6G6 b+ voltage & 5881 tubes

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scooter_trasher
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6G6 b+ voltage & 5881 tubes

Post by scooter_trasher »

I have a new to me 1960s custom built 5E6 tweed bassman with with 6G6 solid state rectification, power transformer & output transformer, it's running all vintage 7025 preamp & pi tubes, with JJ6L6GC power tubes, I'm looking at the 6G6 schematics and it shows 470v b+ with 5881 power tubes, everywhere I look says 5881 are max at 360V is fender using a lower wattage to get away with 5881s , are the schematics wrong , or had I better not use the NOS RCA Tung-sol brown base 5881 tubes I just ordered?
anybody got a 6G6 bassman?
Fender_BASSMAN_6G6A.pdf
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martin manning
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Re: 6G6 b+ voltage & 5881 tubes

Post by martin manning »

Fender regularly ran voltages over the spec sheet limits, but replacement tubes were readily available. What B+ voltage does your amp have?
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Re: 6G6 b+ voltage & 5881 tubes

Post by Roe »

6g6a is 470v and 6g6b is 420v according to factory schematics. Old 5881s, RCAs, tend to handle 450v. I'd use 1k screen resistors at higher voltages. PErsonally, I don't like 470v in a 6g6, 450v is max for me
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scooter_trasher
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Re: 6G6 b+ voltage & 5881 tubes

Post by scooter_trasher »

Thanks for the reply, I'll be checking my B+ v ,hopefully it's down around 425v, my amp is either a test ,( prototype chassis) or a home brew from the 1960s, it has a 6E6 layout,with what appears to be unmolested original caps, resistors and pots, no serial numbers, no rectifier socket holes, 6G6 solid state rectifier circuit, and 6G6a & up big iron, Thordarson T-22 R07 off the shelf power transformer that I found the specs on out of a 1947 catalogue, 350-0-350, (matching fenders 125P7A), a fender Schumacher 022871 ot dating to 1967, it's currently running 3 x 7025 in the pre & pi sockets as you would expect to find in a 6G6 and a pair of JJ 6L6GCs, all in a piggyback with a ported 4x10 cabinet, with 1966 cts alnico & 1965 Jensen concert speakers that I haven't pulled the bell on, I'll likely never find out the story on the amp, but it's definitely all vintage 60s, and it sounds great. Given the time period it almost doesn't make sense to build a Boutique amp morphing features of 5 and 10 year old models, especially given it doesn't have two channels, or maybe that's what the builder was after. a 1955 tweed with the low end punch of a brownface.
Last edited by scooter_trasher on Mon Jun 05, 2017 9:54 pm, edited 1 time in total.
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rooster
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Re: 6G6 b+ voltage & 5881 tubes

Post by rooster »

Schematics are wrong for this amp, the 'A' and 'B', the voltages are more like 500 on the plates. Also note, the 'B' is more desirable than the 'A' due to a design flaw. When you get this working, circle back and post your voltages. Bias the power tubes to 28Ma@ and you should be fine with most power tubes produced these days. The 'Double Mica' 6L6s from Eurotubes.com would be a safe bet as they are very serious about testing their tubes before sale. Oh, and yes, I have owned both the A and B versions as well as worked on a few of them over the years.
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two tone
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Re: 6G6 b+ voltage & 5881 tubes

Post by two tone »

I have a 64 blonde Bassman,6G6B,425v on the plates,great amp with matching cab,Oxford speakers
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Re: 6G6 b+ voltage & 5881 tubes

Post by Stevem »

There is a lot wrong with many Fender voltages on there schematics!
From my 40 plus years of working on Fenders I can tell you with certancy that only Fenders test had over 465 volts on the output tube plates are the super Reverbs , duel Showmans and Twin Reverbs, and as always this hinges on how hot the amp is biased.

Some facts from Fender schematics.

6G6 PT number 125P5A, V+ 460 by means of a 5AR4 recto, -56 bias.

6G6A. PT number 125P7A, V+470 by SS recto, bias -58.

6G6B. PT number 125P7A. V+ 430 by SS recto, bias -54

As you can see in two amps there is a 50 volt difference listed when the same power transformer is used!

