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Post subject: Quick Bias Question
Posted: Mon Apr 10, 2017 4:42 pm
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Hey, guys!
Quick question to confirm my adjustment.

I have a '60 Super. With the GZ34, it was running 471v on the plates. I had it biased at 44.6. Sounds great. I dropped in a 5R4GYB ruectifier to try something different. It lowered the PV to 426 with bias set at 44.9- that's the max adjustment the pot will allow (I have a bias pot installed) That puts me in the 60%-70% bias range.

I feel like the numbers with the 5R4 are within the acceptable range.

Do you guys concur?

Thanks!


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Post subject: Re: Quick Bias Question
Posted: Mon Apr 10, 2017 4:54 pm
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Your B+ is a bit high for a 6G4-A. The schematic shows 448 VDC as a nominal plate voltage.

That said, I see nothing amiss using the 5R4GYB in lieu of the GZ34, especially if you're running 5881's as specified.

Arjay

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Post subject: Re: Quick Bias Question
Posted: Mon Apr 10, 2017 6:02 pm
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Retroverbial wrote:
Your B+ is a bit high for a 6G4-A. The schematic shows 448 VDC as a nominal plate voltage.

That said, I see nothing amiss using the 5R4GYB in lieu of the GZ34, especially if you're running 5881's as specified.

Arjay


Hey, Arjay- Thanks!
Mine is a 5G4 (6G4 schem) with 6L6GC listed. I'm running JJs right now.

Isn't the stock B+ a bit higher on that layout than the 6G4-A?

Also, where on the schematic can I find the listed plate voltage? I must be overlooking it.

Thanks!


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Post subject: Re: Quick Bias Question
Posted: Mon Apr 10, 2017 6:22 pm
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My 6G4 schematic shows a B+ of 456 VDC, driving a pair of 6L6GC's. Still somewhat tamer than your 471 VDC when rectifying via the GZ34.

Arjay

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Post subject: Re: Quick Bias Question
Posted: Mon Apr 10, 2017 7:10 pm
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Retroverbial wrote:
My 6G4 schematic shows a B+ of 456 VDC, driving a pair of 6L6GC's. Still somewhat tamer than your 471 VDC when rectifying via the GZ34.

Arjay


I haven't checked my wall outlet voltage, but I would bet it is pushing my B+ up to 471.
It probably came down a bit after I bumped the bias up a bit with the GZ34. I forgot to measure after the adjustment

I see now on the schem the B+ labeled from pin #3.
Thanks!


Last edited by Bottleneck on Tue Apr 11, 2017 4:51 am, edited 1 time in total.

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Post subject: Re: Quick Bias Question
Posted: Mon Apr 10, 2017 7:59 pm
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Yes, pin #3 of the 6L6GC, is the anode plate.

44.6mA at 471VDC = (0.0446) X (471) = 21 watts.

This is about 70% of 30 watt max for 6L6GC. Most ppl like 60-70% of max dissipation, for push-pull Class AB1, Fender amps using 6L6GC.

You have a very healthy GZ34. The 5R4GYB is a nice rectifier. Direct heated, like the 5U4GB. But, a lot more forward voltage drop. Like 50VDC, versus 40VDC for the 5U4GB. And ~15VDC for the GZ34.

BTW... ever heard of the mil spec Chatham-Tungsol 5R4WGB "potato masher?" :D

Image


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Post subject: Re: Quick Bias Question
Posted: Mon Apr 10, 2017 10:01 pm
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Bottleneck wrote:
I haven't checked my wall outlet voltage, but I would bet it is pushing my B+ up to 471.


+1

The likely culprit. Your amp was designed to operate on 110 VAC and I bet you'll find that your wall-socket voltage is close to ten volts higher.

Arjay

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Post subject: Re: Quick Bias Question
Posted: Tue Apr 11, 2017 5:00 am
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BMW2002Ti wrote:
Yes, pin #3 of the 6L6GC, is the anode plate.

44.6mA at 471VDC = (0.0446) X (471) = 21 watts.

