Plate voltage too low

[C Moore]

I just rebuilt this 1969 Twin. It is pretty quiet, has good tone, the verb and trem work good, but I only have about 408 VDC on the plates. Current on the power tubes is between 41-45 mA.
PT secondary is 622 VAC, and I get about 410 VDC from the rectifier. Does the plate voltage seem kind of low to you guys? Shouldn't it be more in the 450 VDC range?
Did I do something wrong in the rectifier?
Thank You

[topbrent]

The schematic voltage spec for a 1969 AA769 - 1970 AA270 twin is 410v from the rectifier.

Voltage is acceptable. Anything +/- 20% is acceptable.



http://www.prowessamplifiers.com/schema ... ayout.html

http://www.prowessamplifiers.com/schema ... matic.html

[C Moore]

This is an AB763. That layout shows 460 on the plates, and 458 on the screens. But regardless of what is on paper, am I wrong in thinking my plate voltage seems low? Anybody else with a Fender Twin that has less than 415 volts on the plates?
In general, isn't that kind of low for =C= 6L6GC tubes?
Thank You

[Drumslinger]

hired hand,

what are you getting at the choke? you should read the same voltage on the plates as what the choke is reading.

[Andy Le Blanc]

for that type of rectifier and sec. AC V of 622 the most you get is 435V DC.
The good thing about full wave capacitor Input PS is that you get all the current
that the PT is rated for. Double check your wall voltage, mine gets close to 130
yours might be a little low. Hows the power supply caps, new, old...
It can also be how the tubes are biased, if the bias is on the hot side the plate V can be lower than published numbers.
There was always quite a bit of parts transition between production models.
It may have the years model number but still have last years parts.
check the PT #...

[Structo]

What voltage are you getting right out of the diodes at the first primary filter?
Where it shows the 460+?

I would say check your wall voltage but really, as long as it is safely biased and sounds good, and is quiet, mission accomplished.

I've had an amp that measured low at peak usage time then later when demand was down, it was spot on.

I see they don't list the primary voltage for that PT.

Anyway, if it sounds good it probably is alright.

30 watts x 70% = 21
21 / 410 = 51ma is your 70% bias current.

44ma is around 60%
Screen current is only a few ma so you are pretty conservative in the bias.

[Drumslinger]

As Tom said "as long as it is safely biased and sounds good, and is quiet, mission accomplished.". some amps sound good biased a little cold and others sound good biased hot. play it for a while and see if you like the tone coming out of it.

[Structo]

Also if this amp is for a customer (read not you) then consider the way they play the amp.
Do they attack the strings without abandon, or are they a conservative player, use pick dynamics to control tone and attack?

For a player that plays with everything on 10 and ballz to the wallz style I would bias the amp middle to left of center.

For someone that has more finesse in his playing I would spend more time with it and see how they like it when playing.

I never even knew what picking dynamics meant or were until I built a *umble type amp.

[C Moore]

Thanks for all the replys......
All the components are new, including the filter caps.
My wall voltage is always pretty high, about 122 VAC.
Yeah, right at the rectifier I was getting 410 and then about 408 at the plates. There is only 1 VDC across the screens, so I am losing about 2 mA there. I know my bias is a bit low, as far as 70% is concerned. I was hesitant to increase it, because I hated to see the plates go any lower. But I will at least try increasing it and see how it goes. The chassis is still on the bench, with jumper RCA and speaker cables running to the cabinet. So maybe that is why it does not quite seem as loud and punchy as it should be.
I do not play guitar, so I am at a disadvantage. After I do this much work to an amp, I always have one of my friends, that are good players, test drive the amps for me so I can really hear how they sound, and to get that second opinion from a real player.
Let me up the bias a bit, get the chassis and cabinet all buttoned up, and I will get back to you guys. I was concerned mostly because I do not recall working on a 6L6 amp with plate voltage this low. We did have a Carol-Ann with low plates, but that was by design, as he quotes that amp at only 30 Watts for a pair of 6L6. But that is out of the norm in my experience, so I asked you guys......
Thanks Again

[C Moore]

Also if this amp is for a customer (read not you) then consider the way they play the amp.
Do they attack the strings without abandon, or are they a conservative player, use pick dynamics to control tone and attack?

For a player that plays with everything on 10 and ballz to the wallz style I would bias the amp middle to left of center.

For someone that has more finesse in his playing I would spend more time with it and see how they like it when playing.

I never even knew what picking dynamics meant or were until I built a *umble type amp.

I am going to sell this amp ASAP. I probably should have just left it alone, but it looked like such a mess inside. It WAS NOT original. Looked like it had been worked on 6 or 7 times over the years. The lead dress was atrocious, all different brands and colors of caps and resistors, 22 AWG wire on the power tube heaters, scratchy pots, etc. It just looked like an old amp that needed "some" work. So I did this to it, and now I need to sell it. The only thing original is the chassis, trannies/choke, preamp tube sockets, pilot light, and the RCA jacks. Speakers are Eminence Tonker & Weber California ceramic, paper dust cap. I am thinking it is in the 600-800 dollar range. What do you guys think?
Thank You