Adding caps to prevent oscilation in AF 5687

[ervinl]

Hi,

I just build simple AF 5687 tube pre amp as in DIY paradise (but without LED, just traditional Rk, R plate and C coupling).

I also buy some silver mica caps 47pf/400-1000 volt, in order to prevent oscilation that may destroy tube and/or sound. How should I connect to this caps, I see some variation circuit:
- bypass in Input (parallel with R grid)
- bypass in R plate
- another variation, from grid to plate

Any pros-cons for above methods, or should we combine? How is the effect to sound quality (e.g. BW etc)?

Thanks,

Ervin L

[SemperFi]

If you have grid stoppers there should be no reason to add those caps.

[ervinl]

I use 1k of grid stopper (Carbon Composite, T-Rex), before R grid of 470k, Riken RM.

In --> 1k (TRex) --> 1k --> grid
|
470 k
|
Gnd --------------------->

I just see some circuit, paralel those R grid and R plate with C filter. So I just would like to know what is the effect if I add this C with 3 configuration above, which one will be the best but not cut so much bandwidth.

Thx,

Ervin L

[DougL]

Originally Posted by ervinl

I use 1k of grid stopper (Carbon Composite, T-Rex), before R grid of 470k, Riken RM.

In --> 1k (TRex) --> 1k --> grid
|
470 k
|
Gnd --------------------->

I just see some circuit, paralel those R grid and R plate with C filter. So I just would like to know what is the effect if I add this C with 3 configuration above, which one will be the best but not cut so much bandwidth.

Thx,

Ervin L

If you could link to an example, we could help.
SempreFi is correct, in a zero feedback design, additional parts other than a grid stopper are not necessary for stability.
Just build the design on the schematic.

HTH

Doug

[Shoog]

If the anode hasn't got any load on its anode pin its remotely possible that its picking up RF.
In situations where the load is distant I generally add a 100R resistor straight on the pin. It may help.
When I messed about with the 5687 I noticed it was prone to oscillation, which could manifest as hum pickup from the heaters..

Shoog

[ervinl]

Originally Posted by Shoog

If the anode hasn't got any load on its anode pin its remotely possible that its picking up RF.
In situations where the load is distant I generally add a 100R resistor straight on the pin. It may help.
When I messed about with the 5687 I noticed it was prone to oscillation, which could manifest as hum pickup from the heaters..

Shoog

Does it mean more beneficial to add C parallel to R plate to block oscilation from anode?

How is 100 Ohm R connection, serie to coupling caps, or parallel as dummy load? If as load, is not it too smalll, usually I see the schematic with a vary of resistance value, 470k to 1k? Which one is most correct? I use 1uF of coupling caps (WIMA MKP).

[Shoog]

What I was suggesting is a 100R resistor in series with the anode before connection to the load resistor and output. Its only going to work if the load resistor isn't already straight onto the pin of the anode.
Basically any long unterminated leads to the valve socket are liable to become antenna at RF frequencies, and should have some type of load as close to the pin as possible. Its just good design practice.

Shoog

[ervinl]

Hi,

Do you mean like this:

B+
|
R plate
|
A --- R 100Ohm -----| C coupling | ----> Out
---G
K

About terminating, I am also thinking of it, what resistance should be, could we just add R in each of RCA connection in the box/chasis, to prevent oscilation due to long cable, unterminated (I could hear radio station somewhen, afraid it will make tube oscilate unexpectedly that may damage the tube)?

Thanks,

Ervin L

[Shoog]

That would be about it. See the comment below, as an alternative to the 100R resistor you could substitute an inductor which would do the same thing but not add series resistance at audio frequencies.

A small inductor in series with the hot pin of the RCA would isolate the cable from the load (a 10R resistor wrapped with magnet wire 10times soldered across the resistor), or traditionally wire a small ceramic cap from the center of the RCA to its earth shield.
I have never had to resort to this sort of tactic so cannot comment about the practical implementation.

Remember that the anode is a control grid - just a very low power one.

Shoog

[Arnulf]

Ervin, just about every forum software out there supports so-called "code" tag which formats text with fixed width font. This is ideal for drawing ASCII art, such as that in your messages. To use code tag, put [ code ] (without extra spaces) before your ASCII artwork and [ /code ] (without spaces) after it and it will show up as intended.