Best Way to Control Plate Voltage?

[slor]

As I've chimed in on other posts, I'm working on a Bell 2440 integrated amp. It calls for 4 x 7189A, but I'm using EL84M as their economy better suits the job at hand.

The schemo calls for a plate voltage of 420+ and a bias voltage of -19. Right now I'm getting a plate voltage of 462+ with a bias voltage of -18. My question is: How best to control the B+? I'm told that the EL84M can handle plate voltages in excess of 400+, but it seems wiser to be a little lower than where I'm at....

My understanding is that I can:

a) Make the bias voltage more positive (the bias is fixed but non-adjustable, a trim pot is on order so I can mod the circuit).

b) Add a dropping resistor to the power supply to get the B+ more in line with the schemo (keeping in mind that the amp is using a non-original tube type).

I understand, dimly, that changing the bias will affect the B+ (and current draw). Can anyone steeer me in a (more or less) correct direction?

Many thanks,
Seth

[Miles Prower]

Your Vpp is within the magical +/- 10%. VT circuits are quite a bit more "forgiving" than SS. Don't worry about it.

[sorenj07]

If you have room you could always stick a choke in there.

[slor]

Gentlemen: Thanks for the replies!

Miles, I think I'm getting to the point where I can predict your replies 99% of the time: "Don't worry about it!"

Frankly, that's the answer I like to hear, but I am still scratching my head a little over this one. (The amp is for a non-tech friend, so I want to be doubly sure it's running on the cooler side). Plus, I've never done the adjustable bias conversion before, so I may try it for kicks anyway.

And thank you Soren, I don't think there's room for a choke in the amp, maybe on top of the chassis? Seems like that would be a fairly noticeable change in the topology, though, right? Plus, I don't know anything about calculating what value it would be. Do you have any tips?

Thanks,
Seth

[rcavictim]

Seth,

IME with 6BQ5/EL84's and high'ish B+ the tube manual recommended grid one drain resistor to earth seems to be too high and the tubes often hot plate and run away destroying them, especially if their quality is less than perfect. RCA recommends 1 megohm for cathode bias and 300K for fixed bias. I have discovered that reducing the value of this R to 250K or 330K max ohms in cathode bias seems to solve the problem. This will have a small effect on the filter corner frequency developed between the coupling cap to this grid from the driver stage. The solution here is to increase slightly the value of the prescribed coupling cap. 2X value will do.

[slor]

Hey Bob,

Thanks much for the tips...some of what you're saying is a little beyond my comprehension:

the tube manual recommended grid one drain resistor to earth seems to be too high and the tubes often hot plate and run away destroying them, especially if their quality is less than perfect. RCA recommends 1 megohm for cathode bias and 300K for fixed bias. I have discovered that reducing the value of this R to 250K or 330K max ohms in cathode bias seems to solve the problem.

By "grid 1" do you mean the control grid? In any event I don't find any resistors to ground from the output tube sockets, except the 50ohm 2W variables for balance.

Best,
Seth

[rcavictim]

Originally posted by slor
Hey Bob,

Thanks much for the tips...some of what you're saying is a little beyond my comprehension:


By "grid 1" do you mean the control grid? In any event I don't find any resistors to ground from the output tube sockets, except the 50ohm 2W variables for balance.

Best,
Seth

Seth,

I re-read your first post and see my assumption that your circuit was cathode bias was in error. My advise does not apply. Yes, grid one is the control grid.