My take on this is that the listed 470 volts was taken with the amp in standby, and the normal voltage in play mode is 430 volts.

More evedence for this that the BF Bassman and bandmaster uses a PT 125P7D , the last digit change is when Fender switched from Triad made pts to Schumacher , other wise the specs are the same and the V+ voltage is shown as being 425 which is right in the ball park to 430 in the 6G6B amps.

Now to your tube question.

Those RCA 5881s will handle the 430 volts , but as posted to me over 450 is pushing it pretty good!
They will be far more prone to arcing as they age, and even if they do not arc there life will be less to the tune of 10%.

In short with 470 volts on my plates it would be JJ 6L6GC tubes for me!
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john l
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Re: 6G6 b+ voltage & 5881 tubes

Post by john l »

I'm glad this thread came up. I'm trying to decide if there's a problem with my 6g6b I just got. When I variac down to 110 which is what my amp wants to see my plate voltage is 448v. I thought this was pretty high?

Edit this is with all tubes in and biased at 36 mA

If I plug straight into the wall my Plate voltage is 497
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scooter_trasher
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Re: 6G6 b+ voltage & 5881 tubes

Post by scooter_trasher »

Just checked with the old Fluke, 478 & 478 at the plates, I'll be getting one of them Variacs I've been wanting, now the question will be vintage variac or imported grounded, I do like the grounded thing :)
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martin manning
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Re: 6G6 b+ voltage & 5881 tubes

Post by martin manning »

Maybe consider a bucking transformer setup to knock 10% off the line voltage?
scooter_trasher
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Re: 6G6 b+ voltage & 5881 tubes

Post by scooter_trasher »

martin manning wrote: Mon Jun 05, 2017 6:52 pm Maybe consider a bucking transformer setup to knock 10% off the line voltage?
variacs aren't all that much money and I can use it with all my vintage amps + there great for powering up old amps that are suspect.
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Re: 6G6 b+ voltage & 5881 tubes

Post by john l »

scooter_trasher wrote: Mon Jun 05, 2017 6:18 pm Just checked with the old Fluke, 478 & 478 at the plates, I'll be getting one of them Variacs I've been wanting, now the question will be vintage variac or imported grounded, I do like the grounded thing :)
Out of curiosity what's you're wall voltage?
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scooter_trasher
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Re: 6G6 b+ voltage & 5881 tubes

Post by scooter_trasher »

john l wrote: Tue Jun 06, 2017 3:33 am
scooter_trasher wrote: Mon Jun 05, 2017 6:18 pm Just checked with the old Fluke, 478 & 478 at the plates, I'll be getting one of them Variacs I've been wanting, now the question will be vintage variac or imported grounded, I do like the grounded thing :)
Out of curiosity what's you're wall voltage?
My wall voltage is 122.4 v 478v DIV by 122.4= 3.9 x110v = 429v ish, should get me in the ballpark for the 5881 unless my math is off or 478v - 10% = 430.2v
scooter_trasher
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Re: 6G6 b+ voltage & 5881 tubes

Post by scooter_trasher »

john l wrote: Mon Jun 05, 2017 5:33 pm I'm glad this thread came up. I'm trying to decide if there's a problem with my 6g6b I just got. When I variac down to 110 which is what my amp wants to see my plate voltage is 448v. I thought this was pretty high?

Edit this is with all tubes in and biased at 36 mA

If I plug straight into the wall my Plate voltage is 497
sounds like your amp may like 105 at the wall if you run 5881s , your amp only sees what comes off the PT not the wall, mine seems to be running fine at 478 v with 6L6GC jjs, but I don't have but maybe an hour on it, just got it.
scooter_trasher
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Re: 6G6 b+ voltage & 5881 tubes

Post by scooter_trasher »

[quote=Stevem post_id=373780 time=1496680792 user_id=15219]
There is a lot wrong with many Fender voltages on there schematics!
From my 40 plus years of working on Fenders I can tell you with certancy that only Fenders test had over 465 volts on the output tube plates are the super Reverbs , duel Showmans and Twin Reverbs, and as always this hinges on how hot the amp is biased.

Thanks for the reply & info Stevem, sometimes I forget little things like please & thank you, for a bit when I get stuff on my mind.
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