adjustable bias 4 peavey bravo.

[ampstepper]

hi everybody...ive been hanging around for a while and have learned a lot from the people posting here...many thanks to people like enzo and steve connor...anyway to the question in hand...does anybody here know how to do an adjustable bias mod for my peavey bravo.....any help would be most appreciated..thanks in advance..Ampstepper...:

[pontiacpete]

can you post a schematic?

[ampstepper]

re peavey bravo bias

hi pontiac im a bit new to the forum thing ... i have one ill try to attach it ...

[ampstepper]

Bravo_112.pdf tried to attach a pdf of bravo schem ...bear with me if it dont attach..

[ampstepper]

peavey bravo adjustable bias?

right then ....im pretty pleased with myself...ive sussed adjustable bias on bravo.i looked at valveking mod and applied it to bravo.yeeesss it works..but on matched tubes:::::does anybody know of a way to give adjustable bias independantly on each output tube????................................................ ..............incedently if anyones interested substitute resistor 60 which is 33k to 50k cermet.... bravo adjustable bias on matched pair..........resistor 60 is hidden between 390 ohm choke resistor and big can next door,,,,,,,,,,choke resistor is 400 ohm on schematic??...previous owner mod??

[Enzo]

400 ohms is a non-standard value. PV can order any value they want, and often does. But they are not going to shut down production if they run out. The standard resistor value of 390 ohms is within 2.5% of the 400 ohms and is perfectly suitable as a substitute. the factory uses 390 when it doesn't have sufficient 400 ohm parts. Consider that the 400 ohm resistors are 5% at most, which means a good 400 ohm resistor could meaure as low as 380 ohms anyway.

Independent bias seems like overkill to me, but that is merely my opinion. You want two bias controls? Wire a second 18k resistor from the C39 node, and duplicate what you have already for the adjustment. Then rewire so R47 runs off of one of the supplies and R49 off the other.

[ampstepper]

thanks very much enzo,,,,,,will digest and decide whether its worth doin....ampstepper steppin up amps

[Enzo]

Did you follow what I meant to do?

[ampstepper]

hi enzo i had a good look at what you suggested.Only one query,do i have to give the second bias string its own capacitor...{ie cap 40@ 22uf at 25v? or can i piggyback off the original.im also thinkin i have to cut a trace on output board...so all in all......think twice mod once .....thanks in advance for all and any help

[Enzo]

Of course you will have to cut traces or at least have resistors flying by their leads - you want to separate that which is currently joined.

You want two bias supplies. So two R65s. two R60s, two C40s. then do whatever you did to make the whole thing adjustable to the new supply. Change R60 to a pot presumably.

Some advice. Replacing R60 with a pot leaves the amp so that someone COULD turn the bias to zero volts. Not suggesting you would, but someone could. SO instead of a 30k pot, I recommend a pot with series resistor. For example a 25k pot with a 10k resistor in series with it makes something close to 33k. it has two advantages. One is you can only turn the bias down to that 10k safety resistance. Second is that now more of the pot range is useful. With the 50k pot you would only be using the small part of its range. With a 25k pot, you could use a lot more of its range - adjustment wouold be less touchy.

And nothing magic about my numbers. A 22k resistor and a 10k pot might work super.

Of course if your 50k pot works well for you, then don;t worry about it, just consider the safety margin the added resistor gives.

[ampstepper]

yes as expected from what i can see i only have to cut one trace,also thanks for confirming the cap issue...as far as the safety resistor goes this amp is a keeper 4 me after my many varied tinkering..also incedently i only wanted to do a dual bias arrangement because i have a plethora of good but non matched tubes......also like the idea of unbalancing the output stage slightly.............how hard would it be to do some sort of balance control to match tubes instead of dual independant bias......thanks for your help so far enzo......ampstepper

[Grrrg]

Hello, first timer here. Found the thread while researching the Bravo 112. I just got the amp and have an issue with the reverb since it starts screaming when I turn it up a lot. I already found out, that putting it into a reverb bag or some other shielding might help. If someone has any ideas on that issue I would be interested.

Also the amp is humming a lot, but since it still had the peavey tubes in, probably nothing has been done with this amp in a long time. Changing the tubes didnīt help on the humming issue, but I will also change the bigger caps (and probably the smaller ones as well)

But thatīs not why I am here.
Question: if I wanted to install a dual bias on the Bravo: instead of doing all said above wouldnīt it work to just put in pots/trimmers instead of R47 and R49?
Thanks in advance

[Helmholtz]

Question: if I wanted to install a dual bias on the Bravo: instead of doing all said above wouldnīt it work to just put in pots/trimmers instead of R47 and R49?

No, varying R47/R49 won't change grid voltages. As there is no current flowing through them, they don't drop voltage.

[Grrrg]

Thanks. Something else. I just measured the resistance of the output transformer (excuse my English if I donīt know the correct technical terms) to check the bias and one half measures 150 Ohms, the other 200 Ohms. Shouldnīt they be the same? Is the transformer defect?

The amp is working it is just humming a lot

[Helmholtz]

Thanks. Something else. I just measured the resistance of the output transformer (excuse my English if I donīt know the correct technical terms) to check the bias and one half measures 150 Ohms, the other 200 Ohms. Shouldnīt they be the same? Is the transformer defect?

OTs often have different DCRs of the primary halves. Reason is that the outer half has a larger diameter than the inner one. DCR difference is irrelevant.
What matters is the same number of turns.