Bassman 6G6-b voltage problem...

[grtamp]

Hi , a friend of mine have bring me an old Fender Bassman 6G6-b model for first power up after 20 years!

First problem I found is in the bias circuit , there is a 43K res instead 27k and my Vbias was 36V, I change the 25U/50Vcap , and I solve the problem.

Second problem is about the anodic voltage b+ , after the chocke I have 478/480V and this makes me think to bad caps in power filter section.

any suggestion?

Thanks

R

[Milkmansound]

It's probably fine. Voltages may be higher than what's on the schematic because line voltages are a lot higher these days than they used to be in most houses.

As long as the output is properly biased for the new voltage, and the caps are within their rating you should be fine

Unless it sounds bad, in which case there may be a problem

[grtamp]

my wall voltage is 220V , I use a variac!

I set the Vbias for the 470/480 but when turn up the volume ( 1khz 1Vpp input sine-wave), current in the power tubes rise to 65/70mA only , and anodic voltage go down drastically as far as to 420V .

Output power is low.

PT don't seems to me has worked a lot , and all cc res are in good conditions!!!

maybe a lot time with no use, the electrolytic capacitors are exhausted!

[ToneMerc]

Are you sure it's a 6G6-B and not actually a 6G6-A? The reason being, there were 6G6-A's sent out with a 43K( instead of the 56K on the sch.)bias resistor and of course the "A" runs the higher 470V on the plates.

TM

[grtamp]

Hi TM , it's a 6G6-B , the value of the tone stack of the BASS Channel are identical , 6G6-A has different values! can you look to the Tone stack: 6G6-B has 10K/47K w/250Pf and 6G6-A has 27k/27K w/100Pf

One thing seems strange to me , the amp is a blackface with round cream colour knobs , and the head-cab is black ,not blonde!!

[ToneMerc]

One thing seems strange to me , the amp is a blackface with round cream colour knobs , and the head-cab is black ,not blonde!!

Not odd at all!, but rare. You are describing a " Tuxedo Bassman" which is a transitional model of Bassman using left blonde chassis parts. Can you take some pictures?

http://gretschpages.com/forum/other-amp ... 261/page1/

TM

[grtamp]

Yes ,I will post some photo in the afternoon!!

[grtamp]

I found one problem , the screen res of the power tubes was 47k instead 470Ohm!....

another problem I seen is the different current in the power tubes, always 17/18mA. with different tubes also!

[grtamp]

I can't believe ,this amp mount a 022897 / 125A30A OT , and if I'm right , this trafo is for a SingleShowman amp and run with 4x6L6!.... (AB568 - AA763)
I will have to replace it...

The PT is a 125P32B....showman?

[ToneMerc]

Doesn't appear to be stock, take a pic of the entire inside chassis.

TM

[grtamp]

some pics!

[ToneMerc]

Looks like the original Bassman iron was pulled. I don't often push the Merc Mag iron for a given model, but if you have the coin to spend, install the Mercury blonde Bassman clone iron if you want the normal channel to sound like the original when you start to move some air. Also, try to get the plate voltage down into the 425-435V range.

TM

[grtamp]

Thanks for your suggestion TM!!

[ToneMerc]

I like modding the bass channel in these, but the simplest mod you can do is to remove the .002 ceramic cap across V2b plate resistor. That will at least make the bass chan more conducive to guitar.

TM

[Firestorm]

That looks like one of the stock export transformers from the era, currently set up for 220VAC. You might be able to knock the B+ down closer to 6G6B levels if you connect to the 240VAC tap. Make sure the filaments don't go below 5.9VAC.

The 022897 would be roughly a 2K primary into 8 ohms, so assuming you have a stock 2 x 8 ohm cab, until you can get the proper OT, you could connect the speakers in series instead of parallel and at least have the correct primary impedance.

Besides snipping the .002 on V2B, you can also fiddle with the value of the 820R mid resistor on the Bass pot. Try 1K5 or 2K2. You have to be careful; this preamp has way more potential gain than most Fenders. The tone stack was designed to heavily attenuate the signal.

If it were me, I'd replace the electrolytics.