This is about 70% of 30 watt max for 6L6GC. Most ppl like 60-70% of max dissipation, for push-pull Class AB1, Fender amps using 6L6GC.

You have a very healthy GZ34. The 5R4GYB is a nice rectifier. Direct heated, like the 5U4GB. But, a lot more forward voltage drop. Like 50VDC, versus 40VDC for the 5U4GB. And ~15VDC for the GZ34.

BTW... ever heard of the mil spec Chatham-Tungsol 5R4WGB "potato masher?" :D

Image


I saw one of those when I was shopping around. I wasn't sure, so I passed.

I almost got an early 60s RCA 5R4 that was shaped like a big bulb, but ended up with a '71 RCA 5R4GYB that tested good.

I am looking forward to trying out the amp this weekend with the band to see the difference in sound/feel.

It has sounded great with the GZ34, but there is just a hair of harshness in the treble that I think might be the result of too much voltage. We'll see.

Thanks!


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Post subject: Re: Quick Bias Question
Posted: Tue Apr 11, 2017 8:13 pm
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The GZ34 not only puts a lot more B+ onto the 6L6GC ---- but really --- onto the 12AX7 (7025) anodes, too. And you can't easily drop the idle bias in those tubes. My bet is the phase inverter 12AX7 maybe running too hot, with the GZ34 and current outlet voltages. Hitting voltages over 300VDC on each anode. That'll definitely make for a more strident, forced tone.

Maybe, going to a 120K-ohm/140k-ohm anode load instead of the 82/100 that is OEM, may lower the plate voltages back to a more sane value. Or try a 12AT7 in the PI position. It can handle more idle current, than a 12AX7.

http://ampwares.com/schematics/super_6g4a.pdf


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Post subject: Re: Quick Bias Question
Posted: Sat Apr 15, 2017 6:10 pm
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Thanks, Beemer!
I will look into changing the PI to 12at7 if I go back to the GZ34.
I played a gig last night with the 5R4 setup. I can definitely tell a difference in the tone and feel of the amp. I like it, but need to spend more time with this configuration to see if I'm going to keep it this way.

I am running a Klon Clone into my '64 6G15 into the Super. It sounds glorious. I am not sure I like all the sag it is giving up with the 5R4. I do like the bass response from the GZ34 a little better.

I will probably keep the 5R4 to keep the stress on the components down as opposed to the higher PV with the 6L6s.

Only one way to find out...lol


Last edited by Bottleneck on Mon Apr 17, 2017 4:29 am, edited 1 time in total.

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Post subject: Re: Quick Bias Question
Posted: Sat Apr 15, 2017 6:22 pm
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Bottleneck wrote:
Only one way to find out...lol


Just be sure to keep the magic blue smoke within the amp's components.

:wink:

BTW, what speakers are you using?

Arjay

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Post subject: Re: Quick Bias Question
Posted: Sun Apr 16, 2017 4:52 am
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Retroverbial wrote:
Bottleneck wrote:
Only one way to find out...lol


Just be sure to keep the magic blue smoke within the amp's components.

:wink:

BTW, what speakers are you using?

Arjay


LOL...

I have Jensen P10Q in there. The are vintage, but I believe they're reconed.


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Post subject: Re: Quick Bias Question
Posted: Sun Apr 16, 2017 10:56 am
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Great speakers!

Arjay

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Post subject: Re: Quick Bias Question
Posted: Sun Apr 16, 2017 6:00 pm
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What I've discovered over the course of a few years is that there isn't much for options when it comes to rectifier tubes for vintage Fender amps, or any others I guess. Too bad a 5V4 is only rated at 175 mA's, it'd be a great replacement for an indirectly heated 5AR4, would drop the PV just a bit with less sag than a 5U4 or 5R4.

UN


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Post subject: Re: Quick Bias Question
Posted: Mon Apr 17, 2017 2:07 am
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Since the GZ34 (and presumably the 5AR4) are providing better bass response, that'd be the rectifier I'd use. Ten-inch speakers generally need all the LF help they can get.

Arjay

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