC should hire you to build amps in the Custom Shop!

Where's the ballot?! I'd vote you in! Bluesky636 for FMIC Custom Shop Amps President!

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Post subject: Re: My Weber 5F6A Bassman Build-Up Thread
Posted: Sat Feb 11, 2012 8:05 am
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BLUESMAN1904 wrote:
I think FMIC should hire you to build amps in the Custom Shop!

Where's the ballot?! I'd vote you in! Bluesky636 for FMIC Custom Shop Amps President!


Do they have good benefits? :lol:

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