What is a good replacement for ceramic caps (Fender Twin)

[ctcpete]

I have original ceramic caps in my '75 Twin. Are Mallory 150s (tubular metalized polyester) good for this application? Should I just get new ceramics? One of my .1u 600v caps just before the 1500 ohm power tube grid resistor actually crumbled when I moved it slightly. Do you guys recommend replacing all the old ceramics?

Thanks in advance, Pete

[jrfrond]

That .1 cap you are talking about isn't ceramic, it's polyester.

[ctcpete]

ok thanks. It was round and light brown, looked like a ceramic to me I'm still trying to learn this stuff. What are good replacements for poly caps?

[Bruce / Mission Amps]

ok thanks. It was round and light brown, looked like a ceramic to me I'm still trying to learn this stuff. What are good replacements for poly caps?

Sounds like a ceramic cap to me too.
Get a layout from online somewhere and show which one you mean.
I don't think there are any .1uF ceramic caps in the audio chain though.

[tubeswell]

Any high voltage film cap will work in the signal path. (But I wouldn't use disc ceramics, they are more prone to microphonics - although the kV rated ones okay in the power supply for balancing rectifier diode PIVs).

As far as film caps go, some folks swear by orange drops, some by Mallory 150s. I like the sound of silver micas for smaller value (e.g. treble tone) caps myself. Polystyrene caps need care when soldering otherwise they will melt and then they are no good. JM2CW

[ctcpete]

The cap is indeed a .1u, 600 V ceramic (one of the previous posters was probably referring to the poly film Mollory150s I was asking about in my original post). It is located between the 12AT7 (phase invertor I believe) tube and the 1500ohm grid resistors of the 6l6 power tubes (twin reverb, aa270 circuit). Is there any reason or benefit in not using a ceramic in this section? I am leaning towards just replacing it with another ceramic.

I'm a newbie and am not sure if this is the audio path, I'll try later to confirm.

Once again, many thanks. Pete

[ctcpete]

Sorry guys, my mistake. I just had a look at the chassis. I had the value of one of the blue Mallorys mixed up with the ceramic cap. The ceramic cap is labled .01 K, 1Kv (.01u,1000V I suppose). From looking at the schem., one end connects to two 220k resistors, the other lead connects to a 330 k resistor and from there there is a connection to pin 2 of the 12AT7 (phase invertor?) tube. The bottom of the ceramic cap partially crumbled.
Long story short, in general, do you guys replace old ceramics with new ceramics?

Thanks, Pete

[tubeswell]

Sorry guys, my mistake. I just had a look at the chassis. I had the value of one of the blue Mallorys mixed up with the ceramic cap. The ceramic cap is labled .01 K, 1Kv (.01u,1000V I suppose). From looking at the schem., one end connects to two 220k resistors, the other lead connects to a 330 k resistor and from there there is a connection to pin 2 of the 12AT7 (phase invertor?) tube. The bottom of the ceramic cap partially crumbled.
Long story short, in general, do you guys replace old ceramics with new ceramics?

Thanks, Pete

That is the .01uF coupling cap going into the first grid of the PI. A 600V film cap will be ideal. Somebody in the past probably put that 1kV disc ceramic in there as a substitute but it is not necessary to replace it with another disc ceramic cap (in fact I reckon it'd be better if you don't).

[52 Bill]

That makes much more sense now. I think that it is original to that amp.

As tubeswell noted, a good replacement would be a film cap.

Ceramic caps will crumble as you noted, especially right around the leads when they are bent. In that position of your amp, they tend to be microphonic, acting like a contact mike. Sometimes you can hit them with a screwdriver and they will ring through the speaker like a bell.

In this case, I'd replace it with a non-ceramic cap. In the case of non-signal path cap, like in the vibrato oscillator circuit, ceramics are fine.

[ctcpete]

I understand there are several different tpes of film caps, would Mallory 150's (spec sheet:tubular metalized polyester) be a good choice?
Funny thing, the schematic calls for a .001u while the chassis layout diagram calls for a .01u (both are from the ampwares site). I guess I will go with the value of the cap currently installed (.01u). And the 1Kv rating of the original cap is overkill? 600v is ok?
Thanks again for the great information
Pete

[tubeswell]

I understand there are several different tpes of film caps, would Mallory 150's (spec sheet:tubular metalized polyester) be a good choice?
Funny thing, the schematic calls for a .001u while the chassis layout diagram calls for a .01u (both are from the ampwares site). I guess I will go with the value of the cap currently installed (.01u). And the 1Kv rating of the original cap is overkill? 600v is ok?
Thanks again for the great information
Pete

Any film cap of a minimum 400V rating will be fine. Which schematic are you looking at BTW? The aa270 (which as far as I am aware covers '68 - '76 SF TR) shows a .01uF coupling cap going in to the PI.

http://ampedia.redbeartrading.com/da...a270_schem.gif

However the aa769 uses a .001uF there. (Could be that they started doing that before '76 - what values they ultimately use in these amps is always an evolving concept) No matter - either one will work, you'll just get less bass with the .001.

[ctcpete]

Hello Tubeswell, I'm looking at the aa270 schematic rev E dwg# 045377. It looks like Fender would change the rev level as it added ECO's ("engineering change orders" in my last job). I just checked, at the junction of the two 220k resistors the schem shows a .001u before the upper 12AT7 grid and the chassis layout (ie: schematic wiring diagram, same rev and dwg#)included with that schematic shows a .01 which is what my amp has.

Thanks for the reply. Pete

[tubeswell]

Well the scheme on that link in my previous post shows .01uF where you say it shows .001uF, yet according to the drawing reference, its the same drawing 045377 revision E. Go figure.

[ctcpete]

tubeswell, you are correct, the schematic on the ampedia link does show the correct .01value (as does the aa270 schematic in the Kendrick amp book I recently purchased). The aa270 schematics I have seen on the ampwares and schematic heaven sites show the .001 value and the printing is somewhat different and easier to read. Must have been a typo or something, makes me wonder if there are any other differences in the two schematics. As if things aren't confusing enough for a newbie like me. I will add your version to my already considerable collection of amp info. It is amazing what is available on this subject on the web.
Thanks again, Pete